lectric welding is a type of welding process that uses electricity to produce heat and melt metal.

electric welding

1. ELECTRIC WELDING UNIT-II KUMAR SALIGANTI Assistant Professor (C) Department of Electrical and Electronics Engineering JNTUH College of Engineering Manthani

2. CONTENTS: Introduction-welding Advantages and disadvantages of weldin g Types of welding Electric welding equipment Comparison between A.C. and D.C. welding Classification of electric welding Resistance welding – types Arc welding - types

3. The Welding is a process of joining two or more, similar or dissimilar metals by heating them to a suitable temperature , with or without the application of pressure, filler materials and flux. WELDING: Welding is used for making permanent joints.

4. ADVANTAGES OF WELDING: o Welding is the most economical method to permanently join two metal parts. o It provides design flexibility. o Welding equipment is not so costly. o It joins all the commercial metals. o Both similar and dissimilar metals can be joined by welding. o Portable welding equipments are available. DISADVANTAGES: o Welding gives out harmful radiations and fumes. o Welding needs internal inspection. o If welding is not done carefully, it may result in the distortion of workpiece. o Skilled welding is necessary to produce good welding.

5. 2. Fusion Welding or Non-Pressure Welding: The material at the joint is heated to a molten state and allowed to solidify. (Ex) - Arc welding, Gas welding 1. Plastic Welding or Pressure Welding: The piece of metals to be joined are heated to a plastic state and forced together by external pressure. (Ex) - Resistance welding TYPES OF WELDING:

6. ELECTRIC WELDING EQUIPMENT: Electric welding accessories required to carry out proper welding operation are: 1. Electric welding power sets. 2. Electrode holder to hold the electrodes. 3. Welding cable for connecting electrode and workpiece to the supply. 4. Face screen with colored glass. 5. Chipping hammers to remove slag from molten weld. 6. Wire brush to clean the weld. 7. Earth clamp and protective clothing.

7. ELECTRIC WELDING EQUIPMENT :

8. COMPARISON BETWEEN AC AND DC WELDING

9. CLASSIFICATION OF ELECTRIC WELDING

10. RESISTANCE WELDING: It is the process of joining two metals together by the heat produced due to the resistance offered to the flow of electric current at the junctions of two metals. The heat produced by the resistance to the flow of current is given by: H = I 2 Rt

11. Advantages: Welding process is rapid and simple. Localized heating is possible, if required. No need of using filler metal. Both similar and dissimilar metals can be welded. Comparatively lesser skill is required. It can be employed for mass production.

12. Initial cost is very high. High maintenance cost. The work piece with heavier thickness cannot be welded, since it requires high input current. Dis-advantages: Applications: It is used by many industries manufacturing products made up of thinner gauge metals. It is used for the manufacturing of tubes and smaller structural sections.

13. Types of resistance welding The resistance welding is classified as: 1. Spot welding 2. Seam welding 3. Projection welding 4. Butt welding (i) Spot welding: Spot welding means the joining of two metal sheets and fusing them together between copper electrode tips at suitably spaced intervals by means of heavy electric current passed through the electrodes.

14. Spot welding is widely used for automatic welding process, for joining automobile parts, joining and fabricating sheet metal structure, etc. Applications: Fig. Water cooled electrode

15. (ii) Seam welding: Seam welding is nothing but the series of continuous spot welding. If number spots obtained by spot welding are placed very closely that they can overlap, it gives rise to seam welding. Seam welding is obtained by keeping the job under electrodes. When these wheel type electrodes travel over the metal pieces which are under pressure, the current passing between them heats the two metal pieces to the plastic state and results into continuous spot welds.

16. It is usually employed in welding of pressure tanks, transformers, condensers, evaporators, air craft tanks, refrigerators, varnish containers, etc. In general, it is not satisfactory to make a continuous weld, for which the flow of continuous current build up high heat that causes burning and wrapping of the metal piece. To avoid this difficulty, an interrupter is provided on the circuit which turns on supply for a period sufficient to heat the welding point.

17. (iii) Projection welding: It is a modified form of the spot welding. In the projection welding, both current and pressure are localized to the welding points as in the spot welding. The two pieces of base metal to be weld are held together in between the two platens, one is movable and the other is fixed. As current flows through the two metal parts to be welded, which heat up and melt. These weld points soon reach the plastic state, and the projection touches the metal then force applied by the two flat electrodes forms the complete weld. But the only difference in the projection welding is the high mechanical pressure applied on the metal pieces to be welded, after the formation of weld. The electrodes used for such welding are flat metal plates known as platens.

