Effect And Mechanism Of Ultrasonic Impact On The Anisotropy Of Low-carbon Steel Fabricated By Wire And Arc Additive Manufacturing

In recent years,with the increasing requirements for performance,precision,cost and manufacturing cycle of metal parts in the manufacturing field,it is difficult to fully meet the market demand by traditional manufacturing methods.Well,the rapid development of additive manufacturing technology has provided the possibility of low-cost and highly flexible manufacturing of various metal parts,especially in forming complex metal parts with unique advantages.As one of the additive manufacturing technologies,wire and arc additive manufacturing(WAAM)has gained a lot of attention from researchers and manufacturers for its advantages such as high deposition rate,high material utilization,low cost and environment-friendly way.Depositions experience large differences in melting,solidification,heating and cooling conditions in different directions in the process of WAAM.The significant temperature gradients results in a certain degree of anisotropy in the organization and properties of the parts,which seriously limits the application and development of the technology.In this paper,a low-carbon high-strength steel fabricated by WAAM for marine vessels is studied.It starts with the forming process and then reflects in the forming quality,microstructure and mechanical properties of thin-wall parts.As well,the anisotropy of the parts is investigated.Microstructure refinement,mechanical properties improvement,and stress concentration and anisotropy relief of thin-wall parts can be achieved by introducing inter-layer ultrasonic impact reinforcement during the process of WAAM.And on the basis of the existing ultrasonic impact theory,the mechanism of ultrasonic impact on the microstructure of WAAM parts and the mechanism of action are enriched,providing a theoretical basis and technical support for the promotion and application of WAAM assisted by inter-layer ultrasonic impact reinforcement.The main research content and significance are shown as follow:In order to obtain thin-walled parts of excellent quality,a number of studies have been carried out on the deposition process of the parts.Starting from the most basic single-pass single-layer forming process,the relationship between process parameters and forming morphology has been analyzed from the view of energy.The research shows that when increasing linear energy density,the bead width,bead height,penetration and dilution increase.And linear energy density shows the most significant effect on the bead height and the least significant effect on the dilution.According to bead molding results with different process parameters,a process window is established based on linear energy density.It is found that when the linear energy density is greater than 400 J/mm,single-pass single-layer parts can be formed.Based on the study of the single-pass single-layer part forming process,a model between heat input and single-layer part forming morphology is established,and the relationship is analyzed theoretically.In addition,in order to obtain a thin-wall deposition,important forming assessment parameters such as surface roughness,straightness and deposition efficiency of single-pass multi-layer parts are defined to establish a relationship between process parameters and forming quality.Analysis on the variation of forming assessment parameters of single-pass multi-layer parts under different heat input conditions is made.The result shows that when the linear energy density is around 600 J/mm,multi-layer depositions with high quality can be obtained.A 25-layer thin-wall part with uniform dimensions,good surface quality and no obvious deposition defects is deposited under this process,and the deposition efficiency can be up to 80%.After obtaining the thin-wall part made of low-carbon high-strength steel,its surface quality,microstructure characteristics and mechanical properties are evaluated.The surface quality of the thin-wall part is observed by OM and SEM.The results show that the part surface is smooth,and the wire is fully melted.The overall deposition quality is satisfied,and no unfused conditions,cracks,inclusions and porosity are found.The macrostructure of the thin-wall part is dominated by a large number of columnar grains,which shows an obvious directional growth,while the phase composition from the top to the bottom of the microstructured parts varies slightly,resulting in microhardness fluctuations between 290HV0.2 and 270 HV0.2.The transversal properties of the part are 4.2% higher in tensile strength,10.1% higher in yield strength,and 21.4% higher in elongation compared to the longitudinal properties during the process of quasi-static tension,showing anisotropy.In order to explore the causes for anisotropy,the strain evolution is analyzed by DIC during the quasi-static tension process in the transversal and longitudinal directions.It is found that a certain number of local strain concentration regions exist during the longitudinal tension process,and these regions show a periodic distribution.It can be seen that the local strain concentration basically occurs in the inter-layer area,causing local necking.The abrupt changes of the dimples can be observed in the fracture of the adjacent area,which leads to stress concentration and a decrease of longitudinal tensile properties.To further analyze the reasons for stress concentration and anisotropy in the inter-layer area,SEM and EBSD are applied to investigate element distribution,microstructure morphology,phase composition,carbide content and distribution,grain size and texture between inter-layer area and deposited area.The result shows that the main difference in both areas reflecting in microstructure,grain size and texture rather than element distribution.Additionally,compared to the deposited area,the lower number of large angle grain boundaries,larger grain size,inhomogeneous Schmid factor distribution,and more significant texture in the inter-layer area are also responsible for the stress concentration during the longitudinal tension.For phenomena such as stress concentration and anisotropy in the thin-wall part,the inter-layer ultrasonic impact enhancement is introduced to relieve the stress concentration and anisotropy of the thin-wall part.Firstly,the analysis starts from the action depth of ultrasonic impact on single-layer deposition.It is found that inter-layer ultrasonic impact enhancement can promote the formation of a large number of substructures(including subboundary and low angle grain boundary)in the deposition microstructure at a certain depth.The effect decreases with increasing distance from the impact surface.The main reason for this phenomenon is that the ultrasonic impact will break the binding of dislocations in the surrounding microstructure during the process of impacting on the part.And the dislocations will be merged and annihilated during the movement of dislocations,and then a large number of substructures will be formed.The internal substructures will be locally recrystallized by the thermal effect of the subsequent depositions,which can be confirmed by the comparison of the thermal expansion results of the parts before and after the ultrasonic impact.The occurrence of recrystallization can effectively promote the transformation of columnar grains to equiaxed grains in the deposition macrostructure.This transformation is mainly due to the grain refinement caused by recrystallization,which effectively blocks columnar grains growth and leads to the formation of a cellular with a smaller length and width or equiaxed grain.In addition,the ultrasonic impact will induce oxidation effect on the surface of the part,and the oxide formed will promote nucleation in the form of heterogeneous nucleation masses during the solidification of liquid metal in the melt pool under the remelting effect of the subsequent deposition,promoting the transformation of columnar grains to equiaxed grains.The result indicates that in the equiaxed grain area treated by ultrasonic impact,local misorientation is reduced,grain is refined,and the strength of texture is significantly weakened.During the DIC test on the part treated by ultrasonic impact,it is found that the local stress concentration problem is significantly relieved,the transversal and longitudinal tensile properties and elongation are significantly improved,and the anisotropy percentage is significantly reduced,which effectively improves the uniformity of the part.