| Laser cladding, one of the additive manufacture, not only can improve the properties of metal materials, but also produces directly complex three-dimensional parts by the three-dimensional shape data-driven without die. Currently, the study of laser cladding ceramic/intermetallic composites is still in its infancy, the principles of physics and chemistry of the laser cladding process of composite material synthesis mechanisms involved are relatively scarce. Therefore, combining combustion synthesis with laser cladding, laser assisted combustion synthesis (LCS) was proposed for titanium alloy surface modification. The selected powders for LCS were Ti and BN, Ti and A1N, Ti and Si3N4, Al and TiO2as raw material, and were preset on Ti6A14V alloy by an extrusion method. According to above method, TiN ceramic reinforced TiB, Ti3Al and Ti5Si3intermetallic matrix composite coatings and Al2O3ceramic reinforced Ti3Al intermetallic matrix composite coatings were prepared on Ti6A14V alloy. Moreover, the synthesis mechanism and microstructure controlling of composite coatings by laser assisted combustion synthesis was also discussed.A reasonable powder composition ratio of prepare layer was determined by principles of composite coating design theoretical, chemical composition and physical parameters of selected Ti6A14V alloy, ceramic and metal powder. The thermodynamic calculations of combustion chemical reaction between the ceramic and metal powder were carried out. The impacts of laser processing parameters changes on laser energy density changes were investigated by numerical analysis. Laser ignition power of these prepare layers was tested. According to the above calculation or other determining process, laser process parameters of LCS ceramic reinforced intermetallic matrix composite coatings were also carried out! TiN and Al2O3reinforced titanium intermetallic matrix composite coatings, such as TiN/Ti-B, TiN/Ti-Al, TiN/Ti-Si, Al2O3/Ti-Al composite coating, were prepared on Ti6A14V alloy by LCS using high-temperature exothermic chemical reaction between Ti and BN, A1N, Si3N4, Al and TiO2, and so on. The surface quality, macro-morphology of bonding zone, phase composition, microstructure and cross-sectional micro-hardness distribution of composite coatings under different composition ratio and different scanning speeds was characterized. The influence of types of protective gas, such as Ar and N2, and alloying elements on phase composition, microstructure, cross-sectional micro-hardness distribution and high temperature oxidation resistance properties was studied respectively.The microstructure of composite coatings and bonding state between composite coating and Ti6A14V alloy substrate was investigated by optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) attached with SEM. The results indicated that samples showed a good metallurgical bond between the substrate and composite coating. A lot of reinforcements, such as TiN and Al2O3, dispersed in composite coating, and the reinforcements phase appeared three basic shapes, fibrous, fine arborescent crystal and granular.The results indicated that there was an evident influence of protective atmosphere on microstructure and mechanical properties of composite coating. The prepare layer can produced a more fully combustion chemical reaction under N2atmosphere than under Ar atmosphere. Additionally, the composite coating of more uniform microstructure, gentle cross-sectional micro-hardness distribution from the substrate to the coating surface, the higher average micro-hardness can be fabricated by LCS under N2atmosphere. The the relative oxidation resistance values of composite coating under N2atmosphere at600℃and800℃were6.83times and1.94times higher than that of Ti6Al4V alloy, improved by about17.96%and19.75%than that of composite coatings under Ar atmosphere, respectively. The results indicated that N2could be a catalyst and played an similar role in all the laser burning chemical reactions. Therefore, during the LCS process, N2was selected for protective atmosphere.The results also indicated that there was an important influence of alloying elements on microstructure and mechanical properties of composite coatings. The effects of Nb content on phase composition, microstructure and micro-hardness distribution from the substrate to the coating surface were discussed. The composite coatings containing Nb were composed of TiN ceramic reinforcement and Ti2AlNb, TiaAlNb, Ti39Nb, Nb7Al intermetallic. There were many kinds of fine particulate TiN dispersed in the Ti-Al-Nb matrix. With the increase of Nb content in the prepare layer, TiN content of composite coating reduced, species and contents of intermetallic compounds containing Nb increased. Therefore, the average micro-hardness values of composite coating cross-section reduced.Combustion synthesis mechanism of composite coatings were discussed according to the combustion chemical thermodynamic and equilibrium phase diagram. The TiN and Al2O3reinforced titanium metal matrix composite coatings were prepared by the laser combustion chemical reactions occurs between the compound powders. The heat effect of combustion chemical reaction makes the laser melting pool temperature field distribution evenly, so the reinforcements of composite coatings by LCS were uniformly distributed in the intemetallic matrix. |
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