18. The projection welding has the following advantages over the spot welding. Simplicity in welding process. It is easy to weld some of the parts where the spot welding is not possible. It is possible to join several welding points. Welds are located automatically by the position of projection. As the electrodes used in the projection welding are flat type, the contact area over the projection is sufficient. Projection welding is used extensively by auto manufactures for joining nuts, bolts and studs to steel plates in car bodies. This process is especially suitable for metals like brass, aluminum and copper etc. mainly due to their high thermal conductivity. Applications: The projection welding is particularly employed for mass production work, i.e., welding of refrigerators, condensers, crossed wire welding, refrigerator racks, grills, etc.

19. (iv) Butt welding: Butt welding is similar to the spot welding; however, the only difference is, in butt welding, instead of electrodes the metal parts that are to be joined or butted together are connected to the supply. Butt-welding is an economical and reliable way of joining without using additional components. The three basic types of the butt welding process are: 1. Upset butt welding. 2. Flash butt welding. 3. Percussion butt welding.

20. Butt welding machine

21. (a) Upset butt welding: In upset butt welding, the two metal parts to be welded are joined end to end and are connected across the secondary of a welding transformer. When current is made to flow through the two electrodes, heat will develop due to the contact resistance of the two pieces and then melts. By applying high mechanical pressure either manually or by toggle mechanism, the two metal pieces are pressed. When jaw-type electrodes are used that introduce the high currents without treating any hot spot on the job. This type of welding is usually employed for welding of rods, pipes, and wires and for joining metal parts end to end.

22. (b) Flash butt welding: Flash butt welding is a combination of resistance, arc, and pressure welding. This method of welding is mainly used in the production welding. In this method of welding, the two pieces to be welded are brought very nearer to each other under light mechanical pressure. These two pieces are placed in a conducting movable clamps. When high current is passed through the two metal pieces and they are separated by some distance, then arc established between them. This arc or flashing is allowed till the ends of the workpieces reach melting temperature, the supply will be switched off and the pieces are rapidly brought together under light pressure. As the pieces are moved together, the fused metal and slag come out of the joint making a good solid joint.

23. Following are the advantages of the flash butt welding over the upset welding. o Less requirement of power. o When the surfaces being joined, it requires only less attention. o Weld obtained is so clean and pure; due to the foreign metals appearing on the surfaces will burn due to flash or arc.

24. (c) Percussion welding: It is a form of the flash butt welding, where high current of short duration is employed using stored energy principle. This is a self-timing spot welding method. Percussion welding arrangement consists of one fixed holder and the other one is movable. The pieces to be welded are held apart, with the help of two holders, when the movable clamp is released, it moves rapidly carrying the piece to be welded. There is a sudden discharge of electrical energy, which establishes an arc between the two surfaces and heating them to their melting temperature, when the two pieces are separated by a distance of 1.5 mm apart. As the pieces come in contact with each other under heavy pressure, the arc is extinguished due to the percussion blow of the two parts and the force between them affects the weld. The percussion welding can be obtained in two methods; one is capacitor energy storage system and the other is magnetic energy storage system.

25. The capacitor ‘C’ is charged to about 3,000 V from a controlled rectifier. The capacitor is connected to the primary of welding transformer through the switch and will discharge. This discharge will produce high transient current in the secondary to join the two metal pieces. Applications: o It is useful for welding satellite tips to tools, sliver contact tips to copper, cast iron to steel, etc. o Commonly used for electrical contacts. o The metals such as copper alloys, aluminum alloys, and nickel alloys are percussion welded.

26. ARC WELDING PRODUCTION OF ARC : An electric arc is formed whenever current is passed between two metallic electrodes separated by a short distance. Whenever electrode first touches the plate,a large short circuit flows and as it withdrawn later,current continues to flow in the form of spark across the air gap. Due to this spark,air gets ionized and air becomes conducting and so,current is able to flow across the gap. Temperature of arc welding flame is about 3100°c.

27. Types of arc welding: Unshielded arc welding: When a large electrode or filler rod is used for welding, it is said to be un- shielded arc welding. Shielded arc welding: When the welding rods coated with fluxing material are used, then it is called shielded arc welding . Electrodes An electrode is a tool used in arc welding to produce electric arc . Based on their characteristics, arc  welding electrodes can be broadly classified into two types. They are : Consumable Electrode: If the melting point of an arc welding electrode is less, it melts and fills the gap in the workpiece. Such an electrode is called consumable electrode . Non-consumable electrode: If the melting point of the arc welding electrode is high, it does not melt to fill the gap in the workpiece. Such an electrode is called non-consumable electrode

