Date of Award

2021-12-01

Degree Name

Master of Science

Department

Mechanical Engineering

Advisor(s)

Francisco Medina

Abstract

Electron Beam Powder Bed Fusion (EB-PBF) is an additive manufacturing process that allows for the fabrication of nearly fully dense Alloy 625 parts. In this study EB-PBF Alloy 625 samples were fabricated in an Arcam A2X, heat-treated, then machined and tensile tested. Preliminary process tests varying focus offset, and speed function were conducted to establish baseline process parameter conditions to fabricate Alloy 625 parts based on surface finish and structural defects. The microstructure observed in EB-PBF Alloy 625 parts along the build direction in the as-built state consists of irregularly sized columnar grains averaging 105 μm in width that have formed parallel to the build direction and contain γ’’ Ni3Nb columnar precipitates.

Mechanical properties and microstructures of as-built and heat-treated tensile coupons printed vertically and horizontally were compared. A higher yield strength of 0.387 GPa was observed on as-built horizontally printed parts compared to the vertically printed parts, which produced a yield strength of 0.365 GPa. The presence of anisotropic mechanical properties of parts in the as-built state is attributed to the process-induced columnar microstructure and fusion between layers.

The proposed heat treatment resulted in parts with a relative skeletal density of 99.99%, and a major reduction in internal porosity of approximately 90% %, from 0.52 % to 0.05%, when compared to the as-built parts. Moreover, the heat treatment dissolved the γ’’ columnar precipitates and recrystallized the columnar grains into an equiaxed NiCr grain structure with annealing twins in both horizontal and vertical planes. This led to an increase in percent elongation by ~20%, resulting in 69% elongation at fracture. The optimization of process parameters in combination with the proposed heat treatment allowed for defect reduction and microstructure uniformity in EB-PBF Alloy 625 printed parts, which are of value for end-use application.

Language

en

Provenance

Recieved from ProQuest

File Size

116 p.

File Format

application/pdf

Rights Holder

Aldo Rubio Hidalgo

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