Due to lightweight requirements, aluminum alloys are increasingly used in automotive structural parts, which places higher requirements on their mechanical properties. High vacuum die castings can be welded and heat treated to strengthen, are widely used in production of aluminum alloy automotive structural parts that require welding and are subject to larger loads or cyclic loads. AlSi10MnMg alloy is a strong aluminum alloy suitable for high vacuum die-casting. It has characteristics of high tensile strength and high elongation, and can be used in automobile structural parts. Therefore, AlSi10MnMg alloy was selected to analyze mechanical properties of structural parts.
Effect of different heat treatment processes on mechanical properties of AlSi10MnMg shock tower under high vacuum die-casting was studied. Influence of various factors in heat treatment process on mechanical properties of shock tower was analyzed, aiming to provide a reference for its application. Results show that under same heat treatment conditions, mechanical properties of different areas of casting are different, and performance near inner gate is better than that at filling end. It can be seen from the T6 heat treatment test results that lower solid solution temperature and longer artificial aging time can obtain higher mechanical properties. In addition, there are still a few pores in high vacuum die castings, there are some shrinkage cavities and shrinkage porosity in thick parts.
Graphical results
Appearance of shock tower is shown in Figure 1. Parts are symmetrical, with a single piece mass of 3.8kg, an average wall thickness of 3mm, an outline size of 547mm*337mm*318mm, and a projected area of 1802cm2. It is made of AlSi10MnMg die-cast aluminum alloy and produced by Idra 35000kN die-casting machine (vacuum degree is 2000~2400Pa), one mold with two cavities (left and right symmetrical parts, one cavity each), the total projected area is 6226cm2, and pouring weight is 9.27kg. Gating system design is shown in Figure 2.

 

(a) Movable mold

 

(b) Fixed mold
Figure 1 Schematic diagram of three-dimensional structure of shock-absorbing tower

 

Figure 2 Schematic diagram of pouring system design
Put high-vacuum die-casting parts into a high-temperature resistance furnace for T6 heat treatment, then group them according to different heat treatment processes, sample and analyze each group. Samples were taken from right cavity of mold. Sampling location and number are shown in Figure 3. They were then processed into standard tensile specimens using a CNC machine tool, as shown in Figure 4. Castings were tested for mechanical properties on CMT5205 tensile testing machine.

 

Figure 3 Sampling locations and numbers

 

Figure 4 Schematic diagram of tensile specimen
Solid solution temperatures used in test were 490, 500, and 510℃, and heat preservation was for 3 hours. Artificial aging temperatures were 170, 175, and 200℃, and aging was for 3 hours. Seven heat treatment plans respectively formulated are shown in Table 1, and their mechanical properties are shown in Table 2. Mechanical property data analysis of typical positions 1, 2, 4, 6, and 10 is shown in Figure 5. It can be seen from Figure 5 that tensile strength, yield strength, and elongation data at position 1 at filling end are relatively low compared to other positions. Mechanical properties at position 4 near inner gate are higher, indicating that mechanical properties of material at final position where aluminum alloy liquid flows are poor, and should be focused on during die-casting process. In addition, it can be seen that during die-casting production of structural parts, it is not advisable to place important parts of part at the end of alloy liquid filling but should be placed near inner gate. Comparing as-cast and heat treatment schemes 1 to 4, it can be seen that heat treatment can increase yield strength of parts by an average of 60 to 110MPa.

Table 1 Heat treatment scheme

Table 2 Mechanical properties under different heat treatment conditions

 

Figure 5 Strength and elongation analysis curves

 

Figure 6 Mechanical properties under different heat treatment conditions
Under optimized process conditions, compared with without heat treatment, tensile strength is increased by 20-40MPa, and elongation is basically stable. Analyzing mechanical properties data after heat treatment is shown in Figure 6. It can be seen from Figure 6 that tensile strength and yield strength do not change significantly, elongation shows a downward trend with increase of solution treatment temperature and artificial aging temperature. In addition, it can be seen that extending artificial aging time and reducing solution heat treatment temperature can increase material elongation. Mechanical properties of high vacuum pressure structural castings near inner gate after heat treatment are better than those at filling end. Lower solid solution temperature and longer artificial aging time as well as lower artificial aging temperature can achieve better mechanical properties.