D.H. Lammlein PhD Dissertation - Vanderbilt University

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In demonstrating the effectiveness of FSW on small diameter hemispheres, this experiment has shown that FSW is applicable to highly curved surfaces and surfaces with curvature in two degrees of freedom like a sphere. The demonstration of FSW on this particularly difficult geometry extends the range of FSW application. This has applications to things like aerospace structures and pressure vessels. Although the small diameter butted hemisphere joint presents a FSW case with many intrinsic difficulties, in one way it is possibly less difficult than it could be. The sphere is curved equally in two degrees of freedom allowing it to mate ideally with a cupped tool even as the cupped tool is rotated about the long axis of the tool as in welding. This is not the case with a pipe where there is no tool shoulder geometry which maintains this ideal tool to work mate when the tool is rotated. The pipe therefore presents another important FSW situation with inherent difficulties. In this work it was demonstrated that small diameter pipe sections can be joined effectively with a simple FSW tool. The scrolled shoulder and threaded probe tool was used to make full penetration welds on 4.2” diameter and wall 0.2” thickness pipes. The welds were exceptional superficially and achieved tensile strengths in excess of 70% of the parent material over the limited parameter range tested. The work done here extends the use of FSW to small diameter pipe with a wide range of application such as in the petroleum industry. Finally, it was shown that the FSW is well understood in that equations governing the process are refined to the extent that they can reliably produce what is seen experimentally. CFD models utilizing process governing equations of limited complexity can be used to reliably model and predict the relevant aspects of the FSW process. This was demonstrated over a wide variety of FSW work geometries including t-joints, butted plates, butted hemispheres, and butted pipes. Additionally, a variety of tool geometries were tested including conically shaped FSW tools, tools with scrolled shoulders, and tools with threaded probes. It was demonstrated that the thermal contours of the tool and work can be reliably produced in CFD. It was also demonstrated the flow field in the work around the FSW tool can be reliably shown in CFD models. The FSW process forces were also predicted using CFD with a reasonable degree of accuracy. These process equations and CFD models are valuable because the experimental evaluation of a 208
new FSW process environment is resource intensive. CFD models provide a quick and effective means of evaluating an FSW process situation prior to making an actual weld. Future Work The cylindrical and spherical weld geometries presented required a circular weld path. The circular weld path resulted in a secondary heating effect which was exacerbated by the small radius of the work and correspondingly reduced conduction lengths within the work. The sum of these effects resulted in a significant increase in temperature throughout each experimental weld. This steady increase in temperature during the weld cycle is undesirable as the properties of the welded material will vary according to temperature around the circumference of the weld. It is desirable to reach a welding temperature at some point early in the weld and maintain this temperature throughout the weld. This can be achieved by either a reduction in the rotational velocity of the tool, an increase in the traverse rate of the tool, or by the implementation of a heat dissipation system. A reduction in tool rotational velocity or an increase in traverse rate will generally result in a reduction in the power input at the weld seam and the rate of heat dissipation into the tool and work. If a particular FSW process and environment is well characterized, then a predetermined reduction in weld power can be programmed into the weld cycle. This solution is simple and effective but requires prior knowledge of how the FSW process environment will behave thermally. A thermal (or temperature) based feedback control system could alternatively be implemented which adjusts weld parameters in real time in response to a measured temperature exceeding the desired value. This sort of system would require an instrument which provides real time temperature data to the weld controller. One example of this would be a thermal camera calibrated to the emissivity of the tool shank. The thermal camera computer could then send live temperature data from the region of interest on the tool shank to the weld controller. Another example would be a thermocouple in the tool or in the work. 209
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FRICTION STIR WELDING OF SPHERES, C
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TABLE OF CONTENTS TABLE OF FIGURES
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Appendix ..........................
