| With advancing in science and technology, the fine performance of aluminum and its alloys is constantly recognized and used, and their use is expanding. The aluminum and its alloys have some unique features, such as low density, good conductivity, corrosion resistance, thermal performance, high specific strength and easy processing. The history of joining aluminum has near 100 years and the welding technology of aluminum and its alloys has been developed. However, there are many difficulties in welding aluminum by the conventional fusion welding method, such as oxidation, high heat transfer coefficient, pores, cracks and other defects, which limits its application greatly. The friction stir welding (FSW) which provides a new effective way to weld aluminum has been invented. The friction stir welding is a solid-phase joining technology which is efficient, low consumption, low cost and meeting the environmental requirements. Compared with the fusion welding, FSW is no splash, no dust, no pore, no need to add wire and shielding gas. The fatigue performance, tensile properties and bending properties of FSW joints are good. The joints don't produce various defects that there exist in fusion welding joints, and have forming well. The internal stress and deformation in the FSW joints are small after welding. Friction stir welding is very suitable to weld heterogeneous or homogeneous aluminum and its alloys, because the melting point of aluminum and its alloys is not high (482～660℃) and the stirring head materials with working temperature higher than 700℃is easy to access. In addition, the control parameters of FSW are less which is easy to automation. The forms of welded joints are various. FSW is suitable for joining butt, lap, fillet and T lap joints. With further development of friction stir welding, study on the quality of welding affected by the welding processing parameters has become more and more in-depth.The issue mainly includes: the structure and mechanical properties of 3mm thick sheet aluminum alloys FSW butt-joints were investigated, using friction stir welding equipment which was converted from a milling machine and the homemade FSW tool, and the influences of tool rotating speed and welding speed on weld quality, structure and mechanical properties were investigated. In addition, the inherent essence of FSW was researched, in order to provide a theoretical and practical basis for FSW technology applications. Based on the experimental results, some conclusions have been drawn as follows:(1) Similar FSW butt-welds of 6082 Al and 5083 Al are performed by using a milling machine, and the joints with excellent formability and mechanical property are obtained when proper processing parameters are used. Since the joint experienced heat history and stir effect, typical four distinct microstructure zones, i.e., nugget zone (NZ), thermo-mechanically affected zone (TMAZ), heat-affected zone (HAZ), and parent zone are formed. The nugget zone where fine equiaxed grains were found appears basin-shaped and elliptical"onion ring"structure, and has same phases as parent metal; there is an asymmetric phenomenon within TMAZ and boundary and flow lines on advancing side are more conspicuous than retreating side; The HAZ is very narrow, where the microstructure is as same as parent material.(2) The heat input during welding depends on processing parameters, and it increases with increasing the tool rotating speed when the other parameters are constant. The defects such as groove, pores and tunnel are observed when low rotating speed is used. The heat input decreases with increasing the welding speed and the pores and tunnel defects are also observed when the welding speed is too high. In the present work, fine weld structure is formed when welding speed and rotating speed used are 23.5mm/min and 1180rpm, respectively.(3) During friction stir welding, the weld metal becomes plastic state under friction heat cycle, and the plastic metal is transferred with rotating and moving of FSW tool and migrated in all three dimensions of space. The flow modes of plastic metal are different in three directions and also different at the different positions in the same direction, which are related to the morphology of the FSW tool, welding processing parameters, mechanical and thermal properties and flow properties of the welding materials.(4) Micro-hardness profiles of the FSW joints have a U-type or W-type. The minimum micro-hardness appears in the HAZ of the FSW advancing side, and the micro-hardness in the advancing side is lower than the retreating side. The welding speed has little effect on the micro-hardness, but it decreases slightly with increasing the rotating speed.(5) The fracture of FSW joints mainly occurs at HAZ on advancing side. When there are defects in joints, such as"kiss bonding", incomplete penetration, hole and tunnel defect, the weld fracture is observed. The fracture is a mixture of ductile and brittle. Generally, the brittle fracture is observed at top and bottom of the weld, and the ductile fracture is in the middle of the weld. The joint tensile strength decreases with increasing the welding speed but increasing within one limitation with the tool rotating speed increasing. The joint (5083Al) tensile strength reached 216.2MPa that is 68.64% of the base metal strength, when welding speed and rotating speed used are 23.5mm/min and 1500rpm, respectively.(6) When the welding parameters are improper, the defects are observed in joints, some of them can be seen in fusion welding joints and the others are FSW peculiar defects. Defects, observed in FSW joints of aluminum alloys, can be classified into two catalogs: a) Surface defects include flash, surface galling, groove and incomplete penetration. b) Inside defects include hole, tunnel defect, kiss bonding, zigzag line, crack, pores and inclusion. However, the flash and surface galling have little influence on microstructure and property of joints and it was better to have artificial remedy, but groove, incomplete penetration and inside defects severely affect the property of joints and their formation, must be avoided. The formations of these defects are closely associated with heat input during welding, which depend on process parameters. So, as long as setting proper welding parameters, it is possible to form a defect-free joint.
|
PDF Full Text Request