With rapid development of science and technology, China's alloy industry is also constantly moving forward. Research on high-strength die-cast aluminum alloys and their heat treatment is of great significance to alloy industry. Development of aluminum alloy industry has promoted development of industries such as automobiles, but as development of automobiles and other industries has increased demand for aluminum alloys, this in turn has promoted development of aluminum alloy industry. However, current aluminum alloy casting technology is difficult to meet such a large number of market needs. In this study, by elaborating on connotation of high-strength die-cast aluminum alloy and its heat treatment, influencing factors, test process and test results of optimization of high-strength die-cast aluminum alloy were analyzed, as well as influencing factors, test process and test results of heat treatment optimization of high-strength die-cast aluminum alloy, to provide a reference for development of alloy industry.
High-strength die-cast aluminum alloy has greater strength, better corrosion resistance and casting performance than ordinary aluminum alloy. Because of its excellent properties, it is widely used in electrical industry, automobiles, aerospace and other fields. Development of aluminum alloy industry has promoted development of industries such as automobiles, but as development of automobiles and other industries has increased demand for aluminum alloys, this in turn has promoted development of aluminum alloy industry. However, current aluminum alloy casting technology is difficult to meet such a large number of market needs. Then, it is urgent to study optimization of high-strength die-cast aluminum alloys and their heat treatment.

As name suggests, high-strength die-casting aluminum alloy is a process of die-casting aluminum alloy materials through high temperature and high pressure. This is a material casting method that saves energy and improves environment. It is an inevitable result of human production and life and sustainable development of society. High-strength die-cast aluminum alloy can enhance performance of castings through heat treatment, which is the most economical and reliable processing method among all existing material forming methods. High-strength die-cast aluminum alloys are widely used in industries such as machinery manufacturing, aerospace , weapons, and shipbuilding due to their lightweight structural advantages, low material density, good plasticity, easy processing, good electrical and thermal conductivity, and relatively high strength.
Usage of high-strength die-cast aluminum alloys is second only to steel. Materials cast by ordinary gravity casting and other alloy casting methods often suffer from deficiencies such as shrinkage holes and excessively large grains. However, liquid forming technology of high-strength die-cast aluminum alloys that combines characteristics of casting and forging can not only reduce shrinkage porosity, but also refine grains. Therefore, cast parts have relatively excellent mechanical properties. At the same time, high-strength casting methods can improve mold-filling ability of materials with low fluidity, and can produce castings with relatively complex shapes. Moreover, material utilization rate in high-strength casting process is relatively high, surface of manufactured parts is ideal and does not require reprocessing.

 

Material forming method of high-strength die-cast aluminum alloy has a relatively large impact on micro and macro parts of casting. Therefore, finding influencing factors for optimization of high-strength die-cast aluminum alloy is the key to ensuring part quality. Main influencing factors for optimizing high-strength die-cast aluminum alloys include: melt quality of metal material, specific pressure, temperature during pouring, preheating temperature of pouring mold, time interval between pressure application and pressure holding time, etc. Specific pressure is pressure experienced by aluminum alloy melt per unit area. The most obvious feature that distinguishes high-strength casting methods from ordinary castings is that aluminum alloy melt crystallizes under very high pressure conditions, and specific pressure is the biggest influence factor on structure and mechanical properties of parts. Now, most scholars believe that impact of specific pressure on structure and mechanical properties of castings is due to increasing supercooling of aluminum alloy liquid, which increases nucleation rate; or because specific pressure can reduce air gap between part and pouring mold, interface heat transfer rate between part and pouring mold increases, and solidification speed of aluminum alloy accelerates.
In addition, when specific pressure is appropriate, microstructure of part can be significantly improved, and aluminum alloy liquid solidifies under high pressure during high-strength die casting. High pressure during solidification has a forced feeding effect on aluminum alloy material, which can effectively shrink cavities and porosity, improve quality of castings. During high-strength die-casting process, if specific pressure is too small, quality of parts will not meet standard requirements; if specific pressure is too large, effect on enhancing mechanical properties of aluminum alloy will become worse, and casting mold will be easily damaged. If cracks or burrs appear in casting mold, it will increase difficulty of demoulding parts and increase energy consumption during molding process of parts. Therefore, when selecting specific pressure, we must not only meet mechanical properties of casting, but also consider consumption of equipment and molding processing costs during processing. Pouring temperature is another optimization factor in process of high-strength die-casting aluminum alloy. Temperature when liquid aluminum alloy is poured into mold cavity has a great impact on quality of parts and service life of mold. When pouring temperature is too low, fluidity of aluminum alloy liquid is small and mold filling capacity is low. Especially when pouring castings with relatively complex shapes, it is easy to cause insufficient mold filling and defective products with cold insulation on the surface of part; when pouring temperature is higher than standard temperature, aluminum alloy liquid is easily oxidized during transfer process, punch is prone to melt splashing under high pressure, and hot spots or shrinkage cavities may occur in thick part of part.
In addition, excessively high pouring temperature will also have a great thermal impact on pouring mold, and pouring mold is very prone to rupture. Preheating temperature of mold has a great impact on interfacial heat exchange rate during high-strength die casting. Improper mold preheating temperature will destroy microstructure and mechanical properties of part. When preheating temperature of casting mold is lower than standard temperature, quenching of aluminum alloy liquid near mold wall will cause cold isolation on the surface of casting, resulting in insufficient filling of castings with complex shapes, reducing casting qualification rate of parts, thermal stress of pouring mold will also be damaged and prone to premature failure; when preheating temperature of pouring mold is higher than standard temperature, parts will have hot spots and shrinkage holes, pouring mold and parts will easily bond, reducing service life of pouring mold. Generally speaking, holding time during casting process should be controlled until part is completely solidified. If holding time is too long, it will increase difficulty of demoulding, lengthen production cycle, increase consumption of molds and equipment, and increase production costs; if holding time is too short and thick parts of parts have not completely solidified, insufficient feeding of parts will occur and surface will not be smooth, which will reduce quality of parts.

