The use of laser beams has recently been developed for surface cladding with the addition of alloying elements. However, the coating obtained may have some defects, such as porosity, shrinkage, and cracks due to fast cooling, which can be extremely damaging for the applications (wear on materials, blades, etc.). The aim of this study is therefore to gain mastery of all the treatment parameters in order to obtain a defect-free coating and a reliable hard-facing process usual in industry. In this project the coatings consist of two types of carbide powder (WC and TiC) and two substrates (a structural steel and a tool steel). This process requires: - thermal modelling of laser-material interaction; - largescale testing of treatment parameter influence, for validation of modelling and coating optimization; - characterization of the optimal coatings (structures, wear resistance, residual stresses field, etc.).
The use of laser beam surfacial alloying with carbide powder injection gives coatings which may have some defects such as porosity and cracks due to fast cooling. The mastery of this process requires a precise correlation between the treatment parameters and the phenomena induced during the laser material interaction.
To establish this correlation a 3-dimensional heat transfer model with a moving heat source was developed using finite difference technique. The model accommodates the melting and solidification phenomena, the carbide powder injection effect and the surface heat loss by convection and radiation. The results of these calculations are qualitatively consistent with all experimental observations which take into account treatment parameters such as laser power, beam focalization, scanning velocity, powder feeding rate, powder injection geometry and overlap conditions.
From this modelization, optimal treatment conditions were found and applied to laser alloying of 42 CD 4 and high speed M2 steels with tungsten carbide and titanium carbide injections. Identification of solidification structures, using metallurgical and thermal analyses, and characterization of the friction and wear resistance were conducted on these optimized hard surface coatings. The mechanical test results show that the optimized laser alloying process with tungsten carbide improves wear resistance and friction properties.
THE AIM OF THE PROGRAMME IS TO STRENGTHEN THE SURFACE OF A WIDELY USED ENGINEERING STEEL AND A TOOL STEEL BY INJECTION OF TUNGSTEN OR TITANIUM CARBIDE WITH OR WITHOUT A BINDER.
THE PROGRAMME CONSISTS OF THE FOLLOWING STAGES :
1. DESIGN AND CONSTRUCTION OF THE INJECTION ASSEMBLY AND FITTING TO THE ECOLE CENTRALES POWER LASER.
2. STUDY OF THE LASER/MATERIAL INTERACTION, THE EFFECTS OF THE CHARACTERISTICS OF THE POWDER (SPECIFIC SURFACE, GRANULOMETRY, FLOW) AND OF ITS INTRODUCTION INTO THE IRRADIATED AREA IN THE FORM OF ABSORBED ENERGY.
3. SYSTEMATIC STUDY OF THE EFFECTS OF THE LASER INJECTION PARAMETERS ON THE STRUCTURE OF THE DEPOSIT.
4. EXPERIMENTAL DETERMINATION OF THE THERMAL FIELDS AND RESIDUAL STRESS AND THEORETICAL MODELING.
5. IDENTIFICATION OF SOLIDIFICATION MECHANISMS AND METALLOGRAPHIC ANALYSIS OF THE STRUCTURES OBTAINED.
6. STUDY OF THE PROBLEMS OF DEPOSITION BY MEANS OF MULTIPASS INJECTION.
7. CHARACTERIZATION OF THE STRENGTHENED SURFACES IN TERMS OF FRICTION BEHAVIOUR AND RESISTANCE TO WEAR.