Formation Mechanism Of Defects In Direct Laser Deposition Of 24CrNiMoY Alloy Steel And Its Effect On Properties

High-speed rail brake discs play an important role in ensuring the safe operation of high-speed rail.Therefore,higher requirements are placed on the shape,structure and comprehensive performance of the brake disc.The traditional manufacturing method cannot meet the preparation requirements.As a new type of preparation technology,direct laser deposition technology can be used to prepare parts with complex shapes,and it has received widespread attention in the field of brake disc manufacturing.However,the direct laser deposition process is a complex non-equilibrium metallurgy process,the temperature changes dramatically during the process and the solidification rate of the molten pool is extremely high,so it is easy to produce various unpredictable metallurgical defects inside the parts.These defects have major impacts on the properties of the materials,which can lead to a significant reduction in material properties and lead to material failures.Therefore,it is important to study the formation mechanism of typical defects in direct laser deposition process and its influence on material properties.It lays the theoretical foundation for improving material properties and preparing defect-free parts.In this paper,24CrNiMoY alloy steel samples with different process parameters and different layers were prepared by direct laser deposition(DLD)using YLS-1000 W fiber laser.Lack-of-fusion(LOF)defects and crack defects were found in the samples.The relationship between process parameters and these two types of defects were studied by SEM,TEM,EBSD,OM,XRD and other methods.The formation mechanisms of LOF defects and cracks were established and their effects on mechanical properties were further analyzed.The results show that:When the laser power was 600 W,the energy input was lower,the energy obtained by the molten pool was insufficient,which led to the poor wettability of the solid-liquid phase,the remelting depth between the deposited layers was insufficient,and the molten pool parameters could not satisfy the complete fusion formula,so the LOF defects produced in the samples.When the laser power was 700 W,the energy input was too high,the large thermal stress in the alloy steel would lead to the cracking of liquid film,inclusion and solid deposition layers,so the cracks produced in the samples.The presence of LOF and crack defects would reduce the mechanical properties of alloy steel samples.When the laser power was 650 W,the energy input and thermal stress were moderate,good mechanical properties for sample without LOF and crack defects were prepared.The density of sample reached 99%,the average microhardness was 386 HV0.2,the tensile strength was 788 MPa,the yield strength was 699 MPa,the elongation was 9.1%,and the wear capacity was 0.2 x 10’3 g.