friction plug welding
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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
2014 ASIP Conference, San Antonio, TX 4 December 2014
DISTRIBUTION A: Approved for public release; distribution unlimited.
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.
2014 ASIP Conference, San Antonio, TX 4 December 2014
DISTRIBUTION A: Approved for public release; distribution unlimited.
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
2014 ASIP Conference, San Antonio, TX 4 December 2014
DISTRIBUTION A: Approved for public release; distribution unlimited.
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)
2014 ASIP Conference, San Antonio, TX 4 December 2014
DISTRIBUTION A: Approved for public release; distribution unlimited.
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
2014 ASIP Conference, San Antonio, TX 4 December 2014
DISTRIBUTION A: Approved for public release; distribution unlimited.
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
2014 ASIP Conference, San Antonio, TX 4 December 2014
DISTRIBUTION A: Approved for public release; distribution unlimited.
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
2014 ASIP Conference, San Antonio, TX 4 December 2014
DISTRIBUTION A: Approved for public release; distribution unlimited.
FPW Machine
Slide 8
• Weld head contains the
hydraulic motor, flywheel,
and hydraulic cylinder
• Sensors provide feedback
to the controller during the
process
2014 ASIP Conference, San Antonio, TX 4 December 2014
DISTRIBUTION A: Approved for public release; distribution unlimited.
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
2014 ASIP Conference, San Antonio, TX 4 December 2014
DISTRIBUTION A: Approved for public release; distribution unlimited.
Process Development
Slide 10
• Iterative developmental
effort
• Characterize the effects of
altering input variables
• Specimen testing
incorporated throughout
the development process
Plunge
Rate
2014 ASIP Conference, San Antonio, TX 4 December 2014
DISTRIBUTION A: Approved for public release; distribution unlimited.
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
2014 ASIP Conference, San Antonio, TX 4 December 2014
DISTRIBUTION A: Approved for public release; distribution unlimited.
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
2014 ASIP Conference, San Antonio, TX 4 December 2014
DISTRIBUTION A: Approved for public release; distribution unlimited.
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
2014 ASIP Conference, San Antonio, TX 4 December 2014
DISTRIBUTION A: Approved for public release; distribution unlimited.
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
2014 ASIP Conference, San Antonio, TX 4 December 2014
DISTRIBUTION A: Approved for public release; distribution unlimited.
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)
2014 ASIP Conference, San Antonio, TX 4 December 2014
DISTRIBUTION A: Approved for public release; distribution unlimited.
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
2014 ASIP Conference, San Antonio, TX 4 December 2014
DISTRIBUTION A: Approved for public release; distribution unlimited.
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
2014 ASIP Conference, San Antonio, TX 4 December 2014
DISTRIBUTION A: Approved for public release; distribution unlimited.
Slide 18
Tensile Tests
A close-up examination of an optimized process tensile
specimen’s fracture surface reveals no weld defects
2014 ASIP Conference, San Antonio, TX 4 December 2014
DISTRIBUTION A: Approved for public release; distribution unlimited.
Tensile Tests
Slide 19
• Example FPW force vs. deflection plot
- Depicts a sample with an ultimate strength of 64,200
psi (99.2%)
2014 ASIP Conference, San Antonio, TX 4 December 2014
DISTRIBUTION A: Approved for public release; distribution unlimited.
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
2014 ASIP Conference, San Antonio, TX 4 December 2014
DISTRIBUTION A: Approved for public release; distribution unlimited.
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
2014 ASIP Conference, San Antonio, TX 4 December 2014
DISTRIBUTION A: Approved for public release; distribution unlimited.
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
2014 ASIP Conference, San Antonio, TX 4 December 2014
DISTRIBUTION A: Approved for public release; distribution unlimited.
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
2014 ASIP Conference, San Antonio, TX 4 December 2014
DISTRIBUTION A: Approved for public release; distribution unlimited.
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
2014 ASIP Conference, San Antonio, TX 4 December 2014
DISTRIBUTION A: Approved for public release; distribution unlimited.
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
2014 ASIP Conference, San Antonio, TX 4 December 2014
DISTRIBUTION A: Approved for public release; distribution unlimited.
Slide 26
Questions?
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