Microstructure And Properties Of A517Q Alloy Steel Fabricated By Laser And Wire Additive Manufacturing Assisted With Ultrasonic

In recent years,laser and wire additive manufacturing(LWAM)has been widely used in aerospace,energy power,automobile industry and other fields because of its high material utilization,low production cost and high deposition efficiency.However,in the process of LWAM,due to the high energy of the laser,the melting,solidification and cooling of materials are completed in a short period of time,which makes the microstructure in the deposition layer uneven,and it is easy to form thick columnar dendrites,resulting in a decrease in the mechanical properties of deposition layer.Therefore,how to improve the microstructure of the deposition layer and promote the columnar grains to equiaxed grains transition(CET)is one of the current research focuses in the field of metal additive manufacturing.Existing studies have shown that the ultrasonic energy field can improve the microstructure and optimize the mechanical properties by the effects of plastic deformation,sound flow and cavitation on the solid deposition layer and liquid molten pool respectively.However,the research in this area mainly focuses on the effect of ultrasonic energy field on solid deposition layer or liquid molten pool.This mode of action can only play a part of the role of the ultrasonic energy field,that is,it is either dominated by the plastic deformation of the solid deposition layer,or dominated by the sound flow and cavitation effects of the liquid molten pool,both of them can improve the microstructure and properties of additive manufacturing parts to a certain extent,but they all have certain limitations.Therefore,on the basis of the current research,this paper puts forward the method of dual ultrasonic vibration(UV)treatment,that is,during the LWAM process,the ultrasonic energy field has an effect on both the solid deposition layer and the liquid molten pool,thus improving the microstructure of the deposition layer and further improving the mechanical properties of the additively manufactured parts.The influence of LWAM A517 Q alloy steel forming process and ultrasonic energy field application method on the macroscopic morphology and microhardness of the deposited layer was studied.The results show that with the increase of the ultrasonic output amplitude and power,the contact angle and height of the deposition layer decreases,the aspect ratio and dilution rate increase greatly,and the area of equiaxed grains in the deposition layer increases,and the microhardness increased from 365.4 HV to 394 HV(ultrasonic amplitude of 20 μm)and 396.8 HV(ultrasonic power 1000 W)respectively.After dual UV treatment,compared with single UV treatment,the height of the deposition layer is basically unchanged,but the deposition layer has a larger width and a smaller depth,the grain size of the deposition layer is refined,and the microhardness is increased from 365.4 HV to 416.3 HV.According to the morphology and mechanical properties of the deposited layer,the optimized manufacturing parameters of LWAM are laser power of 3300 W,wire feeding speed of 1.2 m/min and scan speed of 288 mm/min,the optimal parameters of the ultrasonic energy field are the ultrasonic output amplitude of 20 μm and the ultrasonic power of 1000 W.Based on the optimization of process parameters,the influence of different ultrasonic application methods on the microstructure and mechanical properties of the single-pass multilayer deposition layer was studied.By comparing the results of texture density,grain size,equiaxed grain ratio,microhardness and tensile properties,it is found that dual UV treatment has the best effect on improving the microstructure and properties of the deposited layer.In the surface ultrasonic action area,the primary austenite grain refinement phenomenon is most obvious in the area where the ultrasonic energy field acts on both the liquid molten pool and the solid deposited layer,and the proportion of low angle grain boundaries is significantly increased by 31.2%,the average grain size is 2.17 μm,reduced by 23.9%,and the proportion of equiaxed grains is as high as 56.1%.Compared with the sample without UV treatment,the average grain aspect ratio of the sample with dual UV treatment decreased from 3.85 to 2.11,and the proportion of equiaxed grains increased to 76.8%,and the average microhardness,tensile strength and yield strength of the deposited layer increased by 8.3%,29.8% and 41.2%,respectively.In order to further reveal the mechanism of ultrasonic sound flow and cavitation effect on molten pool,the effects of different ultrasonic application methods on molten pool solidification process were studied by numerical simulation.The effects of different ultrasonic output amplitudes and powers on the sound pressure field and temperature field of the molten pool were studied.With the increase of the ultrasonic output amplitude and ultrasonic power,the ultrasonic cavitation range increases,while the maximum temperature in the molten pool decreases.Through the comparison of theoretical calculation and numerical calculation results,it is found that after UV treatment,the negative pressure in the molten pool is much greater than the ultrasonic cavitation threshold,indicating that ultrasonic cavitation can occur in the molten pool after UV treatment.The complete solidification time of the molten pool was significantly reduced after UV treatment,from 0.27 s without UV treatment to 0.06 s(surface UV),0.05 s(bottom UV)and 0.03 s(dual UV).The three ultrasonic application methods all promote the columnar grains to equiaxed grains transition,the surface UV treatment contributes more to the reduction of temperature gradient,and the bottom UV treatment contributes more to the increase of solidification speed.Under the dual UV treatment,the temperature gradient decreases and the solidification rate increases at the same time,the solidification structure of the molten pool is almost entirely equiaxed grains,and the grains are obviously refined.