Challenges in welding

Technology: Space scenario

Dr. P. V. Venkitakrishnan

Dy. Director

Materials and Mechanical Entity

Vikram Sarabhai Space Centre, ISRO, Trivandrum

1

CII Welding conference 2016, Mumbai16th November, 2016

Outline of talk

• Introduction : Welding Processes

• Fusion welding and its Applications

• Solid state welding and its Applications

• Brazing and its Applications

• Welding Institutes

• Future directions

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Welding Processes

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Types of Welding ProcessesAlmost 50 types of welding processes exist

Can be categorized into two major classes

Fusion welding : Joining is accomplished by melting of

the two parts to be joined. Filler metal is added if

necessary

Examples : TIG Welding, Laser welding, Electron Beam

welding, submerged arc welding, gas welding

Solid state welding : Heat/pressure are used for joining

but no melting of base metals occurs. No filler material is

added.

Examples: Friction welding, Friction Stir welding,

Ultrasonic welding, Diffusion Bonding, Explosive bonding

COMPARISON OF FUSION WELDING PROCESSES

LB EB

PlasmaTIG

6

Considerations and requirements for

weldments of aerospace systems

- Emphasis on quality with minimum defects .

In general industries there is more emphasis

on productivity.

- Choice of process important. TIG preferred

instead of MIG.

- Joining of dissimilar metals required

- Extensive NDT is done to ensure joints with

acceptable defects

- Repair welding is practiced and is essential

Welded components

in PSLV

Strap on motor case ,15CDV6

steel (GTAW)

PS1 motor case 18Ni250

maraging steel (GTAW)

AA2219 tank (PS2) (GTAW/ FSW)

Bimetallic adaptor (Explosive

bonding/ FSW)

Ti-6Al-4V tanks (PS4) (EBW)

Ti-6Al-4V Gas bottles

(EBW)

BMA (Friction welding)

Battery cases

(GTAW/ LBW/

Ultrasonic welding)

PS2 Engine impellor

(Diffusion bonding)

Important Fusion

welding Techniques

8

Gas Tungsten Arc welding (GTAW)

Electron Beam Welding (EBW)

Laser Beam Welding (LBW)

Gas Tungsten Arc Welding (GTAW)

• Also known as Tungsten Inert Gas (TIG) welding

• Uses an arc between a non consumable tungsten electrode and a work piece

• Shielding is obtained from a inert gas like Argon

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Welding of Motor Cases

Propellant

(Chemical energy)

Igniter Hot Gases

(Heat energy)High velocity Gases

(Kinectic Energy)

Nozzle

Material : M250 Maraging steel

(Fe-Ni-Co-Mo)

This is a high strength steel

[Yield strength- 1800 MPa]

Size : 2800 mm dia

Height of one shell : 1500 mm

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•Total Weld length : 60 Metres

• To have uniformity , Auto-TIG process used

Current, voltage, speed, wire feed are

controlled automatically

Welding of Motor Cases – cont…

12

* Properties required : High strength and high

fracture toughness (75 MPa√ m)

* To achieve consistent properties :

- Close control of parameters , e.g.

Current : 230 + 1 Ampere

- Interpass cleaning

- Overall process cleanliness

- Use of high purity Ar gas (4 ppm of oxygen max.)

Welding of Motor Cases – cont..

WELDING PROCESS

Selected Process: DCSP –TIG

For Propellant Tank realisation

Why DCSP ?

Aim :To achieve 180 MPa( min) without any post weld treatment and with R1 repair.

Change over to DCSP TIG from AC TIG yielded a payload gain of 41 Kg.

Propellant Tank realisation

Why DCSP ?

DCRP DCSP

High Depth to width ratio

15

Welding of LOX, LH2

and L40 tanks

Material : AA 2219 Aluminum

Alloy

This is a Aluminum- copper alloy

with good properties at

cryogenic temperatures

• Aluminum alloys – main

problem during welding is

porosity

• Rigorous cleaning with alkali to

remove oxide

• Mechanical properties reduction

in welding, to minimise this

loss, welding with lowest

possible heat input

DCSP- GTAW process

Key controlled parameters include: •Speed of the electrons (ACCELERATING VOLTAGE) •Number of electrons in the beam (BEAM CURRENT) •Energy density at the joint (FOCUS) •Speed of welding (TIME)

EBW

PS4 gas bottle17

Forged hemispheres

High Pressure Gas bottles/ propellant tanks

are made of titanium alloy (Ti-6Al-4V)

