As a key load-bearing component connecting car body and chassis, shock tower is mainly used to absorb impact load generated during driving of car. Service environment is relatively harsh and has high requirements on strength and toughness of material. In order to meet demand, a lot of research has been carried out, a variety of aluminum alloy materials and corresponding processing techniques have been developed. A357 (AlSi7Mg) alloy developed abroad not only has excellent mechanical properties and corrosion resistance, but also has good casting properties. However, alloy has poor fluidity due to its low Si content, which reduces production efficiency. AlSi10MnMg (Silafont-36) alloy has attracted attention in application of shock towers due to its excellent comprehensive properties such as casting performance, strength, elongation, and welding. Since average wall thickness of shock absorbing tower is generally 2 to 3 mm, and when wall thickness of casting is less than 4 mm, fluidity will be reduced due to surface tension of molten metal, so it is difficult to form shock absorbing tower in mold. Compared with other Al-Si alloys, Si content in AlSi10MnMg alloy is higher, which can effectively reduce its crystallization temperature range, increase number of Si phases in structure, improve its fluidity and tensile strength. Therefore, it is particularly suitable for production of shock towers. At the same time, a trace amount of Mn element (0.4% to 0.6%) is added to alloy, which can form a fine and dispersed AlFeSiMn phase with elements such as Fe, thereby reducing alloy's tendency to stick to mold and improving its overall performance through precipitation strengthening.
In summary, it can be seen that shock-absorbing tower has high requirements on material strength and toughness. Due to low toughness of die-cast AlSi10MnMg alloy, and Mg in alloy cannot be completely dissolved in α-Al solid solution at room temperature, Mg will be in a supersaturated state in α-Al solid solution during quenching, and strengthening phase Mg2Si will precipitate after the aging treatment. This phase can significantly improve strength and hardness of alloy, so in actual production, properties of AlSi10MnMg alloy need to be optimized through heat treatment. Some studies have shown that reasonable heat treatment processes can significantly improve mechanical properties of AlSiMg alloys. However, in actual production applications, mechanical properties of die-cast AlSi10MnMg alloys fluctuate greatly after heat treatment, and are sometimes far lower than test values. Therefore, in order to optimize AlSi10MnMg alloy production process and give full play to its good casting, corrosion resistance and good mechanical properties, and then widely used in automobile lightweight research, die-cast AlSi10MnMg aluminum alloy automotive shock tower was used as object, using hardness testing, tensile testing, optical microscopy, scanning electron microscopy and transmission electron microscopy, effects of different heat treatment systems on microstructure and mechanical properties of die-cast shock tower were studied, aiming to provide reference for its application.

Table 1 Chemical composition of AlSi10MnMg alloy (%)

Table 2 Die casting process parameters
Sampling part of shock tower and tensile specimen are shown in Figure 1. Heat treatment process parameters are shown in Table 3. T4 solid solution treatment and high-temperature T5 treatment at 500℃×2h are carried out in a box-type resistance furnace, water quenched, and aging treatment in an oil bath furnace. Zwick-100kN universal material testing machine was used for mechanical property testing, and average value of 3 samples was taken at each position. Hardness test was carried out on an HVS-30P Vickers hardness tester with a load of 49N and a holding pressure of 15s. Seven points were taken for each sample and average value was taken. Samples for microstructure and scanning electron microscopy analysis were directly taken from tensile specimens, processed by grinding and polishing, etched with HF acid solution with a volume fraction of 0.1%, then their morphology was observed with a Zeiss metallographic microscope and a phenom desktop scanning electron microscope. Samples for transmission electron microscopy analysis are prepared using electrolytic double spray and ion thinning. Electrolytic double spray instrument is a magnetically driven double spray electrolytic thinner. Electrolyte used is composed of 10% perchloric acid and 90% alcohol by volume. Electrolysis temperature is -30~-20℃, voltage is 40~50V, and control current is below 100mA. Ion thinning was performed on GATAN695 instrument. Voltage used was 2kV, angle was 2.5°, and thinning time was about 20 minutes. Microstructure was characterized using a 2100F transmission electron microscope (TEM). ImageProPlus software was used to count size of Si phase.

