| 6061 aluminum alloy belong to the heat treatmentable high-strength aluminum alloy. It has been well applied in the aerospace, automotive and many other fields for its good specific strength, corrosion-resistance and weldability. Previous studies show that the preparation of firm welding joint can be completed through cold metal transfer(CMT) welding, and the internal organization and performance of welding joint can be improved through cryogenic treatment. Additionally, the cryogenic treatment is pollution-free, with important application value.In this research, the same plates of 6061 aluminum alloy are processed by CMT welding and different welding specimens are contained through the changes of different parameters. The optimized welding parameters are contained through the analyses of macroscopic morphology, microstructure and mechanical property. Then,the optimized welding parameters of welded joints with different technological parameters are processed by cryogenic treatment. And a comparative analysis is made to welded joints with cryogenic treatment and without cryogenic treatment respectively from perspectives of microscopic hardness, tensile testing, fracture scanning, metallography, X-ray diffraction and TEM organization, revealing influences of cryogenic treatment on CMT welding joint of 6061 aluminum alloy.The results of welding tests are shown as follows. The same plates of 6061 aluminum alloy for 3 mm are processed by CMT welding, with nine different groups of wire feeding speed changed. When the wire feeding speed is 7.2 mm/s, from the macro and internal organization performance, the resulting welded samples whose tensile strength is 192.8MPa accounting for 78.4% of the base-metal and extension rate is 11.8% accounting for 90.1% of the base-metal, are better.The results of cryogenic tests are listed below, cryogenic time, cryogenic times has significant effect on the improvement of welded joints. Firstly, micro-hardness of weld seam area as well as tensile strength and elongation of welded joint have been improved to different degrees. After cryogenic treatment of three times for 15 h,micro-hardness value researches maximum(94.6 HV) and tensile strength reachesmaximum(217.9 MPa). After cryogenic treatment of two times for 15 h, elongation reaches maximum(13.1%). Secondly, fracture morphology shows that types of fracture are all ductile fracture. After cryogenic treatment of two times for 15 h, the number of dimples on the tensile fracture of welded joints is the largest, and its distribution is uniform as well as dense, showing that the plasticity is the best. Thirdly,results of metallographic, TEM organization show that there is more in the second phase precipitation of weld zone substrates. Also, results of X-ray diffraction show that the diffraction peak intensity becomes bigger on the 44.5 ° location of weld area,indicating that grain rotation phenomenon is contained. TEM organization show that after circulation after cryogenic treatment, helical dislocations, dislocation loops and sub-grains exist in the weld zone. The strength of the welded joint is improved with the interaction between dislocation itself and grain boundary, sub-grain as well as the second phase. The distribution of dispersion of the second phase and generation of the sub-grain refine the organization and improve the plasticity of welded joint. Finally,artificial aging after cryogenic treatment, the tensile strength values of welded joints increase but its plasticity decline significantly. Therefore, the cryogenic treatment and aging treatment cannot improve the strong toughness of welded joints at the same time.The comprehensive analyses of test results from perspectives of mechanical properties and microstructures show that the microstructure and strong toughness of CMT welding joints of 6061 aluminum alloy are improved greatly after cryogenic treatment, among which the organization and performance of welding joints are optimal after twice cryogenic treatment for 15 hours. |
PDF Full Text Request