Stationary shoulder friction stir welding is a newly developed technique currently used for joining plates of relatively soft metals at different angular planes. Although the process is not currently applicable to steel, a study to investigate the theoretical and technical viability of stationary shoulder technology in DH36 steel has been undertaken. Aluminium welds were produced using both conventional rotating shoulder and stationary shoulder friction stir welding techniques, whereas steel welds were produced using only conventional friction stir welding techniques. The effects of stationary shoulder technology on both the microstructural evolution and resultant mechanical properties of aluminium have been evaluated so that the likely effects on steel could be predicted. In the aluminium welds, the stationary shoulder technique results in a distinct transition between stirred and unstirred material, in contrast to the gradual change typically seen in conventional friction stir welds produced with a rotating shoulder. An investigation of weld properties produced in DH36 steel has demonstrated that the microstructure likely to be formed, if the stationary shoulder weld technique was used, would be dominated by a bainitic ferrite phase and so would exhibit hardness and tensile properties in excess of the parent material. It is predicted that if the same abrupt transition between unstirred and stirred material, as seen in aluminium, occurred in steel this would lead to crack initiation followed by rapid propagation through the relatively brittle weld microstructure. Hence the findings demonstrate that stationary shoulder friction stir welding is unlikely to be applicable to steel without further design and process developments

The effect of iron dilution on the elemental segregation of alloying elements in nickel based filler metals. AbstractMicrostructural evolution of Inconel 625 and Inconel 686CPT filler metals, used for the fusion welding of clad carbon steel linepipe, has been investigated and compared. The effects of iron dilution from the linepipe parent material on the elemental segregation potential of the filler metal chemistry has been considered. The results obtained suggest that, in Inconel 686CPT weld metal, the segregation of tungsten is a function of the level of iron dilution from the parent material. The data presented suggest that the incoherent phase precipitated in the Inconel 686CPT weld metal has a morphology that is dependent on tungsten enrichment and, therefore, iron dilution. Furthermore, in the same weld metal a continuous network of finer precipitates was observed. The Charpy impact toughness of each filler metal was evaluated and the results highlighted the superior impact toughness of the Inconel 625 weld metal over that of Inconel 686CPT.