Won-Bae Lee

The improvement of mechanical properties of friction-stir-welded A356 Al alloy

Abstract The joint characteristics of friction-stir-welded A356 alloys, especially concerning the improvement of mechanical properties at the weld zone, were observed with various FSW (friction stir welding) speeds. The microstructures of the weld zone are composed of SZ (stir zone), TMAZ (thermo-mechanical affected zone) and BM (base metal). The BM shows the hypoeutectic Al–Si dendrite structure. The microstructure of the SZ is very different from that of the BM. The eutectic Si particles are dispersed homogeneously in primary Al solid solution. TMAZ, where the original microstructure was greatly deformed, is characterized by dispersed eutectic Si particles aligned along the rotational direction of the welding tool. These regions are mainly formed at the advancing side and the upper region of the retreating side. The mechanical properties of the weld zone are greatly improved in comparison to that of the BM. The hardness of the weld zone shows more uniform distribution than that of the BM because some defects are remarkably reduced and the eutectic Si particles are dispersed over the SZ. The tensile strength of the SZ is also greatly elevated and shows over 178 MPa, i.e. almost 120% that of the BM. Therefore, the FSW is well applied to the joining of the cast Al alloys.

The joint properties of dissimilar formed Al alloys by friction stir welding according to the fixed location of materials

Abstract The mechanical properties of the weld mainly depended on the materials fixed at the retreating side because the microstructure of the stir zone was mainly composed of the materials fixed at the retreating side. The onion ring pattern was observed like lamellar structure stacked by each material in turn.

Joint properties of friction stir welded AZ31B– H24 magnesium alloy

Abstract The weldability of friction stir welded hot rolled AZ31B-H24 magnesium alloy sheet, 4 mm in thickness, was evaluated, varying welding parameters such as tool rotation speed and travel welding speed. Sound welding conditions depended mainly on sufficient heat input during the welding process. Insufficient heat input, which was generated in the case of higher travel speed and lower rotation speed, caused an inner void or lack of bonding in the stir zone. The microstructure of the weld zone was composed of five regions: base metal, heat affected zone, thermomechanically affected zone, stir zone I and stir zone II. Unlike the general feature of friction stir welded aluminium alloys, the grain size of the weld zone was larger than that of the base metal. Stir zones I and II were characterised by partial dynamic recrystallisation and full dynamic recrystallisation, respectively. The hardness of the weld zone was lower than that of the base metal owing to grain growth. A wider range of defect free welding conditions was acquired at higher tool rotation speed and lower welding speed. The maximum tensile strengh was 240 MPa, which was ~85% of the base metal value of 293 MPa. The fracture location was close to the stir zone.

Evaluation of the microstructure and mechanical properties of friction stir welded 6005 aluminum alloy

Abstract The microstructural change related with the hardness profile has been evaluated for friction stir welded, age hardenable 6005 Al alloy. Frictional heat and plastic flow during friction stir welding created fine and equiaxed grains in the stir zone (SZ), and elongated and recovered grains in the thermomechanically affected zone (TMAZ). The heat affected zone (HAZ), identified only by the hardness result because there is no difference in grain structure compared to the base metal, was formed beside the weld zone. A softened region was formed near the weld zone during the friction stir welding process. The softened region was characterised by the dissolution and coarsening of the strengthening precipitate during friction stir welding. Sound joints in 6005 Al alloys were successfully formed under a wide range of friction stir welding conditions. The maximum tensile strength, obtained at 507 mm min-1 welding speed and 1600 rev min-1 tool rotation speed, was 220 MPa, which was 85% of the strength of the base metal.

The mechanical properties related to the dominant microstructure in the weld zone of dissimilar formed Al alloy joints by friction stir welding

The joint properties of dissimilar formed Al alloys, cast Al alloy and wrought Al alloy, were examined with various welding conditions. Friction stir welding method could be applied to join dissimilar formed Al alloys which had different mechanical properties without weld zone defects under wide range of welding condition.The weld zone of dissimilar formed Al alloy exhibited the complex structure of the two materials and mainly composed of the retreating side material.The mechanical properties also depended on the dominant microstructure of the weld zone with welding conditions. The different mechanical properties of the weld zone with welding conditions were related to the behavior of the precipitates of wrought Al alloy and Si particles of cast Al alloy. The higher mechanical properties of the weld zone were acquired when a relatively harder material, wrought Al alloy, was fixed at the retreating side.

