Automotive, rail, defense, aircraft, heavy manufacturing, and shipbuilding original equipment manufacturers (OEMs) and suppliers alike are all being driven by the government and natural resources costs to build more fuel-efficient vehicles. By the use of lightweighting, a vehicle can reduce its weight to increase fuel efficiency and/or increase its load carrying capability. Proper application of aluminum to steel structures can reduce weight up to 25% when compared to a monolithic structure made of only steels. The largest barrier to fundamental joining of these two materials is the formation of brittle intermetallic compounds (IMC) at the joint interface. The formation of IMC during short intense thermal cycles as commonly encountered in welding is not readily understood in literature. Researchers have reported varying intermetallic phases observed at the interface, accelerated growth rates, and conflicting reports of which phases exist and the rate of growth of the IMC. The formation and growth of intermetallic compounds in the aluminum-iron system is not well understood at short times in the solid state as traditional diffusion couple experiments are measured in hours as opposed to seconds.
In this study, a new technique utilizing resistance-based heating was developed via extensive thermal modeling and experimental design to generate short time diffusion couples in aluminum to steel at temperature via the bulk resistive heating. The experiment utilizes a resistance welder to create near-isothermal temperature profiles at the iron-aluminum interface. By monitoring of the dynamic resistance and experimental validation, finite element modeling was used to simulate the temperature profiles of each resistance based-diffusion couple. The resulting interfaces were characterized using electron microscopy techniques which included scanning electron microscopy (SEM) imaging, backscatter imaging, scanning transmission electron microscopy (STEM) imaging, Transmission electron microscopy (TEM) energy dispersive X-ray spectroscopy (EDS), nano-beam diffraction (NBD), and dark field imaging in the TEM. The electron microscopy was performed to determine IMC growth layer thickness, IMC morphology, and ultimately first phase formation and sequential growth order.
Examination of varying couples exposed to different temperatures and times has revealed that planar growth of the IMC does not occur at short times but rather a multi-site nucleation of IMC in the aluminum matrix near the iron-aluminum interface. Discrete grain of Fe4Al13 are nucleated within the aluminum bulk material within 1-Ám of the Fe-Al interface. Along the Fe-Al interface, Fe2Al5 nucleates and grows. As the IMC layer becomes continuous, interdiffusivity is greatly reduced, limiting the nucleation of Fe4Al13 by reducing the available solute supply to the aluminum matrix shifting the mode of growth from multi-site nucleation and growth to planar growth of the IMC. Close formed kinetic equations to describe growth have been generated and the mechanisms of growth at short time have been determined and are presented in the body of the research. The experiments have successfully identified the growth mechanisms of IMC in the iron aluminum system, identified non-planar growth conditions at short time, identified Fe4Al13 as the first phase to form, and provided the basis of fundamental kinetics calculations in the iron aluminum system.