Materials & Advanced Manufacturing (M&AM) Evaluation of - - PowerPoint PPT Presentation

Materials & Advanced Manufacturing (M&AM)

8/10/2018

Evaluation of Candidate Methods for Welding Steel to Other Structural Lightweight Metals

Jerry E. Gould, Mike Eff, and Kate Namola Resistance and Solid State Welding EWI ph: 614-688-5121 e-mail: jgould@ewi.org

Materials & Advanced Manufacturing (M&AM)

Dissimilar Materials Joining Needs for Various Industries

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Key Dissimilar Materials Joining Needs for Military Applications

• Aluminum to steel assembly

– Lightweight composite gears – Transition joints for ship construction – Lightweight blast protection

• Titanium to steel/Ni-base alloy

assembly

– Lightweight torsion shafts – Gas turbine engine shafts – Sub-system bracket assemblies

Page 3

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• Large difference in melting points
• Formation of low melting temperature

constituents

• Difference in crystal structure

– Al is FCC, Fe is BCC

• Multiple intermetallic phases

Issues with Joining Al to Steel

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• Metallurgical factors

– Reduced peak temperatures – Reduced times at temperature

• Geometric factors

– Designed interfacial topography

• Temperature excursions in friction welds

– Yield strength as a function of temperature – Applied contact pressures – Implicit peak temperature variations for different aluminum alloys

• Design of the thermal cycle

– Example for inertia welding

Key Factors in Friction Processing for Aluminum to Steel Joints

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• Process characteristics

– Inertia and direct-drive friction welding variants – Low surface velocities – Short heating times – Forging only in the aluminum

• 500

500 1000 1500 2000 2500 3000 100 200 300 400 500 Time (ms) RPM

Macrosection of an aluminum to steel inertia friction weld Deceleration profile for an inertia weld between aluminum and steel

Friction Welding Aluminum to Steel

Temperature profile of the interface of dissimilar FWed joint

50 100 150 200 250 300 350 400 450 5 10 15 20 25 30 35

Time (s) Temperature (C)

Steel Aluminium

Typical thermal cycle for an Al-to-steel inertia weld

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Example of Designing an Aluminum to Steel Inertia Welding Process



Relationship between part size and process parameters



Scaling is critical to properly size the welding hardware for large cross sections

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Joint Morphology vs Total Cycle Time

320 ms

Average Tensile = 318 MPa Average Tensile = 306 MPa

280 ms

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• Dedicated electric servo-

drive LFW machine

• Working interface ~12-mm

x 12-mm

• Targeting pressures and

heating times to match IFW results

• Effective heating times

<200-ms

• Observed aluminum

material loss ~8-mm Application of Linear Friction Welding to Aluminum – Steel Joints

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• Oscillation of the

steel component

• Use of square joint

faces

• Burn-off on

aluminum side

• nly
• Material loss ~8-

mm Aluminum – Steel LFW Process Observations

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• High contact pressures

used for joining

• Implicit surface

topography

• Enhanced plasticized

material at interface

• Lack of observable

intermetallics

• Joint strengths ~300-

MPa Aluminum – Steel LFW Metallurgical and Mechanical Observations

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• FSW processing for continuous

joints

• Al6061 to 1018 steel
• Specially designed tool

– Zero tilt – Large diameter pin – Flutes and threads

• Materials ~3-mm thick
• Tool offset to the aluminum side
• Tool shoulder 0.25-mm above

steel surface

• Scarfing of the steel interface
• Processing speeds ~8.5-mm/s

Friction Stir Welding of Aluminum to Steel

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• Transverse tensile testing

shows failures in Al HAZ

• Joint strengths ~200-MPa
• Bend performance along

joint

• Macrostructures show:

– Scarfing of the steel surface – Resultant texture – Flow of Al across steel surface

• Residual intermetallics

along bond line

Aluminum to FSW Microstructures and Properties

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Challenges for Titanium to Steel Joining

• Eutectic formation

– Ti-Fe eutectic temperature ~1073oC

• ~500oC melting point suppression

– Ti-Ni eutectic temperature ~972oC

• ~600oC melting point suppression

– Susceptibility to solidification and liquation cracking

• Intermetallics

– Ti-Fe and Ti-Ni intermetallic compounds – σ-phase formation – Bond line embrittlement

• Carbides and nitrides

– Titanium a strong carbide/nitride former

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Key Factors for Joining Titanium Alloys to Steels

• Use of Interlayers for Joining Titanium

Alloys to Steels

• Functionality of interlayers

– Separation of the Ti and steel substrates – Bondability to the substrates

• Arc welding using vanadium

interlayers

– Joint strengths up to 400-Mpa

• Diffusion bonding using nickel

interlayers

– Joint strengths up to 260-MPa – Failure along a Ni-Ti intermetallic layer

• Friction welding

– Use of copper interlayers – Joint strengths up to 375-MPa

• Explosion bonding

– Tantalum, Monel, and Copper all used as interlayers – Ductile components to absorb deformation during processing

