A Seminar on Space Welding

Guided By : Dr. Kulwant Singh Submitted By: Jaiveer Singh Rajput(116109) Bharat Singh(116105) Mtech. WLF

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ContentIntroductionNeed of an in-space weldingTechnological difficulties and their overcomesPhysics behind metal transfer in spaceIdeal performance criteria for in an space welding

processChoosing a welding processWelding Process usedFuture of space weldingApplicationsAdvantages

Till now we heard about welding on earth where conditions are suitable for the welding but ever we think about welding in the space where conditions are unusually complex. Space presents a unique welding environment as well as many technological challenges. The most obvious and dramatic challenges are;

1. Microgravity2. High Vacuum

Instead of such technological challenges we must be thankful to the advance technology which made our “YESTERDAY’S DREAM TOMORROW’S REALITY”.

  Yesterday’s dream of in-space construction, repair, and maintenance by

welding is becoming Tomorrow’s reality for Sky Lab. In comparison with mechanical fastening and adhesive bonding, in-space

welding is accepted as a joining and fabrication process because; In-space joining techniques are currently limited to mechanical fastening

and adhesive bonding. Thus, need of in-space welding is must.

Introduction

Need of an in-space welding

•Higher strength and rigidity

•Better hermetic joint

•Lower mass joint

•Simpler joint manufacturing

•Broader repair versatility

•Higher joint reliability

•Consequent cost saving

As compare to adhesive bonding and mechanical fastening in-space welding would offers:

1. Higher joint strength and rigidity Construction of large truss, antennas, solar panel requires

higher joint strength and rigidity but mechanical fastening or adhesive bonding can’t compete with the superior welding joint’s properties.

2. Better hermetic joint The need of pressurized vessels and modules for thermal,

propulsion, and life support system in the hostile space vacuum environment requires reliable hermetic joint. Mechanical fastening, involving gaskets or O rings, and adhesively bonded joints can’t provide hermetic seal which is as strong or durable as welded joint’s seal.

3. Lower joint mass To achieve desire strength and hermeticity with mechanical

fastening or adhesive bonding which usually adds extra mass to the parent component can also be achieved by welded joints with no or almost negligible mass added.

4. Simpler joint manufacturing Designing in-space joints with mechanical fastening is more time

consuming than welding techniques. Varying mechanical fasteners and adhesive bonders increase the joint components and, therefore, complexity of the manufacturing operations.

5. Broader repair versatility Orbital debris collision or fatigue damage to the lab modules, radiators, fluid

lines etc requires quick repair so that damage will not effect the function of space craft. Quick repair can be provided by welding techniques because it employs less or almost negligible joint preparation as compare to mechanical fastening or adhesive bonding.

6. Higher joint reliability Due to difficulty of repair, construction and maintenance in the hostile

space environment, requirement of joint reliability is maximum to minimize the joint servicing. Mechanical fastening or adhesive bonding are more susceptible to fatigue failure and are less reliable as compare with welded joint, especially with thermal, corrosion and radiation stresses.

7. Consequent cost savings In-space joint cost reduction can be achievable through Higher joint

strength and rigidity, Better joint hermeticity, Lower joint mass, Simpler joint manufacturing, Broader repair versatility, Higher joint reliability.

Technological difficulties and how to overcome

•Lack of electric power supply

•Lack of non destructive testing

•Metallurgy of Low gravity weld

•Lack of metal transfer

•Limited Mass carriage

•Spatters

  1.Lack of electric power supply

Three methods used to produce electric power, fuel cell, batteries and solar conversion. These produce small amount of electric power. Large capacity power generation system based on nuclear power generators are being developed.

2.Lack of nondestructive testing Methods such as radiography and eddy current inspection may be feasible, but no space-qualified equipment has

yet been developed. Even if such equipment was available, the competition for weight and volume onboard a spacecraft might prevent its usage.

3.Metallurgy of Low gravity weld The microgravity environment have significant effect on the metallurgy of welds. Two important effects on the

microstructure of a weld are cooling and fluid flow within the weld pool. Slow cooling and effected fluid flow will result in coarser grain structure and can be prevented from

forced cooling jet of gas. 4.Lack of metal transfer Due to the low gravity, metal transfer due to the application of gravity is not possible. Molten metal droplet will

remain suspended in the space. To overcome this problem Short circuit mode of metal transfer is preferred.

