Metal additive manufacturing (AM) is a disruptive technology, enabling fabrication of complex and near net shaped parts by adding material in a layer-wise fashion. It offers reduced lead production time, decreased buy-to-fly ratio, and repair and remanufacturing of high value components. AM processes are finding applications in many industrial sectors such as aerospace, automotive, biomedical and mold tooling. However, beside tremendous advantages of AM, there are still some challenges that prevent the adoption of this technology into high standard applications. Anisotropy and inhomogeneity in mechanical properties of the as-built parts and existence of pores and lack-of-fusion defects are considered as the main issues in directed energy deposition (DED) parts. Process planning and the utilization of methods that can increase the flexibility of design of DED parts with overhang sections is also of great importance. A robotized laser powder and/or wire directed energy deposition system has been developed at Research Center for Advanced Manufacturing (RCAM) at Southern Methodist University (SMU) in order to address the mentioned issue and eventually to make the robotized DED process more practical for abroad range of industrial applications.
The mechanical and microstructural properties of 316LSi parts were studied. In this regard, two types of coupons, thin-walled and block, of short and long inter-layer time intervals were considered. It was found that different thermal histories caused by different inter-layer time intervals have significant impact on mechanical and microstructural properties. The thin-walled samples with lower cooling rates showed coarser columnar grains, lower ultimate tensile strength, and lower hardness compared to the block samples. The melt pool was monitored in real-time. An empirical correlation between the melt pool area and cooling rate was achieved that could enable control of scale of the final solidification structure by maintaining the melt pool size in real-time. Further, to study the anisotropic behavior, tensile samples were loaded in parallel and perpendicular directions with respect to the deposition direction. The results indicated that samples in the perpendicular direction had lower UTS and elongation for both coupon types, revealing a weaker bonding at inter-layer/bead interface due to the existence of lack-of-fusion pores.
As mentioned earlier, the robotized laser wire directed energy deposition (RLW-DED) has limitations in printing certain complex shape parts. Fabricating parts with overhang sections, depending on the geometry, might cause a collision between the laser head and the buildup. Part segmentation and joining the elements back together has been presented to overcome those limitation. In this study, the welding of additively-manufactured parts by RLW-DED has been proposed. Autogenous laser welding, performed at the same setup used for RLW-DED, was utilized to join the thin-walled 316LSi DED parts. Mechanical and microstructural testing were then performed on the welded samples. The results showed that the mechanical properties of welded DED parts are comparable with those of DED parts. Furthermore, a component of complex shape was fabricated to show the capability of the developed process. Therefore, the welding of RLW-DED parts can expand the application of 3D-printed parts in industry.
Robotized laser powder directed energy deposition is a non-linear process, and the dynamic response of the system varies layer by layer. An adaptable PI-controller with layer-dependent control gains was developed to ensure a constant melt pool width through the entire build. The laser power was selected as the control output variable, and the melt pool width was chosen as the control input variable. The performance of the controller was evaluated through deposition of thin wall samples. The results showed that the controller, by adjusting the laser power in real time, could successfully maintain the melt pool width and produce a more uniform and finer microstructure as compared to the sample with a constant laser power.
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Akbari, Meysam, "Development of A Robotized Laser Directed Energy Deposition System and Process Challenges" (2019). Mechanical Engineering Research Theses and Dissertations. 21.