Magnesium alloy semi-solid forming technology
1. Introduction of magnesium alloy
      Magnesium is a silvery white light metal, with a density of only 1.74g/cm3, less than 2/3 of that of aluminum. Magnesium alloys have high strength, and they were used in various structures (such as aircraft structure) as early as World War II. Applicability on the part) and become the lightest high-quality metal structural material. Magnesium is abundant in nature and it is the third most abundant structural metal, second only to aluminum and iron in the earth's crust. At present, the world mainly extracts magnesium from magnesite, dolomite, salt lake brine and seawater. The total amount of magnesite resources in China is 5.3 billion tons, and the proven reserves of dolomite are more than 4 billion tons. Qinghai Salt Lake has 6.73 billion M3 of intergranular brine is also an important magnesium resource in China. The current reserves of magnesium resources in China rank first in the world. The primary use of metallic magnesium is as an alloying additive; followed by the manufacture of cast and deformed magnesium alloys suitable for structural parts; and finally for the desulfurization of steels. At present, the smelting aluminum alloy accounts for almost half of the world's magnesium consumption. Magnesium alloys are used as structural parts, but they have not been widely used in aluminum alloys due to cost reasons. In recent years, in order to solve the problem of reducing energy consumption and environmental protection, the automobile industry has turned its attention to magnesium alloys that make them lightweight. Magnesium alloy can reduce vehicle weight and reduce fuel consumption, high specific strength, specific rigidity close to aluminum alloy and steel, good casting performance and dimensional stability, easy processing, low reject rate, impact resistance, good damping performance, and reduced Noise, vibration reduction, safety and comfort are superior to aluminum and cast iron. Driven by the automotive industry, die-casting magnesium alloys have continued to expand at an annual growth rate of 15% over the past decade. In addition, magnesium alloy components have been widely used in the casings of mobile phone cases, laptops, advanced audio-visual equipments, and digital cameras, and magnesium alloys have been fully utilized for their light, durable, vibration-reducing, and shield-free functions. At present, the fields and applications of magnesium alloy production and application continue to expand, and there is great potential for growth.
2. Magnesium alloy rheoforming technology
   The rheology technique began in the 1970s and was first proposed by a team led by Professor Flemings of MIT, USA. The basic principle of rheology is to apply stirring to the melt during the solidification of the melt, thereby changing the nucleation and growth laws during solidification. The agitated melt is introduced into the forming apparatus and the resulting microstructure of the casting is a fine uniform equiaxed crystal instead of the conventionally cast coarse dendrites. A variety of molding methods are suitable for rheology, such as continuous casting, die casting, extrusion, forging, and rolling. With the rapid development of magnesium alloy die-casting industry, rheo-diecasting technology has shown a good application prospect. Porosity is often present in conventional liquid die castings, resulting in low production yields and the inability of parts to improve performance through heat treatment. Rheological die-casting can avoid the formation of pores in the castings, the performance of the castings is improved, and the production efficiency is significantly improved.
3, double spiral flow shape technology
     Double helix rheological pulping technology was invented and applied for a number of international patents by Z. Fan and others from Brunel University in the United Kingdom. The Beijing Nonferrous Metal Research Institute cooperated with the inventors and worked together to develop and promote the technology. The double helix rheological pulping apparatus applies high shear rate stirring to the melt through a pair of high speed rotating screws. The melt is directly subjected to die casting, extrusion, rolling and the like after being stirred. Double-screw stirring technology is applicable to many different brands of magnesium and aluminum alloys and has the ability to continuously process melts. It is currently the most successful rheoforming melt processing technology.
4. Analysis of Microstructure and Properties of Semi-solid Thixoforming Magnesium Alloy
     The microstructure and properties of the semi-solid thixotropic injection-molding magnesium alloy AZ91D were analyzed. The results show that the microstructure and mechanical properties of the magnesium alloy produced by this forming method are all superior to that of die-casting products, and thus the semi-solid thixoforming method is applied. To lay the foundation for the production of magnesium alloy auto parts.
5. Thixotropic injection molding Magnesium alloys Mechanical properties
  With the improvement of requirements for green and environmental protection, magnesium alloys stand out from many metal materials due to their light weight, high specific strength, high specific rigidity, good shock absorption, electromagnetic shielding, and easy recycling. In the aviation, aerospace, electronics and automotive industries. At present, two major hotspot industries for the application of magnesium alloys are the electronics industry and the automotive industry. On the one hand, the casing used for "3C" (Computer, Communication, Consumption Electronics Products) products gradually replaces the plastic housing with poor recyclability; on the other hand, it is the lightest structural metal in practical applications. Magnesium alloys can meet the increasingly stringent energy-saving and exhaust emission requirements of the transportation industry, resulting in a new generation of vehicles that are lightweight, consume less fuel, and are environmentally friendly.
The magnesium alloy forming method widely used at home and abroad is a die casting method. Die-casting magnesium alloy products have the advantages of good dimensional stability and high productivity, but also have the disadvantages of multiple inclusions, multiple pores, difficult heat treatment after forming, and poor near-net-shape forming. Parts produced by die casting are difficult to satisfy requirements such as thin-walled housing parts that are widely used in "3C" products and high-performance magnesium alloy parts used in the automotive industry.
    Compared with the die-casting method, semi-solid method products have the advantages of less casting defects, product mechanical properties, dimensional accuracy, surface and intrinsic quality, as well as the advantages of energy conservation, safety, and near-net formability. At present, the semi-solid forming technology of magnesium alloys that has been successfully industrialized in the world is thixotropic injection molding technology [1]. Changchun Huayu Magnesium Industry Co., Ltd. is the earliest manufacturer to introduce this technology in China. This article uses the company's thixotropic injection molding machine to prepare samples, and analyzes the microstructure and mechanical properties of thixotropic injection molding magnesium alloys. The next step is to conduct research and development of high-performance magnesium alloys for automobiles and make appropriate technical reserves.
6, the principle and process of semi-solid thixotropic injection molding technology.
  6.1: Principles of Semi-Solid Thixo Injection Molding Technology
In the ordinary casting process, the primary crystal grows in a dendritic manner. When the solid fraction reaches about 0.2, the dendrites form a continuous network skeleton and lose macroscopic fluidity. Semi-solid forming is performed during the cooling of the liquid metal from the liquid phase to the solid phase, so that the dendritic network skeleton that is easily formed during the ordinary casting process is broken up to retain the dispersed granular structure and suspended in the remaining liquid phase. . The granular non-dendritic microstructure still has a certain rheology at a solid phase rate of 0.5 to 0.6, allowing the use of conventional forming processes such as die-casting, extrusion, and die forging to achieve metal forming [2 ~4]. Semi-solid thixotropic injection molding is a new process developed in recent years. It originated from the United States DOW Chemical Company and was commercialized by THIXOMAT. The process integrates the plastic injection molding principle with the semi-solid metal forming process and integrates the preparation, delivery and forming of semi-solid metal slurries. This method better solves the preservation and transportation of semi-solid metal slurries. Problems such as difficulty in forming control.
  6.2: Process of semi-solid thixotropic injection molding technology
The main process of the injection molding process is as follows: The magnesium alloy raw material (made of dendritic magnesium alloy ingot, whose structure is still dendritic structure) is made into granules. It is added from the hopper; the magnesium alloy raw material in the sleeve is electrically heated. In the semi-solid state, under the shearing action of the screw, the semi-solid metal slurry in the sleeve forms nearly spherical solid particles, which is equivalent to a plastic injection molding machine at the rate of ten times that of the injection cylinder. Press into the mold to form.