A B C D Fig. 7 — Hardness profiles. A — Asreceived samples produced by using 8 kA welding current; B — colddeformed samples by using 3 kA; C — 8 kA; D — 10 kA welding current. and 20–25 cycles welding times should be applied. Discussion Weld button diameter and weld button size ratio are the most critical factors in weld quality in terms of peak tensile shear load and failure mode for TRIP800 spot welds. As compared to the as-received welded TRIP800 samples in previous work (Ref. 15), the critical button diameter size (4√t) for desired strength of 10% for cold-deformed TRIP800 samples was found to be lower than as-received samples’ button diameter size (4.5√t). It could be attributed to the plastic deformation of TRIP800 steels in which austenite transforms into a much harder martensite phase and dislocation density is increased. Therefore, electron flow is prevented due to high dislocation density and disordered crystal structure, thus electrical resistivity is increased. So, heat generation in prestrained TRIP800 spot welds (Q = I2Rt) increased with the same welding parameters as compared with the as-received ones, and more heat generation leads to the formation of a larger weld area in the weldment. Mukhopadhyay et al. (Ref. 28) investigated as-received resistance spot welds and 5% prestrained BH (bake hardening) steels. They reported that prestrain samples’ strength was higher than the as-received samples depending on the increasing density of the dislocations in the deformation and the formation of Cottrell atmosphere on the HAZ–base metal interface during welding. Khodabakhshi et al. (Ref. 29) reported that different rates of deformation applied to low-carbon steel used in the automobile industry caused an increase in electrical resistance. The strength of the colddeformed samples was determined to be higher than the as-received ones using the same welding parameters. More heat input raises the temperature at the electrode-sheet interface, and more plastic deformation is generated even under a constant electrode force (Refs. 29, 30), and for this reason the electrode indentation depth is found to be higher in the cold-deformed TRIP800 steel weldment. It is believed that an increase in electrode indentation caused much more contact surface between the watercooled electrode and weld button so the cooling rate increased in the colddeformed weldment. This caused the weld button to have more fine-grained microstructure and a higher hardness. The grain refining increased the strength of the weldment. The width of the weld lobe gives an indication of good welding parameters and the tolerance of the weld schedule in a production environment. Compared to the as-received welded samples, weld lobe diagram in previous work (Ref. 15), the cold-deformed and then welded samples, weld lobe diagram became narrow. The weld lobe curves set back to lower welding currents with the same welding time. Conclusions From the results given above, the following conclusions can be drawn: 1) The tensile shear strength of the samples increased with heat input due to increased welding time and WELDING RESEARCH 82-s WELDING JOURNAL / MARCH 2016, VOL. 95
Welding Journal | March 2016
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