Advanced WeldingAdvanced Welding

ContentContent

Soldering

Brazing

WeldingGas weldingMetal arc weldingAdvanced welding techniques: Magnetic arc welding, Friction welding, Explosive welding, Ultrasonic welding, electron beam welding, Laser welding

Joining processes which produces coalescence of materials by heating them to a suitable temperature and by using a filler metal having a liquidus not exceeding 450 oC and below the solidus of the base materials.

The filler metal (usually of lead and tin) is distributed between the closely fitted surfaces of the joint by capillary attraction

Soldering

Brazing is the joining of metal without melting them, using a filler metal which has a melting point above 4500C but below that of the parent metal, and which fills the joint by capillarity

Advantages - Brazing is a non-fusion techniques, as base materials does not melt, low distortion- Usually does not effect the properties of the parent metal.

So, post heat treatment are rarely required- Semi-skilled/unskilled labour can be used because of ease of

automation-Wide range of filler metal heating methods are available

BrazingBrazing

Brazing procedure

• Mechanical and chemical cleaning• Heat components• Flux and filler metal melting1. Borax (used above 7500C)-less corrosive than 22. Fluoride (used below 7500C)- used in silver brazing• Post-braze heat treatment• Post-braze cleaning• Inspection

Disadvantages or Difficulties• The nature of the braze component is complex. The

most important consideration as regards strength is the continuity of the bond, which can vary from 0-100%, as it is dependent on the ability of the brass metal to wet the surfaces of the gap

• In general, liquid braze metals will not wet, clean unfilmed surfaces unless

• (a) the liquid metal is intersoluble with the base (parent) metal

• (b) the liquid and solid metal react to form an intermetallic compounds

Fluxes

• Most common method of ensuring good wetting• Generally achieved by dissolving oxides• Same fluxes also deposit metals on to the surface of the

parent metal and reacts with the surfaces, thus preparing it chemically (e.g.:ZnCl2 flux-Zn is deposited on Fe surfaces giving tinning effect)

• Flux also has a blanketing effect on the surface keeping O2 out

• Fluxes are applied over heated area or filler rod is coated in flux

BrazingBrazing

Induction heating Inductor is placed close to the parts to be brazed. In most cases the coil surrounds the components. A high frequency current in the inductor induces a heating current in the work piece.

The brazing cycle can be precisely controlled using timing equipment built into the HF generator.

Advantage of induction heatingRapid and uniform heat-rateCan be used in inert atmosphere or in vacuumGood heating techniques for high qualityMostly used for steel components

CleaningCleaning

Mechanical cleaning–Usually abrasion will be necessary on large components –It is usually less efficient and more costly than chemical cleaning when large numbers of small components are involved in the production process.–Other mechanical methods generally employed are chipping and scratch brushing, rinsing or scrubbing with water, acid or other chemical

Cleaning

Chemical cleaning1. Degreasing using (a) Solvent (Petroleum or chlorinated

hydrocarbons) or (b) Vapour degreasing using stabilised

trichloroethylene, carbon tetrachloric or acetone

2. Scale or oxide removal can than take place by acid cleaning or pickling (salt pickling can also be used)

e.g. : Iron and steel – 10% H2SO4

Brass – 10% H2SO4 acid for 10 min maxm

Stainless steel – 7% HNO3 + 21% H2SO4 in water

Welding• The process of permanently joining two or more metal parts,

by melting both materials. The molten materials quickly cool, and the two metals are permanently bonded.

Advantage:• Higher mechanical properties• Fixing stress cracks• Reinforcing weak joints• Cutting or shaping new parts

Equipments used in gas and oxy-acetylene welding processes

Oxygen

Steel cylinder

Contained in compressed form

Supplied 3.4, 5 and 6.8 m3 capacities

Mild steel-13, 660 kN/m2

Alloy steel-17, 240kN/m2

R. H. thread in valve

Acetylene

Steel cylinder

High pressure acetylene is not stable so it dissolved in acetone, which has the ability to absorb a large volume of gas and release it as the pressure falls.