5. Limited Mass carriage Carrying large amount of mass and volume along space craft is limited due to which it prevents the usage of

such inspection equipments such as X-ray inspection equipment which requires heavy shielding shield to prevent astronaut from the expose of X-rays. Prevention from the amount of X-rays is directly proportional to the mass of the shield.

6.Spatters There may be a problem of spatters in the space because during welding operation

due to the high current, problem of spatter may exists and they will start flying in the space. To avoid such a problem STT mode (Surface tension transfer mode) is required to operate. STT mode is operated at low current input.

Short-circuit transfer mode Surface Tension Transfer mode

Physics behind metal transferMetal transfer which is depends upon the cumulative effect of

the different forces acting on the molten droplet growing at the tip of the cathode.

Forces

include

Gravity

Electro-

magnetic

pinch effect

Drag force

Surface

tension

 Forces include:  1. Gravity (only when gravity is present) 2. Surface tension It tends to retain the molten droplet at the tip of the electrode and due to the effect

of the gasses they will push it downward. After detachment it will touch the weld pool and drawn into molten weld pool by surface tension.

 

3.Electromagnetic Pinch effect   When a current carrying conductor is under the influence of its own magnetic field ,

radial contracting forces are developed which produce pressure within the conductor. The combined effect of these forces is the detaching force acting on the molten droplet at the tip of the electrode and is referred to as pinch effect, force referred to Lorenz force or electromagnetic pinch force.

4. Drag force In order to detach the droplet, it requires the detaching force (downward) must be

greater than supporting force (upward). That is,

Fa + Fem > Fσ

Where, Fa = Drag force due to gasses (Downward) Fem = Electromagnetic pinch force (axially) Fσ = surface tension force (Upward)

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IDEAL PERFORMANCE CRITERIA FOR IN AN SPACE WELDING PROCESS

CONFORM TO REGID OPERATOR (ASTONAUT)/MISSION SAFETY

NOT HAVE FUNDAMENTAL PROBLEM WITH MICRO GRAVITY.

PRODUCE FIRST CLASS QUALITY WELD ON ALL AEROSPACE MATERIALS.

EASILY ADAPT TO MANUAL,SEMI OPERATED,TELEOPERATED AND ROBOTICS OPERATION MODES

TOLERATE JOINT MISMATCH

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Continue………..

OPERATE EFFICIENTLY WITH LOWER POWER LEVELS AVAILABLE IN SPACE .

MINIMIZE USE OF CONSUMABLE MATERIALS.

EXHIBIT HIGH EQUIPMENT RELIABILITY AND SERVICEABILITY

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CHOOSING A WELDING PROCESSThe space environment has constraints that limit

the application of certain conventional welding processes

Developing an in space welding capabilities will naturally employ our gained expertise with terrestrial welding.

The space environment also affects some of the basic physical processes involved in welding, making welding processes operate differently in space than in earth-based environments.

In addition, the weight limitations of orbital payloads on spacecraft will impose constraints on the processes that can be feasibly used in space.

There are four processes which come closet to satisfying the specific requirement for space welding capability.

1 EBW 2. GTAW 3. PAW 4. LBW

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There are four processes which come closet to satisfying the specific requirement for space welding capability.

1. EBW 2. GTAW 3. PAW 4. LBWNOW……….5. FRICTION STIR WELDING PROCESS(FSW)

RECOMMENDED PROCESSES 

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CANDIDATE PROCESSES IN SPACE APPLICATIONCRITERIA/PROCESS EBW GTAW PAW LBW

OPERATOR/MISSION SAFETYMICRO-G WELD QUALITYIVA & EVA FLEXIBILITYWORKPIECE VARIETYOPERATION MODE FLEXIBILITYTOLERANCE FLEXIBILITYPOWER REQUIREMENTENERGY EFFICIENCYCONSUMABLES REQUIREEQUIPMENT SERVICEBILITY

GOOD SATISFACTORY

POOR

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The EBW process consists of an electron-beam gun that generates a high-energy stream of electrons. When directed through a series of focusing electromagnets, this high-energy stream has sufficient power density to melt metals.

