Al-Si alloys have been widely used in machinery manufacturing, transportation, aerospace and other fields. Die casting has characteristics of high casting dimensional accuracy, low surface roughness, high material utilization, less machining, dense organization, high strength and surface hardness, simple process and high production efficiency. Because ordinary Al-Si alloy die castings contain gas, it is generally not suitable to use heat treatment for further strengthening. At the same time, in order to prevent sticking, iron content is generally required to be high, resulting in low comprehensive mechanical properties. In relevant domestic and foreign standards, elongation of cast Al-Si die-casting alloys is also low, which is difficult to meet actual needs of enterprises for their high strength and toughness. Therefore, research and development of new die-casting Al-Si alloys has positive significance for manufacturing of some aluminum alloy products.
Ordinary die-casting Al-Si alloys are mainly modified by introducing refiners, modifiers and strengthening elements to improve comprehensive mechanical properties of cast state. In recent years, in cooperation with enterprises, hypoeutectic Al-Si alloy materials under ordinary die-casting conditions have been systematically studied, among which Al-9Si-0.6Fe-0.2Mn-0.2Mg-0.6Cu-0.3Sc alloy has high tensile strength, but slightly low elongation. Under non-vacuum conditions, tensile specimens of Al-9Si-0.6Fe-0.2Mn-0.2Mg-0.6Cu-0.3Sc-xZr (x=0, 0.1, 0.2, 0.3, mass fraction, %) alloy were prepared by ordinary horizontal die-casting machine, their cast microstructure and mechanical properties were studied by metallographic microscope, scanning electron microscope, electron probe and electronic universal material testing machine. Effect of Zr addition on morphology, size and distribution of primary α-Al phase, eutectic Si phase and intermetallic compound phase in alloy was investigated, influence and mechanism of Zr on mechanical properties of cast alloy were analyzed, aiming to provide a reference for its application.

Raw materials are industrial pure aluminum (99.9%, mass fraction, same below), industrial pure magnesium (99.9%), Al-20Si, Al-10Fe, Al-10Mn, Al-50Cu, Al-2Sc and Al-10Zr master alloys; refiner and modifier are Al-5Ti and Al-10Sr master alloys respectively; refining agent is ZS-AJ01c aluminum alloy degassing and deslagging agent.
Alloy was melted in a graphite crucible resistance furnace. Place weighed pure Al and master alloy in a graphite crucible in a resistance furnace, heat resistance furnace to 750℃ for melting, use a ladle to press pure Mg into aluminum liquid for melting, then add Al-5Ti and Al-10Sr master alloys for refinement and modification. After it is completely melted, use a bell jar to press refining agent into aluminum alloy melt for refining and degassing for 3 minutes, then stand for 5 minutes before slagging. Use a DC280T horizontal cold chamber die-casting machine to die-cast tensile test bars, as shown in Figure 1. Pouring temperature is 680℃, injection pressure is 40MPa, holding time is 5s, and mold preheating temperature is about 280℃.

 

Figure 1 Actual picture and size diagram of die-cast tensile specimen

 

(a)x=0 (b)x=0.1 (c)x=0.2 (d)x=0.3
Figure 2 Metallographic structure of cast Al-9Si-0.6Fe-0.2Mn-0.2Mg-0.6Cu-0.3Sc-xZr alloy

 

Figure 3 Average diameter of primary α-Al phase grains in cast alloy
Phase composition of alloy after adding Zr is same as that of alloy without Zr, which is white α-Al primary + dark gray (α-Al+Si) eutectic + intermetallic compound phase. However, primary α-Al phase in alloy without Zr addition is mainly flower-shaped, and the overall distribution is relatively uniform, as shown in Figure 2a. After adding 0.1% Zr, some flower-shaped primary α-Al phases began to break and break into fine dendrites, which were dispersed between flower-shaped grains, and dendrite grain size was significantly smaller than flower-shaped primary α-Al phase grains, as shown in Figure 2b. When Zr content was 0.2%, more flower-shaped primary α-Al phases transformed into fine dendrites, and grain refinement was obvious, as shown in Figure 2c. When Zr content was 0.3%, fine dendrites in alloy aggregated, and grain refinement effect weakened, as shown in Figure 2d.
As Zr content increased from 0 to 0.2%, average diameter of primary α-Al phase grains in alloy decreased from 17.3μm to 8.2μm, a decrease of about 53%. However, when Zr is added to 0.3%, average diameter of primary α-Al phase grains increases to 10.3μm, indicating that adding an appropriate amount of Zr can promote refinement of primary α-Al phase grains, and refinement effect is best when 0.2% Zr is added. This should be related to lower lattice mismatch between Al3 (Sc, Zr) phase and primary α-Al (only about 0.5%) than Al3Sc phase, which can also serve as α-Al phase nucleation site to refine primary α-Al phase. However, adding too much Zr will increase density of Al3 (Sc, Zr) phase particles in melt, causing them to aggregate and coarsen, segregate and precipitate, affecting refinement effect and causing grains to grow secondary.
As can be seen from Figure 4a, most of eutectic Si phase in alloy without Zr is irregular worm-like. When 0.1% Zr is added, worm-like eutectic Si phase gradually becomes fine and round. At this time, granular eutectic Si phase is majority in alloy and is evenly distributed, as shown in Figure 4b. When Zr content increases to 0.2%, except for a small amount of worm-like eutectic Si phase at the edge of Al/Si interface, the rest is basically granular eutectic Si phase, distribution is more compact and dense, as shown in Figure 4c, indicating that morphology and distribution of eutectic Si phase in alloy are significantly improved. However, when Zr content increases to 0.3%, eutectic Si phase gradually aggregates, and proportion of worm-like eutectic Si phase increases.

