Mg-Zn-Al-Mn alloy was prepared by die casting process, effects of different heat treatment processes on mechanical properties and microstructure of die-cast Mg-Zn-Al-Mn alloy were studied. Results show that when die-cast alloy is directly aged at 190 ℃ for 5 h, alloy strength reaches peak value, its tensile strength, yield strength and elongation are 236 MPa, 149 MPa and 7.3%, respectively. After solution treatment, supersaturated solubility is increased, and most of second phase particles are dissolved into magnesium matrix during solution process, that is, there is a greater driving force for precipitation of second phase. Mechanical properties of alloy are optimal at 335 ℃×4 h+190 ℃×5 h, its tensile strength, yield strength and elongation are 243 MPa, 167 MPa and 5.1% respectively. Immersion test shows that alloy treated with T6 has the best immersion corrosion resistance. Electrochemical test results show that T6 treated alloy has the smallest corrosion current density and the largest high-frequency impedance arc radius, indicating that it has the best corrosion resistance.
With increasing severity of global resource crisis and environmental pollution, lightweighting of structural materials and reducing energy consumption in industrial production applications have become key issues that need to be addressed. With advancement of production technology and reduction of costs, magnesium alloys have shown significant application advantages and broad application prospects as structural materials in important fields such as aerospace, national defense and military, rail transportation, electronic communications, and biomedicine in recent years, are known as "21st century green engineering materials". However, strength of magnesium alloys is not as good as that of aluminum alloys, and die castings need to be heat treated to meet performance requirements.
Among many methods to improve material properties, heat treatment is a widely used one. Annealing and solution aging are the two most commonly used categories in magnesium alloy heat treatment processes. Solution treatment is to place alloy at a constant temperature in its high-temperature single-phase region for a period of time. In a high-temperature environment, alloying elements will be more soluble in matrix, so that a large amount of second phase in organization will be dissolved into matrix, then a supersaturated solid solution will be formed by rapid cooling. After alloying elements are dissolved into matrix, matrix will be lattice distorted, thereby generating a large internal stress in organization, hindering movement of dislocations, and strengthening matrix, which is manifested as an increase in strength of alloy. Aging treatment refers to process in which sample is placed in a container at a certain temperature after solution treatment, kept warm for a certain period of time, then gradually restored to room temperature, so that supersaturated solid solution after solution treatment is decomposed, and second phase is precipitated in this process, so that alloy is precipitated and strengthened.
This paper adopts self-designed magnesium alloy composition and prepares magnesium alloy castings by die-casting forming process. It mainly studies influence of heat treatment process on magnesium alloy organization and performance, so as to improve performance of magnesium alloy and meet performance requirements of modern industry for magnesium alloy materials.

Test material is Mg-Zn-Al-Mn alloy, which is prepared by melting pure magnesium ingot (99.8%, mass fraction, same below), pure zinc ingot (99.8%), pure aluminum ingot (99.8%) and manganese powder. First, cut and weighed pure Zn ingot and pure Al ingot are placed in a preheating furnace and kept warm for 0.5 h to fully remove remaining moisture. Secondly, matrix magnesium is melted and added to furnace to melt pure magnesium liquid. Preheated pure Mg ingot is placed in a pit-type resistance crucible furnace and completely melted under protective gas SF6 and N2, and melt temperature is controlled at 700~720 ℃. After pure magnesium is completely melted, add pure zinc (1.6 kg), pure Al (160 g), and manganese powder (100 g) (the overall melting is about 20 kg). Preheated pure Zn ingots and pure Al ingots are added to melt one by one, kept warm for 25-35 minutes until all pure Zn ingots and Al ingots are melted; slag is removed once for each alloy melted. Then temperature of molten metal is raised to 750℃, and a special refining agent for magnesium alloy is added for refining. Amount of refining agent added is about 0.5% of the total weight of melt. Refining spoon is immersed in 2/3 of molten metal, and molten metal is stirred from top to bottom in an orderly manner for 10-15 minutes, then left to stand for 0.5 hours, and a small amount of slag is left to settle to the bottom. Then, part of magnesium alloy melt on the surface is scooped out and poured into prepared stamping container. Finally, a release agent is sprayed on stamping die to facilitate demolding of sample after stamping. Actual alloy composition is determined by SPECTROMAX photoelectric direct reading spectrometer, as shown in Table 1.

Table 1 Experimental alloy composition wB/%
Die casting test used a 280 t LK IMPRESS-III vertical die casting machine. Specific pressure selected in test was 200 MPa, extrusion speed was 10 mm/s, holding time was 30 s, and mold temperature was 350 ℃. After test, HD-B615A-S computer servo double-column tensile testing machine was used for tensile testing. Size of tensile specimen is shown in Figure 1. Metallographic specimen was cut from tail of tensile test bar for about 20 mm. After grinding and polishing, it was corroded with 5% nitric acid alcohol. Sample structure was observed by OLYMPUS GX-51 optical microscope, second phase morphology, solute element distribution and other information of die casting structure were analyzed by Verios G4 UC scanning electron microscope. Phase composition of alloy was analyzed by XRD. Surface of sample was polished to a smooth surface, and conductivity test was carried out using a FIRST FD-102 eddy current conductivity meter. RST5200F electrochemical workstation was used for open circuit test, impedance test and polarization curve test.

