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i

NASA/TM—2012–217474

Effects of Fusion Tack Welds on Self-Reacting

Friction Stir Welds

A.C. Nunes, Jr., M.L. Pendleton, S.A. Brooke, and C.K. Russell

Marshall Space Flight Center, Huntsville, Alabama

November 2012

National Aeronautics and

Space Administration

Marshall Space Flight Center • Huntsville, Alabama 35812
ii

Available from:

NASA Center for AeroSpace Information

7115 Standard Drive

Hanover, MD 21076 –1320

443–757–5802

This report is also available in electronic form at

<https://www2.sti.nasa.gov/login/wt/>

TRADEMARKS

Trade names and trademarks are used in this report for identification only. This usage does not constitute an official

endorsement, either expressed or implied, by the National Aeronautics and Space Administration.
iii

TABLE OF CONTENTS

1. INTRODUCTION ............................................................................................................. 1

2. PROCEDURE ................................................................................................................................. 3

2.1 Panels ...................................................................................................................................... 3

2.2 Gas Tungsten Arc Tack Weld Surface Preparation ................................................................. 3

2.3 Gas Tungsten Arc Tack Weld Parameters ............................................................................... 3

2.4 Gas Tungsten Arc Tack Weld Panel Configuration ........................................................ 4

2.5 Delay Intervals ........................................................................................................................ 5

2.6 Friction Stir Welding Parameters ............................................................................................ 5

2.7 Test Matrix .............................................................................................................................. 5

2.8 Mechanical Testing Procedures .............................................................................................. 5

2.9 Nondestructive Evaluation Procedures ................................................................................... 5

2.10 Metallographic Procedures .......................................................................................... 5

3. ANALYSIS OF DATA ........................................................................................................ 10

3.1 A Measure of Tack Weld Effect ....................................................................... 10

3.2 Results .......................................................................................................................... 14

3.3 Interpretation of Results ............................................................................... 16

4. CONCLUSIONS ................................................................................................................ 19

APPENDIX—FRACTURE STRESSES AND LOCATIONS ............................................... 21
iv
v

LIST OF FIGURES

1. GTA tack weld panel configuration (dimensions in inches) ......................................... 4

2. Location of tensile test coupons on panel (dimensions in inches) ................................ 7

3. Placement of metallographic specimens on weld panels 

(dimensions in inches) .................................................................................................. 7

4. Transverse section of panel FTA03 (0.327 inches thick panel, 

cold weld parameters, prep/tack/FSW with no delay, no seam offset) 

at the M1 (incorporating a tack weld) position............................................................. 8

5. Transverse section of panel FTA03 (0.327 inches thick panel, cold weld 

parameters, prep/tack/FSW with no delay, no seam offset) at the M2 

(not incorporating a tack weld) position....................................................................... 8

6. Idealized determination of tack weld effect ................................................................. 11
vi

LIST OF TABLES

1. Parameters for GTA tack welds ................................................................................... 4

2. Nominal FSW parameters ........................................................................................... 4

3. Test matrix ................................................................................................................... 6

4. Analytical results of coupon tests ................................................................................ 15

5. Fracture stresses (ksi) and fracture location (retreating side = R, 

nugget = N, and advancing side = A) ........................................................................... 21
vii

LIST OF ACRONYMS AND DESIGNATORS

DCEN direct current electrode negative (arc welding mode)

FSW friction stir welding

FTA01, FTA02, etc. test panel designations (0.327 in thick)

GTA gas tungsten arc

GTAW gas tungsten arc welding

GTA01, GTA02, etc. test panel designations (0.257 in thick)

SRFSW self-reacting friction stir welding
viii
ix

NOMENCLATURE

a position independent constant in linear representation of nontack-containing coupon 

fracture stress within a given panel

aT position independent constant in linear representation of tack-containing coupon 

fracture stress within a given panel

b position slope constant in linear representation of nontack-containing coupon fracture 

stress within a given panel

bT position slope constant in linear representation of tack-containing coupon fracture 

stress within a given panel

N number of data items in a sample

X mean of sample measurements

x position from end of panel where friction stir weld begins

xi mean position from end of panel where friction stir weld begins of ith fracture test 

coupon in a given panel; test measurement datum

DsT interpolated difference between fracture stress of tack-containing and nontackcontaining fracture test coupons in a given panel, i.e., sT–s, measure of the effect 

of a tack weld on a given panel

DsT/s interpolated difference between fracture stress of tack-containing and nontackcontaining fracture test coupons in a given panel, a measure of the effect of a tack 

weld on a given panel, normalized with respect to interpolated fracture stress 

for both test coupons not containing a tack weld in a given panel

e sum of squares of errors, differences between estimated measure and actual measure

