마찰교반점용접을 이용한 알루미늄 합금과 초고강도강(Al5083-O/DP590)의 이종접합 특성 연구

Abstract

The restrictions of energy efficiency, hazardous substances and greenhouse gas exhaustion are getting stricter globally. This rules effects directly and indirectly to international economy. Especially, studies about improvement of body in automobiles and weight lightening are in progress to follow the environmental regulation, the goals of this study are eco-friendly and for the improvement of fuel efficiency. High strength steel and hot stamping are using as weight lightening process but it effects are not significant. High strength steel is replaced by aluminum and magnesium for lighter and harder. Welding these materials with steel by dissimilar materials welding technique is considered as realistic alternative. It is hard to make good properties in welding zone during dissimilar welding between steel and aluminum due to formation of intermetallic compound(IMC). The reason of IMC created is due to difference in melting point between steel and aluminum. Also conventional fusion welding has some defects such as thermal deformation, residual stress, crack are representative defects. For this reason, a new welding method is needed to weld aluminum alloys in automobile industry. This limitation of fusion welding overcame by using of Friction stir joining(FSJ). It is welded by frictional heat, force, plastic flow which appears below melting temperature. This compensate the defects of fusion welding, used similar and dissimilar material welding. In this study, the effect of various conditions (plunge depths, plunge speeds, rotation speeds) on weldability (mechanical and metallurgical characteristics) in dissimilar materials (Al5083-O/DP590) by FSJ has been clarified. Frist, the result did not show a specific trend in the tensile shear strength in accordance with the rotation speed (300~500rpm). When the rotation speed of 300rpm, a maximum tensile shear strength of 3.0kN and gained strength at the lowest rotation speed 400rpm to 2.6kN. When welding in accordance with the plunge speed (0.3~1.0mm/s), 0.3mm/s gained the maximum tensile shear strength of 6.7kN. The faster plunge speed decreased the tensile shear strength, in particular, the plunge speed 1.0mm/s has been reduced more than 30% tensile shear strength of 0.5mm/s. It is expected that the influence on the tensile shear strength affect the softening and bonding properties of the bonding material to avoid friction and lack of stir by rotation of the tool. The results of plunge depth conditions shown that when plunge depth was 0.7mm, the tensile shear strength reached to the maximum failure load of 6.5kN due to the slightly decreased thickness of aluminium and the improved plastic flow, the maximum tensile shear strength could be acquired. At 0.7mm of plunge depth, the intermetallic(IM) layer was around 5~6μm that was the thinnest. At 1.5mm of plunge depth, IM layer increased to 9~30μm. In this case, IM layer largely observed and the thickness of aluminum was decreased by force that made the tensile shear strength to be the lowest.