Fracture toughness characterisation of functionally graded grade 5 titanium alloy with titanium carbide

Abstract: Functionally graded materials (FGMs) are composites consisting of two or more different materials with a gradient composition. These materials are drawing attention because they can provide new combined functions that surpass the characteristics specific to its constituent elements. The aim of this research is to determine the effect of processing parameters on the fracture toughness of laser processed Ti-6Al-4V (Grade 5) with Titanium Carbide (TiC) to form FGM. Other characterisation tests were also conducted, which include the microstructure and microhardness. Titanium alloys have been chosen as the study material in this research work because titanium and its alloys have a high production cost and are difficult to machine. As a result, titanium has a low industrial usage although it has superior mechanical properties when compared to steel. Literature study has revealed limited information on the properties and characteristics of functionally graded Ti6Al4V and TiC, particularly the fracture toughness behaviour. Preliminary design was carried out to obtain optimised laser processing parameters. A total of 13 trials were carried out by varying the laser power (0.6 kW – 2.5 kW), the scanning speed (0.045 – 3 mm/min), laser spot size (2 or 4 mm) and the power flow rate (3.34 g/min or 6.68 g/min). The specimens were then characterised with respect to the evolving microstructure and the microhardness. For certain processing parameters, cracking of the FGM deposit was observed and could be heard during processing. Optimized laser parameters were obtained for laser power between 1.5 kW to 2.5 kW and scanning speed between 2.5 m/min to 3 m/min. Functionally graded grade 5 titanium (Ti6Al4V) with titanium carbide (TiC) was successfully prepared by laser metal deposition (LMD). The TiC fraction was varied from 5 to 40 vol. % in increments of 5 percentage. A total of 8 layers were deposited to form the FGM. The laser power was varied from 1.5 kW to 2.5 kW in increments of 0.5 kW. A total of 3 laser power settings consisting of 5 specimens each were prepared for fracture toughness testing. The FGM specimens were cut using wire electical discharge machining (WEDM) to obtain a compact specimen design for fracture toughness. An alternative method to fatigue pre-cracking was used to produce the pre-crack. WEDM was used as an alternative due to it being less costly compared to fatigue pre-cracking. Also investigations have proven that the fracture toughness measurements were satisfied according to the ASTM E 399-90 ...M.Ing. (Mechanical Engineering