难混溶合金广泛用作中高压（1~100kV）真空断路器的触头材料，在电力行业具有广阔的应用前景。随着国家特高压电网的全面建设，我国电网容量扩大、电压等级不断提高，触头的失效会导致断路器整机爆炸和电网瘫痪等重大事故，成为制约输变电网络发展的瓶颈问题。研究发现，难混溶合金的显微组织对宏观性能有着重要影响，当弥散的、细小的、球形第二相相粒子分布于基体中时，其电性能大幅提升。然而，在地面常规凝固条件下，难混溶合金极易形成偏析型组织，限制了此类合金的应用。激光表面熔凝技术的冷却速度快、能量密度高，能在保持合金成分和物相不变的同时，实现表面的快速熔化和凝固，有助于获得细小、弥散型凝固组织。激光表面熔凝技术已大量应用在低碳钢、钛合金和铝合金等材料上，但是迄今为止，还未见该技术在难混溶合金上的应用。本文研究了高功率密度激光作用下的Cu-Cr系难混溶合金的激光表面快速熔凝过程，研究激光工艺参数对Cu-Cr合金凝固组织的影响，研究Cu-Cr合金中第二相的生长、凝并及快速凝固行为，探索在常规重力条件下均质难混溶合金的制备方法，并针对Cu-Cr合金触头开展零件级的表面熔凝实验，激光表面熔凝处理后弥散型显微组织对Cu-Cr合金触头的电性能的影响。本文的主要内容及研究成果如下：

1. 建立了以高功率密度激光为热源的激光表面快速熔凝实验系统，研究激光功率密度、作用时间等工艺参数对熔凝组织的影响。发现当能量密度位于4.3×10³J/cm²~5.6×10³J/cm²时，Cu-Cr50（wt.%）合金表面能够形成均质、弥散型显微组织。

2. 在建立了相关实验平台的基础上，以Cr相粒子的运动为载体，研究冷却速度对液相分离行为和凝固组织特征的影响。结果表明，当Cu-Cr50(wt.%)合金熔凝层的平均冷却速率由~10⁵K/s增加到~10⁶K/s时，显微组织由分层型结构转化为弥散型结构。弥散型熔凝层中的均匀分布的富Cr晶球平均直径减小到~1μm，其Maragoni对流运动和Stokes沉积运动速率的比值v_M/v_S约~10³。这表明在液相分离过程中，含有过饱和Cu相的α-Cr液滴的运动主要受Marangoni对流运动的控制。虽然α-Cr液滴之间的碰撞和凝并不能消除，但高的冷却速度抑制了第二相的生长。

3. 在Cu-Cr30（wt.%）合金表面制备了一层厚度为300μm的纳米结构层。其熔凝层的平均冷却速度为5.75×10⁶K/s，熔凝层中分离出的富Cr相晶球平均粒径降低至39.5nm，并将Cr在Cu中的固溶体提高到1.96at.%。实验结果表明，当冷却速度升高后，液相分离过程中α-Cr纳米晶球在ɛ-Cu基体中的运动主要由布朗运动控制，使Cr相纳米颗粒做无序运动。在快速熔凝的Cu-Cr合金熔体中，布朗运动的凝并作用主导了液相分离初始时刻分离出的液滴数目，初始分离出的富Cr相液滴数目越多，布朗运动凝并完成的时间越少。

4. 高功率密度激光表面快速熔凝后的Cu-Cr30（wt.%）合金，富Cu基体中分离出大量的富Cr纳米颗粒，其硬度显著提高，是位错强化、弥散强化和固溶强化共同作用的结果。Cu-Cr50（wt.%）合金的基体和熔凝层显示出了不同的断裂机制。基体的断口主要是沿着Cu基体和Cr颗粒的界面断裂和Cr颗粒本身的解理断裂，熔凝层的断口显示出典型的韧窝状断裂特征，断口处出现了大量微米量级的等轴韧窝。经高功率密度激光表面快速熔凝处理后，Cu-Cr合金的综合力学性能得到了提高。

5. 高功率密度激光表面快速熔凝后的Cu-Cr合金触头装的电性能有了大幅提升，主要体现在以下两方面：1）耐电压值大幅提高，其中动端和静端的平均耐受电压分别比未处理的触头分别提高了18.3%和21.0%；2）激光表面熔凝处理后触头的电弧寿命为提高了17.8%，并成功地通过了20次连续的关断操作。结果表明，经激光表面快速熔凝处理后，其最大耐受电压值和稳定性均得到改善。Cr相显微组织的细化可以调节电弧的运动，提高Cu-Cr合金触头的使用性能，证明这一思路可以指导了触头材料的设计，并能够在实际中将思路得以应用。

关键词：Cu-Cr合金，激光表面快速熔凝，液相分离，凝固组织，力学性能，关断性能

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