Patent Publication Number: US-11649557-B2

Title: Method for forming holes, metal product, and metal composite

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims all benefits accruing under 35 U.S.C. § 119 from Chinese Patent Application No. 202011593469.2 filed on Dec. 29, 2020, in the State Intellectual Property Office of China, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The present disclosure relates to metal materials, and more particularly, the present disclosure relates to a method for forming holes, a metal product, a metal composite. 
     BACKGROUND 
     Portable consumer electronic products are being used more and more in people&#39;s lives. Consumers have higher and higher requirements for the appearance of electronic products and the performance of the housing. The existing housing molding process generally involves forming holes in a single metal piece, and then injecting plastic into the holes to form the housing. The holes in the metal piece formed by the traditional hole-forming method is not firmly combined with the material product. 
     SUMMARY 
     In view of the above situation, it is necessary to provide a method for forming holes to improve a bonding strength between a metal product and a material product. 
     According to some embodiments, a method for forming holes to form first holes in a surface of a metal part includes: putting the metal part into a first solution as an anode; applying a first voltage on the metal part to form the first holes in a surface of the metal part; and cleaning and drying the metal part with the first holes. The first solution includes a first organic solvent, chloride, and a phosphoric acid compound. 
     According to some embodiments, the first organic solvent is selected from a group consisting of ethylene glycol, propylene glycol, diethylene glycol, glycerol, and any combination thereof. 
     According to some embodiments, chloride is selected from a group consisting of sodium chloride, potassium chloride, copper chloride, ferric chloride, and any combination thereof. 
     According to some embodiments, the phosphoric acid compound is selected from a group consisting of phosphoric acid, dihydrogen phosphate, monohydrogen phosphate, phosphate, metaphosphate, and any combination thereof. 
     According to some embodiments, the first voltage is output by a gradual DC power supply, an increase rate of the first voltage is in a range of 1 V/s to 2 V/s, and a current density is in a range of 1 A/dm 2  to 10 A/dm 2 ; in the step of applying the first voltage on the metal part, a temperature of the first solution is in a range of 25° C. to 55° C., and a time of applying the first voltage is in a range of 10 min to 25 min. 
     According to some embodiments, before putting the metal part into the first solution, the method for forming holes further includes: putting the metal part into a second solution as an anode; and applying a second voltage on the metal part to form second holes in a surface of the metal part. The second solution includes a second organic solvent and a substance capable of dissociating Cl − . 
     According to some embodiments, the substance capable of dissociating Cl −  includes a compound containing crystallization water. 
     According to some embodiments, the substance capable of dissociating Cl −  is selected from a group consisting of NaCl, KCl, FeCl 3 ·6H 2 O, FeCl 3 , CuCl 2 ·12H 2 O, CuCl 2 , and any combination thereof. 
     According to some embodiments, the second solution further includes a substance capable of dissociating at least one Fe 3+  and Cu 2+ . 
     According to some embodiments, the substance capable of dissociating Fe 3+  or Cu 2+  includes a compound containing crystallization water. 
     According to some embodiments, the substance capable of dissociating Fe 3+  is selected from a group consisting of FeCl 3 ·6H 2 O, FeCl 3 , and any combination thereof, and the substance capable of dissociating Cu 2+  is selected from a group consisting of CuCl 2 ·12H 2 O, CuCl 2 , and any combination thereof. 
     According to some embodiments, the second voltage is output by DC power supply, the second voltage is in a range of 60V to 100V, and a current density is in a range of 1 A/dm 2  to 3 A/dm 2 ; in the step of applying the second voltage on the metal part, a temperature of the second solution is in a range of 50° C. to 70° C., and a time of applying the second voltage is in a range of 5 min to 20 min. 
     According to some embodiments, after putting the metal part into the first solution, the method for forming holes further includes: putting the metal part with the first holes into an electrolyte as an anode; and applying a third voltage on the metal part to form third holes in the metal part. The metal part is made of a material selected from a group consisting of aluminum, aluminum alloy, a composite material of aluminum alloy and stainless steel, and any combination thereof, the third holes are located in a portion of the metal part containing aluminum or aluminum alloy. 
     According to some embodiments, the metal part is made of a material selected from a group consisting of aluminum, aluminum alloy, stainless steel, and any combination thereof. 
     According to some embodiments, a metal product a metal substrate and holes in a surface of the metal substrate. The metal substrate is made of a material selected from a group consisting of aluminum, aluminum alloy, stainless steel, and any combination thereof, and the holes occupy 20% to 90% of an area of the surface of the metal substrate. 
     According to some embodiments, the metal substrate comprises an aluminum alloy portion and a stainless steel portion, the holes are located in a surface of the aluminum alloy portion and a surface of the stainless steel portion. A diameter of an inner cavity of each of at least one of the holes in the surface of the stainless steel portion is larger than a diameter of an opening of the corresponding hole. 
     According to some embodiments, a diameter of each of the holes in the aluminum alloy portion is in a range of 60 μm to 150 μm, a depth of each of the holes in the aluminum alloy portion is in a range of 80 μm to 100 μm, and the holes in the aluminum alloy portion occupy 60% to 90% of a portion of the area of the surface of the metal substrate corresponding to the aluminum alloy portion. 
     According to some embodiments, oxide holes are formed in an inner surface defining the inner cavity, and the oxide holes are nano-sized holes. 
     According to some embodiments, a metal composite includes the above metal product and a material product formed in the holes of the metal product. 
     According to some embodiments, the material product made of a material selected form a group consisting of metal, polymer, ceramic, glass, and any combination thereof. 
     In the method for forming holes, by adding corrosive ions into the first solution mainly composed of at least one organic solvent, the organic solvent can increase the energy required for the migration of corrosive ions (Cl − ) and reduce the diffusion rate of the corrosive ions (such as Cl − ), so that the corrosive ions (such as Cl − ) will not be unevenly distributed due to the influence of reaction activity, but can evenly bind effective ions that corrode to form holes on the surface of the metal part. Based on the small radius and the strong penetrating ability of Cl − , Cl −  can be preferentially adsorbed on the oxides to squeeze out oxygen atoms of the oxides and combine with cations of the oxides to form soluble chloride, thereby forming the first holes in the surface of the metal part. In addition, PO 4   3−  can react with the metal to form the aluminum phosphate film, thereby forming the first holes in a shape of coral. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Implementations of the present disclosure will now be described, by way of example only, with reference to the attached figures. 
         FIG.  1    is a flowchart of a method for forming holes, in accordance with some embodiments of the present disclosure. 
         FIG.  2    is a diagram of applying a second voltage on a metal part as an anode, in accordance with some embodiments of the present disclosure. 
         FIG.  3    is a simplified cross-sectional side view of first holes and third holes sequentially formed on a surface of a metal part, in accordance with some embodiments of the present disclosure. 
         FIG.  4    is a simplified cross-sectional side view of second holes formed on on a surface of a metal part, in accordance with some embodiments of the present disclosure. 
         FIG.  5    is an optical microscope image of a metal composite formed by a metal product with first holes of Example 1-2 of the present disclosure after injection molding. 
         FIG.  6    is a partial enlarged view of the metal composite of  FIG.  5   . 
         FIG.  7    is a partial enlarged view of a stainless steel portion of the metal composite of  FIG.  5   . 
         FIG.  8    is a scanning electron microscope (SEM) image of an aluminum alloy portion of an aluminum alloy-stainless steel metal product with second holes of Example 1-2 of the present disclosure. 
         FIG.  9    is a SEM image of a stainless steel portion of an aluminum alloy-stainless steel metal product with second holes of Example 1-2 of the present disclosure. 
         FIG.  10    is an optical microscope image of an aluminum alloy-stainless steel metal product with second holes of Example 5-1 of the present disclosure. 
         FIG.  11    is a partial enlarged view of an aluminum alloy portion of  FIG.  10   . 
         FIG.  12    is a partial enlarged view of a stainless steel portion of  FIG.  10   . 
         FIG.  13    is a partial enlarged view of the stainless steel portion of  FIG.  12    and an test view of a diameter and a depth of a hole along A-A. 
         FIG.  14    an optical microscope image of a stainless steel portion with first holes and second holes of a metal product Example 6-3 of the present disclosure. 
         FIG.  15    is a partial enlarged view of the stainless steel portion of  FIG.  14   . 
         FIG.  16    is a SEM image of a metal composite formed by an aluminum alloy-stainless steel metal product with first holes and second holes of Example 6-3 of the present disclosure after injection molding. 
         FIG.  17    is a SEM image of a metal composite formed by an aluminum alloy-stainless steel metal product with first holes and second holes of Example 6-3 of the present disclosure after injection molding. 
     
