Aluminum-Steel ETJs Effects of Temperature and Time Upon Mechanical Properties
Aluminum–steel electric transition joints (ETJ) are used in aluminum and magnesium reduction cells for making welded connections between aluminum buss systems and steel anodes and cathodes. Depending upon the cell design and operating procedures, ETJs can operate at temperatures ranging between 200°C and 500°C, and sometimes hotter. Over time, these thermal conditions can have significant deleterious effect upon the ETJ bond strength, resulting in eventual failure.
A detailed study was performed measuring ETJ bond strength as a function of time, temperature and design. Specimens were maintained at temperatures of 300°C to 640°C for up to 300 days, and then tensile tests were performed. Metallographic and spectrographic analyses were performed to analyze the mechanism of bond degradation. Diffusion and subsequent intermetallic formation were the mechanisms causing bond degradation. Theoretical diffusion equations provide a basis for extrapolation of test data to longer time periods.
The addition of a thin interlayer between the aluminum and steel can alter this behavior. Several interlayer materials were evaluated, including chromium and titanium. Titanium provides the most significant improvement in performance. Extrapolations indicate that titanium interlayer ETJ’s can be expected to operate for over 10 years at 450°C without significant degradation.
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