 

Figure 1 Dimensions of tensile specimen

Table 2 shows effect of direct aging treatment at different times on mechanical properties of Mg-8Zn-0.8Al-0.5Mn alloy. It can be seen from test results that after direct aging at 190 ℃, yield strength and tensile strength of alloy have been significantly improved. When aging time is 5 h, mechanical properties reach the peak. Its tensile strength, yield strength and elongation are 236 MPa, 149 MPa and 7.3%, respectively. Compared with cast Mg-8Zn-0.8Al-0.5Mn alloy without direct aging treatment, its tensile strength and yield strength are increased by 12 MPa and 30 MPa, respectively, and its elongation is reduced by 1.1%. Table 3 shows effect of direct aging at different temperatures on mechanical properties of Mg-8Zn-0.8Al-0.5Mn alloy. Test results show that after direct aging at different temperatures, there is no obvious difference in yield strength and tensile strength of alloy. After comprehensive comparison, 190 ℃×5 h was selected as optimal aging process.

Table 2 Mechanical properties of Mg-Zn-Al-Mn alloy after direct aging at different times

Table 3 Mechanical properties of Mg-Zn-Al-Mn alloy after direct aging at different holding temperatures
Figure 2 shows fracture morphology of direct aging at different aging times. A large number of tear edges and a few dimples can be seen. As aging process passes, number of dimples decreases significantly, tearing edges become finer, a small amount of cleavage platforms appear, tensile strength of alloy increases, and plasticity decreases; as can be seen from Figure 2e and f that after 8 h of direct aging treatment, more cleavage platforms appear in fracture, which is manifested as brittle fracture and plasticity of alloy decreases. As aging time increases, cleavage platform of fracture gradually becomes smooth and flat. During tensile deformation process, cracks are mostly generated along grain boundaries, mainly intergranular fracture.

 

Figure 2 Fracture morphology of Mg-Zn-Al-Mn alloy at 190 ℃ with different aging times

Table 4 shows effect of T6 treatment at 315 ℃ with different solution times on mechanical properties of Mg-8Zn-0.8Al-0.5Mn alloy. It can be seen from test results that after T6 treatment, yield strength and tensile strength of alloy are significantly improved compared with cast alloy, but there is no change compared with alloy performance after direct aging. Therefore, solution temperature was increased by 20 ℃ and a set of T6 treatment was performed. According to test results (Table 5), after 335 ℃-T6 treatment, yield strength and tensile strength of alloy were significantly improved. When solution time was 4 h, mechanical properties reached peak. Its tensile strength, yield strength and elongation are 263 MPa, 167 MPa and 5.1% respectively. Compared with cast Mg-8Zn-0.8Al-0.5Mn alloy without T6 treatment, its tensile strength and yield strength are increased by 39 MPa and 48 MPa respectively, and its elongation is reduced by 3.3%. In summary, above experiments show that 335 ℃×4 h+190 ℃×5 h is the best parameter for T6 treatment process.

Table 4 Mechanical properties of Mg-Zn-Al-Mn alloy treated with T6 at 315 ℃ with different solution time

Table 5 Mechanical properties of Mg-Zn-Al-Mn alloy treated with T6 at 335 ℃ with different solution time
Figure 3 shows fracture morphology of Mg-8Zn-0.8Al-0.5Mn alloy under different heat treatment processes. After solution treatment, morphology and quantity of coarse second phase around grain boundary will be greatly improved, and there will be more fine dimples in fracture, showing certain tearing characteristics, strength and plasticity of alloy will be improved. After solution treatment, aging treatment is performed. After aging treatment, second phase dissolved into matrix during solution treatment will re-precipitate on grain boundary and gradually spread to inside of grain, which is easy to produce stress concentration, resulting in a decrease in plasticity of alloy. Compared with fracture of alloy treated at 315 ℃×4 h+190 ℃×5 h, fracture cleavage platform and tearing edge of alloy treated at 335 ℃×4 h+190 ℃×5 h are increased, there are longer tearing edges in fracture, and number of dimples in fracture is reduced; compared with alloy treated at 315 ℃×4 h+190 ℃×5 h, tensile strength and yield strength of alloy treated at 335 ℃×4 h+190 ℃×5 h are improved, while plasticity is reduced.

