| Wire-arc additive manufacturing(WAAM)is a technology,using the arc as the heat source and melt wire as the deposition material,fabaricates metal parts ayer by layer on the substrate.Compared with metal additive manufacturing technology with laser and electron beam as heat source,WAAM technology has the advantages of low equipment cost,high material utilization rate,high deposition rate,and large size of forming parts.However,there are obvious problems to solve.For examble:(1)control of size accuracy and surface roughness of manufacturing parts(aiming at controlling the forming quality,referred to as "shape control");(2)control of internal welding defects(pores and cracks),dendrite segregation,microstructure and other problems of manufacturing parts(to control performance,also known as "controllability").In this study,the current mode,heat input,impose cooling and friction stir processing(FSP)on the forming quality,microstructure and mechanical properties of Al-Si alloy was systematically studied by using CMT arc additive manufacturing experimental platform to form the straight wall of Al-Si alloy.It provides theoretical and experimental data support for the practical application of"shape" and "controllability" in WAAM aluminum alloy.First,the CMT arc additive manufacturing experimental platform was used to five different current modes(common MIG,CMT,CMT-P,CMT-ADV,CMT-PADV)to form Al-5Si alloy thin wall.the CMT-PADV mode is the preferred current mode based on porosity and secondary dendrite arm spacing determine for WAAM manufacturing Al-Si alloys during the study.Next,the CMT-PADV current mode and different welding speeds were used to fabaricate four thin walls(270mm×7-10mm×66mm)with Al-5Si alloy under different heat input conditions.Effect of the forming quality,microstructure and mechanical properties of Al-5Si alloy thin wall.The results show that when the heat input is reduced from 380J/mm to 140J/mm,the effective width of the fabaricate thin wall decreases then the effective width coefficient increases;the porosity increases first and then decreases with the increase of heat input.When the heat input to 220J/mm,porosity is the highest(0.15%);the secondary dendrite arm spacing of the formed thin wall decreases with the decrease of heat input.When the heat input is 140J/mm,the secondary dendrite arm spacing is the smallest(8.3?m);during this study,the effect of welding speed on the morphology and size of eutectic Si is not obvious,but the morphological characteristics of eutectic Si within and between layers are significantly different.The shape of the eutectic Si in the interlayer is a short rod continuously distributed in the ?-Al dendrite gap,with an average size of 0.7?m and an aspect ratio of about 2;the shape of the interlayer eutectic Si is coarse particles with a size of 1.1 ?m.Continuously distributed in the?-Al dendrite gap.With the decrease of heat input,the tensile strength of straight wall along the direction of welding and the direction of additive increases gradually.When the heat input is 140J/mm,the tensile strength in the direction of welding reaches the maximum,and the tensile strength,yield strength and elongation are 169MPa,75MPa and 30.8%.Compared with the straight wall fabaricated by air cooling process,the elongation of the straight wall fabaricated by impose cooling process decreased about 5%along the welding direction and about 30%along the additive direction.The crack originate from the interlayer interface between ?-Al and Si phase,and a number of dimples are observed in the tensile fracture,indicating that it is a ductile fracture.The Al-5Si alloy straight wall is the best comprehensive mechanical properties when the heat input is 140J/mm.Al-12Si alloy straight wall(length 320mm × width 10-12mm × height 52mm)is fabaricateed by wave bead welding and forced water cooling.The influence of different cooling conditions on the microstructure and mechanical properties of Al-12Si alloy straight wall was studied.Studies have shown that with forced water cooling,the porosity increases,the secondary dendrite arm spacing decreases,and the size of eutectic Si decreases.After forced water cooling was applied,the porosity of the fabaricate straight wall increased from 0.17%(air-cooling)to 0.21%;the secondary dendrite arms is reduced from 4.1?m(air-cooling)to 3.4?m(impose cooling),and the average size of the eutectic Si in the layer was reduced from 0.7?m(air-cooled)to 0.4?m,and the aspect ratio was reduced from 2.5(air-cooling)to 2(impose cooling);the average size of the Si phase inter layers was reduced from 1.4?m to 1.1?m,the aspect ratio is 2;It is not obvious that morphology of the inter-layer Si phases by imposing cooling.The impose cooling process improves the yield strength of Al-12Si alloy straight wall,reduces its elongation,and nuchange its tensile strength.Compared with the straight wall prepared by the air cooling process,the yield strength of the straight wall prepared by the forced water cooling process is increased by about 45 MPa in the welding direction and by about 23-32 MPa in the additive direction.the elongation of the straight wall prepared by the impose cooling process is reduced by about 5%in the welding direction and about 30%in the additive direction;cracks originated at the interface between the interlayer of ?-Al and Si,and a large amount of dimples was observed on tensile fracture,so it is judged to be ductile fractures.Al-12Si straight wall with cooling conditions A has the best comprehensive mechanical properties.In order to control the solidification characteristics of WAAM,an FSP process was applied in the process of an Al-12Si alloy straight wall by using the WAAM.The results show that the welding defects and dendrite microstructure characteristics of the straight wall are eliminated by FSP,and the structure is significantly refined.It is worth mentioning that the Si phase in the original WAAM layer was broken by FSP,and the spatial morphology changed from coralline to partical and uniformly distributed in the sample.The average size of the Si phase is 0.6?m,and the aspect ratio is 1.4;the spatial morphology and dimensional change of the interlayer granular Si phase in the original interlayer deposition are not obvious change under the action of FSP,showing a continuous distribution along the direction of metal flow in the stirring zone,and its average size is 1.4?m.Compared with the straight wall formed by the WAAM process,the straight wall formed by the WAAM+FSP process is notable to decrease in yield strength,a significant increase in the elongation,and a significant decrease in tensile strength.Original deposition is changed by FSP.The above experimental phenomenon resulting from morphology and size of the Si phase within the original WAAM interlayer metal. |
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