Microstructure And Mechanical Property Characteristics And Their Control Of GTA Additive Manufactured Al-Zn-Mg-Cu Alloy

These years,as the application of WAAM in the aerospace field has been recognized gradually,there is an increasing demand for using WAAM to fabricate Al-Zn-Mg-Cu alloy structures instead of forging and chemical milling.During WAAM,cathode cleaning can be used to remove oxidation film,so the sensitivity to pores can be reduced.The energy distribution of electric is sparse,while the cooling rate of deposited metal is slow during fabrication,so the sensitivity to hot cracking is also relieved.Comparing with additive manufacturing methods based on high-energy beams,WAAM shows special adaptivity to this kind of aluminum alloy,and a broad prospect.However,this aluminum alloy is highly alloyed,and has complex structure and phases composition.The as-deposited microstructural characteristics,microstructural evolution laws,as well as the microstructural control methods are unknown.Based on this background,this thesis focuses on the microstructures,mechanical properties,and their control of the GTA additively manufactured 7xxx series Al-Zn-Mg-Cu alloys.The characteristics of microstructures and mechanical properties are revealed,the microstructure formation and evolution mechanisms are clarified,and effective controlling method is proposed.Firstly,the microstructures and mechanical properties of as-deposited Al-Zn-Mg-Cu alloy fabricated with typical deposition parameters are investigated.Results show that the top region consist of both twinned dendrites and normal dendrites,which has<011>orientation and<001>orientation,respectively.The middle region shows a periodical arrangement of twinned dendrites and normal dendrites.With the intrinsic thermal cycles during WAAM,theη（Mg（Zn,Cu,Al）₂）phase can precipitate out spontaneously.The average yield strength,elongation of the as-deposited metal are 148.3MPa,and 3.3%,respectively.The yield strength on the height direction is16.8%higher than the horizontal direction,while the difference in elongation is small.The intrinsic mechanisms of growth of twinned dendrites and precipitation driven by intrinsic thermal cycles are investigated combined with experiments and numerical simulation.Results show that,in the aspect of the growing mechanism and evolution laws of twinned dendrites,elements which have low stacking faults energy such as Zn,Mg and Cu is the primary cause for the nucleation of twinned dendrites.On the tail of the molten pool,the temperature gradient and the crystallization rates are high,which provides solidification conditions for the stable continuous growth of twinned dendrites.And stacking faults promote the longitudinal growth of twinned dendrites.The twinned dendrites show both competitive and epitaxial relationship with normal dendrites.The morphological change of dendrites is sensitive to the remelting depth.In the aspect of precipitation behaviors,thermal cycles during WAAM can be classified into two categories:the type I thermal cycle and the type II thermal cycle.Type I thermal cycles have peak temperature higher than the dissolving temperature ofηphases,during which the precipitates experience periodical change of nucleation-dissolve.Type II thermal cycles have peak temperatures lower than the dissolving temperature ofηphases,during which the nucleation and growing of precipitates is promoted.The interlayer temperature control and heating of the heat source during WAAM both promote the precipitation.When the interlayer temperature gets larger,its contribution also increases.However,the mechanical property tests show the deformation of twinned dendrites and normal dendrites is discordant.And the precipitation driven by thermal cycles during WAAM makes the micro-regional mechanical properties of the alloy sensitive to the thermal process.Thus,twinned dendrites and non-uniform precipitates should be eliminated.Microstructure controlling methods are proposed in the aspects of process and metallurgy to eliminate pores,twinned dendrites,and to refine grains.The low frequency pulsed GTA additive manufacturing process is proposed.In this process,the vibration of molten pool promotes the escape of bubbles,leading to lower porosity.The diffusion of solutes is promoted to generate additional constitutional supercooling,while remelting provides condition for the formation of Al₃Zr.Thus,the grains are refined.An in-situ alloying method based on adding Ti C particles into the wire is proposed.The Ti C particles reduces the critical supercooling limits to refine the grains and suppress the formation of pores.This method also lowers the request for process controlling.Although the two methods show minor influences on microhardness and strength,they promote an increase in elongation significantly.The heat treatment is explored to eliminate large second phases and non-uniform precipitates.Results show that in the single stage solution,there are an interplay between the dissolving of second phases and the formation of solution pores.At a temperature above 450℃,the high diffusion rates of Mg and Zn leads to the imbalance diffusion on the interface of matrix and second phases,which results in severe solution pores.A step solution is proposed to relieve the interplay,which successfully realizes the orderly diffusion of alloy elements and reduces the degree of diffusion imbalance.The step solution reduces the solution pores by 54.8%,while the effects of second phases dissolving are the same.After the 430℃/4h+450℃/4h step solution and 120℃/24h artificial aging treatments,dispersed and coherentη’phases precipitate in the matrix of GTA additively manufactured Al-Zn-Mg-Cu alloys.The Al8.1Zn2.1Mg2.3Cu alloy fabricated by the low-frequency pulsed process show an average hardness of193HV_0.2,a yield strength of 475MPa,and an elongation of 6.5%.The Al5.8Zn2.6Mg1.6Cu alloyed by Ti C particles show an average hardness of 187HV_0.2.The average yield strength is 496MPa,while the elongation reaches 5.6%.The overall mechanical properties of alloys after heat treatments are improved significantly.