ABSTRACT
Lasers due to their high monochromacity, high directionality, high coherence, and good spatial heat distribution have been used in materials science for advanced processing of engineering materials. A high-power laser beam can be focused to a power density up to 1010-12 W/cm2, and can rapidly heat a metal surface layer to a temperature more than 10,000 K, which then can rapidly cool at a similar rate, which makes laser processing potential for high-quality, noncontact precisecontrolled processing. Different processing including laser matter interaction, laser surface modi™cation (laser transformation hardening, laser remelting, laser alloying, laser cladding, laser shock peening, laser glazing), laser welding, laser cutting and drilling, laser forming, and manufacturing can be done for laboratory as well as for industrial applications. Fundamental scienti™c research on laser micro/nanoprocessing has attracted more attention in recent years, including nanofabrication, microfabrication, ultrafast processing, micropackaging, and multiphoton processes. Among the many applications of laser processing of material, laser cladding has attracted extensive research over the last two decades. Laser cladding is basically a coating technology, which utilizes a focused or a defocused high-power laser beam to locally melt the thin surface layer of a substrate and the added materials, which form a new surface layer either by alloying with the base metal or new materials coated on the base material. A large area can be covered by overlapping different laser tracks by suitable manipulation of base material or the laser itself. The melting of the substrate is precisely controlled for a metallurgical bond at the interface, so that the dilution from the substrate is minimal, allowing the newly formed layer to retain the original composition and properties of the added materials. The materials can be added mainly by spraying powder or powder is pasted on the work piece and then laser melted. The powder is sprayed
3.1 Laser Processing and Materials ............................................................................................ 109 3.2 Laser Process Interactions, Instrumentation, and Models .................................................... 110 3.3 Cladding for Aerospace Applications ................................................................................... 121
3.3.1 Surface Enhancement by Laser Cladding ................................................................ 121 3.3.2 Laser Cladding and Near Net Forming of Components ...........................................124 3.3.3 Repairing the Aerospace Components in Service .................................................... 128 3.3.4 Laser Joining by Cladding ........................................................................................ 134 3.3.5 Functionalization and Innovative Processing of Surfaces for Aerospace ................ 139 3.3.6 Laser Fabrication of Different Filters for Aerospace Fuel,
Electronic Cooler, and Propulsion System ............................................................... 142 3.3.7 Repairing of Single-Crystal Superalloy Components by Epitaxial Laser Cladding .......................................................................................................... 144
3.4 Future Requirement and Challenges .................................................................................... 145 References ...................................................................................................................................... 145
via a powder feeder and a coaxial nozzle. Laser cladding is a rapid solidi™cation process with a high cooling rate controlled by the heat input, thin laser melt pool, and heat conduction to the bulk substrate. The microstructure of the deposited layer is usually very ™ne, resulting in superior metallurgical properties. Excellent properties can be obtained, such as improved wear resistance, corrosion resistance, erosion resistance, fatigue, and high-temperature resistance, by optimizing the cladding parameters and selecting suitable powder materials. Laser cladding can locally tailor the substrate surface to designed macro/microstructures with desired properties, while maintaining the toughness and strength of the bulk substrates. By integrating with a 3D computer-aided design (CAD) complex, a 3D metal component can also be fabricated via line-by-line and layerby-layer additive laser cladding. Structures such as gradient, step, sandwich, layered, and inner cavity can be produced; strengthening by grain re™nement, extended solid solution, and particulate reinforcement can be achieved. Microstructures with diverse grain sizes and morphology such as columnar, cellular, dendrite, equiaxial, and directionally solidi™ed (DS), and excellent resistance to wear, corrosion, erosion, fatigue, oxidation, and high temperature can be engineered. The initial research work on laser cladding was performed with commercial multi-kW high-power industrial CO2 lasers.1,2 The powder feeding laser-cladding technique, fundamental methodologies such as sensors for in-process monitoring, calculation of powder ef™ciency, computer simulation, hopper powder feeder system for variable composition laser cladding, cladding of aluminum alloys, surface coatings for intergranular corrosion resistance, and the principle of layer-by-layer cladding are different developments that took place in this ™eld along with the different demands of technologies.3-8 The fundamental mechanism and modeling of laser-cladding processes,9,10 the extended solid solubility, microstructural evolution and nonequilibrium phase diagrams,11-14 the corrosion, wear and oxidation behaviors of laser-cladded coatings,15-17 and the formation of novel alloys by laser cladding with mixed powder feeding18-21 have been reported.22-29 A bibliometric analysis of the patents and scienti™c publications on laser cladding for the period ranging from 1985 to 2007 has been reported.30 The cladding parameters such as laser power, energy distribution, beam diameter, scanning speed, and powder feeding rate on the clad geometry decide the cracks, porosity, microstructure, phases, and properties, for which many investigations are available.31-34 Practical industrial applications are also shown in different reports.35-42 In addition, laser cladding has been extensively investigated for repair and manufacture of 3D components.43-54
Laser cladding involves the interaction of the materials and the laser beam, and the resultant complex metallurgical and solidi™cation processes. In the laser-cladding process, it is essential to provide appropriate power density and interaction time between the laser beam and the material. Figure 3.1 shows the range of the power density and interaction time for various laser material processing techniques. As it is seen, the laser-cladding process requires a power density from 70 to 100 W/mm2 and an interaction time of 0.01−1 s; any laser intended for use in the laser-cladding process should provide this level of power density. The selected laser should provide the appropriate beam quality. Another important issue for any laser material processing is the light režection from the surface of metals. The režection is strongly a function of laser wavelength and it varies from metal to metal. Figure 3.2 shows the wavelength dependency of several metals režection factor. It is also important to consider the contribution of temperature in režectivity. As the temperature of the process zone rises, an increase in absorption occurs, this indicates the potential of more energy absorption by the material.
