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September 17, 2014

Development of an Indirect

Resistance Brazing Technology

for Sandwich Metal Panels

Jerry E. Gould

Technology Leader – Resistance and Solid State Welding

EWI

ph: 614-688-5121

e-mail: jgould@ewi.org

Doug Cox

President

CellTech Metals

ph: 858-414-5373

e-mail: dcox@celltechmetals.com

Application and Potential for

Sandwich Based Materials

Aerospace

─ Extensively used for structural components

─ Shrouds, mufflers, thrust reversers, TPS

─ Brazed and welded variants

─ Cost a major consideration ($500 - $1000/m2)

Automotive

─ Needs for structural lightweighting

─ Aluminum a primary focus

─ Sandwich materials seen as a vehicle for steel to compete with aluminum

─ Cost of sandwich product a key

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Low Cost Honeycomb Panel

Concept, Design, and Demonstration

Honeycomb panel design ─ Thin gauge (0.2-mm) face and core ─ Mild and stainless steel variants ─ Brazing as the primary assembly mechanism

Panel formability ─ Stamping similar to similar thickness

monolithic steels ─ Precedent in the aerospace industry ─ Manufacture of stamped structures for

vehicle components

Need for low cost assembly methods ─ Indirect resistance heating methods

Forming Simulation for a 1-5-mm Thick

Panel (Courtesy CellTech Metals Inc.)

Stamped B-Pillar from a 1-5-mm Thick

Panel (Courtesy CellTech Metals Inc.)

Schematic Representation of a 1-5-mm Thick Panel Indicating

the Thicknesses of the Sheets and Locations of the Braze

Joints (Courtesy CellTech Metals Inc.)

Basic Concepts for Indirect

Resistance Roll Brazing (IRRB)

Mechanism of achieving uniform brazing conditions

─ Temperature ─ Geometric position

Use of a linear resistance heating mechanism

Product heating through conduction

Cooling techniques to maintain constrained hot zone

Roll designs to assure intimate contact of components

Local conditions similar to vacuum brazing

Potential for: ─ High processing speeds ─ Uniform final product

Schematic Representation of a Roll Brazing System for Low Cost sandwich Panel Construction (Courtesy CellTech Metals)

Roll Brazing Process Dynamics – Heat

Transfer Modeling

Thermal analyses for predicting strip heating and cooling ─ Closed form solutions ─ Geometric and material

property effects ─ Estimates of heating and

cooling dynamics ─ Temperature-time

relationships at each interface

Estimates of processing requirements ─ Total thermal cycles on the

order of 10s of milliseconds ─ Speeds in the range of m/min ─ Influence of panel design ─ Estimates of currents and

voltages

Graphical Adaptation of the Thermal Model to a Four-

Sheet Thickness Panel

Roll Brazing Process Dynamics – Heat

Transfer Modeling – Thermal Results

0

100

200

300

400

500

600

700

0 0.01 0.02 0.03 0.04 0.05

Location in X Direction, m

Tem

pera

ture

, C

Heating and Cooling Profiles for the Dimple-to-Dimple

Contact Surface in a Four-Thickness Stack-Up Panel.

Processing Speed is 3 m/min.

Temperature Profiles Through the Thickness of the

Sandwich for Various Positions Through the Heating

Rolls. Processing Speed is 3 m/min.

0

0.0001

0.0002

0.0003

0.0004

0.0005

0.0006

0.0007

0.0008

0 100 200 300 400 500 600 700

Temperature (C)

Locatio

ns in

Y D

irectio

n (

m)

0.000080.0004

0.00080.0016

0.0040.008

Roll Brazing Process Dynamics – Heat

Transfer Modeling – Implied Process

Dynamics

Current and Voltage Requirements for Maintaining a

600oC Roll Temperature as a Function of Panel Width

and Line Speed.

Allowable Line Speeds for Full Heat Penetration as a

Function of Strip and Core Thickness. Analysis is

Based on a Roll Temperature of 600oC and a 10-mm

Contact Length.

0

2

4

6

8

10

12

0 0.2 0.4 0.6 0.8 1 1.2

Strip Thickness (mm)

Lin

e S

pe

ed

(m

/min

)

0.5-mm core

1.0-mm core

1.5-mm core

0

2000

4000

6000

8000

10000

12000

0 2000 4000 6000 8000 10000 12000

Travel Speed (mm/min)

Weld

Cu

rren

t (A

)

0

50

100

150

200

250

300

350

400

I (A)

