Resumo
This paper investigates the cracks and porosity defects occurring in high-speed laser cladding of iron-based alloy coatings on the surface of hydraulic support columns. By combining the principles of metallurgical solidification and using Energy Dispersive Spectroscopy (EDS) for point and line scan analysis of the coating’s elements, the study systematically explores the causes of these defects in terms of the cladding material composition and process parameters. The results show that the segregation of B and Si elements, the precipitation of Cr-Mo-C carbides, and the phase transition behavior of the matrix are the primary factors causing cracks and porosity. Process parameters such as laser power, scanning speed, powder feed rate, and the number of cladding layers can exacerbate defect formation if not properly adjusted. This study provides a theoretical basis for the industrial application of high-speed laser cladding technology on hydraulic support surfaces.

Introdução
During operation, hydraulic support columns are subjected to alternating loads, leading to wear and corrosion on the surface. High-speed laser cladding technology has become an effective method for surface strengthening and repair due to its high efficiency, low dilution rate, and localized heat-affected zone. However, cracks and porosity defects within the cladding layer seriously affect the service performance of these coatings, and there is a need for a systematic analysis of their formation mechanisms.

1. Crack Formation Analysis
1.1 Material Factors

B and Si Segregation: When the B content exceeds 0.5%, it forms low-melting-point eutectics with Ni and Si at the grain boundaries, creating a liquid film that weakens the bond at the grain boundaries, thus promoting crack initiation and propagation.

Influence of Si and Mn Elements: Si increases the viscosity of the melt pool, hindering gas expulsion, while Mn promotes the formation of MnS inclusions, which become crack sources.

Cr-Mo-C Synergistic Effect: Cr and C form carbides like Cr₂₃C₆ and Cr₇C₃, while Mo forms Mo₂C. The precipitation of these carbides leads to volume shrinkage, which, combined with thermal stress, increases residual stress and induces cracking.

Phase Transition in 27SiMn Matrix: The transformation of austenite to martensite leads to volumetric expansion and shear stress, increasing the risk of interface delamination.

1.2 Process Factors

Excessive Laser Power: High laser power increases the temperature gradient, concentrating thermal stress.

Fast Scanning Speed: A high scanning speed reduces solidification time and increases the cooling rate, leading to intensified stress concentration.

Excessive Number of Cladding Layers: Too many cladding layers result in cumulative interlayer stress, which, when exceeding the material’s yield strength, triggers cracks.

2. Porosity Formation Analysis
2.1 Material Factors

Reaction of B with O: B reacts with oxygen to form volatile B₂O₃, which creates gas bubbles in the melt pool.

Oxidation of Mo: Mo oxidizes to form MoO₃, which acts as the nucleus for gas bubble formation.

Formation of Composite Inclusions: Si reacts with C to form SiC, while SiO₂ creates composite inclusions that hinder the expulsion of gas bubbles.

Mn Vaporization: Mn vaporization induces turbulence in the melt pool, trapping gases and causing porosity.

Low-Melting-Point Eutectics: The formation of low-melting-point eutectics such as SiO₂ and B₂O₃ traps gas in the material.

2.2 Process Factors

Unstable Gas Flow: Unstable gas delivery leads to poor protection or turbulence in the melt pool.

Excessive Powder Feed Rate: Too much powder feed can cause clumping, trapping gas bubbles.

Mismatch of Laser Power and Scanning Speed: If laser power and scanning speed are not properly matched, it affects the flow of the melt pool and the expulsion of gases.

3. Synergistic Effects of Cracks and Porosity
Porosity acts as a stress concentration source, increasing the stress intensity factor at the crack tip and accelerating crack propagation. During crack propagation, fresh surfaces adsorb gas, further promoting the aggregation and oxidation of porosity, leading to a complex damage network that significantly reduces the material’s fatigue life.

4. Conclusion

Crack Formation: Cracks are primarily caused by the segregation of B and Si, carbide precipitation, and phase transitions in the matrix. Process parameters influence thermal stress and solidification behavior.

Porosity Formation: Porosity is closely related to the volatility, oxidation, and inclusion behavior of elements like B, Mo, Si, and Mn. Process parameters control the expulsion of gases.

Effective Control Measures: Controlling B and Si content to below 0.5%, optimizing the Cr/Mo ratio, and increasing Ni content can effectively suppress defects.

Synergistic Damage Mechanism: Cracks and porosity exhibit a synergistic damage mechanism, requiring a comprehensive approach through material composition design and process optimization.