Controlling hard zone formation in friction stir processed HSLA steel

The present study was undertaken to determine the effects of friction stir process conditions on mitigating the “hard zone” often present in FSW HSLA steels. The primary parameters investigated were travel speed, heat input, and cooling rates. The hard zone consists of primarily fine lath ferrite. There are strong linear correlations between the ferrite lath width, hardness of the hard zone, and weld cooling rate. The hard zone was eliminate at cooling rates below 20 °C s−1 in FSW HSLA-65.