DIFFICULTIES OF EBW IN SPACEThe impingement of electrons onto a metal surface

causes the emission of x-raysHowever, x-ray shielding capacity is proportional to the mass of the shield; therefore, shielding of workers may be difficult due to the high cost of payload weight on spacecraft. Automated or robotic welding may be recommended due to the radiation exposure to workers, particularly for extended periods of welding activity

Electron-Beam Welding (EBW)

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GTAW PROCESSES IN SPACE  The GTAW process consists of a tungsten electrode connected to one

lead of a power source, the other lead of which is connected to the workpiece. When the power supply is activated, current flows to the tungsten electrode through an arc plasma to the workpiece, and returns to the power supply to complete the circuit. The arc plasma is the primary source of heat as electrons move from the metal surface, are accelerated through the plasma column, and impinge on the metal surface at the opposite side of the arc.

DIFFICULTIES INSPACEThe GTAW arc plasma requires a constant flow of gas. This gas

supply, in the form of fairly heavy compressed gas cylinders, adds to the cost and weight of the spacecraft payload. In addition, this supply will need to be constantly replenished as welding progresses.

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FRICTION STIR WELDINGFriction Stir Welding is the most recent upgrade

to the Space Shuttle’s gigantic External Tank, the largest element of the Space Shuttle and the only element not reusable. The new welding technique—being marketed to industry—utilizes frictional heating combined with forging pressure to produce high-strength bonds virtually free of defects. Friction Stir Welding transforms the metals from a solid state into a "plastic-like" state, and then mechanically stirs the materials together under pressure to form a welded joint.

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DEVELOPMENT OF  FSW FOR SPACE WELDINGIn 1993, NASA challenged Lockheed Martin Laboratories in

Baltimore, Md., to develop a high-strength, low-density, lighter-weight replacement for aluminum alloy Al 2219–used on the original Space Shuttle External Tank. Lockheed Martin, Reynolds Aluminum and the labs at Marshall Space Flight Center in Huntsville, Ala., were successful in developing a new alloy known as Aluminum Lithium Al-Li 2195, which reduced the weight of the External Tank by 7,500 pounds (3,402 kilograms).

Friction Stir Welding produces stronger welds—that are easier to make—the External Tank Project Managers chose to use the process on its Super Light Weight Tank, which is made from Al-Li 2195. The Friction Stir Welding process produces a joint stronger than the fusion arc welded joint, obtained in the earlier Light Weight Tank program.

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ADVANTAGES…..OF FSW OVER OTHER PROCESSES

A significant benefit of Friction Stir Welding is that it has significantly fewer process elements to control. In a Fusion weld, there are many process factors that must be controlled–such as pure gas, voltage and ampere, wire feed, travel speed, shield gas, arc gap. However, in Friction Stir Weld there are only three process variables to control: rotation speed, travel speed and pressure, all of which are easily controlled.

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MAJOR APPLICAATIONSWelding applications in space are similar to those on earth in that they both involve the need to assemble structural components that will exhibit sufficient strength, endurance, and reliability during their service lives.Typical space welding applications generally fall into three categories1. ON-ORBIT CONSTRUCTION OF

LARGE STRUCTURES2. REPAIR WELDING ON-ORBIT AND

FOR EXPLORATION FLIGHTS BEYOND THE VICINITY OF THE EARTH

3. WELDING AND REPAIR OF STRUCTURES ON A LUNAR BASE

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CUTWAY VIEW OF EXTERNAL TANK

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Future of Space Welding The most important space-based welding application for long-duration

manned spacecraft is repair welding. This technology offers the possibility of decreasing assembly time and decreasing the weight and cost of space structures.

Following steps must be taken for futuristic development of space welding :

DEVELOPMENT OF EQUIPMENT THAT HAS SUFFICIENT SENSOR TECHNOLOGY TO REPORT IN REAL TIME THE QUALITY OF WELDS AND TO FACILITATE REAL-TIME PROCESS CONTROL AND CORRECTION OF WELDS IN PROGRESS.

 CHARACTERIZATION OF THE DIFFERENCES IN SPACE-BASED WELDS AND

EQUIVALENT EARTH-BASED WELDS TO PROVIDE A BASIS FOR MODELING AND UNDERSTANDING OF THE DIFFERENCES IN THESE WELDS.

 PROCESS DEVELOPMENT FOR WELDS CONDUCTED IN SPACE.

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FUTURISTIC R&D CENTREPROCESSES

RESAERCH CENTRE

EBW Paton Institute Ukraine. And NASA

GTAW Rock Well International Corporation, UNITED STATES

PAW NASA's Marshall Space Flight Center in Huntsville, Ala (USA)

LBW University of Tennessee calspan(for co2 laser) and University of Alabama (for Nd-YAG laser)

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THANKS…