Patent Publication Number: US-8974728-B2

Title: Hot-dip cast aluminum alloy containing Al-Zn-Si-Mg-RE-Ti-Ni and preparation method thereof

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATION 
     The present application is the US national stage of PCT/CN2010/071482 filed on Mar. 31, 2010, which claims the priority of the Chinese patent application No. 200910223768.4 filed on Nov. 19, 2009, which application is incorporated herein by reference. 
     FIELD OF INVENTION 
     The invention relates to hot-dip cast aluminum alloy containing Al—Zn—Si—Mg—RE—Ti—Ni and a preparation method thereof, in particular to hot-dip cast aluminum alloy containing Al—Zn—Mg—RE—Ti—Ni for anticorrosion treatment on engineering parts resistant to marine climate and a preparation method thereof. 
     Background of the Invention 
     With the rapid growth of science and technology, more and more engineering equipment is applied in offshore water and ocean, but its service environment is generally higher than level C5 according to ISO 9225 environmental assessment standard and belongs to extremely harsh environment with rainy, high temperature, salt misty and strong wind. Comprehensive actions or strong atmospheric corrosion, electrochemical corrosion and current scour corrosion on exposed parts cause service lives of various steel structures to be far shorter than that in the common inland outdoor environment. 
     For instance, presently, wind energy has become a renewable and clean energy resource processing the mature technology and conditions of scale development. However, because wind turbines utilize wind energy to generate electricity, and there is rich wind resources at coast lines and offshore waters, most wind power plants are located at coastal or offshore waters. Wind turbines service in marine climate with common protective measures are usually seriously corroded within only a couple of months because the external members, such as engine rooms, engine covers, tower structures, etc., are directly exposed in extremely corrosive atmosphere. Therefore, the problem urgent to be solved is corrosion resistance of the coating for anticorrosion treatment on engineering parts resistant to marine climate. 
     SUMMARY OF THE INVENTION 
     In view of the problems of the prior art, the invention provides hot-dip cast aluminum alloy for anticorrosion treatment on engineering parts resistant to marine climate and a preparation method thereof. 
     In the hot-dip cast aluminum alloy for anticorrosion treatment on engineering parts resistant to marine climate provided by the invention, said cast aluminum alloy contains Al, Zn, Si, Mg, RE, Ti, Ni and nanometer oxide particle reinforcing agent, said nanometer oxide particle reinforcing agent is selected from one or two of TiO 2  and CeO 2 , the mass percentage of the components is as follows: Zn: 35-58%, Si: 0.3-4.0%, Mg: 0.1-5.0%, RE: 0.02-1.0%, Ti: 0.01-0.5%, Ni: 0.1-3.0%, and the total content of the nanometer oxide particle reinforcing agent: 0.01-1.0%; and the balance consists of Al and unavoidable impurities. 
     Wherein, RE is any one of or several rare earth elements. 
     Preferably, if said nanometer oxide particles are spherical particles, the specific surface and the average particle size of the spherical particles satisfy the following relation expression: 
               Specific   ⁢           ⁢   surface     =       6     ρ   ·   D       ⁢     (       m   2     ⁢     /     ⁢   g     )             
Where D is the average particle size; and
 