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Application or Resistance Mash Seam Welding for Titanium Alloy to Steel Joining

• Nominal butt joints
• Defined small overlap between

workpieces

• Application of resistance seam welding
• Forging of components together
• Solid state joining
• Defined levels of uniaxial strain
• Potential for interfacial forging

– Defined material interfaces – Separation of base materials – Series solid state joints

• Materials for study

– 3.5-mm CP Ti hot rolled sheet – 3-mm 304 SS hot rolled sheet

• Beveled edges on the SS sheets

– 65-μm Nb strip (interlayer)

• Continuous seams following welding

0.8 1 1.2 1.4 1.6 1.8 2 2.2 1 2 3 Relative Joint Thickness (Delta) Bond Line Strain (Epsilon) 0.5 T 1.0 T 1.5 T 2.0 T 2.5 T

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Visual and Optical Assessments of the Resulting Joints

• Optical observations of resulting joints

– Welding to within a few 10’s of millimeters to the part edge – Smooth profiles on both the stainless steel and titanium – Ti seen to fill the bevel on the stainless steel – Residual Nb sheet seen to extend to both free edges of the joint

• Optical microscopy observations

– Bonding to within a few hundred microns of the joint free edges – Bi-axial deformation of the titanium sheet – Continuity of the Nb foil across the bond width

Top Surface of an RMSeW made between 304-SS and Ti Sheet. Cross Section of the RMSeW between 304-SS and Ti

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Interlayer Behavior in the Resulting Joints

• Indications of bonding to within

a few microns of the bond line edge

• Continuity of the Nb interlayer

to the bond line edge

• Resulting interlayer ~60-μm

wide

• Minimal diffusion of species

across the interlayer

• Solid state character of the bond

evident on both sides of the interlayer

• Joint strengths 200-MPa to 300-

MPa

Details of the bond line edge for a best practice Ti to SS RMSeW Microstructural and chemical variations across the bond line of a Ti to SS RMSeW with an interlayer

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Microstructural Behavior in Areas

• f Foil Rupture
• Some samples showed

discontinuities near the bond edge

• Discontinuities appeared to

be small ruptures in the Nb foils

• Rupture resulted in

localized constitutional melting

• Melt zones grew into both

the Ti and SS

• Composition of this zone

suggests a Ti-Fe eutectic

Details of the bond line edge for a best practice Ti to SS RMSeW Microstructural and chemical variations across the bond line

• f a Ti to SS RMSeW

with an interlayer

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Upset Welding Titanium Alloys to Steels

• Resistance heating

combined with axial forging

• Rapid thermal cycling
• Use of refractory metal

interlayers

• Resistance heating to

provide softening and plasticity

• Development of high

faying surface strains

• Simple (uni- and bi-axial)

strain paths

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UW Titanium Alloys to Steels – Process Response and Macrostructures

• Constant voltage power
• Two second heating time (2 – 1-s pulses)
• Upslope employed for providing good initial

contact

• Current variations related to changes in

workpiece resistance

• Preferential forging of the titanium
• Observable upset on the SS
• Two stage deformation behavior

– Initial forging of the Ti-alloy – Secondary forging of the SS

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UW Titanium Alloys to Steels – Microstructure and Properties

• Nb-foil distorted along with

the forging base materials

• Retained foil thickness

between 10-μm and 25-μm

• Apparent bond integrity

– Ti-foil interface – SS-foil interface

• Ti-foil separation edges of

workpiece

• Replicate tensile specimens

show >300-MPa joint strengths

• Further process optimization

needed

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Evaluation of Candidate Methods for Welding Steel to Other Structural Lightweight Metals – Summary

• Joining of Aluminum Alloys to Steel

– Suppression of intermetallic compounds – Rapid thermal cycles – Low peak temperatures – Applicability of friction welding processes

• Peak temperatures <475oC
• Heating times <200-ms
• Surface textures on steel

workpieces

– Example technologies

• Inertia welding
• Linear friction welding
• Friction stir welding
• Joining of Titanium Alloys to Steel

– Prevention of:

• Eutectic formation
• Intermetallic compounds

– Use of refractory metal interlayer technology – Simple strain paths – Significant strain between workpieces – Minimum thermal cycle – Continuity of the interlayer in the final joint – Example technologies

• RMSeW
• UW

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Questions?

Jerry E. Gould Resistance and Solid State Welding EWI ph: 614-688-5121 e-mail: jgould@ewi.org