Mg Alloys for Improved Corrosion Resistance Presenter: Michael A. - PowerPoint PPT Presentation

Excimer Laser Processing of Al containing Mg Alloys for Improved Corrosion Resistance Presenter: Michael A. Melia Co-Authors: M.L. Serron a , D.C. Florian a , J.P. Weiler b , J.R. Scully a , J.M. Fitz-Gerald a a University of Virginia b Meridian Lightweight Technologies, Inc. March 2 nd , 2017

Motivation: Need for Light-Weight Vehicles Mg alloys used in Ford F-150 Maupin (2010) Picture from: bp3.ford.com Mg sacrificial anodes. http://www.cathodicme.com/sacp.html 2 http://www.williammaloney.com/aviation/USNavyMuseum/SwiftBoat/images/06SwiftBoatSacrificialAnode.jpg

A Closer Look at Mg-Al Alloys: AZ31B Φ -Mg 6 (Al,Zn) 5 Large γ -Al 8 Mn 5 IMPs γ -Al 8 Mn 5 IMPs 10 μm 1 μm Mg has low solid solubility with alloying elements. Equilibrium Terminal Solid Solubility Range (at. %) Binary System <0.1 0.1-1 1-5 5-25 >25 As, Ba, Ce, Co, Au, Ca, Ir, Ag, Bi, Dy, Ga, Al, Er, Ho, Cu, Eu, Fe, Ge, Mg - X Nd, Th, Gd, Hg, Pu, Sn, Li, Lu, Pb, Tl, Cd La, Na, Ni, Pd, Mn, Ti Y, Yb, Zn, Zr Tm, In, Sc Pr, Sb, Si, Sr K. Schlüter et al. / Corrosion Science 52 (2010) 3973 – 3977 F. H. Froes et al. / Materials Science and Engineering, A 117 (1989) 19 – 32 3 http://www.himikatus.ru/art/phase-diagr1/Al-Mg.php

Issues with Mg: Corrosion Cathodic Reaction (H 2 Evolution): Anodic Reaction (Mg Dissolution): 2H 2 O + 2e -  H 2 (gas) + 2OH - Mg  Mg +2 + 2e - Mg +2 H 2 Corrosion potential of phases present in 0.6 M NaCl: e - α -Mg = -1.65 V SCE β -Mg 17 Al 12 = - 1.20 V SCE γ -Al 8 Mn 5 = - 1.25 V SCE Heterogeneous Mg alloy Microstructure http://iarjset.com/upload/2015/december-15/IARJSET%2012.pdf 4

Alloys Under Investigation Composition of AZ31B-H24, AZ91D, and AM60B alloys. Element (wt%) Al Zn Mn Fe Mg AZ31B-H24 3.0 1.0 0.33 0.005 Bal. AZ91D 7.2 0.79 0.19 0.02 Bal. AM60B 5.9 0.02 0.28 0.01 Bal. AZ31B-H24 AM60B AZ91D 10 μ m 10 μ m 10 μ m Increasing amount of β -Mg 17 Al 12 phase. Decreasing size of Al 8 Mn 5 particles. 5

Excimer Laser Processing Key Parameters: • Laser source: pulsed excimer laser ( λ = 248 nm and FWHM = 25 ns). • Laser fluence. • Pulse per area (PPA) and overlap. • Spot size: cylindrical lens. • Ar backfill gas. Polished surface Laser processed 5 mm 6

Surface Temperature 2 1.5 J/cm 1800 Pure Mg 1500 boiling Temperature (K) γ -Al 8 Mn 5 1200 Pure Mg melting 900 600 β -Mg 17 Al 12 300 0 20 40 60 80 100 120 140 Time (ns) 7

Solution to Micro-galvanic Corrosion of Mg alloys Cathodic Reaction (H 2 Evolution): Anodic Reaction (Mg Dissolution): 2H 2 O + 2e -  H 2 (gas) + 2OH - Mg  Mg +2 + 2e - Homogenized top layer Heterogeneous Mg alloy Microstructure 8

