With improvement of safety and environmental awareness, people are paying more and more attention to lightweight of automobiles. Some steel automobile parts are gradually being replaced by lightweight materials such as aluminum and magnesium alloys. Among them, A356 aluminum alloy is widely used due to its excellent casting properties and mechanical properties. α-Al solid solution and eutectic Si phase are main constituent phases of A356 aluminum alloy. Among them, α-Al phase has good plasticity, while eutectic Si phase is brittle and hard. When alloy is stressed, stress is concentrated at interface between aluminum matrix and eutectic Si phase, which easily leads to generation of microcracks, thereby reducing toughness of alloy and reducing usage conditions of alloy.
However, researchers currently only conduct thermal deformation research on A356 alloys prepared by gravity casting and low-pressure casting. Alloys prepared by above processes have many defects such as shrinkage porosity and shrinkage holes, have a coarse structure and poor mechanical properties. This project uses high-pressure water-cooled casting of A356 aluminum alloy, which can significantly refine grains, improve alloy properties, and improve production efficiency. High-pressure holding can compensate for internal defects in castings, improve density and precision of castings, and improve mechanical properties of castings. At present, there are few reports on thermal deformation research of A356 die-cast aluminum alloy. Therefore, thermal deformation mechanism of die-cast A356 aluminum alloy was studied. A356 aluminum alloy was prepared by water-cooling die casting, and hot compression test was used to study thermal deformation behavior of alloy. Stress-strain curve of alloy was analyzed, constitutive equation was established and thermal processing diagram was drawn. Aimed to provide reference for subsequent plastic processing.

Material used in test is A356 aluminum alloy, which is cast by high-pressure water cooling. Mold material is H13 steel, and cooling water pipes are installed around mold. Cooling rate of A356 alloy during solidification reaches 102~106℃/s, and it cools at a rate of 5℃/s below liquidus temperature of 616℃. During die casting, filling temperature of A356 aluminum alloy is 710℃, and initial temperature of mold is 350℃. First, molten metal is filled with runner and inner gate by punch in a slow injection method with an injection speed of 0.3m/s, then mold is filled with a fast injection method with an injection speed of 5m/s. s, and maintain pressure at 70MPa until casting solidifies and cools. Casting structure is shown in Figure 1, and chemical composition is shown in Table 1. It was processed into a φ8mm*12mm compression specimen by wire cutting.

 

Figure 1 Initial structure of die-cast A356 aluminum alloy

wB
Si	Mg	Ti	Sr	Fe	Mn	Al
6.64	0.236	0.125	0.019	0.113	＜0.004	margin

Table 1 Chemical composition of A356 aluminum alloy (%) Figure 2 Schematic diagram of stress-strain curve position of A356 aluminum alloy during hot deformation at different temperatures
It can be seen that when strain is small, as strain rate increases, stress increases rapidly, showing work hardening behavior; however, when strain increases to a certain value, stress tends to characteristics of steady-state flow. This is because work hardening caused by strain and softening phenomenon caused by dynamic recovery reach a balance, and steady flow stress appears. At the same time, it can also be found that when temperature is constant, as strain rate increases, peak stress also increases. This is because the greater strain rate, the faster dislocations are generated, the greater density, and the greater work hardening. Therefore, peak stress increases with increasing strain rate. When strain rate is constant, peak stress decreases as deformation temperature increases. This is because the higher temperature, the greater deformation activation energy and the more obvious dynamic recovery of material. Figure 3 Relationship between peak stress, strain rate and deformation temperature of die-cast A356 aluminum alloy under different deformation conditions Figure 4 Relationship between parameter Z and flow stress Figure 5 Thermal processing diagram of die-cast A356 aluminum alloy when strain is 0.7
Figure 6 is a schematic diagram of microstructure of alloy in different regions. It can be seen from Figure 6a that structure of alloy is reconstructed, dendrites are destroyed, α-Al phase spheroidizes, and dynamic recovery is found at phase boundary. As can be seen from Figure 6b, instability in region 2 mainly manifests as an adiabatic shear band. This is due to fact that at high strain rates, heat generated by plastic deformation cannot diffuse to cooler parts of specimen in a short period of time, resulting in a reduction in flow stress and an increase in local plastic flow. It can be seen from Figure 6c that deformation of alloy in region 3 is uneven. Therefore, hot processing of die-cast A356 aluminum alloy should be carried out in zone 1 of higher temperature and higher strain rate. Figure 6 Microstructure of alloy under different deformation conditions

(1) From stress-strain curve of hot deformation of die-cast A356 alloy and microstructure after deformation, it can be seen that softening mechanism of hot deformation of A356 aluminum alloy is mainly dynamic recovery.
(2) Analyzing high-temperature deformation of die-cast A356 alloy, average activation energy is approximately 238.6kJ*mol-1. Relationship between peak stress, deformation temperature and strain rate during thermal deformation process can be expressed by following thermal deformation constitutive equation:

 

(3) A thermal processing diagram was established based on dynamic material model DMM. Deformation conditions of alloy during thermal processing were determined through thermal processing diagram and microstructure after thermal deformation. Temperature was 330~380℃ and the strain rate was 5~10s-1. Range of materials has good thermal processing properties.