Abstract
These days, one of the main targets in automotive industry is to reduce the weight of vehicle as well as increase the strength. To accomplish this goal, laser heat treatment has been introduced. Laser heat treatment is a locally hardening process which results in little deformation due to the characteristic of material self-quenching in very small area. In addition, it is more desirable process than hot forming since overall brittle characteristic and large thermal deformation shown in hot forming can be avoided. The objective of this study is to apply laser heat treatment for front side member which needs high performance of crash-characteristic behavior. In general, front side member is manufactured by the tailor welded blank(T.W.B) which is made by laser welding of different thicknesses of two metal sheet. There are some problems at TWB such as spring-back in welding part due to different hardness between high strength sheet and low strength sheet. In this study, front side member has been produced by the metal sheet which has a constant thickness, and locally heat treated with diode-laser where a high strength is required. During laser heat treatment procedure, laser power, beam diameter, and laser scanning speed are very important parameters to achieve desirable results. For mass production, beam diameter and laser scanning speed was predetermined. Thus, laser power(or surface temperature) is the only variable parameter which is controlled during the heat treatment process. In order to find optimized process parameter, tensile tests for each sample have been conducted, and numerical analysis of crash behavior has been carried out to find the vulnerable area at front side member. To validate the results, drop test has also been conducted. Front side member can be divided by two regions; one is a crash energy absorbing region and the other is an intrusion prevention region. To obtain superior crash characteristics compared to those of conventional component, laser power has carefully been controlled to obtain different strength in each area. Thus, the area where crash energy absorbing capability is important has been heat treated, and two times larger strength has been achieved in this area compared to conventional part. In addition, at the area where intrusion prevention is important, three times larger strength has been obtained. To validate the improved performance of laser heat treated part, crash analysis using LS-Dyna and drop test have been carried out. As a result, it is found that total 8% of weight reduction is achieved compared to that of conventional front side member maintaining similar internal energy and obtaining higher hardness.
KEYWORDS – Laser heat treatment, Front side member, Drop test, Internal energy, displacement