Research On Microstructure And Properties Of W-Cu Composites Fabricated By Laser Solid Forming

As a typical type of "pseudo alloy",W-Cu composites combines the advantages of both W and Cu,such as good electrical and thermal conductivity,low thermal expansion coefficient,good abrasion resistance and arc corrosion resistance,as well as the "self cooling effect" at high temperature.It is widely used in electronics,microelectronics,aerospace,military and medical fields due to the excellent performance.However,W and Cu has poor wettability and can not form any solid solution or inter metallic compounds because of the big difference of each other in lattice type,melting point and densification.We can fabricate W-Cu composites only by powder metallurgy(PM)method,which can achieve the final size compacts.PM is not suitable for the complex and thin-wall parts,and the mold is expensive and has long designing cycle.The development of advanced near-net forming is the main solution for the manufacturing of W-Cu composites.In this thesis,two different laser forming system was used for the single track laser cladding of W-Cu composites,and the influence of component and parameters on the morphology,density and mechanical properties of W-Cu cladding was investigated.Then the W-Cu thin-wall parts were fabricated by Laser Solid Forming(LSF)in the optimized process parameters to study the influence of process parameters.The influence on W-40wt.%Cu thin-wall parts of a small amount of Fe and Co addition were investigated as well.The main conclusions are as follows:(1)Under the condition of low energy input(P?200W),the morphology of the cladding layer is mainly affected by the component ratio.When the ratio of Cu increased from 30wt.%to 40wt.%,the microstructure of cladding layer changed from homogeneous structure with W as the skeleton metal to uneven "W-rim/Cu-core" structure.The optimized ratio of Cu is 30wt.%-40wt.%.(2)Under the condition of high energy input(200W<P?1000W),the escaped energy density of W-40wt.%Cu single track cladding on 45#steel substrate is b?1.8×10-3kJ/mm2.The influence of process parameters on the morphology of the single track cladding layer can be attributed to the influence of the effective mass energy density k.When k<10.2kJ/g,the cladding layer had no dilution zone;when k>10.2kJ/g,dilution zone appeared.When k<4.49kJ/g,the special "W-rim/Cu-core" structure could be observed;when 4.49kJ/g<k<9.58kJ/g,W particles distributed along the edge of cladding layer;when 9.58kJ/g<k<13.19kJ/g,W particles distributed uniformly;when k>13.19kJ/g,clusters appeared.The optimized parameters should satisfy the condition of 9.58kJ/g<k<13.19kJ/g.(3)The microstructure and properties of W-Cu thin-wall parts fabricated by laser solid forming are not only affected by the process parameters,but also closely related to the geometrical characteristics of the single track cladding layer.The multi-layer cladding with low value of W/H has higher energy loss and is easier to form pores.In W-40wt.%Cu thin-wall parts,tensile strength was mainly provided by reticular Cu phase.Ductile fracture and brittle fracture both occurred under the condition of stretch.The trends of tensile strength was consistent with micro hardness.According to the performance,we choose P=800W,v=10mm/s,a=0.42g/s and D=3mm as the optimal parameters.Under this condition,the microstructure is more uniform,the value of relative density,microhardness and tensile strength is 80.54%,122.75HV and 122.89MPa.(4)The addition of Fe and Co can improve the properties of W-40wt.%Cu thin-wall parts obviously.The relative density can be increased to over 94%when 5wt.%Fe and Co were added.The addition of Co element can increase the microhardness by 50?60HV.When 5wt.%Fe were added,The microhardness of the formed parts reached 319HV.Tensile strength after the addition of activators was 1.5 to 2 times of the original one.When 1 wt.%Fe,5 wt.%Fe and 5 wt.%Co were added,the fracture surface will cross through the W particles,which provide most of the strength as the framework metal.