Al-Si alloy has good fluidity, small shrinkage and hot cracking tendency, and is widely used. Pressure casting has characteristics of high productivity, high casting precision, and dimensional stability. Based on high-quality, efficient and low-cost manufacturing, there is an actual demand for high strength and toughness of as-cast Al-Si die-casting alloys. When producing rotating ring castings in industrial fire pipe joints, die-cast Al-Si alloys with good comprehensive mechanical properties are required, such as tensile strength greater than 240MPa and elongation greater than 4%. However, current elongation rate of relevant alloys is difficult to meet requirements. Therefore, it is of positive significance to develop new as-cast high-strength die-cast Al-Si alloys.
Refining and modifying modified materials through microalloying is an effective way to improve comprehensive mechanical properties of die-cast Al alloys as cast. Rare earth elements can refine grains through heterogeneous nucleation, and high-affinity rare earth elements can form metal compounds with harmful metals or impurities in matrix, effectively removing defects in matrix, thereby improving the overall performance of alloy. Rare earth element Er has a certain impact on properties of aluminum alloys. Research shows that tensile strength and elongation of Al-Mg alloy first increase and then decrease as Er content increases, while hardness continues to increase; Er will also significantly increase number of rare earth compounds and impurity phases in AA6061 alloy structure, greatly improving strengthening effect of second phase. In addition, strengthening element Zn can be evenly distributed in matrix in the form of solid solution to solid solution strengthen alloy. Some studies have also pointed out that when Zn content is 1%, eutectic Si phase in alloy structure will transform into fine fibers or particles, effectively improving performance of die-cast Al-Si alloys. However, there are few reports on effects of addition of Er, combined addition of Er and Zn on mechanical properties of Al-Si alloys.
In the early stage, this research group obtained a set of basic alloy compositions Al-9Si-0.6Fe-0.2Mn-0.2Mg based on a combination of orthogonal tests of commonly used elements in Al-Si alloys, fluidity of alloy, ordinary die-casting to prevent mold sticking, and other factors. However, its comprehensive mechanical properties still need to be further improved. Based on this, this study prepared ordinary die-cast tensile test bars (numbered Z1~Z4) of four hypoeutectic Al-Si alloys, Al-9Si-0.6Fe-0.2Mn-0.2Mg-X(X=0.2Er,0.4Er,0.6Er,0.4Er+1.5Zn), by single addition of different contents of Er element and composite addition of Er and Zn elements to study their as-cast microstructure and mechanical properties, aiming to provide reference for application of as-cast high-strength and tough die-cast Al-Si alloys.

Raw materials are industrial pure aluminum (99.9%, mass fraction, same below), pure magnesium (99.9%), pure zinc (99.9%), Al-20Si, Al-10Fe, Al-10Mn, Al-20Er, Al-5Ti and Al-10Sr master alloy. Place weighed pure aluminum, Al-20Si, Al-10Fe and Al-10Mn raw materials into graphite crucible in SG-30-10 series well-type crucible resistance furnace, heat to 750℃ and keep for a period of time until all added raw materials melt, then add Mg or Zn. In order to prevent Mg from burning, Mg is completely wrapped with aluminum foil and added. After it melts, Al-5Ti and Al-10Sr master alloys are added in sequence for refinement and modification. Addition amounts of Sr and Ti are 0.03% and 0.05% respectively; then add a certain amount of refining agent to degas, and let it stand for slag removal; use DC280T horizontal cold chamber die-casting machine to prepare tensile test bars. Mold preheating temperature is 280℃. Dimensions of test bars are shown in Figure 1. WAW-10000 universal material testing machine was used to conduct tensile testing on test rods. Tensile rate was 1.0mm/min. Three test rods were tested for each alloy, and results were averaged. Samples were taken from the end of tensile specimen, ground and polished, etched with a HF solution with a volume fraction of 0.5% for 10 s, then microstructure of sample was observed using an Olympus-BHM363U metallographic microscope, and hardness was measured using a DigiVicher-1000A Vickers hardness tester with a load of 50N and maintained for 15s. 10 different positions are randomly selected for measurement in the area and average value is taken. Combined with D/MAX-RB X-ray diffractometer and JXA-8230 electron probe, physical phase of sample was detected and analyzed.

