Welding

Welding

Welding is a material joining process in which two (or more) parts are joined at their contacting surfaces by the suitable application of heat and/or pressure. In some welding processes a filler material is added to facilitate joining. The faying surfaces are the part surfaces that are to be joined. Welding is usually performed on parts made of the same metal.

Importance of Welding Welding provides a permanent joint.

The welded joint can be stronger if a filler metal is used that has strength properties superior to those of the parents.

Welding is usually the most economical way to join components in terms of materials usage and fabrication costs.

Limitation of Welding

Most welding operations are performed manually and are expensive in terms of labor cost

Most welding processes, involving the use of high energy, are inherently dangerous.

Since welding accomplishes a permanent bond between the components, it does not allow for convenient disassembly.

The welded joint can suffer from certain quality defects that are difficult to detect.

In Autogeneous welding processes, no filler metal is added to the interface. e.g. electric resistance welding

In Homogeneous processes, filler metal is added & is of the same type as the parent metal. e. g. welding of low C steel with a low C welding rod

In Hetrogeneous processes, a filler metal is used but is of a diff. type from the parent metal. e. g. brazing & soldering processes.

Applications of Welding

Welding is performed in a variety of locations and in a variety of industries:(1) Constructions (buildings and bridges).(2) Piping, pressure vessels, boilers and storage tanks.(3) Shipbuilding.(4) Aircraft and aerospace.(5) Automotive and railroad

Types of Weld Joints

There are five basic types of joints for brining two parts together for joining.(a) Butt Joint. In this joint type, the parts lie in the same plane and are joined at their edges.

Types of Weld Joints(b) Corner Joint. The parts in a corner joint form a right angle and are joined at the corner of the angle

(C) Lap Joint. This joint type consists of two overlapping parts.

Types of Weld Joints(d) Tee Joint. In the tee joint, one part is perpendicular to the other in the approximate shape of the letter T.

(e) Edge Joint. The parts in an edge joint are parallel with at least one of their edges in common, & the joint is made at the common edge(s).

Types of Welds(a) Fillet Weld is one in which filler metal is deposited at the corner of two intersecting surfaces such as a T or Lap joint.

(b) Bead Weld is one in which when the two surfaces in the same plane.

(c) Groove Weld is one in which the filler material is deposited in a groove formed by edge preparation.

(d) Spot or seam weld used without prepared holes. e.g. arc or resistance.

(e) Plug Weld used with prepared holes.

Edge preparationIt is necessary when thickness increases.Done by bevelling the edges of pieces.Five diff. types of chamfersa. Squareb. Vc. beveld. Ue. J

Butt JointSquare butt: For thickness upto 4.5*more than 4.5mm edge preparation is recommendedSingle V: For thickness upto 16 mmDouble V: For thickness > 16 mmSingle U & Double U: For greater than 20 mm

Lap Joint*Lap joint used to join sheet less than 3 mm thick.*Corner joint used to join sheet upto 5 mm thick.

Corner Joint-Used to join sheet upto 5 mm thick-Welded with or without edge preparation

Tee JointsA- plain teeB- single beveled: 10 to 20 mmC- double beveled: For thicker metalsD- single JE- double J

Edge JointUsed for metals upto 3 mm

Classification of Welding Processes

Welding processes are divided into three basic categories:

(A) Fusion welding.(B) Solid-state welding.(C) Brazing and Soldering welding.

(A) Fusion Welding

Fusion Welding processes use heat to melt the base metals.

In many fusion welding opns, a filler metal is added to the molten pool to facilitate the process and provide bulk and strength to the welded joint.

Classification of Fusion Welding Processes

(a) Arc Welding (AW)

(b) Resistance Welding (RW)

(c) Oxyfuel Gas Welding (OFW)

Arc Welding

Arc Welding (AW) is a fusion welding process in which joining of the metals is achieved by the heat from an electric arc betn an electrode & the work. A pool of molten metal, consisting of base metal(s) & filler metal (if one is used), is formed near the tip of the electrode.

