Abstract: Microstructure evolution and formation mechanism of low superheat inclined plate rheological die-casting ADC12 alloy were studied using metallographic microscope, X-ray diffractometer and scanning electron microscope, and its mechanical properties were tested. Results show that as distance from die-casting machine pressure chamber increases, alloy structure grows from coarse dendrites to spherical or nearly spherical grains. Structure formation mechanism includes dendrite fragmentation and melt explosion nucleation. In addition, rheological die-casting improves mechanical properties of alloy. Tensile strength of rheological die-cast aluminum alloy reaches 248 MPa, and elongation rate is also slightly better than conventional die-casting.
High-pressure die-casting process is widely used in production of parts in automobiles, electronics, aerospace and other fields. Compared with traditional casting processes, it has characteristics of high productivity, low energy consumption and complex formed parts. However, die-casting process involves injecting molten aluminum alloy into mold cavity under high pressure, which causes metal reaction and air to be trapped inside casting, bubbles and oxide inclusions are prone to appear inside part.
In order to reduce above-mentioned defects in traditional die-casting process and improve performance of die-cast products, researchers have introduced a semi-solid die-casting process. A large number of studies have reported that semi-solid die casting improves performance of die castings and improves quality of die castings. Since semi-solid die casting can provide higher viscosity fluids, and temperature of filling mold is low, amount of air entrainment is greatly reduced, thereby reducing pores and oxide inclusions during filling process, achieving effect of improving mechanical properties of casting. Semi-solid forming is generally divided into semi-solid thixoforming and semi-solid flow forming. Due to long thixoforming process, difficulty in storing and transporting semi-solid slurry, cost is too high and industrial production is difficult. Semi-solid flow forming process, which has characteristics of short process flow, energy saving, material saving, and low cost, has become a focus of attention.
ADC12 aluminum alloy is widely used in automotive industry due to its high casting performance, low shrinkage, and good corrosion resistance. However, so far, there are few reports on rheological die casting of ADC12 aluminum alloy. JANUDOM et al. successfully prepared ADC12 aluminum alloy semi-solid slurry using gas-induced semi-solid (GISS) method and die-casted commercial parts. HU et al. successfully prepared ADC12 semi-solid die-cast aluminum alloy by combining mechanical stirring to prepare slurry and high-pressure die-casting. However, these methods require additional equipment and cost also increases significantly.
This project takes ADC12 aluminum alloy as research object. In order to simplify forming process and shorten process, die-casting machine was slightly modified and a die-casting system that combined tilted plate method of semi-solid slurry preparation with high-pressure die-casting was designed. In addition, microstructure of different positions of casting's gating system was studied, and mechanical properties of rheological die-casting ADC12 aluminum alloy were studied.

Material used is commercial ADC12 aluminum alloy. Chemical composition of alloy measured by X-ray fluorescence spectrometer (XRF) is shown in Table 1.

Table 1 Chemical composition of ADC12 aluminum alloy wb/%
Figure 1 shows gravity casting structure of ADC12 aluminum alloy used in test. Its structure is mainly composed of white dendritic α-Al phase and black needle-like eutectic Si phase.

 

Figure 1 ADC12 aluminum alloy gravity casting structure

Inclined plate rheological die-casting process is shown in Figure 2. During test, first place ADC12 ingot in a graphite crucible and melt it at 620℃; at the same time, apply graphite paint on the surface of inclined plate. Inclined plate is made of No. 45 steel, with a length of 100 cm and an inclination angle of 30°. Adjust the bracket to ensure that molten metal flows from rear end of inclined plate to gate of die-casting machine; preheat die-casting machine cavity to 200℃.
After molten liquid is degassed and slag-dried, it is poured from top of inclined plate. After cooling by inclined plate and its own gravity, a semi-solid slurry is formed and flows directly into inlet of die-casting machine. Use a thermocouple to measure temperature of slurry. When temperature drops to 560~570℃, perform die casting and open mold to take out sample.

 

Figure 2 Schematic diagram of principle of inclined plate rheological die-casting ADC12 aluminum alloy

Select gating system part of casting for sampling research and observe its structure at different locations. Take a sample from A to E (filling order is E→A) to observe microstructure. Use a wire cutting machine to cut out 8 mm * 10 mm samples. After grinding with sandpaper and mechanical polishing, polished surface was etched with 0.5% HF aqueous solution for 15 s. After being taken out, it was rinsed with alcohol and metallographic structure of sample was observed using an optical microscope (OM).
DTA curve was measured using a DTG-60H differential thermal analyzer. A cylindrical sample with a diameter of 3 mm, a height of 2 mm, and a weight of 19.67 mg was placed in an alumina crucible, and sample was heated from 450 ℃ to 650 ℃ at a heating rate of 5 ℃/min in an argon protective atmosphere with a gas flow of 50 mL/min. A Bruker D8 advance diffractometer was used to conduct phase analysis on sample using X-ray diffractometer (XRD) with Cu Kα radiation. A JSM-6490LV scanning electron microscope (SEM) was used for microstructure observation and phase composition identification. Mechanical properties were tested. Sampling location of tensile specimen is shown in Figure 3.

