Papers by Author: Woong Seong Chang

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: Magnesium alloys are becoming important material for light weight car body, due to their low specific density but high specific strength. However they have a poor weldability, caused by high oxidization tendency and low vapour temperature. In this study, the welding performance of magnesium alloy was investigated for automobile application. The material was rolled magnesium alloy sheet contains 3wt%Al, 1wt%Zn and Mg balance. The effects of filler wire addition was investigated on 2kW Nd:YAG laser welding. For the results, the mechanical properties of welded specimen were similar with base metal in laser welding with and without filler wire. The bridging ability was improved with filler wire without weld properties deterioration on laser welding of magnesium alloy.

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: Influence of heat input on the tensile strength and impact toughness of multipass weld metal made with AWS E81T1-Ni1 metal-cored wire was investigated. Welding parameters such as current, voltage and travel speed were varied independently to get different heat inputs. When it was increased by varying current, tensile strength of the weld metal increased even if more primary ferrite and wider columnar grains were observed. The increase is attributed to the higher recovery ratio of deoxidizing elements such as carbon, manganese and silicon due to the shorter reaction time in both wire tip and arc column. It also showed that impact toughness was influenced by the formation of reheated weld metal by subsequent passes and it decreased continuously with an increase of the amount of coarse grained region in the reheated weld metal.

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.