Tailoring Mechanical Properties and Porosity in Laser Powder Bed Fusion by Spatial Manipulation of Feedstock Composition

Additive manufacturing (AM) is capable of forming complex geometries, can build custom products, and offers greater versatility than many conventional manufacturing processes. Metal AM has revolutionized the design and production of rocket engines, enabled lightweight robotics and drones, and created heat exchangers that improve the efficiency of energy conversion systems. Typically, the feedstock for metal AM is a single alloy powder, metal powder, or a stochastic mixture of alloys. The design space of AM can be expanded by techniques for local manipulation of the powder feedstock, thereby enabling the tailoring of composition or microstructure gradients to improve mechanical or thermal properties. Current methods for local manipulation of the feedstock incorporate hopper- or vacuum-based methods with spatial deposition resolution on the order of 500 𝜇m and typically require a secondary recoating step after depositing the material, further limiting the resolution and fidelity of property. In this thesis, a new hybrid AM technique combining inkjet deposition followed by laser powder bed fusion (LPBF) is studied, and the technique is demonstrated in exemplary contexts. To begin, a rapid experimental workflow is developed, whereby etched metal substrates are used as templates for spreading a thin layer of metal powder; spreading is preceded by inkjet deposition onto the wells and is followed by laser scanning to form a solid material with tailored characteristics. This workflow facilitates an understanding of the influence of additive concentration, laser power, and scan speed on the process. Using this workflow, the thesis: (1) assesses the resolution (∼ 200 𝜇m) and limitations of the hybrid inkjet-LPBF process; (2) demonstrates the fabrication of porous stainless steel using a polymer-based ink as the deposited additive, achieving spatially controlled porosity upwards of 8% locally with pore sizes of 250-800 𝜇m2 and a spatial resolution of ∼ 400 𝜇m; (3) demonstrates spatial tailoring of the hardness of stainless steel using an ink with carbon black as the additive, achieving a local increase in the microhardness from a control of 178 HV to 196.6 HV with a spatial resolution of 250 𝜇m. In the case of porosity control, a scaling model of thermal effects, combined with an understanding of the melt pool’s flow and stability, rationalizes the experimental outcomes and the process parameter space. This thesis validates the multi-material inkjet-laser process for high-resolution, spatially graded metal AM. These findings lay the foundation for a hybrid inkjet-LPBF fusion system to develop high-precision components and its use for rapid alloy development in metal AM.

Name thesis:degree_name

Doctoral

Department dc:contributor.department

Massachusetts Institute of Technology. Department of Mechanical Engineering

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2023

Author dc:creator

• Lettiere, Bethany Rose

Advisor dc:contributor.advisor

• Hart, Anastasios John

Statement dc:rights

• In Copyright - Educational Use Permitted
• Copyright MIT

Licence dc:rights.uri

• http://rightsstatements.org/page/InC-EDU/1.0/

Handle dc:identifier.uri

https://hdl.handle.net/1721.1/150128

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/150128