Evolution of microstructure and properties of a new high-strength Al-Si-Mg-Mn alloy was studied. Based on JMatPro phase diagram simulation calculations, Al-Si-Mg-Mn alloy compositions with different eutectic integral fractions were designed. Results show that tensile strength of new Al-Si-Mg-Mn alloy after die-casting (as cast) can reach 230-310 MPa, yield strength is 200-240 MPa, and elongation is about 0.5%. As-cast structure contains α-Al, α-AlFeMnSi, binary (α-Al+α-AlFeMnSi/α-AlFeMnSi+Mg2 Si), and quaternary (α-Al+α-AlMnSiFe+Mg2 Si+Si) eutectic . Microstructure observation shows that formation of fine α-AlFeMnSi phase and multi-scale eutectic structure gives alloy high strength; fracture morphology analysis shows that lower elongation of alloy is directly caused by larger pores and coarse second phases.
Die-casting process is a near-net forming process with high centralized production efficiency. Die-casting products have high precision and excellent performance. Therefore, die-casting is widely used to manufacture parts for automobiles, communications, engineering machinery, etc. In the past ten years, many steel parts in automobile parts have been replaced by die-cast aluminum alloy parts, thereby reducing weight of automobile, achieving energy conservation and emission reduction. In the past few decades, there has been a lot of research on high-performance die-casting aluminum alloys. Existing die-casting aluminum alloy series include Al-Si series, Al-Si-Mg series, Al-Si-Cu series, and Al-Si -Cu-Mg series and Al-Mg series. Types and properties of commonly used die-cast aluminum alloys are shown in Table 1. From the table, it can be concluded that yield strength of these aluminum alloys after die-casting is 100 to 190 MPa, but elongation changes greatly. Elements such as Mg, Cu, Mn or Zn are added to die-cast aluminum alloys to form intermediate compounds such as AlMgZn, AlMn or Al 2 Cu to improve strength. Strengthening effects of these alloys are also attributed to solid solution strengthening and precipitation strengthening. Al-Mg-Si-Mn alloy developed by Hu et al. has a yield strength of 183 MPa, Al-5Mg-0.6Mn alloy developed by Zhang et al. has a yield strength of 212 MPa, and Al-10Mg-3.5Zn-3Si alloy die-casting developed by Ji et al. has a yield strength of 320 MPa after heat treatment after die casting. Because heat treatment is required, a certain amount of blistering in die castings is inevitable. At the same time, temperature of heat treatment is generally high (such as 535℃), which will cause surface blistering and dimensional instability of product. Adding an appropriate amount of Mn to alloy can not only reduce sticking pattern, but also change morphology of β-Fe phase. Addition of Cu and Zn alloy not only increases alloy density (without achieving effect of weight reduction), but also increases composition accordingly.

Table 1 As-cast properties of commonly used industrial die-cast aluminum alloys
Because ultrafine eutectic structures can improve strength and plasticity of alloys, research on binary or multicomponent ultrafine eutectic or hypoeutectic structures has received widespread attention in recent years. At present, there are relatively few ultrafine eutectic alloy series, such as TiNbCoCuAl alloy. Due to existence of ultrafine eutectic structure in structure, its specific orientation of eutectic phase hinders dislocation movement, improves strength and plasticity of alloy. But for die-cast Al-Si alloys, it is still a challenge to have a yield strength exceeding 200 MPa in as-cast state.

Raw materials used in test include industrial pure Al (99.7%), metallic Si (99.7%), pure Mg (99.9%) and Al-20Mn master alloy. First, oil stains and other impurities on the surface of industrial pure Al are cleaned, removed and dried, then 200 kg are added to machine side furnace (capacity 300 kg) of die-casting machine. Die-casting machine model is Haitian DC-300 (ordinary cold die-casting machine). Ingredients are prepared according to Table 2, and melting temperature is controlled at (730±10)℃. After all melts, let it stand for 30 minutes and perform degassing refining. Graphite rotor rotates at 380 r/min and degassing time is 15 minutes. Alloy composition was measured using SPECTROLAB M12 direct reading spectrometer. Actual composition is shown in Table 3.

