6061 aluminum pipe has the advantages of medium strength, corrosion resistance, good machining performance and strong weldability. It has been widely used in functional and structural materials. Traditional welding methods (such as MIG, TIG, etc.) are often adopted at home and abroad to weld such alloys, but the welded joints are not strong enough and the welds are prone to defects such as gas, slag inclusion and crack. Ultrasonic welding technology can realize the connection of low melting point materials such as magnesium alloy and aluminum alloy, which are difficult to be welded by traditional welding methods. This paper will detail the study of 6061 aluminum alloy after ultrasonic welding joint microstructure, surface morphology and mechanical properties, and get the better process parameters of 6061 aluminum alloy ultrasonic welding, aluminum alloy surface treatment and analysis of the influence of the welding performance, in order to promote the ultrasonic welding of this advanced connection technology applied in lightweight alloy connection.
Intragranular segregation is the phenomenon of inhomogeneous chemical composition in microstructure.
The microstructure of intracrystalline segregation is characterized by water wavy intracrystalline similar to tree ring. The microhardness in the grain is different, the microhardness near the grain boundary is high, and the microhardness in the grain center is low.
The presence of intracrystalline segregation makes the chemical composition inside the grain and the microstructure of the ingot extremely uneven, which makes the properties of the ingot seriously deteriorate, mainly as follows:
1) the inhomogeneity of chemical composition and the appearance of unbalanced excess phase caused by intra-crystalline segregation of solid solution reduce the stability of alloy to resist electrochemical corrosion.
2) the appearance of non-equilibrium eutectic or low-melting components reduces the melting temperature at the beginning of the alloy, making the ingot prone to local overburning during the subsequent hot deformation or quenching heating process.
3) intra-crystalline segregation not only causes the appearance of non-equilibrium phase and increases the number of second phase, but also, these low-melting phases form a hard and brittle dendrite network around the dendrites, resulting in a sharp reduction in the plasticity and processing performance of the ingots.
4) the inhomogeneity of chemical composition caused by intracrystalline segregation is transmitted to semi-finished products, resulting in the formation of coarse grains in processed materials after annealing.
6061 aluminum alloy of 0.3mm thick was selected in the experiment. The size of the specimen was 160mm x 18mm x 0.3mm. The ultrasonic spot welding machine is used to weld the two cut aluminum sheets. The working frequency is 20kHz, the amplitude is 35um, the welding pressure and welding time are adjustable, and the size of welding head is 8mm x 8mm. The welding time changes from 40 to 140ms, and the cylinder pressure changes from 0.1 to 0.6mpa. The relation between cylinder pressure and welding pressure is: welding pressure =(cylinder area/spot area) x cylinder pressure. In the experimental equipment, the welding area is 88mm2, the cylinder diameter is 53mm, and the welding pressure is 35 times of the cylinder pressure after conversion, so the welding pressure changes from 3.5 to 21MPa. The welding sample stripping test was conducted by CMT2520 new thinking microcomputer to control the electronic tensile testing machine, whose loading rate was 15mm/min. The stripping test was conducted according to the structure adhesive testing standard.
Tensile strength of fibrous segments b(MPa) : 215 ~ 355
Elongation rate: 10(%) : 12 ~ 17
Solid solution treatment temperature: 500 ℃ ~ 510 ℃.
Annealing cold material range: 340 ℃ ~ 350 ℃.
Material after annealing heat treatment temperature: 415 ℃.