Abstract
Titanium and its alloys are some of the most useful rare earth metals in the revolutionized manufacturing industry. Its unique properties of high strength to weight ratio, high corrosion and wear resistance have made its applications in the industry greatly surpass some other metals such as aluminium, copper, zinc, and stainless steel. In the past few decades, research has focused on fusion welding and additive manufacturing (AM) of Ti6Al4V without much attention to joining additive manufactured products using fusion welding. The limitation of building huge AM parts that will not accommodate the machine build space can be solved if AM parts are built in bits and joined using fusion welding. The focal point of this research examines the feasibility of joining these AM parts using laser welding. It further evaluates the microstructural and mechanical properties of the joints made in AM welds through laser welding. To fully understand the material behaviour of Ti6Al4V, the research presented in this thesis has been grouped into three, 1. Heat treatment of Ti6Al4V 2. Fusion welding of Ti6Al4V and 3. Fusion welding of AM product. An extensive review has been done on the thermo-
mechanical processes of Ti6Al4V, which ranges from heat treatment to fusion welding. Annealing process results discussed in article 1 of chapter two and articles 1 and 2 of chapter three gave a proper understanding of the metallurgical and corrosion behaviour of heat-treated Ti6Al4V. It gave an insight into what is expected during fusion welding and the corrosion behaviour of Ti6Al4V after annealing. The microstructure of Ti6Al4V after annealing above the β transus temperature shows the lamellar structure and those annealed within the α +β phases; the equiaxed α/β phases is still retained. The resistance to corrosion of Ti6Al4V annealed around 950 ºC for one hour shows a good corrosion resistance and an enhanced mechanical property.
vi Gas Tungsten Arc Welding (GTAW) or Tungsten Inert Gas (TIG) welding, which is a fusion welding process, was also used to join Ti6Al4V alloy due to its economic and ease of use. The welds were characterized through the microstructure, tensile strength, and microhardness. Results show the microstructure of the welds having martensitic microstructure as a result of cooling above the critical cooling temperature rate of 410 ºC/s. The martensitic microstructure was also responsible for the high hardness within the weld zone compared to the base material. Furthermore, laser welding, a fusion welding process, was used in welding Ti6Al4V alloy due to its advantage of the small width of weld zone over the arc type fusion welding process such as TIG and Metal Inert Gas (MIG) welding. The laser-welded samples were characterized by evaluating the microhardness, tensile strength, and microstructure. Results show an improved mechanical property compared to the TIG welding, with improved ductility and tensile strength. The microstructure of the laser welds was characterized at the fusion zone (FZ) with columnar grains and acicular α’ microstructure as a result of the zone transforming from α + β → β → liquidus → β → α/α’, then cooling above the critical cooling rate. The heat affected zone (HAZ) were classified into two, near HAZ and far HAZ. The former is close to the FZ and comprises an acicular α’ microstructure resulting from the zone attaining a temperature close to the liquidus temperature. The latter is made of blocky α and some original β phase microstructure due to the lower temperature experienced in this zone. The laser welding process was found to be more suitable for welding Ti6Al4V than the TIG process. Additive manufacturing (AM), a non-traditional machining process, has greatly revolutionized the manufacturing industry by improving the way machine parts are produced and enabling the creation of intricate and complex machine component shapes. Laser metal deposition, a direct energy deposition technique that belongs to the AM group, is used to manufacture the AM blocks used in this research. Laser power of 400 W, powder feed rate of 2.4 g/min and hatch spacing of 0.9652 mm were used in manufacturing the AM block, with scanning rotated at 90º between successive layers. The AM block was sliced into thin sheets using wire EDM and joined using a laser welding system. The characterizations on the welds were done in terms of microstructure, microhardness, and tensile properties. The results of the AM welds were further compared with that of the bulk sheet Ti6Al4V welds made using the same laser welding system and same welding parameters. Results of the AM welds show a wider bead width compared to the bead width of the bulk sheets. The microstructure of the AM product after slicing shows columnar grains arranged in a layer-by-layer form as expected, with a needle-like acicular structure. After the AM products were welded using laser welding, the microstructure retains the columnar and acicular α’ microstructure within the FZ. The HAZ is made of α’ martensitic microstructure. The AM welds had a higher hardness than the bulk welds across the material due to the higher volume of martensitic microstructure in AM welds. The AM welds performed lower in terms of tensile strength when compared to the bulk sheet welds. The performance of the tensile strength of AM welds could be attributed to the stress concentration within the material after the LMD process. Therefore, stress-relieving heat treatment processes are recommended after AM build to improve AM welds' mechanical properties. Keywords: Additive manufacturing, Annealing, Corrosion, Fractography, Fusion welding, Heat treatment, Laser metal deposition, Laser power, Laser welding, Microhardness, Microstructure, Porosities, Ti6Al4V, TIG, Welding speed.