A Study On Dissimilar Friction Stir Welding Between Aluminum And Copper

Friction stir welding (FSW) is a new type of solid-state connection technique and has theadvantages of high efficiency, energy saving, environment friendly compared with traditionalfusion welding. FSW is particularly suitable for the welding of aluminum, magnesium andother light alloys. With the development of FSW, its application field has been greatlyexpanded, including high meltingpoint alloys joining and dissimilar welding. Aluminum andcopper are quite different in both physical and chemical properties, so defects are easy to formin their dissimilar welding, and the formation of brittle intermetallic compounds also has greateffects on the properties of the joints. In this paper, microstructure and mechanical propertiesof the monolith and dissimilar welding between1350pure aluminum and pure copperproduced by FSW were studied, and the influence of pin offsetting on performances ofdissimilar welds were also investigated.Effects of the―ω/v‖ratio on the microstructures and mechanical properties of1350purealuminum were investigated. With the welding parameters ranged as follows: rotation speed500~1500rpm, welding speed60~100mm/min. The results showed that defect-free joints wereobtained when ω/v＞8.3r/mm. Equiaxed grain structure was found in the stir zone (SZ), thearea of SZ increased with the increase of ω/v ratio. The profile of hardness distribution alongthe cross-section presented an "U" shape, the hardness of SZ was lower than that of the basemetal. When welding parameters were1000rpm-100mm/min, the joints showed the optimalmechanical properties with a tensile strength of187MPa, and an elongation11.9%, which is91.5%and93%of the base matal, respectively.For the welding of pure copper, the rotation speed was fixed at800rpm with the weldingspeed ranging among60~300mm/min. The results showed four typical regions in the jointsobtained under different parameters. SZ consists of recrystallized microstructure, and thegrain size became smaller as the welding speed increased, the minimum grain size of SZ wasabout8.4μm, grain size in thermo-mechanically affected zone (TMAZ) showed no significantchanges. The hardness curve was similar to an "U" shape, tensile specimens’ fracture locationwas located near the heat affected zone (HAZ) when the welding speed was less than300mm/min. Joints with its tensile strength of282.5MPa, and elongation of21.7%, wereachieveed with a welding speed of100mm/min, which were93.8%and79.2%of the parentmaterial, respectively.Pin insert into the weld centerline and offsetting were studied in the Al/Cu dissimilarwelding, respectively. Welding results showed that defect-free joints were obtained only when the copper placed in the advancing side, and the formability of joints had been greatlyimproved with the pin offsetting to aluminum. Sound joints can only obtained at a smallparameters range when the pin insert into the weld centerline, but when the pin offsetting toaluminum welding parameters had been greatly broaden. With the pin offseting, more uniformstructures can be observed in SZ. At the top of the nugget, small particles of copper andhybrid layered copper structures were observed, while the bottom of the joint was composedof dispersed small copper particles and a continuous vortex-like aluminum. Hardness in mostof area in SZ was ranged between aluminum and copper base metal. Joints with the tensilestrength and elongation were only79MPa and1.7%when the pin insert into the weldcenterline, and tensile properties of the welds increased to152MPa and6.3%when the pinoffsetting to aluminum. The fracture morphology showed a ductile-brittle hybrid mode. Al-Cuintermetallic compounds were not detected in the joints.Welding defects and material flow were analysised during dissimilar welding process inthis paper. Welding defects such as flash, holes, grooves, tunnel-type defects and incompletefusion are easy to form in dissimilar welding. In addition to welding parameters and weldingtool, the placement of base plates also had a great impact in defects generation. Material flowstate in SZ showed unequal in different location in the thickness direction. Materials near thetool shoulder demonstrated a more uniform patten, large blocks of copper were mostlyconcentrated in the middle part of the joints, and in the bottom Cu reduced greatly. In addition,the distribution of Cu in SZ was not continuous, which showed a comprehensive pattern.