Effect Of Additive And Subtractive Hybrid Manufacturing And Ultrasonic Surface Rolling Process On Surface Integrity Of 18Ni300 Maraging Steel

18Ni300 maraging steel has been widely used in aviation,aerospace,medical,chemical,precision mold and other fields because of its excellent mechanical properties.As an excellent tool steel,18Ni300 maraging steel can be shaped into the mold with conformal cooling channels by selective laser melting(SLM)technology to meet the market demand.As a typical laser 3D printing technology,selective laser melting has the advantage of free forming,but also has the defects of poor surface quality and large dimensional errors.Although the traditional machining subtractive manufacturing is not high forming freedom,but its excellent surface quality and dimensional accuracy can make up for the defects of additive manufacturing to a certain extent,so it can be combined with 3D printing,that is,additive technology into Additive and Subtractive Hybrid Manufacturing(ASHM).Ultrasonic surface rolling process(USRP)is the process of rolling and extruding the surface of metal parts by "cutting the peaks and filling the valleys",which causes strong plastic deformation on the surface of the parts in order to obtain good surface integrity.This thesis takes 18Ni300 maraging steel as the research object,and uses selective laser melting process to form 18Ni300 maraging steel,and co MPares the effect of different laser process parameters(laser power,scanning speed)on the surface integrity of SLM formed maraging steel,and obtains the best process parameters for SLM forming maraging steel.Subsequently,the SLM-formed maraging steel was milled to co MPare the effects of different milling process parameters(cutting speed and feed)on the surface integrity of milled SLM specimens and to obtain the optimal process parameters for milling SLM maraging steel.Finally,the surface of milled SLM maraging steel was strengthened by ultrasonic surface rolling process,and the effects of different process parameters(amplitude,static pressure,feed)on the surface integrity of ultrasonic surface rolling process strengthening were co MPared,and the optimal process parameters for ultrasonic surface rolling process of additive and subtractive hybrid manufacturing maraging steel were obtained.The main findings are as follows:(1)In the SLM process,increasing the laser power can improve the surface morphology,but there is a balling effect regardless of the laser power variation.The surface quality of SLM specimens gets worse as the scanning speed increases.SLMformed 18Ni300 maraging steel is dominated by columnar and equiaxed crystals,and Epitaxial structure is observed at the melt pool boundary.Laser power and scanning speed have no significant effect on the microstructure changes of SLM-formed specimens.The microhardness of SLM specimens increases and the error decreases with increasing laser power,and decreases and the error increases with increasing scanning speed.With the increase of laser power,the fluctuation of surface residual stress becomes smaller,and the scanning speed is opposite.When the laser power is 180 W and the scanning speed is300mm/s,the surface is the flattest and the microhardness value is the largest,while the fluctuation of residual stress is the smallest.(2)Milling subtractive can significantly improve the surface integrity of SLM specimens,with a significant reduction in roughness and a surface change from the irregular fusion channel of laser additive to machined texture.The roughness increases when the feed rate increases,the machined texture spacing becomes larger,and the maximum distance between the crater and the bump becomes larger.When the cutting speed increases,the surface roughness increases and then decreases,the surface texture spacing becomes smaller,and the maximum distance between the bump and the crater increases and then decreases.Milling can make the surface layer of SLM specimen plastic deformation and grain refinement.Cutting speed and feed rate will deepen the depth of plastic deformation,increasing cutting speed will strengthen the grain refinement effect,while increasing the feed rate will weaken the grain refinement.Therefore,an increase in cutting speed will also increase the microhardness,and an increase in feed rate will decrease the microhardness.In addition,the residual tensile stress is introduced on the surface of the sample during milling because the mechanical effect is weaker than the thermal effect,which decreases with the increase of feed and increases with the increase of cutting speed.The surface quality is best when the cutting speed is 10m/min and the feed rate is 36mm/min,the roughness value is 2.1μm,and the maximum distance between the convex peak and the crater is 52μm.Microhardness of 465 HV,a 31% increase co MPared to SLM specimens.The lowest residual tensile stress was 245.48 MPa at a feed rate of 98mm/min and a cutting speed of 10m/min.(3)Substantial improvement in surface integrity of 18Ni300 maraging steel made from additive and subtractive hybrid manufacturing after ultrasonic surface rolling process treatment.The surface finish is obviously better,and the surface reaches the mirror effect.When the amplitude increases,the surface roughness decreases,the static pressure increases so that the roughness first becomes smaller and then larger,and when the feed rate increases,the roughness first decreases slightly and then increases.The i MPact of ultrasonic surface rolling process on the specimens of additive and subtractive hybrid manufacturing causes strong plastic deformation and grain refinement,so that the microhardness also increases significantly co MPared to that before ultrasonic surface rolling process,and introduces residual compressive stresses in the surface layer of the material.The increase of amplitude can promote the plastic deformation depth,refined layer depth,microhardness and residual compressive stress in the surface layer of the material.When the static pressure increases,the plastic deformation depth gradually increases,the refined layer depth increases first and then decreases when the static pressure increases,so the microhardness and residual compressive stress also become larger and then decreases with the increase of static pressure.When the feed rate increases,the plastic deformation zone and the grain refinement layer first remain almost the same and then decrease significantly,so when the ultrasonic surface rolling process feed rate increases,the microhardness and the residual compressive stress on the surface of the specimen first increase slightly and then decrease.When the amplitude is 8μm,the static pressure is 600 N,and the feed rate is 0.08mm/r,the surface modification effect of18Ni300 maraging steel specimens made by additive and subtractive hybrid manufacturing is best after ultrasonic surface rolling process.The lowest roughness of ultrasonic surface rolling process sample was obtained at an amplitude of 8μm,a static pressure of 600 N and a feed of 0.08mm/r,which is 0.126μm,95.8% lower than before ultrasonic surface rolling process.Maximum microhardness of 541.6 HV,an increase of19.3% co MPared to before ultrasonic surface rolling process and 52.4% co MPared to the original SLM specimen.Maximum residual compressive stress of-1229.7MPa,a significant improvement co MPared to before ultrasonic surface rolling process.