Mechanical Properties and Microstructural Characterization of Multi Directional Forged and Heat Treated Zn-24al-2cu Alloy

Abstract

Multi directional forging (MDF) is one of the techniques to achieve severe plastic deformation (SPD) which yields ultrafine grained metallic materials. In this technique, the material is compressed in a channel die to a fixed strain, retaining its shape by appropriately rotating it over the three axis. In the present investigation, the MDF process was carried out on selected Zn-24Al-2Cu alloy at two different temperatures, viz., 100 °C for three passes and 200 °C for six passes. Further, the MDF process was subjected to age hardening treatment by following the sequence of solutionizing, MDF processing and post aging. The aging studies were carried out by the choice of aging temperature between 100 to 250 °C and aging curves were generated. Further, all mechanical characterizations of the material were carried out on the test samples which had been aged at 100 °C for one hour as this combination of parameters yielded highest hardness in all the categories of sample. Techniques like optical microscopy, electron microscopy and X-Ray diffractometry were used to characterize and analyze the microstructure of MDF processed and age hardened materials. Microstructural analysis revealed that the MDF processing reduces the grain size, yielding fine grained materials, apart from well-distributed phases. The microstructure is observed to be consisting of three phases, viz., Al-rich α phase, Zn-rich η phase and CuZn4 ε phase. Mechanical characterization of the materials revealed an important fact that strength and hardness increased along with the ductility. It was attributed to fineness of grain and evolution of phases in a favorable manner because of MDF and aging treatment meted out to the material. The creep behavior of the material is investigated by conducting the indentation creep test with 2 kg and 2.5 kg load at different temperatures from 30 to 150 °C. The MDF processed material is found to lose its creep resistance substantially mainly because of microstructural refinement. It was possible to infer from the determination of value ofactivation energy that creep occurred by various mechanisms at different temperature regimes. Those mechanisms include dislocation creep and diffusional creep. Wear behavior of the materials was assessed by conducting the test on pin on disc machine resorting optimization of parameters using Taguchi method. It was proved that processing the materials by MDF followed by post-aging improved the wear resistance of the materials. Wear mechanism was observed to be abrasive type in the case of solutionized samples and adhesive type in MDF-processed samples as revealed by SEM investigations on worn surface.