Experimental And Mechanism Study Of TA2 Fabricated By Laser Surface Modification

Laser surface alloying, laser surface remelting and quenching were performed on commercial pure titanium TA2 with optimized process parameters. Microstructures were characterized using X-ray diffraction, optical microscopy and scanning electron microscopy. The wear properties and hardness were examined using UMT-2MT wear tester and HVS-1000 type microhardness tester.TiC/Ti composite layer was successfully fabricated by laser surface alloying with carbon. The microhardness and wear resistance of laser surface alloyed layer were greatly improved. The microstructure analyses showed that titanium carbide growth morphologies had a well-developed dendrite, cellular dendrite, globular microstructure and cross-petal microstructure and so on. The hardness measurement showed that the microstructure corresponding to the globular morphology for the titanium carbide phase had the highest microhardness. The mechanism of the formation of titanium carbides could be divided into three periods of diffusion, reaction and rapid solidification. The carbide morphology in the alloyed coating was directly dependent upon the solidification cooling rate. At a high cooling rate, the carbide morphology was well-developed dendrite. As the solidification cooling rate decreased, the growth morphology of titanium carbide changed to cross-petal-like with symmetrical arms.Nitrided layers with golden yellow color were fabricated by laser gas nitriding. The result of XRD showed that TiN was detected but no Ti₂N was detected. The microhardness of laser nitrided layer was about 500Hv and gradually decreased to 209Hv for the titanium substrate. Nitriding of titanium occurred via the inward diffusion of nitrogen in the molten pool. The diffusion coefficients of nitrogen in TiN and a-Ti(N) phases were determined.P-Ti to a'-Ti phase transformations occurred with rapid cooling rate by laser surface remelting and quenching. The morphologies of a' martensite in the quenched and laser surface remelted samples were corresponding with the characterization of acicular martensite. The cross-section microstructure of laser melted layer showed that a 3 μm thick TiN continuous thin surface region formed on the top surface of laser remelted sample. The hardness measurement showed that the microhardness of laser remelted layer was improved to425 Hv while the microhardness of the quenched sample was improved to 275Hv from 225Hv for the annealed sample. Wearresistance were improved by laser surface remelting.