Modeling Al-Mg Weld Corrosion

Project Introduction

The next frontier in vehicle body lightweighting could make extensive use of magnesium (Mg) alloy sheet for its outstanding stiffness- and strength-to-weight, with an aluminum (Al) alloy sheet providing low-cost paintable class-A finish surfaces. Already, the Lincoln MKT and Chrysler Pacifica liftgates have used cast Mg inner and Al sheet outer components, and the Magna-FCA next generation ultra-light door will likely use similar materials. However, corrosion of Al-Mg fusion welds is a serious problem, limiting widespread use of this material pair in a vehicle’s body (e.g., steel self-piercing rivets in Mg would create more severe corrosion).

Friction stir welding (FSW) leads to significant grain refinement, second-phase refinement, homogenization, and densification; all characteristics known to have beneficial effects on the corrosion resistance of light metals. Al corrosion studies suggest grain refinement via equal-channel angular pressing improves corrosion resistance primarily via impurity breakup and homogenization leading to reduced microgalvanic current. Similarly, FSW in wrought Mg-Yttrium rare earth and AA5083 Al alloys shows potential for improved corrosion resistance likely due to breakdown and dispersion of intergranular precipitates reducing the mass loss rate.

That said, there is no quantitative link between microstructure and corrosion performance. Closing this missing link in understanding will facilitate design of cost-effective processes for making robust joints whose geometry and microstructure reduces corrosion and joint failure, even when coatings fail.

Objectives

The objective of this project is to provide a quantitative link between FSW microstructure and corrosion performance. In particular, the team aims to develop and validate a grain-level phase field model of microgalvanic corrosion, and coupled micromechanics model of mechanical failure, in FSW Al-Mg alloy joints to predict strength and fatigue lifetime of corroded joints within 10% of measured performance.

This project will develop the models using 6022 Al and ZEK100 Mg alloys, which were selected for the Magna-FCA ultra-light door project. Test joints between these sheet materials will use diffusion bonding and FSW. The team will attempt to validate the model using 7xxx-series Al alloys.

Approach

The phase field corrosion model starts with an expression for free energy as a function of composition in the metal, electrolyte, and oxide and/or hydroxide corrosion product phases. This free energy expression begins with a fit to thermodynamic data on the Al and Mg alloy systems, including the base metals and intermetallic compounds (IMCs), with higher free energy in composition ranges between IMCs to tune the interfacial energies. The electrolyte is an aqueous solution with Al³⁺ and Mg²⁺ ions and dissolved oxygen. Using a representation of the full Al-Mg system free energy function both produces the correct compositions at phase boundaries, and leads to correct chemical potential of both species at metal-electrolyte boundaries, which automatically creates electronically-mediated microgalvanic corrosion reactions between the various phases.

Corrosion model validation begins with fabrication of diffusion-bonded and welded joints. Diffusion-bonded sheets of pure Mg and Al provide an ideal model system for understanding the fundamentals of galvanic corrosion. Diffusion bonded alloy sheets deepen this understanding to include additional phases in each alloy. When the phase field corrosion model is proven for diffusion-bonded couples, we will then try to use it to describe corrosion in a complex FSW microstructure.

Prediction of FSW microstructure is beyond the scope of this study. Instead, advanced characterization techniques such as scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD) will produce maps of composition and grain orientation across a FSW joint. This will include using in situ focused ion beam and plasma etching to expose and characterize multiple layers in a joint leading to a 3-D microstructure. The microstructure thus characterized will provide the initial condition for the corrosion model.

This project consists of four tasks with an end goal of a validated accurate model of corrosion and mechanical failure:

1. Produce Diffusion-Bonded and Welded Coupons for Testing:

Produce coupons with joints between pure and/or alloyed Mg and Al sheets by either diffusion bonding or friction stir and/or fusion welding (PNNL: Piyush Upadhyay)

2. Conduct Corrosion and Mechanical Testing:

Run accelerated corrosion tests and tensile and cyclic loading tests to determine corrosion morphology and its effect on strength and fatigue performance of Mg-Al joints (WPI: Brajendra Mishra, Qingli Ding)

3. Characterize Welded Joints and Corrosion and Mechanical Test Samples:

Use advanced characterization methods including ESM with EDS and EBSD, as well as small angle neutron scattering, to understand the structure of corrosion products and fracture surfaces and provide input geometry/morphology as an initial condition for models (ORNL: Donovan Leonard)

4. Develop Corrosion and Mechanics Model:

Use phase field and crystal plasticity modeling based on PRISMS tools from the University of Michigan to build a grain-level model of corrosion and mechanical deformation of Mg-Al joints (WPI: Adam Powell, Kübra Karayağız)

Researchers: Kübra Karayağız, Adam Powell

Sponsors: US Department of Energy Vehicle Technologies Office, Contract DE-EE0008454

Collaborators: Brajendra Mishra, Pacific Northwest National Laboratory, Oak Ridge National Laboratory

Challenge Problem and Cost Share Partner: Magna Services of America