High-pressure casting is widely used due to its ability to produce thin-walled components, high production efficiency, and high dimensional accuracy. Manufacturing process of high-pressure casting determines that alloy melt has characteristics of rapid solidification, which is of great significance for refining microstructure of cast aluminum alloys (including grains and second phases) and improving mechanical properties. Al-Si-Cu-Mg alloy is widely used in automotive industry due to its excellent casting characteristics and mechanical properties. However, for Al-Si-Cu-Mg alloy, most studies only focus on low Cu content alloys, whose yield strength is about 120MPa. Yield strength of high-pressure cast aluminum alloys can be improved by adding rare earth elements, TiB2, etc. and ultrafine eutectic strengthening. This method has higher requirements on casting process and higher costs. At present, there are relatively few reports on strength and toughness of high-pressure cast Al-Si-Cu-Mg alloys under high Cu content and high w(Cu)/w(Mg) mass ratio. Cu can be used as a strengthening element to further improve mechanical properties of alloy. .
On the other hand, trace elements such as Mo and Zr often form coarse intermetallic phases during traditional solidification process, but fine intermetallic phases can be formed under high cooling rates, so Mo and Zr have potential to strengthen aluminum alloys under rapid solidification conditions. Microalloying is widely used in preparation of high-strength and high-pressure cast aluminum alloys. However, due to influence of turbulence and air inclusions in high-pressure casting, castings cannot be heat treated, which limits application of high-pressure cast aluminum alloys. Therefore, using low-temperature artificial aging, natural aging + low-temperature artificial aging instead of traditional T6 heat treatment to improve strength of high-pressure cast aluminum alloys can produce high-strength die castings with high dimensional accuracy and good safety, avoid heat treatment defects such as blistering. Artificial aging can accelerate precipitation of strengthening phase by heating it to an appropriate temperature and maintaining it for a certain period of time, making alloy stronger and tougher, improving its performance. Natural aging, by placing workpiece in atmospheric environment, will gradually precipitate due to supersaturated solid solution formed at a high cooling rate, and amount of precipitation will gradually increase as time goes by until it becomes stable. Combination of artificial and natural aging can strengthen and toughen high-pressure casting alloys and improve their performance. This study focuses on effects of trace amounts of Mo, Zr elements and low-temperature aging on mechanical properties of high-pressure casting Al-Si-Cu-Mg-Fe-Mn alloy, aiming to provide a reference for expanding its application range.
Graphical results
A total of two chemical composition alloys were designed for this experiment. Materials used for smelting include pure Al ingots (99.85%, mass fraction, the same below), pure Mg (99.9%), pure Cu (99.9%), pure zinc (99.9%), Al -10Mn, Al-10Fe, Al-20Si, Al-10Sr, Al-5Mo and Al-5Zr master alloys. Chemical compositions of the two alloys are shown in Table 1. A 300kg machine-side furnace is used to smelt raw materials. First, add pure aluminum ingots, Al-20Si master alloy, pure Cu ingots, and pure Zn ingots in specified amounts. When preparing M2 alloy, pure Cu and pure Zn ingots must be added before adding Al-5Mo and Al-5Zrmaster alloy, then heat melt to 760℃ and keep it for 10~15min. When melt temperature drops to 740~750℃, add Al-10Fe and Al-10Mn master alloy, keep it for 10~15min. When melt temperature drops to 720-730℃, add pure Mg ingots. Stir melt for 10 minutes to ensure that melt is fully mixed. Subsequently, refining, degassing and slag removal are carried out at 730℃ for 30 to 40 minutes. Subsequently, prepared melt was transferred to injection sleeve of a cold chamber HPDC machine (Haitian Metal DC-300). Mold preheating temperature is 250℃, injection sleeve preheating temperature is about 200℃, and pouring temperature is controlled at 690~700℃. For each chemical composition alloy, tensile test bars with a size of ϕ6.4mm were prepared. Among them, mechanical properties of M1 and M2 alloys were tested after die-casting was completed, then placed in atmospheric environment. Mechanical properties were tested at 24, 48, 96, 168, 360 and 720 hours, and changes were compared. In addition, M1 and M2 alloys were artificially aged at 175℃*6h respectively.

