| Al-Zn-Mg series alloys belong to age-strengthening aluminum alloys and have medium strength,which have good mechanical properties,corrosion resistance,thermal stability,and welding performance.At present,the research on Al-Zn-Mg series alloys mainly focuses on optimization of heat treatment process and effect of alloying elements on its performance.However,the researches on the effects of welding process and thermal cycling on the microstructure and properties of Al-Zn-Mg alloys are still insufficient,especially the evolution of the precipitation of Al-Zn-Mg alloys under thermal cycling,it is necessary to do further in-depth research.In the assembling of aluminum structural components,the Al-Zn-Mg alloys are often joined by welding.Among various welding methods,metal inert gas(MIG)welding is commonly used for welding different dimensional structures owing to its relatively easy operation and better economy and the proportion is around 90%.The A14.5Znl.5Mg alloy with T5 condition and A14.8Znl.2Mg alloy with T4 status was joined through MIG welding and the fracture mechanism,corrosion mechanism of A14.5Zn1.5Mg-T5/A14.8Zn1.2Mg-T4 alloys MIG welded joint are systematically studied;in addition,the effect of heat straightening and repair welding on mechanical properties and corrosion susceptibility is investigated.The evolution mechanism of corrosion susceptibility of A14.5Zn1.5Mg-T5 alloy is evaluated clarified from the evolution of precipitate-free zone,matrix precipitation and intergranular precipitation,which will provide the theoretical basis and scientific basis for the engineering application of Al-Zn-Mg series alloys.The main conclusion as follows:(1)Mechanism research on fracture and corrosion behavior of A14.5Zn1.5Mg-T5/A14.8Zn1.2Mg-T4 aluminum alloys MIG welded joint is investigated and the results show that the cracks initiate at the junction between heat affected zone I(HAZ I)of A14.8Zn1.2Mg-T4 and weld passes,ascribing to gas porosity and inclusions,and the joint finally fails along the weld zone during tensile test,and the fracture morphology shows ductile characteristic.The difference of corrosion behavior of different locations of welded joint is obvious and A14.8Zn1.2Mg-T4 side has the worst corrosion resistance,however,the corrosion resistance of the weld seam is the best.On the one hand,it is caused by the difference in the self-corrosion of different locations of welded joint,namely the high exfoliation corrosion susceptibility is attribute to flattened grains on the A14.8Zn1.2Mg-T4 side,while the equiaxed crystal grains characteristics on A14.5Zn1.5Mg-T5 side has good corrosion resistance.On the other hand,the reason lies in the galvanic corrosion effect of different positions of welded joint:In the welded joint,the difference in composition of Zn,Mg and Cu and the amount of precipitates between A14.5Zn1.5Mg-T5 side and A14.8Zn1.2Mg-T4 side induces the variance of corrosion potential,resulting in the appearance of galvanic corrosion.A14.8Zn1.2Mg-T4 side is an anode during galvanic corrosion because of low corrosion potential,then it is corroded firstly in corrosion medium.(2)The effect of quenching conditions during heat straightening on mechanical properties and corrosion resistance of Al4.5Zn1.5Mg-T5 aluminum alloy is revealed.The results shows that the quenching conditions have no obvious effect on the tensile strength of Al4.5Zn1.5Mg-T5 aluminum alloy during heat straightening process,however the corrosion resistance decreases.The Al4.5Zn1.5Mg-T5 aluminum alloy has the lowest corrosion resistance with air quenching during heat straightening,however,the specimens can get better corrosion resistance when suffered 5 min air cooling followed by water quenching process.(3)The effect of heat straightening number on properties and corrosion evolution of Al4.5Zn1.5Mg-T5 aluminum alloy are revealed based on evolution rule of precipitation.The results indicate that heat straightening number has no obvious effect on tensile strength of Al4.5Zn1.5Mg-T5 aluminum alloy,however the corrosion resistance decreases with the increase of heat straightening number.The evolution of corrosion behavior of Al4.5Zn1.5Mg-T5 aluminum alloy is caused by modification of intergranular precipitates and diffusion of Zn,Mg atom from matrix to grain boundaries.Zn and Mg elements aggregated at the grain boundaries with the types of solid solution and GP zones with the increase of heat straightening number.The electrochemical activity of the GP zones is similar to that of Zn-rich areas at the grain boundaries and a continuous corrosion channel formed,and then resulted in the decrease of corrosion resistance of Al-Zn-Mg alloy.(4)The mechanism of the corrosion evolution of Al4.5Zn1.5Mg-T5 alloy under different heat straightening number is revealed based on the theory of non-equilibrium segregation.The results show that the elements of Zn and Mg formed complex with vacancy,respectively,then the complex diffuse to grain boundaries quickly.The effective time of non-equilibrium segregation of solute elements Zn and Mg during three times of heat straightening is less than the critical time tc.Consequently,the contents of solute elements Zn and Mg at grain boundaries gradually increase with the increase of heat straightening number,then results in the decrease of corrosion resistance of Al4.5Zn1.5Mg-T5 alloy.(5)The effect of repair welding on the performance of Al4.5Zn1.5Mg-T5/Al4.8Zn1.2Mg-T4 MIG welded joint is investigated.The results show that the repair welded joint fractures at the junction of original welded beads and repair welded beads.The tensile strength of repair welded joint has no obvious change,which is 280 MPa.The precipitates in different locations of repair welded joint change and the Zn and Mg elements diffuse toward the grain boundaries,then the corrosion potential difference between the grain boundaries and matrix increases,resulting in the decrease of corrosion resistance at HAZ I locations on both sides of the weld. |
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