Titanium has low density

Titanium – highly reactive to oxygen/

nitrogen

GTAW welding – poor quality welds

Welding in vacuum required

Electron beam welding is the best

choice

TITANIUM ALLOY GAS BOTTLES - EBW

PS4 propellant tank

INSAT/ GSAT tank

PS2 gas bottle

LASER BEAM WELDING

Laser welding

done here

• Lithium–ion batteries for satellites

• Cell casing made of 1mm

thick aluminum alloy

• Separators made of polymer

located close to the weld area

• To minimise damage to polymer,

a low heat input process –

LASER BEAM WELDING used

LASER WELDING – LITHIUM BATTERIES

Wire sizing, Globule formation, flattening& Welding Process

Cryo Initiator Bridge wire Welding

Wire holding fixture

Laser beam

Punch &Die for Globule flattening

Laser beam

Inert Gas chamber

Laser welded Initiators • Source : 400W Pulsed Nd:YAG laser.

•Tailored the workstation for microwelding

application with a power reduction aperture

• Wire : 80micron/ Nichrome

• A dedicated facility established in VSSC

• Flight proven in GSLV F05

Explosive bonding

Ultrasonic welding

Important Solid state Welding

Friction Stir welding

Diffusion Bonding

Friction welding

Explosive bonding

• Welding is accomplished by accelerating one of the components at extremely high velocity through the use of chemical explosives.

Drawbacks:• Low joint strength in tension• High fabrication cost• Material wastage• Low yield because of defects• Hazardous process• Noise and vibration

Explosive bonding for bimetallic adaptors

Features of explosive bonding:

(i) Interfacial pressure of 102 Mbar maintained for few microseconds & diffusion rate is small.

(ii) Jet formation removes surface defects/contaminations.

Al alloy AA2219 – stainless steel 12X18H10T (ICSS 321) with interlayer of Al

Friction welding

Al

SS

Friction Stir Welding

Major Considerations in Tool Configuration

• Strength

• Ratio of swept volume to static volume

• Minimum slip

• Shear layer velocity

• Minimum flashApproximate Heat Input

Linear Velocity at shoulder periphery – 1.3 m/s

Linear velocity at extreme end of probe – 0.5 m/s (Sq probe)

Energy due to friction heat – 4220 W

Energy due to interfacial shear stress – 477 W

NEW RETRACTABLE TOOL

Photographs of the Al-SS joints

Al Al Al Al

SS

SSSS

Friction surfaced coating of AA2014

CMT welding of AA4047

Alternate option for Bi-metallic joining by using Friction stir + Cold metal transfer (FS+CMT)

Examples of joints

Step 1: Annealing of AA2219-T87 base materialStep 2: Bending of AA2219 and AISI321 base materialsStep 3: Joining of AA2219 aluminium alloy (Al/Al)Step 4: Post weld heat treating of AA2219 joints to T6 conditionStep 5: Joining of AISI321 stainless steel (SS/SS) Step 6: Joining of AISI321 and AA2219 joints (SS/Al) - a plus shaped coupon

Microstructure at different rpm of tool

1400 rpm

Diffusion Bonding

• Diffusion bonding is a

solid state bonding

process

• Process under pressure

and temperature

• Bonding happens by

diffusion of atoms

Impeller body and cover parts before bondingMaterial : Titanium alloy

Diffusion bonding parameters

Temp 9600C Time 2.5 Hrs Load 3Ton

Diffusion bonding on LH2 impeller

100X

Vacuum Hot PressCapacity : 250 T

Bond interface

CERAMIC + METAL JOINING

Oxides : Al2O3, ZrO2, Nitrides : Si3N4, AlN Carbide

: SiC,

Al & Ni are used as interlayers

Required duration : 2–4 s Welding pressure : 10–30 MPa

Si3N4 + Al (interlayer) + Cu used as inserts

Al2O3 + Cu Used in terminal seal Li-ion batteries

ULTRASONIC WELDING

Two components are held together, and oscillatory shear stresses of ultrasonic frequency are applied to interface to cause coalescence

• Oscillatory motion breaks down any surface films to allow intimate contact and strong metallurgical bonding between surfaces

Li-ion batteries

•Positive electrodes – aluminum foils of

20 micron thickness

•Negative electrodes - pure copper foils

of 20 micron thickness

• These electrodes have to be joined to

the main tab [1 mm]

