Die-casting aluminum alloy has high specific strength, good casting performance, processing performance, renewable performance, excellent electrical and thermal conductivity, is widely used in the fields of automobile, aerospace and electrical industry. Al-Si die-casting alloys are widely used due to their small crystallization temperature range, large Si phase solidification crystallization latent heat and specific heat capacity, small linear shrinkage, good flow properties, mold filling properties, small thermal cracking and shrinkage tendencies. However, traditional die-casting Al-Si alloy has poor plasticity, and it is difficult to meet requirements of rapid development of automobile industry. In order to meet performance requirements, at present, performance improvement is mainly achieved by introducing strengthening elements, and plasticity of alloy is improved by modifying eutectic Si.
Common strengthening elements in die-casting Al-Si alloys include Mg, Cu, etc. Mg can form Mg2Si with Si, which can be dispersed and precipitated in alloy structure to strengthen and improve yield strength of alloy; Cu can be used as a solid solution element in alloy, and at the same time, it can form Al2Cu phase with Al, which plays a role in solid solution strengthening and precipitation strengthening, improves yield strength and tensile strength of alloy. Zn is rarely studied as a strengthening element in Al-Si alloys. Some studies have explored influence of Zn on structure and properties of Al-Si-Cu-Mg alloys, found that within 1% Zn can promote rounding of Si phase, improve hardness and strength of alloy, but reduce elongation. Some researchers further expanded Zn content and studied effect of Zn with a mass fraction of more than 1% on properties of near-eutectic Al-Si alloys. They found that when Zn content exceeds 1%, strength and hardness of alloy increase with increase of Zn content, elongation decreases, eutectic Si size of alloy increases, flakes and needles appear. In Al-Si alloy, when Zn content exceeds 1%, size of eutectic Si will increase. Obviously, introduction of Zn can increase strength of alloy, but elongation decreases due to coarsening of eutectic Si. Current research mainly focuses on gravity casting, no explanation is given for reason why Zn coarsens eutectic Si, and no solution is given at the same time. Therefore, it is necessary to study effect of Zn on microstructure and properties of Al-Si alloy under condition of rapid cooling in die casting, and at the same time introduce modifying elements to solve problem of coarsening of eutectic Si.
Select Al-10Si-0.3Mg-0.6Mn as research object, adopt die-casting, add 2.5% Zn, then further add Sr and La/Ce mixed rare earth to modify alloy to explore effect of different elements on mechanical properties, microstructure characteristics and fracture behavior of alloy were explored, so as to obtain a high-strength and high-toughness die-casting aluminum alloy.
Graphical results
Select Al-10Si-0.3Mg-0.6Mn aluminum alloy (Z1 alloy) as base alloy, add 2.5% (mass fraction, same below) of Zn, 0.04% of Sr, and 0.1% of La/Ce mixed rare earth (RE) , to obtain Z2, Z3, Z4 alloys, using pure Al, Si, Mg, Zn and Al-10Mn, Al-10Sr, Al-10RE master alloys as raw materials. Actual composition of alloy was determined by spectroscopy and ICP and is shown in Table 1.

Table 1 Measured chemical composition of four alloys (%)

 

Figure 1 Die casting tensile test bar dimensions

 

Fig.2 OM photo of typical structure of alloy Z1

Table 2 Average grain size and area fraction of primary α-Al phase in four alloys

 

Fig.3 SEM morphology of eutectic Si phase of four alloys

 

Figure 4 XRD analysis results

 

Figure 5 Z2, Z4 alloy element analysis surface scanning results and composition analysis
It can be seen that there is no obvious segregation of Zn in alloy Z2 (Figure 5a-5d), distribution is relatively uniform, and distribution of Sr is basically same as that of Si, indicating that Sr plays a role in modifying eutectic Si. After adding Sr and RE (see Figure 5e to Figure 5j), surface scanning results are same as those of alloy Z2, distribution of Si, Sr, and Zn remains unchanged, RE content in circle area is relatively high, so point A is selected for analysis (see Figure 5k), it can be seen that this place is mainly composed of Al, Si, Zn, La, and Ce, that is, part of RE forms a new phase with Al, Si, and Zn. Due to low content, it was not detected by XRD analysis.

 

Fig.6 TEM analysis results of different alloys

 

Figure 7 Line scan of Figure 6a

Table 3 Solid solution amount of Zn element in α-Al phase of different alloys (%)
At the beginning of solidification, primary α-Al phase is formed first, and solute content is low, which increases solute content in remaining molten metal. Solid solution amount of Zn in α-Al phase formed by subsequent eutectic reaction is higher than that of primary α-Al phase. Subsequently, during eutectic solidification, because Zn is mainly enriched at solid-liquid front of Al phase, which hinders growth of Al phase, growth rate of Si phase is relatively improved, and Si phase obtains a larger growth space, thereby making Si phase coarse, as shown in Figure 8. Sr, as a commonly used modifier in Al-Si alloys, mainly affects growth of eutectic Si through adsorption of element Sr on the surface of silicon phase structure of Al-Si alloys, so as to achieve effect of grain refinement.

 

Fig.8 Schematic diagram of solid-liquid interface in eutectic region during solidification

 

Fig.9 Stress-strain curves and mechanical properties at room temperature of four alloys

 

Fig.10 Fracture morphology of four alloys

 

Fig.11 Longitudinal SEM photos of fractures of different alloys
in conclusion
(1) Adding 2.5% Zn to the Al-10Si-0.3Mg-0.6Mn alloy will increase size of eutectic Si, then add Sr and La/Ce mixed rare earths, and size of eutectic Si will be refined again.
(2) Addition of Zn can increase strength of Al-10Si-0.3Mg-0.6Mn alloy, while its elongation decreases; after adding Sr and La/Ce, strength and elongation of alloy are improved. Compared with Al-10Si-0.3Mg-0.6Mn alloy, strength is increased but elongation is not decreased.
(3) Dimples can be observed on fractures of Al-10Si-0.3Mg-0.6Mn, Al-10Si-0.3Mg-0.6Mn-2.5Zn, Al-10Si-0.3Mg-0.6Mn-2.5Zn-0.48Sr, Al-10Si-0.3Mg-0.6Mn-2.5Zn-0.1La (Ce), but after adding Zn, number of dimples at fracture site decreases, some fractured needle-like eutectic Si can be seen in longitudinal observation of fracture; when eutectic Si is refined again, number of dimples increases.