Papers by Author: Heung Ju Kim

Abstract: In attempts to improve the performance of dissimilar joints between AZ31 Mg alloy and different Al alloys, solid state joining processes such as Magnetic Pulse Welding (MPW) and Friction Stir Welding (FSW) were applied for minimizing the formation of brittle intermetallic phases. MPW process has been concentrated mainly on round section tube to tube and tube to bar welds. Mg alloy AZ31 has been successfully welded to pure Al A1070 as well as to Al alloy A3003. Tensile test clearly showed the MPW welds were stronger than the weaker of the base metal so failure occurred in aluminum base metal. While FSW process for the dissimilar joint between AZ31B/A6061 alloys with a thickness of 2mm revealed optimum weldability under the conditions of travel speed of 0.8mm/sec and tool rotation speed of 850rpm. For the sound dissimilar joint, the maximum tensile strength of 179 MPa, which was about 80 % of the Mg base metal tensile strength, has been obtained.

Abstract: Friction spot joining was used to make lap joints on strips of 5052 and 6111 aluminum alloys. The influence of joining parameters such as tool rotation speeds, plunge depths and dwell times on the weld joint properties were evaluated. A wide range of joining conditions could be applied to join Al alloys without defects in the weld except for certain welding conditions with an insufficient heat input. The microstructures and hardness variations in the welds were discussed. The microstructures of welds, corresponding to the pin hole, have dynamic-recrystallized grain similar to stir zone in FSW weld. In hardness distribution, minimum hardness region was located about 6-mm away from the weld center, corresponding to the shoulder radius of the tool. For each weld the results from tensile-shear tests are also presented. For sound joints without defects, tensile shear fractured load of weld joints was higher approximately 230% than acceptable criteria of tensile shear strength of electrical resistance spot-welded joints for aluminum (MIL-W-6858D).

Abstract: Dissimilar friction spot lap joining of Al5052 and Al6022 sheet has been investigated using a combination of joining parameters, thickness and upper plate material. The joining parameters such as tool rotating speed, plunging depth and joining time have been considered. The maximum tensile shear strength has been observed at tool rotating speed of 1000rpm, dwell time of 2.5sec, plunging depth of 1.8 mm and Al 6022 as upper plate. The maximum tensile shear fractured load of the joint was about 80% of that of the similar Al 5052 alloys joint.

Abstract: In an attempt to optimize the friction spot joining process of Al alloys for automobiles (Al 5000 and Al 6000 series, thickness 1mm), effects of joining parameters such as tool rotating speed, plunging depth and dwelling time on the weld joints properties were evaluated. Experimental tests were carried out for lap joined Al plates. A wide range of joining conditions could be applied to join Al alloys for automobiles without defects in the weld zone except for certain welding conditions with an insufficient heat input. The microstructures and hardness variations in the weld regions are discussed. The microstructures of welds, corresponding to the pin hole, have dynamic-recrystallized grain similar to stir zone in FSW weld. In hardness distribution, minimum hardness region was located about 6-mm away from the weld center, corresponding to the shoulder radius of the tool. For each weld the results from tensile-shear tests are also presented. For sound joints without defects, tensile shear strength of weld joints was higher approximately 230% than acceptable criteria of tensile shear strength of electrical resistance spot-welded joints for aluminum (MIL-W-6858D).

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.