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Friction-Stir Spot Welding of Advanced High-Strength Steel

Tsung-Yu PanFord Motor Company

Mike Santella, Zhili FengOak Ridge National Laboratory

Glenn GrantPacific Northwest National Laboratory

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Team Members

DCX :Bill MarttilaGM: Jim Quinn, Jim ChenMittal Steel: Min Kou, Ravir BhatnagarMegaStir: Russ Steel, Scott PackerHardtech: Bryan Cooper, Hakan Andersson

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Friction stir: What is it?

Solid state joining method developed ~ 1990A non-consumable cylindrical tool is usedTool rotation produces frictional heating and deformationTranslation of the tool produces a “stir zone”Can be used for welding or surface modification

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Friction Stir Spot Welding (FSSW)

A novel variant of linear FSWCreate solid-state bond

No bulk meltingTechnical and economic advantages have been demonstrated for Al alloys

Plunging Stirring Drawing out

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FSSW of Aluminum in Production

A Production since 2003Eliminated water, air, and welding power supplies.99% energy saving (vs. RSW of Al) 40% investment reduction (vs. RSW of Al)Weight reduction vs. rivets

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Why FSSW of AHSS?

Steel is still the primary material for body structures of high-volume mass-production carsIncreased use of AHSS for safety and fuel efficiencyResistance spot welding of AHSS is problematic

Solidification related weld defectsInterfacial failure due to defects and brittle and hardened microstructuresDegradation of electrode life

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Charpy V Notch

0

20

40

60

80

100

120

140

-60 -50 -40 -30 -20 -10 0 10 20 30

Temp, C

Ene

rgy,

J

Weld

HAZBase Metal

Linear FSW of High-Strength Steels Improves Dynamic Impact Properties

Girth weld of API X65 steel for natural gas transmission pipelines

Feng and Packer et al., 2005

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Objective of This Study

Explore the feasibility of friction stir spot welding of AHSS

Can existing FSW tool materials be used to make FSSW in AHSS?Can a FSSW be made in a time comparable to resistance spot welding?What are the microstructural characteristics of FSSW AHSS?

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FSSW Tool

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FSSW Tool

DP780 and DP600Polycrystalline cubic boron nitride (PCBN)

toolShoulder dia: 10mmPin length: 2.3mmTapered pinA single tool was used

Displacement controlPlunge to a pre-determined depth in 2 steps

800 rpmTotal cycle time (plunging and dwelling)

1.7 to 9.75 secLaboratory FSW system

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FSSW Process

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FSSW process – displacement control

3 plunge rates + 2 dwells + 7 depths

3.0

2.5

2.0

1.5

1.0

0.5

0.0

Plun

ge d

epth

(mm

)

121086420Time (s)

Examples of weld programs 2.3 mm final depth 2.6 mm final depth

Fast plungeShort dwell

Slow plungeLong dwell

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Variation of Lap-shear Strengthwith Plunge Depth for DP780

14

12

10

8

6

4

2

Tens

ile s

hear

load

(kN

)

2.92.82.72.62.52.42.3Plunge depth (mm)

Plunge rate, dwell 0.4 mm/s, 1 s 0.4 mm/s, 3 s 2 mm/s, 1 s 2 mm/s, 3 s 3 mm/s, 1 s 3 mm/s, 3 s

σf increases with plunge depth, plunge rate, dwell time

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Variation of Lap-shear Strength with Weld Time for DP780

14

12

10

8

6

4

2

Tens

ile s

hear

load

(kN

0

1086420Total weld time (s)

Plunge rate, dwell 0.4 mm/s, 1 s 0.4 mm/s, 3 s 2 mm/s, 1 s 2 mm/s, 3 s 3 mm/s, 1 s 3 mm/s, 3 s

Consistent higher σf obtained with fast plunge rate + long dwell

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FSSW strengths vs. industry standards for high strength steel RSWs

Plot based on JIS Z3140, “Method of Inspection for Spot Welds”

20

15

10

5

0

Tens

ile s

hear

load

(kN

)

2.01.81.61.41.21.00.80.60.4Sheet thickness (mm)

Sheet strength 270-370 MPa > 590 MPa

RSW diameters:4.7 mm minimum5.5 mm average

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Strength depends on joint area,plunge depth

Bonded area is influenced by tool geometry, tool & work materials properties, process parameters

Condition #1

Condition #14

Final depth 2.3 mmTotal time: 6.25 sec

Final depth 2.9 mmTotal time: 9.75 sec

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Welding cross sections

Total welding time: 2.1 sec

(a) – base metal(b) – heat-affected zone(c) – Stir zone (thermal-mechanical affect zone)

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Microstructure of DP600

BMSZ

HAZ

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Microhardness

0

50

100

150

200

250

300

0 5 10 15 20

Position (mm)

HV

, 500

g

1/4t 1/2t 3/4t

exit hole

Tool shoulderDP600

Similar hardness in the weld zone as in the parent metal.

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Tool appearance after hundreds of welding trials

No visible tool wear over hundreds of welds made in this study

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FSSW Joint Appearance

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Conclusions/Summary

PCBN tool material appears to have acceptable durability & performance for DP780

No significant wear or deterioration observedSpot welds of relatively high strength (8-12 kN) were obtained

Strengths compare favorably with JIS standardWeld times of 4 s appear feasible

Bonding region showed similar microstructure and hardness as in the base metalNo attempts were made to optimize any aspect of the materials or processFSSW of DP780 appears feasible

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Acknowledgements

Research sponsored by the U.S. Department of Energy, Assistant Secretary for Energy Efficiency and Renewable Energy, Office of FreedomCAR and Vehicle Technologies, as part of the Automotive LightweightingMaterials Program.Contacts:

DOE Headquarters: Joseph A. CarpenterOak Ridge National Laboratory: Phil SkladPacific Northwest National Laboratory: Mark Smith