Challenges in welding Technology: Space scenario Dr. P. V. Venkitakrishnan Dy. Director Materials and Mechanical Entity Vikram Sarabhai Space Centre, ISRO, Trivandrum CII Welding conference 2016, Mumbai 16 th November, 2016 1
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
Welding Processes 3
Types of Welding Processes Almost 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 EB LB TIG Plasma
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 6
Welded components in PSLV Battery cases Ti-6Al-4V tanks (PS4) (EBW) (GTAW/ LBW/ Ultrasonic welding) Ti-6Al-4V Gas bottles AA2219 tank (PS2) (GTAW/ FSW) (EBW) Bimetallic adaptor (Explosive BMA (Friction welding) bonding/ FSW) PS2 Engine impellor (Diffusion bonding) PS1 motor case 18Ni250 maraging steel (GTAW) Strap on motor case ,15CDV6 steel (GTAW)
Important Fusion welding Techniques Gas Tungsten Arc welding (GTAW) Electron Beam Welding (EBW) Laser Beam Welding (LBW) 8
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
Welding of Motor Cases Nozzle Igniter Propellant Hot Gases (Chemical energy) (Heat energy) High velocity Gases (Kinectic Energy) 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 10
Welding of Motor Cases – cont … • Total Weld length : 60 Metres • To have uniformity , Auto-TIG process used Current, voltage, speed, wire feed are controlled automatically 11
Welding of Motor Cases – cont.. * 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.) 12
For Propellant Tank WELDING PROCESS realisation Selected Process: DCSP – TIG Why DCSP ? Change over to DCSP TIG from AC TIG yielded a payload gain of 41 Kg. Aim : To achieve 180 MPa( min) without any post weld treatment and with R1 repair.
Propellant Tank realisation Why DCSP ? High Depth to width ratio DCRP DCSP
Welding of LOX, LH2 and L40 tanks DCSP- GTAW process 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 15 possible heat input
EBW 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)
TITANIUM ALLOY GAS BOTTLES - EBW 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 PS2 gas bottle Welding in vacuum required Forged hemispheres Electron beam welding is the best choice INSAT/ GSAT tank 17 PS4 gas bottle PS4 propellant tank
LASER BEAM WELDING
LASER WELDING – LITHIUM BATTERIES • 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 welding done here LASER BEAM WELDING used
Cryo Initiator Bridge wire Welding Laser beam Wire sizing, Globule formation, flattening& Welding Process Inert Gas chamber Punch &Die for Globule flattening Wire holding fixture Laser beam • Source : 400W Pulsed Nd:YAG laser. Laser welded Initiators • 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
Important Solid state Welding Explosive bonding Friction welding Friction Stir welding Diffusion Bonding Ultrasonic 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 10 2 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 flash Approximate Heat Input NEW RETRACTABLE TOOL 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
Alternate option for Bi-metallic joining by using Friction stir + Cold metal transfer (FS+CMT) CMT welding of AA4047 Al Al Al Al SS SS SS Friction surfaced coating of AA2014 Photographs of the Al-SS joints
Examples of joints Step 1: Annealing of AA2219-T87 base material Step 2: Bending of AA2219 and AISI321 base materials Step 3: Joining of AA2219 aluminium alloy (Al/Al) Step 4: Post weld heat treating of AA2219 joints to T6 condition Step 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
Diffusion bonding on LH2 impeller Diffusion bonding parameters 960 0 C Temp Time 2.5 Hrs Load 3Ton Impeller body and cover parts before bonding Material : Titanium alloy Bond interface Vacuum Hot Press Capacity : 250 T 100X
CERAMIC + METAL JOINING Oxides : Al 2 O 3 , ZrO 2 , Nitrides : Si 3 N 4 , AlN Carbide : SiC, Al & Ni are used as interlayers Required duration : 2 – 4 s Welding pressure : 10 – 30 MPa Si 3 N 4 + Al (interlayer) + Cu used as inserts Al 2 O 3 + 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
JOINING FOIL ELECTRODES IN Li-ION BATTERIES: ULTRASONIC WELDING 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
Brazing Rotary vacuum brazing Static brazing 35
Sea level Thrust Chamber Assembly : Major parts Manifold Nickel ring • SS sheets 4 mm • Ring forging • Forming • Machining • EDM Convergent inner shell • Cu alloy plates • Deep forming • Profile machining Divergent inner shell • 5 axis Channel • Cu alloy plates milling • Deep forming • Profile machining • 5 axis machining Convergent outer shell Divergent outer shell • SS shaped forging • SS shaped forging • Machining • Machining
TYPICAL BRAZING CYCLES Convergent- Divergent Assembly Second Divergent Assembly 1030 0 C 1230 0 C 1180 0 C 0-8 min 1130 0 C 980 0 C 950 0 C Switch or till Switch off 1-8 min off power temp power 975 0 C 900 0 C VP on till temp 5 min stabilises stabilises 850 0 C VP on 500 0 C 5 min Time Time
FIXTURE FOR ROTARY VACUUM BRAZING
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