    
    
     DETAILED DESCRIPTION 
     It will be appreciated that for simplicity and clarity of illustration, where appropriate, reference numerals have been repeated among the different figures to indicate corresponding or analogous elements. In addition, numerous specific details are set forth in order to provide a thorough understanding of the embodiments described herein. However, it will be understood by those of ordinary skill in the art that the embodiments described herein can be practiced without these specific details. In other instances, methods, procedures, and components have not been described in detail so as not to obscure the related relevant feature being described. Also, the description is not to be considered as limiting the scope of the embodiments described herein. The drawings are not necessarily to scale, and the proportions of certain parts may be exaggerated to better illustrate details and features of the present disclosure. 
     The term “comprising,” when utilized, means “including, but not necessarily limited to”; it specifically indicates open-ended inclusion or membership in the so-described combination, group, series, and the like. 
     According to some embodiments, a method for forming holes is provided to form first holes  30  in a surface of a metal part  100  (shown in  FIGS.  1 - 4   ). In some embodiments illustrated in  FIG.  1   , a flowchart of a method for forming holes includes following steps. 
     Step S 10 , in some embodiments illustrated in  FIGS.  2  and  3   , putting a metal part  100  into a first solution  70  as an anode, then applying a first voltage on the metal part  100  to form first holes  30  in a surface of the metal part  100 . The first solution  70  includes a first organic solvent, chloride, and a phosphoric acid compound. 
     The first solution  70  further includes water. In the first solution  70 , the chloride can dissociate chloride ions (Cl − ) in the water, and the phosphoric acid compound can dissociate phosphate ions (PO 4   3− ) in the water. 
     The first organic solvent may be alcohol that can be miscible with water. The alcohol may be selected from a group consisting of ethylene glycol, propylene glycol, diethylene glycol, glycerol, and any combination thereof. The first organic solvent and water are mutually soluble, so that hydrolyzed ions are uniformly dispersed in the first organic solvent and uniformly loaded on the surface of the metal part  100  during applying the first voltage. 
     The metal part  100  may be made of a material selected from a group consisting of aluminum, aluminum alloy, a composite material of aluminum alloy and stainless steel, and any combination thereof. 
     According to some embodiments, the metal part  100  includes an aluminum alloy portion  10 . During applying the first voltage, a surface of the aluminum alloy portion  10  is oxidized to form oxides. Based on a small radius and a strong penetrating ability of Cl − , Cl −  can be preferentially adsorbed on the oxides to squeeze out oxygen atoms of the oxides and combine with cations of the oxides to form soluble chloride, thereby forming first holes  30  in a shape of coral in the surface of the aluminum alloy portion  10 . That is, the surface of the aluminum alloy portion  10  is uneven due to the holes and a protrusion between any two adjacent holes. Further, an inner wall defining each of the first holes  30  may include at least one protrusion. In addition, PO 4   3−  can react with the aluminum of the aluminum alloy portion  10  to form an aluminum phosphate film on the surface of the aluminum alloy portion  10 , and the reaction formula is Al 3+ +PO 3   3+ ═AlPO 4 , thereby preventing surface of the aluminum alloy portion  10  from excessively corroding. 
     According to some embodiments, the metal part  100  includes the aluminum alloy portion  10  and a stainless steel portion  20 . During applying the first voltage, a surface of the aluminum alloy portion  10  and a surface of the stainless steel portion  20  are oxidized to form oxides. Based on the small radius and the strong penetrating ability of Cl, Cl can be preferentially adsorbed on the oxides to squeeze out oxygen atoms of the oxides and combine with cations of the oxides to form soluble chloride, thereby forming first holes  30  in the surface of the aluminum alloy portion  10  and the stainless steel portion  20 . In addition, PO 4   3−  can react with the aluminum of the aluminum alloy portion  10  to form an aluminum phosphate film on the surface of the aluminum alloy portion  10 , and the reaction formula is Al 3+ +PO 4   3+ ═AlPO 4 , thereby forming holes in a shape of coral in the surface of the aluminum alloy portion  10 . That is, the first holes  30  are formed in the surface of the aluminum alloy portion  10  and the surface of the stainless steel portion  20 , the first holes  30  in the surface of the aluminum alloy portion  10  are coral-shaped, and the first holes  30  in the surface of the stainless steel portion  20  are pitting-shaped. 
     The water may be separately added to the first solution  70 , or may be generated by a compound containing crystallization water added in the first solution  70 . The water can be used to dissociate chloride to form Cl, and to dissociate the phosphoric acid compound to PO 4   3− . 
     Chloride may be selected from a group consisting of sodium chloride, potassium chloride, copper chloride, ferric chloride, and any combination thereof. 
     According to some embodiments, Chloride is ferric chloride, and ferric chloride can not only dissociate Cl − , but also dissociate iron ions (Fe 3+ ). Fe 3+  can interact with iron (Fe) of the stainless steel portion  20  to further form pitted first holes  30 . 
     The phosphoric acid compound may be selected from a group consisting of phosphoric acid, dihydrogen phosphate, monohydrogen phosphate, phosphate, metaphosphate, and any combination thereof. 
     The first voltage is output by DC power supply to make the ions in the first solution  70  migrate in an orderly manner. A current density may be in a range of 1 A/dm 2  to 10 A/dm 2 . According to some embodiments, the current density may be 1.6 A/dm 2 , 3.2 A/dm 2 , 5.6 A/dm 2 , or 7.8 A/dm 2 . 
     The first voltage may also be output by a gradual DC power supply. An increase rate of the first voltage may be in a range of 1 V/s to 2 V/s. According to some embodiments, the increase rate of the first voltage may be 1.2 V/s, 1.4 V/s, 1.6 V/s, or 1.8 V/s. The gradual DC power supply can prevent sudden excessive voltage from causing a concentrated discharge of electric charges and forming large holes that do not meet requirements in the surface of the metal part  100 . 
     In the step of applying the first voltage on the metal part  100 , a temperature of the first solution  70  may be in a range of 25° C. to 55° C., and a time of applying the first voltage may be in a range of 10 min to 25 min. 
     In step S 20 , cleaning and drying the metal part  100  with the first holes  30  to obtain a metal product  80 . 
     The ions and the first organic solvent on the surface of the metal part  100  are cleaned, and then the metal part  100  is dried, thereby obtaining the clean metal product  80  with the first holes  30 . 
     According to some embodiments, before the step S 10 , the method may further include a step S 01 . 
     The step S 01 , in some embodiments illustrated in  FIG.  4   , putting a metal part  100  into a second solution as an anode, then applying a second voltage on the metal part  100  to form second holes  40  in a surface of the metal part  100 . The second solution includes a second organic solvent and a substance that can dissociate Cl. 
     During applying the second voltage, the second organic solvent can increase an energy required for a migration of corrosive ions (Cl − ) and reduce a diffusion rate of the corrosive ions (such as Cl − ), so that the corrosive ions (such as Cl − ) will not be unevenly distributed due to the influence of reaction activity, but can evenly bind effective ions that corrode to form holes on the surface of the metal part  100 , so as to avoid a formation of galvanic cells in an electric field due to a potential difference between the aluminum alloy portion  10  and the stainless steel portion  20 , prevent the corrosive ions (such as Cl − ) from being concentrated on the surface of the aluminum alloy portion  10  serving as the anode to react with the aluminum alloy portion  10 , and avoid a phenomenon that the stainless steel portion  20  is not etched to form holes by the corrosive ions (such as Cl − ). The surface of the metal part  100  is weakly corroded by electrochemical traction. Due to the difference in material between the aluminum alloy portion  10  and the stainless steel portion  20 , an oxide film (alumina) is formed on the surface of the aluminum alloy portion  10  to protect the aluminum alloy portion  10  and avoid excessive corrosion of the surface aluminum alloy portion  10  by the corrosive ions. Therefore, it is possible to form high-density second holes  40  in the surface of the stainless steel portion  20  while protecting the aluminum alloy portion  10 . 
     The second solution further includes water. The water may be separately added to the second solution, or may be generated by a compound containing crystallization water added in the second solution. The water can be used for dissociation to form Cl − . 
     A mass fraction of the water in the second solution is in a range of 7% to 63%. A content of the water in the second solution should not be too much. On the one hand, if the water is excessive (more than 63%), a content of the second organic solvent will be reduced, which will not play a role of restraining the corrosive ions. On the other hand, the water in the second solution is not excessive, which can prevent the corrosive ions from migrating too fast in the excess water and quickly corroding the surface of the aluminum alloy portion  10 . The content of the water in the second solution should not be too low (less than 7%), otherwise a migration rate of the corrosive ions is too slow, resulting in a low reaction efficiency and increasing time cost and energy cost. According to some embodiments, part of the water may also be provided by the crystallization water contained in the compound. 
     The substance capable of dissociating Cl −  may be selected from a group consisting of NaCl, KCl, FeCl 3 ·6H 2 O, FeCl 3 , CuCl 2 ·12H 2 O, CuCl 2 , and any combination thereof. 
     According to some embodiments, the second solution may further include a substance capable of dissociating at least one Fe 3+  and Cu 2+ . 
     The substance capable of dissociating Fe 3+ , Cu 2+ , or Cl −  includes a compound containing crystallization water. 
