TWI Industrial Member Report Summary 1017/2012
By RE Andrews
Background
Friction stir welding (FSW) is a relatively new technology and has rapidly progressed from concept to industrial application for the fabrication of predominantly aluminium alloy structures. Despite the successful industrial use of FSW, the apparent simplicity of the process, particularly as perceived by non-practitioners, can be misleading. For example there are major challenges associated with the limited life of FSW tool probes (pins) when joining high strength aluminium alloys, which has hindered the industrial adoption of the FSW process. In an industrial application of FSW to join a 4mm thick 6xxx series aluminium alloy 1km of weld is produced before the FSW tool probe (pin) is changed but not failed. In contrast, when welding 12mm thick 7xxx series alloy, tool probe failure has occurred after the production of only 2m of weld. This short FSW tool life influences the cost to produce a unit length of weld and thus influences the commercial viability of using FSW for a specific application. Although it has been demonstrated that it is possible to weld all of the high strength aluminium alloys and to achieve excellent weld quality, (as assessed by metallographic examination and mechanical testing), short FSW probe life time remains a concern for very challenging applications.
The challenges associated with using the FSW process for joining high strength aluminium alloys, led to this project initiation. Discussions with TWI member companies suggested that FSW practitioners are looking for a solution to the short life time of FSW tool probes, when welding high strength aluminium alloys. Friction stir welding tool probe design and materials has tended to remain confidential to Research and Development organisations and industrial users of the FSW process. Confidentiality surrounding FSW know how has hindered the industrial adoption of the FSW process. It is interesting to note that the two main FSW standards, as yet, do not include any details of FSW tool probe geometries or materials.
This report describes an analysis of the failure mode of part-used FSW tool probes and a review of the current state-of-the-art with respect to FSW tool material selection. In addition, tool materials with the capability of welding high strength aluminium alloys were selected from the review. In future work these materials will be subjected to lifetime welding trials to establish their capability and investigate the mechanisims of tool failure. This report provides the first comprehensive published comparison and quantification of tool performance and lifetime for challenging FSW operations in high strength aluminium alloys.
Objectives
The objectives of the work described in this report are as follows:
- To investigate the failure mode of current FSW probes used to weld high strength aluminium alloys.
- Review the current state-of-the-art with respect to FSW tool materials selection, and identify a range of candidate FSW tool materials for high strength aluminium alloys.