Failure analysis of tungsten based tool materials used in friction stir welding of high strength low alloy steels

• The process was done in “worst-case set-up” that is, without using shielding gas.
• Identification of microstructural stability of tungsten based tools
• Microstructural stability in W99 tool was due to absence of Fe–Co–Ni phase.
• Tungsten based alloys are cheaper than W–Re/W–Re–HfC/PCBN tools.

A non-consumable tool is a vital requirement for friction stir welding (FSW) of high melting point alloys such as steel and titanium. In this investigation, an attempt was made to understand the pre-weld and post-weld microstructural characteristics of three tungsten based alloy FSW tools viz. 90%W, 95%W and 99%W. High strength low alloy (HSLA) steel plates of 5 mm thickness were welded using the above tools with a tool rotational speed of 600 rpm and welding speed of 30 mm/min. Microstructural characteristics of the FSW tools, before and after welding, were analyzed using optical microscopy (OM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) techniques. From this investigation, it is found that the tool made of 99%W doped with 1% La2O3 exhibited microstructural stability due to absence of Fe–Co–Ni phase formation at elevated temperatures during FSW process.