Research On The Forming Process And Properties Of W-Cu Composite Fabricated By Selective Laser Melting

With the development of large scale integrated circuit and high power electron devices,the packaging materials with high thermal conductivity and low thermal expansion coefficient are in urgent need.In general,it is difficult to combine high thermal conductivity and low expansion coefficient.However,the W-Cu composites have solved this contradiction,at the same time own excellent properties of high thermal conductivity and low thermal expansion coefficient,is the preferred electronic packaging materials.However,due to complicate design,higher forming precision and higher relative density requirement,the traditional manufacturing methods was limited.Therefore,the developments on new forming methods for W-Cu composites become research hotspot attracted much attention.Selective laser melting(SLM)technique is a new forming method based on additive manufacturing method,enable the quick fabrication of arbitrary shaped three-dimensional components by fusing loose powder layer by layer with a high energy scanning laser beam without using of other fixturing or tooling,meanwhile after post-processing,the SLMed components obtain excellent physical and mechanical properties.Therefore,SLM method exhibits a great potential for processing W-Cu component.In this paper,SLM technique was adopted to fabricate W-Cu composite and W-Ni-Cu composite which added the activating agent Ni element.The effect of powder characteristic(such as shape,size,tapping density and apparent density)and process parameters(such as power,scanning velocity,length of scanning line and overlap rate)to formability of composite,microstructure and physical properties was studied.The forming mechanism during selective laser melting was also studied.Because of the high cost of spherical W powder,in order to achieve low cost in industrial applications,Ni powder was added to the non-spherical W-Cu powder.Basing on the numerical simulation and experimental verification of the temperature field,the melting mode of W-Ni-Cu alloy and the cause of forming defects were analyzed.Furthermore,the feasibility of Ni as sintering activator in electronic packaging material was demonstrated.The overall results were as follows.Selective laser melting of three kinds of composite materials with different powder characteristic and the component content was 60W-Cu?70W-Cu and 75W-Cu,respectively.The relative density obtained by spherical powder was higher than blocky-shaped powder,much higher than irregular layer-sheet powder.Increasing with the W content,the balling phenomenon on the surface aggravated,the connection and agglomeration of W particles became evident,and the bonding between W and matrix Cu was closer with the spherical degree of W particle.In the forming precision measurement,the direction of x and y expanded while shrinked in z direction,and the expansion increment and shrinkage of forming spherical powder was lowest,blocky-shaped powder took second place.The hardness of Cu matrix was increasing with W content.Combining with the therotical calculation and experimental observation,determining the mechanism of sintering W-Cu composites was liquid phase sintering.Studying the effect of process parameter on forming relative density and microstructure,adopting spherical powder fabricated by SLM.Indicating that there were some problems existing in the common evaluation parameter "volume energy density":it belongs to thermodynamic parameter,the mass and monentum change cannot be considered.Combination of theory and experimental research,under the same volume energy density value,there would be varied forming results by changing different parameter.Analyzing the force on liquid phase and solid phase in molten pool,further studied the evolvement rule and influence factor of densification behavior.The three-dimensional temperature field model of selective laser melting forming process by finite element method was established,forming W80wt.%-Ni5 wt.%-Cu15 wt.%alloy system with irregular-shaped W mixed powder as an example,combining the experimental results to study the change of temperature field and the evolution of forming mechanism and microstructure under different process parameters.Studing the causes of defects in different process parameter zone,the pore and balling in incomplete melting zone were generated by low temperature and solid phase sintering,the pores in the melted zone were produced because of keyhole effect,the defects in balling zone were caused of the high scanning speed intensified the molten's property of solidification velocity larger than the spreading speed.On the basis of application requirements of electrical package materials,studying the thermal physical properties and roughness of SLMed components which adopted spherical powder and blocky-shaped powder,respectively.The thermal conductivity of SLMed components using two kinds of powders were both lower than theoretical value,the thermal conductivity of SLMed spherical powder was closer to theoretical value.Increasing with the W content,the difference between calculate value and theoretical value increased.The coefficient of thermal expansion of SLMed spherical powder component was lower than blocky-shaped powder component.With the increase of W content,the intensified balling phenomenon caused the roughness increases.