| dc.description.abstract | Despite the tremendous growth in adoption and research interest in additive manufacturing, there remain several unsolved issues regarding the properties and defects associated with the process. In selective laser melting, variability and poor repeatability has been known to be a critical defect that requires costly and time consuming postprocessing to correct. Despite this fact, there has been very little research into this topic. Variability issues are especially critical with regards to NiTi alloys, which are especially sensitive to changes in microstructure and composition.
In the current work we investigate various aspects of variability in the transformation temperatures under stress free conditions as well as the mechanical properties of NiTi alloys manufactured using selective laser melting. The main objectives were to observe and quantify variability in the transformation temperatures and mechanical properties, to investigate the relationship between process parameters and variability, and to investigate the microstructural and compositional mechanisms that control the variability.
Experimental evidence was provided that verified the presence of localized variability on the micron scale as well as poor repeatability in NiTi parts. The variability in transformation temperatures and mechanical properties was found to exceed the values found in conventionally processed alloys found in the literature. Out of the various process parameters investigated such as laser power and speed, the hatch distance was found the be the most effective parameter for controlling the variability. Use of a narrow hatch distance of 35μm resulted in parts with highly homogenous and repeatable transformations, while larger hatch distances of 120μm resulted in parts with inhomogenous and poorly-repeatable transformations. It was also found that the critical hatch distance between variability and repeatability was between 50-60μm.
The mechanisms responsible for variability in the transformation temperatures were found to be precipitate volume fraction and interparticle distance as well as dislocation density. In parts with high variability it was observed that local differences in precipitate volume fraction, size, and area fraction occurred, as well as differences in dislocation density.
Finally, it was shown using uniaxial tensile testing that specimens that had exhibited high variability in transformation temperatures also had higher variability in elastic modulus and yield strength. | en |
