Weldability and Corrosion of 7xxx Series Aluminum Alloys DISSERTATION Presented in Partial

277 Pages·2016·17.51 MB·English

by  Tyler Borchers

Weldability and Corrosion of 7xxx Series Aluminum Alloys

DISSERTATION

Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy
in the Graduate School of The Ohio State University

By
Tyler E. Borchers
Graduate Program in Welding Engineering

The Ohio State University
2016

Dissertation Committee:
Wei Zhang, Advisor
Antonio J. Ramirez
David H. Phillips
Copyrighted by
Tyler E. Borchers
2016
Abstract
Wide-spread implementation of 7xxx series aluminum alloys (AAs) in the mass
production of automotive vehicles can have a broad impact on vehicle structural light-
weighting and fuel efficiency improvement, while maintaining or improving structural
integrity. However, there are two significant technical barriers that limit the
implementation of fusion welded 7xxx series AAs, which are solidification cracking and
stress corrosion cracking (SCC). The present research addresses both technical barriers
through a combination of experimental testing, microstructure characterization, and
computational thermodynamics simulation. First, with respect to AA 7003 base metal
(BM) welded with AA 5356 filler metal (FM), cracking of the weld joint was exhibited
during testing in a saline fog, where fractography confirmed the cracking type as SCC.
The present research established a mechanistic understanding of the aggravated and
accelerated SCC that occurs in fusion welded 7xxx series AAs, when compared to un-
welded parent material. Second, a preliminary investigation into a novel solution for
solidification cracking was conducted for Cu-rich AA 7075 BM.
A novel discovery in the present research is the phenomenon of a fused-overlap zone
(FOZ) existing as a fusion zone layer that overlaps and fuses to the BM. FOZ may, and
typically does, become enriched with solute elements that are constituents of the BM
and/or FM. Moreover, large precipitates form in the solute-enriched FOZ. For instance,
for AA 7003 BM welded with AA 5356 FM, a transmission electron microscopy (TEM)
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investigation identified the FOZ enriched with Mg and Zn and the precipitates there as T
phase [(Al, Zn) Mg ]. The FOZ was found to be a prevalent phenomenon in AA
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weldments, occurring regardless of joint geometry, welding heat input, welding process,
or BM and FM investigated in the present research. The corrosion response of the FOZ
was dependent on BM and FM compositions, which was likely a result of different
precipitates formed in the FOZ that have various electrochemical properties (e.g., anodic,
cathodic, or neutral to the Al matrix).
It is found that the FOZ forms as a result of elemental vaporization during welding.
The vaporized elements deposit directly on the BM adjacent to the weld pool in the form
of a detritus (smut). Shielding gas flow is identified as a plausible driving force for the
detritus deposition. On the HAZ surface of AA 7003 BM welded with AA 5356 FM, x-
ray diffraction showed that Al Mg , a magnesium aluminide, is the most prominent
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component in the detritus. A thermodynamic calculation shows that the melting
temperature of magnesium aluminide (465 °C) is much lower than the HAZ temperature
immediately adjacent to the weld pool; this leads to a liquid film existing atop the HAZ,
which is rich in Mg. The convex weld pool subsequently wets over the BM, and mixes
with the adjacent liquid film. Rapid solidification of this region occurs, trapping the
solute elements and forming the solute-enriched FOZ. Subsequently, the precipitates
form in the FOZ, due to the micro-segregation under a non-equilibrium solidification
condition.
With respect to the SCC susceptibility of AA weldments, a band of the HAZ,
extending up to 1 mm from the fusion line, was markedly more susceptible to pitting than
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the remainder of the HAZ. Strain maps measured by digital image correlation (DIC)
indicated that geometric changes to SCC test specimens consequentially altered the stress
intensity factor (K) experienced by the susceptible HAZ region during testing.
I
Propensity for SCC was linked to K; particularly, if the induced K was greater than the
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HAZ’s critical stress intensity factor for SCC (K ), SCC failure was propagated. A
ISCC
high K could be introduced artificially by a sharp notch from electrical discharge
I
machining (EDM), or naturally by intergranular corrosion in the HAZ during testing;
both cases would result in SCC failures. Although K was found to be the most dominant
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variable for SCC, T phase precipitates in the FOZ were linked to accelerated failures.
Pitting of the T phase, anodic to the Al matrix, caused local solution acidification, and
consequentially accelerated intergranular corrosion crack growth into the HAZ. Because
of the accelerated natural development of K, SCC failures were also accelerated by the
I
presence of T phase precipitates in the FOZ. SCC resistance was promoted by the
complete removal of the FOZ, but SCC was not absolutely suppressed. Additionally, a
simulated paint bake thermal cycle was found to prolong the time-to-failure of tested
specimens, and was hypothesized to subtly increase the HAZ’s K , likely due to
ISCC
microstructural changes such as formation of intragranular precipitates (similar to aging).
Based on the testing results, a quantitative ranking of different factors’ contribution to
SCC in 7xxx AA joint is established.
Among the various options investigated, the most practical solution to SCC was
found through the development of precision additive dressing (PAD), where an inert FM
layer was precisely added to the location that needed corrosion resistance and protection.
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Cold metal transfer (CMT) or other low-heat input processes can be used for PAD.
Testing of PAD specimens, even in aggressive acid testing, showed remarkable corrosion
resistance of the region dressed with PAD.
Finally, preliminary attempts were made to mitigate solidification cracking by the
introduction of TiO nanoparticles into the weld pool, but experienced marked difficulty.
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However, when introduced, the nanoparticles significantly refined the grain size, which
was correlated with a reduction in solidification cracking susceptibility in other literature
studies. A significant decrease in weld penetration was also noted with the introduction of
TiO nanoparticles. The nanoparticles successfully dispersed throughout the entire weld
2
pool, however were more heterogeneously distributed after long weld times. Additional
work is necessary to characterize the effect of nanoparticles on solidification cracking.
In summary, the present research studied the mechanism of FOZ formation, the
catalytic effect of FOZ  phase precipitates on intergranular corrosion, the SCC
susceptibility of HAZ, and the grain refinement by addition of nanoparticles for
solidification cracking resistance. The novel discovery and investigation of the FOZ, and
its contribution to the development of SCC in welded AAs, is noted as a significant
addition of knowledge to the field of welding engineering. Taken as a whole, this work
significantly advanced the fundamental understanding of SCC and weldability of the
fusion weldments of high-strength 7xxx series AAs.

