WELDING

Metal Joining processes

Joining processes and equipments

WeldingAdhesive bonding Mechanical fastening

Fusion Brazing & soldering Solid state

chemical

Electrical

Electrical

Chemical

Mechanical

Oxyfuel gasThermit

ArcResistance

Electron beamLaser beam

FasteningSeamingCrimpingstitching

Resistance

DiffusionExplosion

ColdFrictionUltrasonic

Examples of joints

Fusion welding processesTypes of flames

Neutral flame

Oxidising flame

Carburising flame

For neutral flame ratio of Oxygen and acetylene is 1:1.

For Oxidising flame Oxygen is more and acetylene is less.

For carburising flame Oxygen is less and acetylene is more.

Gas welding – Oxy-acetylene welding

(a) General view and (b) cross section of a torch used in oxy-acetylene welding. The acetylene valve is opened first, the gas is lit with a spark lighter or a pilot light, and then the oxygen valve is opened and the flame adjusted.

Oxy-acetylene welding - Equipments

Basic equipment used in oxyfuel gas welding. Other equipments include safety shields, goggles, gloves, and protective clothing. To ensure correct connections, all threads in the acetylene fittings are left handed, and those for oxygen are right handed. Oxygen regulators are usually painted green, and acetylene regulators red.

Arc – welding processesConsumable Electrodes

Arc – welding processesConsumable Electrodes

• Shielded metal - arc welding

• Sub merged arc welding

• Gas Metal Arc Welding (MIG)

• Flux cored arc welding

• Electro gas welding

Shielded metal - arc welding

• Definition:– Consumable electrode coated with chemicals

that provide flux and shielding– The filler metal (here the consumable electrode)

is usually very close in composition to the metal being welded.

Shielded metal - arc welding

Fig (a). Schematic illustration of shielded metal-arc welding operation, also known as stick welding because the electrode is in the shape of a stick

Fig. (b) weld zone showing the built up sequence of individual weld beads in deep welds

a

b

• Benefits•Simple, portable,& inexpensive•Self flux provided by electrode•Provides all position flexibility

• Shielding Gases•No shield gases added•Lower sensitivity to Wind

Shielded Metal Arc Welding

• Applications•Construction, pipelines, shipbuilding,

fabrication job shops.•Used for: Steels, stainless steels, cast

irons.• Not used for aluminum and its alloys, or copper

and its alloys (energy density is too high).

Shielded Metal Arc Welding Cont.:

Specifications of an Electrode

E6010

Stands for electrodeDesignates the tensile strength in ksi*

Position the electrode can be used 1- all positions 2- horizontal and flat

Electrode coating type

*1ksi = 7 Mpa

Volt – Ampere characteristic in Arc welding

Fig.a Produces only small amount of ampere change when the arc voltage is changed

Fig. a & bFlat and rising volt-ampere curves as used on automatic machines are designedto provide a relatively constant current by balancing the effect of volt X amps.

a b c

Comparison of weld penetration

weld penetration obtained by dc straight polarityweld penetration obtained by dc reversed polarity

An alternating – current welder is, in case, a combinationreverse – and straight – polarity machine.

Reverse polarity: Electrode connected to positive terminal and work connected to negative terminal

straight polarity: Electrode connected to negative terminal and work connected to positive terminal

Wave form of high frequency, low-power current

A super imposed high – frequency current along with capacitors stabilizes the welding current and produces a balanced wave form.

Sub merged arc weldingProcess featuresSimilar to MIG welding, SAW involves formation of an arc between a continuously-fed bare wire electrode and the work piece. The process uses a flux to generate protective gases and slag, and to add alloying elements to the weld pool. A shielding gas is not required. Prior to welding, a thin layer of flux powder is placed on the work piece surface. The arc moves along the joint line and as it does so, excess flux is recycled via a hopper. Remaining fused slag layers can be easily removed after welding. As the arc is completely covered by the flux layer, heat loss is extremely low. This produces a thermal efficiency as high as 60% (compared with 25% for manual metal arc). There is no visible arc light, welding is spatter-free and there is no need for fume extraction

Operating characteristics

Sub merged arc weldingApplications

SAW is ideally suited for longitudinal and circumferential butt and fillet welds. However, because of high fluidity of the weld pool, molten slag and loose flux layer, welding is generally carried out on butt joints in the flat position and fillet joints in both the flat and horizontal-vertical positions. For circumferential joints, the work piece is rotated under a fixed welding head with welding taking place in the flat position. Depending on material thickness, either single-pass, two-pass or multipass weld procedures can be carried out. There is virtually no restriction on the material thickness, provided a suitable joint preparation is adopted. Most commonly welded materials are carbon-manganese steels, low alloy steels and stainless steels, although the process is capable of welding some non-ferrous materials with judicious choice of electrode filler wire and flux combinations

Sub merged arc welding – Equipment layout

Schematic illustration of sub merged arc welding process

Gas Metal Arc Welding (MIG)

• Definition:– The heat source is formed by creating an electric arc

between the work piece and a wire, which is fed continuously into the weld pool.