     ρ is density. 
     If the shape of said nanometer oxide particles is more complex than common spherical particles, the performance and the effect of the coating is more perfect, and thus, the more prefer nanometer oxide particles of the invention have greater specific surface than the calculated value according to the above expression: 
     Preferably, when the nanometer oxide particles are TiO 2 , the average particle size of said TiO 2  is 15-60 nm. 
     Preferably, when the nanometer oxide particles are TiO 2 , the specific surface of said TiO 2  is 20-90 m 2 /g. 
     Preferably, when the nanometer oxide particles are CeO 2 , the average particle size of said CeO 2  is 25-70 nm. 
     Preferably, when the nanometer oxide particles are CeO 2 , the specific surface of said CeO 2  is 10-80 m 2 /g. 
     Preferably, when the nanometer oxide particle reinforcing agent consists of TiO 2  and CeO 2 , the mass ratio of TiO 2  to CeO 2  is 1: (1-3). 
     More preferably, the mass ratio of TiO 2  to CeO 2  is 1:2. 
     Preferably, the mass percentage of said components is as follows: Zn: 41-51%, Si: 1-3.2%, Mg: 1.8-4%, RE: 0.05-0.8%, Ti: 0.05-0.35%, Ni: 1.5-2.6%, and the total content of the nanometer oxide particle reinforcing agent: 0.05-0.8%. 
     Furthermore, the invention provides a method for preparing said hot-dip cast aluminum alloy, which comprises the steps of preparing materials according to the mass percentage of Al, Zn, Si, Mg, RE, Ti, Ni and the nanometer oxide particle reinforcing agent, firstly heating Al to 700-750° C. and melting Al in vacuum or protective atmosphere, stirring evenly, and adding Si; raising temperature to 800-840° C. and then adding RE; raising temperature to 830-850° C. and then adding Zn; raising temperature to 850-880° C. and then adding Ni and Ti; cooling to 750-700° C. and then adding Mg and the nanometer oxide particle reinforcing agent; and cooling to 700-650° C., standing for 10-35 minutes after stirring evenly, and forming ingots by casting or die casting. 
     Preferably, the heating rate is 10-40° C./minute during said heating process, and the cooling rate is 20-60° C./minute during said cooling process. 
     In the hot-dip cast aluminum alloy resistant to marine climate corrosion provided by the invention, metal Al can resist atmospheric corrosion, a layer of dense oxide film can be rapidly formed on the surface of AL, and Al has capacity of rapid damage self-repairing; and Zn has lower electrode potential acting as a sacrificial anode and thus enables steel to have sufficient capacity of resisting electrochemical corrosion. 
     However, if the content of Zn is over high, the toughness and the hardness of the coating will be decreased resulting in the reduction of resistance of the coating to atmospheric corrosion and current scour resistance. In order to solve the problem, in the invention, a certain amount of nanometer oxide particle reinforcing agent is added to greatly fine particles of the coating, thereby improving the capacity of the coating resisting to atmospheric corrosion, electrochemical corrosion and current scour resistance and significantly improving the strength and the hardness of the coating so as to endow the coating with better current scour resistance. Furthermore, through a larger number of repeated experiments and selections, the performance of the coating can be remarkably improved by selecting proper particle size and specific surface of the nanometer oxide particle reinforcing agent. Moreover, the particle size of the nanometer oxide particle reinforcing agent being within the range of the invention can improve the abrasion resistance index of the coating, and the specific surface of the nanometer oxide particle reinforcing agent being within the range of the invention can greatly increase the aggregation degree of the alloy, and thereby the scour resistance of the alloy coasting is remarkably improved. 
     On this basis, microalloy elements such as Mg, Ti, Ni, etc. are added to fine particles better and further improve the toughness and the corrosion resistance of the coating, wherein Mg can improve the affinity, the corrosion resistance and the room-temperature strength of the alloy, Ti enhances the hardening constituent in the coating and has the function of solid solution to the alloy, and Ni not only has further function of solid solution to the alloy but also further improve the toughness and the stability of the alloy. 
     To sum up, coating employing the cast aluminum alloy prepared by the invention has sufficient corrosion resistance and scour resistance in marine climate. 
     In the other aspect, the invention provides a method, in which hot-dip alloy elements are added at different temperature sections to be beneficial to the improvement of the dispersion of the nanometer oxide particle reinforcing agent and the elements along with the raise of temperature, thereby improving the uniformity of the components of the coating and significantly enhancing the binding strength between the coating and a substrate. 
     However, if all the elements are added when the temperature of melt is over high, the coating easily shows high-alumina brittle phase, which goes against bearing contact fretting load. Therefore, in the invention, a part of hot-dip alloy elements are added at different temperature sections, then the nanometer oxide particle reinforcing agent is added after the temperature falls to a certain temperature, and the temperature is decreased and preserved for a certain time, thereby overcoming the above defect to obtain a coating with composition uniformity and better toughness. 
     In summary, compared with the prior art, the coating of the invention remarkably improves the performance of resisting atmospheric corrosion, electrochemical corrosion and current scour corrosion as well as the strength, the hardness and scour resistance, and the coating is firmly bounded to the substrate and totally suitable for extremely harsh environment such as marine environment, and the like. Furthermore, the invention has simplified process and can obtain a coating with composition uniformity and better toughness. In addition, main elements in the alloy, such Al, Zn, etc., are rich in nature, therefore, the invention has the advantages of low material cost, environmental protection and energy conservation. The coating using the alloy of the invention has wide adjusting range of thickness and is suitable for the treatment on parts with different sizes. 
    
