drawing of marker insert placement. The upper drawing is a top view,
and the lower is a view from the side. The dark blue, green and orange
markers are on the retreating side, the light blue, yellow and red
markers are on the advancing side.
Representative Images of the Markers
Schematic drawing of marker insert placement. The upper drawing is a top view, and the lower is a view from the side. The dark blue, green and orange markers are on the retreating side, the light blue, yellow and red markers are on the advancing side.Inserts were placed on the advancing and retreating side, respectively, at three different heights: top, middle and bottom. Inserts at the same height were staggered so that mutual mixing could be eliminated. The length of the markers was such that some of the marker, remote from the weld line, extended beyond the TMZ and was not deformed by the welding process. Markers were placed by milling narrow slots into the faying surface and press-fitting the markers into the slots. For the markers that were placed in the middle third of the plate, a 2195 shim of similar size to the marker was press fit into the upper third of the plate above the marker. Marker flow was elucidated by milling off successive slices, 0.25 mm thick, from the top surface of the weld. After each cut the surface was etched with Keller's reagent and digital images of the region surrounding each marker were obtained. Due to the etching process the base material (which contains copper) turns dark whereas the 5454 stays shiny. The pixel positions of the 5454 markers were extracted from the digital images of each marker (a total of six) at each level (up to 32 per weld). Combining the data from all six markers, a three-dimensional flow visualization similar to a CT scan was obtained. The 3-D plot gives the final position of an originally thin layer that was perpendicular to the welding direction.
positions at the top (left image) and at the middle (right image) of
Vertical Mixing and Full 3-D View
Detected marker positions at the top (left image) and at the middle (right image) of the weld.In the black and white images, the 5454 markers of advancing and retreating side are shown at two representative sections in the weld height; at the top on the left and at the middle at the right side. The pictures of advancing and retreating side markers have been combined to single images, in which the advancing side is the left hand side in both images The images show the final position of the markers in a cold weld, which was performed at low rotational and medium translational speed. Material from the retreating side was transported along with the rotating tool to the advancing side and parts of it even to a position in front of its original position. At the very top of the weld (left image), advancing side marker movement was only in welding direction. No advancing side marker was moved to a position behind its original position closed to the shoulder. However, the image of a marker at one height does not give any details about the vertical flow that is examined later in this paper. The right image shows advancing and retreating side markers at the middle of the weld thickness. Advancing side material in the middle of the weld thickness was mainly transported backwards, dissimilar to the material movement at top levels. The connection of advancing and retreating side behind the pin is not necessarily located at the centerline. Note that material transport backwards is smaller at the top than at the middle of the weld height.
welding direction. The Markers cover the total weld height and
width. During welding the markers are mixed vertically (right
image). 3-D plot of the
markers after welding. View from behind in welding direction. Tool
rotation is clockwise.
Fluid Mechanics based FSW Process Model
Predicted Material Flow in FSW
Projection in welding direction. The Markers cover the total weld height and width. During welding the markers are mixed vertically (right image).As can be seen, material inside the pin diameter (dashed lines) is pushed downwards on the advancing side and moved upwards on the retreating side. This phenomenon is referred as a vertical circular motion. It is initially caused by the movement at the top where retreating side material is transported to the advancing side due to the rotating tool shoulder. Therefore, advancing side material is pushed downwards in the weld when it passes the pin. On the retreating side the material is moving upwards since flow is restricted in x-y-z-direction by the cold base material, the surrounding material flow and the backing plate. Material at the bottom is involved in the vertical circular motion to a smaller extent.
3-D plot of the markers after welding. View from behind in welding direction. Tool rotation is clockwise.The 3-D plots show a continuos marker after welding. At upper levels material transport is due to the shoulder rotation where material is moved from the retreating side to the advancing side behind the pin. Parts of the material from the top of the advancing side is transported in the welding direction to a final position in front of its original. The backward flow of material from the top of the weld is different from the backward flow of material from lower levels. Material originally from the top did not move as far backwards as material from middle heights. Instead, it is dragged along with the translational motion of the pin sometimes even to positions in front (but still below) its original position. The minimum deformation against the welding direction at the top of the weld occurs where the vertical circular material transport interacts with the material flow at the top caused by the rotating shoulder. The two perpendicular circular motions cause the observed flow pattern. The marker insert technique gives insight in the thermo-mechanically affected zone by reconstruction of the piecewise cut-off welded zone. As a result, full three-dimensional plots of the deformed markers provide a good qualitative characterization of the material flow of friction stir welds. However, this technique does not reconstruct the actual flow path of the material to its final position. The material transport in FSW is a result of the two tool motions - the translational and rotational tool movement. Most material is moved around the pin to final positions behind its original position. No material is transported across the centerline at the leading edge of the pin. Material at the top of the weld is moved with the tool rotation from the retreating to the advancing side. At the top of the advancing side material is transported in welding direction. Both major motions cause vertical mixing within the pin diameter.
around the rotating pin; Right: Final Position after passing the
Left: Streamlines around the rotating pin; Right: Final Position after passing the pinThe streamlines in the flow domain suggest that material, originally within the pin diameter, is transported around the pin only in rotation direction of the pin. No material within the pin diameter is passing the pin on the advancing side whereas all the material is transported around the retreating side. Furthermore, streamlines downstream end up at the same distance to the centerline as they were upstream. Since the flow is laminar the streamlines are not crossing each other. The material flow path past the tool, predicted by the model, is not intuitive. Knowing only the final position of the markers in the weld the flow around both sides of the pin is conceivable as well. However, only if the material within the pin diameter flows around one side, e.g. the retreating side, the experimentally observed flow pattern can be matched. To compare the material flow prediction with the experimentally observed flow pattern, the final position in the weld must be determined. The final position in the weld describes the position of particles relative to each other after welding, which were vertically lined up in the in the path of the tool before welding. If particles are released in the flow field upstream at one line in transverse direction, the relative position of those particles to each other downstream at any time describes the final position in the weld. Both, the original position upstream as well as the final position downstream are plotted. The simulation shows that all of the material within the pin diameter is transported to a position behind its original. The last plot shows the marker at their final position at middle height in a cold weld (blue is from the advancing side and red from the retreating side) as well as the predicted final position in the 2-D simulation. The rotation direction of the pin (black circle) is clockwise. The markers are shown in their original and final position. The 2-D simulation is able to predict the material flow in a cold weld at middle heights. Since the amount of vertical mixing is very small in a cold weld, the material flow is nearly two-dimensional in a sufficient vertical distance to the tool shoulder.
Comparison of predicted (black squares)
and experimentally (red and blue marker) observed flow pattern in
FSW. The markers of advancing (blue) and retreating (red) side are
displayed in the undeformed and deformed stage.
Comparison of predicted (black squares) and experimentally (red and blue marker) observed flow pattern in FSW. The markers of advancing (blue) and retreating (red) side are displayed in the undeformed and deformed stage.The 2-D model cannot predict the effect of the shoulder on the flow field and therefore, the simulation approach is considered only as a step towards a three-dimensional model, representing the real tool geometry. Other fields of studies are the tool-fluid interaction as well as the heat and work input.
Student: Dipl.-Ing. Tilman U. Seidel
Advisor: Dr. Anthony P. Reynolds