Effect Of Beam Energy Density Characteristics On M Icrostructure And Mechanical Properties Of Ni-based Alloy By Laser Additive Manufacturing

Laser additive manufacturing technology has the advantages of short processing cycle,no mold,and is not limited by complex structures.It has great application prospects in aerospace,automobile manufacturing,biomedicine,national defense and other fields.Laser additive manufacturing forming is a complex chemical,physical and metallurgical process.The quality of formed parts is affected by various process parameters.Among them,the characteristics of laser beam energy density distribution directly affect the temperature gradient,cooling rate and solidification rate of the molten pool,which in turn affects the microstructure and mechanical properties of the formed parts.It is of great significance to explore the influence of beam energy density distribution characteristics on the main properties of formed metal parts and reveal the relevant mechanisms to improve the quality of laser additive forming.For this reason,this thesis takes GH3625 nickel-based superalloy as the object,and studies the influence of laser energy density distribution characteristics on the microstructure and mechanical properties of additively manufactured nickel-based alloys.Firstly,the basic theory of laser beam energy density distribution characteristics was studied.The energy density of the traditional Gaussian beam and the flat-topped beam was characterized,and the energy density distribution of the Gaussian laser beam and the flat-topped laser beam was visualized using MATLAB software in combination with the formula.The control method of laser beam energy density distribution was discussed,and the influence of beam order and defocus amount on laser energy density distribution is analyzed.The theoretical calculation and image drawing were performed by MATLAB programming software,and the defocus amount and normalized energy density were obtained.The correspondence between the distribution characteristics proves the effectiveness of changing the beam energy density distribution characteristics by controlling the defocus amount.Secondly,the nickel-based alloy additive manufacturing experiment was carried out,and the sample performance test was completed.In the experiment,5 kinds of defocus amount parameters are set,and the laser energy density distribution characteristics are controlled by changing the defocus amount,that is,5 kinds of defocus amounts correspond to 5 kinds of beam energy density distribution characteristics,and 3 kinds of laser power and scanning speed parameters are set at the same time.Additively manufactured nickel-based alloy samples under different parameters were prepared;the samples were observed and analyzed for macrostructure and microstructure,density measurement,microhardness test,and tensile properties and friction and wear properties were tested and analyzed.Thirdly,the influence of laser power,scanning speed,and energy density distribution characteristics on the macroscopic morphology and microstructure of the nickel-based alloy samples manufactured by laser additive manufacturing were analyzed and obtained,and the mechanism of defect generation was explored.Low laser power or excessive scanning speed will cause insufficient energy absorption of the metal powder,resulting in insufficient melting of the metal powder,resulting in an uneven forming surface.Too high laser power or too low scanning speed will cause serious oxidation of the forming surface.When the energy density distribution is changed from feature 1 to feature 4,the energy density distribution is gradually uniform,the temperature gradient is reduced,and the forming quality of the sample is better.When the energy density distribution is feature 5,although the energy density distribution is uniform,the laser energy density is small,the molten pool temperature is low,the fluidity is poor,and the quality of the formed sample is poor.Within a certain range,the smaller laser power and the larger scanning speed will make the grain size of the tissue finer.When the energy density distribution is characteristic 1 and characteristic 2,cellular crystals and a small number of columnar crystals are distributed in the tissue.When the energy density distribution is changed to feature 3 and feature 4,the structure grains are mainly equiaxed.When the energy density distribution is characteristic 5,the structure is the coexistence of columnar crystals and equiaxed crystals.Moreover,the density of nickel-based alloys manufactured by laser additive manufacturing first increases and then decreases with the increase of laser power and scanning speed,and first increases and then decreases with the change of energy density distribution characteristics.Finally,the effects of laser power,scanning speed and energy density distribution characteristics on the microhardness,tensile properties and friction and wear properties of nickel-based alloy formed samples were analyzed and obtained,and the related mechanisms were discussed.The microhardness of nickel-based alloys manufactured by laser additive manufacturing first increases and then decreases with the increase of laser power and scanning speed,and first increases and then decreases with the change of energy density distribution characteristics.When the laser power is 600 W,the scanning speed is 700mm/min,and the energy density distribution feature is 4,the microhardness is the largest.The tensile strength first increases and then decreases with the increase of laser power,and first increases and then decreases with the change of energy density distribution characteristics.When the laser power is 600 W and the energy density distribution characteristic is 4,the maximum tensile strength measured in the parallel scanning direction is 1328 MPa,and the elongation is 10.06%.The maximum tensile strength measured in the vertical scanning direction is 1214 MPa,and the elongation is 9.16%.The friction coefficient and wear rate first decreased and then increased with the laser power and scanning speed,and decreased first and then increased with the change of the energy density distribution characteristics.When the laser power is 600 W,the scanning speed is 700mm/min,and the energy density distribution characteristic is 4,the friction reduction and wear resistance of the sample are the best.