Strong resistance to Zn-assisted liquid metal embrittlement of austenitic-TWIP/martensitic-HSLA multi-layered steel sheets additively manufactured by laser cladding

As a novel design idea to overcome a shortcoming of liquid metal embrittlement (LME) occurred frequently in twinning-induced plasticity (TWIP) steel, multi-layered steel (MLS) sheets consisting of austenitic TWIP and high-strength low-alloy (HSLA) steel grades have been suggested. Two- and four-time clad MLS sheets (C2 and C4) were fabricated by a powder-fed laser additive manufacturing (AM) process using HSLA powders on the TWIP surface, and the LME was investigated in various resistance-spot-welding-time stages. The C2 and C4 sheets consisted of the austenitic TWIP substrate and tempered-martensitic HSLA-clad layer, and the HSLA layer had two or four sublayers having different C and Mn contents reduced step-wise from the TWIP/HSLA interface. LME cracks formed in the C2 spot-welded for 240 or 380 ms, whereas they did not form at all in the C4 spot-welded for 380 ms. This different LME susceptibility depended mainly on the HSLA-sublayer microstructures beneath the Zn coating after their high-temperature exposure during the spot welding. Since LME cracks propagated mainly along prior austenite grain boundaries, the whole austenite existed in the austenite range played a key role in determining whether the LME cracking occurred or not. The total volume fraction of reverted austenite (appeared as fresh martensite at room temperature) and retained austenite was very high at about 68% in the C2 welded for 240 ms, thereby leading to the LME cracking. It was lower than 20% in the C4 even under the severe welding condition of 380 ms, which provided the reason for no LME cracking.