Electric Arc Welding

Electric Arc Welding provides the heat required for melting the parent as well as filler material. The workpiece and the electrode are connected to the power source (The electrode is an electrical conductor used to make contact between cathode and anode to fuse or melt of workpiece) and the arc is started with touching the electrode to the workpiece and then withdrawing it to a short distance (a few mm) from the workpiece. When the electrode and workpiece are in contact the current is flows. The arc is generated by the electrons emitted form cathode and moving towards anode and the arc changes electrical energy into heat and light.

Figure 28 Electric Arc Welding
About 70% of the heat emitted due to flux of electrons at anode raises the anode temperature to high values (5,000 to 30,000oC). This heat melts the base metal as well as tip of the electrode in the area surrounding the arc. A weld is formed when the mixture of molten base and electrode metal solidifies in the weld area.
Both direct current (DC) and alternating currents (AC) are used in arc welding. AC machines are less expensive to purchase and operate, but generally limited to welding of ferrous metals. DC equipment can be used on all metals with good results and is generally famous for better arc control. The used can be either non-consumable or consumable electrodes. Consumable electrodes usually have a coating on its outer surface which on melting emitted gases like hydrogen or carbon dioxide to form covering around the molten pool.
The electrode also reacts to slag which is a liquid and lighter than the molten metal. The slag therefore is rises and floating on the surface and by solidification forms a protective covering over the hot metal. This also slows down the rate of cooling of the weld. The slag layer can be removed by light chipping or small hammering on the slag cover. Electric arc welding of this type is known as (Shielded Metal Arc Welding). More than 50% industrial arc welding is done by this method.
For continuous arc welding operations, the consumables electrode is wire in the form of a coil and the flux us fed into the welding zone, or the weld area is covered by an inert gas. In Submerged Arc Welding the electrode is shielded by granular flux supplied from a source, while is Gas Metal Arc Welding shielding of the area is provided by an inert gas such as argon, helium, carbon dioxide , etc.
Arc Welding (GTAW) is also called as Tungsten Inert Gas (TIG) welding. It uses tungsten alloy electrode and helium gas shield. Because of inert gas atmosphere tungsten is not consumed. Filler materials are supplied by a separate rod or wire in this case.





Friction Welding
Friction welding is a solid-state joining process that produces coalescence in materials, using the heat developed between surfaces through a combination of mechanically induced rubbing motion and applied load. The resulting joint is of forged quality. Under normal conditions, the faying surfaces do not melt. Filler metal, flux and shielding gas are not required with this process. Friction welding is a type of forge welding, i.e. welding is done by the application of pressure. Friction generates heat, if two surfaces are rubbed together, enough heat can be generated and the temperature can be raised to the level where the parts subjected to the friction may be fused together.
Types of friction welding
• Linear friction welding.
• Spin welding.
• Rotary friction welding.
• Inertia friction welding.
• Friction stud welding.
• Friction stir welding.
In conventional friction welding, relative rotation between a pair of workpieces is caused while the work pieces are stuck together. Typically then once sufficient heat is built at the interface between the workpieces, relative rotation is stopped and the workpieces are adhere together under forging force which may be same as or greater than the original adhering force.
In this process the two surfaces to be welded are rotated relative to each other under light normal pressure. When the interface temperature is produced due to frictional rubbing and when it reaches the required welding temperature, sufficient normal pressure is applied and maintained until the two pieces get welded, figure below is explain the process.

Figure 29 Friction welding
With friction welding can be saving cost, time, materials, and dissimilar materials can be joined, a significant cost savings is possible because engineers can design parts that use expensive materials only where is needed. Expensive forgings and castings can be replaced with less expensive forgings welded to bar stock, tubes, plates and the like. Substantial time savings are realized since the process is faster than more conventional methods of welding.
Even dissimilar metals or considered incompatible can be joined by friction welding, such as aluminum to steel, copper to aluminum, titanium to copper and nickel alloys to steel. As a rule, all metallic engineering materials which are forgeable can be friction welded, including automotive valve alloys, maraging steel, tool steel, alloy steels. In addition, many castings, powder metals and metal matrix composites are weldable.
Mechanism of friction welding
It is carried out by moving one component relative to the other along a common interface, while applying a compressive force across the joint. The friction heating generated at the interface softens both components, and when they become plasticised the interface material is extruded out of the edges of the joint so that clean material from each component is left the original interface. The relative motion is then stopped, and a higher final compressive force may be applied before the joint is allowed to cool. The key to friction welding is that no molten material is generated, the weld being formed in the solid state.
The principle of this process is the changing of mechanical energy into heat energy. One component is rotated about its axis while the other component to be welded to it and does not rotate but can be moved axially to make contact with the rotating component. At a point fusion temperature is achieved then rotation is stopped and forging pressure is applied. Then heat is generated due to friction and is concentrated and localized at the interface, grain structure is refined by hot work. Then welding is done, but there will not occur the melting of parent metal.
Advantages
1. Dissimilar metals are joined, even some considered incompatible or non-weldable.
2. The process is much faster than other welding techniques.
3. Friction welders are enough to join a wide range of part shapes, materials and sizes.
4. Joint preparation isn’t critical machining, saw cut, and even sheared surfaces are weldable.
5. Resulting joints are of forged quality, with a 100% butt joint weld through the contact area.
6. The machine-controlled process eliminates human error, and weld quality is independent of operator skill.
7. It’s ecologically clean, no objectionable smoke, fumes, or gases are generated that need to be exhausted.
8. No consumables are required, flux, filler material, or shielding gases.
9. Power requirements are as low as 20% of that required of conventional welding processes.
10. Since there is no melting, no solidification defects occur, e.g. gas porosity, segregation or slag inclusions.
Disadvantages
The disadvantage of friction welding are that not every configuration is feasible, that a machine of sufficient power is needed and that for short runs the process may not be economical. The friction welding process has some costs in tooling and set up that must be taken into account when calculating the costs per weld. Also finishing operations may be requested which sum up to the total cost.