A B Fig. 4 — EDS analysis of nonmetallic inclusions. A — 8 kA and 20 cycles; B — 9 kA and 25 cycles parameters in the fusion zone. A B C Fig. 5 — Failure mode of welded samples. A — IF; B — PIF; C — PF. pulsion due to high heat input causes loss of surface quality as well as strength in the resistance spot weldment (Ref. 22). Hardness measurements of samples were also performed and results are shown in Fig. 7A–D. It was found that deformed TRIP800 base metal hardness values were approximately 100 HV0.5 higher than those of the as-received sample due to transformation of retained austenite into martensite during plastic deformation. The weld button hardness of colddeformed samples increased as compared to as-received samples due to increasing volume of martensite phase and grain refining with much more plastic deformation. This is consistent with the results obtained from the tensile shear test. It is believed that increased hardness in the weld button increases the strength of the weldments. However, the hardness of the cold-deformed sample decreased in the HAZ. The HAZ of the as-received samples (450–500 HV0.5) had slightly higher hardness than the cold-deformed samples (424–475 HV0.5) for the same welding parameters, as seen in Fig. 7. It may be attributed to the severe deformation that decreases the recrystalization temperature; this, in turn, leads to the coarsening of grain in the HAZ during welding. In other words, work hardening is mostly wasted in the HAZ during recrystallization and grain growth. Effect of Deformation and Welding Parameters on the Weld Button Geometry The quality and strength of the weldment can be estimated by measuring its button diameter (Ref. 23). Sun et al. (Ref. 12) reports that AHSS spot welds with a fusion zone size of 4√t (t = thickness of sheet, mm) cannot produce a button with a desired pullout failure mode for both DP800 and TRIP800 spot welds under lap shear loading. Kumar Pal and Bhowmick (Ref. 24) claim that the average button diameter should be equal to or larger than 4√t for button pull out failure mode in dualphase steels for a sheet thickness less than 1.5 mm. The relationship between button size and welding parameters was determined and results are shown in Fig. 8. Figure 9 clearly shows that it is possible to achieve a desired PF mode through a constant button diameter of 4√t by using welding parameters of more than 10 cycles welding time and 3 kA welding current. The button diameter of the spot weld increased with increasing welding currents up to maximum level (i.e., 7 kA for 10 cycles), then it gradually decreased. This phenomenon may be attributed to high heat input resulting in undesired expulsion, which results in the loss of liquid metal from the button. It was also found that the button diameter increased while the button height decreased. Because a higher heat input produces a larger fused vol- WELDING RESEARCH 80-s WELDING JOURNAL / MARCH 2016, VOL. 95
Welding Journal | March 2016
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