Papers by Author: Han Sur Bang

Abstract: In this paper, the characteristics of bond interface and bonding mechanism were investigated with peeling-off method. The fracture was observed and interfacial composition was certified by map scanning of EDX (Energy dispersive X-ray analysis). Based on the features of interfacial characters, the actual joining area mainly distributed at bond periphery; non-bonded at bond center. When the bonding time was lower, the ratio of the bond length to its width was larger and elemental aluminum distributed discontinuously on the bond fracture, primarily at the periphery. After aging, the fractures were also analyzed and Cu2Al3 intermetallic compound (IMC) was identified. The phenomena of bond interfacial tracings were analyzed, and the bonding mechanism was ascribed to plastic flow analyzed by finite element method based on the contact issues.

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: In the present study, an investigation has been carried out on the friction stir welding (FSW) of two magnesium alloys. Hot-rolled and Die-casting plates of AZ type magnesium alloys were successfully joined by friction stir welding. AZ31B-H24 and AZ91C-F plates with the thickness of 4mm were used, and the microstructural development in stir zone (SZ) was investigated using optical and scanning electron microscopes. Hardness of SZ in AZ31B-H24 alloy has been slightly decreased due to the coarse structure. On the other hand, Hardness of SZ in AZ91C-F alloy has been remarkably increased due to very fine recrystallized grain structure. The result of EPMA showed Al Solid solution hardening by Solid solution of β intermetallic compound to α-Mg. While tensile strength of the FSWelded AZ31B-H24 alloy showed about 80% values compare to Base metal, AZ91C-F showed about 68% values due to strongly affected by formation of the intermetallic compounds, β-Al12Mg17.

Abstract: Friction stir welding is being attracted and developed as an efficient joining method in the manufacturing field of Automobile, Aerospace and Ship building industries. As the FSW develops, more scientific research work investigations in this field have also been increased. Recent studies in FSW have revealed that both heat and metal flow characteristics have a non-symmetric complex nature about the tool axis. But until now there is no efficient 3D- heat flow model to be comparable with the experimentally measured values. The body of the work covered FSW of Al6061 and its thermal distribution based on a nonsymmetrical analytical model for the heat input in to the matrix of Al plates from FSW tool due to the effect of combined translation and rotational motion of the tool pin and shoulder. Finally the 3D- finite element heat transfer analysis program has been used to plot the heat distribution at the Friction Stir Welded joint in Al 6061 plate. The work concludes that the heat distribution result obtained from FE analysis has a reasonable agreement with the experimentally measured values.

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.

Abstract: It has been well known that ductile fractures of steels are accelerated by triaxial stresses. The characteristics of ductile crack initiation in steels are evaluated quantitatively using two-parameter criterion based on equivalent plastic strain and stress triaxiality. It has been demonstrated by authors using round-bar specimens with circumferential notch in single tension that the critical strain to initiate ductile crack from specimen center depends considerably on stress triaxiality, but surface cracking of notch root is in accordance with constant strain condition. This study fundamentally clarifies the effect of strength mismatch, which can elevate plastic constraint due to heterogeneous plastic straining under static loading, on critical conditions for ductile cracking from the pre-notch root. In order to evaluate the stress/strain state in the pre-notch root of specimens, a thermal elastic-plastic finite element (FE) analysis has been carried out.