Processing Effects on the Friction This investigation attempts to understand the true temperature at the WELDING JOURNAL 11-s WELDING RESEARCH Introduction Stir Weld Stir Zone As with most welding processes, friction stir welding (FSW) produces a nonhomogenous macrostructure whose regions, illustrated in Fig. 1, include the heat-affected zone (HAZ), thermomechanical affected zone (TMAZ), and weld nugget or stir zone (SZ). Each zone is characterized by a unique microstructure related to different levels of thermomechanical processing. The tool rotation and travel impart a nonsymmetrical flow pattern that is observed in the nonsymmetric weld structure of the transverse section in Fig. 1. The side where the tool rotation and travel vectors are in the same direction is labeled the advancing side (AS), and where they are opposed is labeled the retreating side (RS). Because FSW is a solid-state process, correlation of the temperature at the workpiece/weld tool interface with the processing parameters presents challenges. Understanding of this correlation is needed for control of the processing temperature and optimization of the resulting mechanical properties. Because the maximum temperature in FSW is generally considered to be at the shear interface between the SZ and the TMAZ (Refs. 1–3), understanding the variation in temperature in this region with respect to processing parameters is necessary. Numerous studies report the resulting weld temperature to be most strongly influenced by the tool rotation velocity (Refs. 2–7). In addition to understanding the temperature, the heating rate can also affect the kinetics of the phase changes in age-hardenable alloys such as the AA2xxx series. Since the FSW process is considered to involve a large shear strain at high rates (Refs. 2, 8–12), the heating or up-quenching times associated with the process may be very rapid (Ref. 13). Determining the temperature at the workpiece/weld tool interface was directly approached using embedded thermocouples in 2xxx series aluminum alloys (Refs. 1, 2, 14–20), and it has provided information on the relative homologous temperature in the range of 0.80 to 0.90 Tm (where Tm is the melting temperature of the Al with a value of 933 K). Little variance has been reported with SZ temperature measurements of 525°C in AA2024 (Ref. 17) and 480°–520°C in AA2195 (Ref. 14), where the increase in temperature corresponded to an increase in tool rotation. Positioning the thermocouple close to the shear zone has noted difficulties due to potential displacement by the resulting material flow and response to rapid heating conditions. Thus, most thermocouple measurements have been used to validate a numerical model with extrapolation of measured temperatures outside the SZ to the workpiece/weld tool interface. Attempts to model the temperature in the shear region have often resulted in overprediction of the weld temperature, which has been attributed to slippage occurring at the workpiece/weld tool interface (Refs. 21, 22). While relationships between peak temperature and processing conditions have been shown (Refs. 2, 14), they are not considered to change the overall temperature field significantly (Ref. 22). Conversion of weld power to thermal energy has also being pursued to determine the weld temperature (Refs. 23–27), and may have validity if the temperature does not exceed the eutectic or solidus temperature resulting in tool slippage and reduced efficiency (Refs. 13, 21). Since the processing temperature controls the resulting mechanical properties, as affected by microstructural variations, interpretation of the resulting grain size and precipitate state can be used to verify processing temperatures and provide insight as to the heating conditions, and hence, strain rate experienced during FSW of age-hardenable alloys (Refs. 8–10, 12, 13, 28). Due to the complex nature of the FSW process, various characterization methods at different length scales are often needed to interpret the results. Although much research has been published on the resulting microstructure and mechanical properties of FSW in the agehardenable 2xxx series (Refs. 14, 15, 19, 20, 27–39), these studies generally characterized a single FSW obtained with a single set of processing parameters that covered a range of tool rotations from 120 to 1040 rev/min. Further adding to the difficulty of comparing findings, not all studies document details of the tool design and processing parameters. Thus, assessing whether the microstructural evolution observed is due to the material, tool design, processing parameters, or some combination is difficult and sometimes results in conflicting findings. Studies on 2024 (Refs. workpiece/weld tool interface BY J. SCHNEIDER, R. STROMBERG, P. SCHILLING, B. CAO, W. ZHOU, J. MORFA, and O. MYERS ABSTRACT While many researchers have carefully mapped out the various microstructural regions of a friction stir weld (FSW), concluding that each region undergoes different thermomechanical cycles during the process, these studies generally have only considered one set of FSW parameters. By considering only the shear zone (SZ) over a range of FSW process parameters, it can be observed that material within this region is also subjected to different thermomechanical cycles. Whether this results from a temperature increase with higher rev/min and/or material held for an increased time at temperature, is still not understood. This study, however, does give insight into the often conflicting results published regarding the microstructural evolution in a FSW. KEYWORDS Aluminum Friction Stir Welding Heat-Affected Zone Shear Zone Solid-State Welding J. SCHNEIDER, J. MORFA, and O. MYERS are with Mechanical Engineering Department, Mississippi State University, Mississippi State, Miss. R. STROMBERG is with Hysitron, Inc., Minneapolis, Minn. P. SCHILLING is with Mechanical Engineering Department, University of New Orleans, New Orleans, La. B. CAO and W. ZHOU are with Advanced Materials Research Institute, University of New Orleans, New Orleans, La.
Welding Journal | January 2013
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