     The substance capable of dissociating Fe 3+  may be selected from a group consisting of FeCl 3 ·6H 2 O, FeCl 3 , and any combination thereof. The substance capable of dissociating Cu 2+  may be selected from a group consisting of CuCl 2 ·12H 2 O, CuCl 2 , and any combination thereof. The substance capable of dissociating Cl −  may be selected from a group consisting of NaCl, KCl, FeCl 3 ·6H 2 O, FeCl 3 , CuCl 2 ·12H 2 O, CuCl 2 , and any combination thereof. 
     When the second solution contains at least one of Fe 3+  and Cu 2+ , a potential of Fe 3+  and a potential of Cu 2+  are both higher than a potential of elemental iron (Fe), and both can have a substitution reaction with the iron in the stainless steel portion  20 , which is beneficial to etching the stainless steel portion  20  to further form the second holes  40  having larger diameters. A potential difference (shorted as PD) between Cu 2+  and Fe is greater than a PD between Fe 3+  and Fe. Therefore, the second solution containing Cu 2+  is more conducive to forming the second holes  40  having larger diameters and larger depths than second solution containing Fe 3+ . 
     The second organic solvent may be alcohol that can be miscible with water. The alcohol may be selected from a group consisting of ethylene glycol, propylene glycol, diethylene glycol, glycerol, and any combination thereof. The second organic solvent and water are mutually soluble, so that hydrolyzed ions are uniformly dispersed in the second organic solvent and the corrosive ions are uniformly loaded on the surface of the metal part  100  during applying the second voltage. 
     The second voltage is output by DC power supply to make the ions in the second solution migrate in an orderly manner. The second voltage may be in a range of 60V to 100V. A current density may be in a range of 1 A/dm 2  to 3 A/dm 2 . According to some embodiments, the second voltage may be 65V, 76V, 88V, or 95V, and the current density may be 1.5 A/dm 2 , 1.9 A/dm 2 , 2.3 A/dm 2 , or 2.8 A/dm 2 . 
     In the step of applying the second voltage on the metal part  100 , a temperature of the second solution may be in a range of 50° C. to 70° C. According to some embodiments, the temperature of the second solution may be 55° C., 59° C., 63° C., or 68° C. If the temperature of the second solution is too low, a density of the second holes formed in the stainless steel portion  20  will be low. If the temperature of the second solution is too high, energy is provided for ion migration, thereby corroding the aluminum alloy portion  10 . 
     A time of applying the second voltage may be in a range of 5 min to 20 min. According to some embodiments, the time of applying the second voltage may be 7 min, 9 min, 13 min, or 17 min. The time of applying the second voltage can be adjusted according to factors such as the diameter, the depth, and the density of the second holes  40  required to be formed. 
     According to some embodiments, before forming the second holes  40  in the surface of the metal part  100 , the method may further include a step of applying a surface treatment of the metal part  100  to remove impurities, oil stains, oxide layers, etc. on the surface of the metal part  100 . The surface treatment may includes oil removal treatment and black film peeling treatment. 
     According to some embodiments, after the step S 10  and before the step  20 , the method may further include a step S 12 . 
     The step S 12 , in some embodiments illustrated in  FIG.  3   , putting the metal part  100  with the first holes  30  into an electrolyte as an anode, then applying a third voltage on the metal part  100  to form third holes  50 . The metal part  100  may be made of a material selected from a group consisting of aluminum, aluminum alloy, a composite material of aluminum alloy and stainless steel, and any combination thereof. The third holes  50  are located in a portion of the metal part  100  including aluminum or aluminum alloy. 
     According to some embodiments, the metal part  100  applied in the step  12  may further include second holes  40 . 
     The step S 12  may be an anodic oxidation process. Water is mainly used as a solvent in the electrolyte to from an anodic oxidation film with holes on the aluminum alloy portion  10 . 
     In some embodiments illustrated in  FIG.  3   , a metal product  80  formed by the above method for forming holes is provided. The metal product  80  includes a metal substrate and holes located in a surface of the metal substrate The metal substrate may be made of a material selected from a group consisting of aluminum, aluminum alloy, stainless steel, and any combination thereof. The holes may occupy 20% to 90% of an area of the surface of the metal substrate. The holes are the first holes  30  described above. 
     According to some embodiments, the metal substrate is made of a material composed of an aluminum alloy portion and a stainless steel portion, that is, the metal substrate includes an aluminum alloy portion  10   a  and a stainless steel portion  20   a . The holes are located in a surface of the aluminum alloy portion  10   a  and a surface of the stainless steel portion  20   a . The holes in a surface of the aluminum alloy portion  10   a  are coral-shaped. A diameter of an inner cavity of each of at least one of the holes in the surface of the stainless steel portion  20   a  is larger than a diameter of an opening of the corresponding hole, that is, the at least one of the holes located in the stainless steel portion is barbed-shaped. 
     A diameter of each of the holes in the aluminum alloy portion  10   a  is in a range of 60 μm to 150 μm. A depth of each of the holes in the aluminum alloy portion  10   a  is in a range of 80 μm to 100 μm. The holes in the aluminum alloy portion  10   a  occupy 60% to 90% of a portion of the area of the surface of the metal substrate corresponding to the aluminum alloy portion  10   a.    
     A diameter of each of the holes in the stainless steel portion  20   a  is in a range of 60 μm to 120 μm. A depth of each of the holes in the stainless steel portion  20   a  is in a range of 50 μm to 100 μm. The holes in the stainless steel portion  20   a  occupy 20% to 55% of a portion of the area of the surface of the metal substrate corresponding to the stainless steel portion  20   a.    
     The metal product  80  may further include an oxide film including oxide holes on the aluminum alloy portion  10   a . The oxide holes in the oxide film are the third holes  50  described above. A diameter of each of the oxide holes included in the oxide film may be in a range of 1 nm to 900 nm. A shape of the each of the oxide holes included in the oxide film may be roughly cylindrical. 
     According to some embodiments, the metal substrate is made of the material selected from a group consisting of aluminum, aluminum alloy, and any combination thereof. An inner cavity of each of the holes is coral-shaped. That is, the surface of the metal substrate with the holes is uneven due to the holes and a protrusion between any two adjacent holes. Further, an inner surface defining the inner cavity may include at least one protrusion. 
     According to some embodiments, the oxide holes are formed in the inner surface defining the inner cavity. The oxide holes are nano-sized holes. 
     In some embodiments illustrated in  FIG.  3   , a metal composite  200  is provided. The metal composite  200  includes the metal product  80  and a material product  220 . The material product  220  is formed in the holes to be combined with the metal product  80 , thereby enhancing a bonding strength between the material product  220  and the metal product  80 . 
     The material product  220  may be made of a material selected form a group consisting of metal, polymer, ceramic, glass, and any combination thereof. It should be noted that the polymer includes plastics or resins commonly used. 
     According to some embodiments, the metal product  80  includes the first holes  30 , the material product  220  is formed in the first holes  30  to enhance the bonding strength between the material product  220  and the metal product  80 . 
     According to some embodiments, the metal product  80  may further include the third holes  50 , that is, the oxide holes. The material product  220  may be further formed in the third holes  50  to further enhance the bonding strength between the material product  220  and the metal product  80 . 
     According to some embodiments, after the step S 20 , the method may further include a step S 30 . 
     The step  30 , in some embodiments illustrated in  FIG.  3   , putting the metal part  100  with the first holes  30  into a molding machine (not shown), and forming the material of the material product  220  in the first holes  30  and the third holes  50 , thereby obtaining the metal composite  200 . 
     According to some embodiments, the molding machine may be an injection molding machine for injecting polymer into the first holes  30  and the third holes  50  to form the metal composite  200 . 
     Some specific examples and comparative examples are listed below to better illustrate this application. It should be noted that the metal parts  100  used in Examples 1-1 to 5-4 and Comparative Example 2-1 are workpieces after a surface treatment, and respectively include the aluminum alloy portion made of aluminum alloy 6013 and the stainless steel portion made of stainless steel 316. The metal parts  100  used in Examples 6-1 to 9-3 and Comparative Examples 6-1 to 8-1 are workpieces are all the workpieces processed in Example 2-1. 
     The surface treatment includes: first placing the metal part  100  in an aqueous solution containing a degreasing agent (BONDERITE C-AK 1523R) having a mass fraction of 35% at a temperature of 55° C. to ultrasonic cleaning for 3 min; then putting the metal part  100  in nitric acid having a mass fraction of 35% for 1 min. 
     The metal product  80  obtained after the metal part  100  undergoes a method for forming holes is tested for holes forming condition through an optical microscope. 
     Examples 1-1 to 1-4 
     The metal part  100  after the surface treatment was putted into a first solution  70 . The first solution  70  included 40 wt % propylene glycol, 5 wt % ethylene glycol, 10 wt % phosphoric acid, 5 wt % sodium chloride, and 45 wt % water. In Examples 1-1 to 1-4, rection temperatures of Examples 1-1 to 1-4 were controlled to be 20° C., 30° C., 40° C., and 50° C., respectively. Then, the metal part  100  was used as an anode, a first voltage was applied on the metal part  100  for 15 min to form first holes  30 . A current density was controlled to be 3 A/dm 2 . Finally, the metal part  100  with the first holes  30  was taken out to wash with water, and dried at a temperature of 80° C. for 20 min. 
     The main different conditions between Examples 1-1 to 1-4 and the test results of Examples 1-1 to 1-4 were shown in the following Table 1. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Test results 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 Percentage of 
               