m mean of population of measurements

s	 interpolated fracture stress for both test coupons not containing a tack weld in a given 

panel; standard deviation of population of measurements

si fracture stress of ith fracture test coupon in a given panel

sT interpolated fracture stress for all four test coupons containing a tack weld in a given 

panel
x
1

TECHNICAL MEMORANDUM

EFFECTS OF FUSION TACK WELDS ON SELF-REACTING FRICTION STIR WELDS

1. INTRODUCTION

This Technical Memorandum is based on a report submitted to the Metals Joining and Processes Branch, Materials and Processes Laboratory, Marshall Space Flight Center, July 29, 2011, 

by M.L. Pendleton, Professional Intern Program, and S.A. Brookes entitled “A Study of the Effects 

of Autogenous Gas Tungsten Fusion Tacking of Self-Reacting Friction Stir Welds.” 

It is standard welding practice to join the edges of a weld seam at an array of points along 

the seam prior to welding to prevent slippage and misalignment during welding. The initially joined 

points are called ‘tack welds.’ Tack welds are not necessarily produced by the same welding process 

as welding the seam.

The friction stir welding (FSW) process was originally introduced at Marshall Space Flight 

Center for fabrication of a lightweight version of the space shuttle external tank out of a new alloy 

that proved difficult to weld by fusion processes. FSW is a solid state process in which a rotating 

pin is inserted into the weld seam and literally stirs the sides of the seam together as it is translated 

along the seam. The initial version of the FSW process required a large axial compression force 

on the tool, which had to be balanced by a heavy anvil incorporated into the weld fixture. A later 

version of the FSW process eliminated this large axial force by splitting the tool in two and pulling crown and root shoulders together so as to balance the axial compression within the tool and 

substitute a squeeze for an unbalanced axial force. This was the self-reacting friction stir welding 

(SRFSW) process, now in common use for welding light alloy aerospace structures.

The SRFSW process has not yet been adapted to the production of tack welds. Tack welds 

can currently be made by the old FSW process, which requires heavy fixtures, or by fusion welding, 

which does not require heavy fixtures.

The belief that the tack weld will be swallowed up by the FSW process with no effect upon 

the resulting weld led to the idea to use the much more convenient fusion tack weld process rather 

than the old FSW process. But caution required verification of this belief. Tack welds do not usually penetrate fully through the seam; they are only supposed to hold the seam together under 

limited loading, and if this purpose is fulfilled, the smaller the weld, the better. It was thought 

conceivable that the residual parts of the weld seam near enough to the tack weld to reach a high 

temperature but not incorporated into the tack weld might be oxidized to an extent that the seam 

trace would be weakened (residual oxide defect) and the weld strength reduced.
2

The object of the present study was to see if a tack weld effect could be detected by comparison of the strengths of weld segments cut from fusion tack welded regions with segments cut 

from outside the tack welds. An array of different autogenous (i.e., no filler-wire supplement) gas 

tungsten arc (GTA) tack weld schedules and different FSW process conditions (panel thickness, 

time elapsed after cleaning, pin-tool offset from the seam) were evaluated to obtain a comprehensive grasp of the situation. Nondestructive evaluation (NDE) and metallographic microstructural 

observations supplemented measurements of weld strengths.

Upon completion of this study, no effect of a fusion tack weld on the strength of a friction 

stir-welded seam weld was detected.
3

2. PROCEDURE

2.1 Panels

The welds in this study were made on pairs of 6-in × 24-in 2195 aluminum alloy panels in 

the T8M4 condition. Thicknesses were either 0.257 or 0.327 in. The panels were welded by the 

SRFSW process along the long side to produce panels 12 in × 24 in with a weld down the center 

to be evaluated.

2.2 Gas Tungsten Arc Tack Weld Surface Preparation

The top and bottom surfaces adjacent to the weld seam were cleaned with Scotch-BriteTM 

or a wire brush to a distance at least 1.5 in from the seam. The surface was wiped with acetone 

(followed by isopropyl alcohol when using Scotch-Brite). The abutting surfaces were draw filed 

or scraped. The panels were handled throughout with clean gloves to avoid contamination with 

skin oils. During storage the panels were wrapped in brown paper and kept free of dust and 

contamination.

2.3 Gas Tungsten Arc Tack Weld Parameters

Hot, nominal, and cold tack weld parameters were defined based upon the penetration 

depth of the tack weld into the weld seam. A 30% penetration was deemed a hot weld; 20% 

penetration, a nominal weld; and 10% penetration, a cold weld. 

