Fig. 6 — Tensile shear load bearing capacity of weldment. A — Effect of welding current; B — effect of welding time. ume, the size of the button naturally increases with increasing electrode indentation; therefore, the button height decreases. The button diameter, or button height, is not enough to determine the strength of spot welded joints. However, the button size ratio can be used for this purpose (Ref. 25). Therefore, the relationship between button geometry and welding parameters was determined for all the weldments, as shown in Figs. 9–11. It was found that the button height of the welded specimen decreased with increasing welding parameters, except for a welding time of 5 cycles — Fig. 9. Meanwhile, the electrode indentation depth appeared to increase with increased heat input — Fig. 10. The button size ratio diagram associated with the welding parameters was evaluated from Fig. 11. The (0.14–0.3) button size ratio was found to be sufficient to achieve the desired PF mode. The PIF mode observed when the ratio was lower than 0.14 appears to be associated with the expulsion of molten metal from the weld area resulting from the high heat input, while the IF and PIF modes associated with a ratio higher than 0.3 could be the result of low heat input that is not sufficient enough to create the required button and button size ratio. Effect of Button Geometry on the Tensile Shear Strength of Samples The strength associated with button geometry and electrode indentation depth was also determined and results are shown in Figs. 12–15, respectively. As a result of an increase in button diameter, the strength of the deformed samples increased. However, the strength began to decrease after a critical button diameter (i.e., 8-mm weld button for 20 cycles) — Fig. 12. By increasing welding current and welding time, the amount of the fused metal increased, so the button diameter increased, and the button height nearly reached the sheet thickness but then decreased with increasing electrode indentation. The strength also increased up to a limited value by decreasing button height and increasing electrode indentation depth, and then it started to decrease due to thinning of the cross section of the button as a result of high heat input — Figs. 13 and 14. When the button size ratio vs. tensile shear strength diagram was investigated, it was seen that with increasing button size ratio, strength of the deformed samples decreased — Fig. 15. Determining the Weld Lobe A graphical explanation of the ranges of welding parameters over which acceptable spot welds are formed at a constant electrode force is known as a “spot weld lobe curve” (Ref. 26). The spot welds defining the lower limit of the lobe curve are undersize, resulting in weaker nuggets. On the other hand, the spot welds at the upper limit of the lobe curve are large, resulting in severe expulsion, which decreases the weld WELDING RESEARCH strength. The selection of the lower limit of the weld lobe is determined by the specific demand of automakers. The weld lobe diagram generally is developed with regard to the welding parameters vs. button size. However, some researchers developed a weld lobe diagram based on electrode indentation depth and strength in resistance spot welded steels (Refs. 25, 27). In the present study, weld strength was used as a criterion to assess the weld quality. It can be seen from the left limit of the diagram, when indentation is small, low strength and button size were achieved. High welding current and weld time increase electrode indentation, which may exceed an acceptable limit, causing severe expulsion (Ref. 27). Weld strength reduces due to expulsion. The right limit of weld lobe was drawn according to 80% and 100% of maximum strength, while the left limit of the weld lobe was established according to indentation depth equal to about 25% of sheet thickness. The weld lobe of deformed TRIP800 steel based on welding parameters is shown in Fig. 16. If higher strength (100%) is desired, 9 kA welding current and 15 cycles welding time should be applied. However, 7 kA welding current and 25 or 20 cycles welding times are advised. The 25% sheet thickness limit is exceeded in these conditions. When a high surface quality rather than strength is desired, the welding current range of 3 to 4 kA and 20 to 25 cycles welding times should be applied. When the 80% of strength and acceptable surface quality are desired, the welding current range of 5–6 kA MARCH 2016 / WELDING JOURNAL 81-s A B
Welding Journal | March 2016
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