Fiduciary Backscatter Micrographs of AZ31B-H24 10 μ m Base 2 PPA 10 PPA 20 PPA 100 PPA 200 PPA 9

Fiduciary Backscatter Micrographs AZ91D Base AZ91D 100 PPA 10 μ m AM60B Base AM60B 100 PPA 10 10

Grazing Incident X-ray Diffraction Mg peaks  - Mg 17 Al 12 MgO AM60B Laser Processed. AM60B Base. Intensity (Arb.) AZ91D Laser Processed. AZ91D Base. AZ31B Laser Processed. AZ31B Base. 30 35 40 45 50 55 60 65 Angle (2  ) Grazing angle of 0.5 degrees using Cu-K α radiation, ~2.5 μ m x-ray penetration. 11

Grazing Incident X-ray Diffraction Laser processed Intensity (Arb.) Base material β -Mg 17 Al 12 35 40 45 50 Angle (2  ) Laser processing results in β -Mg 17 Al 12 phase below XRD detection limit for AM60B and 12 AZ91D.

Laser Processed Cross-section of AZ31B 13

Cross-section with EDS line-scan of AM60B 100 90 Weight Percent 80 Mg Al Mn 15 10 5 0 0 2 4 6 8 10 Distance (  m) The cast alloys revealed a uniform distribution of Al throughout the processed region. 14

Open Circuit Potential (OCP) Measurements -1.5 Corrosion of the specimens were -1.6 performed in quiescent 0.6 M NaCl V vs. SCE -1.7 (pH = 5.5). -1.8 AZ31B was measured for 24 hours, Base AZ31B-H24 -1.9 AM60B and AZ91D OCP measured Laser Processed AZ31B-H24 for 72 hours with EIS. -2.0 1 10 100 1000 10000 100000 Time (seconds) -1.50 -1.50 -1.55 -1.55 -1.60 -1.60 -1.65 -1.65 V vs. SCE V vs. SCE -1.70 -1.70 -1.75 -1.75 -1.80 -1.80 -1.85 -1.85 Base AM60B Base AZ91D -1.90 -1.90 -1.95 -1.95 Laser Processed AZ91D Laser Processed AM60B -2.00 -2.00 1 10 100 1000 10000 100000 1 10 100 1000 10000 100000 Time (seconds) Time (seconds) 15

The Cathodic E-log(i) Measurement Anodic branch V vs. SCE E Corr Cathodic branch Expected result of reduced micro- galvanic Log (Current Density) 16

The Cathodic E-log(i) Measurements -1.5 -1.6 -1.7 E(V vs. SCE) -1.8 -1.9 AZ91D Base -2.0 AZ91D Processed AM60B Base -2.1 AM60B Processed AZ31B-H24 Base -2.2 AZ31B-H24 Processed -2.3 -8 -7 -6 -5 -4 -3 -2 10 10 10 10 10 10 10 2 ) i (A/cm Performed in quiescent 0.6 M NaCl (pH = 5.5) after 30 min. at OCP. 17

Electrochemical Impedance Measurements C 1 Measured data Simulated with equivalent circuit C 2 R s R 1 -60 R s +R ct -40 R 2 -20 2 ) L R 3 Z" (  -cm R s +R p 0 R s At low frequencies 20 freq  ∞ reduces to freq  0 40 1 1 1 R p = R 1 +R 2 + 60 R 3 0 20 40 60 80 100 120 140 Z' (  -cm 2 ) EIS measurement conditions: • Frequency range: 100,000 Hz to 0.005 Hz at 3 points per decade. • Frequency amplitude: ± 10 mV. • At OCP 18 Bland (2015) and King (2014)

