Research On Process And Microstructure Propertie Of Laser Preparation High Load Bearing Capacity On The Titanium Alloy Surface

In this research, sufaces of titanium alloy was modified by semi-conductor laser. The nitrided layer and TiC+Ti cladding layer which has excellent load-bearing capacity were obtained on the surfaces of different titanium alloy. The microstructure, composition and phase constitution of the coatings were characterized using scanning electron microscope(SEM), X-ray diffrcaction(XRD), respectively. The generation and mechanism of cracks in the hardened coating are investigated. Corrosion resistance, hardness distribution and load-bearing capacity were measured by electrochemical workstation, micro-hardness meter and friction and wear tester. The indentation depth was measured by optical profiling system in order to characterize its load-bearing capacity. In addition, the wear mechanism was studied by observing the wear morphology of specimens. The COMSOL software was used to simulate temperature distribution in laser nitriding Ti-6Al-4V alloy and to predict the thickness of nitride layer.The results can be summarized as follows. The hardness, corrosion resistance and load-bearing capacity of harden layers on the surfaces of TC4 and TC11 alloy are all improved effectively. Micro-hardness of nitride layers on the surfaces of TC4 and TC11 alloy are 1392 HV and 923 HV, respectively. The average hardness of TiC+Ti cladding layer on the surface of TC4 alloy increases with the increase of TiC content in the powder, and the hardness of the particle phase is fluctuated in 1000~1500 HV. The corrosion currents of the nitride sample of which laser power is 600 W and cladding sample with TiC content less than 60% was smaller than that of the original sample. However, due to the existence of pores and cracks in the hardened layers, the remaining samples corrosion resistance is poor. The corrosion current(I=1.145×10-7A) of sample with TiC content equal to 60% is minimum, and has the best corrosion. Under the contact stress between the ball and sample is equal to 8.5GPa, the most shallow indentation depth on the surfaces of TC4 alloy nitride layer, TC11 alloy nitride layer and TC4 alloy cladding layer after the balls rolling on the surface of hardened layer 120 times are 6um, 13.1um and 20.45 um, respectively. Under the same load, the loadbearing capacity of hardened layers is significantly improved compared with the original sample indentation depth 56.4um. The process parameters which has the most shallow indentation depth and the best load-bearing capacity is: substrate TC4 alloy, laser power 700 W, scanning speed 300mm/min, overlapping ratio 40%. Moreover, the nitride layer thickness of the simulation prediction and measured were nearly same through simulating laser nitriding TC4 alloy molten pool temperature field in the process of laser power 1000 W, scanning speed 300mm/min. The thermal stress which causes the titanium alloy plate to produce plastic deformatin mostly concentrated at the edges on both sides of the molten pool. The maximum deformation of TC4 alloy plate is about 24 um.