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Figure 16: Left(FE streamline), Mid
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Figure 51: A thermal camera image t
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for a similar conventional weld (3/
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Figure 102: Tapered retraction proc
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setting are indicated. Note that st
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Unsupported welds of this kind with
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Figure 157: A thermal camera image
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ottom sample is a tungsten inert ga
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N - newton (force unit) n - visco-p
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INTRODUCTION Friction Stir Welding,
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educed conduction lengths. In small
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CHAPTER I LITERATURE REVIEW: COMPUT
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frictional condition at the interfa
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contact. The contact area can inclu
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For horizontal sections, a heat inp
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(1.11) where r p is the probe (pin)
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This efficiency term, η pd , is kn
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The heat input on the tool surface
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The added complication of these met
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Figure 5: Lateral cross sections of
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Flow stress in aluminum alloys is d
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Figure 8: Three flow fields a) rota
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and post-weld cool-down. This model
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Williams et al. [60] use a 2D-axisy
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Figure 16: Left(FE streamline), Mid
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Figure 18: Temperature contour over
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Figure 22: Temperature distribution
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Figure 24: Lateral velocity contour
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Figure 26: Experimental(white) vers
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Figure 28: Top view schematic of tu
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[5] Taylor RE, Groot H, Goerz T, Fe
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[41] Heinz B., Skrotzki B., Eggler
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CHAPTER II COMPUTATIONAL ANALYSIS O
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Figure 30: Blind t-joint setup with
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Figure 33: Process forces and later
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Figure 35: Experimental Welds, Axia
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Figure 37: FEA boundary conditions
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A 4500 N axial force is applied eve
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axial force with lateral offset dur
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Figure 42: Contour plot of von Mise
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Figure 44: Deformation contour for
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Figure 46: Quarter-plate model for
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Figure 49: Contour plot of deflecti
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where P is the weld power (W), Q is
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Figure 52: Fluent CFD model zones.
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Figure 55: Thermal contour of weld
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Figure 59: Thermal contour of weld
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Figure 61: Velocity vectors for 0.0
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Figure 65: Contour of velocity magn
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CHAPTER III THE APPLICATION OF SHOU
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Figure 66: A shoulder-less, conical
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Figure 68: Typical axial(Z-axis) fo
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Figure 71: 90° conical tool macros
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Figure 75: Run 3, 80° conical tool
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Figure 79: (90º tool) Ultimate ten
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Figure 83: (90º tool) Lateral forc
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Figure 87: (80º tool) Lateral forc
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The Eulerian, finite volume, CFD so
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Figure 92 shows the increasing elem
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Figure 91: CFD model geometry consi
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Figure 95: Lateral cross-section of
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Figure 96: Attempts a probe tapered
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CHAPTER IV THE FRICTION STIR WELDIN
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applications for its high strength
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Figure 97: Experimental weld sample
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Figure 99: Split protrusion type de
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with a cupped recess. It is however
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of tapered retraction (i.e. move th
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The adjustments in vertical positio
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manners the assigned depth was main
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Figure 108: Lateral macrosections f
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Figure 110: (Supported, cupped tool
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apparent strength of 26% parent whi
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with a 100º conical tool (0.025”
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Figure 119: Macrosection view of an
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and tooling. Torque is highly depen
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locally in proportion to the local
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Figure 126: (threaded tool) Contour
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Figure 128: (conical tool) Geometry
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Figure 131: (threaded tool) Lateral
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The results presented here show tha
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Figure 136:(from left: Tapered retr
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[8] Metallurgical analysis of ablat
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CHAPTER V FRICTION STIR WELDING OF
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Collaboration between Advanced Meta
Page 181 and 182: eccentricity, the method of interio
Page 183 and 184: gas tungsten arc welding of small d
Page 185 and 186: Full penetration welds of 4.2” (1
Page 187 and 188: Figure 143: The curvature of the pi
Page 189 and 190: Figure 144: Experimental axial forc
Page 191 and 192: Together, a fine wall thickness tol
Page 193 and 194: Figure 147: Axial force history for
Page 195 and 196: The weld macrosections show complet
Page 197 and 198: Figure 154: Macrosections of welds
Page 199 and 200: Figure 157: A thermal camera image
Page 201 and 202: Figure 159: Average tool shank temp
Page 203 and 204: 471,146 faces. The meshes are fine
Page 205 and 206: Figure 162: A closeup view of mesh
Page 207 and 208: 4 2 3 4 5 16 " 1.745 2 0.24
Page 209 and 210: applied heat locally in proportion
Page 211 and 212: Figure 164: Modeled temperature con
Page 217 and 218: Figure 172: Model pathlines for the
Page 219 and 220: Figure 174: Model pathlines for the
Page 221 and 222: later speed setting, the tool welds
Page 223 and 224: Figure 178: Zoom view of a macro ta
Page 225 and 226: Figure 180: The superficial appeara
Page 227 and 228: Figure 182: The rotary welding appa
Page 229 and 230: Figure 185: A diagram illustrating
Page 231: CONCLUSION AND FUTURE WORK Conclusi
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D.H. Lammlein PhD Dissertation - Vanderbilt University

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