GTA weld parameters necessary to achieve the above penetrations in the experimental 

panels were determined by trial and error. Gas tungsten arc welding (GTAW) as used for tack 

welding is a manual and not an automated process. Constant current power sources are typically 

used for GTAW. The current is set as a parameter, and the voltage (and power) is determined by 

the arc length held by the welder. By observing the weld pool, the welder can estimate the depth of 

penetration, which can be adjusted by lengthening or shortening the arc. Weld speed can also affect 

penetration but is not as sensitive as arc length. In this study, manual tack welding speed is taken 

as roughly constant, about 15 in/min. The GTA parameter determined here amounts to weld current setting. The current settings established are listed in table 1 for the 0.327-in-thick panels and in 

table 2 for the 0.257-in panels. Voltages ranged from 12 to 14 V. These parameters were used to tack 

weld the test panels.
4

Table 1. Parameters for GTA tack welds.

Panel Thickness (in)

0.257 0.327

Current Settings (A)

Hot 120 160

Nominal 113 140

Cold 105 120

Table 2. Nominal FSW parameters.

Panel Thickness 

(in)

Tool Rotational 

Speed (rpm)

Weld Speed

(in/min)

Pinch Force

(lb)

0.257 225 15 2,400

0.327 150 15 4,000

The GTA torch was operated in the direct current electrode negative (DCEN) mode. In the 

DCEN mode, heating action is maximized and there is no reverse polarity surface cleaning effect.

2.4 Gas Tungsten Arc Tack Weld Panel Configuration

The tack welds were applied in the following order:

(1) A 2-in tack weld was applied on each end of the seam to be welded.

(2) A pair of 4-in tack welds separated by three, 4-in untacked intervals filled in the 20-in 

weld segment between the end tack welds. The resultant placement of tack welds on a panel is 

illustrated in figure 1.

0 2

2

6

6

4 4 4

24

4 4 2

6 10 14 18 22 24

Figure 1. GTA tack weld panel configuration (dimensions in inches).
5

2.5 Delay Intervals

Because of the possibility that delay intervals between preparation and tacking or between 

tacking and friction stir welding might give time for detrimental reactions between the weld seam 

and the environment, a delay interval parameter was introduced into the study. One set of panels was prepared, tacked, and friction stir welded the same day (prep/tack/FSW); another set was 

welded with a 5-day delay between preparation and tacking and another 5-day delay between tacking and friction stir welding (prep/5 days/tack/5 days/FSW).

2.6 Friction Stir Weld Welding Parameters

Referring to figure 1 at a distance of 1.5 in, a 0.516-in- (33/64) diameter hole was drilled in 

the panel, either on the weld seam or displaced 0.06 in downwards, i.e., toward the retreating side 

of a clockwise-rotating pin-tool moving to the right so that the pin encounters the seam displaced 

toward its advancing side. Two pin-tool offsets were used in this study: 0 and 0.06 in.

The nominal weld parameters are listed in table 2.

Although the same parameters are used to weld both thicknesses, the machine torque—not 

recorded—will be greater in the case of the thicker panel.

2.7 Test Matrix

Table 3 shows the 24 panels that were tested: 2 thicknesses × 3 tack weld parameters 

× 2 delay intervals × 2 FSW offsets.

2.8 Mechanical Testing Procedures

Welds were evaluated by tensile tests of 1.1-in-wide coupons stressed perpendicular to the 

weld direction. Six coupons from each panel were cut as shown in figure 2 and tested, yielding 

24×6=144 tensile test data tabulated in the appendix. It can be seen in figure 2 that four of the 

coupons on each panel enclose tack welds; two do not.

2.9 Nondestructive Evaluation Procedures

After welding, but prior to cutting test coupons, the panels were inspected using phased 

array ultrasonic techniques. No defects were detected in the welds. The results of ultrasonic testing 

are presented in the Pendleton/Brooke report.

2.10 Metallographic Procedures

Four metallographic samples placed on the panels as shown in figure 3 were cut. Weld transverse sections were ground, polished, and etched and photographs of the weld macrostructure were 

taken.
6

Table 3. Test matrix.

Panel

Thickness

(in)

GTA Tack

Heat Level Delay

Seam Offset

Toward

Advancing

Side 

(in) 

Panel

Designation

0.257 Hot Prep/tack/FSW – GTA01

0.06 GTA04

Prep/5 days/tack/

5 days/FSW

– GTA07

0.06 GTA10

Nominal Prep/tack/FSW – GTA02

0.06 GTA05

Prep/5 days/tack/

5 days/FSW

– GTA08

0.06 GTA11

Cold Prep/tack/FSW – GTA03

0.06 GTA06

Prep/5 days/tack/

5 days/FSW

– GTA09

0.06 GTA12

0.327 Hot Prep/tack/FSW – FTA01

0.06 FTA04

Prep/5 days/tack/

5 days/FSW

– FTA07

0.06 FTA10

Nominal Prep/tack/FSW – FTA02

0.06 FTA05

Prep/5 days/tack/

5 days/FSW

– FTA08

0.06 FTA11

Cold Prep/tack/FSW – FTA03

0.06 FTA06

Prep/5 days/tack/

5 days/FSW

– FTA09

0.06 FTA12
7

24

6

6

0

6.3

7.4

10.5 14.3

15.4

18.5

1

24

2 3 4 5 6

Tack Weld

Friction

Stir

Weld

Figure 2. Location of tensile test coupons on panel (dimensions in inches).

