Microstructure And Mechanical Properties Of GH3039 Superalloy Processed By Laser Metal Deposition Assisted With Ultrasonic Micro-forging Treatment

Additive manufacturing(AM)technology provides a new solution for the processing and manufacturing of high-performance,difficult-to-process material parts due to its characteristics such as no molds and short production cycles.However,the solidification process of AM is non-equilibrium.It is easy to form heterogeneous microstructure,reduce the mechanical properties of parts,and restrict the development and application of AM technology.In recent years,special energy fields have been continuously applied to AM to change the solidification process in the AM process,thereby improving the microstructure and performance of parts.Special energy field-assisted manufacturing technology has great potential in improving the manufacturing problems of high-strength and difficult-to-process materials,and its scientific principles are still today's leading scientific issues.Therefore,this paper investigates the ultrasonic micro-forging treatment(UMFT)assisted laser metal deposition(LMD)technology.The ultrasonic field is introduced in the LMD process.The single-track single-layer cladding layers and multilayer of GH3039 nickel-based superalloy were prepared by LMD assisted with UMFT.The microscopic analysis instruments,such as optic microscope(OM),scanning electron microscope(SEM)and ultrasonic scanning microscope(USM),were used to investigate the defects and microstructure of the deposited cladding layers.The mechanical performance test equipment,such as Nano indentation apparatus,Vickers haedness tester and electronic universal mechanical testing machine,were used to investigate the mechanical properties of the deposited cladding layers.The microstructure and mechanical properties characteristics and evolution of the deposited cladding layers with multi-physics fields(laser field and ultrasonic field)were studied,and the main control process parameter categories for the formation of defects and microstructure were revealed.The results show that there are two morphologies of grains for the deposited cladding layers,which are columnar dendrites mainly distributed at the bottom,and equiaxed grains mainly distributed at the top of the deposited cladding layers.Due to the influence of cavitation and acoustic streaming effects induced by the ultrasonic vibration,the front of columnar dendrites are fractured and detached.The fractured and detached dendrites enter the mushy zone or melting pool as new nucleation cores.Then heterogeneous nucleation happens,and the columnar-to-equiaxed transition(CET)are promoted.The proportion of equiaxed grains are largest when the ultrasonic input power(UIP)is 60%(300W).With UMFT,the grain size is refined to 10.0?m from16.0?m.The granular shaped?'and MC phases with sizes of less than 5?m,were precipitated at grain boundaries.The refined grain and existence of precipitated phase can increase the microhardness of the cladding layer.As UIP incrased from 0 to 90%,the microhardness of the top region of the cladding layer increased from 295HV to430 HV,with an increase of 46%.The existence of defects,such as pores,would lead to a decline of microhardness.The overlap rate of 30%is too small that burrs appear on the surface and collapse appear at the edge passes of the deposited multiple layers.The interlayer prosity,intralayer pores and micro-cracks are the main defects of the deposited cladding layers.With UMFT,the interlayer prosity is reduced by 18.4%?39.6%,the reduction value is proportional to the distance from the substrate.The intralayer pores are reduced and the relative density is increased to 99.4%from 98.3%.The initiation and propagation of microcracks are inhibited with UMFT.As the number of layers increases,the temperature gradient decreases,and the proportion of equiaxed grains at the bottom,middle and top regions of the deposited cladding layers are increased,which are 15.1%,30.0%and 56.3%,respectively.With UMFT,the proportion of equiaxed grains at the bottom,middle and top of the deposited cladding layers increased to 31.9%,39.2%and60.1%,respectively.Due to the limited depth of the ultrasonic field,the effects of UMFT on CET for the bottom to top region of the deposited cladding layers are reduced from 111.3%to 6.7%,but the effects on refining the equiaxed grains are increased from5.0%to 23.7%.The existence of defects would decline the interfacial bonding strength.UMFT could reduce defects and promote the formation of equiaxed grains,improve the distribution uniformity of microhardness and the interfacial bonding between layers of the deposited cladding layers.The deposited cladding layers have excellent plasticity.But the presence of interlayer defects would decline the nominal yield strength(?_0.2)of the deposited cladding layers.In addition,equiaxed grains have better strain coordination ability than columnar dendrites,which could suppress the generation of microcracks during compression deformation.The overlap rate and the matching of laser power,scanning speed and wire feeding speed are the the main control process parameter categories of defect formation for the deposited cladding layers.The laser power,scanning speed and UIP are the main control process parameter categories of microstructure formation for the deposited cladding layers.When the overlap rate increases from 30%to 40%,the scanning speed increases from 2mm/s to 3mm/s,the wire feeding speed decreases from 15mm/s to10mm/s and the laser power maintains at 2.4kW.The thickness of the deposited cladding layers could be decreased.So that the ratio of the depth of UMFT to the thickness of the single layer could be increased,and further reduce defects and refine equiaxed grains.In this paper,the microstructure and mechanical properties of GH3039 superalloy processed by LMD assisted with UMFT were analyzed.The results show that UMFT can reduce defects,promote the formation of equiaxed grains,enhance microhardness and improve the bonding of the layer-by-layer boundary.The ultrasonic field introduced by UMFT can continuously affect the melting pool and is not limited by the forming size,which provides a new approach for improving the microstructure and mechanical properties of high-performance parts with large sizes by AM layer by layer.