Masters Theses

Date of Award

8-2019

Degree Type

Thesis

Degree Name

Master of Science

Major

Materials Science and Engineering

Major Professor

Sudarsanam Suresh Babu

Committee Members

Ryan Dehoff, Hahn Choo, Yanfei Gao

Abstract

Single-crystal nickel superalloy components are widely used in high temperature aerospace applications due to their excellent high temperature strength and creep resistance. Mechanical properties of single-crystal nickel superalloys are affected by chemical composition and the method of manufacture. Production of these components is costly and repair offers opportunity to sustain system performance while reducing costs. Additive Manufacturing by Laser Powder Feed Directed Energy Deposition (DED) is a promising method for repair due to selective application of material and the ability to tailor heat input. However, laser processing of these materials presents several challenges due to the high potential for cracking and recrystallization. Limiting stray grain formation and cracking within repairs is critical to product performance. The evolution of single-crystal nickel superalloy DED microstructures is investigated to assess the influence of DED on microstructural features including stray grain formation and dendrite growth directions. A systematic assessment on the influence of mass flux, travel speed, and laser power on deposit microstructure was completed using design of experiments and thermo-fluid process modeling. Two test cases were considered including single-track and multi-track deposits. The influence of varying substrate thickness was considered. Modeling was used to understand the influence of varying mass flux, laser power, travel speed, and deposit geometry on crystallographic texture. Model calibration was performed using experimental measurements. The influence of varying processing parameters on deposit texture evolution for multiple deposit geometries are presented. Epitaxial growth was observed for multiple deposit geometries. The experimental results suggest that process parameter optimization offers a means to produce highly textured microstructures. Managing equiaxed grain formation is critical to obtaining epitaxial growth which agrees with prior work. The ranges of laser power, travel speed, and mass flux considered in this study can significantly influence deposit texture evolution and appear to be key process parameters. Reducing the thickness of the cap of equiaxed material tends to promote highly textured microstructures with alignment of the solidification structure to [100]. The thickness of the equiaxed cap decreases with reducing mass flux and heat input. The results suggest a variation in the number of nucleation sites for solidification within the deposit.

Download
Files over 3MB may be slow to open. For best results, right-click and select "save as..."

Share

COinS