 

(a) Shock tower

 

(b) Schematic diagram of sampling location of shock tower

 

(c) Tensile specimen
Figure 1 Schematic diagram of sampling location of shock tower and tensile specimen

Table 3 Heat treatment process scheme

 

Figure 2 Microstructure of as-cast and T4 die-casting parts

(a) Cast state (b) T4 state

 

Figure 3 Al-Si binary alloy phase diagram

 

Figure 4 SEM structure of as-cast and T4 die-casting parts

Table 4 Comparison of mechanical properties of as-cast and T4-state castings
Figure 5 shows SEM structure of casting after different aging times at 350℃. It can be seen that after casting is aged at 350 ℃*0.5h, Si element in eutectic structure begins to dissolve into α-Al matrix, form of Si phase begins to change from initial fibrous state to a granular state. As heat treatment time increases, Si phase continues to neck and melt and gradually spheroidizes, interface with aluminum matrix becomes smoother, but average particle size increases from 0.35 μm in die-cast state to 0.44 μm after aging for 2 h. Table 5 shows mechanical properties of die castings after being exposed to different temperatures and times. It can be seen that at a temperature of 350℃, as holding time is extended, strength of alloy gradually decreases, while elongation continues to increase; at the same time, as temperature increases, strength of alloy decreases significantly, but decline gradually slows down, while elongation increases significantly, but increase also gradually slows down.

 

Figure 5 SEM structure of castings aged at 350℃ for different times

Table 5 Mechanical properties of castings under different heat treatment processes

 

Figure 6 Age hardening curve of casting after solid solution at 500℃*2h

Table 6 Mechanical properties of castings subjected to T6 aging treatment at different temperatures and times after solid solution at 500 ℃ * 2 h
Analysis shows that aging process of die-cast AlSi10MnMg alloy is a precipitation process of strengthening phase β-Mg2Si. When aging time is short, precipitation phase of alloy is mainly in G.P. zone, precipitation phase is in a slowly increasing state with a small amount, so strengthening effect on the alloy is limited, and alloy is in an under-aging state at this time; as aging time continues to prolong, number of precipitated phases begins to increase rapidly and gradually begins to transition to β″ phase and β′ phase whose composition and structure are close to second phase. At the same time, coherent relationship is gradually destroyed, causing severe distortion of crystal lattice, and alloy is strengthened. At this time, alloy is in peak aging state; but as aging time continues to increase, semi-coherent phase will gradually transform into equilibrium phase β-Mg2Si, degree of lattice distortion will continue to decrease, and strengthening effect will continue to weaken. At this time, alloy is in an over-aging state.

 

Figure 7 SEM structure of T4 state sample after aging treatment at different temperatures

Table 7 SEM energy spectrum analysis results (%)

 

Figure 8 Die-cast AlSi10Mn alloy in T4 state and T6 state
TEM bright field image

(a) T4 state (b) T6 state

 

Figure 9 TEM high resolution and corresponding Fourier transform images of T6 state AlSi10Mn alloy during peak aging
(a) TEM image (b) Magnification of the box (c) Corresponding Fourier transform diagram
In conclusion
(1) Yield strength of die-cast AlSi10MnMg alloy shock tower is 144.13MPa, tensile strength is 312.58MPa, and elongation rate is 7.67%; compared with low-pressure casting and gravity casting, primary α-Al phase in die-cast AlSi10MnMg shock tower alloy is small, with an average diameter between 5 and 40 μm, and a uniform structure distribution.
(2) Die-cast AlSi10MnMg alloy shock tower has good gas entrainment control, and die-cast structure is relatively dense. Alloy structure and mechanical properties are significantly affected by heat treatment. Through different heat treatment systems, strength and plasticity can be controlled within a reasonable range without bubbling.
(3) During T5 treatment, Si phase in shock tower structure transforms very rapidly. At a temperature of 350℃, Si phase obviously dissolves after 0.5h. When kept for 2 hours, alloy elongation reaches 16.32%, which is 113% higher than that of cast state.
(4) After aging treatment at 175℃ and 200℃, precipitated phases in structure of shock absorbing tower are small in size and uniformly distributed in granular form, and mechanical properties reach extreme values; when process is 500℃*2h+175℃*6h, tensile strength of alloy reaches 294.28MPa, hardness (HV) reaches 100.8, and elongation rate reaches 10.25%; when process is 500℃*2h+200℃*2h, tensile strength of alloy reaches 281.47MPa, hardness (HV) reaches 94.4 , elongation reaches 11.47%. However, when aging temperature is increased to 225℃, eutectic structure and precipitated phases in alloy begin to grow larger, and performance decreases significantly.