Abstract

Laser Powder Bed Fusion (LPBF) is an Additive Manufacturing (AM) method whereby complex parts can be built to near net geometry in an automated environment. Parts formed by LPBF have excellent properties and require little post-processing. While LPBF allows for creative designs to reduce weight and part count by integrating sub-components of assemblies, designers are limited to one material within any LPBF print. Although various materials can be used in LPBF, metals are of special interest to many industrial customers due to their high strength and toughness. The metals commonly used for LPBF include iron, aluminum, and titanium alloys. These alloys are generally designed to fulfill specific needs and even small variations in composition are detrimental to the functionality. However, some alloys are intentionally modified to achieve specific results. For example, Yttria is dispersed in stainless steel to enhance its resistance to radiation damage. The sensitivity of metal alloys to small changes in composition can be exploited to change the properties of LPBF material within a single print. An en-situ doping technique, which is under development, allows for the introduction of small quantities of liquid-suspended additives to any part of the powder bed. The liquid is then evaporated, and these additives integrate with the solid material upon laser fusion to change the properties of the base material. In this thesis, steel-insoluble (zirconia) and soluble (carbon) dopants are introduced into multi-layer parts formed by LPBF. Zirconia significantly increased the porosity of the steel with continuous pores which disrupt the columnar grain structure. The majority of the added zirconia segregated to the outer surface and porous surfaces within the bulk. Although hardness did not increase as expected, the porosity can aid in osseointegration when used for implants, or as a reduced-conductivity thermal barrier in heat sensitive applications. Carbon-doped samples, on the other hand, had nearly 30% increased hardness and more homogeneous microstructure than unmodified material. Hardened surfaces may be a valuable tool for designers who require wear resistance. Although porosity increased from ~0% to over 10% in the worst case, modified parameters resulted in only 1% porosity. The data indicate that changing the processing conditions affects porosity, so the amount of porosity could be adjusted. Finally, carbon was shown to create preferential etching which enables easy removal of support structures. Supports doped with carbon to promote sensitization and etched in an electrolyte bath either broke free without tension, or using no more than 20% of the force required to remove unmodified supports. This is a valuable step for reducing the post-processing required of many LPBF designs.

Degree

College and Department

Ira A. Fulton College of Engineering; Mechanical Engineering

Rights

https://lib.byu.edu/about/copyright/