Study On The Microstructure And Properties Of Cu-Cr-Zr Alloy After Deep Cryogenic Treatment

Cu-Cr-Zr alloy, a kind of copper alloy, has high strength, electrical and thermal conductivity, so it has been used for electrode of resistance welding. However, its electrode life in the welding of aluminum alloy and coated steels is extremely short because the rapid electrode wear, and the properties of Cu-Cr-Zr alloy made in our country are instable. Deep cryogenic treatment is a very effective method for improving the electrode life of aluminum and hot zinc coated steel. Yet, the mechanism of nonferrous properties improved by deep cryogenic treatment is not clear for certain nonferrous materials. So based on the effect of deep cryogenic treatment on the microstructure and properties of Cu-Cr-Zr alloy, the mechanisms of properties improvement of Cu-Cr-Zr alloy and the electrode life improvement by deep cryogenic treatment is clarified in this thesis. Research results of this dissertation are as follows:The effect of deep cryogenic treatment on the microstructure of Cu-Cr-Zr alloy was investigated using optical microscope, transmission electron microscope(TEM), small angle x-ray scattering (SAXS), and quantitative metallography. The results revealed that the Cr and CuxZry particles precipitated during the deep cryogenic treatment, providing Orowan strengthening. During the deep cryogenic treatment, the dislocation loop and twins were formed. The vacancy concentration decreased with the increase of deep cryogenic treatment time by positron annihilation lifetime analysis. Element chemical states in Cu-Cr-Zr alloy were characterized using X-ray photoelectron spectroscopy (XPS). The intensity corresponding to Cu2p increases due to the precipitation of Cr, CuxZry particles, and the decrease of vacancy concentration. Meanwhile, the binding energy also increases after long time deep cryogenic treatment, which is not propitious to the electron movement.Mechanical properties of Cu-Cr-Zr alloy were also investigated in this dissertation systemiccally. Vickers hardness increased slightly because the hardness of Cu matrix can not be enhanced during deep cryogenic treatment. The softeningtemperature decreases slightly with the increasing deep cryogenic treatment time. However, the high temperature hardness of Cu-Cr-Zr alloy after deep cryogenic treatment will be high enough to satisfy the requirement of aluminum resistant welding. The electrical conductivity, tensile strength and wear resistance of Cu-Cr-Zr alloy were also improved during deep cryogenic treatment, and its degree of improvement increases with the increasing deep cryogenic treatment time. Surface of tensile strength fracture and wear resistance were studied by SEM and EDS analysis. Analyses of SEM and EDS combined the variations of microstructure reveal that the precipitation of second phase, the formation of dislocation loop and twins are the main reason for the improvement of strength and wear resistance of Cu-Cr-Zr alloy. Electrical conductivity of Cu-Cr-Zr alloy is improved by deep cryogenic treatment owing to the precipitated phase and the decreased micro defect. Theoretical analyses of electrical and mechanical properties improved by deep cryogenic treatment coincide well with the results of experiment.Based on the contact resistance model of micro-contact deformation, the mechanism of the decrease of contact resistance between electrode and work piece by deep cryogenic treatment was analyzed showing that the improvement of electrical and mechanical properties by deep cryogenic treatment leads to the decrease and the distribution uniform of electrode/sheet, which decrease the ratio of melt of contact between electrode and sheet during the aluminum spot welding, and decrease the deterioration of the electrode.