Electric Arc Welding

Comparison of A.C. and D.C. arc weldingAlternating Current (from Transformer)

More efficiencyPower consumption less Cost of equipment is lessHigher voltage hence not safe Not suitable for welding non ferrous metalsNot preferred for welding thin sectionsAny terminal can be connected to the work or electrode

Comparison of A.C. and D.C. arc weldingDirect Current (from Generator)Less efficiencyPower consumption moreCost of equipment is moreLow voltage safer operationsuitable for both ferrous non ferrous metalspreferred for welding thin sectionsPositive terminal connected to the workNegative terminal connected to the electrode

What is polarity in welding?

Polarity in welding has to do with the direction of the current in the welding process.

1. DC Straight polarity: current flows from the electrode to the weld surface and creates considerable heat in the metal.

2. DC Reverse polarity: current is backwards and is flowing from the metal to the electrode causing a grater concentration of heat at the electrode.

Electrodes

Electrodes used in Arc Welding processes are classified as

Consumable electrodes provide the source of the filler metal in arc welding.

They are available in two principal forms: - Rods (225 to 450 mm long and 9.5 mm or less in dia.) - Wires which can be continuously fed into the welding process

2. Non-consumable electrodes are made of tungsten (or carbon), which resist melting by the arc.

Arc shielding

At the high temperatures in arc welding, the metals being joined are very chemically reactive to oxygen, nitrogen, and hydrogen in the air.

Arc shielding is accomplished by covering the electrode tip, arc, & molten weld pool with a blanket of gas or flux, or both, which inhibits exposure of the weld metal to air.

Mechanical properties of the weld joint can be seriously changed by these reactions.

Common shielding gases include argon and helium, both of which are inert.

Arc Shielding

Flux is a substance used to prevent the formation of oxides & other unwanted contaminants or to dissolve them and facilitate removal.

During welding, the flux melts & becomes a liquid slag, covering the operation & protecting the molten weld . The slag hardens upon cooling & must be removed

The methods of flux application include

(1) Pouring granular flux onto the welding operation.(2) Using a rod electrode coated with flux material.(3) Using tubular electrodes in which flux is contained in the core & released as electrode is consumed.

Types of Arc Welding

Arc Welding (AW) is a fusion welding process in which joining of the metals is achieved by the heat from an electric arc betn an electrode

Arc Welding processes use consumable electrodes(1) Shielding Metal Arc Welding.(2) Gas Metal Arc Welding.(3) Flux-cored Arc Welding.(4) Submerged Arc Welding.

Arc Welding processes use non-consumable electrodes(1) Gas Tungsten Arc Welding.

Shielding Metal Arc Welding

Shielded Metal Arc Welding (SMAW) is an arc welding process that uses a consumable electrode consisting of a filler metal rod coated with chemicals that provide flux and shielded.

Gas Metal Arc Welding

Gas Metal Arc Welding (GMAW) is an arc welding process in which theelectrode is a consumable bare wire & shielding is accomplished byflooding the arc with a gas. The bare wire is fed continuously &automatically from a spool through the welding gun.

Flux-cored Arc Welding

Flux-cored Arc Welding (FCAW) is an arc welding process in which theelectrode is a continuous consumable tubing that contains flux & other ingredients in its core.

Submerged Arc Welding

Submerged Arc Welding (SAW) is an arc welding process that uses acontinuous, consumable bare wire electrode, & arc shielding is provided by a cover of granular flux. The electrode wire is fed automatically from a coil into the arc.

Gas Tungsten Arc Welding

Gas Tungsten Arc Welding (GTAW) is an arc welding process that uses a non-consumable tungsten electrode & an inert gas for arc shielding.The term TIG welding (Tungsten Inert Gas welding) is often applied tothis process.

