Investigations on autogeneous joining of thin sheets of dissimilar materials by micro-plasma transferred arc process

Abstract

To utilize specific properties of different materials to improve structural integrity, safety and corrosion resistance in specific areas and allow greater flexibility in materials selection, it is necessary to use joints of dissimilar material and same or different thickness. It is very difficult to join dissimilar materials due to their properties variation. The present work is focused on autogeneous joining of thin sheets of two dissimilar material namely marine grade austenitic stainless steel (SS 316L grade) and mild steel (MS 513 grade) of same thickness and different thickness in lap configuration due to widespread use of such joint in the shipbuilding, construction equipment, ductwork, etc. The study was conducted though pilot and main experiments conducted in two stages by varying three most important parameters (i.e. current, torch travel speed and stand-off distance) of the newly developed energy-efficient and cost-effective process termed as micro-plasma transferred arc (μ-PTA) process. Experimentations of stage-1 involved investigations on autogeneous joining of thin sheets of SS 316L and MS 513 of same thickness (i.e. 1 mm) and stage-2 was investigated autogeneous joining of 1 mm thick sheet of SS 316L with 2 mm thick MS 513 sheet. The μ-PTA process has been usedbecause of its higher precision due to use of low current ranges with almost no HAZ and thermal distortion and significant reduction in joining times in comparison with conventional joining processes. Pilot experiments of both the stages of experiments aimed to identify those ranges of the variable parameters which resulted in good quality joint of SS and MS thin sheets whereas, main experiments of both the stages were meant to identify that combination of the considered variable parameters which yielded maximum value of ultimate tensile strength of the joint. Such joints were characterized using optical microscopy, scanning electron microscopy, Vickers’s microhardness and energy dispersive X-ray (EDX) analysis.It was found that joint prepared using parametric combination (9.0 A; 110 mm/min; 5.0 mm) yielded sound weld joint with ultimate tensile strength of 325 MPa prepared using thin sheets of same thickness (1 mm) of both the materials. Parametric combination for different thickness sheet joint (i.e. 1 mm SS316L and 2 mm MS 513) to prepare sound weld joint is (13.0 A; 70 mm/min; 7.0 mm), which resulted into ultimate tensile strength of 396 MPa of joint. The value of plasma flow rate was 0.4 Nl/min and shielding gas flow rate was 5 Nl/min in both the stages. Insignificant heat affected zone(HAZ) was confirmed by very small variation in Vickers’s microhardness values across the fusion zone. This study establishes capability of μ-PTA process for joining stainless sheets of same and different thickness with mild steel sheets without any filler material and producing very good quality joint using much less power as compared to conventional fusion joining processes.