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진우성,Lalachan Arun,Murugan Sivaprasad,지창욱,박영도 대한용접접합학회 2022 대한용접·접합학회지 Vol.40 No.6
Although the influence of weld process variables on LME cracking was known to be significant, limited studies have been conducted on the effect of process variables with systematic approaches in the equivalent nugget growth behavior and heat input. This study aimed to identify the effect of weld process variables on LME sensitivity with the equivalent nugget diameter and underlying mechanism with induced tensile stress for cracking. Among the welds with equivalent nugget diameters in the combination of different welding current and time, higher LME sensi- tivity was observed with the high welding current and short welding time combination than that with the low weld- ing current and long welding time combination for the equivalent nugget diameter. Because a high current and short time combination resulted in faster weld nugget growth than the low welding current and long welding time combi- nation, it rapidly increased the surface temperature along with the cooling from the electrode. These combined ef- fects induced a higher thermal gradient and thermally induced tensile stress on the weld surface, satisfying the con- ditions of the LME cracking. The simulation results also confirmed that the critical weld cycle time of the LME cracking (tc), which is the cross point between the nugget growth diameter and a contact diameter of the electrode, could be different with the combination of the weld process variables with the equivalent weld nugget size. Therefore, tc can be applied for the sensitivity index of LME cracking of the resistance spot weldment considering complex weld variables.
Effect of Top Sheet Materials on Joint Performance of Self-Piercing Riveting
Karim Md Abdul,Murugan Sivaprasad,배기만,백종진,지창욱,노우람,이한주,Jang Will,김덕봉,박영도 대한용접접합학회 2022 대한용접·접합학회지 Vol.40 No.6
Three types of self-piercing riveting (SPR) joints, i.e., steel/aluminum, carbon fiber reinforced polymer (CFRP)/ aluminum, and aluminum/aluminum, were constructed using three different top sheet materials with the same alumi- num alloy as the bottom sheet. The effects of the top sheet material on the joint quality and mechanical behavior were evaluated. The top sheet materials’ characteristics dominate the rivet piercing process and the consequent inter- lock distance. The high-strength steel top sheet requires a comparatively higher rivet setting force and induces early flaring of the rivet tail, resulting in a larger interlock distance. Though the CFRP needs the highest rivet setting force to penetrate the rivet through the CFRP fibers, the CFRP/aluminum joint exhibits the smallest interlock distance be- cause of the SPR process-induced damages to the CFRP and subsequently less flaring of the rivet tail. In strength tests, the damaged CFRP sheet resulted in rivet head pullout of the CFRP/aluminum joints, which exhibited the low- est lap-shear and cross-tension strengths. In contrast, the steel/aluminum joints demonstrated the highest strengths because of their comparatively larger interlock distances. In addition to the experimental analysis, simulations re- vealed the rivet penetration and flaring mechanisms with various top sheet materials, and their respective joint qual- ity and strengths.