TIG welding

Resistance Welding Processes

Resistance welding (RW) is a group of fusion welding processes thatutilizes a combination of heat & pressure to accomplish joining, theheat being generated by electrical resistance to current flow at thejunction to be welded.The amount of heat generated at the contacting area to be welded,Q = I2Rt.k joulesWhere I = current in amperes R = resistance of the circuit in ohmst = time during which current flows, in secondsk = a constant < 1

Types of Resistance welding Processes

(1) Resistance Spot Welding(2) Resistance Seam Welding(3) Resistance Projection Welding

Resistance Spot Welding

Resistance Spot welding (RSW) is a resistance welding process in which fusion of the faying surfaces of a lap joint is achieved at one location by opposing electrodes. The process is used to join sheet-metal parts of thickness 3 mm or less. The size & shape of the weld spot is determined by the electrode tip, the most common electrode shapes are round & square.

Resistance Spot Welding

(1) Parts inserted betn open electrodes.(2) Electrodes close & force is applied(3) Weld time (current is switched on)(4) Current is turned off but force is maintained or increased(5) Electrodes are opened, & the welded assembly is removed.

Resistance Spot Welding

Resistance Seam Welding

Resistance Seam welding (RSEW) the stick-shaped electrodes in spot welding are replaced by rotating wheels, & a series of overlapping spot welds is made along the lap joint.

Since the operation is usually carried out continuously, rather than discretely, the seams should be along a straight or uniformly curved line.

Resistance Seam Welding Used in manufacture of tanks, tubes & other articles of steel & non ferrous metals which have airtight joint.Thickness of plate is 0.25 to 3.2 mm.

Resistance Projection WeldingFIG. Resistance projection welding: (a) before and (b) after. The current is concentrated at the spot to be welded, when simple projections are embossed on one of the sheet.

- used for joining nuts, bolts & studs to steel plates in car bodies

Resistance Projection Welding.

Solid- state Welding Processes

In solid-state welding, joining of the part surfaces is achieved by (1)pressure alone or (2) heat and pressure.

Types of Solid-state welding processes

(1) Forge Welding.(2) Cold Welding.(3) Roll Welding.(4) Diffusion Welding.(5) Friction Welding.(6) Ultrasonic Welding.

Solid- state Welding Processes

Forge Welding is a welding process in which the components to bejoined are heated to hot working temperatures & then forged togetherby hammer or other means. used to manufacture seamless piping, shotgun barrels.

Cold Welding (CW) is a solid-state welding process accomplished byapplying high pressure betn clean contacting surfaces at roomtemperature.used to manufacture kitchen utensils (tea kettles, racks), lap & butt welding of wires, trolley wires & home wiring.

Diffusion Welding (DFW) is a solid-state welding process that resultsfrom the application of heat & pressure, with sufficient time allowed for diffusion & joining to occur.Filled gold is obtained by placing gold face sheets over a silver or copper core.

Solid- state Welding Processes Roll Welding (ROW) is a solid-state welding process in which pressure sufficient to cause joining is applied by means of rolls, either with or without external application of heatused in joining dissimilar materials.

Solid- state Welding Processes

Friction Welding (FRW) is a solid-state welding process in which joining is achieved by frictional heat combined with pressure.

The friction is induced by mechanical rubbing betn the two surfaces, usually by rotation of one part relative to the other.

Friction-Welding ProcessFIG. Sequence of operations in the friction-welding process. (a) The part on the left is rotated at high speed. (b) The part on the right is brought into contact under an axial force. (c) The axial force is increased; flash begins to form. (d) The part on the left stops rotating. The weld is completed. Flash can be removed by machining or grinding.

Friction-Welding ProcessUsed for circular part i.e. butt welding of round bars or tubes, dissimilar metals can easily joined e.g. aluminium to steel, copper to steel, copper to aluminium.

Solid- state Welding Processes

Ultrasonic Welding (USW) is a solid-state welding process in which two components are held together under modest clamping force, & oscillatory shear stresses of ultrasonic frequency are applied to the interface to cause joining.

Generally limited to lap joints on soft materials such as aluminum andcopper.

Ultrasonic-WeldingFIG. (a) Components of an ultrasonic-welding machine for lap welds. The lateral vibrations of the tool tip case plastic deformation & bonding at the interface of the workpieces. (b) Ultrasonic seam welding using a roller.

