Numerically Based Phase Transformation Maps for Dissimilar Aluminum Alloys Joined by Friction Stir-Welding

metals Article Numerically Based Phase Transformation Maps for Dissimilar Aluminum Alloys Joined by Friction Stir-Welding Carter Hamilton 1, * ID , Stanisław Dymek 2 ID , Mateusz Kopyściański 2 ID , Aleksandra W˛eglowska 3 and Adam Pietras 3 1 2 3 * Department of Mechanical and Manufacturing Engineering, College of Engineering and Computing, Miami University, Oxford, OH 45056, USA Faculty of Metal Engineering and Industrial Computer Science, AGH University of Science and Technology, 30-059 Krakow, Poland; (S.D.); (M.K.) Department of the Testing of Materials Weldability and Welded Construction, Institute of Welding, 44-100 Gliwice, Poland; (A.W.); (A.P.) Correspondence: ; Tel.: +1-513-529-0722 Received: 31 March 2018; Accepted: 4 May 2018; Published: 8 May 2018



 Abstract: Sheets of aluminum 2017A-T451 and 7075-T651 were friction stir-welded in a butt-weld configuration. An existing computational model of the welding process for temperature distribution and material flow was adapted to estimate the phase transformations that occur across the weld zone. Near the weld center, process temperatures are sufficient to fully dissolve the equilibrium η phase in 7075 and partially dissolve the equilibrium S phase in 2017A. Upon cooling, Guinier–Preston (GP) and Guinier–Preston–Bagaryatsky (GPB) zones re-precipitate, and hardness recovers. Due to the more complete dissolution of the equilibrium phase in 7075, the hardness recovery skews toward whichever side of the weld, i.e., the advancing or retreating side, represents the 7075 workpiece. Phase transformation maps generated by the numerical simulation align not only with the hardness profiles taken across the weld zone, but also with positron lifetimes obtained through positron annihilation lifetime spectroscopy (PALS). Boundaries between the aluminum matrix and the secondary phases provide open volumes to trap positrons; therefore, positron lifetimes across the weld correspond with the phase transformations that occur in 7075 and 2017A during processing. Keywords: friction stir-welding; dissimilar materials; aluminum; material flow; temperature; phase transformations 1. Introduction Friction stir-welding (FSW) is now a common technique utilized to join metallic materials and produce high-quality welds. Since the friction stir-welding process takes place at lower temperatures than traditional welding methods, i.e., below the melting and solidus temperatures of the alloys, FSW generally circumvents the detrimental effects that can arise from melting and re-solidification. Kumar et al. [1] and Mishra et al. [2,3] have published a book series on friction stir-welding/processing that thoroughly details the current state of knowledge for this technology. Despite being a mature technology, however, the majority of FSW research and development efforts have studied the production of single-alloy welds [4,5]. Yet numerous manufacturing sectors, such as the aerospace and automotive industries, require dissimilar-metal welds in order to produce structures that are lightweight and mechanically sound. Common materials for these structures are aluminum alloys from the 2xxx and 7xxx series, which are classified as “non-weldable” with traditional fusion methods. In contrast, because joining occurs in solid-state, FSW mitigates the chemical and Metals 2018, 8, 324; doi:10.3390/met8050324 www.mdpi.com/journal/metals Metals 2018, 8, 324 2 of 14 mechanical incompatibilities between dissimilar metals that typically degrade weld quality during traditional processes. DebRoy et al. [6] and Murr [7] provide comprehensive literature surveys regarding the friction stir-welding of dissimilar alloys, and Kumar et al. [1] recently reviewed the current state of knowledge regarding this topic. Due to tool rotation and forward translation, FSW is an inherently asymmetric process relative to the workpieces. On the advancing side of the weld, tool rotation and forward translation are aligned, but on the retreating side, tool rotation and forward translation are opposed. Not surprisingly, material flow and temperature distribution during friction stir-welding are also asymmetric. When welding dissimilar materials, the asymmetric character of the process is compounded by the discontinuity in material properties across the weld zone. In fact, the placement of the alloys on either the advancing or retreating sides significantly influences the final weld properties. As noted by Ma et al. [8] in their review of friction stir-welding and processing in aluminum alloys, the theoretical prediction of heat generation, material flow and mechanical properties in dissimilar friction stir-welding is challenging. The temperature distribution, material flow and secondary phase transformations during FSW of dissimilar alloys are critical to the overall quality and mechanical performance of the dissimilar-alloy joint. Hamilton et al. [9] investigated the welding of 2017A-T4 and 7075-T6 and successfully developed a numerical simulation of the process to predict material flow and temperature distribution during welding. In addition to mapping the hardness behavior across the weld, they further characterized the weld through positron annihilation spectroscopy (PALS) [10]. The researchers demonstrated that positron lifetimes correlate with the hardness trends across the weld zone and with the microstructural characteristics of the weld. The interphase boundaries between secondary phases and the aluminum matrix provide open volumes that trap positrons. Because the interphase boundaries have a lower electron density than the crystal lattice, a positron trapped within a boundary has a longer lifetime than one trapped in the bulk. Even though many investigations of FSW welds concentrate just on the weld center and its vicinity, their research clearly demonstrated that variations related to phase transformations also occur at larger distances from the weld center (up to 40 mm). Hamilton et al. [10] aligned the temperature distribution predicted by the model with the known phase transformation temperatures in the two alloys (from differential scanning calorimetry) to explain the positron lifetime and hardness curves across the weld. The simulation itself, however, could not directly predict the phase transformations occurring in 2017A and 7075 during processing. The goal of the present study, therefore, is to adapt the computational model developed for the FSW of 2017A-T451 and 7075-T651 such that it can generate maps of the phase transformations that occur in the alloys during welding. These maps can then facilitate the analysis of positron spectra and hardness data and their correlation with the microstructural characteristics of the weld. 2. Materials and Methods The Instytut Spawalnictwa (Institute of Welding) in Gliwice, Poland, friction stir-welded the aluminum 2017A-T451 and 7075-T651 sheets utilizing a conventional milling machine modified for th (...truncated)