Research Of Al-Ti-B Refined Mg-20Al-0.8Zn Magnesium Alloy And Semi-solid Thixocasting Research

Magnesium alloy with light weight, high strength and stiffness than high, while at the same time has good thermal conductivity, good electromagnetic shielding, damping properties and can be recovered using characteristics, which Satisfy the automotive industry that meet the weight loss, energy saving, environmental protection and communications Electronics highly integrated device miniaturization and light requirements. Therefore, regarding the development of magnesium alloy and its forming process will be the practical application of research must have a guiding significance.This article selects the Hypoeutectic magnesium alloy that aluminum-containing respectively 15% and 20%. This composition alloy satisfies the characteristics of semi-solid processing.In this paper, the alloy's preparation, refining, semi-solid slurry (blank) pre paration and thixotropic forming process and die-casting heat treatment were re searched to carry out a systematic study on the outcome of the further applicat ion of magnesium alloy has certain significance.Experiments using the principle of Heterogeneous Nucleation study the refinement of as-cast magnesium alloy, the results prove that: Al-Ti-B alloy at the high temperature can be formed phase TiB₂ and AlB₂, these particles are close-packed hexagonal structure, their lattice structure are the same with as theα-Mg and with small degree of lattice mismatch, interface with a total of Georgian relations, can be used as alloy solidification process at the non-homogeneous nucleation core. Heterogeneous nucleation played the great role of grain refinement.Isothermal treatment process and evolution of organization in the isothermal treatment process of those magnesium alloys which after refinement of semi-solid was studied, the alloy billets of different diameters at isothermal treatment in the process of warming curve and holding time's relationship with the organization were studied. The results showed that: the middle alloy Al-Ti-B accelerates the rate of Isothermal treatment, while the growth rate compared with the before alloying was much slower.In the different diameter billets' isothermal treatment, when the center temperature reaches set temperature, and then insulations 20min, organization achieve the best effect. Isothermal treatment process is in the solid-liquid equilibrium. Low melting point eutectic structure (α+Mg₁₇Al₁₂) first melt and then form induced liquid-phase, which urge solid changes to spherical.Through isothermal treatment, we received suitable semi-solid billet for thixotropic forming, selection of the three groups better isothermal treatment process, and then studied the relationship between isothermal treatment process and die casting process parameters with the mechanical properties. By orthogonal experimental program, we first fixed casting speed of 3m/s, investigated isothermal treatment process on the mechanical properties of alloys and obtained optimum techniques, then studied the casting speed on the mechanical properties of alloys. The final results as follows: when the isothermal treatment process for the 460℃, 160min, casting speed of 2m/s, the comprehensive mechanical properties of die castings higher, at: tensile strength of 190Mpa, an extension rate of 4.5%. Experiments show that: When the semi-solid slurry solid phase is high, the viscosity increased, mobility decreased, which is not conducive to feeding, which may lead to insufficient water, cold traps, the defects such as warm; At the same time, because of viscosity, gas emissions speed in liquid metal reduced, which will increase the number of shrinkage, therefore, poor mechanical properties of alloys. When the liquid is high, the alloy's mobility increased, but because of the liquid alloy has a higher rate, making the process of alloy solidification contraction becomes more apparent. This may result in more shrinkage of the alloy is also detrimental to performance.The solubility of aluminum alloy with the temperature change more obvious, therefore, better mechanical properties of the sample had after heat treatment. After heat treatment, the alloy's tensile strength improves from 190Mpa to 205Mpa, elongation from 3.5% to 5.5%. At 400℃after the solid solution hardening, quenching obtained supersaturated solid solution in a state of imbalance, which has happened decomposition and precipitation of excess solute spontaneous trend. However, because of lower room temperatures, the general had completed only the initial stage of precipitation (Was the second-phase precipitates form); at 180℃insulation 8 hours, resulting in a discontinuous precipitation and continuous precipitation. Dispersed precipitates play a strengthening effect.