Table 1 Chemical composition of test alloy (%)

 

Figure 1 Microstructure and EDS results of as-cast M1 alloy and M2 alloy

 

Figure 2 SEM morphology and element surface scanning of M1 alloy

 

Figure 3 SEM morphology and element surface scanning of M2 alloy

 

Figure 4 AlFeMgMnSiMo phase morphology and EDS

 

Figure 5 α-AlFeMnSi phase STEM surface scanning results
From perspective of second phase particle strengthening, M1 and M2 alloys both contain higher Cu and Zn elements, a certain Mg content and a lower Fe content. Therefore, there are many fine Q-AlCuMgSi and θ-Al2Cu intergranular strengthening phases in as-cast microstructure of M1 and M2 alloys. Under high cooling rate of HPDC, low Fe and high Mn element ratios in M1 and M2 alloys enable alloys to form fine massive α-AlFeMnSi phases, which plays a certain role in improving mechanical properties. In addition, M2 alloy adds an additional 0.12% Mo and 0.15% Zr. 0.12% Mo forms a fine granular AlFeMnSiMo phase in alloy, uniformly dispersed AlFeMnSiMo phase further strengthens alloy.

Table 2 Mechanical properties of M1 alloy as cast and in different natural aging states

Table 3 Mechanical properties of M2 alloy as cast and in different natural aging states

Table 4 Mechanical properties of M1 and M2 alloys after artificial aging at 175℃×6h
Combining relevant research and HRTEM images of θ' phase and G.P. zone phase in Figure 3, it can be seen that long strip-shaped precipitated phase is θ' phase, and short rod-shaped precipitated phase is G.P. zone. Alloy composition of M1 and M2 (high Cu content) results in more stripe θ’ phases, a certain amount of G.P. zone phase precipitates during artificial aging process, which strengthens mechanical properties of alloy. Compared with ordinary gravity casting, high-pressure casting has a high cooling rate (>500℃/s), which causes θ-Al2Cu phase, β-Mg2Si phase and Al3Zr phase to have no time to precipitate and form a supersaturated solid solution. At the same time, both Cu phase and α-Al grains in as-cast structure are significantly refined and distributed more uniformly, which is conducive to full precipitation of high-density θ’ phase during artificial aging process. At the same time, less Cu-containing phase on grain boundaries also helps alloy maintain better elongation.

 

Figure 6 Precipitate phase in M2 alloy after artificial aging (175 ℃×6h)

 

Figure 7 SEM and M2 local EDS analysis of fracture surfaces of M1 and M2 alloys under as-cast conditions
In conclusion
(1) As-cast microstructure of high-pressure casting Al-Si-Cu-Mg-Fe-Mn alloy has a small second phase at grain boundary and is evenly distributed. Addition of Mg, Cu, Zn, Fe, and Mn can form fine, uniformly distributed strengthening phases during rapid cooling process of high-pressure casting. Addition of Mo forms a fine granular AlFeMnSiMo phase in alloy, which is evenly dispersed and distributed in aluminum matrix, which can further improve strength of alloy.
(2) During rapid cooling process of high-pressure casting, Cu, Mg, Zn, Mn, Fe, etc. play role of solid solution strengthening and second phase strengthening. In as-cast or natural aging state, both M1 and M2 alloys show good strength and toughness, tensile strength and yield strength of the two alloys increase with increase of natural aging time, and reach stability after aging for 720h.
(3) Al-Si-Cu-Mg-Fe-Mn alloys with high Cu content precipitate dense G.P. zones and strip-like θ’ nano-strengthening phases during low-temperature artificial aging process, thereby improving yield strength and tensile strength of M1 and M2 alloys. After artificial aging at 175℃×6h, tensile strength and yield strength of M1 alloy increased by 8.8% and 43.0% respectively compared with as-cast state, tensile strength and yield strength of M2 alloy increased by 10.6% and 43.8% respectively compared with as-cast state.