The Influence of Grain Structure on Intermetallic Compound Layer Growth Rates in Fe-Al Dissimilar Welds
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TRODUCTION
JOINING aluminum to steel is an essential technology for lightweight, cost-effective automotive body structures (e.g., Reference 1). Whilst mechanical fastening is a proven method, dissimilar welding has a number of advantages, with solid-state ultrasonic and friction stir methods having the most promise.[2] However, it has been shown that control of the thermodynamically unavoidable interfacial reaction between aluminum and iron is one of the most critical issues in determining the performance of dissimilar Al-steel joints.[1,3,4] The principle reason for poor joint performance is the growth of a brittle intermetallic compound (IMC) reaction layer between the two metals at the joint interface, which
reduces their maximum strength but, more importantly, leads to low energy absorption during failure.[4,5] In order to investigate the growth behavior of the IMC layers and quantify the reaction kinetics, there have been several studies of Al-steel diffusion couples during either welding or isothermal heat treatment (e.g., References 1, 3, 4, 6). In most cases, a two-phase reaction layer consisting of g (Fe2Al5) and h (FeAl3) is reported to form at the interface in dissimilar A—steel combinations. The g phase generally dominates the IMC layer thickness.[6] Therefore, most previous studies have mainly focused on the growth behavior of the g phase and have paid less attention to the contribution of the h (FeAl3) layer. The usual way to interpret IMC layer growth kinetics is to fit the thickness as a function of time to a parabolic law, which is expected for the case of lattice diffusion-controlled growth: x2 ¼ kt;
LEI XU, JOSEPH D. ROBSON, and PHILIP B. PRANGNELL are with the School of Materials, University of Manchester, Manchester, M13 9PL, UK. Contact e-mail: [email protected] LI WANG is with Advanced Manufacturing Engineering, Jaguar Land Rover Limited, Abbey Rd, Coventry, CV3 4LF, UK. Manuscript submitted April 4, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
½1
where x is the thickness of IMC layer (m) after reaction time t (s), while k is the rate constant, with dimensions of m2 s1. The rate constant is temperature dependent and expected to follow an Arrhenius relationship[6]:
Q k ¼ k0 exp : RT
½2
k0 is the pre-exponential factor; Q is an effective activation energy (J mol1); R is the gas constant (J K1 mol1); and T is the reaction temperature (K). It is common practice to fit experimental data at different temperatures to derive values for k0 and Q. Examples of effective activation energies for g phase growth determined in this way are shown in Table I. It is clear that there is no agreement on a single value for the activation energy, with factor 4 differences between the smallest and largest reported values. This means that the simple parabolic law cannot be used in a predictive capacity, since the calculated layer thickness is very dependent on the value for activation energy chosen. Whilst some difference in this parameter may be attributed to the data obtained for differen
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