Joining new materials is always a challenge; at the very least a new set of parameters needs to be developed, but occasionally new techniques are required. High-strength steel can be welded by conventional resistance or MAG welding but the joint performance does not match that of the un-welded sheet. Friction stir spot welding, as used by Mazda on aluminium, would give a huge advantage if it could be performed successfully and at acceptable cost. Unfortunately, the high softening temperature of high-strength steel requires tool materials that have strength and wear resistance when glowing white-hot - not an easy requirement.
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