Investigation on autogenous joining of stainless steel sheets of different thickness using micro Plasma Transferred Arc (μ-PTA) process

Abstract

There are wide varieties of applications of tailored welded blank. A tailored blank consists of materials of different gages, strengths and coating types welded together to produce a single blank prior to forming. Due to overwhelming demand in the electrical goods, packaging and construction markets etc, SS 316L was selected as workpiece material. Properties of SS 316L are similar to those of Type 304 except that this alloy is somewhat stronger at elevated temperatures. Type 316L is an extra-low carbon version of Type 316 that minimizes harmful carbide precipitation due to welding The most interesting and effective advantage of using tailor welded blanks (TWBs) is the proper distributions of weight and material properties in the final stamped part with a consequent reduction in weight and cost. μ-PTA process is used because of higher precision, due to use of low current range with almost no HAZ and thermal distortion and 4- or even 5- fold reduction in welding times in comparison with manual welding process. Different methods, including metallographic observations i.e., optical microscope, scanning electron microscope, uniaxial tension tests, vickers microhardness and energy dispersive X-ray (EDX) analysis were used for determination of mechanical characteristics of the weld joint.This thesis is focused on autogenous joining of tailored blanks, i.e., sheets of different thickness but of same material (SS 316L) i.e., a thin sheet (1 mm) and a thick sheet (3 mm) are joined autogenously using micro-plasma transferred arc process. Effects of current, torch travel speed and SOD were investigated to determine their optimum levels for joint tensile strength.It was found that (19.4 A; 63 mm/min; 1.3 mm), (19.6 A; 80 mm/min; 1.3 mm) and (19.8 A; 100 mm/min; 1.3 mm) with values of plasma flow rate at 0.4 Nl/min and shielding gas flow rate at 5 Nl/min, yielded sound weld joint. The fusion zone was found to have finer grain size. Absence of heat affected zone was confirmed by very small variation in Vickers microhardness values across the fusion zone. This study establishes capability of micro plasma transferred arc process for joining stainless sheets of different thickness without any filler material and producing very good quality joint using much less power as compared to conventional fusion joining processes.