Patent Publication Number: US-2010126639-A1

Title: Magnesium-contained high-silicon aluminum alloys structural materials and manufacture method thereof

Description:
BACKGROUND OF THE PRESENT INVENTION 
     1. Field of Invention 
     The present invention relates to aluminum alloys and their method of preparation, and more particularly to magnesium-contained high-silicon aluminum alloys for use as structural materials, and the manufacture method thereof. 
     2. Description of Related Arts 
     Aluminum-silicon alloys (Al—Si alloys), especially those with high silicon content, are widely used in car and aviation industries, due to their low density, high wear resistance, high anti-corrosiveness, and low thermal expansion coefficient. With the common solidification method for preparation of Al—Si alloys, there usually appear large silicon particles and eutectic plates, resulting in a dramatically increased brittleness of the alloys. Thus, it is difficult to improve the solidified microstructure and to obtain various shaped high-performance structural materials through subsequent plastic deformation, which poses a bottleneck for more general applications of these alloys. Traditionally, Al—Si alloys are categorized into the casting aluminum alloy series. In order to overcome their poor deformation ability, some new processing methods based on rapid solidification or powder metallurgy have been explored. However, the rapid solidification method can only allow preparation of tiny ingots with a diameter less than 10 mm. If a large-sized Al—Si alloy ingot is desired, a further process is then required. As for the powder metallurgy method, it enables to obtain a relatively large-sized part, but suffers from a complicated process with high fabrication cost. 
     In the existing methods for casting pure aluminum or aluminum alloys, the direct chill (DC) casting is most common in use. The main concerns are focused on how to reduce the chemical composition segregation, refine the solidification microstructure, and improve the ingot surface quality. Based on the DC casting route, an efficient manufacture method has been developed by one of the present inventers to prepare large-sized ingots of high silicon-containing aluminum alloys without adding any modifiers like P, Na, and Sr, and received a China Invention Patent with the patent number ZL200510119550.6. The present inventers have further found out that while narrowing the range of the Si content (i.e. increasing the lower limit to 8% by weight and reducing the upper limit to 18% by weight of Si concentration, and adjusting the contents of Mg and other alloy elements, the above method can be incorporated with thermoplastic processing and subsequent heat treatment, so as to produce Mg-containing high-silicon aluminum alloys with relatively high plasticity and strength, including profiles, bars, sheets, and forgings, for use as advanced new structural materials. 
     SUMMARY OF THE PRESENT INVENTION 
     An object of the present invention is to provide high-silicon aluminum alloys (Al—Si alloys) that contain magnesium (Mg) and have good plasticity and high strength for use as structural materials, and the manufacture method thereof. Without adding any modifiers, the Al—Si alloys are manufactured at low cost with the direct chill casting followed by the thermoplastic process and heat treatment. 
     Accordingly, in order to accomplish the above object, the present invention presents the aluminum alloys containing Mg and high Si, which comprises profiles, bars, sheets, and forgings, wherein the aluminum alloys are made by a process comprising the steps of:
         (a) casting an alloy ingot by a method of direct chill casting;   (b) preheat-treating the ingot to disperse eutectic Si phase particles; and   (c) thermal-plastic processing and heat-treating to obtain the alloy with a final shape and a modified microstructure, wherein the strengthening mechanisms of the alloy are referred to the grain refinement strengthening of aluminum matrix, the dispersion strengthening of silicon particles, and the precipitation strengthening of second phase particles.       

     The Mg-contained high-silicon aluminum alloys for use as structural materials contain 0.2˜2.0 wt % of Mg and 8˜18 wt % of Si, wherein they have an evenly refined microstructure: the aluminum matrix is fine equiaxed with an average grain size less than 6 μm, and the silicon and second phase particles are dispersed with an average size less than 5 μm. 
     The Mg-contained high-silicon aluminum alloys may contain at least one of Cu, Zn, Ni, Ti, and Fe elements, wherein a total weight percentage of the Cu, Zn, Ni, Ti, and Fe is less than 2 wt %. 
