Study On The Formation Mechanism Of Pure Tungsten Materials Manufactured By Selective Laser Melting

Selective Laser Melting(SLM)has been extensively studied as a kind of metal additive manufacturing technology which can fabricate complex structures.However,owing to less applicable materials and insufficient understanding of the forming mechanism of relevant materials,widely application of this technology has been restricted.This thesis mainly focused pure tungsten material.The forming mechanisms of SLM process were investigated.The effects of tungsten powder properties on the spreading process and SLM process were studied.The result indicated that the spreading process was mainly affected by the mobility and layer thickness of the powder.High mobility would be benefit of spreading process,and the powder bed would become even with the increase of layer thickness after spreading.The heat conductivity coefficient of the powder bed was affected by the apparent density of the powder,and it had an influence on the stay time of the melted metal.At last,the difference in powder properties affected the SLM parts.SLM laser parameters applied on the tungsten materials were studied and optimized in this study.Tungsten thin wall parts,which could be used as high performance collimator devices,were fabricated and the effects of laser power and scan rate on wall thickness and surface morphology of tungsten thin wall parts were discussed.The results showed that the wall thickness of tungsten thin wall parts was relative to the laser power and laser scan rate in SLM process.The wall thickness was increased with the enhancement of laser power in a linear relationship.On the contrary,the wall thickness was decayed exponentially with the acceleration of laser scan rate.However,laser power had great influence on wall thickness,and high laser power could make the hot cracks occur.The top surface roughness could be reduced by increasing input laser energy and the re-melting process.To get rid of the big adhesive parts on the wall,the laser power should not be too high.Meanwhile,the wall thickness fabricated by laser double pass melting was thinner than the one fabricated by laser single pass melting.The formation mechanism of pure tungsten bulk materials was investigated in this paper.The result indicated that the number of the internal pores in the SLM samples was decreased with the increase of input laser energy density.The density was increased with the improvement of laser power and the decrease of laser scanning speed.The pure tungsten block sample with a relative density of 97% was prepared by using laser scanning parameters of 300 W and 200 mm/s.The grain was enlarged and elongated along the building direction with the increase of input laser energy density.The inside pores were increased and distributed along the building direction by utilizing laser strategy 2.Moreover,some of the grain inside the sample which was manufactured by strategy 2 was elongated along the building direction.The formation mechanism of SLM TC11 and 7075 alloys were also investigated in this paper,and the forming mechanism and crack initiation mechanism of these alloys during SLM process were compared and analyzed.It was found that main defects of SLM TC11 alloy were pores,and the shape and generating mechanism of the pores were different owing to different laser parameters.The main defects of SLM 7075 alloy were cracks,and laser parameters had significant influence on the crack morphology and the performance of SLM samples.The crack initiation mechanism of pure tungsten and 7075 alloy and the good formability of TC11 alloy were analyzed from the aspects of crystallization temperature interval,grain size,composition,cooling speed and the constraint degree during SLM process.