Study On Strengthening Phase Evolution And Fatigue Properties Of CMT Welded Joint Of Al5.5Zn2.5Mg2.2Cu High Strength Aluminum Alloy

With China's"Revival"EMU achieving 350 km/h operation,China has become the country with the highest speed of commercial operation of high-speed railways in the world.It also marks that China's high-speed trains have reached the world advanced level.Al-Zn-Mg-Cu aluminum alloy is a high-strength aluminum alloy material with excellent overall performance,which is developed for advanced manufacturing industries such as high-speed trains.It is mainly used for bearing system of body and component.Welding is an indispensable means of connection in the manufacture of trains and aircraft and is an important factor in the commercialization of materials.It is difficult to achieve high quality welding of high strength aluminum alloy.So the breakthrough of this welding bottleneck has always been the direction of the researchers.Cold metal transition welding technology?CMT?has the characteristics of low heat input,stable welding arc,droplet transfer without spatter and easy realization of automatic control.It is a brand new,high efficient and high quality welding method,and is applied more and more widely in advanced manufacturing field.In this paper,3 mm thick Al5.5Zn2.5Mg2.2Cu high strength Aluminum Alloy as research object,developed CMT Advanced 4000R type NC welding system used for welding,the welding process,joint macro forming,microstructure,phase evolution and mechanical properties,fatigue properties of joints were studied,and the static tensile and fatigue fracture observation and analysis.The results show that the minimum heat input required for penetration welding is 137.7 J/mm.At the same time,the reduction of welding heat input not only can reduce the amount of weld porosity,but also obviously improve the forming effect of the welded joint.Al5.5Zn2.5Mg2.2Cu high-strength aluminum alloy CMT welded joints are divided into weld zone,heat-affected zone and base metal,compared with the traditional arc welding,the heat-affected zone width narrowing;fusion zone to columnar grain structure was typical co-occurrence of crystallization characteristics;weld zone structure is mainly equiaxed and dendritic,with the welding heat input gradually decreased,the weld microstructure of varying degrees of refinement.The hardness distribution of the joints was uneven and the hardness of the weld area was the smallest after 360 hours of natural aging.The hardness of over aging softening area decreased by 10 HV compared with the solid solution area due to the coarsening of the matrix precipitation phase and grain boundary precipitation.Tensile specimens of joints under different welding parameters were all broken in the weld seam,with the decrease of the heat input,the tensile strength of the joint decreases.When the heat input is 137.7J/mm,the tensile strength of the joint is the optimal and the value is 334.9 MPa.The SEM observation of tensile fracture shows that the main fracture surface is the equiaxial dimple,which is the typical fracture morphology of the dimple.The Mg₂Si diffraction peak is also found in the weld in addition to the typical?-Al diffraction peak.Some?'-metastable phases dissolve into the matrix and only weak Mg Zn₂phase diffraction peaks are observed in the solid solution zone due to the high temperature of the solid solution zone close to the weld.On the contrary,there are obvious Mg Zn₂phase diffraction peaks in the overaged softening zone.The microstructure evolution of the heat-affected zone was analyzed by TEM image,compared with the base metal,the fine GP zone precipitated in the solid solution zone and the coarse precipitates increased,but the?'-metastable phases decreased obviously,the size of phase precipitation and precipitate free zone width did not change significantly;In the aging zone,the precipitation phase of the matrix and grain boundary obviously increase,but the amount of precipitation in the matrix decreases,the precipitation of the grain boundary is discontinuous,the size of the precipitation decreases with the increase of the?phase in the grain boundary,The width of the tape increased and there was no precipitation of solute atoms on both sides.The evolution of the heat affected zone leads to the obvious change of its hardness and tensile properties,compared with the base metal,the heat affected zone is softened obviously.Under the condition of 10⁷ cycles,the stress-fatigue life relationship of Al5.5Zn2.5Mg2.2Cu aluminum alloy base metal and the heat input of 137.7 J/mm and148.3 J/mm of CMT welding are ???????????respectively.The fatigue limit of the specimen under this cycle is166.4 MPa,109.3 MPa and 102.2 MPa.The results of the joint fatigue test show that the joint with a heat input of 137.7 J/mm can obtain higher fatigue life at the same stress level.It is observed that the fatigue fracture is composed of the fatigue source zone,extension zone and instantaneous zone,the fatigue source region for welding joint is mainly pore,extension fatigue striations visible band spacing differences in the crack propagation process,strip across the second phase particles tend to bypass the particles after expansion;The morphology of the transient fracture zone is the same as that of the static tensile fracture,which is made up of dimples of different sizes.