     The step (a) of direct chill casting is subjected to the cast ingot preparation for a given Al—Si alloy, at a relative casting temperature of 150˜300° C. above the liquidus line, a casting speed of 100˜200 mm/min, and a cooling water flux of 5˜15 g/mm·s on the periphery of the solidified ingot, wherein no modifier is added to the alloy. 
     The step (b) of preheat-treating is subjected to the formation of dispersed eutectic Si phase particles in the ingot, at a heating rate of 10˜30° C./min, a heating temperature of 450˜520° C., and a holding time of 1˜3 hours. 
     The preheat-treated ingot is subjected to a thermal-plastic processing in the step (c), at a hot-deformation temperature of 400˜520° C., followed by cooling naturally or forcedly. The hot-deformed product is then heat-treated after the thermal-plastic processing. 
     For the product thermal-plastic processed with natural cooling, the heat treatment in the step (c) further comprises a step of solution treatment and a step of artificial aging process. The solution treatment is performed at a heating rate of 10˜30° C./min, a solution treatment temperature of 500˜540° C., and a solution treatment time of 0.5˜3 hours, followed by quenching. The artificial aging process is performed at an aging temperature of 160˜200° C., and an aging time of 1-10 hours. 
     For the product thermal-plastic processed with forced cooling, the heat treatment in the step (c) further comprises a step of artificial or natural aging treatment, wherein the artificial treatment is performed at an aging temperature of 160˜200° C., and an aging time of 1˜10 hours. 
     A hot rolling process is adapted in the step of thermal-plastic processing, wherein the ingot is hot deformed at a total rolling reduction of more than 40%. 
     A hot extrusion process is adapted in the step of thermal-plastic processing, wherein the ingot is hot deformed at an extrusion ratio of more than 15. 
     A hot forging process is adapted in the step of thermal-plastic processing, wherein the ingot is hot deformed at a forging ratio of more than 40%. 
     The present invention overcomes the cognition prejudice traditionally imposed on Al—Si alloys. Without adding any modifiers, an unexpected effect has been reached on the magnesium-contained high-silicon aluminum alloys prepared by incorporating conventional direct chill casting method with thermal-plastic process and heat treatment. They are typically of fine-dispersed silicon particles and second phase at equiaxed Al matrix, associated with a relatively high strength and good plasticity for potential use as structural materials 
       FIG. 14  gives a comparison of mechanical properties between the Al—Si extrusion alloys of the present invention and the China National Standard extrusion alloy 6063 at the T5 and T6 states, wherein the alloys of the present invention are Al-8.5Si-1.8Mg-0.27Fe, Al-12.7Si-0.7Mg-1.5Cu-0.3Ni-0.3Ti-0.3Fe, and Al-15.5Si-0.7Mg-0.27Fe, respectively. 
     Notably, the yield strength and tensile strength of the Al-8.5Si-1.8Mg-0.27Fe, Al-12.7Si-0.7Mg-1.5Cu-0.3Ni-0.3Ti-0.3Fe, and Al-15.5Si-0.7Mg-0.27Fe extrusion alloys at the T6 state are higher than the China National Standards for the extrusion alloy 6063 at the T6 state. The mechanical properties of these alloys at the extrusion state (T1), especially the elongation rate, are also higher than the China National Standards for the 6063 alloys at the T5 state. 
     As the most common aluminum extrusion alloys, the 6063 alloys have been widely used in architectures, vehicles, and decorations etc., which have great need in the existing market. Once the 6063 alloys are partially replaced by the magnesium-contained high-silicon aluminum alloys of the present invention, it will bring great economic benefits. In addition, the use of an increased amount of Si in the alloys can dramatically conserve the aluminum resource. 
     These and other objectives, features, and advantages of the present invention will become apparent from the following detailed descriptions, the accompanying drawings, and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a device of direct chill casting according to preferred embodiments of the present invention. 