    
     DETAILED DESCRIPTIONS OF THE INVENTION 
     The invention provides hot-dip cast aluminum alloy for anticorrosion treatment on engineering parts resistant to marine climate, in which said cast aluminum alloy contains Al, Zn, Si, Mg, RE, Ti, Ni and nanometer oxide particle reinforcing agent, said nanometer oxide particle reinforcing agent is selected from one or two of TiO 2  and CeO 2 , the mass percentage of the components is as follows: Zn: 35-58%, Si: 0.3-4.0%, Mg: 0.1-5.0%, RE: 0.02-1.0%, Ti: 0.01-0.5%, Ni: 0.1-3.0%, and the total content of the nanometer oxide particle reinforcing agent: 0.01-1.0%; and the balance consists of Al and unavoidable impurities. 
     Furthermore, through a larger number of repeated experiments and selections, the performance of the coating can be remarkably improved by selecting proper particle size and specific surface of the nanometer oxide particle reinforcing agent, and if said nanometer oxide particles are spherical particles, the specific surface and the average particle size of the spherical particles satisfy the following relation expression: 
               Specific   ⁢           ⁢   surface     =       6     ρ   ·   D       ⁢     (       m   2     ⁢     /     ⁢   g     )             
Where D is the average particle size; and
 
     ρ is density. 
     Furthermore, if the shape of said nanometer oxide particles is more complex than common spherical particles, the performance and the effect of the coating is more perfect, and thus, the prefer nanometer oxide particles of the invention has greater specific surface than the calculated value according to the above expression: Preferably, when the nanometer oxide particles are TiO 2 , the average particle size of said TiO 2  is 15-60 nm. 
     Preferably, when the nanometer oxide particles are TiO 2 , the specific surface of said TiO 2  is 20-90 m 2 /g. 
     Preferably, when the nanometer oxide particles are CeO 2 , the average particle size of said CeO 2  is 25-70 nm. 
     Preferably, when the nanometer oxide particles are CeO 2 , the specific surface of said CeO 2  is 10-80 m 2 /g. 
     Prefer embodiments of the mass percentage of the components of the invention are hereinafter given in tables 1-3, however the contents of the components of the invention are not limited to the values in the tables, and those skilled in the art can carry out reasonable generalization and deduction on the basis of the value range listed in the tables. 
     It is necessary to be specifically described that although relative values of the particle size and the specific surface of the nanometer oxide particle reinforcing agent are simultaneously listed in the tables 1-3, these two conditions are not described as essential technical characteristics. As for the invention, the core content lies in obtaining the objects of fining the particles of the coating, improving the toughness and different corrosion resistances and eliminating bad effects caused by over high content of zinc by adding a certain amount of nanometer oxide particle reinforcing agent microalloy elements. On this basis, further selection of proper particle size and specific surface just enables the technical effect to be more prominent and more superior, and thus, although listed in the tables 1-3 simultaneously the two parameters are merely described as more superior conditions for more detailed technical information of the invention but not being necessary conditions. 
     Embodiment 1 
     Hot-dip cast aluminum alloy for anticorrosion treatment on engineering parts resistant to marine climate contains Al, Zn, Si, Mg, RE, Ti, Ni and TiO 2  nanometer oxide particle reinforcing agent, the mass percentage of the components is as follows: Zn: 35-58%, Si: 0.3-4.0%, Mg: 0.1-5.0%, RE: 0.02-1.0%, Ti: 0.01-0.5%, Ni: 0.1-3.0%, TiO 2  : 0.01-1.0% and Al: the balance, and unavoidable impurities. The specific mass percentages and relative parameters are shown in table 1: 
                     TABLE 1                  Mass Percentage (%) of the Components in Total Weight and Relative Parameters                         Element                         TiO 2                                                                                               Particle   Specific       Serial                                   size   surface       number   Al   Zn   Si   Mg   RE   Ti   Ni   content   (mm)   (m 2 /g)                                                                 1   balance   35   4.0   0.1   1.0   0.5   0.1   1.0   15   90       2   balance   36   3.9   0.3   0.9   0.48   0.2   0.9   18   85       3   balance   37   3.8   0.5   0.8   0.45   0.3   0.8   20   80       4   balance   39   3.6   0.8   0.6   0.40   0.5   0.6   23   75       5   balance   41   3.2   1.0   0.4   0.35   0.7   0.4   25   70       6   balance   43   2.8   1.3   0.3   0.30   1.0   0.3   28   65       7   balance   45   2.5   1.8   0.2   0.25   1.3   0.2   30   60       8   balance   47   2.2   2.2   0.15   0.20   1.5   0.15   35   55       9   balance   49   1.8   2.6   0.13   0.15   1.8   0.13   40   50       10   balance   51   1.5   3.0   0.11   0.1   2.0   0.11   45   45       11   balance   53   1.0   3.5   0.09   0.08   2.4   0.09   50   40       12   balance   55   0.8   4.0   0.07   0.05   2.6   0.07   53   35       13   balance   56   0.5   5.4   0.05   0.03   2.8   0.05   55   30       14   balance   57   0.4   4.8   0.03   0.02   2.9   0.03   58   25       15   balance   58   0.3   5.0   0.02   0.01   3.0   0.01   60   20                    
Embodiment 2
 
