Welding

SOLDERING, BRAZING & WELDING INTRODUCTION  Some products cannot be manufactured as a single piece. The desired shape and size of such products can be obtained by joining two parts of same or different materials. These parts are manufactured individually and are joined together to obtain the desired product.

For example, aircraft and ship bodies, welded machine frames, furniture, computers, bridges and the transmission or electric towers etc., are all fabricated by joining several different parts.

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Classification of joining processes:Temporary JointPermanent Joint

A temporary joint can be easily dismantled separating the original parts without any damage to them

In case it is a permanent joint, an attempt to separate the parts already joined will result in the damage of the parts. In a permanent joint, the joint is made such that it has properties similar to the base metal of the two parts. These parts cannot be separated into their original shape, size and surface finish 2

Based on the process used for making the joint, the joining processes can be further classified as:

Soldering.

Brazing.

Welding.

Mechanical Fasteners - bolts, nuts, rivets, screws

Adhesive bonding.

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Welding is one of the most extensively used fabrication method. The joint strength obtained in welding is being equal to or some times more than that of the parent metal.

Welding is not only used for making structures, but also for repair work such as the joining of broken castings.

The choice of a particular joining process depends on several factors such as application, nature of loads or stresses, joint design, materials involved and size and shape of the components

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WELDING

Welding is a process of metallurgical joining of two pieces of metals by the application of heat with or without the application of pressure and addition of filler metal. The joint formed is a permanent joint.

It is extensively used in the fabrication work in which metal plates, steel sections, castings of ferrous metals are joined together. It is also used for repairing broken, worn-out or defective metal parts.

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Modern methods of welding may be classified under two broad headings.• Plastic or pressure welding process • Fusion or non-pressure welding process

In plastic or pressure welding process the pieces of metal to be joined are heated to a plastic state and then forced together by external pressure. This procedure is used in forge welding, resistance welding, spot welding in which pressure is required.

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In the fusion or non-pressure welding, the material at the joint is heated to a molten state and allowed to solidify. This includes gas welding, arc welding

The surfaces of the metal which are to be joined by any of the welding processes must be sufficiently clean to permit clean metallic surfaces to come in to contact.

Fluxes are applied to the parts being welded to dissolve the oxides or to prevent the formation of oxides.

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Types of Welding Joints The choice of the type of joint is governed by the kind of metal to be welded, its thickness and technique of welding

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GENERAL WELDING PROCEDURE

1.Surface Cleaning: Surfaces of the parts to be welded need to be thoroughly cleaned to remove dust, dirt, oil, grease etc.

2.Edge Preparation: Preparing a contour at the edges of the pieces to be joined. It may involve beveling or grooving. This is done in order to get the fusion or penetration through the entire thickness of the member.

3.Clamping: Pieces to be welded are clamped suitably so that there are no undesirable movements during welding.

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4. Safety Devices: Goggles & shields to protect the eyes, Apron to prvent

the sparks and flying globules of molten metal, shoes, hand gloves etc.

5. Initial Weld: Initial tack welds are done at the opposite corners of the

joint to secure the pieces together. Any cracks at this stage must be removed as they cause residual stresses.

6. Intermediate and Final Welding: The weld joint is formed through various weaving

movements (weld beads). During this process, filler metal and a suitable flux are used. After the intermediate run of welding, final run is taken.

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7. Removal of Excess Material: Extra material on the weld surface can be removed

using tongs and chipping hammer. The weld is allowed to cool and then cleaned.

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ARC WELDING

Joining of metals with heat produced by an electric arc.

Heat necessary to melt the edges of the metal to be joined is obtained from an electric are struck between the electrode (filler rod) and the work, producing a temperature of 50000C, in the welding zone.

The heat of the arc melts the base metal or edges of the parts fusing them together.

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Filler metal, usually added melts and mixes with molten base metal to form the weld metal.

The weld metal cools and solidifies to form the weld.

A typical arc welding setup consists of:   An arc welding circuit - power supply

An electrode to conduct the electricity to the arc.

Cables which connect the power supply to the electrode and workpiece to complete the welding circuit.

The arc itself provides the heat for welding.

The workpieces to be welded are kept on a metallic table.

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Arc Welding Setup

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Arc Welding Process

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The arc must be shielded because - as it hardens the molten metal combines with oxygen and nitrogen to form impurities that weaken the weld.

The electrodes are usually coated with a flux. This coating forms a gaseous cloud that shields the molten metal from the atmosphere.

The coating also forms a protective slag. The slag floats on the molten pool and hardens as the weld cools. This keeps impurities out of the weld.

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ARC WELDING MACHINE: AC or DC

AC Arc Welding Machine:

• A step down transformer - receives the AC supply between 200 to 440volts and transforms it to the required low welding voltage in the range of 80 to 100volts.