Void Free Friction Stir Weld Zone Of The Dissimilar 6061 Aluminum And Copper Joint By Shifting The Tool Insertion Location

Abstract Dissimilar 6061 aluminum alloy and copper were friction-stir welded with various tool rotations and welding speeds to achieve a void free weld zone. We could not acquire the joint without void by changing the tool rotation and welding speed. However, a sound joint of the friction-stir welded dissimilar Cu/ 6061 Al alloy without voids was finally achieved by shifting the tool insertion location, where it deviated from the centerline of the two materials. The weld zone produced a very complex structure mixed with a dynamic recrystallized structure of each material, intermetallic compound and unpredictable oxides. These compounds in the weld zone were formed by the chemical reaction and the absorption of oxygen during the welding process.

Effects of fine fiber structures on the mechanical and electrical properties of cold rolled Cu-Ag sheet

The recent progress in the electrical, electronic, and automobile industries requires conductive materials to possess not only a high conductivity, but also high strength, as well as reasonable cost. In order to reach this goal, extensive studies have been carried out on various Cu alloys including Nb [1], Fe [2], Cr [3] and Ag [4] in situ composite with cold deformation and intermediate heat treatment. Among many Cu composite alloys, Cu-Ag alloy has a eutectic two-phase structure. During the cold deformation such as rolling, drawing, forging and swaging, two phases composed of Cu-rich solid solution and Ag-rich solid solution, are compressed and elongated along the direction of deformation; the two phases form a fine fiber microstructure. Severe cold deformed Cu composites have ultra-high tensile strength which is larger than the value predicted by the rule of the mixture of the two materials. The ultra high strength can be explained by the very fine fibrous microstructure and the high dislocation density between the interfaces of two materials. The electrical conductivity is generally inversely proportional to the mechanical property. This problem can be overcome by an intermediate heat treatment which results in the precipitation of a second phase. It has been reported that the mechanical and electrical properties of cold rolled Cu composite mainly depended on the diameter of the fiber and the inter fiber distance without intermediate heat treatment [5, 6]. This study aims to produce information on the relation between fiber shape and the mechanical and electrical properties with Ag contents and reduction ratios. Cu-Ag alloys containing 10, 20, 24, 30 wt% Ag were melted in VIM (vacuum induction furnace). Cu-Ag composite alloys were prepared from the pure oxygen free copper and pure Ag (99.97 wt%). Each charge was melted and then cast in a steel mold. After holding for 2 h, the ingots were quenched. For the homogeneous treatments, any surface defects were cut from the cast ingot and then the ingot was heat-treated at 873 K for 20 h. Each ingot was sectioned to 8 mm thick-

Effects of the local microstructures on the mechanical properties in FSWed joints of a 7075-T6 Al alloy

The grain structure, dislocation density, and precipitate behavior in various regions including the dynamically recrystallized zone, the thermo-mechanically affected zone, and the heat-affected zone of a friction stir welded Al 7075-T6 were compared with those of the base metal. The mechanical properties of the friction stir weld zone were significantly affected by the microstructural issues. In the dynamically recrystallized zone, the grain structure was fine and equiaxed and the grains were separated by high-angle grain boundaries, which acted as obstacles for dislocation movement. The dislocation density was high and precipitates with diameters of about 70 nm were homogeneously distributed throughout the dynamically recrystallized zone. This zone showed the highest mechanical strength as measured by Vickers hardness and tensile tests. However, in the heat-affected zone the presence of coarsened grains and precipitates lead to failure during tensile tests. This zone had the lowest mechanical strength. The transversal tensile strength of the joints without weld defects attained 97 % of that of the base metal with a tool rotation speed of 1600 rpm.