Plastic processing of high strength rare earth magnesium alloy
Entering the 21st century, the industry has an unprecedented demand for lightweight, environmentally low-load materials. Magnesium alloys, which are 30% lighter than aluminum and are harder and stronger per unit weight, have great potential in applications that focus on weight, performance and efficiency. And magnesium is the eighth-most abundant element in the Earth's crust. Magnesium alloy has appropriate mechanical strength, the specific strength is higher than steel and aluminum alloy, only lower than titanium. Specific rigidity is higher than steel, aluminum alloy and titanium. Magnesium alloys will play an increasingly important role in the growing demand for lightweight, environmentally low-load materials. However, compared with iron and steel aluminum alloy, the practical achievements of magnesium alloy are much less. This is due to the poor ductility of magnesium alloy, plastic processing is difficult. Due to the inherent defects of casting process, it is difficult to make high reliability parts.
At present, hot forging is a powerful way to produce high strength and light weight magnesium alloys. Magnesium, however, is highly chemically active at high temperatures and tends to stick to the surface of the mold. Once the magnesium sticks to the die, the forming pressure will increase rapidly, which makes it more difficult to fill the fine shape part of the die, make it difficult to produce precise forging products and form a beautiful forging surface. Therefore, in the hot forging process, the development of wrought magnesium alloy, the setting of forging temperature and the choice of lubricant should be solved first. The adhesion of magnesium alloy to the mold is reduced. Secondly, the forging process is designed through the analysis of material flow, so that the adhesion of lubricant is reasonable, and the possibility of the adhesion of magnesium alloy to the mold will be lower. Finally, a safe forging die is designed by analyzing the stress of the die.
Development of forged magnesium alloy materials
In order to obtain high strength forged products, the mechanical properties and forging process of the forged products should be considered comprehensively. Forging technology of metal materials is mainly embodied in the plasticity and deformation resistance of materials. The plasticity and deformation resistance of materials are related to the chemical composition and microstructure of materials. In addition, it is also related to forging temperature, deformation speed, lubricant and other processing conditions. ZK60A is a typical wrought magnesium alloy, but it has low strength and poor plasticity. It is easy to crack and stick in forging. To solve the above problems, Allite added specific rare earth elements on the basis of the traditional ZK60A wrought magnesium alloy, and the forged magnesium alloy developed was ZE62 (Table 1). The addition of rare earth elements can enhance the solution, fine grain, dispersion and aging precipitation of ZE62 wrought magnesium alloy matrix. The rare earth elements prevent the grain growth during ingot solidification and forging and recrystallization, so that the microstructure of ZE62 material is fine and uniform, the ductility of the material is improved, and the strength and plasticity of the forged magnesium alloy are improved. Meanwhile, the rare earth elements can remove harmful impurities such as Fe and Ni in the magnesium alloy, and improve the purity and compactness of the material. The strong cathodic action of Fe, Ni and other harmful impurities is reduced, so that ZE62 has a good corrosion resistance. Compared with the tensile strength, yield strength and elongation of casting ZE62 magnesium alloy of 270MPa, 175MPa and 8%, the tensile strength and yield strength of extruded forged ZE62 magnesium alloy of 350MPa and 303MPa, the elongation reaches 21%, and the mechanical properties of extruded magnesium alloy are better. The most important effect of addition of rare earth elements is to improve the forging property of magnesium alloy. The surface quality of forged products is improved by improving the material fluidity. Because of the rare earth elements are surface active elements, with the oxygen in the melt easily in the process of smelting, hydrogen, sulfur and other impurities reaction, formation of rare earth oxides or rare earth hydrides, in addition to the oxide inclusions or hydrogen, reduce pores, pinhole, casting defects such as shrinkage, improved material purity, reduced the incidence of forging forming process of crack source. Rare earth elements form rare earth compounds with high melting point with Mg, Zn, Zr and other elements in matrix magnesium alloy. These rare earth compounds exist in grain boundaries and grain interior and increase grain boundary strength. The ZE62 rare earth magnesium alloy is not easy to crack during forging. More importantly, the rare earth compound can form a kind of dense high temperature resistant rare earth oxide film on the surface of the material. The adhesion resistance and forgability of ZE62 rare earth magnesium alloy are enhanced in the forging process.
Composition of wrought magnesium alloy ZE62 (%wt)
Development of forging technology
In our opinion, the reasonable forging method of magnesium alloy is to control grain size through large deformation of forging to produce forged products with high precision and high strength (as shown in Figure 1). The key to control grain size is to control forging temperature. High temperature is easy to cause material and mold adhesion. However, the process of recrystallization and grain refinement cannot be carried out smoothly at too low forging temperature, and high strength forging products are out of the question. Secondly, the strain rate also has a great influence on the grain size of forged products. The excessively high strain rate increases the temperature of the material, increases the dynamic crystallization rate, and affects the grain size. However, the high strain rate increases the forging pressure, which is easy to cause the adhesion between the magnesium alloy and the die. Too low strain rate means reduced production efficiency. Therefore, to find the appropriate forging speed in future production is an important issue besides forging temperature.
