Research On The Process,Microstructure And Property Of Tungsten Alloys Fabricated By Laser Direct Deposition

Tungsten alloys consist of tungsten as the hard phase and copper,nickel,iron,etc.,transition metals as the binder phase,which can be assumed as composite materials.Since tungsten has a series of outstanding performance such as high strength,high hardness,low coefficient of thermal expansion,excellent corrosion resistance and radiation shielding property,tungsten alloys have been widely used in the fields of aerospace,electronics,nuclear and defense industries.However,tungsten has high hardness,high melting point and brittleness at room temperature.As a result,it is difficult to form tungsten alloys parts via the traditional machining processes.At present,sintering is the main method to fabricate tungsten alloys parts.Nevertheless,this method has the shortcomings of long production time and high cost.Also,sintering is unable to fabricate complex shaped parts.These shortcomings have limited the application range of tungsten alloys parts.The laser direct deposition,a kind of additive manufacturing technologies,has the advantages of short production time and low cost.In particular,it can fabricate complex-shaped parts without molds.Therefore,the laser direct deposition can replace part of traditional sintering technology and thus become the new technology to fabricate the tungsten alloys parts in the future.In this paper,laser direct deposition was used to prepare W-Cu and W-Ni heavy alloys.The microstructure and properties under different laser processing parameters were investigated.Then the optimal processing parameters were obtained.The combination of high-speed imaging and numerical simulation was conducted to analyze the temperature rise of powder particles under the laser radiation,the temperature and velocity field change of the melt pool,the effect between the falling particles and the melt pool.Through this research,we can understand the mechanism between laser and powder particles,and the evolution law of microstructure.The research result can provide theoretical guidance for producing tungsten alloys components with high quality.The main research works and results are as follows:（1）The influence laws of laser processing parameters on the microstructure,relative density and tensile property of 60W40Cu thin walls were investigated.The optimal parameters were obtained.Based on that,increasing the W content and adding the activating element Ni were applied to reduce the microstructure defects.The results show that the W particles distributed ununiformly in the 60W40Cu alloy deposition layer.A lot of W particles were distributed on the outer surface of the deposition layer.The W particles were prone to accumulate and then caused residual pores in the interlayer bonding area under the scanning speed of 2 mm/s.As a result,the relative density of the thin walls produced with the scanning speed of 2 mm/s was lower than those produced with the scanning speed of 3 mm/s.Similarly,the tensile strength of the thin walls produced with the scanning speed of 2 mm/s was lower than those produced with the scanning speed of 3 mm/s.Both increasing the W content and adding the activating element Ni were beneficial to make the distribution of W particles more uniform and increase the relative density.By comparison with increasing the W content,adding Ni was more effective in densification and could also increase the tensile strength.（2）The geometrical profile of the W-Cu powder flow and the flying speed of powder particles were measured via high-speed imaging.Based on that,a laser-powder flow interaction model was constructed.The calculation results show that there is a huge temperature difference between W and Cu particles due to the great difference between their laser absorptivity.When the laser power increases from 100 W to 900 W,the temperature of the Cu particles reach the laser beam center on the substrate rises slightly from 317 K to 535 K,which is far below the melting point of Cu.By contrast,the temperature of the W particles rises significantly from 654K to 3538 K.（3）According to the evolution process of the W-Cu melt pool observed by high-speed imaging,a two-phase（W-Cu deposition layer and air）fluid dynamics model was constructed.The simulation results indicate that the W and Cu particles begin to deposit on the substrate surface when the substrate surface is beyond its melting point under laser irradiation.However,the W-Cu composite phase has lower laser absorptivity but higher thermal conductivity than the substrate,resulting in a decrease in temperature.Afterward,the temperature of the deposition layer gradually rises above the melting point of Cu under the continuous laser radiation,which leads to the formation of a melt pool.When the laser radiation is over,the molten Cu begins to solidify with a cooling rate of about 10⁴ K/s.（4）A three-phase（W,Cu particles,and air）fluid dynamics model was constructed to simulate the microscopic particle motion during laser direct deposition.The simulation results show that a fallen W particle can float on the melt pool since the inertial force of the falling W particle and the surface tension between Cu and Wσ_Cu-W are roughly balanced.In comparison,the Cu particle that falls on the floating W particle can be melted rapidly and finally flow into the melt pool,which is caused by the laser radiation and the heat conducted from the W particle.As a result,the number of W particles floating on the melt pool will gradually increase over time.The simulation results can be used to interpret the reason why many W particles were distributed on the outer surface of the 60W40Cu deposition layer.（5）The influence laws of laser processing parameters on the microstructure and property of 85W15Ni tungsten heavy alloy thin walls were investigated.The optimal parameters were obtained.The results show that increasing the laser volume energy density was beneficial to increase the relative density and decrease the residual pores of the samples.When the laser volume energy density was more than 380 J/mm³,the densification curve became smooth and the relative density stabilized between 95%and 97%.The optimal tensile property was achieved when the laser volume energy density was 395 J/mm³.The excessive laser energy density would promote the bonding between W particles or W dendrites,forming more W-W bonding interfaces.Since the bonding between W particles or W dendrites was fragile and easy to be separated,the excessive laser energy density would decrease the tensile strength and elongation.Through the investigation above,the laser direct deposition process of tungsten alloys was disclosed.Meanwhile,the interaction mechanism between the laser and powder flow was understood,the optimal processing parameters were obtained.This research results can provide a foundation for the application of laser direct deposition technology in the field of fabricating tungsten alloys in the future.