| The composite material of W-Cu has been widely used in microelectronics and military fields since it inherits good properties of both tungsten and copper,such as high density,good thermal conductivity,good electrical conductivity as well as low expansion coefficient.However,due to the high melting point and high hardness of the reinforcing phase metal W in W-Cu composites,it is difficult to manufacture dense W-Cu composites with excellent performance by traditional methods.In addition,the distribution of the reinforcing phase tungsten and the bonding phase copper is not uniform,causing pores in the composites.In this paper,the technique of selective laser sintering(SLS)is used to fabricate W-Cu composites.The formation process of W-Cu layer was studied.Firstly,the laser absorptivity and light intensity distribution of W-Cu mixed powders were measured with an absorptivity test system and calculated by the Raytracing method.Afterwards,the W-Cu composites were prepared by SLS.The distribution rule of tungsten and copper in the structure of samples were discussed.Finally,based on the theory of heat transfer and fluid mechanics,the temperature and flow simulation of W-Cu powders were analyzed.The distribution mechanism of W and Cu in the structure was revealed.The main conclusions are shown as follows;(1)The absorptivity of the stainless steel was measured with the absorptivity testing system.The variations of the absorptivity with the incident angle and the polarization state were in accordance with the Fresnel absorption theory.The obtained refractive index was consistent with the reported result,showing the reliability of the testing system.(2)With the modified testing system of absorptivity,the absorptivities of copper and tungsten powders were measured to be 0.19 and 0.51,respectively.For the mixed powders of tungsten and copper,the absorptivity decreases linearly with the increase of the mass ratio of copper.The obtained results agree well with that measured by spectrophotometer.(3)The absorptivity was influenced by the spot size and particle size.In addition,the absorbed intensity distribution was determined by the type of material and the sizes of particles.In the central area with the particle diameter as the radius,the absorbed light intensity increased several times by comparing it with that of a plane plate,which was the main reason for causing the enhancement of absorptivity.What is more,the distribution of the mixed powder is the third aspect that determines the absorptivity.(4)When the heat input of laser was strong,the vapor due to the evaporation of copper will be captured.With low heat input,the particles of tungsten are hardly to be rearranged because of the weak flow of the melted copper.Both of these two aspects lead to many pores in the W-Cu composites.The optimized processing parameters of W-40 wt.% Cu composites were laser power of 400 W and scanning speed of 15 mm/s.(5)According to the transient diagnosis of the high-speed camera,the particles around the molten pool fell into the central area.In these particles,copper melted causing further expansion of the molten pool.But the tungsten particles floate on the surface due to the effect of surface tension and flows to the boundary of the pool along with the flow of Marangoni.When the forming layer cooled,the tungsten particles flow back to the center which eventually leads to the W-rim-Cu-core phenomenon around the surface of the layer.(6)The numerical research shows the molten copper would flow around tungsten particles and then agglomerated when it was started to be irradiated by laser,causing a regional of W-rim-Cu-core phenomenon and formation of pores.To prohibits the appearance of pores,it is necessary to rearrange the particles of tungsten with the assistance of Marangoni flow.In our work,not only the absorptivity of W-Cu mixed powder was obtained but the flow behavior of copper and tungsten particles was discussed,which provides a reference for improving the quality of fabricated W-Cu composites with SLS. |
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