Study On Thermal Compression Behavior And Properties Of Vacuum Hot-press Sintered W/Cu-Al₂O₃ Composites

W-Cu alloys have been widely used in electricity, electronics, metallurgy, machinery, etc. due to the superior characteristics of both tungsten and copper metals. With the rapid development of industrial technology,high performances are proposed for the W-Cu alloys. It is difficult to achieve high density by means of the traditional sintering process. In order to achieve high performance, the studies on fabricating technology and chemical composition of the W-Cu alloys, have been focused at home and abroad.According to the properties requirements of contact materials, the W(25)/Cu-Al₂O₃ and W (50)/Cu-Al₂O₃ composites are developed by vacuum hot-press sintering-internal oxidation method in this paper. Study on influence of sintering process on the density, bend strength, and hardness of the composites. The microstructure and bend fracture morphology were observed in order to analysis densification and fracture mechanisms. The experimental results show that the composites achieve excellent comprehensive properties by means of vacuum hot-press sintering at 950℃for 2hours and 30MPa.With the increase of W content, the bend strength and hardness of W/Cu-Al₂O₃ composites increase except the electrical conductivity. The properties of W(25)/Cu-Al₂O₃ composite as sintered are as follows, microhardness of 128HV, electrical conductivity of 62.8%IACS and bending strength of 286.5Mpa, respectively. That of the W(50)/Cu-Al₂O₃ composites are 135HV, 46%IACS and 291.3MPa, respectively. The microstructure observation shows that there are a mass of fine nanoγ-Al₂O₃ particles dispersed in the copper base with the size of 5²0nm and spatial distance of about 20¹50nm. The W particles are uniform dispersed in the copper base with less small pores and high density. The densification mechanism of the composite is diffusion and plastic slip. The main mechanism of bend fracture is the W grains cleavage and a part of W-Cu interfaces separation. The more W content increase, the more cleavages occur obviously.The plastic flow characteristics and microstructure evolution of the Cu-Al₂O₃, W(25)/Cu-Al₂O₃ and W(50)/Cu-Al₂O₃ composites during hot compression deformation were investigated by thermal simulation test on the Gleeble-1500D thermal simulator, respectively. The results show that the three composites show the typical dynamic recrystallization characteristics under the test temperature varying from 750℃to 950℃and the strain rate varying from 0.1s^-1to 5s^-1 with the true strain of 0.7. Both the deformation temperature and the strain rate have obvious influence on the plastic flow stress. The flow stress increases with increasing the strain and tends to be constant after the peak value. The peak stress value increases with the strain rate increasing and with the deformation temperature decreases. With the increasing temperature and the strain rate decreasing, the strain-free recrystallization fine grains appear in the intersected locations of grain boundary and the grains number increases. The average thermal deformation activation energy of the Cu-Al₂O₃, W(25)/Cu-Al₂O₃ and W(50)/Cu-Al₂O₃ composites is calculated as 220.7kJ/mol, 185.3kJ/mol, 176.1kJ/mol, respectively. The thermal compression behaviors of the composites can be represented in the hyperbolic constitutive equations.Based on the materials dynamic model,the processing maps for subtransus deformation of Cu-Al₂O₃, W(25)/Cu-Al₂O₃ and W(50)/Cu-Al₂O₃ composites were established, respectively. The results indicate that the composites possesses appears the peak of dissipation efficiencyη(over 40%) when deformed at 650℃⁹50℃and the primary strain rate of 0.01s^-15s^-1, while this region has a special microstructure mechanism or mechanisms of flow instability, i.e., loss of area for crack. And the minimum dissipation efficiencyη(less than 20%) appears in the strain rate of 1s^-15s^-1 and deformation temperature of 650℃⁷50℃, where in this region, the composites are prone to grain boundary along wedge-shaped formation of micro-cracks and holes in the other phase, which is also for regional instability. With deformation at 750℃⁸50℃and primary strain rate of 0.01s^-10.1s^-1, the corresponding power dissipation efficiency was about 20%⁴0% in this region.The softening mechanism of the composites is typical dynamic recrystallization, accompanied grains growth.