| Laser cladding is a surface modification technology that deposits a thin layer of alloys with the desired properties onto the surface of the substrates by using laser as the heat source. Laser cladding has attracted extensive researches over the past 30 years due to its unique features such as metallurgical bonding at the interface, low dilution rate, fast heating and cooling rates, and small distortion. To date, laser cladding has been widely used to improve the corrosion, wear, and erosion resistance of the components in industries. The recent development of the high power direct diode lasers (HPDDL) has been opening a new opportunity for laser cladding process due to its unique advantages in comparison to the CO2 and YAG lasers. The HPDDL is characterized by a high absorption rate, uniform distribution of power along the width of laser beam, high deposition efficiency and low clad dilution ratio.;AISI 420 stainless steel (SS) is one of the most popular martensitic stainless steels and has been widely used in structural parts, automotive components, surgical and dental instruments, and pipelines in oil & gas industry because of its high toughness and high strength. However, the poor hardness and low corrosion resistance of AISI 420 SS restrict its performance in highly corrosive and abrasive environments. Therefore, two different alloying compositions were designed in this research to improve the erosion and corrosion resistance of AISI 420 SS stainless steel.;Vanadium carbide (VC) was added as a reinforcement phase into the AISI 420 SS to fabricate a metal matrix composites (MMCs) for the purpose of improving erosion resistance. The main processing parameters of the laser cladding, namely the laser power, scanning speed, and powder feed rate, were optimized with the consideration of multiple clad characteristics (clad height, carbide fractions, and metal matrix hardness) by using the Taguchi based Grey relational method. Then, by using the optimized processing parameters, the AISI420-VC MMCs with different fractions (0 wt.%--40 wt.%) of VC was deposited by using the high power direct diode laser. The effect of VC fraction on the erosion resistance of MMCs was investigated by using the high pressure abrasive waterjet. The optimal addition fraction of VC with respect to the erosion resistance was found to be 30 wt.%. Furthermore, the effect of reinforcements category on the erosion resistance of AISI 420 based MMCs was studied. Three carbides (VC, TiC, and WC) were added into the AISI 420 SS, respectively, according to the optimized addition fraction (30 wt.%). Their erosion resistance was evaluated at the impingement angles of 30°, 45°, and 90°, respectively. It was found that the VC reinforced MMCs performed the highest erosion resistance among the studied MMCs at all impinging angles.;Molybdenum (Mo) and nickel (Ni) are two of the most commonly used alloying elements in stainless steel and are believed to have positive effects on the improvement of corrosion resistance. Therefore, Mo (1--4 wt.%) and Ni (1--4 wt.%) were added into AISI 420 in order to improve its corrosion resistance. By using the electrochemical testing method, the corrosion resistance of the AISI 420 with the addition of Ni and Mo was investigated. The effects of Mo and Ni on the microstructure, phase distribution, and microhardness on AISI 420 SS were also studied.;The thermal results such as the temperature history, temperature gradient, and solidification rate are the main factors deciding the microstructure and mechanical properties of the clads. Thus, in order to study the heat transfer during laser cladding process, a three dimensional (3D) finite element (FE) model was developed. In this model, the interactions between the laser beam and powder streams were considered. A method based on the mass balance was adopted to predict the geometry of the clads. The thermocouples and CCD camera were used to monitor the temperature history and molten pool size. Then, the simulated results were validated by the experiments. |
