Processing,Microstructure And Properties Of Of Alloy Steel Fabricated By Wire And Laser Additive Manufacturing

Wire and Laser Additive Manufacturing(WLAM)is a digital deposition technology based on the "discrete-stacking" principle,which is characterized by high deposition rate,high material utilization rate and high flexibility,and has been widely used in the manufacture of large metal components by stacking layers to produce structurally complex parts.Compared with the traditional forged parts,the mechanical properties of the parts formed by WLAM are lower,and there are problems such as coarse microstructure,significant anisotropy,high residual stress and thermal deformation,which largely limit the application of WLAM technology in the manufacturing field.Therefore,in this paper,different treatments(heat treatment,electric pulse treatment and ultrasonic microforging treatment)are used to improve the surface quality,microstructure and mechanical properties of the fabricated parts for WLAM of GHS785 L low-carbon high-alloy steel.By summarizing and analyzing the morphology of formed single-pass single-layer and single-pass multi-layer fabricated parts under different process parameters,the influence of process parameters on the tissue properties of the deposited layer is explored to achieve process optimization for the deposition of thin-walled fabricated parts.In addition,three means of heat treatment,electric pulse and ultrasonic microforging are used for the deposited layer to treat the formed alloy steel thin-walled fabricated parts for the purpose of improving the microstructure and mechanical properties.The mechanism analysis of the effect of the deposited layer under different treatment processes on the microstructure and mechanical properties are also studied and the results are shown as follow:For single-pass single-layer deposition,with the increase of laser power,the melt width,the width-to-height ratio and the dilution of the deposited layer gradually increases,while the melt height and the contact angle decreases.The bottom of the deposited layer is consisted of coarse columnar crystals,and the microstructure in the middle and bottom of the deposition shows obvious directionality.With the increase of laser power,the martensite content of the deposited layer gradually decreases,and the average hardness decreases from 440 HV to410 HV.With the increase of scanning speed,the melt width and height of the deposited layer decrease.The width to height ratio changes at a small extent,and the contact angle decreases at a large extent.The dilution rate shows a gradually decreasing trend,in general,the scanning speed has no obvious effect on the microstructure and hardness of the deposited layer.For single-pass multilayer deposition,the bottom of the deposited layer is in contact with the substrate,leading to high-temperature tempering,which generates tempered sorbite.With the increase of the number of layers,the role of multiple thermal cycles between the layers prompts to generate many granular bainite.The higher the laser power is,the stronger the direction of granular bainite shows.Lath martensite can be found at the deposited layer surface,which distributes as block.At the same time,the higher the power is,the less martensite exists.With increasing the power of single-pass multilayer deposition,average hardness varies from 380 HV to 360 HV.The change of scanning speed shows little effect on microstructure and hardness.Based on the above summary and analysis,the optimal forming process parameters are obtained,i.e.,laser power 2500 W,scanning speed 4.5mm/s,and wire feeding speed 20mm/s.Different tempering temperatures are selected to treat the thin-walled parts formed by WLAM.With the increase of tempering temperature,the martensitic characteristics of the slats gradually disappears and the tempered sorbite was obtained.While the tempering treatment shows no effect on the phase composition and element content and distribution of the thin-walled parts.Meanwhile,with the increase of tempering temperature,the microhardness gradually decreases and the maximum value reaches to 36.7 HV.The hardness distribution of the deposited layer is more uniform after tempering treatment.With the increase of tempering temperature,the yield strength and tensile strength of the thin-walled parts decrease by 77.09 MPa and 145.8 MPa,respectively,and the elongation increases by5.72%.Different pulse voltages are selected for the electric pulse treatment of the thin-walled parts.When the pulse voltage is set at 64 V,the electric pulse treatment could refine the grains and make the microstructure distribute more uniform.As the pulse voltage increases to 81 V,the microstructure grows significantly and distributes in a blocky manner.The blocky pearlite is also generated.The electric pulse could promote the diffusion of elements and enrich the elements toward the grain boundaries.With the increase of the pulse voltage,the average hardness of the deposited layer continues to increase,and the maximum value can reach to69.2 HV.The hardness of the thin-walled parts distributes more uniformly after the low-voltage electric pulse treatment is applied.With the increase of pulse voltage,the yield strength of the thin-walled parts increased by 33.36 MPa,the tensile strength increased by58.72 MPa,and the elongation decreased by 8.32%.The ultrasonic microforging technique is used to treat the single-pass single-layer and single-pass multi-layer deposited layers.The results show that the technique could cause significant plastic deformation on the top of single-pass single-layer deposited layers,leading to significant hardness increase.While the microstructure and mechanical properties of single-pass multi-layer deposition are not significantly affected.By comparing the effects of three different treatment methods on the mechanical properties of thin-walled parts,it is found that better results in terms of property improvement can be obtained by using the electric pulse treatment.