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Microstructural evolution of semi-solid 7075 Al alloy manufactured by strain-induced melt activation (SIMA) process was investigated. The effects of different processing parameters, such as isothermal temperature and holding time on the semi-solid microstructures (the liquid volume fraction, average grain size, and degree of spheroidization of the solid particles) during partial remelting have been investigated on 7075 Al alloy that was extruded by an extrusion ratio of 20 before remelting. Experiments of remelting were carried out in the range of 560-610°C for 10, 20, and 30 min holding time and then the specimens were quenched in cold water. Microstructure of quenched samples were observed under optical microscope and then analyzed via image analysis. The results showed that high semi-solid isothermal temperature would increase the liquid volume fraction and accelerate the spherical processing of the solid particles. Furthermore at long holding time, the globular grains coarsened slightly and the average grains size are increased. The experimental results showed that the optimum process parameters, should be chosen at isothermal temperature of 580°C with the holding time, <30 min.

Thixo-joining as a method of joining dissimilar materials was introduced recently. In this method, specimens in a semi-solid condition are brought into contact with each other under compression. As a result, diffusion bonding takes place between the specimens. In this research, two kinds of tool steels, AISI D2 and AISI M2, which are difficult to join using conventional methods, were joined together through the thixo-joining process. While specimens were being heated in a furnace with argon-controlled atmosphere through the direct partial remelting method, a uniaxial constant compressive load was applied to them. The Microstructure and mechanical properties of the joint were analyzed using scanning electron microscopy, shear, and hardness tests. The results showed a joint with good quality regarding the microstructure and mechanical properties. Moreover, energy dispersive spectroscopy revealed the diffusion of elements during the joining process. The fracture surface of the joint was also analyzed and characterized as intergranular.

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Thixojoining has been developed for D2 and M2 tool steels. The suitable globular microstructure and excellent bonding quality are obtained through this work. Scanning electron microscopy (SEM) observation along the joint interfaces showed a smooth transition zone with no cracks. In addition, fracture surface of the shear test samples showed that the fracture mode was transgranular. Finally, based on obtained results, this method presented high quality joint with nonequilibrium diffusion interface.