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Effect of Thermal Cycling on Friction Stir Welds of 2195 Aluminum Alloy
Postweld thermal cycling of friction stir aluminum alloy welds leads to complex microstructural evolution
BY G. OERTELT, S. S. BABU, S. A. DAVID AND E. A. KENIK

ABSTRACT. The microstructure in friction stir welded (FSW) aluminum Alloy 2195 was investigated in the as-welded and postweld thermal cycled conditions. Theas-welded microstructure in the dynamically recrystallized zone (DXZ) contains both dislocated and recovered grains. This DXZ region was subjected to thermal cycling. Thermal cycling led to a decrease in dislocation density and precipitation of the second phase within and along the grain boundaries. These results show the DXZ region is supersaturated with alloying element. The grain growth kinetics inthe DXZ region were complicated because of the interaction of precipitation and the recovery of deformed grains.

Introduction
Friction stir welding (FSW) involves plunging a rotating shouldered pin tool into the faying surface of two plates and traversing the tool along its length (Ref. 1). Welding, which is in solid state, is achieved by plastic flow of frictionally heated material.Extensive research has been accomplished on developing the friction stir welding process for aluminum alloys used in aerospace applications (Refs. 2–6). The process has been applied to both precipitation-strengthened (Refs. 1–9) and nonprecipitation-strengthened aluminum alloys (Ref. 10). Even Alloy 7075 (Al-Zn-Mg type), which is considered difficult to weld with conventional welding processes, wassuccessfully welded with FSW and exhibited good properties (Ref. 7). In addition, friction stir welds of 5454 (Al-Mg) alloys have shown potentially good corrosion properties (Ref. 10). Results of these investigations
G. OERTELT is with the University of Leoben, Leoben, Austria. S. S. BABU, S. A. DAVIDand E. A. KENIKare with the Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge,Tenn.

show the FSW process yields better properties than conventional welding processes for aluminum alloys. The details of the microstructural evolution during the severe thermomechanical conditions imposed by this welding process are far from being completely understood. For example, the grains in the dynamically recrystallized zone are not completely recrystallized and there exists a highdensity of dislocations within these grains (Ref. 6). The precipitation of various phases may also complicate the microstructural evolution; these precipitation reactions are determined by the initial state of the base metal (Ref. 8) and alloy composition. In addition, the precipitation characteristics may influence the final grain size of these welds. Besides the microstructural evolution duringwelding, the stability of the microstructure during subsequent heat treatment is also not understood. In particular, the response of DXZ microstructure to another weld thermal cycle is not known. This subsequent weld thermal cycle may be due to repair welding or subsequent weld overlaps. In addition, the stability of this microstructure during high-temperature exposure is not known. In particular, theinitial state of the DXZ may affect the grain growth characteris-

tics. Therefore, in this work, microstructural evolution in the DXZ, stability of the microstructure to multiple thermal cycles and grain growth characteristics of DXZ regions were investigated in friction stir welds of 2195 aluminum alloy.

Experimental
Aluminum Alloy 2195 (Al-4.0 wt-% Cu-1.0% Li-0.5% Mg-0.4% Ag-0.1% Zr) wasused in this investigation, and the plate was in the T8 (solution treated, cold worked and artificially aged) condition (Refs. 6, 11) before the FSW operation. The friction stir welding was done at the Lockheed Martin Manned Space System Complex, New Orleans, La. The 5.8mm-thick (0.23-in.) plates were friction stir welded with the following process parameters: 10.9-mm (0.43-in.) pin-tool...