Determination of Optimum Process Parameters and Residual Stress in Friction Welding of Thixocast A356 Aluminum Alloy
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INTRODUCTION
TODAY’S automobile industries extensively use a variety of aluminum-silicon alloys for producing cast products. To improve the strength of these alloys, several methods like gravity die-casting, pressure die-casting, and squeeze casting are being used. However, a significant drawback of these processes is the formation of dendritic type microstructure, cracking, and porosity that eventually degrades the mechanical properties. In order to overcome these difficulties, semisolid manufacturing (SSM)[1]was invented, which involved stirring of liquid metal during solidification to produce ingots having a non-dendritic or globular microstructure. In a review article, Fan in 2002[2] described the origin and various technologies for semisolid component shaping. In recent years, several research groups[3–6] have successfully demonstrated the application of thixocasting in the production of automobile parts with excellent mechanical properties.
SHAILESH KUMAR SINGH and K. CHATTOPADHYAY are with the Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India. PRADIP DUTTA is with the Department of Mechanical Engineering, Indian Institute of Science, Bangalore 560012, India. Contact e-mail: [email protected] Manuscript submitted March 24, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS B
Joining thixocast components with conventional fusion welding would tend to bring back the dendritic microstructure and create gas porosity, oxides, and air entrainment, and crack, which eventually makes the mechanical properties of the weld joint weak. On the other hand, friction welding, being a solid-state joining process, eliminates all types of defects related to the conventional fusion welding process. Additionally, the process is fast with a narrow heat-affected zone and environmentally friendly. This process produces weld under frictional heating and plastic deformation at the interface of two workpieces. In a recent study by the present authors,[7] the weldability of a semisolid processed component using friction welding is demonstrated. The uniformity of the heat-affected zone, strength, and absence of defects are desirable features of welded components. It is also essential to evaluate the magnitude and nature of residual stress present in the welded sample. Residual stress comes into existence during thermo-mechanical processing of metals, which ultimately influences the performance of the components produced. This is a result of self-equilibrating forces present in the components when the external forces are removed.[8] These stresses not only lead to the distortion of the welded components but also are superimposed on the externally applied forces during the in-service condition, which further deteriorates the performance of components. However, the residual stress present in the component can be beneficial or harmful to the material performance, depending upon its magnitude and nature. Generally, compressive residual stresses are useful as they minimize
defects such as cracks in the comp
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