With increasing prominence of energy and environmental issues, reducing carbon emissions has become an important measure for countries to alleviate resource shortages and environmental pollution. In the field of automobile production and manufacturing, in order to reduce fuel consumption and carbon emissions, development of lightweight new energy vehicles has become a development trend in automobile industry. Among them, under premise of ensuring automobile safety, achieving automobile weight reduction and low-cost manufacturing has become development goal of automobile manufacturers. Main ways to reduce automobile weight are to optimize body structure and develop light alloy materials. In terms of body optimization, Tang Chun and others optimized car wheel hub through finite element simulation, reducing weight by 0.97kg. In terms of body material selection, aluminum alloy has become one of the most ideal materials for automobile lightweighting due to its low density, high specific strength and low cost. At present, aluminum alloy materials can be used to produce automotive parts such as shock towers, wheels, steering wheels, dashboards, engine blocks and anti-collision beams. Common hypoeutectic Al-Si alloys include A356, A380, ADC12, Silafont-36, Castasil-37 and Silafont-38. Among them, Silafont-36 (also known as AlSi10MnMg) alloy is widely used in die-cast automotive parts. It is not only cost-effective (no precious elements added), but also has good casting performance and excellent mechanical properties after heat treatment. In terms of low-cost manufacturing, Tesla proposed an integrated body design, which achieved integrated forming of dozens or hundreds of automotive parts, improving efficiency while greatly reducing high cost caused by riveting and welding.
As a near-net forming process with extremely high production efficiency, high-pressure casting (HPDC) is widely used in production and manufacturing process of automotive parts because of its high dimensional accuracy and low surface roughness of automotive parts it produces. However, in die-casting process, due to characteristics of low-speed advancement in pressure chamber, high-speed filling and high-pressure solidification in cavity, there are heterogeneous tissue structures in die-casting, which are manifested as irregular skin layers, large-scale defect belts, coarse pre-crystallized structures (ESCs), large-sized pores and shrinkage. Researchers simulated high-speed filling process of liquid flow through simulation and found that holes in die-casting parts could not be eliminated. Defect formation and evolution behavior of filling process were calculated through numerical simulation of die-casting filling, it was found that central distribution and annular distribution of ESCs led to generation of defect belts. Development of vacuum die-casting technology has greatly reduced porosity in die-casting products and improved quality of castings. Study found that after adding a vacuum device, porosity in die-casting was reduced from original 8.5% to 3.7%. Relationship between vacuum degree and porosity in cavity was studied, and it was believed that low cavity vacuum could effectively reduce pore content. In recent years, our research group has established a high vacuum die-casting system by continuously improving high vacuum die-casting technology (double vacuum machines for coordinated vacuuming), optimizing die-casting parameters (multi-stage low speed), and improving mold structure (structural optimization of runners and diverter cones). It can ensure that vacuum degree of cavity during die-casting process is less than 10kPa, providing strong process guarantee for die-casting parts. However, coarse pre-crystallized primary α-Al phase (ESCs-αI) formed in pressure chamber can cause large-scale shrinkage in die-casting parts and reduce mechanical properties. Other researchers have found through in-situ tensile tests that large-scale shrinkage is prone to occur on ESCs grain boundaries, which often act as crack sources to accelerate failure of castings. In addition, Fe, as an impurity element in aluminum alloys, is easily introduced during smelting process and is difficult to remove. In die-casting, Fe can promote demolding, which is beneficial to prolong life of mold and reduce production cost of mold, but primary Fe-rich phase (ESCs-IMCI) formed in pressure chamber during die-casting process is relatively brittle and harms mechanical properties. Focus of this study is to explore role of pre-crystallization structure in fracture process of AlSi10MnMg alloy. On this basis, ESCs-αI is refined and ESCs-IMCI is optimized by microalloying, a new heat-treatment-free die-casting alloy is developed and its trial production on shock tower parts is realized, aiming to provide a reference for its application.

