Microstructure engineering of fusion zone in resistance spot welding of martensitic stainless steels: The role of Ni interlayer thickness

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Martensitic stainless steel (MSS) spot welds show low fracture toughness due to the formation of a brittle microstructure at the joint area. To realize the full advantage of this class of steels in industrial applications, the strategy of altering the microstructure of the weld nugget through introduction of a Ni interlayer was investigated. For this purpose, Ni interlayers with variable thicknesses of 50 to 400 μm were utilized. The microstructural evolutions were thoroughly scrutinized using Scheil-Gulliver solidification modeling and electron backscatter diffraction (EBSD) analysis. Conventional quasi-static mechanical tests, namely, tensile-shear (TS) and cross-tension (CT) were used to correlate microstructure-mechanical properties relationships. In the absence of Ni interlayer, the weld nugget was predominantly composed martensite-M and presumably residual delta ferrite-δ (M+ δ-95%) phases with minor amounts of austenite-γ (5%), failing to meet the minimum peak load requirements under TS and CT loading conditions. By applying Ni interlayers of 50 and 75 μm, a noticeable increase in the fraction of austenite phase (up to 31%) in the weld nugget was detected; nevertheless, a slight to moderate improvement in the mechanical properties was observed since the solidification mode was still primary δ, and M+δ were still the predominant phases in the microstructure. A significant improvement in mechanical properties was achieved in 100 μm Ni interlayer owing to change in the solidification mode from primary δ to γ along with the formation of a mainly austenitic microstructure. However, due to the incomplete mixing of melted base metals and Ni interlayer, a different microstructure consisting of M, δ and γ was also occasionally perceptible in the weld nugget of this sample. The best mechanical properties in terms of strength and ductility occurred with increasing thickness to 200 μm and above, which coincided with the formation of a fully austenitic microstructure in the entire weld nugget.