-Effect of Alloying Elements on Expulsion in RSW of AHSS-
This study examined the effects of alloying elements, such as Cr, Si, Al, C and Mn, which are normally used to modify the composition of advanced high strength steels(AHSS), on the expulsion phenomenon during electric resistance spot welding(RSW) as well as the effects of alloying elements on the expulsion limit current (CExp.). Anequation for the CExp. prediction was derived from the correlation between the CExp. and alloying element: CExp. (kA) = 9.85 ? 0.74Si ? 0.67Al ? 0.28C ? 0.20Mn ? 0.18Cr. The coefficient of determination between the calculated and measured values was sufficiently high (R2=0.94) so that this equation could be used to predict the CExp. of AHSS by RSW. The extent of the effects of the alloying elements on the CExp. was in the order of Si, Al, C, Mn and Cr. From the correlations among the CExp. and resistivity (R), the coefficient of thermal expansion (α) and yield strength (Y.S1200℃), the CExp. was calculated to be CExp. (kA) = 9.3-0.057R -0.035α + 0.22Y.S1200℃ and the coefficient of determination was sufficiently high (R2=0.96). This equation could also be used to predict the CExp. of steel from its properties. Over the given composition range, the expulsion shape was a spray type in steel, where more than 0.5% Si was added due to the lower viscosity, melting point and high temperature yield strength.
-Effect of Alloying Elements on Tensile Properties in RSW of AHSS-
Regardless of the alloying elements, the specimens under 5t1/2(t=sheet thickness) were fractured at the interfacial failure mode.
Pull-out failure occurred at the softened zone which was the interface of base metal and HAZ. This zone was the place where hardness was the lowest in the welds and generated by tempering of the martensite due to the welding heat. Tensile-shear load (TSL) of the welds which showed pull-out failure (PF) mode was increased with FDsz and HvSZ. Interfacial failure (IF) occurred along the interface between two spot welds specimens. Shear load (SLIF) of IF mode was dependent only on the fractured diameter. From the equation to predict SLIF of the specimen showed IF mode and TSL of the PF mode, the SLIF=β?0.5?σBM.W.Q (HvN / HvBM.W.Q) ?π?(FD/2)2and TSL= α?t?σBM?(HvSZ/HvBM) ?(ND+1.5)). Correlation coefficients between measured value and calculated value were 0.98 and 0.97 respectively. In the low current (under 6.4kA) condition, IF occurred because the real nugget diameter (ND) was smaller than the critical nugget diameter (NDCri) regardless of the alloy elements, while the PF was in the high current (over 7.4kA) condition because the ND was smaller than the NDCri.