Characterisation of friction stir welded 7042-T6 extrusions through differential scanning calorimetry

Abstract

Extruded and T6 tempered plates of 7042, an Sc modified Al-Zn-Mg-Cu alloy, were joined by friction stir welding (FSW) at a constant weld velocity and various pin rotation speeds (PRSs). At a low PRS, hardness decreased from each edge of the weld to a local minimum at the weld centre, but at a high PRS, the hardness initially decreased from each edge of the weld, but then rose towards the weld centre. The transition in the hardness profile was related to different heating and cooling conditions during FSW. Differential scanning calorimetry of baseline and welded samples revealed that the volume fraction of strengthening particles within the weld regions strongly depended on the weld conditions. For FSW heating rates, secondary phase dissolution/precipitation temperatures are in proximity to the welding temperatures. The maximum welding temperature during FSW increased from y250 to 450uC as the PRS increased from 175 to 400 rev min 21 .

Monitoring of Weld Quality in Friction Stir Welding Based on Spindle Speed and Motor Current Signals

“…UTS, YS, and E were reported to increase with the increase in the revolutionary pitch and then decrease after a certain value of the same. Similar reporting on the variation of weld quality in FSW process can be found in the related works by different researchers (Li and Liu 2013;Lim et al 2004;D'Urso et al 2009;Kamp et al 2009;Buffa et al 2013;Hamilton et al 2012;Miyazawa et al 2012). Researchers have presented the effect of various process parameters over the quality of the weld, but none has reported on the inconsistency of the weld quality over the same range of process parameters.…”

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

“…Transformation Temperatures Estimated (From DSC Data) for Different Weld Conditions at the Weld Edge (Retreating Side) and Weld Center. [28] Welding Condition (rev min Figure 7a presents the material flow behavior of surface material tracers during the 250 rev min À1 condition with color shading indicating the temperature history. As seen in the figure, surface tracers under the shoulder and near the weld edge can reach temperatures of 618 K (345 °C), temperatures just greater than or approximately equal to the weld edge exothermic reaction temperature [604 K (331 °C)] for this weld condition.…”

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

“…This flow of material effectively raises the advancing side temperature above that of the retreating side. In a separate study of friction stir welded 7042-T6, Hamilton et al [28] noted that tensile samples extracted from the welded plates fractured toward the advancing side, suggesting that the advancing side was softer than both the weld center and retreating side. Through DSC analysis of samples taken from the weld, they hypothesized that the advancing side temperature reaches an endothermic transformation temperature before the retreating side, i.e., the advancing side is hotter, and becomes depleted in strengthening precipitates.…”

“…In their differential thermal analysis through DSC, Hamilton et al [28] demonstrated that three phase transformations occur in the 7042-T6 alloy during heating at 10 K/min between room temperature and 673 K (400°C). These are (i) an endothermic transformation occurring near 438 K (165°C) and caused by the dissolution of the non-equilibrium GP zones and g¢ particles, (ii) an exothermic transformation occurring near 489 K (216°C) and caused by the formation and growth of the equilibrium g and/or T phases, and (iii) an endothermic transformation occurring above~523 K (250°C) due to dissolution of these equilibrium phases.…”

“…These are (i) an endothermic transformation occurring near 438 K (165°C) and caused by the dissolution of the non-equilibrium GP zones and g¢ particles, (ii) an exothermic transformation occurring near 489 K (216°C) and caused by the formation and growth of the equilibrium g and/or T phases, and (iii) an endothermic transformation occurring above~523 K (250°C) due to dissolution of these equilibrium phases. [10][11][12][13] Hamilton et al [28] further noted that since the dissolution and formation of secondary phases are thermally activated processes, the positions of the endothermic and exothermic peaks, corresponding to the maximum rates of the reactions, will shift to higher temperatures with increasing heating rates. During friction stir welding, therefore, the high heating rates can shift these phase reaction temperatures to values in the proximity of the welding temperatures, significantly impacting the final microstructure.…”