Development and Evaluation of Cathodic Arc PVD Coated Friction Stir Welding (FSW) Tools

Friction stir welding (FSW) is a recent innovation in the solid state joining process invented in 1991 by The Welding Institute (TWI). Within the past two decades, FSW has rapidly developed from experimentation stage into advanced technology deployed in aerospace, automotive and defense industries with huge research and development investment. Unlike in fusion welding, the absence of melting during FSW results in defect-free and excellent joint and this has been the major grounds for its successful and continuous use in industries. The combination of high frictional heat generated and extensive plastic deformation required to create the weld, FSW tools are subject to severe wear and deformation leading to premature failure and hence high running cost. Considerable effort has been directed to develop FSW tools with enhanced properties especially for welding of high strength engineering alloys. High strength tools such as CBN, W-Re, and WC-Co tools have been developed, however, abrasive, adhesive and oxidation wear, chemical dissolution of tool into the joint as well as the high cost of these tools, have greatly limited their use. The use of metallurgical coatings for FSW tools to improve wear resistance has been considered as a viable and economical approach to improve tool-life and hence the process economics. Stemming from the success in the cutting tool industries, physical vapour deposition (PVD) coatings exhibit great potential to improve tools performance. Studies investigating PVD coated FSW tools are limited. In our preliminary study, Al6061-T6 workpiece, 4041 steel tool, and AlCrN coating were investigated. Tool wear and weld quality after FSW of the workpiece were analyzed and compared with uncoated tool. Compared to uncoated FSW tool, the AlCrN coated FSW tool exhibited improved wear resistance. Examination of AlCrN coated tool wear showed up to 55% reduction in tool wear as compared to the uncoated tool.