• EXTRACTION AND FRAGMENTATION OF SUBSTRATE

The first step of the stirring action is the fragmentation of the solid grains, from the body of the workpiece. The frictional heat from this grain fragmentation can soften the mass and form the plasticized mass which enters into the stirring zone. Hot material is broken into large fragments by the tool features (threads and flats). These fragments may be partially attached to the weld mass, or they may be completely severed but retained within the confines of the weld. These fragments are evident in the lacerations on the internal weld surfaces at the entry of the weld, and as loose granules that fall out of the weld. The fragments are cut from the workpiece in an arc from the leading edge of the tool, from the advancing to retreating sides. We propose that the fragments are delineated by the flow layers, i.e. the region between any two flow arms represents a fragment of material that once had exposed surfaces in the region of the flow arms.
