Study On The Microstructure And Properties Of 2196 Al-Li Alloy Fabricated By Wire And Arc Additive Manufacturing

Wire and Arc Additive Manufacturing(WAAM),which is an additive manufacturing technique based on welding,is attracting increasing interest for practical application in the fields of aerospace and military industry.The main advantages of the WAAM technique are the high materials deposition rate and high materials utilization rate,as well as its ability to fabricate large structures.WAAM of high strength aluminum alloys is considered as an ideal method to produce large-scale lightweight structures with complex geometries.Aluminum-lithium alloys are promising materials for fabricating lightweight structure,due to the fact that Al-Li alloys have a low density,high specific stiffness and strength,as compared with other Al alloys.Cold Metal Transfer(CMT)is an ideal welding method for aluminum alloys,which is featured with low thermal input,high welding speed and nearly spatter free.In this work,WAAM of 2196 Al-Li alloy was achieved using a CMT welder and a six-axis robot.The influence of WAAM process parameters and the inter-layer cooling time on the microstructure and mechanical properties of 2196 Al-Li alloy was studied.The main content of this work is summarized as:(1)In order to optimize the WAAM process parameters,the single-track and multi-layered 2196 Al-Li alloy thin walls were fabricated using different wire feed speeds and torch travel speeds,while the inter-layer cooling time was fixed at 120s.The influence of input energy on the microstructure and mechanical properties was studied.The results indicated that a lower input energy density is beneficial to improve the macro morphology of the thin walls,and the refined microstructure could be obtained.However,all the samples showed limited ductility(total elongation<6%),despite of the wide-range of the selected WAAM process parameters.(2)The influence of inter-layer cooling time(5～120s)on the microstructure and mechanical properties of WAAM fabricated 2196 Al-Li alloy was studied,by fixing the wire feed speed and torch travel speed at 3m/min and 0.45m/min.respectively.The computerized tomography(CT)technique was used to visualize the distribution of pores inside the samples.It was found that both the short(e.g.5s)and long(e.g.120s)inter-layer cooling time could lead to the formation of high volume fraction of pores(0.4～0.5vol.%),while the sample fabricated with an inter-layer cooling time of 15s showed a low porosity of 0.14vol.%.Under a short interlayer cooling time of 5s,most of the pores are spherical,while the number of irregular pores increases with the increase of inter-layer cooling time.Also,a small amount of cracks was observed with the increase of inter-layer cooling time.The mechanical tests showed that the ductility of WAAM fabricated 2196 alloy decreased with the increase of inter-layer cooling time.Since a low volume fraction of pores was observed in the sample,which was fabricated with inter-layer cooling time of 15s.A good reproducibility of the tensile tests was obtained,with ductility of>12%and strength of>210 MPa.(3)The influence of post heat treatment on the microstructure and mechanical properties of WAAM fabricated 2196 Al-Li alloy was studied,especially,the aging temperature and aging time were optimized.After heat treatment,the high strength(>380 MPa)and ductility(>12%)were obtained for the WAAM fabricated 2196 alloy.