Effect Of Heat Treatment On Fracture And Intergranlar Corrosion Of A6N01S-T5 Aluminum Alloy And Hybrid Laser-MIG Welding Joints

In this research,A6N01S-T5 aluminum alloy,which is used in high speed trains,was taken as the investigated subject.The effect of artificial-ageing heat treatment on the fracture surface and intergranular corrosion?IGC?performance of A6N01S-T5 aluminum alloy was investigated.The softening effect of heat affected zone of A6N01S-T5 aluminum alloy hvbrid fiber laser-Metal Inert Gas?MIG?welding joint was studied.And using the best heat treatment technology to solve the softening problem of weld joints and improve the intergranular corrosion resistance of the weld joints.The solution heatrment test results showed that the tensile strength of the alloy was improved with the increasing solution temperature from 490°C to 530°C.However,the tensile strength decreases due to the grain growth when the temperature exceeded 550°C.The SEM,TEM,HRTEM,and DSC test results showed that prolonging the aging time would promote GP zones transform to?",which would increase the hardness and strength.Meanwhile,the increasing aging temperature during the aging process accelerated the transformation of the SSSS?GP zones??".The transformation of GP zones??"phases during the aging process would lead to the fracture morphology of alloy changing from transgranular fracture to intergranular frature,and the increase of aging temperature would accelerate the transformation of fractura morphology.The distribution of?"and Al–Fe–Si phases on the grain boundaries were the main reason for the induction of intergranular fractures.A6N01S-T5 aluminum alloy was susceptible to IGC,which was caused by chemical property differences between the Al matrix and the large numbers of Mg₂Si and Al-Fe-Si phases on the grain boundary.The combined action of Mg₂Si and Al-Fe-Si phases contributed to the maximum IGC depth of the material.The decrease of density of Mg₂Si resulted in the decrease in IGC depth.In addition,the changing mechanism model of the fracture surface of the A6N01S-T5 alloy during the tensile process is presented.The thermal cycling test,TEM,HRTEM and electrochemical corrosion tests showed that the softening of the heat affected zone of the welded joint was caused by the input of the welding thermal energy,which had resulted in the transformation of?"phase into Q phase.And the decrease in the number of?"phases reduced strength of the heat affect zone.The formation of the Q phases made IGC depth deeper.And the continuous strip-shaped eutectic Al+Mg2Si formed continuous IGC channel.Due to the GP zones and more beta?"phases precipitated out in the 530°C/75 min+160°C/540 min heat treatment,the hardness and strength of the heat affected zone was improved,and the position of fracturing had transferred from the heat affected zone to the weld and the base metal.And the softening of the heat affected zone had been sovled.The eutectic structure changed from its original strip-shaped distribution to an intermittent spherical uniform distribution after the solution and aging heat treatment.This hindered the extending of the intergranular corrosion depth and improved the intergranular corrosion resistance of the joint.At the same time,the dissolved Q phase in the heat-affected zone in the solid solution stage did not precipitate out in the aging stage,which reduced the cathode area on the surface of the sample and improved the corrosion resistance of the heat affected zone.