               
                   
                   
                   
                   
                   
                 area occupied 
               
               
                   
                   
                   
                   
                   
                 by the first 
               
               
                   
                   
                 Distribution 
                 Depth of 
                 Diameter 
                 holes on the 
               
               
                   
                 Reaction 
                 of the first 
                 the first 
                 of the first 
                 surface of the 
               
               
                 Example 
                 temperature 
                 holes 
                 holes (μm) 
                 holes (μm) 
                 metal part 
               
               
                   
               
               
                 Example 1-1 
                 20° C. 
                 communication 
                 50 to 80  
                 300 to 500  
                 15% 
               
               
                   
                   
                 between the 
               
               
                   
                   
                 first holes, the 
               
               
                   
                   
                 holes with 
               
               
                   
                   
                 barbed-shape 
               
               
                   
                   
                 are sparse 
               
               
                 Example 1-2 
                 30° C. 
                 the holes with 
                 60 to 100 
                 50 to 100 
                 25% 
               
               
                   
                   
                 barbed-shape 
               
               
                   
                   
                 are formed in 
               
               
                   
                   
                 the surface of 
               
               
                   
                   
                 the stainless 
               
               
                   
                   
                 steel portion, 
               
               
                   
                   
                 the holes near 
               
               
                   
                   
                 the aluminum 
               
               
                   
                   
                 alloy portion 
               
               
                   
                   
                 are sparse 
               
               
                 Example 1-3 
                 40° C. 
                 the holes with 
                 70 to 120 
                 60 to 120 
                 30% 
               
               
                   
                   
                 barbed-shape 
               
               
                   
                   
                 are formed in 
               
               
                   
                   
                 the surface of 
               
               
                   
                   
                 the stainless 
               
               
                   
                   
                 steel portion, 
               
               
                   
                   
                 the holes near 
               
               
                   
                   
                 the aluminum 
               
               
                   
                   
                 alloy portion 
               
               
                   
                   
                 are sparse 
               
               
                 Example 1-4 
                 50° C. 
                 the holes with 
                 80 to 140 
                 70 to 130 
                 35% 
               
               
                   
                   
                 barbed-shape 
               
               
                   
                   
                 are formed in 
               
               
                   
                   
                 the surface of 
               
               
                   
                   
                 the stainless 
               
               
                   
                   
                 steel portion, 
               
               
                   
                   
                 the holes near 
               
               
                   
                   
                 the aluminum 
               
               
                   
                   
                 alloy portion 
               
               
                   
                   
                 are uniform 
               
               
                   
               
            
           
         
       
     