 

Figure 3 Fracture morphology of T6 treatment of Mg-Zn-Al-Mn alloy at 335 ℃ with different solution treatment time

Figure 4 shows structure morphology of Mg-Zn-Al-Mn alloy with different solution treatments. It can be seen that compared with solution treatment at 315 ℃×4 h, second phase in Mg-Zn-Al-Mn alloy after solution treatment at 335 ℃×4 h is dissolved in large quantities. Figures 4a and b are scanning electron microscope microstructures of cast Mg-Zn-Al-Mn magnesium alloy. Mg7Zn3 phase is concentrated around grain boundary in the form of coarse strips and blocks. After solution treatment, Mg7Zn3 phase around grain boundary is greatly reduced, remaining small amount of Mg7Zn3 phase exists in the form of dots and thin strips, indicating that after solution treatment, morphology and quantity of Mg7Zn3 phase in alloy are significantly improved. During tensile process of alloy, coarse alloy phase at grain boundary becomes starting point of internal cracks of sample, resulting in low tensile strength and elongation of alloy. After solution treatment, dissolution of Mg7Zn3 phase at grain boundary reduces stress concentration and cutting effect at grain boundary, improves performance of alloy. In addition, there are many defects in magnesium alloys, such as dislocations. During solution treatment of alloy, internal solute atoms are more active and easy to diffuse, and will interact with defects such as dislocations, so that impurities in alloy are segregated near dislocations, thereby forming "Coriolis gas clusters". When alloy is subjected to tensile deformation, "Coriolis gas clusters" have a resistance effect on movement of dislocations inside alloy, play a role in solid solution strengthening, and thus improve strength of alloy. Composition of alloy is mainly Mg, Al and Zn elements, and contains a small amount of Mn elements. It can be seen from Table 6 that after solution treatment at 335 ℃×4 h, content of Al and Zn elements in matrix increases, Al and Zn contents increase from 0.65% and 2.36% to 1.5% and 11.71%, respectively. From element content analysis, it can be seen that content of Zn element in matrix is significantly increased, indicating that Mg7Zn3 phase is dissolved into matrix.

 

Figure 4 Microstructure morphology of Mg-Zn-Al-Mn alloy with different solid solution treatments

Table 6 Analysis of element content in matrix of Mg-Zn-Al-Mn alloy at%

Figure 5 is a macroscopic morphology of surface corrosion of cast alloy, which was immersed in 3.5% NaCl corrosion solution for 1d, 3d, 7d and 14d after direct aging treatment and T6 treatment. From macroscopic morphology, it was found that alloy sample showed obvious pitting pits, and corrosion degree of alloy increased with extension of time. Comparing macroscopic photos of cast alloy and aged alloy samples after corrosion, it was found that corrosion degree of alloy after T6 treatment was relatively light, whether it was 1d, 3d, 7d or 14d, that is, corrosion resistance of alloy after T6 treatment was better.

 

Figure 5 Surface macroscopic morphology of immersion corrosion of Mg-Zn-Al-Mn alloy under different heat treatments
Table 7 is polarization curve and EIS electrochemical impedance tested by open circuit potential, self-corrosion potential (Ecorr) and self-corrosion current density (Icorr) of alloy are fitted according to Tafel law. It can be seen from Figure 6 and Table 7 that after T6 treatment, self-corrosion current density decreases, and alloy after T6 treatment has the best corrosion resistance, with a self-corrosion potential of -1.604 V and a self-corrosion current density of . Therefore, alloy after T6 treatment has the best corrosion resistance, which is consistent with results of above immersion test.

Table 7 Electrochemical parameters of polarization curves of Mg-Zn-Al-Mn alloy under different heat treatments

 

Figure 6 Polarization curves of Mg-Zn-Al-Mn alloy after different heat treatments
Figure 7 is Nyquist diagram of Mg-Zn-Al-Mn alloy under different heat treatments. High-frequency capacitive reactance arc and low-frequency inductive reactance arc constitute impedance spectrum. Radius of high-frequency capacitive reactance arc is usually used to indicate strength of alloy's corrosion resistance. The larger arc radius, the greater impedance value and the better corrosion resistance of alloy. As shown in Figure 7, radius of high-frequency capacitive reactance arc is from large to small: T6 treatment > direct aging treatment > cast state. Alloy after T6 treatment shows stronger corrosion resistance than cast state and aged state, which is consistent with test results of immersion corrosion.

 

Figure 7 Nyquist diagram of Mg-Zn-Al-Mn alloy after different heat treatments

(1) Tensile strength of cast alloy is 224 MPa, yield strength is 119 MPa, and elongation is 8.4%. After direct aging treatment, 190 ℃×5 h is selected as the best aging process. After direct aging, its tensile strength can reach 236 MPa, yield strength can reach 149 MPa, and elongation is reduced to 7.3%.
(2) After T6 treatment, 335 ℃×4 h+190 ℃×5 h was selected as the best T6 treatment process, its tensile strength increased to 243 MPa, yield strength increased to 167 MPa, and elongation decreased to 5.1%. After solution treatment, supersaturated solid solubility increased, and most of second phase particles dissolved into magnesium matrix during solution process, that is, there was a greater precipitation driving force. After alloy elements were dissolved into matrix, matrix would be lattice distorted, thereby generating a large internal stress in structure, hindering movement of dislocations, and strengthening matrix, which was manifested as an increase in strength of alloy and a decrease in plasticity.
(3) After immersion corrosion, corrosion resistance of alloy after T6 treatment was significantly stronger than that of alloy without heat treatment and alloy directly aged. Electrochemical tests showed that alloy after T6 treatment had the best corrosion resistance, with a self-corrosion potential of -1.604 V and a self-corrosion current density of , which was consistent with results of above immersion test.