Microstructure And Properties Of Ultrafine And Nanosized Laser Cladding Layer On Titanium Alloy Surface

As an excellent structural material,the titanium and titanium alloy occupy an important position in the manufacturing field because of their excellent comprehensive properties such as high strength to weight ratio and good fatigue toughness.Although TA2 titanium alloy has been widely used in various fields,its low micro-hardness and poor wear resistance are still important factors restricting its further development in the field of tribology.At present,more researchers are committed to the surface modification of the TA2 titanium alloy,and plan to prepare composite coatings with better microstructure and performance on its surface,which can effectively reduce costs while improving the performance.Laser cladding/re-melting technology,as a laser nano-modification technology,can prepare the ultrafine nano clad layer on the surface of the titanium alloy,effectively enhancing the surface wear resistance and micro-hardness of titanium alloy.Laser clad materials usually use self-fluxing alloy powders.Among them,Stellite6,as a Co base alloy powder with good wettability,wear resistance and oxidation resistance,was selected as the main cladding of the preset coating in this study material,combined with La₂O₃and Ag to prepare ultrafine nano laser clad layer.In order to improve the properties of the clad layer,laser re-melting technology can be used to study the clad layer.In this thesis,the improvement of surface microstructure and properties of TA2 titanium alloy was studied.The surface of the samples was strengthened by laser nanotechnology,and characterized by XRD,OM,SEM,EDS and HRTEM.The research includes three parts:first of all,the microstructure evolution of Ag modified Stellite6 based laser clad layer was studied.The results showed that the Stellite6-La₂O₃-Ag laser clad layer and TA2 matrix exhibited a good metallurgical bond after Ag-added.The dendrite structure was fine,also nanocrystalline particles were distributed in the laser clad layer.The effect of different content of Ag on the structure and performance of the laser clad layer was explored.It was noted that increasing the Ag content appropriately can retard growth of dendrites in puddle to a certain extent,and further refine the nanocrystalline grain size,favoring to the formation of the ultra-fine nanocrystalline structure,the existence of ultrafine nanocrystals had the strengthening effect on the laser clad layer.On the contrary,additions of an excessive amount of Ag led the coarse structure and microcrack to be formed in the laser clad layer,reducing the performance of the laser clad layer.Further,the Stellite6-La₂O₃laser clad layer can be prepared on the surface of the TA2 titanium alloy;the effect of the La₂O₃content on the formation of Stellite6 base ultrafine nano clad layer was also explored.The results showed that increasing of the content of La₂O₃in a certain range was beneficial to obtain a more uniform also compact laser clad layer,favoring to the formations of ultrafine nanocrystals.With increasing of scanning speed,the microstructure of Stellite6-La₂O₃laser clad layer was refined,the micro-hardness increased.However,the cracks,pores and other defects may be produced in the laser clad layers,making the laser clad layer easy to fall off,and then affect the performance of the laser clad layer,which should improve these defects.Then the microstructure performance of Stellite6-La₂O₃-Ag re-melted layer was analyzed.The experimental results showed that there were still large number of ultra-fine nanocrystals in the laser re-melting layer,and the coarse dendrites were evenly distributed in the middle-layer.The pores in the bonding area between the laser clad layer and the substrate became smaller.The micro-hardness of the laser re-melting layer decreased due to the dilution effect of the TA2 substrate on the puddle induced by the laser beam.This project provides the theoretical and research basis for the preparation of the ultrafine nano laser clad layer on the TA2 titanium alloy and the application of the laser nano technology in the industrial practical productions.