1 volume acetone-25 volume acetylene

Pressure 1, 552 kN/m2

Danger of explosion-porous substance

Welding gas mixture

Fuel Gas Maximum Flame temperature with air (degree C) with oxygen (degree C)

Acetylene 1 755 3 200

Butane 1 750 2 730

Coal gas 1 600 2 000

Hydrogen 1 700 2 300

Propane 1 750 2 500

Oxy-acetylene flame

Neutral flame

Oxidising flame

Carburising flame

www.twi.co.uk

Gas welding

Effect of welding on the structure

Ref: Basic fabrication and welding enginering, F. J. M. Smith, LST

Temperature distribution during oxy-acetylene welding 10 mm thick mild

steel

Arc weldingThe most common and economical method is AC arc welding

Arc

•Highly luminous and intensely hot discharge of electricity between two electrodes

•Discovered early 19th cent. by Sir Humphry Davy

•High current and low voltage

•When electrodes are parted, strong electric forces draw electrons from one electrode to the other, initiating the arc

Shielding gases in arc welding

Tungsten inert gas welding (TIG)

•Tungsten electrode-30000C

•Argon and Helium

•Filler material is added as in gas welding

Schematic of TIG

Shielding gases in arc welding

• Metal inert gas welding (MIG)

• Consumable electrode

• Argon, Helium and Carbon Dioxide

• No filler materials

MIG weld area:

Positions and arc welding

Risk involved in arc welding

1. Exposure to radiation

2. Flying sparks

3. Electric shock

4. Fumes

5. Damage to eyes

6. Burns

Safety

• Make sure to work on a dry floor. • Wear thick rubber shoes and dry leather welding gloves. • Be sure to use insulated electrode holders. • Check to make sure that your equipment is all properly

grounded. • Keep your work area properly ventilated to avoid

inhaling any potentially toxic fumes. • Be on the look out for flying bits of melted metal. • Most importantly, be aware of any other people who are

around you.

Types of weld

Types of weld

Welding processes-groups-letter designation

Magnetic Arc Welding

• Arc is rotated around the weld line by the force which results from the interaction between the magnetic field and the current

• CO2 or inert gas shielding is used

Steps in MIAB

• Faces to be joined are brought together and internal magnetic coil is put in place

• Welding current, magnetic coil system is put in place and shielding gas are turned on

• Work pieces are retracted to a defined gap to produce the arc

• Arc rotates about interface-melting faces to be joined

• Faces are pressed together

• Welding current, magnetic field and shielding gas are switched off

Magnetic Arc Welding

MIABFaster than arc fusion welding and conventional welding

Used industrially

Accurate-No further finishing machining operation are required

Allows quality control

MIAFNon-consumable electrode

Suitable for welding of thin wall pipes or tubes certain pressed sheet fabrication

Friction welding

-Friction heat caused by the motion of one surface against another enables plastic deformation and atomic diffusion at the interface

-Used by the automotive industry for decades in the manufacture of a range of components

-The weld is formed across the entire cross-sectional area of the interface in a single shot process

Advantages of friction welding

• Narrow HAZ

• Dissimilar metals can be joined

• No fusion zone

• Can be used under water

• Very high reproducibility - an essential requirement for a mass production industry

• Excellent weld quality, with none of the porosity that can arise in fusion welding

• environmentally friendly, because no fumes or spatter are generated, and there is no arc glare or reflected laser beams with which to contend

Variations of friction welding

• Rotary Friction Welding• Linear Friction Welding• Friction stir welding

Direct or continuous drive

Pre-determined time of motion determined by the size and type of material

Inertia friction welding

One of the work pieces is connected to a flywheel and the other is restrained from rotating

Flywheel used to provide energy and is disengaged before the work pieces are pushed together

Less drive power required than with direct drive welding

Linear friction welding

±1-3mm

Frequency 25-125Hz

Maximum axial force 150kN

Friction surface / Friction stir welding

Steps in friction stir welding

• A non-consumable rotating tool is pushed into the materials to be welded and then the central pin, or probe, followed by the shoulder, is brought into contact with the two parts to be joined.

• The rotation of the tool heats up and plasticises the materials it is in contact with and, as the tool moves along the joint line, material from the front of the tool is swept around this plasticised annulus to the rear, so eliminating the interface.

•Welding produced by explosively forcing one plate (or component) against the one to which it is to be joined at an approximate angle of incidence, known as the impact angle

•Methods: 1. Inclined gap method 2. Parallel gap method

In parallel gap method, detonation velocity should be equal to or less than the speed of sound in the metal being welded

Explosive Welding

 

Explosive Detonation velocity, m/s

RDX (Cyclotrimethylene trinitramine, C3H6N6O6 8100

PETN (Pentaerythritol tetranitrate, C5H8N12O4) 8190

TNT (Trinitrotoluene, C7H5N3O6) 6600

Tetryl (Trinitrophenylmethylinitramine, C7H5O8N5)

7800

Lead azide (N6Pb) 5010

Detasheet 7020

Ammonium nitrate (NH4NO3) 2655

Explosives

Explosive welding

Inclined gap method

• Various detonation speeds are possible with the inclined gap method

• A jet is formed. The jet is a thin layer of metal stripped from the surfaces of both plates, which in turn exposes the uncontaminated metal surfaces which are then welded in the high pressure zone, known as stagnation point

