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Materials & Advanced Manufacturing (M&AM) DISSIMILAR FRICTION STIR WELDING OF A SOLID SOLUTION-STRAIN HARDENED ALUMINUM 5083 ALLOY AND PRECIPITATION STRENGTHENED ALUMINUM 2139 AND ALUMINUM 7085 Martin McDonnell, US Army TARDEC Dr.

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Materials & Advanced Manufacturing (M&AM)

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DISSIMILAR FRICTION STIR WELDING OF A SOLID SOLUTION-STRAIN HARDENED ALUMINUM 5083 ALLOY AND PRECIPITATION STRENGTHENED ALUMINUM 2139 AND ALUMINUM 7085

Martin McDonnell, US Army TARDEC

  • Dr. Nelson Martinez, Concurrent Technologies Corporation

DISTRIBUTION STATEMENT A. Approved for public release; distribution unlimited

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Materials & Advanced Manufacturing (M&AM)

8/10/2018 DISTRIBUTION A: UNLIMITED DISTRIBUTION 2

Background & Objectives

  • In this study, dissimilar aluminum alloys were joined using a solid phase welding

process known as friction stir welding (FSW)

  • The most common aluminum alloy employed by the US Army, AA5083-H131, was

friction stir welded (FSW) to AA2139-T8 and AA7085-T721

  • AA2139 and AA7085, two of the newest high strength aluminum armor alloys

developed for increased ballistic protection, were also welded to each other as part

  • f this study
  • FSW was selected as the most feasible joining process due to the precipitation

strengthened alloys not being advantageous to fusion welding processes

  • The objective of the study was to evaluate the weldability of dissimilar aluminum

alloys and study the affects of material placement on the advancing side vs. the retreating side of the FSW tool

  • Mechanical characterization results of the weldments are presented here
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Materials & Methods

MP159 FSW Tool

  • Aluminum alloys
  • Solid solution, strain hardened AA5083-H131
  • Precipitation strengthened AA2139-T8 and AA7085-

T721

  • FSW tooling
  • MP159 Ni-Co based alloy
  • Tapered w/ 14 threads per inch (TPI)
  • 5 flats
  • Joint design
  • Plates were 2 in. thick, 24 in. long and 6 in. wide
  • Butt joint configuration
  • Testing performed to date
  • Tensile, side bends and microhardness
  • Macroetching and optical microscopy

FSW Process

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Friction Stir Welded Hull

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Materials & Advanced Manufacturing (M&AM)

2-inch thickness AA7085-T721 (advancing side) welded to AA2139-T8 (retreating side)

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Macroetching

  • Macro cross-sections were taken at 5 and 18 inches from the start of the weld
  • A visual inspection of the macroetch samples showed cracks and voids in the weld

nugget as shown in the images below

Spindle Speed: 110 rpm Tool Velocity: 2 ipm

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Bend Testing

  • Bend specimens were also taken from welded plate just before of each macro cross-

section

  • The results of the testing showed cracking in the weld as observed from the figures

below

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Materials & Advanced Manufacturing (M&AM)

  • Tensile tests showed a 43.2 ksi UTS, 29.6ksi YS, 9.6% elongation with a 65% joint

efficiency

  • Failure occurred in the weld nugget

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Tensile Testing

Sample Ultimate Stress (ksi) Yield Stress (ksi) % EL Joint Eff. 7085-T721 68 60 12

  • 2139-T8

67 64 9

  • 2139-T8 to

7085-T721 43.2 29.6 9.6 65%

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Observations

  • Testing results showed that AA7085-T721 in the advancing side and AA2139-T8 in

the retreating side led to a weld with cracks and voids in the weld nugget thus resulting ineffective joint efficiencies

  • This ensued to another weld in which AA7085-T721 was placed in the retreating

side and the AA2139-T8 was placed in the advancing side of the tool and the same characterization was performed

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Materials & Advanced Manufacturing (M&AM)

2-inch thickness AA2139-T8 (advancing side) welded to AA7085-T721 (retreating side)

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Macroetching

  • Same welding parameters were used but the advancing side of the tool was placed

in the AA2139-T8

  • Macro cross-sections were taken at 6 and 17 inches from the start of the weld
  • Images showed no cracks or voids in the weld nugget indicating a good mixing of the

two alloys

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Bend/Tensile Testing

  • Bend specimens were taken from weld plate next to macro cross-sections as

previously done on the first weldment

  • Bend specimens show very minor cracking in the weld as observed from the figures

below

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Materials & Advanced Manufacturing (M&AM)

  • Tensile tests showed an 8.6% increase in UTS, a 5.1% increase in YS, a 32.3%

increase in elongation with a joint efficiency increase of 7.7%

  • This time the failure appeared in the Heat Affected Zone (HAZ) of the AA7085-T721

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Bend/Tensile Testing

Sample Ultimate Stress (ksi) Yield Stress (ksi) % EL Joint Eff 7085-T721 68 60 12 2139-T8 67 64 9 2139-T8 to 7085-T721 46.9 31.1 12.7 70% 7085-T721 to 2139-T8 43.2 29.6 9.6 65%

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Tensile Observations

  • Clearly there is a difference in mechanical properties depending on

which metal is placed on the advancing side.