Fig. 1 Relationship between yield strength and grain size of AZ91 and A5083 *
In addition, forming analysis plays a very important role in the development of magnesium alloy forging. Through the analysis of the results of forging analysis, the forging steps can be reasonably arranged, which can not only reasonably distribute the forming pressure, but also make the forging products filling and filling. More importantly, through the analysis of material flow, the retention degree of lubricant is analyzed to avoid the adhesion of magnesium alloy to the mold. Allite developed a unique forging method for the ZE62 based on its unique characteristics. This includes the selection of lubricants and temperature setting, forming analysis, mold stress analysis of a series of process parameters. Through a large number of tests, the forging temperature is set at more than 340℃. Choose lubricants containing graphite. By analyzing the flow of materials, the forging procedure is designed to reduce the possibility of adhesion between magnesium alloy and die. After analyzing the stress of the mould in the forming process, the mould was designed reasonably.
The following is an example of forging of ZE62 rare earth magnesium alloy.
Magnesium alloy forging cases
To sum up the above, in order to make magnesium alloy forging products with good precision and high strength, the speed and pressure of forging press are controllable. Therefore, based on the controllable speed and pressure of the hydraulic forging press, the 1000T hydraulic forging press used for development this time (as shown in Figure 2) is suitable for magnesium alloy forging press. The maximum die closing pressure is 10000KN, and the working speed is 5 ~ 25mm/s.
Fig. 2 1000t hydraulic forging machine
Figure 3 shows the magnesium alloy products developed this time. After cutting, the product weighs about 0.75kg, and before forging, the raw material weighs about 2.5kg. Considering the forging formability and cutting allowance, the forging shape was set (see Fig. 4).
Figure 3 ZE62 magnesium alloy forging products
Fig. 4 Forge sample shape
In order to allocate the forming pressure and filling ability reasonably, we repeated the analytical calculation of forming. And through the analysis of material flow, the retention degree of lubricant is estimated, so as to avoid the mold sticking as far as possible. The final forging process (see Fig. 5).
Fig. 5 Forging procedure of ZE62 magnesium alloy
The forging process includes; Profile cutting preparation forming final forming a total of three steps.
Through a series of forging tests, we set the forging temperature above 340℃ to ensure the material fluidity and improve the surface quality of forging products. In terms of the choice of lubricants, considering that graphite has good high temperature lubrication, lubricants containing graphite were selected. According to the above forging process and forging conditions, forging was performed, and forging results close to the analytical results were obtained (see Fig. 6). The forging precision was measured. After machining, a sample was produced (see Fig. 7)
Fig. 6 Actual forged ZE62 magnesium alloy sample
Fig. 7 sample processed by magnesium alloy forging machine
By sampling and analyzing the forging structure, the tensile strength, yield strength and elongation of the forged ZE62 magnesium alloy are 310MPa, 250MPa and 12%. The tensile strength, yield strength and elongation of wrought ZE62 magnesium alloy are increased by 14.8%, 42% and 50% compared with those of wrought ZE62 magnesium alloy in casting state, respectively, by 270MPa, 175MPa and 8%. By analyzing the forging structure (see 8), it is effectively verified that the forging products of ZE62 rare earth magnesium alloy have good fluidity. Moreover, it can refine the grain structure of forged products by forging large deformation, and has strong anti-adhesion force and high forgability. Good for forming high quality forging surface, suitable for making high precision, high strength magnesium alloy products.
CONCLUSIONS: The addition of rare earth elements to ZE62 rare earth magnesium alloy can improve the material fluidity, improve the forging property of magnesium alloy, and has strong adhesion resistance and good forgability during forging. Ensure the good surface quality of forging products. The large deformation of forging refines the grain structure. It is good for forging high precision and high strength products. The hydraulic forging press with controllable speed and pressure is suitable for magnesium alloy forging.
In order to promote the practical application of magnesium alloy forging, we have made some attempts in material and forging technology. In terms of materials, ZK60A wrought magnesium alloy was developed by adding specific rare earth elements. In the forging process, the results of forging analysis were fully utilized to reasonably allocate the forming pressure and estimate the retention degree of lubricant, so as to avoid the adhesion of magnesium alloy to the die. Plan a reasonable forging process. In the end, precision, strength and combined metal structure are achieved in the forged product. In the future, Allite's R&D and engineering teams will build on this foundation and continue to conduct new research and development to enable magnesium alloy forging to be applied in a wider range of fields. Zn Zr Re Mg
5.5 ~ 6.5 0.3 ~ 0.6 0.5 ~ 2.0 Surplus