Precipitation and grain refinement in a 2195 Al friction stir weld
- PDF / 695,037 Bytes
- 12 Pages / 612 x 792 pts (letter) Page_size
- 91 Downloads / 209 Views
ICTION stir welding (FSW) is a solid-state joining process that was developed in 1991,[1] but has already become an important process for joining aluminum alloys. Friction stir welding uses a rotating tool to heat the material along the joint line and plastically deform (stir) the two sides together. This process produces microstructurally different regions in the weld that are each characterized by their thermomechanical history.[2–10] In the outer regions of the weld, the frictional and plastic deformation produces a heat-affected zone (HAZ) similar to that observed in conventional welds. Closer to the weld center, where the material undergoes mechanical deformation in addition to being exposed to higher temperatures, is the thermomechanically affected zone (TMAZ). At the center of the weld is a region called the weld nugget, containing fine, equiaxed grains that are generally viewed to be the result of dynamic recrystallization. The process of FSW induces a complex plastic deformation in the surrounding material that varies as a function of welding tool geometry, weld speed, and tool rotation rate. This material flow has been examined experimentally by tracking the flow of embedded tracer materials[11–19] and by examining the material distribution in welds of dissimilar materials.[16,20] These studies reveal that the rotating tool produces a circumferential flow and threading on the tool can cause a downward component to this material flow, which can induce a counterflow extrusion toward the top of the weld. Of particular interest have been the studies employing stop-action weld techniques,[11,17–23] in which the welding process is stopped and the weld tool is either retracted or left embedded in the workpiece to preserve the material surrounding the tool. Such studies allow an indepth examination of the microstructural development occurring around the tool, from the initial deformation of the base material ahead of the welding tool to the deposition of that material behind the tool. The current article is a R.W. FONDA, Metallurgist, is with the Naval Research Laboratory, Washington, DC 20375. Contact e-mail: [email protected] J.F. BINGERT, Technical Staff Member, is with Los Alamos National Laboratory, Los Alamos, NM 87545. Manuscript submitted November 16, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
companion to one such study[21] and is intended to compare the characteristics that can be determined in a conventional transverse cross section of a deposited weld to those that can be observed around the welding tool. In addition, this article will discuss details of the material flow, texture evolution, and grain refinement mechanisms in light of the prior observations made around the tool. The 2195 alloy was selected primarily because of its strong initial texture and resistance to recrystallization. This alloy can contain a wide variety of precipitates, depending upon heat treatment conditions, including Guinier–Preston (GP) zones, u0, u9, u, d9, b9 (or a9), T, T1, TB, and T2,[24–27] and possibly V and S9
Data Loading...