     Hot-dip cast aluminum alloy for anticorrosion treatment on engineering parts resistant to marine climate contains Al, Zn, Si, Mg, RE, Ti, Ni and CeO 2  nanometer oxide particle reinforcing agent, Hot-dip cast aluminum alloy for anticorrosion treatment on engineering parts resistant to marine climate contains Al, Zn, Si, Mg, RE, Ti, Ni and CeO 2  nanometer oxide particle reinforcing agent, the mass percentage of the components is as follows: Zn: 35-58%, Si: 0.3-4.0%, Mg: 0.1-5.0%, RE: 0.02-1.0%, Ti: 0.01-0.5%, Ni: 0.1-3.0%, CeO 2 : 0.01-1.0% and Al: the balance, and unavoidable impurities. Specific values are shown in table 2: 
                     TABLE 2                  Mass Percentage (%) of the Components in Total Weight and Relative Parameters                         Element                         CeO 2                                                                                               Particle   Specific       Serial                                   size   surface       number   Al   Zn   Si   Mg   RE   Ti   Ni   content   (mm)   (m 2 /g)                                                                 1   balance   35   4.0   0.1   1.0   0.5   0.1   1.0   25   80       2   balance   36   3.9   0.3   0.9   0.48   0.2   0.9   28   75       3   balance   37   3.8   0.5   0.8   0.45   0.3   0.8   30   70       4   balance   39   3.6   0.8   0.6   0.40   0.5   0.6   35   65       5   balance   41   3.2   1.0   0.4   0.35   0.7   0.4   40   60       6   balance   43   2.8   1.3   0.3   0.30   1.0   0.3   45   55       7   balance   45   2.5   1.8   0.2   0.25   1.3   0.2   50   50       8   balance   47   2.2   2.2   0.15   0.20   1.5   0.15   53   45       9   balance   49   1.8   2.6   0.13   0.15   1.8   0.13   55   40       10   balance   51   1.5   3.0   0.11   0.1   2.0   0.11   58   35       11   balance   53   1.0   3.5   0.09   0.08   2.4   0.09   60   30       12   balance   55   0.8   4.0   0.07   0.05   2.6   0.07   62   25       13   balance   56   0.5   4.5   0.05   0.03   2.8   0.05   65   20       14   balance   57   0.4   4.8   0.03   0.02   2.9   0.03   68   15       15   balance   58   0.3   5.0   0.02   0.01   3.0   0.01   70   10                    
Embodiment 3
 
     Said hot-dip alloy contains Al, Zn, Si, Mg, RE, Ti, Ni and nanometer oxide particle reinforcing agent, wherein the nanometer oxide particle reinforcing agent consists of TiO 2  and CeO 2 , the mass ratio of TiO 2  to CeO 2  is 1: (1-3); the mass percentage of the components is as follows: Zn: 35-58%, Si: 0.3-4.0%, Mg: 0.1-5.0%, RE: 0.02-1.0%, Ti: 0.01-0.5%, Ni: 0.1-3.0%, total content of the nanometer oxide particle reinforcing agent consisting of TiO 2  and CeO 2 : 0.01-1.0%, and Al: the balance, and unavoidable impurities. Specific values are shown in table 3: 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Mass Percentage (%) of the Components in Total Weight and Relative Parameters 
               