         FIG. 2  is a microstructure of ingot of Al-12.7Si-0.7Mg-0.3Fe alloy (#3) at cast condition during the direct chill casting process according to a first preferred embodiment of the present invention, wherein a casting temperature is 730° C., a casting rate is 180 mm/min, and a cooling water flow rate is 8 g/mm·s. 
         FIG. 3  is a high magnification microstructure of ingot of Al-12.7Si-0.7Mg-0.3Fe alloy (#3) at cast condition during the direct chill casting process according to the first preferred embodiment of the present invention, wherein a casting temperature is 730° C., a casting rate is 180 mm/min, and a cooling flow rate of the surrounding water is 8 g/mm·s. 
         FIG. 4  is a microstructure of Al-12.7Si-0.7Mg-0.3Fe alloy (#3) after pre-heated to 500° C. for 2 hours, heat extruded at 470° C. (having extraction ratio of 15) according to a second preferred embodiment of the present invention. 
         FIG. 5  is a T6 state microstructure of Al-12.7Si-0.7Mg-0.3Fe alloy (#3) after pre-heated to 500° C. for 2 hours, heat extruded at 470° C. (having extraction ratio of 15) according to a third preferred embodiment of the present invention, wherein said T6 state is at a solution temperature 540° C. for one hour, and at an artificial aging temperature 200° C. for 3 hours. 
         FIG. 6  is a microstructure of ingot of Al-15.5Si-0.7Mg-0.27Fe alloy (#5) at cast condition during the direct chill casting process according to the first preferred embodiment of the present invention, wherein a casting temperature is 800° C., a casting rate is 140 mm/min, and a cooling water flow rate is 10 g/mm·s. 
         FIG. 7  is a high magnification microstructure of ingot of Al-15.5Si-0.7Mg-0.27Fe alloy (#5) at cast condition during the direct chill casting process according to the first preferred embodiment of the present invention, wherein a casting temperature is 800° C., a casting rate is 140 mm/min, and a cooling water flow rate is 10 g/mm·s. 
         FIG. 8  is a microstructure of Al-15.5Si-0.7Mg-0.27Fe alloy (#5) after pre-heated to 500° C. for 2 hours, heat extruded at 470° C. (having extrusion ratio of 45) according to the second preferred embodiment of the present invention. 
         FIG. 9  is a microstructure of Al-15.5Si-0.7Mg-0.27Fe alloy (#5) after pre-heated to 500° C. for 1 hour, heat rolled at 500° C. (pressing amount of 60%) according to the second preferred embodiment of the present invention. 
         FIG. 10  is a T6 state microstructure of ingot of Al-15.5Si-0.7Mg-0.27Fe alloy (#5) after pre-heated to 500° C. for 2 hours, heat extruded at 470° C. (having extraction ratio of 45) according to the third preferred embodiment of the present invention, wherein said T6 state is at a solution temperature 520° C. for 2 hours, and at an artificial aging temperature 180° C. for 4 hours. 
         FIG. 11  is a T6 state microstructure of rectangular ingot of Al-15.5Si-0.7Mg-0.27Fe alloy (#5) after pre-heated to 500° C. for 1 hour, heat rolling at 500° C. (pressing amount of 60%) according to the third preferred embodiment of the present invention, wherein said T6 state is at a solution temperature 520° C. for 3 hours, and at an artificial aging temperature 200° C. for 4 hours. 
         FIG. 12  is a T6 state of high rate microstructure of Al-15.5Si-0.7Mg-0.27Fe alloy (#5) after pre-heated to 500° C. for 2 hours, heat extruded at 470° C. (having extrusion ratio of 45) according to the third preferred embodiment of the present invention, wherein said T6 state is at a solution temperature 520° C. for 2 hours, and at an artificial aging temperature 180° C. for 4 hours. 
