Laser Cladded Surface Hardening Coating With Gradient of Mechanical Properties

The present dissertation “Laser Cladded Surface Hardening Coating with Gradient of Mechanical Properties” is devoted to the research of laser cladding process for obtaining high quality surface hardening coating, which can be used for hardening and refurbishment of both machine elements and pressure processing technological equipment. The aim of the research was to investigate the possibility of creating a laser cladded single layer surface hardening coating with a gradient of mechanical properties in the normal direction from the surface to the base material, which would allow increasing the crack resistance of the cladded coating and consequently extend the workability of product in manufacturing and refurbishment of machine elements and tools. The present dissertation contains an introduction, a literature analysis part, a description of research methods, a description of technical facilities and materials, and experimental-analytical parts. Chapter 1 covers literature analysis of laser cladding technologies and concepts for the development of advanced coatings with gradient of properties. The current situation in laser cladding of surface hardening and wear resistant coatings is analysed, including information on the parameters of the laser cladding process, as well as current information on achievable coating properties, coating quality characteristics and typical defects. Chapter 2 provides information on the research methodology, equipment, methods and materials. In this paper, metallography of laser cladded beads and coatings was investigated in detail, as well as the morphology, chemical composition, and mechanical and tribo-technical properties of coatings. Chapter 3 describes the experimental research on forming conditions of the laser cladded high-speed steel (AISI M2) single beads with a keyhole in penetration quality characteristics (shape geometry, porosity and amount of carbide-forming elements) in respect to the technological parameters (cladding speed and laser beam defocusing) of the process. In chapter 4 the experimental research of the influence of laser cladding process regimes on the quality characteristics (thickness, depth of penetration, amount of carbide forming elements in the facing layer), morphology and change in mechanical properties (microhardness) in coating-substrate system of high-speed steel (AISI M2) coatings with keyhole in penetration is reviewed. In chapter 5 experimental study of laser cladding process parameters (overlap ratio, cladding speed and laser beam defocusing direction) and anisotropic structure influence on surface properties (hardness and friction coefficient) of high-speed steel coatings after mechanical treatment is described. Chapter 6 describes the experimental study, in which comparative analysis of differently laser cladded high-speed steel coatings using conventional and the new laser cladding method is carried out. As a result of the research, regime frames of the new laser cladding method for deposition of a single bead and a coating with keyhole penetration in to substrate were determined. The results are summarized in figures, tables and graphs. On the basis of regression analysis empirical models of laser cladding process are obtained, which can be used to predict the values of the cladded bead and coating quality characteristics. In the dissertation a new laser cladding method was developed (Latvian patent application P–19–84, date of filing 30.12.2019.), which allows to create single-layer surface hardening coatings with improved properties in the coating and base material transition zone, which may have a positive influence on the residual stress redistribution, especially when using materials with different coefficients of thermal expansion. Thus, this method eliminates the need for interlayer application in the cladding of dissimilar steels. Consequently, it can contribute to increasing the fatigue resistance of items as well as to increasing the durability under thermocyclic conditions. Technological instruction for hardening and refurbishment of surfaces of pressure shaping equipment by laser cladding of coatings with keyhole in penetration was elaborated as the basis for the present dissertation. At the end of the dissertation information about the trial production of the refurbished parts with coatings obtained by new laser cladding method in Latvia and Belarus; and approbation of the results, as well as an insight regarding the development prospects of this method in further research is given.