The Research On Mechanical Surface Treatment And Laser Alloying Of Titanium

Titanium and its alloys have low density, high strength, high corrosion resistance and high chemical stability. These properties are very useful for many applications in various fields, such as biomedicine, aerospace and mechanical industries. However, the application of titanium and its alloys restricted because of their poor tribological properties such as high friction coefficient and low hardness. In order to improve the fatigue resistance and hardness, surface modification of commercial purity (CP) titanium TA2 was performed by mechanical surface treatment and laser surface alloying.For mechanical surface treatment, shot peening and cold rolling were used. Shot peened and cold rolled samples were marked as SP and CR for a-Ti, QSP and QCR for a'-Ti (martensitic). The fatigue specimens were cyclically deformed in three point bending under stress ratio R = 0.1, in an Amsler-5100 high frequency machine at room temperature in air. Numerical value of 10⁶-cycle fatigue life was defined as fatigue strength. The treated surface was characterized by using transmission electron microscopy (TEM), X-ray diffraction (XRD), and surface roughness measurements. Based on the three factors (substructure, residual stress, and surface roughness), the mechanisms of mechanical surface treatment were discussed.Experimental results show that SP and CR increased the fatigue strength of a-Ti and a'-Ti (martensitic) because the deformation twins formed. Due to the deformation velocities quite different in CR and SP, the numbers of dislocation and deformation twins were very different in CR and SP. However, the improvement of fatigue strength is disproportional with the number of deformation twins.A comparative study of SP and CR on fatigue strength of a-Ti has been conducted, the following results were obtained: (1) The improvement of fatigue strength is related to the formation of deformation twins in strengthened layer. Before or after fatigue, the samples strengthened by SP or CR not only have twin shape and number change, but also have twin interactions in the SP and twin-grain boundary interactions in the CR. (2) XRD measurement demonstrated that surface compressive residual stress after SP are much higher than those after CR. Surface compressive residual stress has higher relaxation in the SP than in the CR condition during cyclic loading, then the surface compressive residual stress has the same values after fatigue deformation. (3) Surface roughness resulting from SP is ten times of CR.In QSP samples, there are high density dislocations, deformation twins and deformation bands in the strengthened surface layers, and twin-twin interweave resulting of twin paling effects. High compressive residual stress occurs, and then partially relaxes under fatigue loading.In QCR samples, a small amount of deformation twins occur. When the samples were cyclic loaded, the twins could traverse the martensite laths. Therefore, the deformation twins in strengthened layer divided matrix into many small areas. The compressive residual stress was not relaxed after cyclic loading, and the surface roughness decreased.For laser surface alloying, alloying materials including various elements (N, C, Ti+C, Ti+W+C), ceramics (WC, TiC) and metal-ceramics (Ti+WC, Ti+TiC, Ti+C+WC) were used to modify the surface of a-Ti. Laser surface alloying was conducted on a continuous wave CO₂ laser. The microstructures, compositions, phase changes and microhardness profiles were investigated by optical microscopy, SEM, EDS, X-ray diffractometry and microhardness tester.Laser nitrided layer consists of dendritic TiN and martensitic a' structure. The laser remelted layer consists of acicular a' martensitic structure. The mechanism of titanium nitrides formation can be described by the following reactions: surface absorption of nitrogen, nitrogen decomposition, interface reaction, nitrogen diffusion, TiN precipitation and solidification.Laser surface alloying of CP Ti with activated carbon powder, the alloyed layers contain TiC and a' -Ti. The growth morphologies of TiC show a well-developed dendrite, cellular dendrite, globular, cross-petal microstructure and globular microstructure. The change in microstructure is related to cooling rate and carbon concentration. The reaction mechanism of Ti and C is discussed. Laser alloying of Ti+C mixtured powders enhanced characteristics of bond interface. The microstructure of Ti+C is similar to the C addition because of multi-track laser scanning.When laser alloyed TiC or WC, porosity defaults occur in the interfaces between HAZ and alloyed layer. In situ formed TiC in alloyed layers is a solution/precipitation model. WC particles were decarburized by the following procedures: Firstly, WC led to decarburization and resulted in the formation of W₂C or in the appearance of metallic W within the coatings. Secondly, the extraction of C from WC or W₂C reacted with molten titanium, which resulted in TiC formation. Finially, W, WC, W₂C dissolved within Ti matrix.It is noteworthy that functionally gradient material was produced by laser alloying with 2Ti+WC. From alloyed surface to bonded interface, unmelted irregular shape WC, in situ formed dendrite TiC and partially melted round WC, and complex carbides with intermittently net shape were observed.Phases in laser alloyed Ti+C+WC coatings are composed of a '-Ti, W, W₂C, TiC and WC. Ti/C ratio in Ti+C+WC powders remarkably affected the microstructures of laser alloyed layers. Dendrite TiC formed when Ti/C ratio was equal to 1:2; irregular shape WC and round TiC coexisted when Ti/C ratio was equal to 1:1; acicular shape, dendrite TiC and a very small amount of round WC occured when Ti/C ratio was equal to 2:1.Different (Ti,W)C microstructures are observed in laser alloying with TiCW powders. Further investigations are needed for more details of (Ti,W)C.Additionally, Simple models were established to explain the microstructures formation procedures during laser surface alloying of studied alloying powders. In all studied cases, one of the most important conclusions is that TiC in-situ formed and hardness largely increased.