| Each year, several billion dollars have been used in the defense, nuclear, aerospace, and automotive industries for repairing or replacing the functionally value-added components. The laser surface modification techniques (LSMTs) such as phase transformation hardening, cladding, and alloying produce localized surfaces with superior resistance to heat, wear, fatigue, fracture, erosion, and corrosion. The LSMTs can be used to modify/repair the surfaces of the value-added components in order to enhance the length of their service life. For the past three decades, a CO2 and a Nd:YAG based LSMTs have been quite well-established; but, recently, a high power direct diode laser (HPDDL) has been attracting more attention by the industry to perform localized hardening or one-step cladding. The HPDDL combines higher wall-plug efficiency and better absorption by metals due to operating in shorter range of wavelengths (808--940 nm) with lower capital and operating costs. When a larger surface area needs to be hardened, a multiple scans of laser beam with slight overlap should be applied. During this condition, a tempering effect in addition to hardening is generated in the overlapped zone that will affect the homogeneity of phase transformations, microstructures and hardness uniformity. The phase transformations, microstructure homogeneity and uniform hardness are highly influenced by heat management defined by the length of scan and size of overlap. The one-step cladding process is affected by the number of processing parameters such as laser spot size, absorptivity, laser power, scanning speed, powder feed rate and its direction, convective boundary conditions, thermo-physical properties, and the geometry of the substrate.;In this dissertation, a two-fold scheme of process study, one based on modeling the thermo-kinetics of the laser hardening based on phase transformations and cladding based on solidification rate is developed. The other one based on optimization of process parameters incorporating the (a) numerical, (b) experimental, and (c) on-line monitoring is proposed to achieve the defect-free cladding, and uniform hardening. In the numerical simulations, a thermo-kinetic (TK) phase transformation model will be developed to serve as a cost-effective tool for investigating the effect of length of scan and size of overlap on tempering, hardness uniformity of the microstructures resulting from the transformation hardening. A TK solidification model will be developed to obtain a defect-free, good metallurgically-bonded clad geometry of uniform hardness with minimum dilution. The experimental study of microstructures and hardness measurements will assist the optimization process of numerical results. The real-time temperature measurement system that consists of a laser-assisted infrared (LAI) pyrometer and an infrared camera will be used to predict the laser process parameters in order to achieve indirectly the uniform hardness. The machine vision system that consists of a CCD camera, a green laser, and optical filters will be used to monitor the powder feeding and the molten pool in real-time. |