6.3 10.5 14.3 18.5

1 2 3 4 5 6

1 2 3 4

Metallographic Samples

(0.5 in Thick)

Figure 3. Placement of metallographic specimens on weld panels 

(dimensions in inches). 
8

Transverse structures of weld FTA03 (0.327 inches thick panel, cold weld parameters, prep/

tack/FSW with no delay, no seam offset) at the M1 (incorporating a tack weld) and M2 (not incorporating a tack weld) positions are shown in figure 4 and figure 5 respectively.

Figure 4. Transverse section of panel FTA03 (0.327 inches thick panel, cold 

weld parameters, prep/tack/FSW with no delay, no seam offset) at 

the M1 (incorporating a tack weld) position.

Figure 5. Transverse section of panel FTA03 (0.327 inches thick panel, cold 

weld parameters, prep/tack/FSW with no delay, no seam offset) at 

the M2 (not incorporating a tack weld) position.

The weld structure may be interpreted as follows.

Stuck to the friction stir pin and shoulder is a rotating plug of metal separated from the 

nonrotating weld metal by a shear surface with contours selected by nature to allow the tool to 

rotate with minimal torque. For a self-reacting tool, the shear surface takes an hour-glass shape, 

narrow at the pin midpoint and widening toward the shoulders. The width at the shoulder will, in 

general, be less than that of the tool shoulder by the amount of slippage taking place at the outer 

edge of the tool shoulder. Hour-glass asymmetry along the pin axis is associated with temperature 

asymmetry. If the shoulder on the crown side is colder than that on the root side, the crown side 

shear stress should be higher; a higher shear stress makes it easier for slip to occur (in preference 

to shear), and a relatively smaller crown side hour-glass diameter would be anticipated.
9

Weld metal encountering this shear surface is highly deformed at the forward surface so as 

to take a fine-grained structure, rotated to the rear surface, and abandoned to the wake of the weld. 

Thus, the basic structure observed on the transverse weld section is a fine-grained, hour-glass shape 

with top and bottom (root and crown) approaching the width of the shoulder and middle somewhat wider than the pin.

Pin threads and shoulder scrolls induce a gradual rotation in the weld metal flow along 

the tool axis toward the middle of the pin, then outward and around and back toward the pin at 

the tool shoulders. This (ring vortex) flow distorts the hour-glass shape, bulging it out at the middle 

of the pin and retracting it closer to the shoulders.

Eccentricity of the tool pumps metal back and forth along hot, soft-metal channels following the shear surface so as to produce internal textural bands and surface ripples customarily called 

‘tool marks.’ In the transverse weld section of a traditional friction stir weld, these bands appear 

as the ‘onion ring’ pattern. In the transverse weld section of a self-reacting weld, the middle bulge 

of the hour-glass shape is not as pronounced and the bands appear as a series of hyperbolas in the 

crown and root expansions of the hour-glass shape.
10

3. ANALYSIS OF DATA

3.1 A Measure of Tack Weld Effect

An analysis is only as good as the concepts upon which it is based. Here, the concepts on 

which the present analysis of data rests will be made clear, and based on these concepts, a quantitative measure of the effect of a tack weld will be established.

First, weld segments incorporating a tack weld (coupons 1, 2, 4, and 5) and segments not 

incorporating a tack weld (coupons 3 and 6) may be distinguished. Other things being equal, the 

‘effect of a tack weld’ is taken as the difference in fracture strength of a coupon containing a tack 

weld and one not containing a tack weld. If the tack weld is detrimental, this effect is negative.

But other things are not equal, so it will be necessary to attempt to estimate and eliminate the 

effect of ‘other things’ for purposes of extracting the effect of a tack weld.

Within each panel, a monotonic weld strength variation is anticipated due to the gradual heating 

of the panel, a periodic variation due to local heat sink variations as each hold-down clamp is bypassed

and a chaotic variation due to local variations in weld metal past processing and weld fit-up and cleaning. For present purposes, only an attempt to compensate for monotonic variations will be made, and an 

error band due to the other or further overlooked causes of variation will be anticipated. The error band 

will manifest itself as multiple data are collected.

