Grain Refinement of Hypereutectic Immiscible Cu-50Cr Alloy during Rapid Melting and Solidification Induced by High Power Density Laser Beams

Litian Zhang,Xiuli He,Chongxin Tian,Shaoxia Li,Gang Yu

doi:10.3390/met9050585

Litian Zhang, Xiuli He + Show 3 more

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https://doi.org/10.3390/met9050585

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Abstract

The microstructure of hypereutectic immiscible Cu-50Cr alloy (wt%) was effectively refined and homogenized by a high power density Nd:YAG continuous laser beam (power density 104–105 MW/m2). The average grain size of Cr was effectively refined from ~100 μm to ~1 μm, and some of the Cr particles even decreased to a few hundred nanometers. The high cooling rate (7.29 × 106 K/s) effectively inhibited the coarsening effect on Cr particles during liquid phase separation (LPS). The spherical Cr particles were preferably dispersed in the melt layer, and the thickness of the layer was up to ~150 μm. The refinement and dispersion of the Cr phase contributed to improving the interruption capability of the Cu-Cr contacts. Compared with the untreated samples, the arc duration and the withstanding voltage of the laser surface melting (LSM) treated contacts with refined microstructure increased to 21% and 33%, respectively. The results demonstrated that the LSM method was an effective approach to optimize the microstructure of Cu-Cr alloy, which made it a promising modification method for Cu-Cr vacuum contact applications.

Highlights

Hypereutectic Cu-Cr alloys with high Cr content (Cr mass ratio from 20–60%) have been widely accepted as electric contact materials in most the high voltage commercial vacuum interrupters (VIs) [1]
Cu-Cr vacuum electrical was produced by laser surface melting, and the microstructure of the layer was characterized by the contacts, which were fabricated through the PM process
The withstanding voltage was produced by laser surface melting, and the microstructure of the layer was characterized by the refined spherical Cr spheroids and Cr-rich sites embedded in Cu matrix

Summary

Introduction

Hypereutectic Cu-Cr alloys with high Cr content (Cr mass ratio from 20–60%) have been widely accepted as electric contact materials in most the high voltage commercial vacuum interrupters (VIs) [1]. Recent investigations revealed that the performances of Cu-Cr alloy are significantly affected by the grain size and distribution of the Cr phase. When refined Cr spherical particles are dispersed uniformly in the Cu matrix, it was helpful to reduce chopping currents, sustain high voltage breakdown strength capacity, and effectively avoid failures during switching off operations [2,3,4,5]. Traditional preparation methods for Cu-Cr alloys, including powder metallurgy, casting, and arc melting, often fail to control the size of the Cr phase effectively and avoid microstructural segregation [6]. Some small-scale rapid solidification methods, for example melt spinning [7], gas atomization [8], electromagnetic levitation with splat-quenching [9], and selective laser melting [10], were applied to refine the microstructure.

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