Abstract
The thickness of the intermetallic compound (IMC) layer that forms when aluminum is welded to steel is critical in determining the properties of the dissimilar joints. The IMC reaction layer typically consists of two phases (η and θ) and many attempts have been made to determine the apparent activation energy for its growth, an essential parameter in developing any predictive model for layer thickness. However, even with alloys of similar composition, there is no agreement of the correct value of this activation energy. In the present work, the IMC layer growth has been characterized in detail for AA6111 aluminum to DC04 steel couples under isothermal annealing conditions. The samples were initially lightly ultrasonically welded to produce a metallic bond, and the structure and thickness of the layer were then characterized in detail, including tracking the evolution of composition and grain size in the IMC phases. A model developed previously for Al-Mg dissimilar welds was adapted to predict the coupled growth of the two phases in the layer, whilst accounting explicitly for grain boundary and lattice diffusion, and considering the influence of grain growth. It has been shown that the intermetallic layer has a submicron grain size, and grain boundary diffusion as well as grain growth plays a critical role in determining the thickening rate for both phases. The model was used to demonstrate how this explains the wide scatter in the apparent activation energies previously reported. From this, process maps were developed that show the relative importance of each diffusion path to layer growth as a function of temperature and time.
Highlights
JOINING aluminum to steel is an essential technology for lightweight, cost-effective automotive body structures (e.g., Reference 1)
In the Ultrasonic Metal Welding (UMW) joints, a nearly continuous intermetallic compound (IMC) layer had already formed in the pre-welding stage, which had an average thickness of 0.8 lm (Figure 2(a))
The IMC layer became thicker during annealing, but was still not fully uniform even after annealing for 30 minutes at 773 K (500 °C) (Figure 2(c)); such an uneven IMC layer has been commonly observed in the Al-Fe system in several previous studies
Summary
JOINING aluminum to steel is an essential technology for lightweight, cost-effective automotive body structures (e.g., Reference 1). Whilst some of the studies were performed with aluminum in the liquid state, this does not explain the differences, since the same diffusion path through the (solid) IMC will control growth rate It is clear from the data that there is no systematic variation in activation energy that can be explained by either the state of the aluminum or details of the alloy composition. By explicitly considering grain boundary and lattice diffusion pathways, it is possible to determine the significance of both mechanisms and how this is influenced by growth conditions (e.g., temperature) As part of this exercise, detailed analysis was performed by electron microscopy of the growth rate and microstructure of the IMC phases that formed, including measurements of the grain size evolution within each IMC layer as it grows and the composition gradients within each phase
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