Papers by Author: Sung Min Joo

Abstract: Analytic procedure for dissimilar materials welding problem by using developed in-house solver is applied on butt and lap jointed model. In addition, the procedure of data transferring between commercial package and in-house solver for the preparation of input data for in-house solver has been developed. Therefore we can use the commercial package as pre and post processor for in-house solver and the results from in-house solver, for example, welding residual stress can be exported to commercial package as initial value to the model and then further analysis with the application of external loading can be carried out. For the similar material welding the welding residual stress has been decided by temperature dependent material properties that are input to the source program. In the case of dissimilar welding problem due to the difference of expansion and shrinkage rate between aluminum and steel there has been a slight variation in this dependency. Since the aluminum has large thermal expansion coefficient and the mechanical melting point is lower than steel, the order and level of mechanical behavior like stress history become different. The degree of mechanical deterioration of dissimilar materials welded model has been assessed with various view aspects, namely, welding residual stress, plastic strain, equivalent plastic strain and plastic work distribution and it has been revealed that Al5052 is mechanically more sever than SPCC for same heat input.

Abstract: With the intention of improving butt or lap joint of dissimilar materials, specially devised weld beads together with lap and butt-joints were produced between A5052 and SPCC, where A5052 butt-joint was melted by heat-conduction of SPCC weld bead in addition to the formation of a limited weld fusion zone at the lap part in A5052 alloy. The thickness of intermetallic compounds at the butt-joint interface was approximately 2 μm and free of cracks. It was also revealed that crack-free lap weld metals were formed between aluminum alloy and steel when the penetration was controlled to be of less than 0.3 mm in depth at small heat input. It was moreover found that the majority of a laser weld fusion zone solidified as alpha(bcc)-iron phase containing small amount of aluminum, and cracks were absent in the case of hard intermetallic (AlxFey type) layer of less than 10μm zone. It was confirmed that a weld with lap and butt joints possessed high strength (leading to the load 3500 N to 4,380 N for 40 mm width specimen). In addition, SPCC and A1100 or A5052 were subjected to lap welding with a cw YAG laser, where one to three passes were performed to produce wider bonded areas. Dissimilar steel and aluminum joints with good mechanical properties were obtained, since the fracture occurred in the aluminum alloy base metal in the tensile test. It is concluded that welded joints of high strength can be produced between aluminum alloy and steel with proper devices.