Lightweighting automobiles is one of the most effective measures to reduce energy consumption and pollutant emissions. Due to its high specific strength and high cost performance, aluminum alloy is preferred material for lightweighting automobiles. It is used in structural parts and components such as bodies, engines, wheels, etc., and its strength increases. High-pressure die casting has characteristics of high product dimensional accuracy, high production efficiency and considerable economic benefits, and is suitable for production of various aluminum alloy complex castings. Based on cold chamber die-casting process, researchers proposed theory of externally solidified crystals (ESCs) in pressure chamber. It is believed that melt stays in pressure chamber, especially when it is in slow injection stage, superheat of melt dissipates quickly due to contact with colder pressure chamber wall and punch, nucleation and growth conditions of primary phase have been reached. Studies believe that a large part of coarse dendrite structure formed near core of die casting comes from ESCs in pressure chamber, and as melt fills cavity, fluid force is eventually distributed in core area of casting. At the same time, effects of different pressure chamber sizes, pressure chamber residence times, addition of refiners on content and distribution of ESCs in castings were studied for magnesium-aluminum alloy and aluminum alloy die-casting. It was found that the greater heat transfer coefficient of pressure chamber and the longer residence time of pressure chamber, the higher ESCs content in casting. Adding a refiner will increase number of rounded ESCs in casting. Studying Mg-Al alloy die-casting process found that the lower pouring temperature, the higher ESCs content in die-casting parts; the higher speed, the rounder and finer ESCs in die-casting parts. Some researchers have studied effect of pouring temperature on microstructure, casting performance and mechanical properties of aluminum alloys, and found that fluidity of molten metal is poor at lower pouring temperatures, possibility of eddy currents and air entrapment during injection process is reduced. Intrinsic quality of die castings is improved, but structure of die castings is coarsened. In actual production process of cold chamber high-pressure casting, ESCs are inevitable due to mutual constraints between process parameters, volume fraction of ESCs can only be reduced and their morphology controlled by optimizing process parameters. In summary, there are currently few research reports on ESCs. In actual production process of high-pressure casting, there are few reports on systematic study of impact of die-casting process on microstructure morphology and mechanical properties of Al-Si-Mg alloys.
When testing body performance of aluminum alloy structural castings produced by high-pressure casting, it was found that when there is a considerable amount of coarse pre-crystallized structure in microstructure, yield strength and hardness of casting body will be significantly reduced, making it impossible to meet customer standards [yield strength ≥ 120MPa, hardness (HB) ≥ 75]. In order to improve performance of casting body and meet customer standards, effects of pouring temperature, injection waiting time and chemical composition on volume fraction, size and morphology of pre-crystallized structure in microstructure, as well as on mechanical properties were studied, aiming to provide a reference for its application.
Graphical results
An aluminum alloy beam (structural casting) was selected as sample, an IDRA-1600 die-casting machine equipped with a vacuum system and an aluminum alloy quantitative pouring furnace was used for test. By changing process parameters such as pouring temperature, injection waiting time and chemical composition of aluminum liquid(pouring temperature is 670, 680℃; injection waiting time is 3, 4, 6 and 8s; chemical composition of aluminum liquid is Al-7Si-0.25Mg, Al-7.5Si-0.25Mg), samples under different process conditions are obtained. Samples were cut from body, processed into tensile and hardness samples. After rough grinding, fine grinding, and polishing, metallographic samples were corroded using HF aqueous solution with a volume fraction of 0.5%. ZEISSAxio Observer 7m automatic inverted metallographic microscope was used for microstructural observation. ZWICK Z100 tensile testing machine was used for mechanical property testing. Tensile rate was 0.08%/s. Sample dimensions are shown in Figure 1. XHB-3000Z digital display Brinell hardness tester was used for hardness testing. The load was 62.5kN and held for 15s. Test conditions were HB 2.5/62.5. A DSC0901 differential thermal analyzer was used to analyze solidification behavior of alloys with different components. Cooling rates were 10℃/min and 30℃/min respectively.

Table 1 Chemical composition of Al-Si-Mg alloy (%)

 

Figure 1 Tensile specimen dimensions

 

Figure 2. Microstructure of Al-7Si-0.25Mg alloy at different pouring temperatures

Table 2 ESCs area fraction, average diameter and mechanical properties of casting body at different pouring temperatures

 

Figure 3 Microstructure of Al-7Si-0.25Mg at 670℃ under different injection waiting times.

 

Figure 4 Area fraction and average diameter of ESCs in Al-7Si-0.25Mg alloy under different injection waiting times
It can be seen that as injection waiting time increases, pre-crystallized structure in microstructure gradually becomes coarser and dendrites grow fully. Figure 4 shows changes in area fraction and average diameter of ESCs of Al-7Si-0.25Mg alloy under different injection waiting times. It can be seen that area fraction of ESCs increased from 15.8% to 30.2%, and average diameter increased from 122.37 μm to 158.32 μm. Figure 5 shows mechanical properties of casting body under different injection waiting times. It can be seen that yield strength, tensile strength and Brinell hardness of castings decrease as injection waiting time increases, while elongation shows an upward trend. Therefore, in order to improve yield strength and hardness of castings, injection waiting time should be reduced. However, when injection waiting time is in the range of 3 to 8 seconds, yield strength and hardness of casting still cannot meet requirements.

 

Figure 5 Mechanical properties of Al-7Si-0.25Mg casting body with different injection waiting times

 

Figure 6 DSC curves of Al-7Si-0.25Mg and Al-7.5Si-0.25Mg alloys at different solidification rates
On the basis of adjusting pouring temperature and injection waiting time, impact of chemical composition on performance of casting body was further analyzed. Si content in alloy was adjusted from 7.0% to 7.5%. First, DSC was used to test solidification behavior of Al-7Si-0.25Mg and Al-7.5Si-0.25Mg alloys under different cooling rates. Figure 6 shows DSC curves of Al-7Si-0.25Mg and Al-7.5Si-0.25Mg alloys at cooling rates of 10℃/min and 30℃/min. It can be seen that under solidification condition of 10℃/min, liquidus temperature of the two alloys differs by about 8℃. As cooling rate increases to 30℃/min, liquidus temperature of the two alloys differs by about 13℃.

 

Figure 7 Microstructure of alloy under different Si contents

Table 3 ESCs area fraction, average diameter and mechanical properties of casting body under different Si contents
In conclusion
(1) Pre-crystallized structure existing in microstructure is main reason for reducing yield strength and hardness of Al-7Si-0.25Mg castings.
(2) Increasing pouring temperature can significantly reduce ESCs in microstructure of Al-7Si-0.25Mg alloy, promote transformation of its morphology from dendritic to spherical, and improve bulk performance of casting.
(3) As injection waiting time increases, area fraction and average size of ESCs in Al-7Si-0.25Mg alloy microstructure show an increasing trend, yield strength and hardness show a decreasing trend.
(4) Increasing Si content, under appropriate cooling rate conditions, can significantly reduce liquidus temperature of alloy, thereby inhibiting precipitation of pre-crystallized structures (ESCs) and significantly improving mechanical properties of casting body.