Masters Theses

Abstract

"A lack of understanding of corrosion fatigue in friction stir welded aluminum joints prevents friction stir welding from being implemented in aerospace applications. Fatigue testing reveals a 60-75% reduction in the fatigue life of friction stir welded aluminum lap joints immersed in 3.5% NaCl solution (corrosion fatigue) compared with that of lap joints tested in ambient air. The loss in fatigue life is attributed to accelerated fatigue cracking due to hydrogen environment embrittlement. Two polymer sealant candidates are investigated: silicone rubber and nylon-11. Both sealant candidates can be applied prior to welding and seal the faying surface gaps in lap joints upon welding. The rubber sealant cures at room temperature after welding and can be welded with the same parameters as without the sealant. The 50% sample population corrosion fatigue life is increased by 22% with the use of the rubber sealant, but the effectiveness of the rubber sealant is limited by its cohesive mechanical properties, e.g. elongation to failure. In ambient fatigue, the nylon sealed welds exhibit twice the 50% sample population fatigue life of other welds. Finite element modeling predicts a reduction in the stresses in the weld due the stiffness contribution of the nylon sealant. The effectiveness of the nylon sealant is limited by its adhesive bond strength. When immersed in water, as in corrosion fatigue, the adhesive bond strength is reduced, the sealant bond fails within 500 fatigue cycles, and the mechanical benefits of the nylon sealant are negated. The corrosion fatigue life of nylon sealed welds is 26% less than that of welds without sealant because of the more severe hook defect associated with hotter welding conditions required to melt the nylon. Finite element modeling results indicate an increase in stress intensity factors of about 10% in welds with more severe hook defects--Abstract, page iii.

Advisor(s)

Van Aken, David C.

Committee Member(s)

Schlesinger, Mark E.
OKeefe, Matt
Chandrashekhara, K.

Department(s)

Materials Science and Engineering

Degree Name

M.S. in Metallurgical Engineering

Sponsor(s)

Air Force Research Laboratory (Wright-Patterson Air Force Base, Ohio)
Boeing Company
Missouri University of Science and Technology. Center for Aerospace Manufacturing Technologies

Publisher

Missouri University of Science and Technology

Publication Date

Spring 2009

Pagination

xiv, 128 pages

Rights

© 2009 Kenneth Thomas Nathaniel Doering, All rights reserved.

Document Type

Thesis - Open Access

File Type

text

Language

English

Subject Headings

Aluminum -- CorrosionFinite element methodFriction stir weldingWelded joints -- Fatigue

Thesis Number

T 9482

Print OCLC #

436086276

Electronic OCLC #

318114818

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