Insight into hot working conditions encountered in the FSW process

While FSW has been rapidly implemented into the aerospace industry for joining of aluminum alloys, its introduction into other markets has been slower. Even after three decades of development, our physical understanding of the FSW process is still limited. From a metallurgical viewpoint, the process is similar to hot working in which thermomechanical processing results in a 10 fold reduction in grain size within the stir zone. Optimization of the process would benefit by linking the processing parameters with hot working conditions for the metal thereby reducing the reliance on costly trial and error studies. Various modeling approaches have predicted that the material is subjected to strains ranging from 2 to 100 at strain rates ranging from 10⁰ to 10⁶ s^-1. Texture analysis indicates the stir zone material has been subjected to shear deformation. Thus it is hypothesized that the conditions during a FSW may be similar to those encountered in Orthogonal Metal Cutting (OMC). Using a specially designed, high strain rate metal cutting apparatus, chips were formed at shear strains from ~1 to 4 and shear strain rates from 10⁴ to 10⁶ s-1. Although these conditions were within the predicted range for the hot working conditions in the FSW process, no significant grain refinement was observed in the chips formed. This implies shear strains greater than 4-10 are required to achieve the grain refinement found in FSW. Quantifying the amount of shear strain has implications for FSW tool design and provides insight into the variation of the process parameter window size with tool design and geometry.