150

1000

2000

Vo

ltag

e

Use of Zinc and Zinc Alloys as Braze

Fillers – Preliminary Evaluations

| 2-mm | | 2-mm

|

| 2-mm |

Formed dimples on skin rolled galvanized

steel core sheet

Poor wetting of the re-melted zinc coating

during resistance brazing

Cracking of the skin rolled coating on an

individual dimple prior to resistance

brazing

Successful resistance weld between skin

rolled dimpled core sheets – joining of the

steel without zinc re-flow

G30 material Skin rolled

product Cold working of

the coating Localized

cracking during forming

Oxidation of substrate

Poor wetting characteristics

Lack of flow during brazing

Joining only by direct spot welding

Use of Zinc and Zinc Alloys as Braze

Fillers – Non Skin Rolled Flat Stock

Zinc morphology on the non-skin rolled

sheet stock Zinc re-flow at the edge of a braze joint at

best practice conditions

Zinc re-flow at the center of a braze joint

at best practice conditions Over-welded condition showing

complete displacement of the zinc

coating at the bond line

Uniform substructure free coating

Observed wetting during brazing

Flow to the periphery of the joint

Best braze practice ~5-μm zinc layer

Bonding below A1 temperature

Over-welding leads to expulsion of zinc

Solid state welding Potential for spot weld

nuggets Results not yet

demonstrated for dimpled panels

Use of Zinc and Zinc Alloys as Braze Fillers –

Intermetallic Formation and Implicit Thermal

Cycle

0

100

200

300

400

500

600

700

800

900

1000

1.E+01 1.E+02 1.E+03 1.E+04 1.E+05

Time (ms)

Tem

pera

ture

(C

)

Intermetallic Formation

Roll Braze Cycle*

*

1exp

QA

t R T

Holloman Jaffee Equation Defining

Kinetics for Second Phase Formation

Approximate Relationship between Kinetics of Intermetallic Phase

Formation Resulting from Galvanizing and the Implicit Thermal Cycles

from Roll Brazing

Resistance Bar Brazing – Basic

System Configuration

0

5000

10000

15000

20000

25000

30000

0 0.2 0.4 0.6 0.8 1

Current On-Time

Weld

Cu

rren

t

0

5

10

15

20

25

30

I (A)

V 150

Vo

ltag

e

Power analysis indicating the current and voltage

requirements for bar resistance brazing as a function

of heating time

Tooling configuration showing orientation

of the panel between the resistance

heating bars

Layout showing panel positioning tooling

and configuration relative to the

resistance weld platens

Characteristics of the AlN

Ceramic

Material characteristics ─ Compressive strength: 1000-

MPa ─ Electrical resistivity: 1.8 × 1013

-cm ─ Thermal conductivity: 100-

W/m-K ─ Coefficient of thermal

expansion: 5.2-μm/m-oC ─ Maximum service temperature

1000oC – 1900oC ─ Machinable ─ Brazable to metals ─ Commercially available

Major concern ─ Thermal expansion mis-match ─ Compatible with refractory

alloys

Advantages for heater bar applications ─ High resistivity: 20 orders of

magnitude greater than metals ─ High thermal conductivity: 3-4

times that of steel (28-W/m-K) ─ Resistant to wetting by zinc ─ Available in thin sheets (<1

mm) ─ Directly brazable to metals

─ Reactive braze alloys ─ Success in preliminary trials

Resistance Bar Brazing –

Machine Configuration

4-kA MFDC power supply 14-V max secondary

voltage Constant current control Initial bar heater material

316SS Bar width 150 mm

Copper sub-blocks Heater bars in parallel

configuration Nominal 10% compression

of the sandwich

Resistance Bar Brazing – Current

and Temperature Waveforms

Best practice peak currents in the range of 30 kA

Rise times ~30 ms Best practice times ~125 ms Current drop at end of cycle

─ Increasing heater resistance

Consistency with planned roll-brazing operation

Thermocouples on heater bar at end and center locations

Measurements taken after current termination

Temperature uniformity within ~10%

Consistency with preliminary models

-5

0

5

10

15

20

25

30

35

0 50 100 150 200

Time (ms)

Cu

ren

t (k

A)

0

100

200

300

400

500

600

700

800

900

150 160 170 180 190 200

Time (ms)

Tem

pera

ture

(o

C)

Bar end temp

Bar center Temp

Resistance Bar Brazing - Zinc

Reflow from Preliminary Trials

Uniform brazing across the strip width

Demonstrated melt and contact zones between face sheet and core

Interpass buckling indicating concerns with braze strengths

Local areas of braze attachment

Observed zinc re-flow ─ Face sheet to core

─ Core to core

Evidence of attachment

| 2-mm | | 2-mm |

Resistance Bar Brazing – Implications

for Sandwich Appearance

No edge heating on samples Uniform zinc melting across the

sandwich width Minimal surface quality disruption No evidence of sticking to the

heater bar

No cross sandwich current flow

Some zinc re-flow to interior side of face sheet

No evidence of attachment Apparent lack of intimate

contact Better core sheet design

required

Top Surface Showing Zinc Melting at the Contact

Location of the Heater Bar

| 2-mm |

Interior Face Sheet Surface Showing Localized

Zinc Melting

| 6-mm | | 4-mm |

Resistance Bar Brazing – Flow and

Wetting at the Various Interfaces

Zinc Flow Characteristics at the edge of

a Core to Core Attachment Zinc Flow Characteristics at the edge of

a Face Sheet to Core Attachment

Details of the Resolidified Zinc Morphology at

a Face Sheet to Core Attachment

Development of a Prototype

Roll Brazing System

Machine configuration ─ Working width up to 150-mm ─ Rolls nominally 160-mm diameter ─ Roll frame to support rolls and bearings ─ Integrated cooling for entry and exit of

workpieces

Power supply ─ Small scale (10-kVA) system ─ 48 available tap positions for voltage control ─ No active current control ─ Intention of demonstration purposes ─ Scalable as the process is developed