Within a given panel, it is anticipated that the fracture stress, σ, is a function of the position along 

the panel, x. This function will be approximated by the simplest function the data will bear, a straight 

line. Data from tack-containing coupons is labeled with a subscript T.

For the coupons containing a tack weld,

sT =aT+bTx . (1)

For the coupons not containing a tack weld,

s=a+bx . (2)

The effect of the tack weld is ∆sT, such that

∆sT=sT –s =(aT –a)+(bT –b)x . (3)

∆sT captures the effect of tack welds on a single panel including a first-order variation with position. 

This is illustrated in figure 6.
11

24

6

6

0

6.3

7.4

10.5 14.3

15.4

18.5

1 2 3 4 5 6

Position on Panel

6.8 7.9 11 14.8 15.9 19 x

Effect of

Position

Effect of

Tack Weld

Tacks

Excluded

Tacks

Included

Fracture Stress 

σ

Tack

Weld

Friction

Stir

Weld

Figure 6. Idealized determination of tack weld effect.
12

The effect of tack welds is anticipated to vary not only along panels but also from panel to panel. 

Normalizing the measurement with respect to the strength for individual panels, ∆σT /σ, can reduce 

obscuring of the tack weld effect caused by panel-to-panel variation:

ΔσT

σ = aT ( − a) + bT ( − b)x

a + bx . (4)

The estimates of σT and σ can be obtained from a least-squares data fit. The least-squares error, 

ε, is determined by:

ε ≡ a + bxi − σ ( i)

2

i

∑ , (5)

∂ε

∂a = 2 a + bxi −σ ( i) = 2 aN + b xi − σi

i

∑

i

∑ ⎛

⎝

⎜

⎞

⎠

⎟

i

∑ = 0 , (6)

∂ε

∂b = 2 a + bxi −σ ( i)xi = 2 a xi

i

∑ + b xi

2 − σi

xi

i

∑

i

∑ ⎛

⎝

⎜

⎞

⎠

⎟

i

∑ = 0 , (7)

a =

σi

i

∑

N −

σ ixi xi

i

∑

i

∑

N xi

2

i

∑

1−

xi

i

∑

⎛

⎝

⎜

⎞

⎠

⎟

2

N xi

2

i

∑

, (8)

and

b =

σi

i

∑

xi

i

∑ −

N σi

xi

i

∑

xi

i

∑

⎛

⎝

⎜

⎞

⎠

⎟

2

1−

N xi

2

i

∑

xi

i

∑

⎛

⎝

⎜

⎞

⎠

⎟

2

. (9)
13

For this particular experiment a measure of tack weld effect for a particular panel can be 

obtained by evaluating the quantities in equations (8) and (9). For coupons 3 and 6 not containing 

the tack welds:

a =

σ 3 +σ ( 6 )

2 − 11σ 3 +19σ ( 6 )(11+19)

2 112 +192 ( )

1− (11+19)

2

2 112 +192 ( )

= 19

8

σ 3 − 11

8

σ 6 = 2.375σ 3 −1.375σ 6 (10)

and

b =

σ 3 +σ 6 ( )

(11+19) −

2 11σ 3 +19σ 6 ( )

(11+19)

2

1−

2 112 +192 ( )

(11+19)

2

= σ 6 −σ 3

8 = 0.125 σ 6 −σ ( 3) . (11)

For coupons 1, 2, 4, and 5 containing the tack welds,

aT =

σ1 + σ 2 + σ 4 +σ ( 5 )

4 − 6.8σ1 + 7.9σ 2 +14.8σ 4 +15.9σ ( 5 )(6.8 + 7.9 +14.8 +15.9)

4 6.82 + 7.92 +14.82 +15.92 ( )

1− (6.8 + 7.9 +14.8 +15.9)

2

4 6.82 + 7.92 +14.82 +15.92 ( )

=1.042s1+0.85s2 – 0.35s4 –0.542s5 (12)

and

bT =

σ1 +σ 2 +σ 4 + σ ( 5 )

(6.8 + 7.9 +14.8 +15.9)

− 4 6.8σ1 + 7.9σ 2 +14.8σ 4 +15.9σ ( 5 )

(6.8 + 7.9 +14.8 +15.9)

2

1−

4 6.82 + 7.92 +14.82 +15.92 ( )

(6.8 + 7.9 +14.8 +15.9)

2

=–0.0698s1 –0.0529s2 – 0.0529s4+0.0698s5. (13)
14

Note that for a single value of fracture stress, σi= σ, the slope, b, is zero and the value of the leastsquares fit is the constant a=σ; hence, the a-coefficients of the stresses must sum to 1 and the 

b-coefficients must sum to zero. This property of the expressions for a and b provides a quick check 

on the computation. The condition is satisfied in expressions (10)–(13).