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Acknowledgments
The present research was generously supported and guided by Alan Seid and Patrick
Shafer of Honda R&D Americas, Inc. through Manufacturing and Materials Joining
Innovation Center (Ma2JIC), formerly known as CIMJSEA, a NSF industry/university
cooperative research center (I/UCRC).
I am indebted to my advisor, Dr. Wei Zhang, for his magnificent support, guidance,
and mentorship in the many aspects of life. I am equally grateful to my previous advisor,
Dr. Sudarsanam Suresh Babu, for the same, and for initiating this dissertation’s research.
The impeccable work conducted by, and fruitful deliberations had with, each of my
mentees, Andrea Peer, Brett Worrell, Owen Oesterling, and Joline Tran, is tremendously
appreciated. Additional thanks to Daniel Tung for his perpetual willingness to debate
anything anytime that I look to the left. Each professor in Welding Engineering deserves
acknowledgement for their eagerness to educate; our numerous conversations always
resulted in my added knowledge. Additionally, the corrosion perspectives that I gained
through conversations with Drs. G.S. Frankel and J.S. Locke of the Fontana Corrosion
Center were invaluable. Many thanks go to Don McAllister, Henk Colijn, and Cameron
Begg for their assistance in electron microscopy. I thank Ed Pfeifer and Ken Copley for
smooth laboratory operations; productivity is stunted without them. Special thanks to
Drs. Antonio Ramirez and David Phillips for participating in my dissertation committee.
Finally, I must acknowledge The Ohio State University for allowing me this opportunity.
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Dedication

To my mother and father
For without them, I would be nothing
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Vita
November 1989 ......................Born in Wapakoneta, OH
2004 - 2008 ............................Wapakoneta High School
2006 - 2008 ............................Post-Secondary at Wright State University, Lake Campus
2008 - 2012 ............................B.S. in Welding Engineering, The Ohio State University
2011 - 2015 ............................M.S. in Welding Engineering, The Ohio State University
2011 - present .........................Graduate Research Assistant, The Ohio State University
Publications
T.E. Borchers, W. Zhang: “Fused-Overlap Zone in Aluminum Arc Welds: Tendency of
Formation and Effect on Corrosion,” in Proceedings of Trends in Welding Research:
10th International Conference, Tokyo, 2016
T.E. Borchers, A. Seid, S.S. Babu, P. Shafer, W. Zhang: “Effect of Filler Metal and Post-
Weld Friction Stir Processing on Stress Corrosion Cracking Susceptibility of Al-Zn-
Mg Arc Welds,” Sci. Technol. Weld. Join., 2015, vol. 20, pp. 460-467
T.E. Borchers, D.P. McAllister, W. Zhang: “Macroscopic Segregation and Stress
Corrosion Cracking in 7xxx series Aluminum Alloy Arc Welds,” Metal. Mater.
Trans., 2015, vol. 46A, pp. 1827-1833
T.E. Borchers, A. Seid, W. Zhang, P. Shafer, S.S. Babu and D.H. Phillips: “Stress
Corrosion Cracking Susceptibility of Gas Metal Arc Welded 7xxx Series Aluminum
Alloys,” in Proceedings of Sheet Metal Welding Conference XVI, Detroit, 2014
Fields of Study
Major Field: Welding Engineering
Minor Field: Materials Science and Engineering
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Table of Contents

Abstract ............................................................................................................................... ii
Acknowledgments.............................................................................................................. vi
Dedication ......................................................................................................................... vii
Vita ................................................................................................................................... viii
Publications ...................................................................................................................... viii
Fields of Study ................................................................................................................. viii
Table of Contents ............................................................................................................... ix
List of Abbreviations ....................................................................................................... xvi
List of Tables ................................................................................................................. xviii
List of Figures .................................................................................................................. xix
Chapter 1 Motivation for Study ..................................................................................... 1
Chapter 2 Literature Review.......................................................................................... 5
2.1 Aluminum Alloys in the Automotive Industry .................................................... 5
2.2 Physical Metallurgy of Aluminum Alloys ........................................................... 7
2.2.1 Strengthening Mechanisms ........................................................................... 7
2.2.2 A Microstructural Comparison of AA 7003 and AA 5356......................... 17
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