Visualization of the Material Flow in AA2195 Friction-Stir
Welds Using a Marker Insert Technique
T.U. SEIDEL and A.P. REYNOLDS
The material flow in solid-state, friction-stir, butt-welded AA2195-T8 was investigated using a marker
insert technique (MIT). Markers made of AA5454-H32 were embedded in the path of the rotating
friction stir welding (FSW) tool and their final position after welding was detected by metallographic
means. Changes in material flow due to welding parameter and tool geometry variations were examined.
The method provides a semiquantitative, three-dimensional view of the material transport in the
welded zone. Because of the placement of markers at different positions at the weld centerline, the
material transport in the longitudinal, transverse, and the vertical directions could be studied. Markers
embedded in the path of the tool remain continuous after welding. The material transport, which is
not symmetrical about the weld centerline, was such that the bulk of the material was transported to
a position behind its original position. Superimposed on the primary motion of material in the horizontal
plane of the weld is a circulation about the longitudinal axis of the weld. This circulation is found
to increase with increasing weld energy.
I. INTRODUCTION
The weld nugget is the region that has undergone the most
severe plastic deformation and is characterized by a fine,
relatively equiaxed, recrystallized grain structure. The width
of the nugget is normally similar to, but slightly greater
than, the diameter of the pin. Outside of the nugget on either
side is a second region, one that has been deformed to a
lesser extent and that, depending on the alloy, may or may not
show signs of recrystallization. In Figure 1, the delineation
between the nugget and the rest of the TMAZ is quite sharp
because of the recrystallization resistance of the AA2195-
T8 base metal. The deformation of the base metal grains
manifests itself as bending in the plane of the metallographic
section (the grains are also bent in the horizontal plane
perpendicular to the section). The third region of the TMAZ
is the “flow arm.” This is the region of material above the
nugget. The flow arm is formed when the rotating tool
shoulder passes over the weld. It should be noted that the
FSW process is not symmetric about the centerline: the
advancing side of the weld is defined as the side on which
the rotational velocity vector of the welding tool has the
same sense as the translational velocity vector of the tool
relative to the workpiece. The retreating side is where the
two vectors are of opposite senses. The leading side is the
front of the tool and the trailing side indicates the back of
the tool. The crown is the top surface of the weld and the
root is the bottom surface.
F
RICTION Stir Welding (FSW), developed at The
Welding Institute (Cambridge, UK) in 1991,[1] is especially
well suited for joining high-strength aluminum alloys. The
FSW process is a solid-state joining process combining fric-
tion, deformation heating, and mechanical work to obtain
high-quality, defect-free joints. The required heat is produced
during transport of material from the two plates to be joined
around a nonconsumable, rotating tool. The shape of the
tool promotes a high hydrostatic pressure along the joint
line, causing consolidation of the material plasticized due
to heat generation. Although significant effort has been
expended in putting FSW to use in the full-scale production
of such products as ferry boats, rocket fuel, and oxidizer
tanks, the operating mechanisms and, in particular, the mate-
rial flow during FSW are not fully characterized.
The microstructure of a typical FSW has been described
in numerous previous publications[2–6] but will be briefly
reviewed here. Figure 1 shows the microstructure resulting
from the FSW of AA2195-T8. The weld microstructure
features can be separated into two broad categories: the
thermomechanically affected zone (TMAZ), and the heat-
affected zone (HAZ). This HAZ is similar to HAZs resulting
from conventional, fusion welding processes. Depending on
the alloy, its initial heat treatment, and the proximity to the
weld centerline, processes occurring in the FSW HAZ might
include precipitate coarsening, precipitate dissolution, recov-
ery, recrystallization, and grain growth. The TMAZ of a
friction stir weld might be considered analogous to the fusion
zone of a conventional weld except that, instead of being
melted, the material in the TMAZ has been mechanically
worked.
II. BACKGROUND—FSW FLOW
VISUALIZATION
Several friction stir weld flow visualization studies have
been conducted. Midling[6] investigated the influence of the
welding speed on the material flow in welds of dissimilar
aluminum alloys. He was the first to report on interface
shapes using images of the microstructure. The flow visual-
ization, however, was limited because no other detail except
the interface between dissimilar alloys was investigated. Li
et al.[7] described patterns observed on metallographic cross
sections in friction stir welds made both between dissimilar
aluminum alloys and between aluminum alloys and copper.
Within the TMAZ, there are three somewhat distinct
regions. The first and most obvious is the “weld nugget.”
T.U. SEIDEL, Graduate Research Assistant, and A.P. REYNOLDS,
Associate Professor, are with the Department of Mechanical Engineering,
University of South Carolina, Columbia, SC 29208.
Manuscript submitted February 1, 2001.
METALLURGICAL AND MATERIALS TRANSACTIONS A
VOLUME 32A, NOVEMBER 2001—2879