• Typically the weld surfaces are wavy• Weld is mainly solid state with small pockets of melted jet

material (on the front and back slopes of the waves)• Some welding may also be enhanced by friction due to the

difference in the velocity of the plates

Application of explosive welding• Cladding plates• Joining of pipes and tubes• Major areas of the use of this method are heat exchanger

tube sheets and pressure vessels• Tube Plugging• Remote joining in hazardous environments• Joining of dissimilar metals - Aluminium to steel,

Titanium alloys to Cr – Ni steel, Cu to stainless steel, Tungsten to Steel, etc.

• Attaching cooling fins• Other applications are in chemical process vessels, ship

building industry, cryogenic industry, etc.

Advantages of explosive welding

1. Can bond many dissimilar, normally unweldable metals.

2. Minimum fixturing/jigs.

3. Simplicity of the process.

4. Extremely large surfaces can be bonded.

5. Wide range of thicknesses can be explosively clad together.

6. No effect on parent properties.

7. Small quantity of explosive used.

Disadvantages of explosive welding1. The metals must have high enough impact resistance, and

ductility. 2. Noise and blast can require operator protection, vacuum

chambers, buried in sand/water. 3. The use of explosives in industrial areas will be restricted by

the noise and ground vibrations caused by the explosion. 4. The geometries welded must be simple – flat, cylindrical,

conical. 5. Area should be cleaned and sound grounded for explosion6. Licences are necessary to hold and use explosives

Ultrasonic welding• A solid state process for metal and plastics• Energy required comes in the form of mechanical vibrations• Most operates at 20, 30, 40 kHz• Weld is produced when the work pieces are clamped together

between an anvil and a high frequency vibration probe (sonotrode)• Empirical relation for a ultrasonic welding:

E=k(HT)3/2

Where, E = Electrical energyk = Constant for given welding systemH = Vickers hardnessT = Thickness of the work piece in contrast with the sonotrode

Types of ultrasonic welding

Wedge-Reed method – where the transducer is coupled through a resonant bar

Direct couple methods

Ultrasonic welding

• Sonotrode induces lateral vibration and local movement between the frying surfaces

• This tends to disrupt any surface oxide film present and also raises the temperature, extending an area of plastic flow, and a solid-phase type of pressure is formed

• Morphology of the weld is similar to the friction weld

Variants:• Spot welding- elliptical “spots”• Ring welding – hollow sonotrode tip• Line welding – linear sonotrode tip• Continuous welding – Rotating wheel shaped sonotrode

and a roller type of anvilApplication:• Largest growth area for ultrasonic welding is micro

miniature welding and micro joining in micro electric applications

• Capable of joining very fine wires to electrical components

Ultrasonic welding control

Ultrasonic welding control

Ultrasonic welding control

Ultrasonic welding

Advantages:

•Energy efficiency

•High productivity with low costs and ease of automated assembly line production

Disadvantages:

The maximum component length that can be welded by a single horn is approximately 250 mm. This is due to limitations in the power output capability of a single transducer, the inability of the horns to transmit very high power, and amplitude control difficulties due to the fact that joints of this length are comparable to the wavelength of the ultrasound.

Electron beam (EB) welding• EB welding is a fusion joining process in which the work

piece is embedded with a dense stream of high velocity electrons. Welding usually takes place in an evacuated chamber.

• Advantage: Very deep penetration can be achieved. For example, joining of 200 mm aluminium plates requires 600 passes when conventional gas metal arc process requires over 100 passes even using specially developed narrow-grove process. By using the EB process, the same plate can be welded in only 2 passes.

• Disadvantage: Dealing with the vacuum needed for the process

Laser welding

• Possible application is the fabrication of stiffened panel structures commonly used for ships, aircraft, and other structures. Stiffeners can be laser welded on to panels with no filler materials.

• No doubt that laser will be used in various ways in metal fabrication industries.

• It is still difficult to predict how extensively they will be used and how soon.

Ref: Metals hand book. Ninth edition. Vol 6: Welding brazing and soldering

Welding defects

Oxides in weldingOxides in welding

Difficulties: Form tenacious filmMelting point oxides higher than the parent metalRapid formation

Unless the oxides are removed:Fusion may be difficultInclusions may be present in the weld metalJoining will be weakened

Factors that contribute to the weld distortion

and their relation to each other and

to the total distortion

Ref: International series on materials Science and

Technology. V33: Analysis of welded structures

Combating distortion