• Benefits:– Long welds can be made

without starts and stops – Minimal skill required – Minimal cleaning of

surface before weld – Allows welding in all

positions

• Shielding Gases:• Inert

– Argon, Helium» Used for aluminum alloys and stainless steels.

•Active

– 1 to 5% Oxygen, 3 to 25% CO2

» Used for low and medium carbon steels

• Applications• Gas Metal Arc Welding (MIG) is used to weld all

commercially important metals, including steel, aluminum, copper, and stainless steel.

Gas Metal Arc Welding (MIG) Cont.

Gas metal arc welding - Equipment

Schematic illustration of gas metal arc welding process

Flux cored arc welding

Schematic illustration of Flux cored arc welding process

Electro gas welding

Schematic illustration of Electro gas welding process

Electro slag weldingUnlike other high current fusion processes, electro slag welding is not an arc process. Heat required for melting both the welding wire and the plate edges is generated through a molten slag's resistance to the passage of an electric current.

Schematic illustration of Electro slag welding process

Electro slag welding

A view of Electro slag welding process in component

Electro slag welding

Layout of equipment used for electro slag welding operations.

Electro slag weldingBenefits

The principal benefits of the process are: speed of joint completion; typically 1 hour per meter of seam, irrespective of thickness

lack of angular distortion

lateral angular distortion limited to 3mm per meter of weld

high quality welds produced

simple joint preparation, i.e. flame-cut square edge

major repairs can be made simply by cutting out total weld and re-welding

Electro slag welding

Limitations

Electro slag welding is not one of the major welding processes because the high heat input generates large, coarse grained weld metal and HAZs which lead to poor fracture toughness properties in these areas. Toughness improvements can only be achieved by post-weld normalising treatment. Additionally, the near parallel-sided geometry of the weld, combined with the coarse grains, can make it difficult to identify defects at the fusion boundary by standard ultrasonic NDT techniques.

Arc welding processes

Non consumable Electrodes

Arc welding processes(non consumable Electrodes) are

• Gas tungsten arc welding process

• Plasma arc welding processes

• Thermit welding

• Electron beam welding• Laser beam welding

Gas tungsten arc welding process• Definition:

– TIG welding is an arc that is formed between a non-consumable tungsten electrode and the metal being welded.

– Gas is fed through the torch to shield the electrode and molten weld pool.

• Benefits:– Welds with or without filler metal – Precise control of welding

variables (heat) – Low distortion

• Shielding Gases:– Argon – 2 to 5% Hydrogen – w/Helium

• Applications• Most commonly used for aluminum and

stainless steel• For steel

– Slower and more costly than consumable welding

– Except for thin sections or where very high quality is needed

Tungsten Inert Gas Welding (TIG)

Gas tungsten arc welding process Equipment

Plasma arc welding processes

Plasma welding is very similar to TIG as the arc is formed between a pointed tungsten electrode and the work piece. However, by positioning the electrode within the body of the torch, the plasma arc can be separated from the shielding gas envelope. Plasma is then forced through a fine-bore copper nozzle which constricts the arc. Three operating modes can be produced by varying bore diameter and plasma gas flow rate:

Thermit welding

Thermit welding (TW) is a welding process which produces coalescence of metals by heating them with superheated liquid metal from a chemical reaction between a metal oxide and aluminum with or without the application of pressure.

Filler metal is obtained from an exothermic reaction between iron oxide and aluminum. The temperature resulting from this reaction is approximately 2500°C. The superheated steel is contained in a crucible located immediately above the weld joint. The superheated steel runs into a mold which is built around the parts to be welded. Since it is almost twice as hot as the melting temperature of the base metal melting occurs at the edges of the joint and alloys with the molten steel from the crucible. Normal heat losses cause the mass of molten metal to solidify, coalescence occurs, and the weld is Completed.

Thermit welding

Few images of rails, joined by thermit welding

Thermit welding

Few images of rails joined by thermit welding

Electron beam welding

Laser beam welding

Cutting

• Definition:– A stream of oxygen is directed against a piece of

heated metal, causing the metal to oxidize or burn away.

• Making a Cut– Mark a line as a guide.– Turn on acetylene as for welding and light.– Turn on oxygen adjusting flame to neutral.– Make sure the oxygen lever flame remains neutral.– Place metal on the cutting table so metal will fall

clear.