 

Figure 3 Gating system and cross-section of ADC12 aluminum alloy rheological die-casting casting

DTA curve of ADC12 aluminum alloy is shown in Figure 4. A red area can be observed, which is endothermic peak, that is, Al-Si binary reaction. This area is solid-liquid interval. Alloy begins to melt at 539 ℃, completely melts at 578 ℃, and begins to solidify at 590 ℃, so solid-liquid range is 539~590 ℃.

 

Figure 4 DTA curve of ADC12 aluminum alloy
XRD spectra of different sampling positions of casting are shown in Figure 5. Spectrum shows that it is mainly composed of three phases: Al, Si and Al2Cu. For ADC12 aluminum alloy, eutectic phase of α-Al phase and α-Al+Si initially crystallizes, and due to high Cu content in alloy, L→α-Al+Si+Al2Cu eutectic reaction will occur, so final structure is mainly composed of three phases: α-Al, Si, and Al2Cu.

 

Figure 5 XRD patterns of ADC12 aluminum alloy at different sampling locations

 

 

Figure 6 Metallographic structures at different locations under a 50x optical microscope (Figures a-e correspond to locations A-E)
Figure 6 shows metallographic structure at different locations. It can be seen that structure of each position of gating system has significant differences. A and B have more uniformly distributed and rounded primary α-Al, C and D have less primary α-Al, while primary α-Al at E, material handle closest to pressure chamber, is in the form of white coarse dendrites and has the highest solid phase rate. Analysis believes that main reason for this structural difference is influence of cooling rate and mold filling process. Temperature of pressure chamber is 560~570 ℃, while mold preheating temperature is 200 ℃. The closer to pressure chamber, the smaller cooling rate. Position E of material handle is closest to pressure chamber, has the largest thickness, and has the slowest cooling rate, so α-Al at E is in the form of coarse dendrites. And because size of gate connecting E to D is small, it hinders flow of primary coarse dendrites. Only fine primary α-Al and liquid phase can pass through, so solid phase at E is the largest. As filling distance increases, cooling rate at position D→position A gradually increases, and the overall number of white primary α-Al grains increases, among which position C has the smallest primary α-Al content. This is inconsistent with phenomenon in Li Ming et al.'s study that the farther away from pressure chamber, the smaller solid content. Reason is that on the one hand, liquid phase in slurry during filling process encircles small primary α-Al crystal grains, causing liquid phase segregation, resulting in more solid phases at A and B and less solid phase at C and D; on the other hand, due to large cooling rate at A and B, liquid phase nucleates and grows into small and round primary α-Al grains.
In addition, it can also be observed from Figure 6 that there are some obvious black holes in tissue, which are micro pores. During die-casting process, alloy slurry fills mold cavity at high speed, so that gas in cavity that is not discharged in time is trapped by alloy melt. As alloy liquid solidifies, pores remain in specimen, that is, black holes are formed. In addition, cause of micropores may also be due to formation of metal inclusions during alloy smelting process.
Figure 7 shows metallographic structure at different locations under a high-power microscope. It can be observed that coarse dendrites in structure disappear and evolve to form primary α-Al of different sizes and shapes. Shapes are mainly spherical, nearly spherical, and broken dendrites. In Figure 7e, primary α-Al appears in the form of coarse dendrites, which is consistent with Figure 6e. Primary α-Al observed in Figure 7a~Figure 7d can generally be divided into three types: one is large spherical and nearly spherical particles, labeled α1-Al; the other is smaller spherical or nearly spherical particles, labeled α2-Al; in addition to these two types, there are also fine dendrites that are finally solidified, labeled α3-Al. Among them, α1-Al and α2-Al are numerous in Figure 7a and Figure 7b, but are rarely distributed in Figure 7c and Figure 7d, mainly consisting of fine dendrites α3-Al.

 

