Fatigue Behavior of Resistance Spot-Welded Unequal Sheet Thickness Austenitic Stainless Steel

This paper presents a comparative study on the fatigue strength of resistance spot-welded unequal and equal sheet thickness austenitic stainless steel. Lap joints of 3.0-1.0 mm and 1.0-1.0 mm thick austenitic stainless steel were made using the same resistance spot welding schedule with current, weld time and electrode force of 4.7 kA, 20 cycles and 6 kN respectively. The sinusoidal wave form with a constant stress amplitude was selected in the fatigue tests whereas the stress ratio and frequency used were 0.1 and 8 Hz respectively. Fatigue strength and tensile-shear load bearing capacity of 3.0-1.0 mm joint were higher than that of 1.0-1.0 mm joint, although its nugget diameter was smaller. The joint stiffness was the controlling factor of the fatigue strength of resistance spot-welded unequal sheet thickness austenitic stainless steel.


Introduction
Austenitic stainless steels are used for a very broad range of applications especially in automotive, railway vehicle, ship body, and airplane structures when an excellent combination of strength and corrosion resistance in aqueous solutions at ambient temperature is required.Stiffened thin plate construction where the thinner plate is reinforced by thicker plate called a frame, is generally applied to the structures.Gean et al. (1999) have claimed that it is a cost-effective way of achieving a high-performance vehicle structure because it remains suited to low-volume manufacture.This structure is typically joined by the resistance spot welding (RSW) process.The advantages of using RSW are that it is a quicker joining technique, suitable for automation, no filler material is required, and that the low heat input implies less risk for altered dimensions during welding.
Many standards and recommendations are developed by individual companies, such as Ford Motor Company and General Motors.Professional organizations such as the American Welding Society (AWS), Society of Automotive Engineering (SAE), the American National Standards Institute (ANSI), and International Organization for Standardization (ISO) also contribute to a significant portion of the standards.Because of the drastic differences in design, understanding and perception of weld quality, automobile manufacturers and others tend to have very different requirements on weld quality.Zhang and Hongyan (2006) have concluded that in general, spot weld size is enveloped between 3√ and 6√ (t is the thickness of the sheets in millimeters).This recommendation is very useful in finding good weld schedules for equal sheet thickness welding.However, in automotive body application, the majority of welds are between two dissimilar thicknesses.In this case, schedules for welding unequal sheet thickness are generally developed by and practiced within individual manufacturers.Some researchers also have proposed the spot welding unequal sheet thickness researches to evaluate these recommendations.The joint of unequal thickness of the same metal may produce a strength problem due to the heat unbalance (Hasanbasoglu & Kacar, 2007) and have the unique failure mechanism (Pouranvari & Marashi, 2010).
Despite various applications of spot welded unequal thickness in automotive body, reports in the literature dealing with its mechanical behaviors, especially the fatigue behaviors are limited.In fact, Gean et al. (1999) have fo and thu the pre weldin

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Results and
The weld prof   Given SEM views in Figure 13 and Figure 14 are the comparison of the last fracture surfaces of the 3.0-1.0mm and 1.0-1.0mm joint specimens which were subjected to same load of 3.4 kN.As seen in Figure 13(a), crack of 3.0-1.0mm joint initiated on the inside of thin sheet.On the more half of thickness, it propagated slowly due to low peel stress and induced ductile fracture characterized by intergranular cracking.Ductile fracture changed to brittle fracture characterized by transgranular cracking on the remaining thickness due to increased peel stress.The embrittlement of stainless steel was attributed to strain-induced martensite forming during the fatigue tests (Vural et al., 2006).Different view was given by fracture surface of 1.0-1.0mm joint specimen as shown in Figure 13 On the crack propagation zone, brittle fracture characterized by transgranular cracking was observed on both 3.0-1.0mm and 1.0-1.0mm joint specimens.They displayed wave of plastic deformation as shown in Figure 14.However, plastic deformation intensity of 1.0-1.0mm joint was higher than that of 3.0-1.0mm joint.It was indicated by the number of waves in the same observation area as seen in Figure 14.

Conclusions
Fatigue of resistance spot welded unequal sheet thickness austenitic stainless steel has been studied.Due to significant thickness difference, the asymmetric weld nugget, high microhardness on the edge of nugget and tearing fatigue fracture mode were observed.The fatigue strength of 3.0-1.0mm joint was higher than that of 1.0-1.0mm joint, although its nugget diameter was smaller.The endurance limit of 3.0-1.0mm and 1.0-1.0mm joint were 2.9 kN and 0.7 kN respectively.Ductile and brittle fractures were observed on 3.0-1.0mm fatigue specimens whereas 1.0-1.0mm fatigue specimens failed to fully brittle fracture mode.The joint stiffness was the controlling factor of the fatigue strength of resistance spot-welded unequal sheet thickness austenitic stainless

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