Laser welding of dissimilar metals aided by unsteady thermal conduction boundary element method analysis

Abstract

The development of highly efficient power and improved fuel costs are much desired for reasons concerned with global environment and energy saving, particularly in the automobile industry and for transport machinery. Accordingly, in recent years, materials such as aluminium and magnesium alloys, which are lightweight and have superior specific strength, are enthusiastically employed with the aim of reducing car body weight. There are only a few examples where most of the entire car body is constructed using lightweight aluminium alloys; in reality, a combination of conventional steel and light metal materials is generally employed from aspects of workability and cost. Where there is a combination of two or more types of materials which are metallurgically quite different, techniques to join them are, of course, crucial. This is particularly so for combinations of dissimilar metals, such as steel/aluminium and steel/titanium, where it is known that brittle and hard intermetallic compounds are formed at the fusion zone following fusion welding using conventional techniques, the joint performance is markedly reduced1, and it is extremely difficult to produce practical and reliable weld joints. Amongst various joining techniques, solid-state welding is applicable for combinations of most metal materials;2 for example, even for combinations of steel materials and aluminium alloys, there are detailed reports concerning diffusion joining3,4, friction welding5–9 and ultrasonic welding10,11. However, there are some cases where it is difficult to apply most solid-state welding techniques due to restrictions of the arrangement and member configuration so it is hoped to develop more practical and highly applicable dissimilar metal joining techniques. Accordingly, the spot welding technique has been proposed where aluminium clad steel is employed as an insert metal.12–14 By contrast, a report was released by E. Schubert and others15 on a dissimilar welding process for the direct joining of steel and aluminium alloy with the use of a YAG laser. According to this report, the joints thus produced indicate adequate performance in both strength and fatigue tests. In addition, Katayama and others16 carried out lap welding of aluminium alloy/stainless steel and pure aluminium/pure iron, and demonstrated that the combination of pure aluminium/pure iron can tolerate a wider range of welding conditions and achieve a more satisfactory joint. However, a detailed investigation is required for individual cases in order to apply to combinations of various other materials and configurations of members; in practice, a very large number of experiments are required in order to find joining conditions, so an enormous amount of time and effort is required.