            
           
           
               
               
            
               
                   
                 Element 
               
            
           
           
               
               
            
               
                   
                 TiO 2  and CeO 2   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Total 
                 Particle 
                 Specific 
               
               
                 Serial 
                   
                   
                   
                   
                   
                   
                   
                 content 
                 size 
                 surface 
               
               
                 number 
                 Al 
                 Zn 
                 Si 
                 Mg 
                 RE 
                 Ti 
                 Ni 
                 (TiO 2 :CeO 2 ) 
                 (mm) 
                 (m 2 /g) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 1 
                 balance 
                 35 
                 4.0 
                 0.1 
                 1.0 
                 0.5 
                 0.1 
                 1.0 
                 (1:1) 
                 20 
                 80 
               
               
                 2 
                 balance 
                 36 
                 3.9 
                 0.3 
                 0.9 
                 0.48 
                 0.2 
                 0.9 
                 (1:1.2) 
                 23 
                 75 
               
               
                 3 
                 balance 
                 37 
                 3.8 
                 0.5 
                 0.8 
                 0.45 
                 0.3 
                 0.8 
                 (1:1.3) 
                 25 
                 70 
               
               
                 4 
                 balance 
                 39 
                 3.6 
                 0.8 
                 0.6 
                 0.40 
                 0.5 
                 0.6 
                 (1:1.4) 
                 28 
                 65 
               
               
                 5 
                 balance 
                 41 
                 3.2 
                 1.0 
                 0.4 
                 0.35 
                 0.7 
                 0.4 
                 (1:1.3) 
                 30 
                 60 
               
               
                 6 
                 balance 
                 43 
                 2.8 
                 1.3 
                 0.3 
                 0.30 
                 1.0 
                 0.3 
                 (1:1.5) 
                 35 
                 55 
               
               
                 7 
                 balance 
                 45 
                 2.5 
                 1.8 
                 0.2 
                 0.25 
                 1.3 
                 0.2 
                 (1:2) 
                 40 
                 50 
               
               
                 8 
                 balance 
                 47 
                 2.2 
                 2.2 
                 0.15 
                 0.20 
                 1.5 
                 0.15 
                 (1:3) 
                 45 
                 45 
               
               
                 9 
                 balance 
                 49 
                 1.8 
                 2.6 
                 0.13 
                 0.15 
                 1.8 
                 0.13 
                 (1:2) 
                 50 
                 40 
               
               
                 10 
                 balance 
                 51 
                 1.5 
                 3.0 
                 0.11 
                 0.1 
                 2.0 
                 0.11 
                 (1:1.8) 
                 53 
                 35 
               
               
                 11 
                 balance 
                 53 
                 1.0 
                 3.5 
                 0.09 
                 0.08 
                 2.4 
                 0.09 
                 (1:1.5) 
                 55 
                 30 
               
               
                 12 
                 balance 
                 55 
                 0.8 
                 4.0 
                 0.07 
                 0.05 
                 2.6 
                 0.07 
                 (1:2) 
                 58 
                 25 
               
               
                 13 
                 balance 
                 56 
                 0.5 
                 4.5 
                 0.05 
                 0.03 
                 2.8 
                 0.05 
                 (1:2.5) 
                 60 
                 20 
               
               
                 14 
                 balance 
                 57 
                 0.4 
                 4.8 
                 0.03 
                 0.02 
                 2.9 
                 0.03 
                 (1:2.8) 
                 65 
                 18 
               
               
                 15 
                 balance 
                 58 
                 0.3 
                 5.0 
                 0.02 
                 0.01 
                 3.0 
                 0.01 
                 (1:3) 
                 68 
                 15 
               
               
                   
               
            
           
         
       
     
