Laser Powder Bed Fusion Process, Structure, and Properties: Holistic Approach to Establishing Metallurgical Quality
Abstract
The advent of metal additive manufacturing (AM) was posed as a disruption to casting, forging, machining, and forming with the notion “complexity is free”. However, since invention in the late 1990’s the marketed potential has not been realized. Metal based AM is best viewed from the process-structure-properties-performance (PSPP) paradigm taught in material science and engineering, which links the process history to the part performance. Understanding the complex and localized process control made available by AM creates a significant challenge in defining the materials structure, properties, and performance. The lack of holistic understating of inputs and corresponding results has been identified as a major roadblock limiting the promised impact of metal AM. Metal laser powder bed fusion is the most widely adopted form of metal 3D printing and has produced parts certified for use in space, aerospace, and biomedical applications. These success stories prove the technology is capable of high-quality material creation. The focus for AM now shifts to reproducible quality and predictable properties. America Makes partnered with ANSI to create a standards road map, with 93 gaps identified in 2018. A major theme for metal AM was to develop knowledge to fill gaps in the understanding process parameters, machine variables, geometry, and properties. Through extensive literature review the Qualification Test Artifact was developed to quantitatively address knowledge gaps. The defining characteristic of the artifact is its singular body enabling a holistic evaluation of processing condition against the resulting structure/properties across relevant geometric features. The complexity of the artifact created a need for a complete data management solution leading to the development and funding of the Global Test Artifact Data Exchange Program (GTADExP). GTADExP is designed to bring an end to qualitative assertions in the LPBF community through pedigree data driven and holistic analysis of process to properties. Analysis of artifacts from different machines uncovered variability that could not be explained based on the known input parameters. Investigation of microsecond controls of the LPBF process, which are typically hidden from the user, revealed macroscopic impacts to include porosity, spatter, microstructure, and mechanical properties. Utilizing an AconityMIDI+ open architecture LPBF system power ramp rates from <10µs to>400µs were evaluated with plate melting tests. A method to determine optimal ramp rate was developed, and two qualification test artifacts (QTA) were built from IN718 with and without power ramping. Holistic evaluation of the artifacts revealed laser power ramping prevents one form of back spatter but only reduced total spatter by 5%. Laser power ramping also increased near surface porosity which led to reduced elongation to failure in uniaxial tensile tests. The impacts of microsecond control were apparent, and this work highlights the need for transparency in LPBF process control if broadly accepted material property data is to be developed. Ultimately, the combination of applying the PSPP lens to LPBF, development of a holistic test artifact, and the deep-dive into the microsecond impacts provide the breadth and depth necessary to generate strong conclusions on why LPBF struggles with reproducible and reliable production. Industry has adopted a proprietary approach to process parameter development which is directly tied to material properties. This cascaded into every aspect of the process from academic research, small business adoption, and even industry standards. A review of traditional manufacturing standards reveals no similarities in the make/model/serial number material property development. Utilizing the knowledge ascertained from this work, a path to repeatability and reliability is proposed. Finally, the only discernible path forward is build file transparency, along with shifting of LPBF OEM’s and user value away from parameter development.
Subject Area
Materials science|Engineering
Recommended Citation
Taylor, Hunter Cole, "Laser Powder Bed Fusion Process, Structure, and Properties: Holistic Approach to Establishing Metallurgical Quality" (2022). ETD Collection for University of Texas, El Paso. AAI30000346.
https://scholarworks.utep.edu/dissertations/AAI30000346