The document summarizes research on coating magnesium alloys with plasma electrolytic oxidation (PEO) and joining them via friction stir spot welding (FSSW) for automotive applications. The objectives were to synthesize and characterize PEO coatings on monolithic and FSSW magnesium alloys and evaluate their corrosion performance. Testing showed that optimized PEO coatings reduced the corrosion rate of monolithic samples by 16 times and FSSW samples by over 5 times compared to the uncoated magnesium alloy. Corrosion occurred preferentially at weld discontinuities.

1. TEMPLATE DESIGN © 2008
www.PosterPresentations.com
Plasma Electrolytic Oxidation Coated, Friction Stir Spot-Welded
AZ31 Magnesium Alloy for Automotive Applications
Sarah Busef, Steven J. Thorpe
Department of Materials Science and Engineering
1.0 Background and Introduction
Lightweight materials benefit the automotive industry by
enhancing fuel economy and vehicle performance.
Magnesium alloys are strong candidates for
applications in this industry due to their low density and
high strength to weight ratio. However, poor corrosion
resistance and joining properties limit its current uses.
Friction stir spot welding (FSSW) uses frictional heat to
join magnesium alloys without porosity, induced stress,
or hot cracking caused by other joining methods [1]. It
results in the formation of 3 microstructurally unique
zones with different susceptibilities to corrosion.
Plasma electrolytic oxidation (PEO), also known as
micro arc oxidation, is a method of enhancing corrosion
resistance of metallic substrates by applying a sintered
oxide layer under an applied electric field. This results in
the formation of a
3-layered oxide coating
whose thickness,
compactness and
stability influence
the alloy’s corrosion
behaviour [2].
2.0 Objectives and Purpose
3.0 Materials and Methods
5.0 Conclusions and Future WorkThe objectives of this research are to:
1. Synthesize PEO coatings on monolithic and FSSW
AZ31 magnesium alloys
2. Characterize the microstructure of the coating and
optimize processing parameters
3. Identify the corrosion performance of the coating
d)
6.0 References and Acknowledgements
[1] Savguira, Y., Liu, W. H., Miklas, D., North T., and Thorpe, S.,
2014, Corrosion, Vol. 70, pp. 858-866.
[2] Arrabal, R., Matykina, E., Hashimoto, T., Skeldon, P.,
Thompson, G., 2009, Surface and Coatings Technology, Vol. 203,
pp. 2207-2220.
I would like to thank my supervisor Professor Steven J. Thorpe, my
mentor Yuri Savguira, the members of the SEE lab, and Sal Boccia
for their efforts in making my project a success.
Uncoated samples exhibited general, severe
attack. PEO coatings formed in less optimal
conditions provided some protection but
coating degradation was eminent. Optimized
PEO coatings suffered from minimal,
localized attack, or pitting. Welds corroded
preferentially at geometrical discontinuities,
on the shoulder and in the center of the back
face opposite the keyhole. This weld zone-
dependent corrosion behaviour indicated
potential non-uniform PEO coating corrosion
resistance.
1. Microstructural characteristics of porosity and thickness were
determined to give significant indication of corrosion
behaviour of PEO coatings.
2. Corrosion rates of PEO-coated samples were found to be 16
times lower for monolithic samples and over 5 times lower for
FSSW when compared with AZ31 base metal corrosion rates.
Further research is required to confirm the dependence of PEO-
coated FSSW corrosion behaviour on weld zones and to resolve
reproducibility issues with coating performance indices.
Mass loss in 5 wt.% NaCl after 5 days revealed that
corrosion rate decreases for both monolithic and
FSSW samples in the following order: uncoated;
coated under non-optimal conditions; coated under
optimal conditions. The lowest corrosion rate of
approximately 2.7 µg cm-2 h-1, was achieved by
monolithic samples under optimal coating conditions.
These results are in accordance with thickness,
porosity and roughness analyses.
Figure 5: Graph of corrosion rates for uncoated samples,
samples coated in non-optimal conditions, and samples
coated in optimal conditions.
4.0 Results and Discussion
Corrosion Mechanism
Mass Loss Test
Parameters of processing time (a), polyethylene glycol (PEG) additive concentration (b), and applied current density
(c) were analyzed to determine optimal PEO coating porosity. Smoother coating with smaller and less frequent pores
resulted from increased time and lower current processing in the presence of PEG additive.
• PEO coatings were produced in 500 ml of aqueous,
silicate-based electrolyte using a dual-walled beaker
cooling system and a DC power source
• FSSW were made at 3000 RPM and 1 s dwell time
Figure 3: Percentage porosity at (a) 5, 10 and 15 minute processing time (b) 0 and 10 g/L PEG (c) 10 and 15 mA/cm2.
Optimal Coatings
Effect of Processing Parameters
Figure 4:
Surface morphology,
cross-section
morphology, and
surface roughness of
coatings processed
for 15 minutes, 0 g/L
PEG, and 10 mA/cm2
(a), (c), (e); and
coatings processed
for 30 minutes, 10 g/L
PEG and 10 mA/cm2.
(b), (d), (f).
Optimal coatings, whose key morphological characteristics are illustrated below, were processed at 10 mA/cm2 for 30
minutes in PEG-containing electrolyte. This was due to high-energy discharges and thermal stresses at increased
current densities, resulting in defects such as microcracks and micropores. High processing time resulted in the most
developed coating.
• Mass loss testing was done in 5 wt. % NaCl for 5
days at 25°C
Figure 2: Uncoated and coated FSSW samples (left); and
PEO experimental set-up (right).
Figure 1: Three-layered PEO coating
structure [2].
Figure 5: Corroded samples of uncoated monolithic (a), uncoated FSSW (b);
monolithic (c) and FSSW (d) coated in non-optimal conditions; and monolithic (e)
and FSSW (f) coated in optimal conditions.