Friction Stir Welding in Wrought and Cast Aluminum Alloys: Weld Quality Evaluation and Effects of Processing Parameters
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INTRODUCTION AND LITERATURE REVIEW
FRICTION stir welding (FSW) is a complete solid-state welding technique, introduced in 1991 by Thomas et al.[1,2] at The Welding Institute in the U.K. The process is used for both similar and dissimilar metals and alloys, producing welds with very few defects, such as shrinkage porosity, that is often found in parts joined by fusion welding.[3] Understanding its fundamentals and advantages on weld quality and mechanical properties has been a subject of investigation in numerous studies. Arbegast et al.[4] addressed weld quality describing several types of large-scale welding defects, and among those, the ‘‘wormhole’’ and ‘‘surface lack of fill’’ were the two most important defects near the welding line. These specific defects are primarily associated with voids, cavities, and lack of material in local areas, as well as surface roughness. The potential causes of these welding issues are related to overly hot or cold processing conditions, and some other factors related to the workpiece. The weld quality is strongly affected by processing parameters, which are dependent on the type of working materials.[5] Hot or cold processing defects are caused by excessive rotation
YI PAN and DIANA A. LADOS are with Integrative Materials Design Center, Worcester Polytechnic Institute, Worcester, MA, 01609. Contact e-mail: [email protected] Manuscript submitted August 29, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
and/or traverse speed combinations. Kim et al.[6] studied the effects of processing parameters on the weld quality of die-cast Al ADC12 (Al-Si-Cu-Mg-Zn) alloy and proposed three types of FSW defects: (1) a large mass of flash due to the excess heat input; (2) cavity or groove-like defect caused by an insufficient heat input; and (3) cavities produced by abnormal stirring. Doude et al.[7] also found that the same defects formed in wrought Al AA2219 alloy at high rotation and traverse speeds. Tutunchilar et al.[8] and D’Urso et al.[9] reported possible defect formation mechanisms using finite element analysis (FEA) simulation models. The material flow in FSW is limited by insufficient heat input, which results in voids and cavities forming closer to the advancing side of the weld (AS, where tool rotates in the same direction to the traverse direction) rather than the retreating side (RS, where tool rotates in opposite direction to the traverse direction). The results have consistently shown in all these studies that the energy input and stirring rate are strongly affecting defects formation, and their proper selection is key to controlling weld quality. However, no quantitative models have been found in the literature relating processing parameters to the weld quality or types and locations of defects. Microstructure refinement occurs during mechanical stirring in the dynamically recrystallized zone (DXZ or nugget).[2–4] This is an another important effect contributing to improvements in weld quality and application performance. Numerous studies[4,10–14] discussed
the intense plastic deformation
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