The effect of processing variables and tool material on the microstructure of friction stir welded 6mm thick HSLA-65 steel plate has been investigated. Quantitative optical microscopy and Vickers microhardness measurements have been used to qualify and examine the microstructure through the weld cross section. Two tool materials were used: 1. Polycrystalline Cubic Boron Nitride (PCBN) and 2. Tungsten-Rhenium (WRe). Prior austenite microstructure/grain size has been revealed for all welds and in all cases there is a non uniform distribution of grain size in the stirred zone and a much bigger grain size in the centre of the weld than what would be expected from heavily deformed material. However, there is a significant increase in the hardness profile in the stirred zone suggesting that cooling rate is the dominant parameter controlling transformed microstructure and thus weld mechanical properties.
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