Homogenization heat treatment of magnesium alloy

Homogenization heat treatment and thermal conductivity of magnesium alloy

The microstructure, composition and hardness of the new mg-4zn-1mn (zm41) magnesium alloy were studied by means of optical microscope, X-ray diffraction, Brinell hardness test, SEM and EDS. The thermal diffusivity in different states was measured by laser flash method, and the thermal conductivity was calculated. Based on the mechanism of vacancy diffusion, the kinetics of homogenization diffusion was studied and the homogenization diffusion equation of the alloy was established. The results show that the dendrite segregation of as cast structure is serious, there are many coarse Mg7Zn3 non-equilibrium crystal phases on the grain boundary, and Mn exists in the alloy as a single substance. After homogenization heat treatment at 370 ℃× 12 h, most of the Mg7Zn3 phase has dissolved into the matrix. According to the experimental results and homogenization kinetics calculation, the optimal homogenization process is 370 ℃× 12 h. The thermal conductivity of the alloy at room temperature is 125.5 w / (m.k), which is about 1 times higher than that of common magnesium alloys such as AZ system, am system and as system.

In the application of Aeronautics and Astronautics, the thermal properties of magnesium alloy are very important, and thermal conductivity is one of the important properties of thermal properties. However, compared with mechanical properties, there are few reports on thermal properties of magnesium alloy. According to the literature, the thermal conductivity of pure magnesium is 155 w / (m · K), AM50A is 65 w / (m · K), as41a is 68 w / (m · K), AZ91 is 58.6 w / (m · K), and mg-2zn-2y alloy is 53 w / (m · k). In this paper, the room temperature thermal conductivity of mg-4zn-1mn magnesium alloy will be measured to prepare for the follow-up study. Homogenization heat treatment is to dissolve and diffuse the undissolved phase at high temperature, so as to eliminate the eutectic structure of low melting point, improve the dendrite segregation, improve the plastic deformation ability of the alloy, and improve the solid solution degree of alloy elements in the matrix, so as to prepare the structure for precipitation and precipitation in the process of thermal deformation and aging treatment. The dissolution of Mg7Zn3 phase is the key factor to control the microstructure and properties of the alloy. In other alloys [9], the calculation of diffusion kinetic equation can provide theoretical reference for the formulation of homogenization heat treatment parameters, but the data of this alloy are few, and the kinetic model of this alloy has not been reported.

In this paper, the homogenization heat treatment of mg-4zn-1mn magnesium alloy is studied with the goal of optimizing the homogenization heat treatment. On the basis of vacancy diffusion mechanism, the diffusion process of Zn element in mg-4zn-1mn alloy is studied, and the temperature time relation diagram of homogenization process is drawn according to diffusion kinetic equation, which provides theoretical basis for homogenization treatment of mg-4zn-1mn alloy.

Experimental materials and methods

Alloy preparation

The experimental alloy is mg-4zn-1mn magnesium alloy, which is melted in a 5 kW well furnace at 680-720 ℃, in which Mn is added as Mg Mn master alloy and Zn is added as pure metal. The actual composition content is: Z4. 06%, Mn1. 08%, the rest is Mg and trace impurities. The alloy ingot was cut into 12 mm × 12 mm × 10 mm samples along the concentric circle of ingot by wire cutting. The heat treatment and microstructure analysis were carried out.

Experimental method

Netzsch sta 409 C / CD thermal analyzer is used to determine the differential scanning thermal analysis curve of as cast alloy, as shown in Figure 1, and the heating rate is 10 ℃ / min. It can be found that the low melting point transformation range of zm41 magnesium alloy is 337.5-362 ℃, which is accompanied by the decomposition and formation of intermetallic compounds. According to the empirical formula of homogenization annealing, TC = 0.9-0.95tm, the homogenization temperature is 310, 330, 350 and 370 ℃, and the treatment time is 4, 8, 12, 16, 20 and 24 hours respectively. The homogenized samples were aged at 150 ℃× 10 h. The microstructure was observed by Carl Zeiss axiovet 2000 mat metallographic microscope and zeissevo18 scanning electron microscope. The phase identification and composition analysis were carried out by means of xpert Pro polycrystalline X-ray diffractometer and energy spectrum analysis. The hardness test was carried out on the hbs-62.5 small load Brinell hardness tester. The loading time was 30 s, and the average value was obtained by repeating 4 times.

The thermal diffusivity of zm41 alloy was measured by netzschlfa 447 laser flash method at room temperature. Before measurement, the alloy surface shall be blackened with carbon coating to improve the absorption of light pulse, and the test shall be conducted according to ASTM E1461-01 standard. The density of the sample was measured by drainage method. The thermal conductivity is obtained by the following formula of thermal diffusion and specific heat capacity: k = α ρ CP. Where α is the thermal diffusion coefficient, ρ is the density and CP is the specific heat capacity at constant pressure. The resistivity of the as cast alloy was measured at room temperature, heated to 100, 150, 200, 250, 300 and 350 ℃ respectively, and the resistivity at room temperature (t = 20 ℃) was measured after quenching after holding for 2 hours. The enthalpy of vacancy formation of zm41 magnesium alloy was determined according to the difference of resistivity, which provided experimental data for the calculation of diffusion activation energy.