• A high current of 100 to 400A will be suitable for general arc welding work.

• In AC arc welding, there is no choice of polarity since they change in every cycle.

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DC Arc Welding Machine:

• In DC welding, the workpiece is connected to the positive pole of a DC generator and the electrode to the negative pole in order to melt greater mass of the metal in the base material. This is called straight polarity.

• When the less heat is required at the base material, the polarity is reversed. This is called reversed polarity.

• We can select the polarity depending upon the type of the job. Hence, in DC arc welding, it is possible to melt many metals which require more heat to melt.

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ARC WELDING ELECTRODES

1.Consumable Electrodes: Melt along with the workpieces & fills the joint.

They are either Bare or Coated.

When the bare electrodes are used, the globules of the molten metal while passing from the electrodes absorb oxygen and nitrogen from the atmospheric air to form non-metallic constituents which gets trapped in the solidifying weld metal and thereby decreasing the strength of the joint.

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Coated Electrodes facilitate:

1. Protection of molten metal from oxygen and nitrogen of the air by providing a gaseous shield around the arc and the molten metal pool.

2. To establish & maintain the arc throughout welding

3. The formation of slag over the joint, thus protects from rapid cooling.

4. Addition of alloying elements

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2. Non-consumable Electrodes:

When non-consumable electrodes are used, an additional filler material is also required. The advantage of using this type of electrode is that the amount of the metal deposited by the filler rod can be controlled.

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Advantages of Arc Welding1.Applicable to an infinite variety of work & can be executed in any position.

2.There is less buckling and warping of the work.

3.It produces strong sound and ductile welds.

4.Satisfactory welds can be produced in heavy & light sections.

5.Low cost process & Low accuracy in setting up required.

6.Excellent joint properties can be obtained in mild, low alloy and stainless steels, nickel and copper-base alloys.

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Disadvantages of Arc Welding

1.Basically a manual process requiring adequate operator skill for good results.

2.Electrodes require frequent changing.

3.Multi run welds necessary on thick plate-slag chipping necessary after each run.

4.The principal disadvantage has been the high heat of the metal arc which makes it unsuitable for use on materials less than 1.55 mm thick.

5.High initial cost of welding equipment. 24

Sl No. Aspect AC Welding DC Welding

1. Arc Stability Lower/ Unstable Higher/Stable

2. Cost Low High

3. Electrodes Only Coated Both Bare & Coated

4. Suitability Non-ferrous metals cannot be joined

Suitable for both ferrous & nonferrous metals

5. Electrical energy Consumption

Less energy consumption per kg of the metal deposited (3 to 4 kWh)

More energy consumption per kg of the metal deposited (6 to 10 kWh)

6. Efficiency High Low

7. Polarity No choice of polarity Straight or reversed polarity can be used depending on the type of job and heat required at the base metal

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GAS WELDING Gas welding is a fusion welding process

Flame produced by the combustion of gases is employed to melt the metal

The molten metal is allowed to flow together thus forming a solid continuous joint upon cooling.

By burning pure oxygen in combination with other gases, in special torches, a flame up to 33000C can be attained.

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Oxy-acetylene Gas Welding Equipment

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Oxy-acetylene Gas Welding Equipment

1.Welding torch & tip2.An acetylene cylinder – 15.5 bar, red or maroon3.An oxygen cylinder – 125 bar. Blue or black4.Pressure regulator – Cylinder pressure to delivery pressure5.Pressure gauge- One shows cylinder pressure & the other shows the working or delivery pressure6.Rubber hoses - black/green hose for oxygen & red/orange hose for acetylene7.Safety devices – Goggle with coloured glasses, hand gloves, helmet, apron, sleeves, shoes etc.

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The oxy-acetylene flame is used to pre heat the parts to be welded around the joint and also to melt the filler metal.

A jet of oxy acetylene flame issuing from the nozzle of a burner is played on the junction of the two pieces to be welded.

At the same time a filler rod is held in the zone of jet and its melt is deposited on the fused junction.

A weld is obtained after the molten metal solidifies. The coating on the filler rod acts as a flux to keep the joint clean.  29

GAS WELDING FLAMES (OXY-ACETYLENE FLAMES)

(Based on Gas Ratio)1.Neutral Flame: (Gas ratio is 1)Oxygen & acetylene are supplied to the torch in nearly equal volumes Maximum temperature of 32000C. This neutral flame is desired for most welding operations. Used for welding steel, stainless steel, cast iron, Cu etc.

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2. Carburizing or Reducing Flame: ( Gas ratio 0.95 to 1) Excess of acetylene is present, Low temp flame The excess unburnt carbon is absorbed in ferrous metals,

making the weld hard and brittle. An intermediate flame feather exists - reddish in colour.