 

(a)x=0 (b)x=0.1 (c)x=0.2 (d)x=0.3
Figure 4 SEM image of eutectic Si phase in cast Al-9Si-0.6Fe-0.2Mn-0.2Mg-0.6Cu-0.3Sc-xZr alloy

 

Figure 5 Average aspect ratio of eutectic Si phase in cast alloy structure

 

Figure 6 SEM microstructure of cast Al-9Si-0.6Fe-0.2Mn-0.2Mg-0.6Cu-0.3Sc-xZr alloy sample

 

Figure 7 EPMA distribution diagram of cast Al-9Si-0.6Fe-0.2Mn-0.2Mg-0.6Cu-0.3Sc-0.3Zr alloy
Figure 8 shows hardness of Al-9Si-0.6Fe-0.2Mn-0.2Mg-0.6Cu-0.3Sc-xZr cast alloy samples with different Zr addition amounts. It can be found that Vickers hardness of alloy increases first and then decreases with addition of Zr. Figure 9 shows mechanical properties of die-cast alloy sample. It can be seen that tensile strength and elongation of alloy are basically consistent with change law of Vickers hardness, reaching maximum when 0.2% Zr is added, which are 289.3MPa and 4.9% respectively. Compared with tensile strength and elongation without Zr addition, they are increased by about 3% and 16% respectively. When Zr content increases to 0.3%, tensile strength and elongation of alloy decrease, while its yield strength does not change significantly with addition of Zr. Addition of an appropriate amount of Zr enhances alloy refinement and modification effect, making alloy structure fine, uniform and round. At the same time, it also forms a nano-scale Al3 (Sc, Zr) phase, which is dispersed in matrix, pins dislocations, hinders grain boundary movement, and improves mechanical properties of alloy. However, when Zr is added in excess, it will have a negative impact on alloy structure morphology, resulting in a decrease in its performance.

Figure 8 Vickers hardness of cast Al-9Si-0.6Fe-0.2Mn-0.2Mg-0.6Cu-0.3Sc-xZr alloy

Figure 9 Mechanical properties of cast Al-9Si-0.6Fe-0.2Mn-0.2Mg-0.6Cu-0.3Sc-xZr alloy

 

Figure 10 Tensile fracture morphology of cast Al-9Si-0.6Fe-0.2Mn-0.2Mg-0.6Cu-0.3Sc-xZr alloy

(1) Under ordinary non-vacuum die-casting conditions, the best effect of refinement and modification is achieved when 0.2% Zr is added to Al-9Si-0.6Fe-0.2Mn-0.2Mg-0.6Cu-0.3Sc alloy. At this time, refinement effect of primary α-Al phase in alloy is ideal, transforming from flower-like to fine dendrites, with an average grain diameter of 8.2μm; modification effect of eutectic Si phase is also optimal, transforming from worm-like to granular phase, and average aspect ratio drops to 1.42. However, addition of Zr has no significant effect on morphology and microstructure of the Fe-rich and Cu-rich phases in the alloy.
(2) Tensile strength, elongation and hardness (HV) of cast alloy all show a trend of first increasing and then decreasing with increase of Zr addition, and yield strength has no significant change. When Zr addition is 0.2%, tensile strength, elongation and hardness (HV) of alloy all reach maximum value, which are 289.3MPa, 4.9% and 114.3 respectively. At this time, tensile fracture mode of alloy is ductile fracture.