Contactors ─ Sliding contacts for introducing current into the

rolls ─ Electrical path configures the rolls in series ─ “Overmatched” conducting area ─ Flood cooling of contactors ─ Use of GlidCop® type alloys for wear resistance

Development of the Braze

Rolls

Rolls produced from commercially pure Molybdenum

─ Availability from commercial castings ─ Pure rather than alloyed material based on

availability

Roll segmentation for current flow localization

─ Rolls made up of nominally 10-mm wide segments

─ Roll segments 5-mm deep ─ Segments expanded on the ends for increased

contact area ─ AlN insulation between segments

Application of AlN coating to rolls ─ Mixed ceramic flame spray coating ─ Rolls ground to final size ─ Nominal 0.5-mm coating thickness

Attachments to a substrate mandrel ─ Stainless steel substrate ─ Mechanical clamping of the moly segments

onto the substrate roll

Preliminary Roll Brazing

Trials

Roll braze trials done with galvanized steels

Parts three-high ─ Face sheet ─ Core ─ Face sheet

The zinc coating acts as the braze alloy

Trials done at slow speeds based on the transformer size

Results show good reflow over limited lengths of product

Consistency was limited by power supply controls

Product also showed excellent side to side heat balance

First level capability of the roll braze concept demonstrated

Process refinements needed to improve product consistency

| 15-mm |

| 5-mm |

Feeding of 50-mm Wide Galvanized

Steel into the System

Full width (50-mm) Three high Sandwich

Panel Brazed over Several cm.

Details of the Brazed –

Galvanized Surface

Development of Resistance Roll Brazing

for Sandwich Construction – Next Steps

Demonstration of roll brazing at the pilot level

Improved power capacity on existing system ─ Larger transformers ─ Phase shift control ─ Separate power systems for

the two rolls

Improvements in supply cable design ─ Duty cycle assessments ─ Water cooled cables ─ Standard flex cables

Next generation roll coatings ─ Compositions ─ Application techniques

Production trials to scale up welding speeds ─ Higher current levels ─ Potential adaptation to DC

power ─ Target speeds of m/min ─ Comparisons of three and four

high sandwiches

Investigations of demonstrator material systems ─ Steels ─ Ti-alloys ─ Ni-base alloys

Joint EWI – CellTech Metals Joint

Industry Project (55347CPQ)

Definition of candidate applications ─ Aerospace application ─ Automotive application ─ Appropriate braze layers

Design of core profiles ─ Three high design ─ Four high design

Adaptation of the 150-mm wide roll system ─ Integration at EWI ─ Equipment configuration with

high capacity power supplies

Initial product/performance demonstrations ─ Process development trials ─ Product performance and

quality evaluations

Design of a pilot scale system ─ Design intent ~300-mm wide

rolls ─ Roll design ─ Frame and drive system

design ─ Electrical considerations

System component purchase and final assembly

Process adaptations from pilot work ─ Process relationships ─ Performance validation

Single sponsored work for proprietary customer products

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Development of an Indirect Resistance Brazing

Technology for Sandwich Metal Panels – Summary

Performance and economic drivers for low cost sandwich materials

Demonstrated component designs using core sheet arrangements

Conceptualization of indirect resistance brazing for panel construction

Demonstration using laboratory based single shot heating element arrangements

Definitions of power requirements and heating times

Adaptation to an indirect resistance roll heating arrangement

Demonstration of technical viability using corrugated galvanized panels

Planned joint industry project (JIP) for development of a pilot scale system

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| 6-mm |

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Questions?

Jerry E. Gould

Technology Leader

Resistance and Solid State Welding

EWI

ph: 614-688-5121

e-mail: jgould@ewi.org

Doug Cox

President

CellTech Metals

ph: 858-414-5373

e-mail: dcox@celltechmetals.com

EWI is the leading engineering and technology organization in North America dedicated to advanced materials joining and

allied manufacturing technologies. Since 1984, EWI has provided applied research, manufacturing support, and strategic

services to leaders in the aerospace, automotive, consumer products, electronics, medical, energy & chemical, government,

and heavy manufacturing industries. By matching our expertise in materials joining, forming, and testing to the needs of

forward-thinking manufacturers, we are successful in creating effective solutions in product design and production.