Analysis of the Relationship Between Cracks and Residual Internal Stress in the Laser Cladding Process
As a key technology in fields such as mechanical manufacturing and aerospace, laser cladding is commonly used for parts repair, surface strengthening, and functional coating preparation. While its core characteristic of "rapid heating and rapid cooling" enables efficient cladding, it easily induces residual internal stress between the cladding layer and the substrate. When the internal stress exceeds the yield strength of the cladding layer, cracks initiate, which seriously affects product quality and service life. This article systematically analyzes the types, causes of residual internal stress in laser cladding and their correlation with cracks, providing theoretical references for solving crack problems in the industry.
Laser Cladding Thermal Stress: The Main Driving Factor for Crack Formation
Thermal stress is the residual internal stress that has the greatest impact on cracks in laser cladding, and its formation is directly related to "temperature difference" and "difference in coefficients of thermal expansion". During laser action, the cladding layer is instantly heated to a molten state, while the substrate temperature is close to room temperature, resulting in a significant temperature difference between the two. In the cooling phase, the cladding layer needs to shrink rapidly; however, due to the substrate having a lower coefficient of thermal expansion than the cladding layer (e.g., cladding alloy and carbon steel substrate), the substrate's shrinkage is much smaller. The shrinkage of the cladding layer is constrained by the substrate, leading to the accumulation of tensile stress. Simulation studies on multi-pass laser cladding show that this "thermal expansion and contraction" also causes uneven deformation of the cladding layer, intensifying stress concentration and ultimately becoming a core inducement for crack initiation.
Laser Cladding Structural Stress: A Potential Hazard Caused by Microscopic Phase Transformation
Structural stress originates from the crystalline phase transformation of "liquid metal to solid metal" during the laser cladding process, resulting from uneven microscopic structure transformation. After the cladding material and substrate are melted, the liquid metal forms different types of solid structures (such as martensite and austenite) during cooling and solidification, and there are differences in the specific volume of different structures. Meanwhile, the compositional difference between the cladding layer and the substrate (e.g., the cladding layer contains elements such as tungsten and chromium, while the substrate is ordinary steel) further expands the difference in microscopic structure after crystallization, leading to "incoordination" in volume change. This incoordination is constrained within the cladding layer and eventually transforms into structural stress. Although its impact on cracks is weaker than that of thermal stress, it creates stress-weakened areas inside the cladding layer, providing channels for crack propagation.
Laser Cladding Constraint Stress: A Fracture Risk Point with Two-Stage Transformation
Constraint stress is the stress generated when the cladding layer is constrained by the substrate during the two stages of "heating and expansion" and "cooling and shrinkage", and there is an obvious stress transformation process. In the heating stage, the first melted material in the molten pool expands when heated, but the surrounding substrate remains in a low-temperature rigid state, blocking the expansion, thus generating compressive stress inside the cladding layer. In the cooling stage, the heated cladding layer (and composite coating) needs to shrink, but is restricted by the substrate with lower temperature and higher rigidity, so the compressive stress gradually transforms into tensile stress. Studies have shown that the toughness of laser cladding layers is generally low, and the ultimate stress they can withstand is much lower than the critical value for brittle fracture induced by tensile stress, making constraint stress an important risk point for crack initiation.
Differences in the Impact of Three Types of Residual Internal Stress on Cracks in Laser Cladding
Although the three types of residual internal stress in laser cladding all induce cracks, their causes and impacts are significantly different. From the perspective of core causes: thermal stress originates from macroscopic temperature difference and material property differences; structural stress comes from volume changes caused by microscopic phase transformation; constraint stress is the expansion and contraction response under substrate constraint. From the perspective of impact degree: thermal stress has the strongest driving effect on cracks, which can penetrate the cladding layer-substrate interface and cause interfacial cracks; constraint stress directly triggers brittle fracture due to tensile stress, with the second strongest impact; structural stress is mainly concentrated inside the cladding layer, mostly causing local microcracks. From the perspective of distribution characteristics: thermal stress is unevenly distributed; constraint stress has obvious stage transformation; structural stress is concentrated in phase transformation active areas.
Directions for Laser Cladding Crack Control
In summary, the essence of laser cladding cracks is the result of residual internal stress exceeding the bearing capacity of the cladding layer. Among them, thermal stress is the core driving factor, constraint stress is the direct triggering factor, and structural stress creates conditions for crack propagation. Based on this, the control of laser cladding cracks should focus on "reducing thermal stress", such as optimizing laser power and scanning speed to slow down the cooling rate, or selecting cladding materials with a coefficient of thermal expansion matching that of the substrate. At the same time, structural stress can be reduced by adjusting the composition of cladding materials to minimize differences in microscopic structure, and constraint stress can be alleviated by preheating the substrate. By analyzing the relationship between the three types of residual internal stress and cracks, this article provides a clear direction for the process optimization of laser cladding technology, helping to improve the reliability and service life of laser-cladded products.