Friction Stir Welding in Wrought and Cast Aluminum Alloys: Heat Transfer Modeling and Thermal History Analysis
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FRICTION stir welding (FSW) is a solid-state welding technique developed at The Welding Institute (TWI) in the UK in 1991 by Thomas et al.[1,2] The technique allows for both similar and dissimilar materials joining, with no solidification cracking, oxidation, shrinkage, or porosity that are often found in parts joined by fusion welding. Later on, in 2005, friction stir processing (FSP) was developed by Mishra et al.[3,4] and applied to refine surface microstructure, reduce surface defects, control surface residual stresses, and improve mechanical properties. Nowadays, FSW and FSP are commonly applied to Al alloys and other materials, which bring important energy savings in the transportation industry both during processing and vehicle operation.[4] Even though FSW has been around for many years, the magnitude of the heat that is generated during processing and the effects on the resulting microstructure and properties are still not well understood. To soften the material and create enough material flow in welds, a suitable amount of heat is required with the right ratio of rotation and traverse speeds. Studies on material and plastic flow suggested that the distribution of secondary phases may vary even when processed under similar parameters.[5] The generated heat during friction stirring has a direct influence on the weld’s microstructure. Previous investigations have focused on
YI PAN and DIANA A. LADOS are with the Integrative Materials Design Center, Worcester Polytechnic Institute, 100 Institute Road, Worcester, MA 01609. Contact email: [email protected]. Manuscript submitted September 30, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
identifying the relationship between the peak temperature and microstructure changes in Al alloys during processing.[6,7] The heat during processing is primarily related to the processing parameters. It has already been established that the rotation speed plays the most significant role on heat generation. Chen et al.[8] established a finite element model that shows the effect of rotation on friction behavior in their simulation work. In the model, the heat generation rate is associated with the tool radius, angular velocity, and friction coefficient at the interface between the tool and workpiece, and is mathematically calculated as the integration of these factors. The simulation of the thermal and stress history is studied on the Al alloy 6061-T6 at rotation/traverse speeds of 500 RPM and 2 mm/s used in this study. Cartigueyen et al.[9] applied the same heat generation equations for predicting temperature changes around the tool shoulder for copper alloys. The study shows good results in predicting temperature profiles at 700 RPM. The authors also pointed out that the peak temperature is strongly affected by the rotation speed, while the heating rate is affected by the traverse speed. It was also reported in their study that there is a large increase in the peak temperature from when the rotation speed was increased from 500 to 1000 RPM. There was no indication, however, if the rel
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