A commercial AlSi10MnMg alloy is used, and its composition is shown in Table 1. Ingot composition is within standard AlSi10MnMg alloy composition range, without major defects such as oxidation inclusions, and quality is qualified. At the same time, ideal composition range of new THAS-1 alloy designed independently is shown in Table 2. On the basis of AlSi10MnMg alloy, Si content and Mn content are appropriately reduced, Mg is removed, Zr and V are added to optimize alloy structure. Mold used is a three-bar one-piece mold. Referring to literature, a TOYO BD-350V5 horizontal cold chamber die-casting machine equipped with a high vacuum device is used. Metal ingot is added to melting furnace and further quickly heated to temperature range of 700~720℃ to prevent segregation of alloy elements. After melting, it is kept warm for 30 minutes, then argon is introduced and stirred continuously for 30 minutes. After completion, melt is cooled to 680℃ and left to stand for 30 minutes, followed by slagging. Process parameters used in die casting are: pouring temperature of 680℃, initial mold temperature of 120℃, low speed of 0.05~0.4m/s, and high speed of 1~3m/s. Structural parts and sampling positions of automotive shock tower are shown in Figure 1. Die casting test uses Fulai 28000kN high vacuum die casting machine, and die casting process is formulated according to parameters of die casting machine.

Table 1 Chemical composition of AlSi10MnMg alloy (%)

Table 2 Chemical composition of THAS-1 alloy (%)

 

Figure 1 Structure of shock tower and sampling location

 

Figure 2 Microstructure of die-cast AlSi10MnMg alloy
In Figure 2a, abnormally coarse ESCs-αI has a high degree of dendrite formation, while fine αI-Al exists in a petal-like morphology. Dark area in figure is eutectic structure. Both ESCs-αI and αI-Al belong to primary α-Al phase, but ESCs-αI nucleates in pressure chamber and grows rapidly during punch advancement process. With rapid filling of high-speed liquid, it remains in casting after solidification. Among them, some ESCs-αI will be broken and remelted under action of high-speed liquid flow, and its shape becomes spherical or rod-shaped. αI-Al nucleates and precipitates under conditions of rapid solidification of liquid filling cavity, and high cooling rate limits its further growth. Therefore, size is small, about 5μm. In Figure 2b, two primary Fe-rich phases [ESC-IMCI and (P-IMC) II] have regular polyhedral morphology, but size difference between the two is huge. Similar to primary α-Al phase, nucleation positions of two primary Fe-rich phases are also different. ESC-IMCI nucleates and grows in pressure chamber, while (P-IMC) II nucleates and precipitates rapidly in cavity.

 

Figure 3 Fracture of AlSi10MnMg alloy and microstructure near main crack

 

Figure 4 Microstructure of heat-treated THAS-1 alloy

 

Figure 5 Mechanical properties of two alloy shock absorbers at different positions

 

Figure 6 Fracture morphology of die-cast THAS-1 alloy

(1) Pre-crystallization microstructure of pressure chamber in die-cast AlSi10MnMg alloy includes ESC-αI (primary α-Al phase) and ESC-IMCI (primary Fe-rich phase). Compared with primary α-Al phase (αI-Al) and primary Fe-rich phase [(P-IMC) II] in cavity, pre-crystallization microstructure of pressure chamber is large in size and degree of dendrite formation of ESC-αI is high. They are enriched together to form a dendrite network, which is very easy to cause difficulty in shrinkage compensation of residual liquid phase during solidification, forming large-scale shrinkage, promoting extension of intergranular fracture cracks, and reducing performance. Coarse ESCs-IMCI has poor deformation coordination with surrounding matrix, which can easily cause stress concentration and promote crack propagation.
(2) Based on control of pre-crystallization structure of pressure chamber, a new heat-treatment-free THAS-1 alloy was developed. Compared with AlSi10MnMg alloy, ESCs-αI in die-cast THAS-1 alloy has a higher degree of rounding, spheroidization trend of ESCs-IMCI is more obvious, strength and elongation are higher than those of heat-treated AlSi10MnMg alloy. There are a large number of dimples on fracture of THAS-1 alloy, and comprehensive mechanical properties are excellent.