Semi solid forming technology of magnesium alloy

1. Brief introduction of magnesium alloy

Magnesium is a light metal silver, the density is only 1.74g/cm3, less than 2/3 of the aluminum magnesium alloy, and the strength is also very high, as early as before the second world war with its structure (such as in a variety of aircraft structure) on the applicability, and has a high quality the lightest metal structural material. Magnesium is abundant in nature and is second only to aluminum and iron in the earth's crust. It is the third most abundant structural metal. Magnesium is mainly extracted from magnesite, dolomite, Saline Lake brine and sea water at present. The total amount of magnesite resources in China is 53 tons, and dolomite has proved reserves of more than 40 tons. Saline Lake in Qinghai has 6 billion 730 million M3 of intercrystalline brine, and it is also an important magnesium refining resource in china. The reserves of magnesium resources in China are the first in the world.

The primary use of magnesium metal is to be used as alloying elements; followed by the manufacture of cast and wrought magnesium alloys suitable for structural parts; and finally for steel desulphurization. The melting Aluminum Alloy accounted for nearly half of the country's global magnesium content. As a structural member, magnesium alloys have not been widely used in the past, except for aerospace applications, because of cost reasons. In recent years, in order to solve the problem of reducing energy consumption and environmental protection, the automotive industry has turned its attention to the lightweight magnesium alloys. Magnesium alloy can reduce weight and reduce fuel consumption, high specific strength and stiffness ratio close to Aluminum Alloy and steel, stability, good casting properties and size of easy processing, low reject rate, impact resistance, good damping property, in reducing noise, reduce vibration and safety and comfort is better than that of aluminum and iron.

Driven by the automotive industry, the die casting magnesium alloy has been expanding in the past ten years with an average annual growth rate of 15%. In addition, the mobile phone shell, laptop computers, advanced audio-visual equipment and digital cameras in the shell has been widely used magnesium alloy parts and components, and give full play to the quality of magnesium alloy light, durable, vibration reduction, shielding, non-magnetic and other functions. At present, the field and application of magnesium alloy production and application continue to expand, and there is a big trend in the ascendant.

2 、 rheo forming technology of magnesium alloy

Rheological technology began in the 1970s and was first proposed by the research team led by Professor Flemings of MIT in the United states. The basic principle of rheological forming is to stir the melt during the solidification of the melt, thus changing the nucleation and growth law in solidification. The stirring melt is introduced into the forming equipment, and the microstructure of the casting obtained is a small uniform equiaxed crystal, rather than a coarse dendrite made by conventional casting. There are many kinds of forming methods suitable for rheological forming, 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. Conventional liquid die castings often have porosity, resulting in low rate of finished products, parts can not be improved by heat treatment. Rheo diecasting can avoid the formation of porosity in castings, and the performance of castings is improved, and the production efficiency is improved remarkably.

3 、 double helix rheological forming technology

Double helix rheological pulping technology invented by Brunel University of England, Z.Fan and others, and has applied for many international patents. Beijing non ferrous Metal Research Institute and inventor cooperate with each other, and are committed to the two development and application of this technology. Double helical rheological pulping equipment uses a pair of high-speed rotating screw to mix the melt with high shear rate. After stirring, the melt is directly subjected to die casting, extrusion and rolling. Twin screw stirring technology is suitable for many different grades of magnesium and aluminum alloy. It has the ability to deal with the melt continuously. It is the most successful melt forming technology.

4,Microstructure and properties of semi solid thixotropic forming magnesium alloy

Microstructure and properties of semi solid Thixomolded magnesium alloy AZ91D was analyzed, the results show that the microstructure and mechanical properties of the forming method of the production of magnesium alloy products are better than that of die casting products, and magnesium alloy auto parts for the application of semi solid thixo molding method of the production basis.

5. Mechanical properties of magnesium alloy by thixotropic injection molding

With the green environmental protection requirements such as the improvement of magnesium alloy, with its light weight, ratio of high strength, high specific stiffness, good damping performance, resistance to electromagnetic shielding, easy recovery from a number of metal materials in talent shows itself widely used in aviation, aerospace, electronics and automobile industry, etc.. At present, the two hot spots in the application of magnesium alloys are electronics and automotive industry. On the one hand, for "3C" (Computer, Communication, Consumption Electronics Products) products have gradually replaced the plastic shell, shell Recyclable poor trend; on the other hand, as the practical application of the light metal structure, magnesium alloy can meet the transportation industry increasingly stringent energy-saving and emission requirements. In order to produce light weight, less fuel consumption, a new generation of environmentally friendly vehicles.

The forming method of magnesium alloy widely used at home and abroad is die casting method. Die casting magnesium alloy products have the advantages of good dimensional stability and high productivity, but they also have many defects, such as inclusion, porosity, heat treatment after forming and near net shape difference. Parts produced by die casting are difficult to meet the requirements for thin-walled shell parts such as those used in 3C products as well as high performance magnesium alloy parts used in the automotive industry.

Compared with die casting method, manufacturing method of semi solid products with less casting defects, mechanical properties, dimensional accuracy, surface and the inherent advantages of high quality products, in addition to energy saving, good safety, the advantages of near net forming. At present, the semi solid forming technology of magnesium alloy, which has been successfully industrialized in the world, is thixotropic injection molding technology [1]. Changchun China Yumei Industry Co., Ltd is China's first introduction of this technology in the preparation of test specimens of manufacturers, the mechanism of Thixomolded the company, microstructure and mechanical properties of Thixomolded magnesium alloy were analyzed, so as to carry out the research and development of high performance magnesium alloys for automobile technical reserves appropriate for the company step.

6. The principle and process of semi-solid injection molding technology.

6.1: the principle of semi-solid injection molding technology
In the ordinary casting process, primary dendrite grows up, when the solid fraction reaches about 0.2, the dendrite formed continuous network framework, macro liquidity lost. Semi solid forming is in liquid metal from liquid to solid phase during the cooling process of strong mixing, the common casting dendritic network forming the skeleton to be broken and easy to form granular tissue morphology retained, suspended in the remaining liquid. The granular non dendritic microstructure has a certain rheological property when the solid phase rate reaches 0.5 to 0.6, so that metal forming can be realized by conventional forming processes, such as die casting, extrusion and die forging, [2 to 4]. Semi-solid semi-solid injection molding is a new technology developed in recent years. It originated from DOW chemical company of the United States and is commercialized by American THIXOMAT company. This is the process of the plastic injection molding principle and forming process of semi-solid metal combination in process preparation, transportation, forming of semi-solid metal slurry in one, this method solves the preservation of semi-solid metal slurry conveying, forming control problems.

6.2: the process of semi-solid injection molding technology
Injection molding process of magnesium alloy is made as follows: raw material particles (made from dendritic magnesium alloy ingot is still the organization dendrite) is added from the hopper; in the sleeve of the magnesium alloy material by electric heating into a semi solid state in the screw shear were used, in the sleeve semi solid metal forming slurry solid particles near the ball shape, in the injection cylinder under the action of the equivalent of ten times the rate of plastic injection molding machine injection to mold forming.