       FIGS.  5 ,  6  and  7    are tested optical microscope images of the metal composite  200  formed by injection molding the aluminum alloy-stainless steel metal product  80  obtained after the method for forming holes of Example 1-2 of the present disclosure. The first holes  30  are formed in both of the surface of the aluminum alloy portion  10  and the surface of the stainless steel portion  20 . The material product  220  surrounds the metal product  80  and is formed in the first holes  30 . The diameter and the depth of the first holes  30  can be measured from  FIG.  7   . The measured depths of some of the first holes  30  are, for example, 46.96 μm, 50.58 μm, 39.74 μm, 62.62 μm, 55.39 μm, 48.17 μm, and 59.00 μm. It should be noted that  FIG.  7    is only a structure of part of the first holes  30  in the aluminum alloy-stainless steel metal product  80 , and the range of the depth of the first holes  30  of Example 1-2 is the range of an average depth of all the first holes  30  in the the aluminum alloy-stainless steel metal product  80 . Referring to  FIG.  8   , coral-shaped first holes  30  are formed in the surface of the aluminum alloy portion  10 . Referring to  FIG.  9   , pitting-shaped first holes  30  are formed in the surface of the stainless steel portion  20 . 
     It can be seen form the test results in Table 1: in Examples 1-1 to 1-4, the first holes  30  can be formed both in the surface of the aluminum alloy portion  10  and the surface of the stainless steel portion  20 . As the reaction temperature increases, the depth of the first holes  30  and the percentage of area occupied by the first holes  30  both are increased. The main reason is that as the reaction temperature increases, the corrosion activity of Cl increases, which leads to an increase in the depth of the first holes and the percentage of area occupied by the first holes. It can be seen from the test results of Example 1-1 and Example 1-2 that since a crystallization temperature of phosphoric acid is 21° C., a viscosity of phosphoric acid increases at a temperature (such as 20° C.) lower than the crystallization temperature. During the reaction process, phosphoric acid will adhere to the newly formed first holes  30 , so that Cl cannot contact the inner wall of each of the newly formed first holes  30  to deepen the first holes  30 , and Cl will corrode the metal at the edge of each of the first holes  30  to causing a communication between the first holes. As a result, the diameter of the first holes increase but the depth of the first holes is relatively small. 
     Examples 2-1 to 2-5 
     The metal part  100  after the surface treatment was putted into a second solution. The second solution included 1 L propylene glycol as a solvent. The second solutions of Examples 2-1 to 2-5 respectively further included FeCl 3 ·6H 2 O with a concentration of 50 g/L, 75 g/L, 100 g/L, 150 g/L, and 200 g/L. A temperature of the second solution was 55° C. Then, the metal part  100  was used as an anode and graphite was used as a cathode, a second voltage of 80V was applied on the metal part  100  for 10 min to form second holes  40 . Finally, the metal part  100  with the second holes  40  was cleaned and dried at a temperature of 80° C. to obtain the metal product  80  with the second holes  40 . 
     Comparative Example 2-1 
     Different from the above Example 2-1, water was used as the solvent. 
     The main different conditions between Examples 2-1 to 2-5 and Comparative Example 2-1, and the test results of Examples 2-1 to 2-5 and Comparative Example 2-1 were shown in the following Table 2. 
     
       
         
           
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Test results 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 Percentage of 
               
               
                   
                   
                   
                   
                   
                 area occupied 
               
               
                   
                   
                   
                   
                   
                 by the second 
               
               
                 Example or 
                   
                 Concentration of 
                 Distribution 
                 Depth of 
                 holes on the 
               
               
                 Comparative 
                   
                 FeCl 3 •6H 2 O 
                 of the second 
                 the second 
                 surface of the 
               
               
                 Example 
                 Solvent 
                 (g/L) 
                 holes 
                 holes (μm) 
                 metal part 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Example 2-1 
                 propylene 
                 50 
                 Relatively 
                  5-10 
                 45% 
               
               
                   
                 glycol 
                   
                 uniform 
               
               
                 Example 2-2 
                 propylene 
                 75 
                 Relatively 
                 10-20 
                 60% 
               
               
                   
                 glycol 
                   
                 uniform 
               
               
                 Example 2-3 
                 propylene 
                 100 
                 Relatively 
                 15-30 
                 70% 
               
               
                   
                 glycol 
                   
                 uniform 
               
               
                 Example 2-4 
                 propylene 
                 150 
                 Relatively 
                 15-35 
                 75% 
               
               
                   
                 glycol 
                   
                 uniform, the 
               
               
                   
                   
                   
                 area where holes 
               
               
                   
                   
                   
                 communicate 
               
               
                   
                   
                   
                 with each other 
               
               
                   
                   
                   
                 occupies 5% 
               
               
                 Example 2-5 
                 propylene 
                 200 
                 Relatively 
                 20-40 
                 80% 
               
               
                   
                 glycol 
                   
                 uniform, the 
               
               
                   
                   
                   
                 area where holes 
               
               
                   
                   
                   
                 communicate 
               
               
                   
                   
                   
                 with each other 
               
               
                   
                   
                   
                 occupies 20% 
               
               
                 Comparative 
                 water 
                 50 
                 No holes, the 
                 — 
                 — 
               
               
                 Example 2-1 
                   
                   
                 aluminum 
               
               
                   
                   
                   
                 alloy portion is 
               
               
                   
                   
                   
                 corroded a lot 
               
               
                   
               
            
           
         
       
     
     It can be seen form the test results in Table 2: the solvent in Comparative Example 2-1 is all water. After a same method for forming holes applied in Example 2-1 to 2-5, the aluminum alloy portion  10  of Comparative Example 2-1 is largely corroded, and no holes can be formed in the stainless steel portion  20  of Comparative Example 2-1. Relatively uniform second holes  40  are respectively formed in the metal part  100  of Examples 2-1 to 2-5, and the second holes  40  are mainly distributed in the stainless steel portion  20  of Examples 2-1 to 2-5. The reason for the above results is: when in an aqueous solution, an activity of the aluminum alloy portion  10  is higher than an activity of the stainless steel portion  20 , if an voltage is applied, the aluminum alloy portion  10  is preferentially corroded by ions, and uniform holes cannot be formed in the surface of the stainless steel portion  20 ; when in an organic solution, the organic solvent increased an energy required for ion migration, and the effective ions that corrode to form holes are evenly bound on the surface of the metal part  100 , and the surface of the metal part  100  is weakly corroded by electrochemical traction. If an voltage is applied, an oxide film (alumina) is formed on the surface of the aluminum alloy portion  10  to protect the aluminum alloy portion  10 . When the voltage is continuously applied subsequently, the ions cannot continue to react with the aluminum alloy portion  10  coated by the oxide film, and the ions react with the stainless steel portion  20  to form evenly distributed second holes  40  in the surface of the stainless steel. It can be seen from Examples 2-1 to 2-5 that as the mass fraction of FeCl 3 ·6H 2 O changes, the distribution and the depth of the second holes  40  and the percentage of area occupied by the second holes  40  are affected, and the percentage of area occupied by the second holes  40  can be increased as the mass fraction of FeCl 3 ·6H 2 O increases. 
     Examples 3-1 to 3-4 
     The metal part  100  was putted into a second solution. The second solution included 1 L propylene glycol. The second solutions of Examples 3-1 to 3-4 respectively further included different salts that can dissociate Cl with a concentration of 300 mmol/L. A temperature of the second solution was 65° C. Then, the metal part  100  was used as an anode, a second voltage of 80V was applied on the metal part  100  for 8 min to form second holes  40 . Finally, the metal part  100  with the second holes  40  was cleaned and dried to obtain the metal product  80  with the second holes  40 . 
     The main different conditions between Examples 3-1 to 3-4 and the test results of Examples 3-1 to 3-4 were shown in the following Table 3. 
     
       
         
           
               
               
             
               
                   
                 TABLE 3 
               
             
            
               
                   
                   
               
               
                   
                 Test results 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                   
                   
                 Percentage of 
               
               
                   
                   
                   
                   
                 area occupied 
               
               
                   
                   
                   
                   
                 by the second 
               
               
                   
                   
                 Distribution 
                 Depth of 
                 holes on the 
               
               
                   
                 Second 
                 of the second 
                 the second 
                 surface of the 
               
               
                 Example 
                 solution 
                 holes 
                 holes (μm) 
                 metal part 
               
               
                   
               
               
                 Example 3-1 
                 300 mmol/L 
                 uniform, part of 
                 2-8 
                 15% 
               
               
                   
                 NaCl 
                 the area where 
               
               
                   
                   
                 holes communicate 
               
               
                   
                   
                 with each other 
               
               
                 Example 3-2 
                 150 mmol/L 
                 uniform 
                 20-50 
                 10% 
               
               
                   
                 CuCl 2 •12H 2 O 
               
               
                 Example 3-3 
                 100 mmol/L 
                 uniform 
                  5-20 
                 55% 
               
               
                   
                 FeCl 3 •6H 2 O 
               
               
                 Example 3-4 
                 50 mmol/L 
                 uniform 
                 15-30 
                 75% 
               
               
                   
                 FeCl 3 •6H 2 O 
               
               
                   
                 and 150 mmol/L 
               
               
                   
                 NaCl 
               
               
                   
               
            
           
         
       
     
