Mechanical Engineering ETDs

Publication Date

Spring 4-10-2017

Abstract

An adage within the Additive Manufacturing (AM) community is that “complexity is free”. Complicated geometric features that normally drive manufacturing cost and limit design options are not typically problematic in AM. While geometric complexity is usually viewed from the perspective of part design, this advantage of AM also opens up new options in rapid, efficient material property evaluation and qualification. This Thesis demonstrates how 100’s of miniature tensile bars can be produced and tested for comparable cost and in comparable time to a few conventional tensile bars. With this technique, it is possible to evaluate the stochastic nature of mechanical behavior and capture the statistical nature of mechanical properties. As a proof of concept, the technique is demonstrated on a precipitation hardened stainless steel alloy, commonly known as 17-4PH, produced by two commercial AM vendors using a laser powder bed fusion process, also commonly known as selective laser melting. Using two different commercial powder bed platforms, the vendors produced material that exhibited slightly lower strength and markedly lower ductility compared to wrought sheet. After demonstrating vendor to vendor variability, one vendor was chosen to produce 1000’s of tensile samples to explore within-build and between-build variability. Such a large dataset is seldom available in conventional materials evaluation and revealed rare defects that were only present in ~2% of the population. Worst-case failures were associated with residual porosity. To address the deleterious effect of porosity, the study examined a hot isostatic pressing process that collapsed a vast majority of the internal voids. Lastly, hardness testing which is an alternative high-throughput material evaluation technique was used to make a comparison between strength values obtained by tensile tests to those approximated by hardness testing. It is shown that hardness testing can be an appropriate technique for estimating the strength of wrought 17-4PH, but has a non-conservative error in strength estimations for AM 17-4PH.

Keywords

Additive Manufacturing, Mechanical Properties, Stochastic, 17-4PH, Porosity, 3D Printing

Degree Name

Mechanical Engineering

Level of Degree

Masters

Department Name

Mechanical Engineering

First Committee Member (Chair)

Shen, Yu-Lin

Second Committee Member

Tehrani, Mehran

Third Committee Member

Boyce, Brad

Document Type

Thesis

Language

English

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