|Alternative Title||Investigations on Microstructure Evolution and Properties of Immiscible Alloy Subjected to a High Power Density Laser Beam|
|Place of Conferral||北京|
Immiscible alloys are widely used as contact materials for medium and high voltage (1~100kV) vacuum interrupters, and has been widely accepted in the power industry. With the comprehensive construction of national utral-high voltage power grid, the capacity of power grid in China is expanding and the voltage level is continuously improving, the failure of contacts will lead to accidents, such as the explosion of circuit breakers and the paralysis of power grid, which become the bottleneck problem which restricts the development of power industry. It is found that the microstructure of the immiscible alloys has an important influence on the macroscopic properties. The electrical properties of the immiscible alloys are greatly improved when dispersed, refined second phase spheroids distributed in the metallic matrix. However, it is easy to for immiscible alloys to form segregated microstructure under conventional gravity conditions, which limits their applicaitons. Laser surface melting technology has the advantages of fast cooling and high energy density. Rapid melting and solidification of local surface can be realized while keeping the composition or phase unchanged, which is helpful to obtain dispersive solidification microstructure. Laser surface melting technology has been widely applied in low carbon steel, titanium alloy and aluminium alloy, but up to now, the application of this technology in immiscible alloys has not been reported. In this paper, the effects of laser processing parameters on the solidification microstructure of Cu-Cr alloy were studied. The growth, coagulation and rapid solidification behavior of second phases of the immiscible alloy were studied. The preparation methods of homogeneous immiscible alloy under conventional gravity conditions were explored. The surface melting experiments of part-level specimens were carried out to verify the effect of dispersive microstructures after laser surface melting on the electrical properties of Cu-Cr alloy contacts. The main contents and research results of this paper are as follows:
1. An experimental system of laser surface rapid solidification with a high power density laser which acted as a heat source was established. The effects of laser power density and interaction time on the solidification microstructure were studied. It is found that when the energy density is from 4.3×103J/cm2 to 5.6×103J/cm2, the dispersive microstructures can be obtained on the surface of Cu-Cr50 (wt. %) alloys.
2. Based on the establishment of relevant experimental platform, the effect of cooling rates on liquid phase separation and solidification structure characteristics was revealed by studying the movements of Cr particles. Layered structure transformed into a dispersed structure when the cooling rate increased from ~105K/s to ~106K/s. The average diameter of the Cr-rich spheroids in the homogenous layer was reduced to less than 1μm, and and the migration velocity induced by Marangoni convection was ~103 times higher than that induced by Stokes sedimentation. This indicates that the movements of α-Cr droplets containing supersaturated Cu phase is mainly controlled by Marangoni convection during liquid phase separation. Although collision and accumulations between α-Cr droplets can not be eliminated, the growth of alpha-Cr droplets is inhibited by high cooling rate.
3. A nanostructured surface layer of ~300μm thickness was fabricated on Cu-30Cr (wt. %) hypereutectic alloy by a continuous laser beam with high power density (1.08×107W/cm2). The average cooling rate of the solidified layer is 5.75×106K/s. The average size of the Cr-rich spheroids in the solidified layer is reduced to 39.5 nm, and the solid solution of Cr in Cu is increased to 1.96at. %. The experimental results show that the movement of α-Cr nanospheres in the ɛ-Cu matrix during liquid phase separation is mainly controlled by Brownian motion, which resulting in disordered movements of Cr-rich nanoparticles. In the rapidly solidified Cu-Cr alloy melt, the the number of droplets separated at the initial time of liquid phase separation has a significant effect on Brownian motion. The more Cr-rich droplets separated at the initial time, the less time it takes for Brownian motion to complete collisions and coagulations.
4. Compared with untreated Cu-Cr30 (wt. %) alloy, the hardness of Cu-Cr30 (wt. %) alloy irradiated by a high density laser is significantly improved, which is the result of dislocation strengthening, dispersion strengthening and solid solution strengthening. After tensile tests, the fracture morphology of Cu-Cr50 (wt. %) alloy was observed, and the fracture mechanism of the matrix and melt layer was obviously different. The fracture surface of the matrix is mainly along the interface fracture of the Cu matrix and the Cr particles and cleavage fracture of the Cr particles. The fracture surface of the melt layer shows typical dimple fracture characteristics, large number of micron-sized equiaxed dimples appeared on the surface. After After laser surface treatment, the comprehensive mechanical properties of Cu-Cr alloy have been improved.
5. After laser surface treatment, the Cu-Cr contacts were assmebled into vacuum interrupters. The interruption performance has been greatly improved, which is mainly shown in the following two aspects: 1) The average withstanding voltage of the moving end and the static end are 18.3% and 21.0% higher than that of the untreated contacts; 2) The average arc duration of the contact after laser surface melting treatment increases by 17.8%, so the contact has successfully passed 20 sucessive swithc-off operations. The motion mode of vacuum arc can be adjusted by refining the Cr phase to improve the performance of the electrical contacts. It is proved that this method can guide the design of the contact materials and can be applied in practical applications.
Key words: Cu-Cr alloy, laser surface rapid melting and solidification, liquid phase separation, solidification microstructure, mechanical properties, interruptering capability
|张犁天. 高密度激光作用下难混溶合金的组织演变及性能研究[D]. 北京. 中国科学院大学,2019.|
|Files in This Item:|
|张犁天+高密度激光作用下难混溶合金的组织（13659KB）||学位论文||开放获取||No License||Application Full Text|
|Recommend this item|
|Export to Endnote|
|Similar articles in Google Scholar|
|Similar articles in Baidu academic|
|Similar articles in Bing Scholar|
Items in the repository are protected by copyright, with all rights reserved, unless otherwise indicated.