 

Figure 1 Tensile specimen dimensions

 

Figure 2 Metallographic structure of Z1~Z4 die-cast alloys

 

Figure 3 XRD patterns of Z2 and Z4 alloy samples
Figures 4 and 5 respectively show EPMA morphology of Z2 die-cast alloy structure, main element surface scanning and EDS energy spectrum analysis results of corresponding points. It can be seen that narrow and elongated phase at point A is mainly composed of Al, Si, Fe and a small amount of Mn elements, and it is initially determined to be β-Al5FeSi. In addition, part of Mn element will enter Fe-rich phase and promote transformation of β-Al5FeSi phase into agglomerated α-AlSiMnFe phase. Skeleton phase at point B mainly contains Al, Si, Mn, Fe and Er elements. After removing influence of Mg element, it is preliminarily judged to be α-AlSiMnFe phase. A higher content of Er element was detected. This is because affinity between Er-Si atoms is greater than that between Fe-Si atoms. Therefore, Al-Er-Si intermetallic compound is more stable than Al-Fe-Si. When Er element is added to Al-9Si-0.6Fe-0.2Mn-0.2Mg alloy, part of Er will replace Fe in α-AlSiMnFe phase into a higher melting point and more stable α-AlSiMnFeEr phase, thus changing Fe-rich phase composition and grain boundary stability.

 

Figure 4 EPMA morphology and EDS analysis results of die-cast Z2 alloy

 

Figure 5 Main element surface scanning

 

Figure 6 EPMA photos and EDS analysis results of Z4 alloy
It can be seen from Figure 7 that as rare earth Er content increases, hardness of alloy first increases and then decreases. When 0.2% Er is added to alloy, its hardness (HV) is 74.7. When 0.4% Er is added, grain refinement is the most obvious, second phase is distributed most uniformly, and hardness (HV) of alloy reaches 84.6, which is 13.25% higher than that of Z1 alloy. When Er content continues to increase to 0.6%, hardness (HV) decreases to 79.2. This is because addition of Er element will cause excessive precipitation of strengthening phase Al3Er and promote improvement of alloy hardness. On the other hand, as Er content increases, refined grains tend to partially grow, and larger second phase containing Er will also precipitate, weakening grain refinement effect, thereby reducing hardness of alloy.

 

Figure 7 Mechanical properties of alloys with different Er addition amounts

 

Figure 8 Mechanical properties of Z2 and Z4 alloys

Compared with Z2 alloy, tensile strength (σb), yield strength (σ0.2), and elongation (δ) of Z4 alloy with Er and Zn increased by 22.96%, 12.3%, and 50% respectively. From a microscopic perspective, addition of 1.5% Zn to Z4 alloy significantly promotes refinement of phases in structure, and generates more uniformly distributed secondary strengthening phases, which reduces splitting effect on matrix, and significantly improves mechanical properties of alloy. Due to solid solution strengthening and precipitation strengthening effects of Zn, hardness of Z4 alloy is 8.62% higher than that of Z2 alloy. In addition, it may also be due to increase in eutectic Si fraction of hard phase, resulting in higher hardness values. In summary, Z4 alloy with combined addition of Er and Zn shows better comprehensive mechanical properties, can meet mechanical property requirements of castings such as rotating rings.
Main phases in Al-9Si-0.6Fe-0.2Mn-0.2Mg-X (X=0.2Er, 0.4Er, 0.6Er, 0.4Er+1.5Zn, %) alloy include α-Al, eutectic Si, Al3Er, Mg2Si and Fe-rich phases (α-AlSiMnFe and β-Al5FeSi). In alloy with a compound addition of 0.4% Er+1.5% Zn, in addition to above phases, an α-AlMgZn phase will also be produced in alloy.