Effect of solution treatment on microstructure and hardness of rheo-forming AZ91-Y alloy

Abstract

M g-Al series alloys have been recognized as ideal materials for lightweight applications in the automotive industry with great development potential because of their low density, high specific strength and stiffness, and excellent cast-ability [1][2] . But their applications are still limited due to the intermetallic phase Mg 17 Al 12 of easy brittleness and unstable structure at elevated temperatures [3][4] . Some efforts have focused on adding some elements like Ca, Si, rare earth (RE) into the alloy matrix for generating new phases to limit the growth of β-Mg 17 Al 12 and modify its microstructure, and eventually improve the mechanical properties of Mg-Al series alloys [4][5][6][7] . Simultaneously, T6 treatment that can also achieve similar goals has attracted more attention due to its low cost and convenience. This is done by making Mg 17 Al 12 dissolved into α-Mg matrix and then precipitated once again.Semisolid metal (SSM) processing is recognized as Abstract:The microstructure and hardness of rheo-forming AZ91-Y alloy before and after solution treatment (ST) have been investigated by means of optical microscope (OM), scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and Vickers. The experimental results showed that the β-Mg 17 Al 12 phase of alloy was nearly dissolved after ST for 5 min. With the increasing of ST duration to 28 h, both the primary and secondarily solidified α-Mg grains faded away. At the same time, the alloy exhibited a much smoother surface due to the diffusion of solute atoms (Al). During ST, the thermal stable phase of Al 2 Y produced by ultrasonic vibration retained its size and morphology. As the ST duration was increased, the alloy hardness decreased sharply at first, and then gradually reached a minimum level. an outstanding technique for Mg alloys. Its advantages are compared to the conventional die casting processes, including the potential of producing near-net shape and high-integrity components, low processing temperature that resolved the problem of oxidation, and effective burning in Mg alloys processing [8] . Several works have studied the microstructure evolution of AZ91D alloy fabricated by SSM processing during heat treatment [9][10] , but few studies related to the SSM processed Mg-Al series alloys containing RE have been reported. Also, it is well known that different casting processes will result in a variation of the microstructure, which could influence dissolution and precipitation kinetics of Mg 17 Al 12 during subsequent heat treatment. In this present study, solution treatment (ST) at 410 °C was used to process a rheo-forming AZ91-Y alloy. The aim was to detail the alloy's microstructure and hardness before and after ST, and to acquire the appropriate solution duration for subsequent aging treatment. The effect of RE (Y) on kinetics of Mg 17 Al 12 dissolution and the effect of the rheo-forming technique on alloy composition homogenization were also analyzed.

Influence of different tool polarity on short electric arc machining of GH4169

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