Study On Friction Stir Welding Of Aluminum Alloys

The traditional fusion welding for aluminum alloys usually brings some welding defects, such as cracks and porosities. It’s hard to join Al to steel due to their differences in thermophysical properties and the formation of brittle intermetallic compounds in the welding. Friction stir welding(FSW) is a solid-state joining technology, which means the heat generated during the welding process does not reach the melting point of base materials. Hence, the base materials are not melted, and they only reach a plastic state. Thus, FSW can be an effective method to solve the welding problems for aluminum alloys, Al/Steel dissimilar metals.Firstly, 4-mm-thick 7075 aluminum alloy plates were used in the butt welding. Well-formed joints were obtained with a certain range of welding parameters. The maximum of tensile strength can reach 80% of the base metal. A variety of testing methods were used to study the microstructure and macro profile of the joint, the influence of welding parameters on mechanical properties and hardness distribution.Al/Steel dissimilar metals lap welding were respectively carried out with 2-mm-thick Al plate to 2-mm-thick steel plate and 3-mm-thick Al plate to 1-mm-thick steel plate. The results showed that steel on both sides of the joint interface were stirred into the aluminum alloy and bent to hook-like structure. Steel inclusion particles, laminated organizations and intermetallic compounds were continuously distributed throughout the connection interface area, which became the important factors to the joint quality. XRD analysis of the joint fractured interface showed the existence of AlFe, AlFe3 intermetallic compounds. There are three kinds of fracture positions for joints in tensile shear test. They are heat affected zone, Al/Steel interface and steel base material. SEM analysis of fracture surface showed that fracture at the heat affected zone was ductile fracture and fracture at the Al/Steel interface was brittle fracture. A sharp increase of hardness of 340 HV appeared in the Al/Steel interface center, which was related to the hard brittle intermetallic compounds.A heat source model during the FSW process of 7075 aluminum alloy was established based on the theory of sticky friction. The distribution of temperature fields were analyzed by numerical simulation. High temperature was in the welding nugget zone just below the shoulder and temperature distribution on the cross section along the welding direction presented like a bowl. The temperature gradient in front of the tool was larger than the rear area. Also, the area of high temperature in the rear was narrower. The peak temperature did not reach the base metals’ melting points under all the welding parameters, which showed that the FSW process had been in a solid connection status. Rotation speed and welding speed had a certain influence on the peak temperature.