Abstract
Laser metal deposition (LMD) is a manufacturing process, which can be used to manufacture a complete, fully functional part – by building it up layer-by-layer using the data from a Computer-Aided-Design (CAD) file. The layer-by-layer addition can also be used to rebuild worn-out sections of existing parts, as well as to deposit protective coatings to protect parts in surface engineering. The process involves laser heating a substrate, on which a metal powder is deposited. The powder solidifies, when mixed with the substrate, thereby creating a metallurgical bond. In order to produce parts with high geometrical tolerances and desirable material properties, the process parameters have to be carefully controlled. Since the LMD process requires the interaction of parameters, it is not always easy to predict the output geometry. In this dissertation, the laser-metal-deposition process was modelled in ANSYS Parametric-Design-Language (APDL), using a transient thermal analysis, in order to determine the geometrical properties of the clad, that is to say, the width and the height of the clad. The simulated results were then compared experimentally by depositing Commercially Pure (CP) titanium powder onto a Ti-6Al-4V substrate, in order to verify the simulation. The varying parameter in the experimental process was the powder-flow rate, which varied between 0.5-2.5g/min. In addition, to the geometrical properties, the microstructure, microhardness; and the porosity levels of the deposited clads were also analyzed, in order to better determine the clad quality and integrity. The model showed good agreement in predicting both the height and the width of the clads. Porosity was noticed in all the samples – with the exception of the clad deposited at the lowest powder-flow rate setting of 0.5 g/min. An increase in the powder-flow rate also led to a smaller fusion zone, due to a lower laser-material interaction period, which was the result of the increase in the quantity of powder causing attenuation of the beam, and less laser power being absorbed by the substrate. The smaller fusion zone meant that the clads could not bond to the substrate properly, which led to the clad in the sample with the highest powder-flow rate falling off the substrate. There was a significant increase in the microhardness of the clad zone, which was due to a combination of alloying with Ti-6Al-4V and a change in the microstructure to an acicular alpha martensite microstructure; while the Heat-Affected-Zone (HAZ) in the substrate only showed a slight increase in microhardness.
M.Ing. (Mechanical Engineering Science)