Study On Microstructure And Mechanical Properties Of Additively Manufactured Al-Cu-(Mg) Alloys With The CMT Process

Based on "dispersion-deposition" theory,the process of the Wire+arc additive manufacturing(WAAM)technology slices the CAD model into 2D layers normal to the horizontal plane,then freeformly deposits the arc melted metal along the presetted pathway to make the targeted 3D solid parts.This technique is suitable for rapidly manufacturing large scaled metal components having low level of complexity.Thus WAAM,which is a promising rapid prototyping process,can potentially replace the traditional subtraction methods with the benefits of great materials,cost and time saving,and environment friendly.WAAM attracts great interest from the aerospace industry for producing components with aluminum alloys,particularly Al-Cu alloy of the 2000 series such as 2219 alloy.However,the application is restricted by the high level of defects,low strength properties and low efficiency of the as-deposited WAAM metal.In this paper,the pulse advanced Cold Metal Transfer(CMT-PA)was applied to the preparation of the WAAM Al-Cu-(Mg)alloy.The process parameters,microstructure and properties of the as-deposited,cold worked and heat treated WAAM 2319(Al-Cu6.3)alloy were investigated.In addition,the influence of the cold working and heat treatment on porosity was studied.The tandem power source was used for the deposition process with two wires.When two different alloying wires(Al-Cu + Al-Mg)were used,ternary(A1-Cu-Mg)alloys with various compositions can be obtained,by adjusting the wire feed speed of the two wires.According to the optimized ternary composition,one non-standard Al-Cu4.3-Mg1.5 alloy wire(diameter in 1.2 mm)was made.The preparation,testing and analysis were performed on the WAAM alloy with this wire.The key findings of this thesis are presented as follow:The CMT-PA process delivers excellent performance with excellent welding quality,low heat input,high deposition rate,refined grains and is nearly spatter-free.Parameters such as contact tip to work distance,gas flow rate,cooling time,wire feed speed and travel speed had great influence on the formation and microstructure of the WAAM 2319 aluminum alloy.The microstructure of the as-deposited WAAM 2319 alloy is fine,but inhomogenously behaved.The Cu element was mainly solid solutionized or formed as second phases of Al2Cu.All the other elements were solid solutionized,or existed as second phases,or doped into the Al2Cu phase.The mechanical properties were nearly isotropic along the two directions.The minimum ultimate tensile strength(UTS),yeild strength(YS)and elongation for this alloy were 260 MPa,106 MPa and 12%.Although all these properties are better than the 2219-0 tempered wrought alloy,it is still lower than the T tempered wrought alloy.Thus the as-deposited metal generally cannot be applied as the structural alloy in the industry.Inter-layer rolling was applied to the WAAM process with the loads of 15 kN,30 kN and 45 kN.The deformation rate,micro hardness and strength of the rolled metal were nearly linearly increased along with the increase of loads.The rolled WAAM 2319 alloy was strengthened by the high dislocation density and great amount of sub-grain structures.The micro hardness of the 45 kN rolled alloy reached 102 HV;while the UTS and YS were 315 and 245 MPa,respectively.The main strengthening mechanism for this alloy after T6 heat treatment is precipitation hardening,due to the fine needle-like densely and homogeneously precipitates.The T6 treated WAAM 2319 alloys with or without rolling had similar mechanical properties.The micro hardness of both increased by 110%.The minimum UTS,YS and elongation are 450 MPa,305 MPa and 14%,respectively.The elongation of the rolled+heat treated alloy is 23%higher than the heat-treated alloy without pre-rolling.In all,the tensile properties of T6 heat-treated WAAM 2319 alloys were all better than the 2219-T6,T8 wrought alloy.However,"secondary porosity" was generated in the WAAM alloy after high temperature exposure.The total amount and area percentage of the pores were increased.It was found that inter-layer rolling could effectively control the size and amount of the pores,which were gradually flattened to oblate spheroids or closed with increased pressure.The closure rate of pores is related to the deformation rate and original pore size.The hydrogen was absorbed by the dislocation or vacancy defects after rolling.After heat treatment,pores with size of 2-3?m are in evidence in the 45 kN rolled samples due to dislocation release,local strain recovery,vacancy migration or re-growth and spheroidization of the flattened pores.It proved that the inter-layer rolling process is effective in eliminating porosity defects in the additively manufactured as well as the welded aluminum materials.Compared with the single wire WAAM 2319 alloy,the grains were coarser and the tensile properties were lower for the tandem WAAM 2319 alloys due to their higher heat inputs.During the WAAM process,the two feeding wires of Al-Cu and Al-Mg can prepare Al-Cu-Mg alloys with various compositions.When the Cu content was fixed as 4.3%,the WAAM Al-Cu-Mg alloy has the lowest cracking sensitivity with 1.5%-2.5%Mg added in the alloy.Compared with the single wire WAAM 2319 alloy,the micro hardness and yield strength of the tandem Al-Cu-Mg alloy were increased by 58%and 34-57%,respectively.The non-standard Al-Cu4.3-Mg1.5 wire was produced based on the optimized composition of the tandem building experiments.The UTS,YS and elongation of the as-deposited WAAM Al-Cu4.3-Mg1.5 alloy were excellent in the horizontal direction,reaching to 280 MPa,185 MPa and 12%,respectively.However,the great amount of micro-cracks in the vertical direction leads to inferior plastic properties along this direction.The mechanical properties were obviously improved after rolling for this alloy.The elongation was 8.2%and 8.4%for the 45 kN rolled WAAM alloy with or without T6 heat treatment.The UTS and YS,which were higher than 495 MPa and 400 MPa,were all better than the 2024-T6 wrought alloy.