3.2 Results

From fracture stresses of the individual coupons tabulated in table 5 in the appendix, the 

measure of the tack weld effect for each of the 24 weld panels was computed using equations (4) 

and (10)–(13) and tabulated in table 4 on the following page.

Some (positive) measures show a strengthening effect correlated with an embedded tack 

weld. Some (negative) show a weakening tack weld effect. 

The fusion weld effect in the thinner 0.257-in-thick GTA panels differs markedly from that 

in the thicker 0.327-in-thick FTA panels. Thick and thin panels will be considered separately.
15

Normalized Effect of

Tack Weld

–0.05 + 0x

0.07 – 0.01x

–0.04 + 0.01x

GTA01

GTA02

GTA03

GTA04

GTA05

GTA06

GTA07

GTA08

GTA09

GTA10

GTA11

GTA12

FTA01

FTA02

FTA03

FTA04

FTA05

FTA06

FTA07

FTA08

FTA09

FTA10

FTA11

FTA12

38.77 + 1.26x

66.45 – 0.45x

56.89 + 0.37x

63.38 + 0.08x

59.29 + 0.32x

64.54 + 0.02x

64.23 + 0.01x

63.04 + 0.1x

58.37 – 0.39x

68.60 – 0.84x

63.62 – 0.33x

60.05 + 0.25x

64.57 + 0x

63.15 + 0.06x

63.25 + 0.06x

62.41 + 0.13x

63.41 + 0.05

63.97 + 0.03x

63.16 + 0.08x

63.38 + 0.03x

64.07 + 0.01x

62.40 + 0.12x

63.28 + 0.06x

63.97 + 0.07x

59.02 – 0.24x

62.97 + 0.1x

64.61 – 0.1x

60.2 + 0.23x

66 – 0.23x

65.52–0.13x

61.78 + 0.18x

67.38 – 0.41x

72.1 – 1.04x

58.99 – 0.42x

66.22 – 0.61x

57 + 0.42x

64.02 + 0.01x

63.02 + 0.06x

63.43 + 0.05x

63.36 + 0.08x

63.75 + 0.04x

63.69 + 0.06x

63.58 + 0.04x

64.3 –0.09x

63.5 + 0.04x

63.75 + 0.06x

63.79 + 0.02x

64.52 + 0.03x

0.52 – 0.04x

1 + 0.03x

0.14 – 0.01x

1 + 0.01x

–0.05 – 0x

1 + 0x

0.11 – 0.01x

1 + 0.01x

0.02 – 0x

1 + 0x

–0.04 + 0x

1 + 0x

1 + 0x

0.24 – 0.01x

1 – 0x

–0.14 + 0.01x

1 – 0.01x

0.04 – 0x

1 – 0.00x

1 + 0x

–0.01 – 0x

1 + 0x

0 + 0x

1 + 0x

0 – 0x

1 + 0x

0.02– 0x

1 + 0x

0.01 – 0x

1 + 0x

0 + 0x

1 + 0x

0 – 0x

1 + 0x

0.01 – 0x

1 + 0.00x

–0.01 – 0x

1 + 0x

0.02 – 0x

1 + 0x

0.01 – 0x

1 + 0x

0.01 – 0x

1 + 0x

1 – 0.01x

= – 0.05

= – 0.04

= 0.02

= 0.04

= –0.05

= –0.01

= 0.00

= 0.00

= 0.02

= 0.01

= 0.00

= 0.01

= 0.01

= –0.01

= 0.02

= 0.01

= 0.01

Panel

Fracture Stress

σ of Coupons 

Without Tack 

Weld (ksi) 

x = Position

(in)

Tack Weld

Effect:

Median

(x = 12)

Range

(x = 0) (x = 24)

Fracture Stress

σT of Coupons 

Tack Weld (ksi) 

x = Position

(in)

3%

52% –26%

9%

–4% –26%

2%

14% –8%

–1%

11% –10%

–5%

7% –17%

12%

24% 0%

–2%

–14% 13%

–5%

2%

–4%

4%

–5%

–1%

0%

0%

2%

1%

0%

1%

1%

–1%

2%

1%

1%

σT – σ

σ

Table 4. Analytical results of coupon tests.
16

3.3 Interpretation of Results

Scientific hypotheses can never be confirmed by experimental data, only rejected. The entire 

body of scientific knowledge comprises unrejected hypotheses. The more stringent the tests for 

rejecting a hypothesis, the more confidence that can be accorded to an unrejected hypothesis. 

The methodology used here to evaluate the data is as follows. The hypothesis to be tested 

(the null hypothesis) is that there is no tack weld effect, i.e., that the mean value of the measure 

taken for the tack weld effect for the entire infinite population of all possible measurements is zero.

