Study On High-efficiency Welding Of7N01Aluminum Alloy With Shielding Of Mixed Gases

High-speed train is one of the most important indexes of transportation in a country.Since2008, there has been a rapid expansion in high-speed train enterprise in China, butthe security of these fast trains should be paid close attention. At present, pulsed metalinert-gas welding with the shielding of traditional argon is still the main weldingmethod for the production of high-speed trains. If the shielding gas could be changed inorder to improve the mechanical property of joints, there would be a huge boost to fasttrains, and it would be beneficial to realize the high-speed ability, security andlocalization of the transportation.New Ar-He-N2ternary gases were applied in this study to be the shielding gas ofPMIG welding of7N01aluminum alloy. Joints of bead welding and butt welding werestudied. Metalloscope, EBSD, SEM, EDS, STEM and TEM were applied to analyze themicrostructures, phases and element compositions. Vickers micro-hardness, tensilestrength and fatigue properties were also measured.In the bead welding, ternary gases could compress arc more effectively than pureargon. Ternary gases could enhance the maximum heat flux. Occurrence probability ofglobular and spatter were decreased with mixed gases, and droplet transfer was morestable. Welding efficiency was enhanced remarkably with ternary gases.7N01aluminum alloy was sensitive to heat-input, and tensile strength woulddecline sharply when heat input was too large or too small. The welding speed wasincreased by20%with ternary gases close to the fusion degree under pure argon. Morerefined grains were found in the welding center of ternary gases shielded joints. Therewere two main phase,-Al(FeMn) and Al(ZnMg), in the weld center, and part of themdistributed along the grain boundaries. The second phase precipitating at the grainboundaries weakened in the ternary gases shielded joints. There was no N elementduring elements scanning, which stated N did not take part in the metallurgical reaction.The main strengthening phases in ternary gases shielded joints were η’ and GP zone, butη and η’ were the majority in the argon shielded joints. The hardness was increased andthe width of HAZ was decreased in the ternary gases shielded joints. Tensile strengthand coefficient of elongation of ternary gases shielded joints were287.28MPa and7.5%respectively, which exceeded argon shielded joints’256.76MPa and6.2%. Ternarygases shielded joints had a lower crack growth rate under different ΔK.