UC San Diego

In recent times, research efforts have been focused on Additive Manufacturing (AM) technologies, be it high or low energy-based, due to the ability to produce very complex and near-net shaped components. The overall aim of any AM technique is to produce parts that have industrial applicability. However, additively manufactured parts exhibit low service life as a result of part failure during use. These part failures usually arise due to the presence of defects such as excessive porosity, part distortion and property anisotropy. In order to meet the aspirations of using parts manufactured via AM without the fear of catastrophic failure, there is therefore the need for research efforts to be geared towards developing novel approaches to mitigating and eliminating unintentional defects in additively manufactured parts. This body of research work aims to improve on the current understanding of the defect mitigation strategies in additively manufactured parts by contributing towards accelerating new discoveries of innovative approaches in eliminating detrimental defects in high and low energy AM processes. We explored the Binder/Solvent Jetting and the Selective Laser Melting AM technologies when considering low-energy and high-energy based AM processes respectively, with a focus on mitigating excessive porosity and part distortion defects. Our approach to the research work was fundamental in nature, involving both experimental and analytical considerations from both a macroscopic and microscopic point of view and has taken the form of understanding the influence of process parameters on defect formation. We developed novel and innovative approaches to optimizing the AM processes for the production of defects free parts, and further systematically developed empirical relations between process parameters and output variables such as final part density and degree of distortion to help control defect formation from experimental observations and analytical considerations. For example, An empirical equation describing the distortion strain as a function of powder spreading parameters during binder jetting AM is suggested as an approximation of the numerical modeling results, while the results on selective laser melting of metal-matrix-composites show that the filling of the pores between rigid inclusions by the molten matrix is dependent on the laser dwell time, which in turn, depends on the volume fraction of ceramic reinforcement, initial pore sizes between inclusions and materials properties of the matrix phase such as viscosity, surface energy and initial pore sizes between rigid inclusions. Our developed process parameters were used with great success in manufacturing defects free AISI 316L austenitic stainless steel (SS316L) alloy, (SS316L)-WC and functionally graded (SS316L)-WC metal-matrix-composites. Furthermore, a comparative study was conducted to investigate the influence of the various AM processing route on the process-structure-property relationship of processed parts, with results indicting that due to the differences in the kinetics of the various processing routes investigated, different microstructures and hence mechanical properties can be realized.