Research On Crack Inhibition Mechanism And Performance Of Laser Cladding Layer Based On Low Expansion Characteristics

Laser cladding technology is an advanced material surface modification technology,which is an important part of green remanufacturing technology and has a broad application prospect.However,laser cladding layer cracking has become a technical bottleneck,which greatly limits the application and promotion of laser cladding technology.Due to the characteristics of rapid heating and cooling in the laser cladding process and the difference in thermophysical properties between the laser clad material and the substrate,it is easy to cause large thermal stresses,which in turn leads to defects such as cracks in the laser cladding layer,and there is no ideal crack inhibition theory and method in this field.The appearance of cracks will greatly affect the service life of the laser cladding layer.At present,crack inhibition theories and methods in this field mainly focus on optimizing process parameters,preheating and post-treatment,etc.These methods reduce crack sensitivity to a certain extent.However,further exploration and research are still needed for the preparation of high-quality laser cladding layers with low crack susceptibility.Reducing the thermal stress during the laser cladding process is an important way to reduce the crack sensitivity of the laser cladding layer.Inspired by the low expansion phenomenon of invar alloys,this paper introduces the low expansion property into laser cladding technology and constructs a new theory and method for crack inhibition in laser cladding based on the low expansion property.In this paper,an external magnetic field induced and magnetically enhanced elemental excitation method is used to generate the low expansion property in the laser cladding process.The low expansion property reduces and regulates the thermal expansion difference between the laser cladding material and the substrate material,eliminates the thermal stress and promotes element uniform distribution,thus reducing the crack sensitivity.This paper analyzes the influence of low expansion characteristics on the crack inhibition and performance of laser cladding layers from the aspects of lattice parameter variation,magnetic transition,microstructure evolution,mechanical properties and wear of the laser cladding layers.This paper mainly includes the following contents:（1）The negative expansion material Zr W₂O₈ is added to the laser cladding material to adjust the thermal expansion of the cladding layer and achieve low expansion characteristics,thereby reducing the thermal stress of the cladding layer.The trends of thermal stresses in the high and low temperature regions were analyzed.The thermal stress in the low temperature region is determined by the combination of the morphological size,thermal expansion and elastic modulus of the laser cladding layer.The thermal stress in the high temperature region is mainly determined by the magnitude of thermal expansion.The laser cladding layer morphology analysis,microstructure analysis,physical analysis,thermal expansion test,mechanical and frictional properties tests reveal that it is feasible to introduce low expansion characteristics in the laser cladding layer to achieve thermal stress regulation.The low expansion characteristics significantly improve the wear resistance,and the friction coefficient and wear volume of the laser cladding layer is significantly reduced.（2）The low expansion characteristics of laser cladding layers were achieved by using alternating magnetic fields.The physical criterion for the low expansion properties of laser cladding layers induced by an external magnetic field was revealed to be lattice shrinkage,reduction of elastic modulus and magnetic transition from ferromagnetic to the paramagnetic state by the change of magnetic properties,lattice parameters and elastic modulus.It was found that the transition from the ferromagnetic to the paramagnetic state of the laser cladding layer produced a contraction of the lattice parameters,which caused a negative magnetostriction phenomenon that neutralize the thermal expansion of the material to a certain extent,leading to the generation of the low expansion characteristics.It was found that the generation of low expansion characteristics reduced the thermal expansion coefficient of the clad layer and the thermal stress was significantly reduced.In addition,the stirring effect of the alternating magnetic field refines the microstructure of the laser cladding layer,resulting in a more uniform distribution of elements and a shift from coarse dendrites to fine equiaxed crystals,which moderates the temperature gradient and reduces the emergence of thermal cracks.The low expansion characteristic can effectively reduce the thermal expansion and thermal stress of the laser cladding layer and reduce the crack sensitivity.Under the combined influence of low expansion characteristics and magnetic stirring,the crack susceptibility of the laser cladding layer is significantly reduced.（3）The effect of material phase characteristics on the formation of low expansion characteristics and its influence on crack inhibition and properties were revealed.By adjusting the content of Cr and Nb in the laser cladding material,the relationship between the external magnetic field induced low expansion characteristics and the physical phase characteristics is revealed.The influence of Cr and Nb on the wear resistance and corrosion resistance of the laser cladding layer was revealed by dry friction wear experiments and electrochemical corrosion experiments.When the content of Cr and Nb is 5 wt%,the low expansion characteristic of the laser cladding layer is obvious,and the elastic modulus of the laser cladding layer decreases significantly while keeping the hardness increased,which improves the wear and corrosion resistance.The microstructure with low expansion characteristics was significantly optimized,and the mechanical properties were significantly improved,enhancing the toughness and crack resistance of the laser cladding layer.（4）The mechanism of low expansion property on micro interface crack inhibition is revealed.The influence of alternating magnetic fields on the morphology and element distribution of the cladding layer is analyzed.The influence of the low expansion property induced by the external magnetic field on the mechanical properties of the metal and ceramic phases was analyzed.It is found that the low expansion characteristic can significantly optimize the microstructure of the cladding layer and reduce the lattice mismatch phenomenon and crack susceptibility of the crystal during the growth process.The magnetic field improves the hardness of the laser cladding layer by optimizing the microstructure.The generation of low expansion characteristics leads to high hardness and low elastic modulus,which makes the interface more elastic and thus improves the toughness of the cladding layer.At the same time,the magnetic field reduces the defects such as porosity generated during the solidification,thus contributing to the improvement of the mechanical properties of the laser cladding layer.The low expansion characteristic induced by the external magnetic field reduces the plastic deformation during fiction and improves the wear resistance of the material.