• Ultrasonic welding used to to make a

bunch of 10-20 foils

JOINING FOIL ELECTRODES IN Li-ION BATTERIES:

ULTRASONIC WELDING

Brazing

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Rotary vacuum brazing

Static brazing

Sea level Thrust Chamber Assembly : Major parts

Convergent inner shell

• Cu alloy plates

• Deep forming

• Profile machining

• 5 axis Channel

milling

Nickel ring

• Ring forging

• Machining

• EDM

Divergent inner shell

• Cu alloy plates

• Deep forming

• Profile machining

• 5 axis machining

Manifold

• SS sheets 4 mm

• Forming

Convergent outer shell

• SS shaped forging

• Machining

Divergent outer shell

• SS shaped forging

• Machining

TYPICAL BRAZING CYCLES

Time Time

5000C

10300C

9800C

9750C

11800C

12300C

950 0C Switch off power

850 0C VP on

5 min

5 min

1130 0C Switch off

power900 0C VP on

1-8 min till temp stabilises

0-8 min or till temp

stabilises

Convergent- Divergent Assembly Second Divergent Assembly

FIXTURE FOR ROTARY VACUUM BRAZING

TECHNOLOGY DEVELOPMENT

PROBLEMS FACED

Cracks noticed on the steel outer shell.

Blockages noticed inside the channels.

Bulging during hydro test (de-bond).

Thermocouple failure.

Fixture problems.

Induction heating related problems.

PLC problems.

Facility related issues.

Initial hardware lost dueto brazing failure

Material, Structural, Thermal,Instrumentation, Vac. system,Furnace Engg, Electrical,Electronics, Process Engg.

SYNERGISING MULTI-DISCIPLINARYEXPERTISE IN-HOUSE& OUTSIDE

Modifications in Process,material, fixture design, facilitytune-up, procedure, review

SUCCESSFUL BRAZING

ROTARY VACUUM BRAZING for Cryo Thrust Chamber

CHAMBER BRAZING

AFTER BRAZING

BEFORE BRAZINGINNER CHAMBER

BRAZE FOIL OUTER SHELL

COOLING CHANNELS

Complex process as it involves precise control of many variables like temperature, time, vacuum, pressure and rotational speed

VACUUM 10

-2

VACUUM 10

-2

RADIATION

RADIATION

INDUCTION COILS

ARGON Pr.

Pr.

Pr.

Pr.

MUFFLE

VACUUM BRAZING OF 1151-130 AY IN RVBF

ROTATION

VACUUM

Facility

Static vacuum compression brazing furnace

Static brazing process using electroplated Cu and Ag for the Semi-Cryo engine realization

Hardware after copper and silver

coating

a) Outer shell (03X12H10T)

b) Inner shell (Cu-Cr-Zr-Ti)

a) b)

Hardware as

dismantled Hardware welded for vacuum lines

Hardware assembly wrapped with heating

pad for brazing

Major welding Institutions in India• WRI/ BHEL, Trichy• IGCAR, Kalpakkam• VSSC, Triavndrum• BATL, Trivandrum• BARC, Mumbai• Godrej, Mumbai• Walchandnagar industries Ltd.• L&T Mumbai• ARCI, Hyderabad• NML, Jamshedpur• DMRL, Hyderabad• IISc, Bangalore• IIT, Madras, IIT, Kharagpur, Other IITs and NITs• R&D Tata Steel Jamshedpur• Professional institutions: IIW Kolkata, ISW Delhi, ESAB

Objectives of professional institutionsApplication of various welding processes catering to the appropriate needs of industries. Dissemination of knowledge in welding and allied areas through Training, documentation services, publications. Collaborative research in areas specific to any industry or a cluster of industries. Technology diffusion to Industry and the individual. Promote health & safety in welding.

Future Directions……….• Several institutes and industries are involved in welding. But,

there is a need to create common forum to resolve challenges of dissimilar metal and advanced material joining.

• Evolution of alternate and cost effective method of fabrication involving metal joining can be explored for costly and strategic materials.

• Indigenisation of welding consumables and equipment can be taken up to further reduce the cost of welding.

• Concerted efforts required in the area of non-destructive testing of weldment and post weld heat treatment especially to provide additional confidence of crack free weld using techniques like ultrasonic inspection.

• Towards human resources, average age of people involved is on higher side ~ 50 yrs, which need to be brought down to younger generations.

Thank You