         FIG. 13  is a microstructure of ingot of Al-17.5Si-0.7Mg-1.0Cu-0.27Fe alloy (#7) at cast condition during the direct chill casting process according to the first preferred embodiment of the present invention, wherein a casting temperature is 850° C., a casting rate is 120 mm/min, and a cooling water flow rate is 10 g/mm·s. 
         FIG. 14  is a table showing the comparisons mechanical properties of extrusion Si Al alloy of the present invention and the China standard extrusion of 6063 alloy. 
         FIG. 15  is a table showing the compositions of an alloy made from the ingot via the casting process. 
         FIG. 16  is a table showing the parameters of different alloys through casting process. 
         FIG. 17  is a table showing the parameters of the pre-heating process and extraction process of each of alloys. 
         FIG. 18  is a table showing the parameters of the pre-heating process and rolling process of each of alloys. 
         FIG. 19  is a table showing the parameters of the pre-heating process and forging process of each of alloys. 
         FIG. 20  is a table showing the parameters of the extrusion process for different alloys. 
         FIG. 21  is a table showing the parameters of the rolling process for different alloys. 
         FIG. 22  is a table showing the parameters of the forging process for different alloys. 
         FIG. 23  is a table showing the mechanical properties under variety of deformation processes and heat treatments situations. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 1  of the drawings, a step of casting ingot via the direct chill casting method according to a first preferred embodiment of the present invention is illustrated. 
     A device designed for the direct chill casting process is shown in  FIG. 1  of the drawings, wherein the device comprises a cooling water inlet  1 , a crystallizer  2 , a hot top  4 , and a graphite ring  5 , wherein a raw material  3  of the ingot and a liquid metal  6  are separately received within a container of the device. A plurality of compositions of an alloy made from the ingot via the casting process is shown in  FIG. 15 . A plurality of parameters of the casting process is shown in  FIG. 16 . 
     Referring to  FIGS. 17 ,  18 , and  19 , a step of preheating, followed by hot extruding, or hot rolling, or hot forging the ingot according to a second preferred embodiment of the present invention is illustrated. 
     In the preheating process, the ingot is heated in an oven at a predetermined heating rate. After the predetermined temperature is reached, the ingot is held for a predetermined time. Then, a hot extrusion device, or a hot rolling device, or a hot forging device is used to complete a thermal-plastic processing. A plurality of parameters of the preheating and hot extruding for each of the alloys is shown in  FIG. 17 . A plurality of parameters of the preheating and hot rolling for each of the alloys is shown in  FIG. 18 . A plurality of parameters of the preheating and hot forging for each of the alloys is shown in  FIG. 19 . 
     Referring to  FIGS. 20 ,  21 ,  22 , and  23 , a step of heat treatment after hot deformation of the alloys, such as hot extrusion, hot rolling, and hot forging, according to a third preferred embodiment of the present invention is illustrated. 
     After the hot extrusion, hot rolling, or hot forging process of the ingot, the heat treatment is applied to the product at a predetermined temperature. A plurality of parameters of the hot extrusion, hot rolling, and hot forging processes are shown in FIGS.  20 ,  21 , and  22  respectively. A plurality of mechanical properties of the alloys after the heat treatments is shown in  FIG. 23 . 
     The present invention provides the industrial use of the Mg-contained high silicon aluminum alloys (Al—Si alloy), and the manufacture method thereof. Without adding any modifiers, the Al—Si alloys having good plasticity and relatively high strength are manufactured at low cost with the direct chill casting followed by the thermal-plastic process and heat treatment, for use as structural materials. 
     One skilled in the art will understand that the embodiment of the present invention as shown in the drawings and described above is exemplary only and not intended to be limiting. 
     It will thus be seen that the objects of the present invention have been fully and effectively accomplished. The embodiments have been shown and described for the purposes of illustrating the functional and structural principles of the present invention and is subject to change without departure from such principles. Therefore, this invention includes all modifications encompassed within the spirit and scope of the following claims.