Mechanical Properties and Microstructure of Resistance Spot Welded Joints of AISI 409M Ferritic Stainless Steel

Abstract

Resistance spot welding is widely used as a sheet metal joining process, especially in automotive and rail coach manufacturing industries. Ferritic stainless steel (FSS) is used for structural applications in these areas, owing to its superior corrosion resistance compared to conventional carbon steels. In this paper, mechanical properties and microstructure of resistance spot welded FSS joints to grade AISI 409M was investigated. Tensile shear test and micro hardness test were carried out to assess the mechanical properties and failure characteristics of weld joints. Microstructure was evaluated with optical microscopy. The effect of welding current on nugget growth, peak load, fusion zone (FZ) hardness, failure energy and failure mode were analyzed at various current ratings, keeping electrode force, weld cycle time and electrode tip diameter as constants. The results showed that, with the increment in welding current, tensile shear strength increased. Nugget diameter was found to be increasing with increase in current, in expulsion free welds. At low currents, failure mode was found to be interfacial and the pull out mode of failure occurred, at relatively higher current values. Average FZ hardness was found to be more than that of base metal (BM) but less than that of maximum hardness at heat affected zone. Grain enlargement was noticed at high temperature heat affected zone. FZ microstructure consists of predominantly columnar ferrite. Failure occurred at the BM region in all the samples welded above 10 KA current.

Effect of Multi-Objective Optimization for Double Pulse Resistance Spot Welded Joints of Ferritic Stainless Steel

“…Higher weighting factor was assigned to tensile shear strength, as it is the most significant attribute in a spot welded joint, directly affecting the safety and reliability of the product, unlike indentation. Furthermore, weighting factors of 0.8 and 0.2 were used in some of the reported works in the past, on similar situations 8,12,15 .…”

“…2). Also, the parameter with which, the difference between maximum and minimum S/N ratios (represented by delta in Table 5) is the largest, will have the largest influence on the output parameters 12 . Accordingly, here, second pulse current is the largest influencing parameter, followed by second pulse weld time and cooling time in succession ANOVA has been used to determine the level of significance of each of the control parameters in both maximizing the tensile shear strength and minimizing the indentation simultaneously.…”

“…Based on the reported literatures 8,11,12,15 on RSW and preliminary trials, three control parameters, second pulse weld current, second pulse weld time and cooling time, were chosen for optimization. Basic parameters such as, First pulse current value, first pulse weld time value and the corresponding electrode force were kept same for all the experiment trials.…”

“…At the same time, the amount of indentation made by the electrode on the surface of the sheet during spot welding is another quality characteristic that needs to be minimized, to improve the surface finish and aesthetic value. Achieving maximum weld strength, while keeping indentation at the minimum level is one of the major challenges in spot welding process [12][13]. One of the ways to improve the mechanical performance of a resistance spot welded joint is by adding a second pulse current in addition to single pulse current [14].…”

Influence of electrode tip diameter on metallurgical and mechanical aspects of spot welded duplex stainless steel

“…In the past, detailed study about spot welding of austenitic stainless steel [9][10][11], ferritic stainless steel [2,12,13] and martensitic stainless steel [14][15][16] has been carried out. However, very limited literature is available about the investigation of duplex stainless steel (DSS).…”