28. coated electrodes are being extensively used for shielded arc welding . They consist of a metal core wire surrounded by a thick flux coating applied by extrusion, winding or other processes. Electrode coating contributes a lot towards improving the quality of the weld. The principal advantages of using electrode coating are as under : 1.It stablizes the arc because it contains ionizing agents such as compounds of sodium and potassium . 2. It fluxes away impurities present on the surface being welded . 3 . It forms slag over the weld which (i) protects it from atmospheric contamination (ii) makes it cool uniformly thereby reducing the changes of brittleness and (iii) provides a smoother surface by reducing ‘ripples’ caused by the welding operation. Coated electrodes

29. Arc blow Arc blow  is the, usually unwanted, deflection of the arc during arc welding. There are two types of  arc blow  commonly known in the electric welding industry:  magnetic  and  thermal . Magnetic arc blow  or "arc wander" : It is the deflection of welding filler material within an electric arc deposit by a buildup of magnetic force surrounding the weld pool. Arc blow tends to occur if the material being welded has residual magnetism at a certain level, particularly when the weld root is being made, and the welding current is direct current (DC positive or negative). This is due to interaction between the directional magnetic field of the welding arc and the directional field of the residual magnetism. Magnetic arc blow is popularly attributed to a change in the direction of current as it flows into and through the workpiece.  It is experienced most when using currents above 200 A or below 40 A.

30. Thermal arc blow: It  is widely attributed to variations in resistance within the base metal created by the  weld pool  as it is moved across the workpiece . Thermal arc blow can occur because of : 1.Improper surface preparation 2.Improper travel speed Due to arc blow, heat penetration in the required area is low which leads to incomplete fusion.Hence it is needed to be suppressed. Measures to avoid arc blow: Arc blow can be avoided by using a.c. rather than d.c. welding machines because reversing currents in the welding leads produce magnetic fields which cancel each other out thereby eliminating the arc blow . R educing the rate of travel of the electrode. S hortening the arc column length etc.

31. Carbon arc welding was the first electric welding process. It’s difference is that it uses non- consumable carbon or graphic electrodes instead of the consumable flux-coated electrodes. Graphite electrodes are harder, more brittle and last longer than carbon electrodes. They can withstand higher current densities but their arc column is harder to control . The main advantage of this process is that the temperature of the molten pool can be easily controlled by simply varying the arc length. Since arc serves only as a heat source, it does not transfer any metal to help reinforce the weld joint. The major disadvantage of the carbon-arc process is that blow holes occur due to magnetic arc blow especially when welding near edges of the workpiece. Carbon arc welding

32. Common types of arc welding used in practice: Shielded metal arc welding: Shielded metal arc welding (SMAW), also known as manual metal arc welding (MMA or MMAW), flux shielded arc welding  or informally as stick welding, is a manual arc welding process that uses a consumable  electrode covered with a flux to lay the weld .

33. Gas Shield Arc Welding: In this fusion process, welding is done with bare electrodes but weld zone is shielded from the atmosphere by a gas which is piped to the arc column. Shielding gases used are carbon dioxide, argon, helium, hydrogen and oxygen. No flux is required .

34. It is fundamentally a heat and squeeze process. The term ‘resistance welding’ denotes a group of processes in which welding heat is produced by the resistance offered to the passage of electric current through the two metal pieces being welded . These processes differ from the fusion processes in the sense that no extra metal is added to the joint by means of a filler wire or electrode . Advantages: Some of the advantages of resistance welding are as under : Heat is localized where required No filler material is needed. Requires comparatively lesser skill Parent metal is not harmed Only disadvantage is with regard to high initial as well as maintenance cost. Resistance welding

35. Plasma Arc Welding : Basic Principle is It consists of a high-current electronic arc which is forced through a small hole in a water cooled metallic nozzle. The plasma gas itself is used to protect the nozzle from the extreme heat of the arc. The plasma arc is shielded by inert gases like argon and helium which are pumped through an extra passageway within the nozzle of the plasma torch. As seen, plasma arc consists of electronic arc, plasma gas and gases used to shield the jet column. Disadvantages: Since it uses more electrical equipment, it has higher electrical hazards .

36. Laser Welding: It uses an extremely concentrated beam of coherent monochromatic light i.e. light of only one colour (or wavelength). It concentrates tremendous amount of energy on a very small area of the workpiece to produce fusion . Some of the advantages of laser welding process are as follows : 1 . It does not require any electrode . 2. It does not heat the workpiece except at one point. In fact, heat affected zone is virtually non-existent. 3.Liquidus is reached only at the point of fusion . 4.It produces minimal thermal distortion and shrinkage because area of heat-affected zone is the minimum possible.