     In embodiments 1-3, preferably, the percentage of the components in total mass is as follows: Zn: 14-51%, Si: 1-3.2%, Mg: 1.8-4%, RE: 0.05-0.8%, Ti: 0.05-0.35%, Ni: 1.5-2.6%, and total content of the nanometer oxide particle reinforcing agent: 0.05-0.8%. 
     More preferably, the content of said Zn is 45%, the content of said Si is 1.8%, the content of said Mg is 3.5%, the content of said RE is 0.6%, the content of said Ti is 0.25%, the content of said Ni is 2%, and total content of the nanometer oxide particle reinforcing agent: 0.2%. 
     In addition, a large number of experiments show that if the loose packed density of the nanometer oxide particle reinforcing agent is appropriate, the performance and the effect of the final resulting coating is more ideal. 
     If using TiO 2 , preferably, the loose packed density of said TiO 2  is not more than 3 g/cm 3 . 
     If using CeO 2 , preferably, the loose packed density of said CeO 2  is not more than 5 g/cm 3 . 
     If using TiO 2  and CeO 2 , preferably, the average loose packed density of said TiO 2  and CeO 2  is 0.6-4.5 g/cm 3 . 
     Furthermore, the invention provides a method for preparing said hot-dip alloy, which comprises preparing materials according to the mass percentage of Al, Zn, Si, Mg, RE, Ti, Ni and the nanometer oxide particle reinforcing agent, heating Al to 700-750° C. and melting Al in vacuum or protective atmosphere, stirring evenly, and adding Si; raising temperature to 800-840° C. and then adding RE; raising temperature to 830-850° C. and then adding Zn; raising temperature to 850-880° C. and then adding Ni and Ti; cooling to 750-700° C. and then adding Mg and the nanometer oxide particle reinforcing agent; and cooling to 700-650° C., standing for 10-35 minutes after stirring evenly, and forming ingots by casting or die casting. 
     Preferably, preparing materials according to the mass percentage of Al, Zn, Si, Mg, RE, Ti, Ni and the nanometer oxide particle reinforcing agent, heating Al to 720-750° C. and melting Al in vacuum or protective atmosphere, stirring evenly, and adding Si; raising temperature to 820-840° C. and then adding RE; raising temperature to 840-850° C. and then adding Zn; raising temperature to 860-880° C. and then adding Ni and Ti; cooling to 730-700° C. and then adding Mg and the nanometer oxide particle reinforcing agent; and cooling to 690-650° C., standing for 10-30 minutes after stirring evenly, and forming ingots by casting or die casting. 
     Preferably, cooling to 720-700° C. and then adding Mg and the nanometer oxide particle reinforcing agent; and finally cooling to 690-660° C. and preserve the temperature for 22-28 minutes to obtain the alloy. More preferably, cooling to 710° C. and then adding Mg and the nanometer oxide particle reinforcing agent; and finally cooling to 680° C. and preserve the temperature for 25 minutes to obtain the alloy. 
     During the heating process, the heating ratio is 10-40° C. per minute, and the cooling ratio is 20-60° C. per minute during the cooling process. Preferably, during the heating process, the heating ratio is 20-30° C. per minute, and the cooling ratio is 30-50° C. per minute during the cooling process. 
     More preferably, during the heating process, the heating ratio is 25° C. per minute, and the cooling ratio is 40° C. per minute during the cooling process. 
     Experimental Results of Corrosion Resistance 
     Embodiment 4 
     A key part of a certain inshore wind turbine, a flange gasket at blade root (size: Φ2200×30 mm, material: Q345), which adopted common protective coating treatment is obviously corroded after only a few months. The results of accelerated corrosion simulation experiments show that taking the hot-dip alloy of the invention as coating material to form a diffusion coating with the thickness of 150 μm and then coating a layer of polysiloxane with the thickness of 20 ∞m, the flange gasket at blade root has the durability persisting for over 20 years in seawater splashing environment. 
     Embodiment 5 
     A key part of a certain inshore wind turbine, a connecting screw bolt (size: M36×1000 m, material: 40CrNiMo), which adopted common protective coating treatment is obviously corroded after only a few months. The results of accelerated corrosion simulation experiments show that taking the hot-dip alloy of the invention as coating material to form a diffusion coating with the thickness of 100 μm and then coating a layer of polysiloxane with the thickness of 15 μm, the connecting screw bolt has the durability persisting for over 20 years.