       FIGS.  10 ,  11 ,  12  and  13    are tested optical microscope images of the metal product  80  of Example 3-1. Referring to  FIG.  11   , the surface of the aluminum alloy portion  10  is basically not corroded. Referring to  FIG.  12   , high-density and unform second holes  40  are formed in the surface of the stainless steel portion  20 . 
     The diameter and the depth of the second holes  40  in the stainless steel portion  20  are measured from  FIG.  13   . The diameter of one of the second holes  40  is 35.23 μm, the depth of one of the second holes  40  is 6.72 μm. The approximate range of the diameter and the approximate range of the depth of the second holes  40  can be calculated from  FIG.  13   . 
     It can be seen form the test results in Table 3: the evenly distributed second holes  40  can be formed in the surface of the stainless steel portion  20  by different kinds of chlorides. When sodium chloride (Example 3-1) is used as a source of corrosive ions (Cl − ), the diameter and the depth of the second holes  40  in the surface of the stainless steel portion  20  are both small, and the communication between the second holes  40  is easy to occur. In Example 3-2 to 3-4, in addition to Cl − , the corrosive ions further includes Fe 3+  or Cu 2+ , and Fe 3+  or Cu 2+  reacts with elemental iron during applying the voltage to form the second holes  40  with larger diameter and larger depth. Wherein, the depth of the second holes of Example 3-2 is significantly greater than the depth of the second holes of Example 3-3. 
     Examples 4-1 to 4-5 
     The metal part  100  was putted into a second solution. The second solution of Examples 4-1 to 4-5 respectively included different organic solvents with a mass fraction of 95%. The second solution further included 5 wt % FeCl 3 ·6H 2 O. A temperature of the second solution was 50° C. Then, the metal part  100  was used as an anode, a second voltage of 70V was applied on the metal part  100  for 5 min to form second holes  40 . Finally, the metal part  100  with the second holes  40  was cleaned and dried to obtain the metal product  80  with the second holes  40 . 
     The main different conditions between Examples 4-1 to 4-5 and the test results of Examples 4-1 to 4-5 were shown in the following Table 4. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                   
                   
                   
                 Test results 
               
               
                   
                   
                   
                 Percentage of 
               
               
                   
                   
                   
                 area occupied 
               
               
                   
                   
                   
                 by the second 
               
               
                   
                   
                   
                 holes on the 
               
               
                   
                   
                   
                 surface of the 
               
               
                 Example 
                 Organic solvent 
                 mass fraction 
                 metal part 
               
               
                   
               
             
            
               
                 Example 4-1 
                 propylene glycol 
                 95% 
                 50% 
               
               
                 Example 4-2 
                 ethylene glycol 
                 95% 
                 28% 
               
               
                 Example 4-3 
                 diethylene glycol 
                 95% 
                 25% 
               
               
                 Example 4-4 
                 glycerol 
                 95% 
                 50% 
               
               
                 Example 4-5 
                 propylene glycol 
                 50% and 45% 
                 55% 
               
               
                   
                 and ethylene glycol 
               
               
                   
               
            
           
         
       
     
     It can be seen form the test results in Table 4: the evenly distributed second holes  40  can be formed in the surface of the stainless steel portion  20  by different kinds of alcohols as organic solvents. Different alcohols have an influence on the percentage of area occupied by the second holes  40 , but no aluminum alloy portions  10  are corroded by the different alcohols. 
     Examples 5-1 to 5-4 
     The metal part  100  was putted into a second solution. The second solution included 95 wt % propylene glycol and 5 wt % FeCl 3 ·6H 2 O. A temperature of the second solution was 60° C. Then, the metal part  100  was used as an anode, second voltage of 20V. 60V, 80V, and 100V were respectively applied on the metal parts  100  of Examples 5-1 to 5-4 for 15 min to form second holes  40 . Finally, the metal part  100  with the second holes  40  was cleaned and dried to obtain the metal product  80  with the second holes  40 . 
     The main different conditions between Examples 5-1 to 5-4 and the test results of Examples 5-1 to 5-4 were shown in the following Table 5. 
     
       
         
           
               
               
             
               
                   
                 TABLE 5 
               
             
            
               
                   
                   
               
               
                   
                 Test results 
               
            
           
           
               
               
               
               
            
               
                   
                   
                   
                 Percentage of 
               
               
                   
                   
                   
                 area occupied 
               
               
                   
                   
                   
                 by the second 
               
               
                   
                   
                 Distribution 
                 holes on the 
               
               
                   
                 Second 
                 of the second 
                 surface of the 
               
               
                 Example 
                 voltage (V) 
                 holes 
                 metal part 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 Example 5-1 
                 20 
                 a small number of 
                 5% 
               
               
                   
                   
                 the second holes 
               
               
                   
                   
                 appear on the edge 
               
               
                   
                   
                 of the stainless 
               
               
                   
                   
                 steel portion 
               
               
                 Example 5-2 
                 60 
                 the second holes 
                 15% 
               
               
                   
                   
                 appear on the edge 
               
               
                   
                   
                 of the stainless 
               
               
                   
                   
                 steel portion 
               
               
                 Example 5-3 
                 80 
                 the second holes 
                 45% 
               
               
                   
                   
                 uniformly appear 
               
               
                   
                   
                 in the surface of the 
               
               
                   
                   
                 stainless steel 
               
               
                   
                   
                 portion 
               
               
                 Example 5-4 
                 100 
                 the second holes 
                 10% 
               
               
                   
                   
                 appear in the 
               
               
                   
                   
                 surface and on the 
               
               
                   
                   
                 edge of the 
               
               
                   
                   
                 stainless steel 
               
               
                   
                   
                 portion, the second 
               
               
                   
                   
                 holes on the edge 
               
               
                   
                   
                 of the stainless 
               
               
                   
                   
                 steel portion 
               
               
                   
                   
                 communicate with 
               
               
                   
                   
                 each other 
               
               
                   
               
            
           
         
       
     
     It can be seen form the test results in Table 5: as the second voltage increases, it is beneficial to increase the percentage of area occupied by the second holes. If the second voltage is too high, the edge of the stainless steel portion  20  is easy to excessive corrosion. So that the excessive corrosion can be avoided by reducing the second voltage. 
     Examples 6-1 to 6-3 
     The metal part  100  treated in Example 2-1 was putted into a first solution  70  at 35° C. The first solution  70  included 40 wt % propylene glycol, 5 wt % ethylene glycol, 10 wt % phosphoric acid, and 45 wt % water. The first solutions  70  of Examples 6-1 to 6-3 further included sodium chloride with a concentration of 2 g/L, 4 g/L, and 8 g/L, respectively. Then, the metal part  100  was used as an anode, a first voltage was applied on the metal part  100  for 15 min to form first holes  30 . A current density of the first voltage was controlled to be 2 A/dm 2 . Finally, the metal part  100  with the first holes  30  was taken out to wash with water, and dried at a temperature of 80° C. for 20 min. 
     Comparative Example 6-1 
     Different from the above Example 6-2, the first solution  70  does not include any organic solvent, and the mass fraction of the water is 90 wt %. 
     Comparative Example 6-2 
     Different from the above Example 6-1, the first solution  70  does not include sodium chloride. 
     The main different conditions between Examples 6-1 to 6-3 and Comparative Examples 6-1 to 6-2, and the test results of Examples 6-1 to 6-3 and Comparative Examples 6-1 to 6-2 were shown in the following Table 6. 
     