The length of the flame feather is an indication of the excess

acetylene present. Carbonizing flame is used for welding high carbon steels

and cast iron, alloy steel.

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2. Oxidizing Flame: (Gas ratio 1.15 to 1.5) Excess of oxygen is present, similar to the neutral flame Inner white cone is some what small, giving rise to higher

tip temperatures. Excess of oxygen causes the metal to burn/oxidize quickly. Desirable for welding only brass because a thin layer of slag

forms over the molten metal. Widely used for oxyacetylene cutting and not suitable for

welding since the weld metal will be oxidised.

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Advantages of Oxy-acetylene Gas Welding

1.The equipment is inexpensive, simple and is easily portable.

2.Useful for welding light metals such as automobile bodies and repair works.

3.A large variety of material can be welded.

4.Welds can be produced at reasonable cost.

5.Compared to electric arc welding this provides greater flexibility with respect to heat impact and cooling rates.

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Disadvantages of Oxy-acetylene Gas Welding

1. Equipment must always be handled carefully as in certain circumstances acetylene is explosive as oxygen when used in an oily atmosphere (such as an old dirty garage floor pit). 2. A high temperature flame from a hand held torch is dangerous when handled carelessly.3. It is much slower than electric arc welding and does not concentrate the heat close to the weld. Thus, the heat treated area is larger, which causes more distortion.4. Highly skilled operators are required to produce a good weld.5. If electric arc welding is available gas welding is seldom used for work over 3.2mm thick.6. The process is not satisfactory for heavy sections

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WELDING DEFECTS

1.Cracking: Due to incorrect electrodes or wrong working procedure, locked up stresses set up by non-uniform heating & cooling, excess sulphur or phosphorous present in the weld metal. Cracked welds must be cut out & rewelded.2. Incorrect edge preparation: Results in poor fusion, slag inclusion, weak weld, overheating and build up of residual stresses in the weld.3.Craters: Concave depressions in the external surface of the welded joints which reduces the strength of the joint.4.Under cutting: Excess melting of the parent metal due to the non-uniform feed of the welding rod, improper position of the electrode etc.

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WELDING DEFECTS

5.Porosity: Formation of blowholes, gas pockets or roughness on the weld surface due to the presence of gases in the weld metal, moisture in the flux or rust on the welded edges.6.Over welding: Welding over an already welded layer. Results in overheating of the earlier layer of weld and also in improper fusion.7.Slag inclusions: It is the presence of non-metallic substances in the weld. This is due to the contamination of the base & deposited metal by oxides, non-uniform melting of the electrode coating & high viscosity of the slag.8. Poor Fusion: The lack of thorough & complete union between the deposited & parent metal.

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WELDING-MERITS1.Homogeneity and homogeneous properties.

2. Joints withstand elevated stresses.

3.Durable joints – unaltered due to environmental changes or temperature changes.

4. leak-tightness

5.Portable equipment, cheap & economical process.

6.Less noisy operation compared to some of the mechanical joining processes.

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WELDING-DEMERITS

1.Causes residual stresses & distortion of the workpiece.2. Eyes of the operator get strained due to continuous welding or due to light radiations emitted during welding.3.If the welding area is less ventilated or if the air circulation is minimum, fumes that may come out can be suffocating to the welders.4.Requires edge preparation before welding.5.Stress relieving is essential after welding.6.Requires skilled operator for producing a neat job.

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WELDING-APPLICATIONS

1.In aircraft building industry.2.Cylinders, boilers & vessel manufacturing.3.Structural constructions like bridges, buildings, ships etc.4.Manufacture of various machineries like mechanical, food processing, agricultural, earth moving & textile machineries.5.Building of automobile bodies and other parts.6.Manufacturing of furnaces, tanks, cranes, hoists, railway equipments, steel furniture etc.

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SOLDERINGMethod of joining similar or dissimilar metals by the

application of heat and using a filler metal or fusible alloy called solder, whose liquidus temperature is below 4500 C

The molten filler metal is made to flow between the

two closely placed adjacent surfaces by the capillary action.

Strength of the joint is limited by the strength of the filler metal used.

Soldering is used for obtaining a leak proof joint or a low resistance electrical joint.

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The soldered joints are not suitable for high temperature applications because of the low melting temperatures of the filler metals used.

The purpose of using the flux is to prevent the formation of oxides on the metal surface when the same is heated.

The fluxes are available in the form of powder, paste, liquid or in the form of core in the solder metal.

It is necessary that the flux should remain in the liquid form at the soldering temperature and be reactive to be of proper use

The composition of solder used for different purposes are as given below:

Soft solder - lead 37% tin 63%Medium solder - lead 50% tin 50%Plumber’s solder - lead 70% tin 30%Electrician’s solder - lead 58% tin 42%

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Used extensively in sheet metal work for joining parts that are not exposed to the action of high temp & are not subjected to excessive loads and forces or vibrations.

