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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
2
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
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
10
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
11
•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.
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 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
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
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
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
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.