The Microstructure And Mechanical Properties In Weld Heat Affected Zones Of 6xxx Aluminum Alloys

Al-Mg-Si-Cu alloys(6xxx series) are widely used in industrial manufacture due to its excellent properties, such as high strength to weight ratio, good formability, weldability and corrosion resistance. For the 6xxx series aluminum alloys, springback after forming are not easy to occur, and quick enhancement in strength and dent resistance after paint baking is typical for these alloys, therefore they are widely used in car-body panels. Welding is an important connection method to join components made of metallic materials and imposes a rapid heating and cooling process on the base metal. The non-isothermal heating process undergone by the alloys during the thermal cycle of welding can change the local microstructure and the local mechanical properties of the base metal. For the demand of industrial application of 6xxx aluminum alloys in automobile and high-speed strains, the effect of welding process on the local microstructure and mechanical properties as well as the local paint bake response of the body panel are investigated in this paper. At the same time, post-weld heat treatment is carried out to improve the mechanical properties of joints. Moreover, the change of the microstructure is characterized systematically. The main conclusions are summarized as follows:(1) The hardness of the heat affected zone near the base metal laser-welded in T4 temper exhibits a groove before paint bake and a peak after paint bake. It can be explained by the fact that the clusters forming in T4 temper transform to GP zones during the welding process. It is worth mentioning that not all the clusters has transformed to GP zones due to the fast welding speed. The GP zones can act as nucleus of precipitates and directly transform to pre-???/??? phases during the bake process, contributing to the experimentally observed hardness peak. The paint bake response is greatly decreased after welding process in the heat affected zone near fusion zone welded in T4 P temper. It can be explained by that the GP zones forming in T4 P temper will dissolve into the Al matrix during the welding process because of the high weld thermal cycle temperature.(2) The mechanical properties of the joint after post-weld aging treatment can be enhanced compared to the as-welded joint, while the hardness of the weld seam is still the lowest region because the formation of massive second particles during the welding process consumed many solute atoms needed to form strengthening precipitates. The mechanical properties of heat affected zone in the joint under-aged before welding can be fully recovered after post-weld heat treatment, while the heat affected zone in the joint peak-aged and over-aged before welding can not be fully recovered after post-weld heat treatment. It can be proved by the hardness grooves in the hardness profiles, which is characteristic of softening zones. The precipitates in the softening zone coarsen apparently as revealed by the transmission electron microscopy(TEM) observation.(3) An abnormal grain growth is observed on both sides of fusion zone through the EBSD characterization when a solid solution treatment for 0.5 h at 560 ? is carried out on the weldments. Furthermore, the abnormal grain is a single crystal with a random orientation. Moreover, this phenomenon is not observed when the solid solution treatment is carried out at 350?, 450 and 500? ?. The abnormal grain growth seems to occur in a few seconds during solution treatment at 560 for a cert? ain period of time. Once occurring, abnormal grain growth does not continue to grow with time during solution treatment at 560?. This singular phenomenon is speculated to be related with the interface between heat affected zone and fusion zone.(4) The hardness of the heat affeated zone decrease drastically in the Al-Mg-Si side of the MIG-welded Al-Mg-Si and Al-Zn-Mg dissimilar joint. It is explained by the fact that precipitates formed before welding dissolve totally into matrix. Clusters form during the room temperature storage after welding but are not as effective as precipitates in strengthening the alloys.