Study On The Mechanism Of The Hydrogen Induced Aging Treatment Of Heat Treatable Aluminum Alloy Joints By Multi-pass Mig Welding

As extremely important materials in engineering,heat treatable aluminum alloys have been widely used in fields of railway transportation,engineering machinery,aerospace,and automotive industry.Due to their high strength,heat treatability,and good formability,the 6 series and 7 series aluminum alloys in extruded profiles are generally applied in heavy-duty structures.However,the softening problem of welded joints has constrained their application and expansion.Therefore,it is of significant engineering importance to develop excellent post weld heat treatment regimes for 6-series and 7-series thick plate aluminum alloys joints.In this paper,two joints with different contents of Hydrogen,which was introduced by adding 0.05% and 0.20% hydrogen into shielding gas Argon respectively,were obtained by multi-pass MIG welding of 6005 A and 7N01 aluminum alloy thick sheets.After that the hydrogen-induced aging treatments were performed.Compared with the welded specimens and single-stage aging specimens,experimental analysis was conducted on the microstructure,microhardness,tensile properties,impact toughness,fracture morphology,and micro zone composition of welded joints by Hydrogen induced aging treatment,aiming at exploring the mechanism of hydrogen induced aging treatment.By adjusting the process parameters,the form of the weld bevel and the number of passes,a good and stable weld appearance was obtained.6005 A and 7N01 aluminum alloy multi-pass MIG welded joints weld area under the thermal influence of multiple passes makes the weld area grain morphology and orientation is more complex,and defects such as porosity and slag are prone to exist between different passes.7N01 aluminum alloy near the fusion line of equiaxial crystal zone and weld in the pattern is contrary to that of common welds,where the columnar crystal zone is usually present,but grows as an equiaxed crystal zone.Overall,the hydrogen-induced aging has a dual nature,with the fracture of the hydrogen-induced aged specimens containing more second-phase particles to strengthen the matrix,but also a significantly higher number of hydrogen pores,and controlling the hydrogen content is effective in finding a balance between high aging heat treatment efficiency and excellent product performance.In the mechanical properties of multi-pass MIG welded specimens of 6005 A and 7N01 aluminum alloys,the 0.05% hydrogeninduced aging treatment was generally superior to the 0.2% hydrogen-induced aging treatment.Hardness tests showed that hydrogen-induced aging had a small increase in the hardness of the weld zone of the 6005 A and 7N01 aluminum alloy multi-pass MIG welded joints,while the hardness of the heat affected zone was well recovered.In the tensile test,the 0.05% hydrogen-induced aging treatment improved the tensile yield strength of the 6005 A and 7N01 aluminum alloy multi-pass MIG welded joints by 47.0%and 44.2% of the corresponding weld state specimens,respectively,ahead of the specimens with conventional single-stage aging.However,in terms of elongation at break and shrinkage at section,the specimens of 6005 A were substantially lower compared to the welded state,while the specimens of 7N01 aluminum alloy showed an increase.In the impact test,the 0.05% hydrogen-induced aging treatment improved the impact toughness of the 6005 A aluminum alloy multi-pass MIG welded joint by 50.9%relative to the welded specimen,but decreased for the 7N01 aluminum alloy multi-pass MIG welded joint.After hydrogen-induced aging,6005 A and 7N01 aluminum alloy multi-pass MIG welded joints of the weld zone are generated short columnar,discshaped and headphone-like second phase,short columnar second phase mainly contains Al,Mn,Si and Fe and other alloying elements;disc-shaped second phase mainly contains Al,Mg,Si and Cu and other alloying elements,there are also more internal dislocations;headphone-like second phase by the disc-shaped with The long stripshaped second phase mainly contains Al and Si and other alloying elements.The difference is that the weld zone of the 7N01 aluminum alloy multi-pass MIG welded joint also contains a needle-like second phase,whose main alloying elements are Al and Si.