Welding Process

Welding Process

Welding is a metal joining process two or more, similar or dissimilar metals by heating them to a suitable temperature, with or without the application of pressure and with or without the use of filler metal.

It can also be defined as, welding is a physical & chemical mixing or coalescence of the interfaces by the application of heat and pressure.

Welding is used for making permanent joints. Heat is generated by chemical reaction, electric arc, electrical resistance and frictional heat etc.

The large bulk of materials that are welded are metals and their alloys, although  the term welding is also applied to the joining of other materials such as thermo  plastics.

Welding joins different metals/alloys with the help of a number of processes in  which heat is supplied either electrically or by means of a gas torch.

In order to join two or more pieces of metals together by one of the welding  processes, the most essential requirement is Heat. Pressure may also be  employed.

Since a slight gap usually exists between the edges of the work pieces, a ‘filler  metal’ is used to supply additional material to fill the gap. But, welding can also  be carried out without the use of filler metal.

The filler metal is melted in the gap, combines with the molten metal of the  work piece and upon solidification forms an integral part of the weld.

Basic Terms in Welding Process

Several technical terms used in welding technology:-

Figure:- Basic welding terms
  1. Base Metal or Parent metal– The metal to be joined called parent metal or base metal.
  2. Filler Metal– It is a metal or alloy used for filling the weld cavity. Filler metals usually have the same composition as that of the base metal.
  3. Weld Metal– It is the metal that solidifies in the weld cavity. It may be only metal or a mixture of base metal and filler metal.
  4. Root– It is the narrow region at the bottom of the welded joint. The gap between the metal pieces at the root region is called the root opening.
  5. Weld Face– It is the convex shape of the weld deposit.
  6. Weld Pass– A single movement of the welding torch or electrode along the length of the joint is called a weld pass.
  7. Penetration– The depth up to which the weld metal combines with parent metal is called metal penetration. It is measured from the top surface of the joint.

Also readTypes of Welded JointsLaser Beam Welding

Classification of Welding

Welding processes can be classified as follows-

 

Figure- Classification of welding processes

Another criterion used for a classification of the joining processes is based on the composition of the joint. According to this scheme, all joining processes can be grouped into three different categories, namely :

  1. Autogeneous
  2. Homogeneous
  3. Hetergeneous

In Autogeneous, no filler material is added during the joining. All types of solid phase welding and resistance welding are the examples of this category.

In Homogeneous joining processes, the filler material is used to provide the the joint is the same as the parent materail. Arc, gas and thermit welding belong to this category.

In Heterogeneous joining processes, a filler material different from the parent material is used. Soldering and brazing are two such joining processes. When two materials which are insoluble in each other, such as iron and silver, can be joined by a heterogeneous process. This may be achieved by using a filler material (e.g. copper and tin) which is soluble in both the parent materials (i.e. iron and silver)

Gas Welding

Gas welding is a welding process that melts and joins metals by heating their edges with a flame resulting from the burning of gas fuel and oxygen.

The oxyacetylene welding in which high flame temperature is used, in which oxygen acetylene combination is used.

In gas welding, the flame is produced at the tip of the welding torch.

It is used for heating the metal. Gas welding is widely used for welding all ferrous and non-ferrous metals.

Oxy-acetylene Welding (Oxy-fuel welding)

In oxy-acetylene welding, the flame is produced by burning a mixture of oxygen and acetylene.

It utilizes oxygen and fuel gas to heat metal and fuses multiple pieces together.

It can be used with or without the filler metals. The temperature generated during the process is 3480°C.

The apparatus used in gas welding consists of:-
  1. An oxygen source and a fuel gas source contained in cylinders
  2. Two pressure regulators
  3. Two flexible hoses (one for each cylinder)
  4. A torch viz. welding torch head is used to weld metals and a cutting torch head is used to cut materials. 
Figure:- Welding torch (top) and cutting torch (bottom)

Welding metal results when two pieces are heated to a temperature that produces a shared pool of molten metal. 

The molten pool is generally supplied with an additional metal called filler.

Filler material depends upon the metals to be welded.

Acetylene is the primary fuel for oxy-acetylene welding and is the fuel of choice for repair work and general cutting and welding.

Oxygen is not fuel. It is what chemically combines with the fuel to produce the heat for welding.

This is called ‘oxidation’, but the more specific and more commonly used term in this context is ‘combustion’.

This process of combustion occurs in two stages:

(1) The innermost blue, luminous core;

(2) The outer envelope.

In the first stage, the acetylene combines with the oxygen supplied to form carbon monoxide and hydrogen:

In the second stage, the carbon monoxide burns and forms carbon dioxide, while the hydrogen formed from the above actions combines with oxygen to form water:

The flame is applied to the base metal and held until a small puddle of molten metal is formed. The types of flame are shown in the figure given below

Figure: -Types of flame

which shows that when the proportion of oxygen supplied in the blowpipe is increased, the feather first disappears to leave the neutral flame, and further increases in the oxygen give an oxidizing flame. The inner cone is shorter and more pointed, and the excess of oxygen will react with the metal being welded.

Figure: -Equipment used in oxy-acetylene welding

In this method, the welding of two components starts with the heating of the interface by means of a torch using (typically) an oxyacetylene-gas mixture. After the interface begins to melt, the torch is withdrawn. A force is applied to press the two components together and is maintained until the interface solidifies.

Note the formation of a flash due to the upsetting of the joined ends of the two components.

Also see: Pattern Allowances

Electric Arc Welding

Arc welding is the process of joining two metal pieces by melting their edges with an electric arc.

Principle of Arc

An arc is generated between cathode and anode when they are touched to establish the flow of current and then separated by a small distance. In this process, around 65% to 75 % of heat is generated at the anode. The temperature of the arc is about 3870°C.

 

Figure: – Basic circuit for electric arc welding

Figure exhibits a schematic arrangement of an arc welding equipment in which an alternating current or a direct current source can be used for the arc welding process.

Direct current arc welding process can be classified into two types when the electrode is connected to the negative terminal of the power source and workpiece to the positive terminal,known as direct current straight polarity (DCSP) arc welding.

In another case, if the electrode is connected to the positive terminal of the power source and the workpiece to the negative terminal, known as direct current reserve polarity (DCRP) arc welding.

Working of the Electric Arc Welding

Following are steps involved in the process of  arc welding:

Figure: -Arc welding

 

  • The electrode is held by means of an electrode holder and the workpieces are kept on a metal work table.
  • Electrode used in arc welding may be a consumable or non-consumable electrode.
  • A consumable electrode is used to produce an arc and is also melted to fill the weld cavity it also serves as a filler metal. A non-consumable electrode is used only for producing an electric arc. Electrodes may be bare, flux or heavy coated.
  • The electrode holder and the metal work table are connected to different terminals of the welding power source by means of long insulated cables.
  • Then the metal workpiece is touched with the electrode and then separated leaving a small gap between the electrode tip and the workpiece.
  • In arc welding, the gap between the electrode and workpiece is important for the arc to exist continuously, it is known as arc length.
  • Once the arc has been initiated, the electrode is moved along the length for completing the welding process.

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