Oxy fuel gas cutting

Oxy fuel gas cutting

Process fundamentalsThe cutting process is illustrated in Fig. Basically, a mixture of oxygen and the fuel gas is used to preheat the metal to its 'ignition' temperature which, for steel, is 700°C - 900°C (bright red heat) but well below its melting point. A jet of pure oxygen is then directed into the preheated area instigating a vigorous exothermic chemical reaction between the oxygen and the metal to form iron oxide or slag. The oxygen jet blows away the slag enabling the jet to pierce through the material and continue to cut through the material.

Oxy fuel gas cuttingThere are four basic requirements for oxy-fuel cutting:• the ignition temperature of the material must be lower than its melting point otherwise the material would melt and flow away before cutting could take place

• the oxide melting point must be lower than that of the surrounding material so that it can be mechanically blown away by the oxygen jet

• the oxidation reaction between the oxygen jet and the metal must be sufficient to maintain the ignition temperature

•a minimum of gaseous reaction products should be produced so as not to dilute the cutting oxygen As stainless steel, cast iron and non-ferrous metals form refractory oxides ie the oxide melting point is higher than the material, powder must be injected into the flame to form a low melting point, fluid slag

SOLID STATE WELDING PROCESSES

SOLID STATE WELDING PROCESSES ARE

• Cold Welding

• Ultrasonic Welding• Friction welding• Resistance welding - Resistance spot welding - High frequency resistance welding - Resistance projection welding• Flash welding• Stud welding• Explosion welding• Diffusion welding

Cold Welding

Schematic illustration of the roll bonding, or cladding process

Ultrasonic Welding

Fig. a) Components of an ultrasonic welding machine for lap welds

Fig. b) ultrasonic seam welding using roller

Friction welding

Sequence of operations in the friction welding process

Fig. (a) Left part is rotated at high speed

Fig. (b) Right part is brought into contact under an axial force

Fig. (c) Axial force increased; flash begins to form

Fig. (d) Left part stops rotation. Weld is completed. Flash can be removed by machining or grinding.

Friction welding

Shape of fusion zone in friction welding, as a function of force applied and rotational speed

Friction welding of a bolt head

Resistance spot welding

Fig. a) Sequence in resistance spot welding processFig. b) cross – section of a spot weld, showing weld nugget and light indentation by the electrode on sheet surfaces. This is one of the most common processes used in sheet – metal fabrication and automotive body assembly.

Spot welding machine

Schematic illustration of an air operated rocker arm spot welding machine

Spot welding operations for complex shape

Types of special electrodes designed for easy access in spot welding operations for complex shapes

High frequency resistance welding

Fig. a) seam welding process

Fig. b) overlapping spots in seam weld

Fig. c) roll spot weld

High frequency resistance welding of tubes

Methods in high frequency butt welding of tubes

Resistance projection welding

Schematic illustration of resistance projection weldingThe projections are produced by embossing operations

Before welding After welding

Resistance projection welding

Fig. a) projection welding of nuts (or) threaded bosses

Fig. b) projection welding of studs

The projections are produced by forging or machining operations

Flash welding

Fig. a) Flash welding process for end-to-end welding of solid rods

Fig. b) Design guidelines for Flash welding

Stud welding

Sequence of operations in stud welding, which is used for welding bars, threaded rods, and various fasteners on metal plates.

Explosion welding

Schematic illustration of the explosion welding process

Fig. a) constant interface clearance gap

Fig. b) angular interface clearance gap

Explosion welding

Cross – sections of explosion – welded joints

Fig. a) Titanium (top) on low carbon steel

Fig. b) Incoloy 800(iron nickel based alloy) on low carbon steel

Explosion welding

Explosion welding of tube on head plate for heat exchangers and boilers

Before welding During welding completed weld

Note: The tube ends are expanded by placing and detonating the explosives inside the tube

Diffusion welding

Sequence of diffusion bonding between two titanium-alloy sheets. Temperature;925 C: time: 1 hr;

Pressure: 700-3500kpa magnification; 500X

Why Would You Weld Under Water?

• It saves time.

• It can be cheaper

How Is It Done?There are 3 main ways:

1. Preparing an enclosure filled with gas (helium) under pressure and have the welder fitted with protective equipment.

2. Wet underwater method, where the power of the arc generates a bubble of mixture of gases and the welding is done, more or less, normally, with special electrodes.

3. Build a pit, near where the welding will occur and take the water out.

DISADVANTAGES

Electric ShockDangerous Gases

Underwater Hazards

New Processes

A relatively new underwater welding process is “Friction Stud Welding”, which is used for military and commercial use.

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