Abstract
D.Phil. (Mechanical Engineering)
Three alloy particles; Ti6Al4V alloy, Boron Nitride (BN) and Boron Carbide (B4C) particle were compacted and deposited to the surface of Ti6Al4V alloy substrate using laser metal deposition, an advance additive manufacturing laser technology system attached with three hopper feeders for powder delivery to form a hybrid metal matrix composite. The high demand for improved properties of Ti6Al4V alloy metal matrix composites has led to the fabrication of Ti6Al4V metal with good hardness and most importanly for wear application. Combined properties of Ti6Al4V alloy, Boron Nitride (BN) and Boron Carbide (B4C) particles with unique mechanical properties may result in enhancing the resistance to matrix cracking and the forming of a Ti6Al4V metal - matrix hybrid composite. The combination of three different reinforcement powder particles is often very difficult due to extreme hardness property that often affects the homogenization and bonding mechanism, yet such hybrid composites has a number of benefits in service. However, good combination of properties was possible with the aid of an Nd: YAG laser system attached with three hopper system that delivered powder particles into the melt pool created on the surface of the substrate. Boron carbide / nitride additions were less than 10 volume percent with Ti6Al4V alloy powder having more than 90 volume percent to enable grain growth control, which resulted in good metallurgical bonding in the composites. Double tracked Ti6Al4V - BN - B4C composites were fabricated at different laser power and composition / variations, which were used to form the basis for the experiments. Ten samples were fabricated using laser power ranging between 1700 W and 2800 W while the scanning speed was kept constant at 1.0 m/min. This project however focused on Ti6Al4V - BN - B4C powder coatings on titanium alloy substrate at different process parameters. Optimized samples were therefore cut to experimental sizes and the effect of laser power on the deposition process and powder flow rate were investigated. The resultant microstructures revealed an excellent and homogeneous distribution of the martensitic metal matrix composite structure (MMCs). The Ti6Al4V - BN - B4C composites were crack and pore free, with enabling phase transformation in the martensites. The optimized samples were selected for 2000 W and 1400 W due to good surface integrity, absence of porosity good metallurgical and mechanical properties such as fine grain structure with excellent bonding strength...