Oxyfuel Gas Welding

Oxyfule Gas Welding (OFW) is the term used to describe the group of fusion operations that burn various fuels mixed with oxygen to perform weldingSound weld is obtained by selecting proper size of flame, filler material and method of moving torch

The temperature generated during the process is 33000c

Oxyacetylene Welding

Oxyacetylene Welding (OAW) is a fusion welding process performed by a high-temperature flame from combustion of acetylene and oxygen. The flame is directed by a welding torch. A filler metal is sometimes added.

Oxyacetylene Welding (OAW)OAW is a manual process in which the welder must personally control the the torch movement and filler rod application

Cylinders contain oxygen and acetylene gas at extremely high pressure.

Typical Oxyacetylene Welding (OAW) Station

GAS WELDING EQUIPMENT...1. Gas CylindersPressureOxygen 125 kg/cm2Acetylene 16 kg/cm2 2. RegulatorsWorking pressure of oxygen 1 kg/cm2Working pressure of acetylene 0.15 kg/cm2 Working pressure varies depends upon the thickness of the work pieces welded.3. Pressure Gauges4. Hoses5. Welding torch 6. Check valve

Oxy-Acetylene welding

RegulatorEach regulator has two gauges mounted on a single manifold, one indicates cylinder pressure and the other indicates working pressure for the torch

purpose:Reduce high cylinder pressures to suitable low working pressures.2. Maintain an even flow of these pressures.

Regulator

Regulator Hoses Hoses are are fabricated from rubberOxygen hoses are green in color and have right hand thread.Acetylene hoses are red in color with left hand thread. Left hand threads can be identified by a groove in the body of the nut.

Check Valves &Flashback ArrestorsCheck valves allow gas flow in one direction onlyFlashback arrestors are designed to eliminate the possibility of an explosion at the cylinder.Combination Check/ Flashback Valves can be placed at the torch or regulator.

Parts of a welding torchTorch body is the part of the torch that is held like a pencil, it contains two needle valves to control flow of gasWelding head contains mixer, mixing throat, and the welding tip

Three basic types of oxyacetylene flames used in oxyfuel-gas welding and cutting operations:

(a) neutral flame; (b) oxidizing flame; (c) carburizing, or reducing flame.

neutral flameburns equal amounts of oxygen and acetylene and has a clear edged inner cone (32000c)

Used for welding steels, aluminium, copper and cast iron

If more oxygen is added, the cone becomes darker and more pointed, while the envelope becomes shorter is called Oxidizing flameHas the highest temperature about 34000cUsed for welding brass and brazing operation

oxidizing flame

Excess acetylene with an acetylene feather two to three times the length of the inner cone (30000c)used some in hard surfacing, adds carbon to metal

carburizing, or reducing flame.

Oxyfuel welding (Gas Welding)

Thermit Welding

Thermit Welding8Al + 3Fe3O4 9Fe + 4Al2O3 + Heat

Brazing

Brazing is a joining process in which a filler metal is melted &distributed by capillary action betn the faying surfaces of the metalparts being joined

No melting of the basemetals occurs in brazing,only the filler melts.

In brazing, the fillermetal has a meltingtemperature that isabove 450 oC but belowthe M.P. of the base metals to be joined.

Advantages of Brazing

Any metals can be joined, including dissimilar metals.

(2) Brazing can be applied to join thin-walled parts that can not bewelded.

(3) In general, less heat & power are required than fusion welding.

(4) Problem with the heat affected zone in the base metal near the joint is reduced.

(5) Joint areas that are inaccessible by many welding processes can bebrazed.

Disadvantages of Brazing

joint strength is generally less that that of a welded joint.

(2) high service temperatures may weaken a brazed joint.

Soldering

Soldering is similar to brazing & can be defined as a joining processin which a filler metal with melting point not exceeding 450 oC is melted& distributed by capillary action betn the faying surfaces.

Advantages of Soldering(1) Low energy input relative to brazing and fusion.(2) Variety of heating methods available.(3) Good thermal & electrical conductivity in the joint.(4) Capability to make airtight & liquid-tight seams for containers(5) easy to repair and rework.

Disadvantages of Soldering(1) Low joint strength(2) Possible weaking or melting of the joint in elevated temperature

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