Date of Award

2014-01-01

Degree Name

Master of Science

Department

Mechanical Engineering

Advisor(s)

Ryan B. Wicker

Abstract

The focus of this research was to fabricate parts composed of a nickel-based superalloy containing high levels of aluminum and titanium (NSAT) by using electron beam melting (EBM) additive manufacturing (AM) technology and utilizing thermal feedback from a multi-wavelength pyrometer to perform process improvements leading to near defect-free parts. EBM is an AM technology that utilizes metal powder to fabricate parts in layer-by-layer manner. A multi-wavelength pyrometer was implemented in an Arcam S12 (Arcam AB, Sweden) EBM system to observe and record surface temperatures throughout fabrication. Temperature data from the EBM system and the multi-wavelength pyrometer were graphed using MATLAB (The MathWorks, Inc., Natick, MA) to obtain temperature feedback leading to cooling rate determination with the ultimate goal of modifying EBM build parameters to fabricate defect-free NSAT parts. Metallography of fabricated parts was performed to detect defects such as cracks and porosity.

Nickel-based superalloy materials are of special interest to aerospace and nuclear industries due to their mechanical properties that allow such alloys to withstand severe operating conditions such as high temperature and pressure. Their best properties include creep resistance, high tensile strength, and oxidation resistance (Ekrami et al, 2006). However, nickel-based superalloy materials are susceptible to surface cracking during traditional fabrication processes such as casting due to thermal stresses during solidification (Wang et al, 2011). Processing of such alloys by utilizing an advanced manufacturing method such as AM powder bed fusion technology can further help control processing, reduce time-to-market, and reduce fabrication costs. NSAT alloys are challenging to process in powder bed fusion systems due to their propensity to crack during solidification or immediately after solidification. By minimizing thermal gradients throughout the part (e.g. by using powder bed fusion that preheats powder before melting), it was hypothesized that thermal stresses would be decreased which would lead to fabrication of parts containing reduced cracking. It was the ultimate goal of this research to determine if EBM could fabricate defect-free NSAT parts.

A main objective of this research was to fabricate parts from NSAT powder precursor using the EBM process. To achieve dense parts and continuously improve the system parameters in an effort to reduce cracking, a multi-wavelength pyrometer was externally installed atop an Arcam S12 EBM system. The multi-wavelength pyrometer is a non-contact device capable of measuring the temperature of an object without the need of knowing emissivity, or the object's surface ability to emit radiant energy (Felice, 2002). To automatically determine an emissivity, the instrument measured the target object's wavelengths to calculate a temperature whose radiance curve is compared to an ideal Planck curve. If the curve matches, the target is said to be an ideal blackbody (emissivity=1), otherwise the radiance corresponds to a non-blackbody and emissivity is automatically calculated to match a Planck curve. Since the Arcam S12 system emits X-rays, the installation of the multi-wavelength pyrometer included a radiation shield to safely protect the operator from radiation exposure. The instrument allowed the characterization of the EBM process that consisted of various steps during fabrication (e.g., heating of the start plate, raking, powder distribution, and melting).

The EBM system parameter modifications examined included beam speed, beam current, beam focus, line order, line offset, number of repetitions, and the preheat cycle (all of which affect thermal behavior during fabrication). Systematic modification of these system parameters was explored to identify parameters that significantly reduce the amount of cracking observed in fabricated parts when compared to the initial parameters. Two major modifications were found that produced a significant reduction in cracking which included utilizing unidirectional scanning and performing multiple melt cycles. It was theorized that unidirectional scanning reduces the thermal gradient between hatch lines (due to the scanning pattern that inherently reduces localized heating) and that the multiple melt cycles re-melt the forming surface which has shown to reduce defects such as porosity (Medina, 2013). The work presented here showed the strategy for developing system parameters used, the improved parameters developed for reduction in cracking, as well as possible recommendations for future work.

Language

en

Provenance

Received from ProQuest

File Size

179 pages

File Format

application/pdf

Rights Holder

Jonathan Minjares

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