Also employed for joining wires and small parts. The solder is mostly composed of lead and tin whose melting temperature range is 150-350°C.

ZnCl2- commonly used flux- quick acting & produce efficient joints. As they are corrosive,joint should thoroughly cleaned of all the flux residue from the joint.

A soldering iron is used to apply the heat produced from the electrical source.

SOFT SOLDERING

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HARD SOLDERING

An alloy of copper, tin and silver known as hard solder is used for stronger joints.

Hard soldering employs solder which melts at higher temperatures ( 6000 C to 9000 C) is stronger than used in soft soldering.

German silver, used as a hard solder for steel is an alloy of copper, zinc and nickel.

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SEQUENCE OF OPERATIONS

Shaping and fitting of metal parts together: The two parts to be joined are shaped to fit closely so that the space between them is extremely small and filled completely with solder by capillary action.

Cleaning of surfaces: surfaces to be soldered are cleaned to remove dirt grease or any other foreign material.

Application of flux: The flux is applied when the parts are ready for joining. This cleans the surfaces chemically and helps the solder in making a bond.

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SEQUENCE OF OPERATIONS

Tinning & application of solder: After the soldering iron has been heated to the desired heat, its surface is cleaned and then dipped in a mixture of flux and solder. The solder is then melted into the joint is smoothed over and finished by the use of the soldering iron. Another practice is to first dip it in a mass of flux followed by the application of solder. This enables the solder to melt and spread over the hot surface of the bit to form a coating over it. This operation is known as tinning.

Final clean-up: Clean the soldered joints with solvent to remove the remaining traces of flux.

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SOLDERING-MERITS

1.Simple method & less costlier.

2.Joints are easy to repair or rework.

3.Joint can last for many years.

4.Energy required for the process is less.

5.A high degree of control can be obtained over the process by a skilled operator.

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BRAZING

Brazing is a process of making joints where incoalescence is produced by heating to suitable temperatures above 5000 C and by using a non-ferrous filler metal –SPELTER- having a melting point (up to 9000 C) below that of the base metal

Filler metal being distributed between the closely fitted surfaces of the joint by capillary action.

Both similar and dissimilar metals can be joined.

Brazing gives a much stronger joint than soldering.

Borax is the most widely used flux 48

Spelter may be a silver based alloy or copper based alloy

Copper base alloys – Brasses ( Cu & Zn), sometimes with up to 20% tin are mostly used-melting range of 850°C-950°C - used for brazing ferrous metals.

Silver base alloys – (Ag & Cu or Ag, Cu & Zn) - melting range of 600°C-850°C - clean finish & a strong ductile joint.

Types of Brazing:1.Torch Brazing2.Furnace Brazing3.Resistance Brazing4.Immersion Brazing 49

Brazing is a much widely used joining process in various industries because of its many advantages.

Dissimilar metals, such as stainless steel to cast iron can be joined by brazing.

Almost all metals can be joined by brazing except aluminium and magnesium which cannot easily be joined by brazing.

Because of the lower temperatures used there is less distortion in brazed joints.

The brazed joints are reasonably stronger, depending on the strength of the filler metal used.

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Sl. No

WELDING SOLDERING BRAZING

1. High temperature operation & the base metals are melted

Low temperature operation in which the base metals are not melted

Temperature range is higher than that of soldering, but lower than the welding temperature. Base metals are not melted.

2. Applicable for joining similar metals only

For joining both similar and dissimilar metals

For joining both similar and dissimilar metals

3. Filler material is of similar composition as that of the parent metal

Solder is the filler material which is an alloy of lead & tin ( soft solder) or an alloy of copper, tin & silver (hard solder)

Spelter is the filler material which may be either a copper base alloy or a silver base alloy

4. Electrode is coated by the required flux materials

Flux is applied on the surface to be soldered separately

Flux is applied on the surface to be brazed separately

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Sl. No

WELDING SOLDERING BRAZING

5. Joint is formed by the solidification of the molten filler metal with the molten base metal

Joint is produced by the formation of an intermetallic compound between the metals & filler material due to diffusion or alloying process

Joint is produced by the formation of an intermetallic compound between the metals & filler material due to diffusion or alloying process

6. Heat affected zone is large Heat affected zone is nil Heat affected zone is negligible

7. Very stronger joint Joint formed is Less stronger than welded or brazed joint

Joint formed is stronger than soldered joint, but weaker than the welded joint

8. Used to join thick metals, in the fabrication work, structural constructions etc.

Mainly to join thin sheet metals, pipes, wires etc.

For non-structural applications

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