| Due to excellent strength, corrosion resistance, processing performance, fine electrical and thermal conductivity properties, die casting aluminum alloys are widely used in electric appliance, automobile, rail transportation, aerospace and other fields. However, conventional fusion welding are generally difficultly applied on die casting aluminum alloys, due to the formation of blowholes by entrapped H2gas and hot crack. Friction stir welding (FSW) was invented as a new type of connection method by The Welding Institute (TWI) in1991. Its characteristic is the welding temperature which is below the welding workpieces’melting point. As a result. FSW provides a new way for welding some low melting point non-ferrous metals, such as aluminum alloys and magnesium alloys.In the article, the ADC12and6061aluminum alloys are welded by MIG and FSW respectively, using different welding process parameters. Then the mechanical properties, corrosion resistant and aging properties of the friction stir welded joints have been analyzed.Through the hardness test, it is found that friction stir welding joints have four zones: nugget zone(NZ), thermo-mechanically affected zone(TMAZ). heat-affected zone(HAZ) and base metal(BM). The NZ is mainly consisted of trivial equiaxial grain, exhibiting a higher average hardness. The TMAZ undergones great degree of bending deformation and the hardness is lower than NZ. The HAZ has mild coarsening phenomenon, leading to the minimum hardness.Through the tensile test, it is found that the strain of the FSW joints is above10%and the tensile strength is about65%of BM. The cracking occurred at the junction of NZ and TMAZ. The fracture has apparent torn edges and is full of different sizes of round or oval dimple.The V-type gap impact test was used to investigate the FSW joints’ impact toughness. In NZ, the crack is difficult to extend because of the tiny crystal grain. When extending, cracks have to change direction many times, which will consume more energy, exhausting more impact energy. The TMAZ’s impact toughness decline greatly. The HAZ’s impact absorbing energy is the lowest as the weakening area of the joints, only about50%of BM.. The impact fracture of NZ is composed of shear plane and fibrous zone. Shear plane is relatively smooth at the bottom of the gap, instantly formed due to the shear stress parallelling to the fracture direction. Fibrous zone was also formed under tensile stress. Fibrous zone’s surface was hackly and consisted of a number of "small peak". SEM was used to observe the morphology of impact fracture. It is founded that that the NZ impact fracture distributed sizes of dimple, still presenting some ductile fracture characteristic. Distinctly, part area of HAZ’s impact fracture appeared cleavage steps, and the fracture mode is the mix of ductile and brittle.Neutral salt spray test was employed to investigate the corrosion resistance properties of FSW joints. It is found that FSW don’t affect the corrosion resistance of ADC12aluminum alloy and of6061aluminum alloy.The artificial aging test was used to research the effect of aging on the hardness of FSW joints. It is found that the NZ’s hardness had significantly increased after aging while the TMAZ and HAZ’s hardness of ADC12aluminum alloy doesn’t change obviously. On the other hand, the TMAZ and HAZ’s hardness of6061aluminum alloy rises about2-3HV. |