Figure 7 Metallographic structures at different locations under a 200x optical microscope (Figures a-e correspond to locations A-E)
At present, there are two mechanisms for formation of semi-solid microstructure: one is that dendrites are broken under action of external force, and broken dendrites eventually evolve into spherical grains; the other is that melt produces a large number of crystal nuclei in chilled state, these crystal nuclei inhibit growth of dendrites and grow directly in a spherical shape.
In this project, formation mechanism of spherical grains in microstructure of low superheat inclined plate rheological die casting has both. First, alloy melt is chilled by inner wall of inclined plate, causing a large number of primary α-Al crystal nuclei to be generated in melt close to inner wall. These crystal nuclei enter die-casting machine for mold filling with melt. During mold filling process, crystal nuclei continue to grow, and finally grow into spherical or nearly spherical α1-Al or α2-Al. In addition, alloy melt continuously flows downward under action of gravity on inclined plate, forming a shearing effect on melt. Dendrites in melt are sheared and destroyed, forming broken dendrite arms. At the same time, due to chilling effect of inclined plate, these broken and fine dendrite arms are retained.
During die-casting process, melt flowing into pressure chamber from inclined plate is subjected to dual effects of quenching and high pressure during injection and filling. On the one hand, a large number of primary α-Al crystal nuclei formed by flowing through inclined plate continue to grow into spherical or nearly spherical α1-Al or α2-Al; on the other hand, under high pressure and high speed, melt has an effect similar to strong stirring, dendrites remaining in melt continue to be sheared and destroyed. These small dendrite arms eventually solidify into spherical or nearly spherical grains. However, cooling rate in die-casting process is high, but due to different cooling rates at each position, position E is closest to pressure chamber and has the slowest cooling rate, allowing dendrites to grow. However, due to high cooling rate at positions A to D, dendrites have no time to grow, solidifies into fine and uniform α3-Al.
In summary, low superheat inclined plate rheological die casting can destroy growth of dendrites in melt. Under shear effect of flowing through inclined plate and stirring effect of die casting filling process, primary α-Al eventually grows into spherical or nearly spherical grains.

 

 

 

Figure 8 SEM images of intermetallic compounds in alloy (Figures a-e correspond to positions A-E)
Figure 8 is an SEM image of intermetallic compounds in alloy. It can be determined from XRD and metallographic structure that rheological die-casting ADC12 aluminum alloy is mainly composed of α-Al, Si, and Al2Cu phases. However, it can be seen from Figure 8 that in addition there are some unknown intermetallic compounds with different morphologies. EDS point analysis results of various places in Figure 8 are shown in Table 2. Compounds with different morphologies can be determined based on molar ratio: white polygonal ones are α-Al15(Fe,Mn)3Si2, white Chinese character-shaped ones are Al2Cu, dark gray blocks or strips are Si-rich phases, and a small amount of white thick needles are β-Al5FeSi. At point 11, Al2Cu is mixed with part of β-Al5FeSi.
In Figure 8, number of Al2Cu in different positions is the largest, which is strengthening phase. It is generally believed that thick needle-shaped β-Al5FeSi can split matrix and reduce mechanical properties. Vast majority of iron phase in picture appears as white polygonal α-Al15(Fe,Mn)3Si2, which improves destructive effect caused by a small amount of β-Al5FeSi.

Table 2 EDS point analysis of intermetallic compounds

Figure 9 shows mechanical properties of ADC12 conventional die-casting and rheological die-casting samples sampled at same position. It can be seen that tensile strength of rheological die-cast aluminum alloy is 248 MPa, which is significantly better than 210.7 MPa of conventional die-cast aluminum alloy. At the same time, change in yield strength is relatively small, and yield strengths of the two are relatively close. It can also be seen that elongation of rheological die casting is 2.05%, which is also slightly better than 1.46% of conventional die casting.

 

Figure 9 Mechanical properties of ADC12 conventional die casting and rheological die casting
Figure 10 is a comparison of microstructure of conventional die-casting and rheological die-casting samples at point B. It can be seen that conventional die-casting state shows uniform fine dendrites and chilled grains, with many black micropores, while rheological die-casting structure contains a large amount of spherical or nearly spherical α-Al with a high solid phase content. Research shows that mechanical properties of semi-solid die castings increase as solid phase ratio increases, while conventional die castings are prone to air entrainment and inclusions during filling process, which reduces mechanical properties. Reason is that rheological die casting reduces defects such as pores inside sample, makes structure dense and uniform, so its strength and plasticity are better than conventional die casting.

 

Figure 10 Comparison of microstructure at B of the gating system: (a) conventional die casting; (b) rheological die casting

(1) Solid-liquid range of ADC12 aluminum alloy was measured to be 539~590 ℃. Components of rheological die-casting ADC12 aluminum alloy are mainly composed of Al, Si and Al2Cu phases.
(2) During rheological die-casting process, as distance from pressure chamber of die-casting machine increases, alloy structure evolves from coarse dendrites to spherical or nearly spherical grains. Structure formation mechanism includes dendrite fragmentation and melt explosion nucleation.
(3) Composition of metal compounds with different morphologies was determined: white polygonal ones are α-Al15(Fe,Mn)3Si2, white Chinese character-shaped ones are Al2Cu, dark gray blocks or strips are Si-rich phases, and a small amount of white thick needles are β-Al5FeSi.
(4) Tensile strength of rheological die-cast aluminum alloy is 248 MPa, which is better than 210.7 MPa of conventional die-cast aluminum alloy. Yield strengths of the two are relatively close, and elongation of rheological die casting is also slightly better than that of conventional die casting.