Friction stir welding (FSW) is a solid state welding process invented in 1991 at The Welding Institute in the United Kingdom. A weld is made in the FSW process by translating a rotating pin along a weld seam so as to stir the sides of the seam together. FSW avoids deleterious effects inherent in melting and is already an important welding process for the aerospace industry, where welds of optimal quality are demanded. The structure of welds determines weld properties. The structure of friction stir welds is determined by the flow field in the weld metal in the vicinity of the weld tool. A simple kinematic model of the FSW flow field developed at Marshall Space Flight Center, which enables the basic features of FSW microstructure to be understood and related to weld process parameters and tool design, is explained

Nunes, Arthur C., Jr.

NASA Technical Reports Server

Abstract 
For Presentation at 
The University of Alabama in Huntsville 
October 17, 2008 
A Brief Introduction to the Theory of Friction Stir Welding 
By Arthur C. Nunes, ]r 
Marshall Space Flight Center 
Materials and Processes Laboratory, EM30 
Huntsville, AL 35812 
Friction stir welding CFSW) is a solid state welding process invented in 1991 at The 
Welding Institute in the United Kingdom. A weld is made in the FSW process by 
translating a rotating pin along a weld seam so as to stir the sides of the seam 
together. FSW avoids deleterious effects inherent in melting and is already an 
important welding process for the aerospace industry, where welds of optimal 
quality are demanded. The structure of welds determines weld properties. The 
structure of friction stir welds is determined by the flow field in the weld metal in 
the vicinity of the weld tool. A simple kinematic model of the FSW flow field 
developed at Marshall Space Flight Center, which enables the basic features of FSW 
microstructure to be understood and related to weld process parameters and tool 
design, is explained. 
https://ntrs.nasa.gov/search.jsp?R=20110006308 2019-08-30T14:34:41+00:00Z
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A Brief Introduction 
to the Theory of 
Friction Stir Welding 
Arthur C. Nunes, Jr. 
Metals Engineering Branch 
Marshall Space Flight Center 
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Friction stir welding 
• Is a solid-state seam welding process. Melting does not normally 
occur. Welding occurs automatically upon placing clean metal in 
contact. 
• Was invented in 1991 at The Welding Institute, Cambridge, 
England (GB Patent Application No. 9125978.8, 6 December 
1991). 
• Avoids melting problems inherent in fusion welding: loss of 
strengthening structure in residual cast weld metal component; 
disturbance of dispersed strengthening phases (MMCs); safety 
issues (heat sources, spatter, molten metal); precision issues (wide 
fusion penetration variation due to Marangoni circulations); 
contamination issues (metal vapor may coat optics); etc. 
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Basic friction stir welding configuration 
Longitudinal section courtesy of r.e. McClurefUniversity of Texas at El Paso 
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------- --- -
Plan view mid-sectional macrostructure of FSW in 0.317 inch thick 2219-
T87 aluminum alloy plate. The pin-tool has been removed and replaced with 
bubble-filled mounting medium. Rotation is in the counterclockwise 
direction. The spindle speed was 220 RPM and the travel speed 3.5 inches 
per minute. 
~~~--. - ----- ~~--.-
Shear surface, rotating plug model, 
and wiping transfer mechanism 
• Shear occurs at a "discontinuity" in velocity 
between a plug of metal attached to the pin-tool 
and the weld metal bulk. 
• The forward part of the moving shear surface picks up 
weld metal, which is rotated (wiped) around the tool in 
the rotating plug, and left behind as the tool moves on. 
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Mathematical implementation 
of rotating plug model 
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Velocity field of metal in rotating plug (relative to pin;tool) is taken to be a superposition 
of two fields: 1) rotating cylinder and 2) uniform translation . 
r dr r d8 r r . r 
v = - ur + r - ue = (v r - V cos8)u r + (reo + V sm8)ue dt dt 
Trajectory of weld metal element inside plug is then: 
v 1 r -cos8 dr = rI V d8 
rw . 8 
- +sm 
V 
If v r « V and rw » V , 
dr "" _ V dsin8 
w 
In this case the maximum depth is & max "" - 2V on the retreating edge. If the shear 
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surface radius is constant, there is no lateral (y) displacement at exit from the tool. 
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Two-dimensional planar "wiping" flow streamlines for a FSW flow field . The width of 
the pattern has been greatly exaggerated to reveal the structure more clearly. In the 
typical FSW case the flow around the tool takes place within a thin sliver of rotating 
metal just beneath the shear surface 
----. -- - ------
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f 
Shear surface 
~ 0.008" 
Recrystallized 
weld metal 
(N ote banding) 
Retreating edge 
Weld metal backflow 
~.. Parent metal 
Advancing edge 
V [3.5in/min] 
2- = 2 ::::.:: 0.005" 
Q [2nrad I rev ][ 220rev I min] 
Transverse 
Marker Longitudinal 
Marker 
Bimetallic 
Weld 
PLANAR MARKER PATTERNS DUE TO 
TRANSLATING, ROTATING FIELD 
----_._-- -- - - --

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Shear Zone 
Shear strain rate 
in friction stir welding 
is comparable to that of 
metal cutting (103 to 105 sec-I). 