       
         
           
               
               
             
               
                   
                 TABLE 6 
               
             
            
               
                   
                   
               
               
                   
                 Test results 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                 Percentage of 
               
               
                   
                   
                   
                   
                   
                   
                 area occupied 
               
               
                   
                   
                   
                   
                   
                   
                 by the first 
               
               
                 Example or 
                   
                   
                 Distribution 
                 Depth of 
                 Diameter of 
                 holes on the 
               
               
                 Comparative 
                   
                 Concentration 
                 of the first 
                 the first 
                 the first 
                 surface of the 
               
               
                 Example 
                 Solvent 
                 of NaCl 
                 holes 
                 holes (μm) 
                 holes (μm) 
                 metal part 
               
               
                   
               
               
                 Example 6-1 
                 propylene 
                 2 g/L 
                 the holes with 
                 20-30 
                 20-30 
                 10% 
               
               
                   
                 glycol and 
                   
                 barbed-shape 
               
               
                   
                 ethylene 
                   
                 is formed in 
               
               
                   
                 glycol 
                   
                 the surface of 
               
               
                   
                   
                   
                 the stainless 
               
               
                   
                   
                   
                 steel portion 
               
               
                 Example 6-2 
                 propylene 
                 4 g/L 
                 the holes with 
                 30-80 
                 30-50 
                 20% 
               
               
                   
                 glycol and 
                   
                 barbed-shape 
               
               
                   
                 ethylene 
                   
                 is formed in 
               
               
                   
                 glycol 
                   
                 the surface of 
               
               
                   
                   
                   
                 the stainless 
               
               
                   
                   
                   
                 steel portion 
               
               
                 Example 6-3 
                 propylene 
                 8 g/L 
                 the holes with 
                  50-100 
                  60-120 
                 30% 
               
               
                   
                 glycol and 
                   
                 barbed-shape 
               
               
                   
                 ethylene 
                   
                 is formed in 
               
               
                   
                 glycol 
                   
                 the surface of 
               
               
                   
                   
                   
                 the stainless 
               
               
                   
                   
                   
                 steel portion 
               
               
                 Comparative 
                 water 
                 4 g/L 
                 No hole is 
                 — 
                 — 
                 — 
               
               
                 Example 6-1 
                   
                   
                 formed in the 
               
               
                   
                   
                   
                 surface of the 
               
               
                   
                   
                   
                 stainless steel 
               
               
                   
                   
                   
                 portion, the 
               
               
                   
                   
                   
                 aluminum 
               
               
                   
                   
                   
                 alloy portion 
               
               
                   
                   
                   
                 is excessively 
               
               
                   
                   
                   
                 corroded 
               
               
                 Comparative 
                 propylene 
                 0 g/L 
                 the surface of 
                 — 
                 — 
                 — 
               
               
                 Example 6-2 
                 glycol and 
                   
                 the stainless 
               
               
                   
                 ethylene 
                   
                 steel portion 
               
               
                   
                 glycol 
                   
                 is polished 
               
               
                   
               
            
           
         
       
     
       FIGS.  14  and  15    are tested optical microscope images of the stainless steel portion  20  of the metal product  80  of Example 6-3. The diameter and the depth of the first hole  30  can be measured from the enlarged view.  FIGS.  16  and  17    are tested optical microscope images of the metal composite  200  formed by injecting material product  220  on the metal product  80 . Pitting-shaped first holes  30  and barbed-shaped first holes  30  are formed in the surface of the stainless steel portion  20 . 
     It can be seen form the test results in Table 6: comparing Examples 6-1 to 6-3 and Comparative Example 6-1, the barbed-shaped holes can be formed in the surface of the stainless steel portion  20  of each of Examples 6-1 to 6-3. Due to addition of the organic solvent, the difference in chemical characteristics between the stainless steel portion  20  and the aluminum alloy portion  10  can be balanced. In the aqueous solution system, since a chemical activity of the aluminum alloy portion  10  is higher than a chemical activity of the stainless steel portion  20 , the reaction system will mainly chemically corrode the aluminum alloy portion  10 , and the surface of the stainless steel portion  20  cannot be corroded. 
     Comparing Examples 6-1 to 6-3 and Comparative Example 6-2, the barbed-shaped holes can be formed in the surface of the stainless steel portion  20  of each of Examples 6-1 to 6-3, and the surface of the stainless steel portion  20  of Comparative Example 6-2 is polished. The above situation shows that the holes can be formed on the surface of the stainless steel portion  20  by Cl. Comparing Examples 6-1 to 6-3, as a concentration of Cl increases, a hole-forming ability, the diameter of the holes, and the depth of the holes increase. When the concentration of sodium chloride is higher than 8 g/L, the diameter of the first holes  30  and the depth of the first holes  30  will no longer change significantly. 
     Examples 7-1 to 7-3 
     The metal part  100  treated in Example 2-1 was putted into a first solution  70  at 30° C. The first solution  70  included 40 wt % propylene glycol, 5 wt % ethylene glycol, 10 wt % phosphoric acid, and 45 wt % water. The first solutions  70  of Examples 7-1 to 7-3 further included 200 mmol/L potassium chloride, 66.7 mmol/L ferric chloride, and 100 mmol/L copper chloride, respectively. Then, the metal part  100  was used as an anode, a first voltage was applied on the metal part  100  for 5 min to form first holes  30 . A current density of the first voltage was controlled to be 4 A/dm 2 . Finally, the metal part  100  with the first holes  30  was taken out to wash with water, and dried at a temperature of 80° C. for 20 min. 
     The main different conditions between Examples 7-1 to 7-3, and the test results of Examples 7-1 to 7-3 were shown in the following Table 7. 
     
       
         
           
               
               
             
               
                   
                 TABLE 7 
               
             
            
               
                   
                   
               
               
                   
                 Test results 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 Percentage of 
               
               
                   
                   
                   
                   
                   
                 area occupied 
               
               
                   
                   
                   
                   
                   
                 by the first 
               
               
                   
                   
                 Distribution 
                 Depth of 
                 Diameter 
                 holes on the 
               
               
                   
                   
                 of the first 
                 the first 
                 of the first 
                 surface of the 
               
               
                 Example 
                 Chloride 
                 holes 
                 holes (μm) 
                 holes (μm) 
                 metal part 
               
               
                   
               
               
                 Example 7-1 
                 200 mmol/L 
                 the holes with 
                  50-100 
                 60-120 
                 15% 
               
               
                   
                 potassium 
                 barbed-shape 
               
               
                   
                 chloride 
                 are formed in 
               
               
                   
                   
                 the surface of 
               
               
                   
                   
                 the stainless 
               
               
                   
                   
                 steel portion 
               
               
                 Example 7-2 
                 66.7 mmol/L 
                 the holes with 
                 20-30 
                 30-60  
                 10% 
               
               
                   
                 ferric 
                 barbed-shape 
               
               
                   
                 chloride 
                 are formed in 
               
               
                   
                   
                 the surface of 
               
               
                   
                   
                 the stainless 
               
               
                   
                   
                 steel portion 
               
               
                 Example 7-3 
                 100 mmol/L 
                 the holes with 
                 25-35 
                 60-120 
                  8% 
               
               
                   
                 copper 
                 barbed-shape 
               
               
                   
                 chloride 
                 are formed in 
               
               
                   
                   
                 the surface of 
               
               
                   
                   
                 the stainless 
               
               
                   
                   
                 steel portion 
               
               
                   
               
            
           
         
       
     