If the members of the population of measurements xi are distributed in a normal distribution about a mean,

μ = limN→∞

xi

i=1

i=N

∑

N

⎛

⎝

⎜

⎜

⎜

⎜

⎜

⎞

⎠

⎟

⎟

⎟

⎟

⎟

(14)

with a standard deviation σ such that

σ2 = limN→∞

xi ( − μ)

2

i=1

i=N

∑

N

⎛

⎝

⎜

⎜

⎜

⎜

⎜

⎞

⎠

⎟

⎟

⎟

⎟

⎟

, (15)

then means of samples of size N are expected to be distributed about population mean μ with standard 

deviation s/ N . As sample sizes approach the population size, their means converge to μ. Means of 

smaller samples show greater variation, and the variation of single measurements (N=1) becomes that 

of the population itself. 

To test the hypothesis that tack welds produce no effect, it is assumed that for the population 

of all measurements of tack weld effect the mean is zero (μ=0). The standard deviation of the population is estimated from the data in the samples such that 
17

σ ≈

xi ( − X )

2

i=1

i=N

∑

N , (16)

where X is the sample mean such that

X =

xi

i=1

i=N

∑

N . (17)

The hypothesis is maintained unrejected by the experiment (and can be accepted pending further study) 

if the sample mean X is sufficiently close to the hypothetical population mean, μ =0 in the present case, 

as not to be too improbable. What degree of improbability is cause for rejection of the hypothesis is 

a subjective matter. A common criterion for rejection of a hypothesis is a less than 1 chance in 20, less 

than a 5% chance, of the data being compatible with the hypothesis. This is said to be a test at a 5% level 

of significance. Given a normal distribution, the hypothesis rejection criterion in the present case is

−2 >

X

σ

N

⎛

⎝

⎜ ⎞

⎠

⎟

> 2 . (18)

Twelve thicker (0.327 in thick) FTA sample panels exhibited a mean strengthening effect of 0.6% 

with a standard deviation of 1%. The standard deviation for the sample of thick panel welds can then 

be estimated at 1%/ 12 =0.3%. The sample mean lies 0.6/0.3=2 units away from the hypothetical zero 

effect. To a 5% level of significance, the tack weld effect is just on the border for rejection. The hypothesis may be considered verified to a 5% level of significance, and rejected if more stringent, higher levels 

of significance are required. 

Twelve thinner (0.257 in thick) GTA sample panels exhibited substantially greater variation 

in fracture stress for unknown reasons. Computations of the mean and standard variation of the fracture stress ranged from 6.8% and 17.5%, respectively, at x=0 through 0.8% and 5.6%, respectively, at 

x=12 to –6.8% and 11.6%, respectively, at x=24. The significantly larger standard deviations at the end 

points are understandable for extrapolation sites; the interpolation site at x=12 presents a substantially 

smaller standard deviation, is anticipated to be a better indicator of tack weld effect, and is taken as the 

tack weld effect indicator for the thinner panels. The sample standard deviation is estimated at 5.6%/

12 =1.6%. The estimate of the population mean from the sample data lies at a distance 0.8/1.6=0.5 

from the zero tack effect hypothesis, well inside the acceptance level for a 5% level of significance.
18

Hence, to an approximate 5% level of significance, this study detects no detriment in weld 

fracture strength because of the inclusion of a fusion tack weld.

A more stringent test, say, demanding only an estimated 1 in 10 probability or less of the 

experimental results for rejection of the hypothesis, i.e., a 10% level of significance, would reject the 

hypothesis for –1.6 >X/(σ / N ) >1.6. The thinner GTA panels still easily confirm the hypothesis, 

but now the thicker FTA panels reject the hypothesis; however, hypothetical population means of 

0.6%±0.5% with a minimum of 0.1% would be acceptable to a 10% level of significance. This is a 

small effect, and it is a strengthening, not a weakening, effect.

The tack weld effect data were examined to see if there might be an effect of tack weld heat 

level, processing delay, or seam offset, but no obvious effect was found.
19

4. CONCLUSIONS

A measure of the effect of fusion tack welds embedded in friction stir welds in test panels 

was devised. Twelve thinner (0.257 in thick) and twelve thicker (0.327 in thick) panels were fusion 

tacked and friction stir welded under various conditions and a measure of the fusion weld effect 

was computed for each panel from the fracture stresses of six coupons cut from the panel, four 

containing a tack weld and two not containing a tack weld. 

The hypothesis that the embedded fusion tack welds had no effect on weld fracture strength 

was confirmed to a level of significance of 5%.

The more stringent confirmation requirements of a 10% level of significance would reject 

the zero effect hypothesis for the thicker panels, but would not reject effects in the range of 

0.6%±0.5%, indicating a slight strengthening effect.