Table 2 JMatPro6.0 simulation calculation of alloy composition wB/% with different eutectic integral fractions

Table 3 Actual chemical composition of alloy wB/%
After alloy composition is determined to be qualified, a die-casting test is carried out. Tonnage of die-casting machine is 300 t. During die-casting, melt temperature is kept above liquidus line (50±5)℃. Main process parameters of die-casting machine are shown in Table 4, and die-casting mold is shown in Figure 1.

Table 4 Main process parameters of die casting machine

 

Figure 1 Schematic diagram of die casting mold
Sample for tissue observation was taken from middle part of Φ 6.4 tensile test bar, after standard grinding and polishing, microstructure was observed with an OLYMPUS GX53 inverted metallographic microscope; ImagePro6.0 was used to count volume fraction of second phase in five different areas of sample from edge to center; scanning electron microscope (Nova NanoSEM450) sample was etched in 0.5% HF for 30 s; Ultima V multifunctional X-ray diffractometer (XRD) was used to test phase composition of Al-Si-Mg-Mn alloy. Sample was taken from vertical plane in the middle of Φ 6.4 tensile test bar. Test step was 0.02°, scanning rate was 20°/min, scanning range was 10°~90° (2 θ), and target material selected for test was a copper target. (Cu, K α , λ =0.154 059 8 nm); Use NETZSCH DSC204 f1 differential calorimeter scanner to test melting temperature and latent heat of fusion of Al-Si-Mg-Mn alloy. Heating and cooling rates are both 8 K/min, argon gas flow rate is 60 mL/min; tensile test is carried out on a DDL-200 series testing machine, tensile rate is 1 mm/min, tensile test rod gauge length is 50 mm, standard diameter of test rod is 6.35 mm. Test results were taken as average of 5 tensile test bars.

In Al-Si-Mg ternary alloy, when composition is Al-13.9Si-5.55Mg, L→α-Al+Mg 2 Si+Si ternary eutectic reaction occurs. Based on this ternary eutectic composition, compositions of hypoeutectic Al-Si-Mg alloys containing volume fractions of 40% and 60% were calculated. In order to demould in die-casting process standards, content of Fe is generally not less than 0.7wt.%, but Fe is generally harmful to its performance in aluminum alloys. Therefore, Mn is used to replace Fe in the industry. Mn not only helps with demoulding but also forms a dispersed Mn-containing strengthening phase during die-casting process. Therefore, solidification path of eutectic and hypoeutectic Al-Si-Mg-Mn-(Fe) is predicted based on Scheil solidification model (solidification path is calculated based on measured composition) as shown in Figure 2.

 

Figure 2 Simulation of solidification path of Al-Si-Mg-Mn alloy
As Mg and Si contents change, eutectic integral fraction changes from 0.4 to 1.0. The three alloys undergo a quaternary eutectic reaction at the end of solidification (about 556.9 ℃): L→α-Al+α-AlFeMnSi+Mg 2 Si+Si. When ternary L→α-Al+α-AlFeMnSi+Mg2 Si occurs, alloy A forms a solid phase of 0.58 from L→α-Al and L→α-Al+α-AlFeMnSi; alloy B forms a solid phase of 0.22 from L→α-AlFeMnSi and L→α-Al+α-AlFeMnSi; alloy C forms a solid phase of 0.02 from L→α-AlFeMnSi and L→α-AlFeMnSi+Mg2 Si; At the end of ternary reaction, solid phase ratios of the three alloys were 0.67, 0.54, and 0.04 respectively.

Figure 3 is XRD pattern of Al-Si-Mg-Mn alloy with different eutectic integral fractions. Analysis shows that main diffraction peak phases are calibrated to α-Al, Si, Mg2 Si and α-AlFeMnSi phases. It is worth pointing out that some tiny peaks are π-AlFe(Mn)MgSi phase.

