Study On The Process,Microstructure And Properties Of Tungsten And Tungsten-based Materials Fabricated By Selective Laser Melting

Tungsten is the metal with the highest melting point in nature.It has excellent properties such as high strength,high thermal conductivity,low vapor pressure,and low thermal expansion coefficient.It has broad application prospects in the fields of energy,power,military and other fields.Due to its low toughness at room temperature and high toughness-brittle transition temperature,tungsten has always been fabricated by powder metallurgy and thermoplastic deformation processes.Therefore,it is difficult to fabricate tungsten-based materials with complex structures,which greatly limits its application.Selective laser melting(SLM)is one of the additive mafucturing technolyges,has been applied in manufacturing metal parts with complex structures.However,tungsten has the characteristics of high melting point,high thermal conductivity,high surface tension and high viscosity,which makes it difficult to form tungsten by SLM.Based on this,this paper first selected tungsten powder prepared by plasma spheroidization with high sphericity and small particle size as the raw material,and prepared high-density SLM tungsten by optimizing the process parameters.This paper intends to prepare high-density pure tungsten by optimizing the process parameter.And then the formation mechanism and suppression method of crack defects in SLM were explored.The specific research work and results are as follows:Near fully dense W samples with a relative density of 98.71%were obtained for the first time through a series of optimization experiments during the SLM process.The characteristics of the surface and the formation mechanism of the micro defects were systematically elucidated.Additionally,it was found that the typical microstructures of horizontal and vertical planes experienced successive changes,where coarser columnar grains changed to uniform finer grains when increasing the laser scan speed from 50 mm/s to400 mm/s.The compressive strength,micro hardness and thermal conductivity of the optimal SLM sample was improved to 1523 MPa,428 HV₃ and 148 W/m·K,which were superior to the sample produced by the conventional methods.The relationship of processing parameters to the surface morphology and microstructure evolution and material properties associated with fusion reactors was established in order to optimize the performance of SLM pure W and explore the possibility of further application in fusion reactors.For the first time,a comparative study on the influence of different manufacturing methods(SLM and hot rolling)on the microstructure,mechanical and thermal behaviours of W was presented.The results indicated that SLM W exhibited a lower strength ductility,hardness and thermal conductivity compared to hot-rolled W.The main reason for this outcome was that the laser underwent rapid heating and cooling when it was used to melt W powder with high power,resulting in large internal stress in the sample after forming.Subsequently,the internal stress was released,leading to the generation of micro-cracks at the grain boundary,thereby affecting the performance of samples.In addition,the higher fraction of high-angle grain boundaries(HAGBs)of SLM W was found to be the key factor for intrinsic brittleness.In SLM pure tungsten,three types of columnar grains were observed,which provided the propagation paths for cracks.However,ZrC/W composites had more refined equiaxed grains by the addition of ZrC nanoparticles,which could provide more heterogeneous nucleation sites.Transmission electron microscopy confirmed that ZrC could react with oxygen impurities and form stable ZrO₂ and ZrW₂O₈ to improve the cohesion strength of grain boundary.As a result,ZrC/W composites simultaneously exhibited a reduced crack density and higher Vickers hardness compared with pure tungsten.The strengthening and tounghening mechanisms of ZrC reinforcements were discussed in detail.The key to the improved hardness was attributed to Orowan strengthening and GB strengthening.The purified GBs and the columnar to equiaxed transition(CET)of grains played an important part in inhibiting the formation and propagation of cracks.This study provides a novel approach for tailoring the microstructure and suppressing the cracking susceptibility of SLM tungsten.