Friction Plug Welding

2014 ASIP Conference, San Antonio, TX 4 December 2014

DISTRIBUTION A: Approved for public release; distribution unlimited.

Friction Plug Welding of

2024-T3 Aluminum

Michael Lange

Mercer Engineering Research Center

Warner Robins, GA 31088

Slide 1

2014 Aircraft Structural Integrity Conference

2 - 4 December 2014

San Antonio, TX

Dr. Stephen Schwenker

Air Force Research Laboratory

Dayton, OH 45433



Background

Slide 2

• Friction plug welding (FPW) is a solid phase welding

technique in which a round plug is rapidly spun, with an

applied force, to fill a hole

• FPW was first developed by The Welding Institute in the

1990’s

• FPW offers a solution for aircraft panel fastener hole repair

- Current repair options cannot restore panels to original condition

- Impacts fleet availability as spares are exhausted

- FPW offers a repair method without compromising structural

integrity, durability, or corrosion resistance.



Friction Plug Welding

Slide 3

FPW Process Steps:

• Hole preparation, fixturing

• The plug is spun at a

predetermined speed and

forced into the hole

• After the rotational motion has

stopped, an axial force is

applied for a short period of

time

• Excess plug material is

removed



Advantages

Slide 4

• High strength / good mechanical properties

• Low heat input

• Automated / high repeatability

• No special training required / not operator dependent

• Quick process times

- Estimated 85% time savings for panel repair over new

procurement lead time

• Low Cost

- Estimated 90% savings for panel repair over new procurement

costs ($400K for this specific application)



Slide 5

Other Uses

• Termination hole from a circumferential friction

stir weld (2014 / 2219 aluminum)

• Airframe structural component repair (Ti 6Al-4V):

- Vertical stabilizers

- Wing spars



Process Development

Slide 6

• MERC was funded by

AFRL to develop a FPW

repair process for 1/8”

thick 2024-T3 aluminum,

using friction plug welding

• 11/16” hole size

• Alternate to doubler plate

repair methods

Damaged Hole

Doubler Plate Repair



FPW Machine

Slide 7

• Overhead lift mechanism

supporting the weld head

• Hydraulically operated,

inertia driven, push-weld

machine

• Locks into fixture holding

the item to be welded



FPW Machine

Slide 8

• Weld head contains the

hydraulic motor, flywheel,

and hydraulic cylinder

• Sensors provide feedback

to the controller during the

process



Process Variables

Slide 9

• Plugs were fabricated from

2024-T351 aluminum rods

• Plug geometry – diameter,

taper angle, height

• Hole geometry – straight vs.

tapered

• Plug rotational and axial

velocity

• Forging force / duration



Process Development

Slide 10

• Iterative developmental

effort

• Characterize the effects of

altering input variables

• Specimen testing

incorporated throughout

the development process

Plunge

Rate



Parametric Studies

• Parametric studies were performed to better understand

the effects of varying input parameters.

- Tensile strength increased as plug RPM increased.

- Tensile strength peaked at mid-level plunge rates

Slide 11



Parametric Studies

• Tensile strength increased as the plug height (base to

shoulder) decreased.

- Decreasing the plug height allowed the shoulder to better retain

flash material exiting the top of the joint, enabling improved weld

strength

Slide 12



Cross-Section Examination

Slide 13

• Specimens were cut into cross-sections, polished, and etched to reveal

grain structure

- Examinations were made under a microscope

- Inspected for hairline cracks, voids, or other defects

- Defects were readily apparent on non-optimal weld samples

- No defects could be seen on optimized process weld samples



Cross-Section Examination

Slide 14

• Specimens were also sectioned vertically, such that the

weld line could be inspected for cracks or voids

- In non-optimized welds, hairline cracks were evident around the ring

of the weld

- In optimized process welds, no defects were visible



Bending Tests

Slide 15

• Specimens were bent as a

quick means to provide a

qualitative assessment of

weld quality

• Non-optimized welds resulted

in breakage of the weld joint

early in the bending process

(low ductility)

• Optimized process welds

remained intact and showed

no visible cracking (high

ductility)



Tensile Tests

Slide 16

• Tensile specimens were fabricated

and tested to failure

• Non-optimized welds would

typically break at the center of the

weld

• Optimized process specimens

would break offset from the center

of the weld

• Ultimate strength data was

compared to data from non-

welded specimens fabricated from

the same sheet of aluminum



Slide 17

Tensile Tests

Tensile Specimen 37.1 ksi (57%) – smooth surfaces,

centered on plug

66.2 ksi (102%) – rough surface, fully

offset from plug center

57.5 ksi (89%) – rougher surfaces,

partially offset from plug center

Evaluation of the weld specimens’ fracture surface helped to guide the process development.

Non-optimized; plug length / RPM

Non-optimized; plug taper angle Optimized process behavior



Slide 18

Tensile Tests

A close-up examination of an optimized process tensile

specimen’s fracture surface reveals no weld defects



Tensile Tests

Slide 19

• Example FPW force vs. deflection plot

- Depicts a sample with an ultimate strength of 64,200

psi (99.2%)



Test Results

Slide 20

• The process resulted in welds that were close in

strength to the base material’s strength

- Tensile tests: The average ultimate strength of

welded samples using the optimized process was

90% of measured baseline, non-welded, material

values (65 ksi)

- No defects were observed within the etched cross

sections when viewed under a microscope



Thermal Characterization

Slide 21

• The temperature of the panel

surrounding the weld location was

characterized

• Goal: stay below a threshold value

(250o F), 1” from the weld line

- Avoid damage to skin and

honeycomb core bond lines in

close proximity to the weld

repair location

5

4

3

2

1

#1: Adjacent to the weld flash formation

#2: 7/16” radially from the weld

#3: 11/16”: radially from the weld

#4: 1” radially inboard from the weld

#5: Centered between 3” spaced holes



Thermal Characterization

Slide 22

• The average maximum

temperature was below 100o F,

1” from the weld

• The table provides the average

maximum values of four tests; a

typical temperature vs. time

plot is shown

• Low panel temperature will not

affect material bond lines in

vicinity of the weld joint

Location #1 #2 #3 #4 #5

Average Maximum

Temperature

(° F)

191 116 90 93 95



Conclusions

Slide 23

• A friction plug weld panel repair method has been

developed for use on 1/8” thick 2024 Aluminum

- Welds are close in strength to the baseline material’s strength

- Weld joints are free from cracks or voids

- The temperature of the material surrounding the weld joint is not

significantly increased during the process

• Friction plug welding is a cost and lead-time effective means

for aircraft panel repair



Lab Analysis (AFRL)

Slide 24

• Inspections and testing being

performed by AFRL:

- Fluid Penetrant Inspection

- Radiographic Inspection

- Tensile

- Bearing

- Stress Corrosion Cracking

- Salt Fog

- Micrographic Inspection

- Hardness testing



Future Efforts

Slide 25

• Additional specimens for fatigue testing

• Full-scale aircraft panel repair demonstration

• Exploration of other applications

- The process is scalable for other combinations of material alloys,

thicknesses, and hole diameters

- Potential applications: Fixed and rotary wing structure and panel

repair

- Minimize costs through component repair instead of purchasing

new

- Minimize depot maintenance times for increased aircraft

availability



Slide 26

Questions?

Friction Plug Welding

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