 

Figure 3 XRD pattern of Al-Si-Mg-Mn alloy

Figure 4a shows low-magnification overall morphology of alloy A as cast, and its microstructure is shown in Figure 4b. It can be intuitively seen from SEM overall morphology that there are many tiny pores in Alloy A, with a diameter of 20 to 40 μm; low-magnification morphology of Alloy B is similar to Alloy A, but its pore size is about 230 μm, as shown in Figure 4c. Two α-Al phases with different morphologies can be observed in OM structure. One is a coarse dendrite structure, defined as primary α1. This α-Al phase is formed when aluminum liquid is poured into injection cylinder during die casting (temperature of aluminum liquid is higher and temperature of injection cylinder is lower); the other α-Al phase is in the shape of fine spheres, which is formed by rapid filling of aluminum liquid into mold and is defined as secondary α2. These two α-Al phase can be observed in both alloys A and B, as shown in Figure 4b and d. Compounds with dense polygons and irregular shapes were also observed in Alloy B, which were gray in color. From subsequent SEM-EDS analysis, it was found that this was α-AlFeMnSi phase, as shown in Figure 4d. Same phase also existed in Alloy C, as shown in Figure 4f. There are obvious differences in microstructure of Alloy C and Alloys A and B. There are a large number of black lump compounds in Alloy C. Combining alloy composition and SEM-EDS analysis, it is found that black compound is primary Mg2 Si, while in solidification simulation analysis, there is no primary Mg2 Si phase (block-shaped), only eutectic Mg2 Si (Chinese character-shaped). This may be due to very fast cooling rate of die casting and non-equilibrium solidification.

 

(a), (b) Alloy A; (c), (d) Alloy B; (e), (f) Alloy C
Figure 4 Microstructure evolution of Al-Si-Mg-Mn alloy

Figure 5 is DSC analysis curve of alloys A, B, and C. From solidification curve in figure, it can be analyzed that final solidification temperature is 557.5℃, which is close to temperature of recent quaternary eutectic reaction calculated by simulation in Figure 2. Alloy A has two obvious endothermic peaks, corresponding to formation of α-Al dendrites, ternary eutectic and quaternary eutectic. Among them, ternary eutectic reaction and quaternary eutectic reaction are very close. Alloy B showed three endothermic peaks, corresponding to α-AlFeMnSi, binary eutectic, ternary eutectic and quaternary eutectic reactions. For alloy C, only one peak was found from DSC curve, which is mainly eutectic reaction (including binary, ternary and quaternary reactions) and a trace amount of α-AlFeMnSi, which is consistent with simulation analysis. This is because cooling rate in DSC analysis is very slow and is almost close to equilibrium solidification, so some intermediate compounds or ternary eutectic reactions will precipitate during final eutectic reaction. Alloy A has the highest solidification temperature (632℃), while alloy C has the lowest, only 582.1℃.

 

Figure 5 DSC analysis curve of alloy (cooling rate 8 K/min)

SEM-EDS was used to further analyze eutectic structure, morphology and composition of α-AlFeMnSi particles in alloy, as shown in Figure 6. Figure 6a and c are backscattered electron images of alloys A and B respectively. In Figure 6a, it is observed that α-AlFeMnSi phase is very fine, polygonal, approximately spherical, and dispersed on matrix, with a diameter of about 0.25 to 1.75 μm. In observation area of Alloy B, number of α-AlFeMnSi phases increased significantly, and size was relatively concentrated, about 0.5-0.75 μm. It can be observed that pores in Alloy B were also increasing, as shown in Figure 6c.

 

(a), (b) Alloy A; (c), (d) Alloy B; (e), (f) Alloy C
Figure 6 SEM-BSE image of alloy structure
In Alloy C, as shown in Figure 6e, there are not many dispersed and fine α-AlFeMnSi phases found, only very few α-AlFeMnSi phases, they are coarse and irregular, with a length of about 20 μm; at the same time, a large number of coarse tetragonal Mg2 Si phases appear in structure of alloy C. Average chemical compositions of different intermediate phases in alloy are shown in Table 5. There are two eutectic structures in Figure 6b and d, labeled Eu1 and Eu2. Morphology and approximate size of the two eutectic structures in alloys A and B are basically similar, but Eu1 in alloy B is slightly wider. Eu1 contains branched Mg2 Si, and branch spacing of Eu1 in alloy A is about 0.4 to 1.2 μm, while branch spacing of Eu1 in alloy B is about 0.7 to 3 μm. Eu2 eutectic of alloys A and B contains α-Al, Si, Mg2 Si and needle-like π-AlMnFeSiMg phases. Morphology of Eu2 in alloy C is similar to that in alloys A and B, but size is smaller, as shown in Figure 6e; different from alloys A and B, alloy C contains a large amount of dense bulk Mg2 Si phase, with a size of about 15 Around μm, as shown in Figure 6f.

