2 Content Soldering Brazing Welding Gas welding Metal arc welding Advanced welding techniques: Magnetic arc welding, Friction welding, Explosive welding, Ultrasonic welding, electron beam welding, Laser welding
3 SolderingJoining 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
4 BrazingBrazing 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 capillarityAdvantages- 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
5 Brazing procedure Mechanical and chemical cleaning Heat components Flux and filler metal meltingBorax (used above 7500C)-less corrosive than 2Fluoride (used below 7500C)- used in silver brazingPost-braze heat treatmentPost-braze cleaningInspection
6 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 gapIn 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
7 Fluxes Most common method of ensuring good wetting Generally achieved by dissolving oxidesSame 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 outFluxes are applied over heated area or filler rod is coated in flux
8 Brazing 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
9 Cleaning Mechanical cleaning Usually abrasion will be necessary on large componentsIt 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
10 Cleaning Chemical cleaning 1. Degreasing using (a) Solvent (Petroleum or chlorinated hydrocarbons)or(b) Vapour degreasing using stabilised trichloroethylene, carbon tetrachloric or acetone2. 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% H2SO4Brass – 10% H2SO4 acid for 10 min maxmStainless steel – 7% HNO3 + 21% H2SO4 in water
11 WeldingThe 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 propertiesFixing stress cracksReinforcing weak jointsCutting or shaping new parts
12 Equipments used in gas and oxy-acetylene welding processes OxygenSteel cylinderContained in compressed formSupplied 3.4, 5 and 6.8 m3 capacitiesMild steel-13, 660 kN/m2Alloy steel-17, 240kN/m2R. H. thread in valveAcetyleneSteel cylinderHigh 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 acetylenePressure 1, 552 kN/m2Danger of explosion-porous substance
13 Welding gas mixture Fuel Gas Maximum Flame temperature with air (degree C) with oxygen (degree C)AcetyleneButaneCoal gasHydrogenPropane
16 Effect of welding on the structure Ref: Basic fabrication and welding enginering, F. J. M. Smith, LST
17 Temperature distribution during oxy-acetylene welding 10 mm thick mild steel
18 Arc weldingThe most common and economical method is AC arc welding
19 ArcHighly luminous and intensely hot discharge of electricity between two electrodesDiscovered early 19th cent. by Sir Humphry DavyHigh current and low voltageWhen electrodes are parted, strong electric forces draw electrons from one electrode to the other, initiating the arc
20 Shielding gases in arc welding Tungsten inert gas welding (TIG)Tungsten electrode-30000CArgon and HeliumFiller material is added as in gas weldingSchematic of TIG
21 Shielding gases in arc welding Metal inert gas welding (MIG)Consumable electrodeArgon, Helium and Carbon DioxideNo filler materialsMIG weld area:
23 Risk involved in arc welding 1. Exposure to radiation2. Flying sparks3. Electric shock4. Fumes5. Damage to eyes6. Burns
24 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.
28 Magnetic Arc WeldingArc is rotated around the weld line by the force which results from the interaction between the magnetic field and the currentCO2 or inert gas shielding is used
29 Steps in MIABFaces to be joined are brought together and internal magnetic coil is put in placeWelding current, magnetic coil system is put in place and shielding gas are turned onWork pieces are retracted to a defined gap to produce the arcArc rotates about interface-melting faces to be joinedFaces are pressed togetherWelding current, magnetic field and shielding gas are switched off
30 Magnetic Arc Welding MIAF MIAB Faster than arc fusion welding and conventional weldingUsed industriallyAccurate-No further finishing machining operation are requiredAllows quality controlMIAFNon-consumable electrodeSuitable for welding of thin wall pipes or tubes certain pressed sheet fabrication
31 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
32 Advantages of friction welding Narrow HAZDissimilar metals can be joinedNo fusion zoneCan be used under waterVery high reproducibility - an essential requirement for a mass production industryExcellent weld quality, with none of the porosity that can arise in fusion weldingenvironmentally friendly, because no fumes or spatter are generated, and there is no arc glare or reflected laser beams with which to contend
34 Direct or continuous drive Pre-determined time of motion determined by the size and type of material
35 Inertia friction welding One of the work pieces is connected to a flywheel and the other is restrained from rotatingFlywheel used to provide energy and is disengaged before the work pieces are pushed togetherLess drive power required than with direct drive welding
36 Linear friction welding ±1-3mmFrequency HzMaximum axial force 150kN
38 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.
39 Explosive WeldingWelding 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 angleMethods: 1. Inclined gap method2. Parallel gap methodIn parallel gap method, detonation velocity should be equal to or less than the speed of sound in the metal being welded
42 Inclined gap methodVarious detonation speeds are possible with the inclined gap methodA 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 pointTypically the weld surfaces are wavyWeld 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
43 Application of explosive welding Cladding platesJoining of pipes and tubesMajor areas of the use of this method are heat exchanger tube sheets and pressure vesselsTube PluggingRemote joining in hazardous environmentsJoining of dissimilar metals - Aluminium to steel, Titanium alloys to Cr – Ni steel, Cu to stainless steel, Tungsten to Steel, etc.Attaching cooling finsOther applications are in chemical process vessels, ship building industry, cryogenic industry, etc.
44 Advantages of explosive welding Can bond many dissimilar, normally unweldable metals.Minimum fixturing/jigs.Simplicity of the process.Extremely large surfaces can be bonded.Wide range of thicknesses can be explosively clad together.No effect on parent properties.Small quantity of explosive used.
45 Disadvantages of explosive welding 1. 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
46 Ultrasonic welding A solid state process for metal and plastics Energy required comes in the form of mechanical vibrationsMost operates at 20, 30, 40 kHzWeld 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)3/2Where, E = Electrical energyk = Constant for given welding systemH = Vickers hardnessT = Thickness of the work piece in contrast with the sonotrode
47 Types of ultrasonic welding Direct couple methodsWedge-Reed method – where the transducer is coupled through a resonant bar
48 Ultrasonic weldingSonotrode induces lateral vibration and local movement between the frying surfacesThis 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 formedMorphology of the weld is similar to the friction weld
49 Variants:Spot welding- elliptical “spots”Ring welding – hollow sonotrode tipLine welding – linear sonotrode tipContinuous 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 applicationsCapable of joining very fine wires to electrical components
53 Ultrasonic welding Advantages: Energy efficiency High productivity with low costs and ease of automated assembly line productionDisadvantages: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.
54 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
55 Laser weldingPossible 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
57 Oxides in weldingDifficulties: Form tenacious film Melting point oxides higher than the parent metal Rapid formation Unless the oxides are removed: Fusion may be difficult Inclusions may be present in the weld metal Joining will be weakened
58 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
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