A Coupled Thermal/Material Flow Model of Friction Stir Welding Applied to Sc-Modified Aluminum Alloys

CARTER HAMILTON 0 MATEUSZ KOPYS CIAN SKI 0 OLEG SENKOV 0 STANISLAW DYMEK 0 0 CARTER HAMILTON, Associate Professor, is with the Depart- ment of Mechanical and Manufacturing Engineering, Miami University , Oxford, OH . Contact A coupled thermal/material flow model of friction stir welding was developed and applied to the joining of Sc-modified aluminum alloy (7042-T6) extrusions. The model reveals that surface material is pulled from the retreating side into the weld zone where it is interleaved with in situ material. Due to frictional contact with the shoulder, the surface material is hotter than the in situ material, so that the final weld microstructure is composed of bands of material with different temperature histories. For this alloy and the associated FSW heating rates, secondary phase dissolution/precipitation temperatures are in proximity to the welding temperatures. Therefore, depending on the surface and in situ material temperatures in relation to these transformation temperatures, disparate precipitate distributions can develop in the bands of material comprising the weld nugget. Based on the numerical simulation and on thermal analysis data from differential scanning calorimetry, a mechanism for the formation of onion rings within the weld zone is presented. - alloys 2024, 7449, and 6013. The current investigation presents a coupled thermal/flow model of friction stir welding applied to Sc-modified Al-Zn-Mg-Cu extrusions (Al alloy 7042-T6). Additions of scandium (Sc) and zirconium (Zr) to 7000 series alloys stabilize the microstructure at temperatures greater than 423 K (150 C) through the formation of fine, secondary strengthening phases such as Al3(Sc,Zr).[7,8] The nanometer-sized Al3(Sc,Zr) particles also stabilize the microstructure formed during hot working operations and inhibit recrystallization during heat treatment, thus potentially enhancing the residual properties after joining operations such as FSW.[9] These additions also affect the kinetics of precipitation and growth of the primary strengthening precipitates (GP zones, g), thus modifying heat treatment conditions for enhancing the mechanical properties of these alloys.[10] The numerical simulation proposed here gives insight into the material flow and temperature distribution of the weld zone during the joining of 7042-T6 extrusions. Combined with thermal analysis data from differential scanning calorimetry (DSC), the precipitation behavior within the weld is discussed in terms of the volume fraction of the metastable (GP zones and g) and equilibrium [g (MgZn2) and/or T (Al2Mg3Zn3)] strengthening particles found in the 7042 aluminum alloy.[11] It is assumed that FSW does not change the size and volume fraction of the Al3(Sc,Zr) precipitates due to their high thermal stability.[12] DSC is a powerful technique for the investigation of precipitation and dissolution processes in Al alloys.[10,13] By detecting the heat variations due to the phase transformations, the technique is able to identify the temperature ranges in which they occur. For example, Dixit et al.[14,15] utilized DSC to study the nucleation of precipitates within the nugget of friction stir welded aluminum 2024 and to correlate the weld microstructure to mechanical properties. In the present work, results of the DSC thermal analysis of the FSW regions of the 7042-T6 Al alloy, together with a developed coupled thermal/material flow model of FSW, were used to propose a mechanism of onion ring formation within the weld zone. The model reveals that surface material is pulled from the retreating side into the weld zone where it is interleaved with in situ material. II. EXPERIMENTAL PROCEDURE A. Alloy Chemistry and Heat Treatment The chemical composition of the 7042-T6 Al alloy used in this work is given in Table I. This alloy utilizes the synergistic combination of scandium and zirconium to stabilize the microstructure and enhance mechanical properties. For this investigation, a 76-mm diameter 7042 billet was produced by direct chill casting and then hot extruded into a bar with a rectangular cross section of 50.4 mm 9 6.35 mm, thus providing the extrusion ratio of 14:1. Following extrusion, the bar was heat treated to a T6 temper through the following schedule: (1) solution heat treat at 733 K (460 C) for 1 hour followed by an additional hour at 753 K (480 C), (2) rapid quench in water to room temperature, and (3) age at 393 K (120 C) for 19 hours. B. Friction Stir Welding After heat treatment, the bar was cut into twelve, 305mm-long pieces and sent to the Edison Welding Institute (EWI, Columbus, OH) to produce six longitudinal friction stir welds. The diameter of the FSW tool shoulder was 17.8 mm, the pin diameter tapered linearly from 10.3 mm at the tool shoulder to 7.7 mm at the tip, and the pin depth was 6.1 mm. With a constant weld velocity of 2.1 mm s 1 and a constant applied force of 22 kN, unique welds were produced at the following pin rotation speeds (PRS): 175, 225, 250, 300, 350, and 400 rev min 1. The temperature profile across the weld surface was experimentally recorded for each condition using a Mikron M7815 Infrared Thermal Imaging Camera during welding. These data were used to verify the temperature Zn Mg Cu Mn Zr Sc Cr Ti Other, Total Al predictions of the coupled thermal/flow simulation developed during this investigation. The uncertainty in these measurements was 2 pct (or approximately 9 K). The thermal emissivity for the infrared data was calibrated by imaging an extrusion length heated to 733 K (460 C) and adjusting the emissivity value until the recorded temperature of the camera matched the reference temperature. The appropriate thermal emissivity value was determined to be 0.285. C. Post-Weld Investigation Subsequent to joining, the welded panels were stored at room temperature and allowed to naturally age for at least 30 days prior to testing and investigation. Small samples (approximately 20 to 50 mg) were extracted from the T6-tempered baseline material and from the weld center of each welded sample for thermal analysis. The samples were sealed in Al pans and analyzed in a Perkin Elmer Jade differential scanning calorimeter, using an argon atmosphere. Depending on the data desired, samples were heated from room temperature to 673 K (400 C) at a constant heating rate that ranged from 10 to 100 K min 1. A polarized optical microscope was used to study the microstructure of the welds. To enhance the appearance of precipitate distributions and grains, the studied surfaces of the weld samples were polished and anodized in an electrolytic solution of 1.8 pct fluoroboric acid in water at room temperature and an electric current of 0.15 A. The anodizing time was 2.5 to 3 minutes. III. COUPLED MODEL FOR FLOW AND TEMPERATURE BEHAVIOR A. Materials Properties and Boundary Conditions for Flow A coupled thermal/flow model was developed for friction stir welding utilizing the Comsol multi-physics sof (...truncated)