Date on Master's Thesis/Doctoral Dissertation

8-2023

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Industrial Engineering

Degree Program

Industrial Engineering, PhD

Committee Chair

Chou, Kevin

Committee Co-Chair (if applicable)

Starr, Thomas

Committee Member

Starr, Thomas

Committee Member

Yang, Li

Committee Member

Gerber, Erin

Author's Keywords

Powder bed fusion; porosity; transient length; ti-6Al-4V; surface roughness

Abstract

Laser powder bed fusion (L-PBF) process represents a form of metal additive manufacturing (AM) where micron-level powdered material is selectively melted and fused layer by layer to create intricate three-dimensional parts. This process involves rapid melting and solidification, leading to intense thermocapillary convection within the molten pool. The melt pool is a crucial element of the L-PBF process and refers to the localized region where the powder particles are melted and solidified to form each layer of the printed part. The shape and dimensions of the melt pool directly influence the accuracy and surface finish of the printed part. Precise control of the melt pool geometry is essential for achieving the desired dimensions and avoiding defects in the final part. Optimizing process parameters and achieving high-quality printed parts require a deep understanding of the dynamics governing the melt pool. In this regard, both experimental and simulation methods were employed to study the melt pool geometry and its variations, considering various parameter combinations and different length scales. The printed parts were also examined for defects like porosity and to analyze their surface characteristics. The study started with an initial implementation of a simplified three-dimensional model of a powder bed using ANSYS Fluent. The simulation setup was based on a custom user defined function that integrated a volumetric heat source, temperature-dependent material properties, and volume of fluid method for identifying the free surface. The simulation setup was then employed to investigate the impact of varying powder size distributions on the formation of a melt pool. The results show that the size and distribution of particles in the powder mixture play a crucial role in shaping the evolution and geometry of the molten pool. Smaller particles encourage a consistent and uninterrupted flow within the molten pool. However, the presence of voids promotes fluid convection in the downward direction, leading to a temporary increase in the depth of the molten pool. This finding highlights the importance of understanding the role of particle size and distribution in shaping the characteristics of the melt pool during the L-PBF process. The evolution of the melt pool in the L-PBF process is closely related to pore formation in the final printed parts. Porosity refers to the presence of voids or empty spaces within the printed parts during the AM processes. Several factors related to melt pool dynamics, such as insufficient energy input, balling phenomenon, insufficient overlapping, gas entrapment, and overheating can contribute to pore formation. When the energy density is high, there is a possibility of forming keyhole pores. This occurs when excessive energy input from the laser causes deep penetration of the molten material into the powder bed, resulting in an elongated shape cavity resembling a keyhole shape. Multiple tensile coupons were printed with various parameters to understand pore morphology before and after fracture. A non-destructive technique of micro-CT scan method was utilized to analyze the porosity. The findings indicate that the energy density and build orientation significantly influence the porosity of the as-built printed samples, while the impact of the location change is observed to be minimal. After undergoing tensile testing, the samples exhibit a notable increase of more than nine percent in both pore volume and porosity percentage compared to their initial as-built counterparts. These results emphasize the importance of carefully controlling energy input and optimizing build orientation to mitigate porosity and enhance the quality of L-PBF printed parts. During the L-PBF process, a laser with a spot size ranging in the hundreds of microns interacts with metal powder to form tracks. The laser follows predefined scan paths in each layer, and the behavior of the melt pool during each scan is heavily influenced by the laser power and scan speed. The melting process can occur in three modes: incomplete melting, conduction mode melting, and keyhole mode melting, depending on the combination of these two parameters. Only conduction and keyhole mode melting result in the formation of continuous and complete tracks. The length of the scan determines whether the scanned track reaches a quasi-steady state or not. Regardless of the laser power and scan speed values used, a transient region exists at the start and end of long scan vectors. The melt pool geometry in this transient region displays distinct characteristics compared to the quasi-steady region in the middle. The physics of the melt pool dynamics in these transient states remain largely unexplored. Understanding the characteristics of the melt pool in the transient region is essential to ensure the quality of smaller-dimensional parts in the L-PBF process. Improving our knowledge in this area can lead to better control over the printing process and enhanced quality of the final printed components. The research involved conducting experiments on one-dimensional line scans, referred to as single tracks, using an extensive design of experiment (DOE) approach for process parameters and multiple replicates with several builds. The surface data from these as-built tracks were collected using a non-contact optical profilometer, the WYKO NT1100. The analysis of the single tracks revealed that the track width and surface height varied along the scan line. Particularly, the track width at both the start and end of the track demonstrated distinct characteristics with significant fluctuations. The region in the middle, referred to as quasi-steady region, showed uniform width with relatively low variation. The transient and quasi-steady regions were quantified based on the track width variations along the scan line. Additionally, the length of the transient region was not constant for all the parameters but varied with power and scan speed settings. The analysis results show that the transient length at the start ranged from 300 microns to 1400 microns, for the power and scan speed values used in the experiment. Following the analysis of results from the single track experiment, the research progressed to fabricating two-dimensional raster-area scans. The findings of the single track experiment were integrated into the experimental setup of the raster scans to refine the DOE. The two-dimensional prints incorporated extra process parameters like hatch spacing and the number of scan lines. The results show that the parameters that resulted in larger transient regions for raster scans also resulted in higher surface roughness. The shorter scan lengths which only consisted of transient zones resulted in higher surface roughness and the increasing scan length reduced the average roughness. In addition to examining the surface characteristics of the transient region, another set of experiments was conducted to analyze the surface roughness of raster scans in the quasi-steady regions. This experiment involved assessing the impact of process parameters, including laser power, scanning speed, and hatch spacing, on the surface characteristics of single-layer raster scan areas through the design of experiment and multiple replicates. The result revealed that raster scan areas with lower laser power, higher scanning speed, and higher hatch spacing have higher surface roughness. Moreover, among the three main parameters, laser power played the most significant role in determining the surface roughness value.

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