It is concluded that for the range of panel thicknesses and welding practices tested, no detrimental effects are to be anticipated from embedded fusion tack welds. 
20
21

APPENDIX—FRACTURE STRESSES AND LOCATIONS

Table 5 shows the panel/coupon for fracture stresses and locations.

Table 5. Fracture stresses (ksi) and fracture location (retreating side = R, 

nugget = N, and advancing side = A).

Panel

Coupon

1 2 3 4 5 6

GTA01 55.78 N 59.24 N 52.6 N 53.64 N 56.61 N 62.66 N

GTA02 63.69 N/A 63.62 N 61.54 N 65.27 R 63.89 A 57.97 A

GTA03 63.55 A 64.48 R 60.96 N 61.14 N 64.61 R 63.92 N

GTA04 63.71 R 59.83 A 64.29 A 63.53 A 64.22 A 64.95 R

GTA05 64.63 R 63.85 N 62.84 N 63.92 N 61.34 N 65.42 R

GTA06 64.44 N 64.65 N 64.77 R 64.47 R/N 62.75 N 64.94 R

GTA07 63.29 A 62.88 N 64.30 R/N 64.58 R 64.60 R/N 64.35 N/A

GTA08 64.20 R 64.20 R 64.14 R 64.33 R 58.25 N/A 64.94 R

GTA09 64.17 N/A 64.56 N 54.08 N 58.51 N/A 53.81 N 50.96 N

GTA10 60.13 N 58.72 N 59.36 N 65.09 N 56.81 N 52.64 N

GTA11 64.58 R/N 57.73 N 59.95 N 63.28 N/A 51.65 N 57.28 N

GTA12 63.64 R/N 56.00 N 62.76 N 64.23 R 63.53 N 64.73 R/N

FTA01 64.14 R 64.05 R 64.58 R 64.32 R 64.11 R 64.59 R

FTA02 63.19 R/N 63.74 R/N 63.82 R/N 63.90 R/N 63.91 R/N 64.31 R/N

FTA03 63.84 R 63.68 R/N 63.95 R/N 64.32 R/N 64.03 R/N 64.46 R/N

FTA04 63.95 R 63.87 R/N 63.88 R/N 64.53 R/N 64.53 R/N 64.95 R

FTA05 64.09 R/N 63.93 R/N 63.90 R/N 64.51 R 64.20 R/N 64.26 R

FTA06 64.09 R/N 64.09 R/N 64.34 R/N 64.85 R/N 64.33 R/N 64.61 R/N

FTA07 64.10 R/N 63.64 R/N 63.98 R/N 64.40 R 64.14 R/N 64.58 R/N

FTA08 63.82 R/N 63.54 R/N 63.75 R/N 61.69 N 63.87 R/N 64.02 R/N

FTA09 63.95 R/N 63.63 R/N 64.12 R/N 64.17 R/N 64.13 R/N 64.16 R/N

FTA10 64.22 R/N 63.85 R/N 63.76 R/N 64.46 R/N 64.36 R 64.75 R/N

FTA11 64.10 R/N 63.64 R/N 63.90 R/N 64.44 R/N 63.74 R 64.35 R

FTA12 64.85 R 64.54 R/N 64.74 R/N 65.25 R 64.72 N 65.30 R
22

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Effects of Fusion Tack Welds on Self-Reacting Friction Stir Welds

A.C. Nunes, Jr., M.L. Pendleton, S.A. Brooke, and C.K. Russell

George C. Marshall Space Flight Center

Huntsville, AL 35812

National Aeronautics and Space Administration

Washington, DC 20546–0001

Unclassified-Unlimited

Subject Category 37

Availability: NASA CASI (443–757–5802)

Prepared by the Materials & Processing Laboratory, Engineering Directorate

M–1350

Technical Memorandum

NASA/TM—2012–217474

tack welds, friction stir welds, welding, metals joining, metals processing

01–11–2012

UU 36

NASA

U U U

In order to know whether fusion tack welds would affect the strength of self-reacting friction stir seam 

welds in 2195-T87 aluminum alloy, the fracture stresses of 144 tensile test coupons cut from 24 welded 

panels containing segments of friction stir welds were measured. Each of the panels was welded under 

unique processing conditions. A measure of the effect of the tack welds for each panel was devised. An 

analysis of the measures of the tack weld effect supported the hypothesis that fusion tack welds do not 

affect the strength of self-reacting friction stir welds to a 5% level of confidence.

STI Help Desk at email: help@sti.nasa.gov

STI Help Desk at: 443–757–5802

National Aeronautics and

Space Administration

IS20

George C. Marshall Space Flight Center

Huntsville, Alabama 35812