A A transformed into martensite. The microstructure of the weldment was also evaluated and results are shown in Fig. 3. The structure of the weld button contains the martensite phase due to its high cooling rate — Fig. 3A. The structure of the HAZ transformed to predominantly martensite with small areas of ferrite, lath bainite, and retained austenite, depending on the ultimate temperature reached and cooling rate — Fig. 3B and C. Results imply that an increase in welding current and time increases heat input, which leads to expansion of the HAZ region. Since the deformation decreases the recrystalization temperature of the steel, grain growth proceeds more rapidly. Briefly, the deformation process also extends to the HAZ. In the case of a very rapid cooling of the spot weldment, an insufficient diffusion occurs in the precipitated solid crystals and the remaining liquid, so the inhomogeneity of the alloying elements within the fusion zone produces a sharp gradient in the composition distribution through microsegregation (Ref. 16). In TRIP steels, due to the strong affinity for oxygen, the added Al and Si readily form oxides during welding, while leaving the weld pool depleted of these elements (Refs. 17, 18). It is also known that the reaction between the dissolved alloying elements in the weld pool with the available oxygen, nitrogen, and carbon forms nonmetallic inclusions (Ref. 18). Inclusions were observed in the weld nugget and HAZ. The energy-dispersive spectroscopy (EDS) analysis of inclusions is presented in Fig. 4. The result indicates that inclusions could be a mixture of aluminum and silicon oxide — Fig. 4A and B. Similar oxide inclusions were observed in GTA welded high-Al and high-Si TRIP steels (Ref. 18). Effect of Deformation and Welding Parameters on Properties and Failure Mode The macrographs of the failure mode of the tested samples are shown in Fig. 5. Three distinct failure modes were observed (Refs. 19–21). 1) Interfacial failure (IF), in which fracture propagating through the fusion zone was observed for all welding currents at 5 cycles and for all welding times at 1 kA. 2) Pullout failure (PF), in which failure occurs by the effect of the withdrawal of the weld button from a single sheet, was observed from 3 to 9 kA welding currents, at 15 to 25 cycles welding time. The fracture of the weld WELDING RESEARCH made with high expulsion current is through the full thickness of the base metal. The nugget diameter exceeds the electrode diameter and the fracture occurs outside the indented area (PF mode) (Ref. 22). 3) Partial interfacial failure (PIF), in which the fracture first propagates in the fusion zone, was observed for 5 cycles from 5 to 10 kA and 10 kA at all welding times. With higher welding times at higher welding currents, such as 10 kA, the amount of molten metal increases and fused metal ejects and the weld nugget diameter starts to decrease, so the fracture occurs at the edge of the button through the fusion zone and causes IF or PIF modes. No satisfactory joint is expected due to excessive heat input over 11 kA. The strength of the weldment varies with failure types. It increases in the order of IF, PIF, and PF modes. In determining the effect of welding parameters on the strength of deformed steel, all welded samples were tensile shear tested in order to evaluate the weld quality. Test results are shown in Fig. 6A and B. As seen in Fig. 6A and B, the maximum strength is obtained for 9 kA welding current at 20 cycles welding time. It was found that the strength increased with increasing welding time and current up to maximum level, then it gradually decreased due to increased molten metal and spreading of fused metal as well as uneven electrode indentation. The desired button diameter can only be obtained by proper adjustment of welding current vs. welding time. When time is short, the button diameter decreases. On the contrary, when it is long, the amount of molten metal increases and fused metal spreads out and as a result the strength of the welding joint decreases (Ref. 25). Kimchi also reports that ex- MARCH 2016 / WELDING JOURNAL 79-s Fig. 2 — XRD results. A — Asreceived; B — colddeformed TRIP800 steel. Fig. 3 — Microstructure across the resistance spot weld of 10% colddeformed TRIP800 steel. A — Weld nugget; B — HAZ; C — grain growth in HAZ. B B C
Welding Journal | March 2016
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