When experimentally analyzing impact of different influencing factors on optimization of high-strength die-cast aluminum alloys, single-factor analysis method was chosen here. When testing effect of specific pressure on microstructure and mechanical properties of aluminum alloy, specific pressure was controlled between 0-74 MPa, tensile capacity, yield capacity and elongation of aluminum alloy were observed. It was found that tensile capacity, yield capacity and elongation of aluminum alloy increased with increase of specific pressure. When control pressure exceeds 74 MPa in experiment, it is found that tensile capacity, yield capacity and elongation of aluminum alloy do not change significantly, indicating that impact of specific pressure on mechanical properties of aluminum alloy was reduced when pressure exceeded 74 MPa. Other conditions are same and pouring temperature is controlled. As pouring temperature increases, quality of aluminum alloy casting first increases and then decreases. Quality of casting is optimal when pouring temperature is about 100 degrees Celsius, maximum crystallization temperature of aluminum alloy. Therefore, pouring temperature of aluminum alloy should choose the highest crystallization temperature of alloy. Control other conditions to be same, change preheating temperature of mold, observe quality of casting and reaction of mold. Optimal temperature for preheating mold during high-strength casting of aluminum alloy is generally 200-300 degrees Celsius.
In addition, during pressure test, we also found that when pressure is increased and temperature of aluminum alloy liquid exceeds aluminum alloy liquidus, this pressure will increase size of grains, and when temperature of aluminum alloy liquid is basically equal to aluminum alloy liquidus, this pressure will have a very obvious effect on supercooling of aluminum alloy liquid, and a relatively high-quality microstructure can be obtained. When temperature of aluminum alloy liquid is lower than aluminum alloy liquidus, microstructure of parts with relatively small size can be obtained. When experimentally studying liquid temperature of aluminum alloys, other conditions should be controlled to be same and liquid temperature of aluminum alloy should be changed. Results show that for high-strength die-cast aluminum alloys, a temperature slightly greater than 680 degrees Celsius is helpful for improving performance of top of part, while a temperature slightly below 680 degrees Celsius is helpful for performance of the bottom of part. Therefore, optimal temperature for liquid aluminum alloy is approximately 680 degrees Celsius.

