전기자동차 배터리 팩 케이스 용 A6061-T6의 균열무해화 평가

Abstract

As a solution to the problem of global warming, research and development of eco-friendly electric vehicles is being actively conducted. In particular, efforts are being made to reduce air pollution by using electricity or hydrogen as energy sources for electric vehicles to reduce their weight. Battery packs for electric vehicles are made of aluminum to reduce their weight. Aluminum is a useful material for solving low-carbon and recyclable problems. This is because aluminum can be used to save fuel through weight reduction, and can be recycled when scrapped or dismantled. If the weldability and fatigue properties are improved through friction stir welding (FSW) and shot peening (SP), it would be effective to apply aluminum to structural materials subjected to variable stresses. First, in terms of the mechanical properties of A6061-T6 through FSW and SP, the Vickers hardness, tensile strength, and yield stress were all smaller than those of the base metal (BM) specimen. As the heat input of the FSW test specimen increased, the nugget zone (NZ) area expanded, and there was a difference in the hardness by area. The mechanical properties were not proportional to the heat input, and the thermomechanically affected zone (TMAZ) on the advancing side (AS) was the weakest under all the conditions. This is because the material flow speed increased on the AS in the direction of the welding progress and tool rotation, forming a clear boundary with the mechanical properties. Second, using A6061-T6 for an electric vehicle battery pack, we developed electrical discharge machining (EDM) cracks with a crack depth (a) ranging from 0.1 to 0.3 mm. SP was performed after EDM, and a four-point bending fatigue test was conducted. The surface of the SP specimen was rough regardless of the SP time. Moreover, it was more than twice as rough as that of the BM specimen. The fatigue limits of the BM and SP BM specimens were similar. However, the BM crack specimen decreased as the crack depth increased, and the SP BM crack specimen exhibited a fatigue limit similar to that of the SP BM specimen at approximately a=0.2mm. Notably, harmless cracks with crack depths of approximately a=0.2mm could be observed with SP. The size of the harmless crack was evaluated from the relationship between the stress intensity and threshold stress intensity factors of the SP BM crack specimen. The harmless crack size was determined by the crack depth (a), and the influence of aspect ratio was negligible in the diagram of harmless crack size evaluation Third, recently, a technique has been developed to render harmless surface cracks, which reduces the fatigue limit of metals by 20-60% by the effects of compressive residual stress induced by SP. This technique is called surface crack nondamaging technology. For the purpose of contributing to reliability improvement of high strength steel equipment by surface crack non-damaging technology, the following research was carried out. The tensile residual stress of FSW was added by SP, resulting in a larger compressive residual stress than that of the BM. The effect of the surface crack aspect ratio on the maximum harmless crack depth (a_hml) of A6061-T6 is evaluated for the residual stress distribution. The detectable depth was evaluated in terms of the relationship between a_hml and the maximum detectable crack depth (a_NDI) by non-destructive inspection.