     It can be seen form the test results in Table 7: the depth of the holes is relatively reduced in Examples containing ferric chloride or copper chloride. This is because Fe 3  in the ferric chloride and Fe element in the stainless steel part  20  will undergo an oxidation-reduction reaction to form Fe 2+ , Cu 2+  in the copper chloride will undergo a substitution reaction with the Fe element in the stainless steel part  20  to form elemental copper. The above reaction will be rapid in the early stage of the reaction, so that the diameter and the depth of the holes can be enlarged. However, Fe 2+  generated by the oxidation-reduction reaction and the elemental copper generated by the substitution reaction will accumulate in the holes, which will affect the further progress of the above reaction, resulting in a relatively low final depth and a relatively low percentage of area occupied by the first holes. Since the stainless steel part  20  contains more Fe element, under the same Cl −  dissociation concentration, the molar concentration of Cu 2+  will be greater than the molar concentration of Fe 3+ , more Cu 2+  will undergo substitution reaction with Fe element at the beginning of the etching reaction, and the diameter of the holes etched by the first solution  70  containing copper chloride will be larger than the diameter of the holes etched by the first solution  70  containing ferric chloride, and has no negative effect on the aluminum alloy portion  10 . 
     Examples 8-1 to 8-3 and Comparative Example 8-1 
     The metal part  100  treated in Example 2-1 was putted into a first solution  70  at 45° C. The first solution  70  included 40 wt % propylene glycol, 5 wt % ethylene glycol, 5 wt % sodium chloride, and 45 wt % water. The first solutions  70  of Examples 8-1 to 8-3 and Comparative Example 8-1 further included phosphoric acid with a concentration of 50 g/L, 100 g/L, 200 g/L, and 0 g/L, respectively. Then, the metal part  100  was used as an anode, a first voltage was applied on the metal part  100  for 10 min to form first holes  30 . A current density was controlled to be 6 A/dm 2 . Finally, the metal part  100  with the first holes  30  was taken out to wash with water, and dried at a temperature of 80° C. for 20 min. 
     The main different conditions between Examples 8-1 to 8-3 and Comparative Example 8-1, and the test results of Examples 8-1 to 8-3 and Comparative Example 8-1 were shown in the following Table 8. 
     
       
         
           
               
               
             
               
                   
                 TABLE 8 
               
             
            
               
                   
                   
               
               
                   
                 Test results 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 Percentage of 
               
               
                   
                   
                   
                   
                   
                 area occupied 
               
               
                   
                   
                   
                   
                   
                 by the first 
               
               
                 Example or 
                 Concentration 
                 Distribution 
                 Depth of 
                 Diameter 
                 holes on the 
               
               
                 Comparative 
                 of phosphoric 
                 of the first 
                 the first 
                 of the first 
                 surface of the 
               
               
                 Example 
                 acid 
                 holes 
                 holes (μm) 
                 holes (μm) 
                 metal part 
               
               
                   
               
               
                 Example 8-1 
                  50 g/L 
                 he holes with 
                 20-30  
                 20-30 
                 10% 
               
               
                   
                   
                 barbed-shape 
               
               
                   
                   
                 are formed in 
               
               
                   
                   
                 the surface of 
               
               
                   
                   
                 the stainless 
               
               
                   
                   
                 steel portion 
               
               
                 Example 8-2 
                 100 g/L 
                 he holes with 
                 50-100 
                  60-120 
                 35% 
               
               
                   
                   
                 barbed-shape 
               
               
                   
                   
                 are formed in 
               
               
                   
                   
                 the surface of 
               
               
                   
                   
                 the stainless 
               
               
                   
                   
                 steel portion 
               
               
                 Example 8-3 
                 200 g/L 
                 the holes with 
                 60-100 
                 150-300 
                 15% 
               
               
                   
                   
                 barbed-shape 
               
               
                   
                   
                 are formed in 
               
               
                   
                   
                 the surface of 
               
               
                   
                   
                 the stainless 
               
               
                   
                   
                 steel portion, 
               
               
                   
                   
                 part of the 
               
               
                   
                   
                 area where holes 
               
               
                   
                   
                 communicate 
               
               
                   
                   
                 with each other 
               
               
                 Comparative 
                  0 g/L 
                 The holes are 
                 5-10 
                 10-30 
                 25% 
               
               
                 Example 8-1 
                   
                 spherical 
               
               
                   
               
            
           
         
       
     
     It can be seen form the test results in Table 8: a proper amount of phosphoric acid can increase the percentage of area occupied by the barbed-shaped holes, but an excessively high content of phosphoric acid will cause electrochemical polishing on the surface of the stainless steel part  20 , and the barbed-shaped holes cannot be formed without phosphoric acid. 
     Examples 9-1 to 9-3 
     The metal part  100  treated in Example 2-1 was putted into a first solution  70  at 25° C. The first solution  70  included 40 wt % propylene glycol, 5 wt % ethylene glycol, 5 wt % sodium chloride, 45 wt % water, and 5 g/L phosphoric acid. Then, the metal part  100  was used as an anode, a first voltage was applied on the metal part  100  for 12 min to form first holes  30 . Current densities of Examples 9-1 to 9-3 were controlled to be 2 A/dm 2 , 5 A/dm 2 , and 8 A/dm 2 . Finally, the metal part  100  with the first holes  30  was taken out to wash with water, and dried at a temperature of 80° C. for 20 min. 
     The main different conditions between Examples 9-1 to 9-3, and the test results of Examples 9-1 to 9-3 were shown in the following Table 9. 
     
       
         
           
               
               
             
               
                   
                 TABLE 9 
               
             
            
               
                   
                   
               
               
                   
                 Test results 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 Percentage of 
               
               
                   
                   
                   
                   
                   
                 area occupied 
               
               
                   
                   
                   
                   
                   
                 by the first 
               
               
                   
                 Current 
                 Distribution 
                 Depth of 
                 Diameter 
                 holes on the 
               
               
                   
                 density 
                 of the first 
                 the first 
                 of the first 
                 surface of the 
               
               
                 Example 
                 (A/dm 2 ) 
                 holes 
                 holes (μm) 
                 holes (μm) 
                 metal part 
               
               
                   
               
               
                 Example 9-1 
                 2 
                 the holes with 
                 40-80  
                 40-80 
                 25% 
               
               
                   
                   
                 barbed-shape 
               
               
                   
                   
                 are formed in 
               
               
                   
                   
                 the surface of 
               
               
                   
                   
                 the stainless 
               
               
                   
                   
                 steel portion 
               
               
                 Example 9-2 
                 5 
                 the holes with 
                 60-100 
                  50-120 
                 35% 
               
               
                   
                   
                 barbed-shape 
               
               
                   
                   
                 are formed in 
               
               
                   
                   
                 the surface of 
               
               
                   
                   
                 the stainless 
               
               
                   
                   
                 steel portion 
               
               
                 Example 9-3 
                 8 
                 the holes with 
                 70-110 
                 100-300 
                 25% 
               
               
                   
                   
                 barbed-shape 
               
               
                   
                   
                 are formed in 
               
               
                   
                   
                 the surface of 
               
               
                   
                   
                 the stainless 
               
               
                   
                   
                 steel portion, 
               
               
                   
                   
                 part of the 
               
               
                   
                   
                 area where holes 
               
               
                   
                   
                 communicate 
               
               
                   
                   
                 with each other 
               
               
                   
               
            
           
         
       
     
     It can be seen form the test results in Table 8: the barbed-shaped holes can be formed in the surface of the metal composite under a proper current density. If the current density is too low, structures of the holes cannot be formed, and if the current density is too high, it is easy to electrochemically polish the stainless steel part  20 , thereby causing the communication between the holes. 
     In the method for forming holes, by adding corrosive ions into the first solution  70  mainly composed of at least one organic solvent, the organic solvent can increase the energy required for the migration of corrosive ions (Cl − ) and reduce the diffusion rate of the corrosive ions (such as Cl − ), so that the corrosive ions (such as Cl − ) will not be unevenly distributed due to the influence of reaction activity, but can evenly bind effective ions that corrode to form holes on the surface of the metal part  100 . Based on the small radius and the strong penetrating ability of Cl − , Cl −  can be preferentially adsorbed on the oxides to squeeze out oxygen atoms of the oxides and combine with cations of the oxides to form soluble chloride, thereby forming the first holes  30  in the surface of the metal part  100 . In addition, PO 4   3−  can react with the metal to form the aluminum phosphate film, thereby forming the first holes  30  in a shape of coral. 
     It is to be understood, even though information and advantages of the present embodiments have been set forth in the foregoing description, together with details of the structures and functions of the present embodiments, the disclosure is illustrative only; changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the present embodiments to the full extent indicated by the plain meaning of the terms in which the appended claims are expressed.