Micro-Mechanism of Surface Friction and Wear Performance Improvement of Magnesium Alloy by High-Energy Laser Cladding of Cobalt-Based Alloy

Literature Overview

Magnesium alloys are increasingly used in aerospace, automotive, and biomedical applications due to their exceptional specific strength and lightweight characteristics, but their poor surface wear resistance and limited corrosion resistance severely restrict their application in tribological environments. This study investigates the micro-mechanism by which high-energy laser cladding of cobalt-based alloy on magnesium alloy substrates improves the surface friction and wear performance, providing fundamental insights into the tribological behavior of laser-clad magnesium alloy components.

Cladding Process and Microstructural Characterization

The cobalt-based alloy cladding layer is deposited on AZ91D magnesium alloy substrates using a high-power fiber laser with a power of 4 kW and a beam spot diameter of 1.0 mm. The cladding powder consists of CoCr-based alloy with additions of tungsten carbide and chromium carbide to enhance wear resistance. The process parameters are optimized to achieve a cladding layer thickness of 0.5 to 1.0 mm with a dilution rate below 10 percent, ensuring that the cobalt-based microstructure and properties are preserved in the cladding layer.

The microstructure of the cladding layer consists of a dendritic gamma-Co matrix with M7C3 and M23C6 carbide particles dispersed throughout. The carbide particles have a size distribution of 0.5 to 3.0 micrometers and are uniformly distributed, providing effective hardening and wear resistance enhancement. The interface between the cladding layer and the magnesium substrate shows a narrow diffusion zone of 20 to 50 micrometers with a gradient composition, indicating good metallurgical bonding without excessive intermetallic compound formation.

Friction and Wear Mechanism Analysis

The friction and wear behavior of the laser-clad magnesium alloy is evaluated through pin-on-disk testing under dry sliding conditions at loads of 5, 10, and 20 newtons. The coefficient of friction is reduced from 0.6 to 0.35, and the wear rate is reduced by more than 90 percent compared to the unclad magnesium alloy. The wear mechanism analysis reveals that the unclad magnesium alloy exhibits severe adhesive wear with large material transfer to the counterface, while the clad surface exhibits mild abrasive wear with uniform wear tracks and no material transfer.

The micro-mechanism of wear resistance improvement is attributed to three synergistic effects: the high hardness of the cobalt-based matrix provides resistance to plastic deformation, the dispersed carbide particles provide resistance to abrasive wear through micro-ploughing and micro-cutting resistance, and the compressive residual stress at the surface inhibits crack initiation and propagation. The tribological film formed on the clad surface during sliding consists of a mixed oxide layer of Co, Cr, and Mg oxides, which provides a protective barrier against further material removal.

Study Insights and Conclusions

This study provides a comprehensive understanding of the micro-mechanism by which high-energy laser cladding of cobalt-based alloy improves the surface tribological performance of magnesium alloy substrates. The key insight is that the improvement is not simply due to the increased surface hardness but is a synergistic effect of hardness, microstructure, residual stress, and tribological film formation. For engineers designing magnesium alloy components for wear-critical applications, this study demonstrates that laser cladding with cobalt-based alloys is a highly effective surface engineering solution that can extend component life by more than an order of magnitude while maintaining the lightweight advantages of the magnesium substrate.