Table 5 Chemical composition of mesophase in alloy (SEM-EDS)
There is a large amount of eutectic Si phase in Eu2 eutectic structure, and its morphology is still needle-shaped. Aspect ratio and equivalent diameter are as shown in Table 6. It can be seen that aspect ratios are similar. Comparing equivalent diameter, minimum equivalent diameter of Si in alloy A is (0.43±0.34) μm, while maximum equivalent diameter of Si in alloy B is (0.64±0.43) μm. Under same cooling conditions, size of eutectic Si in this test alloy is much smaller than that in binary Al-Si alloy.

Table 6 Aspect ratio and equivalent diameter of Si particles in alloys A, B, and C

Figure 7 shows size distribution of α-AlFeMnSi particles in the three alloys. Size of α-AlFeMnSi particles in alloys A and B is relatively concentrated and very small, ranging from 0.5 to 1.5 μm; while size of α-AlFeMnSi particles in alloy C is larger and dispersed, with sizes ranging from 4 to 18 μm. In addition, Figure 7d statistics aspect ratio of α-AlFeMnSi particles. From statistical results, it can be seen that aspect ratio of α-AlFeMnSi particles in alloys B and C is smaller than that of alloy A.

 

Figure 7 α-AlFeMnSi phase size distribution and α-AlFeMnSi phase aspect ratio-frequency distribution curve in alloy

Figure 8a shows tensile stress-strain curve of die-cast Al-Si-Mg-Mn alloy. As-cast yield strength of the three alloys exceeds 200 MPa, but the three alloys have almost no plasticity. Figure 8b is a comparison chart of tensile properties of three Al-Si-Mg alloys with different eutectic fractions. Yield strength of alloy A reaches 236 MPa, and elongation is only 0.36%; yield strength of alloy B is 229 MPa, and elongation is approximately 0.16%; yield strength of Alloy C is only 202 MPa, and elongation is 0.3%. Compared with common grades of die-cast aluminum alloys in Table 1, yield strength of these three alloys is higher, and there are no other alloying elements in this alloy, such as Cu, Zn and other strengthening elements.

 

Figure 8 Tensile properties of cast Al-Si-Mg-Mn alloy

Figure 9a, c, and e are morphology of middle part of fracture of alloys A, B, and C respectively. A large number of bright white α-AlFeMnSi phases can be observed in the pictures. From pictures, it can also be observed that fracture starts from primary phase and eutectic. There are still holes in fracture surfaces of alloys B and C, as shown in Figure 9c and e. Alloys A and B fractured in eutectic region, as shown in Figure 9b and d, while Alloy C not only fractured in eutectic region, but also fractured on coarse Mg2 Si particles, as shown in Figure 9f. At the same time, it is worth pointing out that fracture of alloy A is tear-like; while fracture of alloys B and C is very smooth and linear.

 

(a), (b) Alloy A; (c), (d) Alloy B; (e), (f) Alloy C
Figure 9 Tensile fracture morphology of alloy and side morphology of fracture perpendicular to tensile direction

(1) Three high-strength die-casting aluminum alloys were designed based on ultrafine multi-element second phase strengthening mechanism. Al-Si-Mg-Mn alloy containing 35% eutectic has a yield strength of 237 MPa, a tensile strength of 301 MPa, and an elongation of 0.36%; Al-Si-Mg-Mn alloy with 55% eutectic fraction has a yield strength of 229 MPa, a tensile strength of 257 MPa, and an elongation of 0.18%.
(2) Structural composition of this high-strength Al-Si-Mg-Mn die-cast aluminum alloy is relatively complex, including α-Al phase, α-AlFeMnSi phase, binary (Al+Mg2 Si) eutectic and quaternary Al+Mg2Si+Si +α-AlFeMnSi eutectic.
(3) Average size of eutectic Si in Al-Si-Mg-Mn alloy containing 35% eutectic is 0.43 μm. α-AlFeMnSi phase is very fine, polygonal, approximately spherical, and dispersed on matrix, with a diameter of approximately 0.25 to 1.75 μm.
(4) Pores and coarse hard second phases in alloy are main reasons for poor plasticity of alloy.