(RQL\t) [0.19inches J[2n rad . 220RPM . _1 min] ~ dy = L\y = 0 = RQ ~ rev 60 sec ~ 0.8xl04 sec-I 
dt L\t (M) 0 [O.0005inchesJ 
Comparison of friction stir welding 
and metal cutting 
Flow Trajectory 
Friction Stir Welding 
Shear Interface 
I 
Low-cost manufacturing process 
for nanostructured metals and alloys 
Travis L. Brown, Srinivasan Swaminathan, Srinivasan Chandrasekar, 
w. Dale Compton, Alexander H. King, and Kevin P. Trumble 
Schools of Engineering, Purdue University, West Lafayette, Indiana 
47907-1287 
Journal of Materials Research 17 (10) (2002). 2484-2482 
In spite of their interesting properties, nanostructured materials have 
found limited uses because of the cost of preparation and the limited 
range of materials that can be synthesized. It has been shown that most of 
these limitations can be overcome by subjecting a material to large-scale 
deformation, as occurs during common machining operations. The chips 
produced during lathe machining of a variety of pure metals, steels, and 
other alloys are shown to be nanostructured with grain (crystal) sizes 
between 100 and 800 nm. The hardness of the chips is found to be 
significantly greater than that of the bulk material. 
Typical friction stir shear produced grains ~ 10,000 nm. 

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Viscous vs . plastic flow models 
• Plastic metal flow is less sensitive to strain rate. 
• Metals are subject to shear instabilities (inhomogeneities) 
as denoted on processing maps. 
Local rearrangement mechanism (viscous flow) 
( E 
1: "" l kTe kT \ . 
nvl1y t 
k = Boltzmann's Constant 
T = Absolute temperature 
E = Activation energy of process 
l1y = Shear strain per unit activation 
n = Number of activation elements per unit volume 
v = Oscillation rate of activation element 
Dislocation slip mechanism (plastic flow) 
1: "" E _ kT In( nl1:v) 
v v 2y 
k = Boltzmann's Constant 
T = Absolute temperature 
E = Activation energy of process 
v = Activation volume for process 
l1y = Shear strain per unit activation 
n = Number of activation elements per unit volume 
v = Oscillation rate of activation element 
A.P. Reynolds and Wei Tang, " Alloy, Tool Geometry, and Process Parameter Effects on Friction Stir 
Weld Energies and Resultant FSW Joint Properties," Friction Stir Welding and Processing, ed. K.Y. 
Jata et ai. (Warrendale, PA: The Minerals, Metals & Materials Society, 2001) 15-23. 
Tool Rs R L 
Configuration (mm) (mm) (mm) 
1 12.5 5 8.1 
2 12.5 4 8.1 
3 12.5 6 8.1 
FSW Pin-Tool 
4 10.0 5 8.1 Shear Zone f---- Rs 
5 15.0 5 8.1 
3 {I h M = 2:rtR -c - + - + 
3 R (L~hr +(R ,~RrHR';RH(R,;Rn) 
R 
~ = 1 _ -=---_ =L 
L [l+~(R s;R)l 
L~_~ 
Table I. Effect of Friction Stir Welding Tool Configuration on Power Requirement. 
Percent error in the comouted value in oarenthesis next to com oared val 
Tool Power Requirement 
Configuration (watts) 
240 RPMl1.3 mm/sec 240 RPM/2.4 mm/sec 390 RPMl3.3mm/sec 
1: = 2.01 ksi* 1: = 2.18 ksi* 1: = 1.75 ksi* 
Measured Computed Measured Computed Measured Computed 
I 2060 1870 (-9%) 2230 2030 (-9%) 2790 2660 (-5%) 
2 1950 1670 (- 14%) 2100 1840 (- 12%) 2790 2400 (- 14%) 
3 2270 2050 (- 10%) 2390 2230 (-7%) 2860 2910 (+2%) 
4 1110 11 80 (+6%) 1260 1280 (+2%) 2290 1670 (-27%) 
5 23 10 29 10 (+26%) 2560 3160 (+23%) 3040 4130 (+36%) 
--
* Chosen for optimal data fit. 
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Fusion Welding 
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,...., Melting 
Temperature 
at Melt Pool 
Surface 
Friction Stir Welding 
,...., 0.8 Melting 
Temperatu re at 
Shear Surface 
Heat Losses of Fusion Welds and Friction Stir Welds Are Similar 
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Courtesy of R. CarteriMSFC Metals Engineering Branch 
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Minimization of torque (assuming constant t) 
() f Sheariog2nr2-r;,h + r2dz = 0 
Surface 
:. z = aF( (jl \ a) 'P = cos-,(: ) 
PIN 
ISOTHERMAL SHEAR SURFACE 
FOR MINIMAL TORQUE 
a = 45° 
The computed shear sunace shape (solid line) yielding minimal torque/power for 
isothermal/constant flow stress FSW tool environment. F( cp/a) is an elliptic integral of 
the first kind. A simplified linear approximation (dashed line) is also shown. 

--
Fragmentation 
(T 
Expanded 
- Radius 
-
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----- - - -~-- -
Scatter 
-
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Shear 
.... ~ Surface 
Oscillation 
+4-- Reduced 
Radius 
-

Advancing 
Side 
Ring Vortex 
Flow 
Component 
t 
( 
Shear Surface 
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Retreating 
Side 
Radial Flow 
Component 
(of Ring 
Vortex) 
SEAM TRACE 
(as def~rmed in rotating plug) 
Advancing Side ~ Retreating Side 
- - ------- - -------------------
Displacement of Weld 
Seam Trace due to 
Retention in Rotating 
Plug by Inward Radial 
Flow 
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Defect causes 
• Parameters (rpm, weld speed, plunge force): 
flash, trenches, surface fissures, wormholes 
• Joint preparation (e.g. , seam cleanliness): 
residual oxide defect 
• Tool design (e.g. , pin bottom-anvil clearance): 
lack of penetration 
- ---- ----------
L __ _ 
---------------- ----- --- -
CONCLUSION: 
Control friction stir welds 
using a kinematic model 
• Weld structure controls weld properties. 
• Weld flow field controls weld structure. 
• Specific aspects of weld process parameters and 
tool design control components of weld flow field 
(in kinematic model). 
• We control weld process parameters and tool 
design. 


A Brief Introduction to the Theory of Friction Stir Welding

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