 

Influencing factors for optimization of heat treatment of high-strength die-cast aluminum alloy include following points: solution temperature, solution time, aging temperature, aging time, quenching speed and addition of trace elements. During heat treatment of high-strength die-cast aluminum alloys, under same solid solution temperature and solid solution time conditions, intergranular second phase in high-strength die-cast aluminum alloys is more fully dissolved than in ordinary die-casting methods. High-strength die-casting alloys have better stability after heat treatment and aging. This is mainly due to fine grains of high-strength die-casting alloys, small and large number of dispersed precipitated particles. Addition of scandium during heat treatment of high-strength die-cast aluminum alloys has a timely and strengthening effect on precipitation behavior of aluminum alloy liquids. When solid solution temperature is 540 degrees Celsius, scandium trialuminum in melt can basically dissolve into casting, and in subsequent uniform precipitation strengthening, distance between effective crystal particles for strengthening aluminum alloy casting can be significantly reduced, greatly enhance mechanical properties of casting; when solid solution temperature is 500 degrees Celsius, most of scandated aluminum in melt has not yet dissolved into casting, which will cause scandated aluminum to disperse and precipitate directly on the surface of casting during aging process, uniform precipitation of scandated aluminum will be inhibited, thereby reducing mechanical properties of casting.
In addition, it is more appropriate to control solid solution time to 12 hours. A solid solution time of >12 hours will not improve mechanical properties of casting. For precipitation strengthening of stretched aluminum alloys, hardness value of aluminum alloy castings reaches maximum 6 to 10 hours after aging at 190 degrees Celsius. If aging continues, hardness of castings will decrease; while under aging at 150 degrees Celsius, aluminum alloy castings will precipitate in orthorhombic system 48 hours after aging, that is, hardness of aluminum alloy castings enters second stage of strengthening. Sulfur phase is main factor for aluminum alloy castings to enter second stage of strengthening. When hardness of aluminum alloy castings is maximum, sulfur phase will form dense precipitation structures on the surface of aluminum alloy castings. When aging temperature is about 250 degrees Celsius, aging response speed of aluminum alloy melt is also relatively large, but due to change of precipitated phase, peak strength of casting at this temperature is smaller than that of aluminum alloy casting at aging temperature of 165 degrees Celsius, which shows that high-temperature aging will inhibit precipitation of casting; in contrast, short-term artificial aging may be more beneficial to high-temperature durability of aluminum alloy castings. In addition, when aluminum alloy castings are subjected to solution treatment, as solution temperature increases, mechanical properties of aluminum alloy castings will first increase and then decrease. Relative to solution temperature, effect of solution time on microstructure and mechanical properties of aluminum alloys is not very obvious.

When experimentally analyzing impact of different influencing factors on optimization of heat treatment of high-strength die-cast aluminum alloys, single-factor analysis method was chosen here. Heat treatment process of aluminum alloys is controlled under same conditions except for single factors, such as different solution temperatures, different solution times, different aging temperatures, different aging times, different quenching speeds, addition of different trace elements, and performing heat treatment separately, observing occurrence of precipitation during aluminum alloy displacement, mechanical properties of final casting, reaction of molds and other equipment. Test results found that aluminum alloy produced by optimized heat treatment has the best mechanical properties when solid solution temperature is 540 degrees Celsius, solid solution time is 12 hours, aging temperature is about 170 degrees Celsius, and aging time is 8 hours. In addition, tests have found that adding scandium during heat treatment of high-strength die-cast aluminum alloys has a timely and strengthening effect on precipitation behavior of aluminum alloy liquids, and hardness of aluminum alloy castings will increase. When an appropriate amount of silver is added to aluminum alloy melt, precipitation phase precipitated in aluminum alloy casting with added silver is significantly more than that of aluminum alloy without added silver; aluminum alloys with 0.6% silver added can precipitate in large amounts under heat treatment, making aluminum alloy castings harder, while aluminum alloys with 0.9% silver added can also precipitate in large amounts during heat treatment, but there is not much precipitation, and hardness of aluminum alloy castings is not that great. Results show that aluminum alloy castings with 0.6% silver added have the best quality.

This study used single factor analysis to test effects of different influencing factors on high-strength die-cast aluminum alloys and their heat treatment, obtained optimal treatment conditions. When die-casting high-strength aluminum alloys, using optimal die-casting conditions can greatly improve mechanical properties of original alloy and effectively protect molds and other processing equipment. After appropriate heat treatment, mechanical properties of die-cast aluminum alloys can also be effectively improved. In short, advanced science and technology should be continuously used to explore optimization of high-strength die-cast aluminum alloys and their heat treatment, so as to achieve the best comprehensive mechanical properties of aluminum alloys, thereby driving development of other industries.