Electrochemical Impedance Measurements -400 2 ) Z" (  cm -200 -15000 0 200 -10000 400 0 200 400 600 800 2 ) Z' (  cm -5000 2 ) Z" (  cm 0 5000 AZ31B Base - 12 Hr 10000 AZ31B Laser processed - 12 Hr 15000 0 5000 10000 15000 20000 25000 30000 2 ) Z' (  cm -15000 -15000 -10000 -10000 -5000 -5000 2 ) Z'' (  -cm 2 ) Z'' (  -cm 0 0 AZ91D Base 1 Hr 5000 5000 AM60B Base 1 Hr AZ91D Base 60 Hr AM60B Base 60 Hr 10000 10000 AZ91D Processed 1 Hr AM60B Processed 1 Hr AZ91D Processed 60 Hr AM60B Processed 60 Hr 15000 15000 0 10000 20000 30000 0 10000 20000 30000 Z' (  -cm 2 ) 2 ) Z' (  -cm

Summary of Impedance Measurements – Corrosion per year 10 Thickness Loss (mm/year) Laser Processed Base Alloy 1 0.1 0.01 AZ91D AZ31B AM60B Corrosion rate (thickness per year) based on EIS estimated anodic charge consumed after 24 hours for base and laser processed Mg alloy samples (AZ31B, AM60B, and AZ91D). 20

Conclusions • Dissolution and complete homogenization of the cast alloys was easier. – lower melting temperature and more uniform distribution of secondary phases. • Less MgO on alloys with more Al present. • Benefits to corrosion are evident but less pronounced for cast alloys. • Control over the ablation environment is critical to reduction of the large γ -Al 8 Mn 5 phase due to plasma induced pressure wave effects. 21

Acknowledgements • U.S. Army Research Laboratory under agreement number W911NF-14-2-0005 with Joe Labukas as project manager. • Students: Michael Serron, David Florian, Fritz Steuer, Patrick Steiner, Bruce Briglia, Michael Purzycki, Michael Serron, Philip Grudier, Chase Weaverling, Ethan Keyser, Bailey Kraft, and Liam Agnew. 22

Questions? 23

Backup Slides 24

Plasma Induced Pressure Wave 40 μ m The effective pressure at surface during irradiation is approximately 40 MPa. Potential to move material 10’s of μ m per pulse. M. Von Allmen, A. Blatter, Laser-Beam Interactions with Materials, 2nd ed., Springer Series in Materials Science 1987. P. Schaaf, Laser nitriding of metals, Progress in Materials Science, 47 (2002) 1-161. 25 M. Han, K.P. Lieb, E. Carpene, and P. Schaaf, Laser-plume dynamics during excimer laser nitriding of iron, J. Appl. Phys., 93 (2003).

Plasma Induced Pressure Wave Base Material After 2 PPA 5 μ m 26

Plasma Induced Pressure Wave Base Material After 2 PPA 10 μ m • Laser fluence: 1.5 J/cm 2 . • 1.2 mm x 27 mm spot size (cylindrical shape). • 95% pulse overlap. • Ar backing pressure (Torr): 810. 27

Secondary Electron Micrograph of Laser Processed Surface 28

Laser Processing Cross-section of AZ31B 29

Table 5: Melting temperatures for common phases found in Mg-Al-Zn- Mn alloys Melting Temp. (K) Phase α -Mg 923 β -Al 12 Mg 17 728 1,253 Al 11 Mn 4 γ -Al 8 Mn 5 1,321

Current Mg Alloy Applications Rolls Royce RB211 Seat Cushion and Frame New, highly efficient turboprop engines incorporate these alloys in many structural parts and in integrated reduction gearboxes Rear Door Liftgate Transfer Cover (Drive Train) Pictures from: http://www.meridian-mag.com/ bp3.ford.com http://www.airteamimages.com/bo eing-747__saudi-arabian- airlines_90395.html http://www.autoblog.com/2010/09 /13/first-drive-2011-lincoln-mkx/ 31