As society pays more attention to energy consumption, lightweight of automobiles has attracted more and more attention. Use of aluminum alloy die castings instead of iron castings can effectively reduce weight of automobiles. However, die-casting process is prone to gas entrainment, and gas trapped in casting forms holes that endanger mechanical properties of material. During subsequent heat treatment process, surface layer of die-casting will blister, and in severe cases, casting may even be scrapped. High-vacuum die-casting can reduce cavity gas pressure during liquid metal filling, greatly reduce amount of gas involved, reduce difficulty of subsequent heat treatment, and further improve performance of castings.
AlSiMg series alloys have characteristics of good casting performance, comprehensive mechanical properties and corrosion resistance, and their mechanical properties can be further improved by solution aging treatment. AlSi10MgMn is the most common AlSiMg-based die-casting alloy. Reducing Si content can increase α-Al matrix and reduce eutectic phase content. Through microalloying, various dispersed precipitates in α-Al matrix can be regulated to improve performance of AlSiMg alloy. In Al-Si casting alloy with certain Fe and Mn content, addition of Cr element will generate coarse precipitate phase and endanger performance of alloy. However, some studies have pointed out that addition of Cr has a positive effect. Adding an appropriate amount of Cr to die-casting alloy can reduce sticking tendency of mold, can also transform needle-like β-Al5FeSi phase between Al-Si alloy dendrites into less harmful α-Fe phase. Some researchers added 0.5% Cr and 0.8% Fe to Al-Si alloy. It was found by XRD that newly formed α-Al-(Cr, Fe)-Si phase in alloy has a body-centered cubic structure and is conducive to improving plasticity of alloy. In addition, Cr can form a dispersed precipitate phase during homogenization of wrought aluminum and solution treatment of cast aluminum alloy. Since it is not coherent with aluminum matrix, it is generally considered to have no strengthening effect. Adding 0.28% Cr and 0.12% Mn to Al-3Si-0.6Mg cast aluminum alloy, it was found that Cr-containing dispersed precipitate phase formed during solid solution process increased hardness (HV) by 5, and the lower heating rate made dispersion phase is more uniform. Adding 0.25% Cr to Al-5Si-0.5Mg alloy, it is found that hardness (HV) of alloy after solution treatment and water quenching is increased by more than 10, and Cr-containing precipitates inhibit subsequent solid solution aging strengthening effect. However, there are few reports on addition of Cr to high vacuum die-casting low-silicon aluminum alloys. Therefore, alloy samples with different Cr contents were prepared by high-vacuum die-casting, Fe phase and Cr-containing precipitated phase were analyzed by SEM and TEM to study effect of Cr element on microstructure and mechanical properties of alloy.
Graphical results
Raw materials used are pure aluminum ingots, Al-10Si-0.6Mn-0.3Mg ingots, Al-50Mg, Al-10Mn and Al-10Cr master alloys. TOYO BD-250 die-casting machine is used, equipped with Wanda vacuum equipment, and vacuum degree actually measured during die-casting process is 4.8kPa. Specific test steps are as follows: First, put weighed pure aluminum ingot, Al-10Si-0.6Mn-0.3Mg ingot, Al-10Mn and Al-10Cr master alloy into a resistance furnace and heat it up to 200℃ for 15 minutes to remove water vapor, continue to heat up until melting; add Al-50Mg master alloy to molten aluminum temperature to 700℃ and stir evenly, and keep it warm for 10 minutes; add slag remover and then pass high-purity argon to degas; mold temperature is controlled at about 180℃, and temperature of molten aluminum reaches 690℃ for high-vacuum die-casting tests (high speed is 2m/s, boost pressure is 80MPa).

 

Fig.1 Assembly drawing of high vacuum die casting test mold

Table 1 Chemical composition of AlSiMgMnCr alloy (%)

 

Figure 2 Dimensions of tensile specimen

 

Fig.3 DSC curve of No.1 alloy and aging hardness curve of No.1-3 alloy at 180℃

 

Fig.4 Metallographic structure of high vacuum die-casting samples with different Cr contents

 

Fig.5 T6 metallographic structure of high vacuum die-casting samples with different Cr contents
It can be seen that alloy structure is mainly α-Al matrix and Al-Si eutectic structure. Among them, α-Al has two forms: α1-Al with larger size and α2-Al with smaller size but rounder shape. Coarse α1-Al is called pressure chamber pre-crystallization. Reason for its formation is that a small part of aluminum liquid begins to nucleate in pressure chamber. Due to its relatively long growth time, a relatively coarse α-Al is finally formed; while α2-Al is nucleated in mold cavity by molten metal, growth time is short and cooling rate of mold in which it is located is fast, resulting in a small and rounded size.

 

Fig.6 Backscattered photos of high vacuum die-casting samples with different Cr contents

Table 2 Statistics of Fe phase characteristics of high vacuum die-casting alloys with different Cr content

 

Fig.7 TEM bright-field image and β″ high-resolution Fourier transform image of No. 3 alloy at peak aging time

 

Fig.8 TEM bright field image and selected area electron diffraction pattern of square precipitates in T6 state

 

Fig.9 Energy spectrum curves of Cr-containing precipitates

 

Fig.10 Mechanical properties of high vacuum die-casting alloys with different Cr contents

 

Fig.11 Tensile fracture morphology of No.1 sample
(1) With increase of Cr content, yield and tensile strength fluctuations of high vacuum die-casting AlSi7MgMn samples in as-cast state are small, and elongation rate decreases; in T6 state, yield strength increases, tensile strength remains basically unchanged, and elongation rate is obvious decline.
(2) When Cr content increases from 0 to 0.18%, Fe phase area fraction and equivalent circle diameter in AlSi7MgMn alloy microstructure increase, Fe phase agglomerates, agglomerated Fe phase particles act as crack formation or expansion position, which greatly reduces plasticity of material.
(3) Dispersed and precipitated β″ phase of high vacuum die-casting AlSiMgMnCr alloy after T6 heat treatment is main reason for improvement of its tensile strength, and precipitation of square-shaped Al(Mn,Cr,Fe)Si has an inhibitory effect on formation of β″ phase, and distribution of β″ phase around it is uniform decline.