  • As was observed by the tensile tests, failure occurred in the HAZ of

the AA7085-T721.

  • This indicates that the dissipation of heat is more detrimental on the

AA7085-T721 alloy.

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Macrohardness Testing

80 100 120 140 160

  • 50
  • 30
  • 10

10 30 50 HV 0.3 Distance (mm) Top of Weld…

7085 2139

80 130

  • 50
  • 30
  • 10

10 30 50 HV 0.3 Distance (mm) Middle of Weld (1") 7085

2139

80 100 120 140 160

  • 50
  • 30
  • 10

10 30 50 HV 0.3 Distance (mm) Bottom of Weld…

2139 7085

  • There is a clear division in hardness values observed in the nugget (the values

average 130 HV in the AA2139-T8 nugget while the AA7085-T721 average 160 HV)

  • Clearly there is a division between the two aluminum alloys in the nugget
  • As observed from the hardness profiles, the weakest part of the weld is in the HAZ
  • f the AA7085 (~88 HV)
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Base Metal Microscopy

500X 100X 100X 500X AA2139 AA7085

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Weld Nugget Microscopy

c e

a b c d e f g 2139 7085

b a d f g

  • It does appear there is a locking mechanism in place joining the two different alloys.
  • However, a clear division between the AA2139-T8 and AA7085-T721 within the nugget is
  • bserved.
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Weld Nugget Microscopy

a b c d e f g

  • Grains refinement over both base materials is clearly observed in the weld nugget
  • From the images, there appears to be recrystallization of the existing parent

material grains into finer grains

  • However, the optical images do show a clear separation of the grains between the

AA2139-T8 and AA7085-T721

500X AA2139 500X AA7085

e d

as welded as welded base

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2-inch AA5083-H131 / AA7085-T721

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Macroetching

  • Macro cross-sections were taken at positions 6, 13 and 18 inches from the start of the weld
  • No cracks or voids observed in the macroetches indicates that there is good mixing of the

two alloys

  • Weld parameters started with spindle speed of 130 RPM and decreased spindle speed of 110

RPM, since it was expected that it would provide less heat with reduction in rotational rate

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Bend/Tensile Testing

Spindle Speed (rpm) Ultimate Stress (ksi) Yield Stress (ksi)

  • Elon. (%)

Joint Eff. 5083 Joint Eff. 7085 AA7085 68 60 12 AA5083 45 37 8 130 RPM 42.8 26.3 10.6 95% 63% 120 RPM 46.2 25.7 16.1 103% 68% 110 RPM 46.4 26.2 19.1 103% 68%

  • The 130 RPM section showed no cracks in the weld nugget (start of the weld)
  • None of the weld nugget cracks extended through the bend specimen thickness
  • Specimen from 130 RPM broke in the weld nugget on the advancing side (AA5083)
  • Specimens from 120 rpm & 110 rpm broke outside the weld nugget on the retreating side

(AA7085) in the HAZ

110 rpm 120 rpm 130 rpm

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  • A division in hardness values is observed in the nugget
  • 120 HV in the AA5083-H131 weld nugget and 160 HV in the AA7085-T721 weld nugget
  • The hardness profiles show three minima values
  • The nugget, 7085 HAZ, and 5083 HAZ & failure will occur at any of these locations
  • Failure in the HAZ of AA7085 and the nugget occurred during tensile testing
  • The strength in the HAZ is ultimately dictated by the heat dissipated throughout the weld

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Macro/Hardness Testing

80 100 120 140 160

  • 50
  • 30
  • 10

10 30 50 HV 0.3 Distance (mm) Top of Weld (.25") 7085

5083

80 100 120 140 160

  • 50
  • 30
  • 10

10 30 50 HV 0.3 Distance (mm) Middle of Weld…

7085 5083

7085 5083

80 100 120 140 160

  • 50
  • 30
  • 10

10 30 50 HV 0.3 Distance (mm) Bottom of Weld…

5083 7085

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Base Metal Microscopy

100X 500X 500X 100X AA5083 AA7085

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Weld Nugget Microscopy

a bc d e f

  • Grain refinement is observed

in the weld nugget

  • Recrystallization of the

existing parent material grains into finer grains is also

  • bserved
  • However, a clear

separation of the grains between the AA5083 and AA7085 f e a b c d

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2-inch thickness AA5083-H131 welded to AA2139-T8

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2-inch AA5083 / AA2139

Sample Ultimate Stress (ksi) Yield Stress (ksi) Elon (%) Joint Eff. 5083 Joint Eff. 2139 5083 45 37 8 2139 67 64 9 2139-5083 48.6 28 11.8 108% 73%

2139 5083

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Base Metal Microscopy

100X 500X 500X 100X

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Weld Nugget Microscopy

a b c d e f a c e b d f

  • Grain refinement is also
  • bserved in the weld

nugget

  • Recrystallization of the

existing parent material grains into finer grains is also observed

  • However, a clear

separation of the grains between the AA5083 and AA7085 2139 5083

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Next Steps

To be presented at MS&T 2018 Poster Session

  • X-ray diffraction for residual stress measurement
  • Grain size and distortion mapping
  • inclusion content and chemical micro-segregation
  • Additional optical microscopy
  • Scanning electron microscopy (SEM)
  • Charpy v-notch testing
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