Resistance spot-welding of AISI-1008 steel joints with MWCNT coating interlayer

Inclusion of multi-walled carbon nanotubes (MWCNTs) as an inter-layer sandwiched between the mating sides of AISI 1008 steel joints in resistance spot-welding and its impact on microstructure/mechanical behavior of the subsequent lap joint is discussed here. A thin layer of MWCNTs approximately 50 μm is incorporated as a coating at one of the mating surfaces of the lap joint. It was found that the MWCNTs get entrapped in weld-nugget during the spot-welding procedure, which subsequently leads to weld strengthening through various mechanisms. While the weld strength is reliant mainly on the welding current, time, and also on the nugget diameter, an improvement of ~45% in weld strength is attained in lap-shear tensile tests due to the MWCNT interlayer. A detailed microstructural and XRD studies corroborate the experimental observations. Peak shifts were observed in Raman spectroscopy which confirms the compressive stress associated with the weld nugget-entrapped MWCNTs. Transmission electron microscopic studies of the nugget region elucidate the prevailing strengthening mechanisms including dislocation pile up, interfacial stress transfer, and mechanical interlocking.