Research On Forming Technology Of Pure Tungsten Material By Selective Laser Melting

Tungsten is an irreplaceable material for preparing key components of CT equipment because of its good absorption and shielding ability to high-energy rays.Tungsten Anti-Scattering Grid(ASG)is used to absorb the scattered X-rays generated by Compton Effect,so as to improve the imaging quality and clarity.ASG with high density,thin walls,microporous and complex structure will be the development direction in the future.This kind of structure is difficult to be prepared by traditional process,especially for tungsten with high melting point,high hardness and high brittleness,which further increase the difficulty of preparation.It is possible to fabricate tungsten ASG with complex thin wall structure by metal additive manufacturing.Selective Laser Melting(SLM)has the characteristics of high energy density and small spot diameter,which is one of the metal additives manufacturing with most accurate at present.This paper established the energy transmission model during SLM process,and theoretical calculated the laser energy required to form tungsten parts.The result is used to guide the single laser scanning forming,multi lasers overlapping forming and blocks forming.The forming process,microstructure,density,mechanical properties,defect control and other aspects of the samples were studied,and some preliminary results were obtained:Based on energy conversion and conservation principle,the energy transmission model during SLM process was established.The laser energy required in SLM process was theoretical calculated.According to the calculation result,several sets of parameters of laser power and scanning rate were designed to be used in single laser scanning forming.The experiment results show that the laser line energy density(?)required during SLM process should be greater than 0.64J/mm when the pre-paving powder thickness is 30?m,while the theoretical calculation results show that should be greater than 0.22J/mm.According to the experiment results,the original model was modified and improved,and the average specific heat of tungsten powder during heating process was about 517J/(Kg·K).Furthermore,additional energy is required to heat the melted tungsten to a temperature suitable for flow and spread forming,which is about 6.25% of the total energy of heating and melting the tungsten powder.According to the modified model,the ? should be greater than 0.34J/mm when the pre-paving powder thickness is 20?m,and the experiment result show it should be greater than 0.35J/mm.The experimental results are in good agreement with the theoretical calculation,which indicates the modified model can be approximately used for subsequent energy prediction and deduction.According to the modified model,that is the ? required for the formability is larger than 0.64J/mm when the pre-paving powder thickness is 30?m,the single laser scanning formed thin wall samples is studied.The results show that the wall thickness of the samples is thinner when ? is not more than 0.88J/mm,and the deviation between the effective wall thickness and the theoretical wall thickness is less than 10%.Based on the characteristics of molten pool expansion and solidification shrinkage,the laser scanning surface morphology of the sample formed by multi lasers overlapping was analyzed.It is found that after laser scanning,the molten pool solidified into a ridge shaped molten trace.If two lasers overlapping,the first molten trace is higher than the second one.If three lasers overlapping,the first molten trace is the highest,the third one is second and the middle is the lowest.The common phenomenon of powder adhesive on both sides of thin wall samples will affect the dimensional accuracy and surface roughness of thin wall.It is found that when the scanning rate is from 200 to 350mm/s,the adhesive thickness of single side of the wall is between 45 and 80?m,which formed by laser power is from 180 to260 W and pre-paving powder thickness is 30?m.The adhesive thickness of single side of the wall is between 40 and 60?m,which formed by laser power is from 120 to 200W and pre-paving powder thickness is 20?m.The author considers that,on the one hand,the temperature gradient between the edge and the center of the molten pool is large,the surface tension of edge molten is large,and the balling phenomenon occurs due to preferential solidification.On the other hand,the partly melted powder on the edge adheres to both sides of the wall.By reducing the powder particle size,the pre-paving thickness and laser power,the adhesive thickness of single side can be reduced by 5 to 20?m.According to the morphology of laser scanning surface of block samples,it was found that the balling phenomenon can be inhibited,a laser scanning surface with continuous and well overlapped molten trace can be formed and the relative density of samples can be increased by increasing the laser power and decreasing the scanning rate.The maximum relative density of the block sample is close to 98%.When the scanning rate is 300mm/s,even the laser power is increased to 200 W,the relative density of the sample decreases,due to the strong capillary convection on the molten pool,which causes protuberance on the surface.The microstructure of the samples shows that the internal grains are uneven lamellar.In the section parallel to laser beam and perpendicular to scanning direction,the grain is columnar or long strip,and the length direction is nearly parallel to the laser beam direction,because the heat in the molten pool mainly diffuses downward through the solidified part pf the lower layer.On the laser scanning surface,the grains are striped at a certain angle with the scanning direction,because part of the heat in the molten pool diffuses from center to edge perpendicular to the laser scanning direction,and the other part diffuses along the opposite direction of laser scanning.It is also found that the larger the scanning rate,the smaller the grain size and the greater the microhardness of the laser scanning surface.