Tribological coating of titanium alloys by laser processing

Abstract

Titanium-based alloys have been used for aerospace materials for many years. Recently, these alloys are now being increasingly considered for automotive, industrial and consumer applications. Their excellent creep resistance, corrosion resistance and relative higher specific strength ratio are attractive for many applications. However, the main obstacle for the wide adoption of Ti alloys in various industries is their poor tribological properties. In slide wear, Ti deforms and adhesive wear readily occurs. The abrasion resistance of Ti alloys is much lower than that of D2 steel, which in itself, is insufficient for many engineering requirements. Their poor tribological properties are mainly due to low hardness and absolute values of tensile and shear strength. Surface treatments of titanium alloys for improving their surface hardness and wear resistance have recently attracted a great deal of research interest. Different surface modification techniques have been studied in order to improve the tribological characteristics of Ti alloys, i.e. PVD, nitrding, carburizing, boriding, plating etc. All these treatments are aimed at producing a hard surface layer on Ti alloys. Coatings produced by these techniques have their own limitations such as thermal distortion and grain growth. A different approach is to introduce hard particles in the Ti alloy matrix to form a MMC coating, which has tailor-made hardness and wear resistance properties. Laser cladding or laser alloying techniques facilitate the fabrication of surface MMC on Ti alloys without thermal distortion to the substrate. Most published papers have focused on increasing the surface hardness of titanium to improve the surface wear resistance, little of these have focused on the reduction of surface friction. This project is focused in both of these two directions. In this project, the fabrication of hard and wear resistant layers of metal matrix composite on titanium alloys substrate by laser surface alloying was investigated. Powder mixtures of Mo and WC were used to form the MMC layer. By optimizing the processing parameters and pre-placed powder mixture compositions, surface MMC of different properties have been successfully fabricated on CP-Ti and Ti6Al4V respectively. The structure and characteristics of the MMC surface were investigated by metallography, SEM, XRD, and E-DAX. It was found that the hardness of the laser alloyed Mo/WC MMC surface was 300% higher than that of the CP-Ti substrate. Excellent metallurgical bonding with the MMC layer of the substrate has also been achieved. The relative kinetic frictional tests were carried out and the results showed that the kinetic coefficient of friction of the laser fabricated Ti-Mo-WC MMC coatings was much lower that that of the CP-Ti and Ti6Al4V substrate. The wear resistance of this Ti-Mo-WC MMC was found to be between 60 to 150 times better than those of CP-Ti. The optimum MMC coating with respect to hardness, wear resistance and frictional coefficient was fabricated using powder mixture composition 60%WC - 40%Mo and processed by an optimum set of laser process parameters. The experimental results also identified a key issue of mixing efficiency that the density of WC was much higher than that of Ti and Mo and this rendered poor mixing between the different constituents. This resulted in a relatively poor repeatability of the coating quality. To solve this issue, a lower density carbide phase, TiC, was used. The hardness and dry sliding wear resistance of the Ti- Mo-TiC MMC was found to be lower than those of Ti-Mo-WC MMC. The mixing efficiency of the TiC in the Ti-Mo-MMC was found to be better than the WC in Ti-Mo-WC MMC. This combination gave a harder and more wear resistant MMC with more uniform properties across the width and depth of the alloyed layer formed. A mathematical model for predicting the melt depth of the alloyed MMC layer was developed together with a heat transfer mechanism between layers of powders under the laser irradiation. The model facilitates the prediction of melt depth of the alloyed layer and the dilution ratio of the MMC layer. It thus enabled the composition of the MMC layer to be predicted and tailor made. The project results contribute significantly to the knowledge of improving the wear and frictional properties of one of the most important engineering metals for the automobile and aerospace industries.