Welding Defects And Mechanical Properties Of Friction Stir Welded 2219 Allminum Alloy Joints

Friction stir welding (FSW) is a new solid-state joining technique, which has lots of prominent advantages, such as high quality, low cost and low deformation. With the development of this technique and the requirement for enhancement of joints, the study of defects in FSW joints and the influence on joints properties is focused on. 2219 aluminum alloy plates, 2.5-mm-thickness and 7.5-mm-thickness are friction stir welded in order to disclosure the formation mechanism of FSW defects and the correlation of defects, welding parameters, microstructures and mechanical properties.The analyses of joints properties demonstrate that the room-temperature tensile properties of both thick joints share the same law of variation, and the welding parameters have an optimum range. Within this range, the maximum tensile strength of 2.5-mm thick FSW joint is 295 MPa, which is 65% of that of the base metal (BM), as for 7.5-mm thick joints, which is 343 MPa, and the strength factor is up to 0.8. The mechanical properties tested in liquid nitrogen surrounding are higher than that in room temperature. The impact properties of both thick joints are obviously increased comparing to those of BMs. And the bend property of 2.5-mm thick joint is lightly lower than that of BM, and as for 7.5-mm thick joint it is higher than it's BM.The experiment results indicate that there are two basic types of defects in FSW joints, external and internal. The external defects include groove and fash, and the internal defects include cavity and kissing bond. The welding parameters, including welding speed and rotation speed of welding tool, and experiment condition are the main factors that influence the generation of FSW defects, when neither of them is proper it'll cause overheat of weld or inadequate plastic flow of materials, and lead to defects generation finally.Micro-structural analyses show that the cross section of FSW joint can be divided into weld nugget zone (WNZ), thermal mechanically affected zone (TMAZ), and heat affected zone (HAZ). WNZ is composed of fine equiaxed grains because of significantly dynamic recrystallization, and due to the strong breaking effect from the welding tool, the precipitates are fine and homogeneous in this zone. In TMAZ the effects of heat and breaking are relatively lower, the grains are deformed to bend and the precipitates are relatively grown. Grains and precipitates are coarsened in HAZ, where there is only heat effect in this zone, so HAZ becomes the weakest area in the joint. The dimension of grains in different areas in the joint and the cross section pattern of FSW joint are influenced by the welding parameters.