Question

a) Discuss the heat generation mechanism in electric arc welding and oxy-acetylene gas welding.

b) Discuss the generation of neutral, reducing and oxidizing flame in gas welding along with the application of each type of flame.

c) Draw schematics of Metal Inert Gas (MIG) and Tungsten Inert Gas (TIG) welding process, and discuss the difference between them. Also, very briefly mention the different between these processes and submerged arc welding processes.

Answer 1

A) Heat Generation Mechanism of: -

(1) Elecric Arc Welding

Arc welding comes under the category of fusion welding. It uses an electrical occurrance called 'arc discharge' to generate heat. Arc discharge means releasing current in air. When voltage is applied between two electrodes seperated at a specific distance, and the voltage is increased gradually, the insulation of air between the electrodes breaks and the current jumps from one electrode to the other discharging bright light and high heat along.

In the electric welding process, welding rod or wire and the metal on which the welding is to be done acts as electrodes. Positive voltage os applied on the welding rod and negative voltage to the base metal. And the heat as a result of arc is generated between them.

(2) oxy-acetylene gas welding:-

Oxy-acetylene welding uses heat generated by the mixture of gas to weld the metal. When acetylene is mixed with oxygen in proper proportions and then of provoked to ignite, the flame temperature of the mixture can reach as high as 5800 degree Celsius. At such a high temperature any commercial metal can melt very easily. Following reactions takes place when acetylene is mixed with oxygen.

Stage 1:- acetylene breaks

2C₂H₂ + 2O₂ = 4CO + 2H₂ + Heat

Stage 2:- reaction of CO and H_{​​​​​2} With more oxygen

4CO + 2H₂ + 3O₂ = 4CO₂ + 2H₂O + Heat

As it is seen both the stages produces heat.

(B) Types of flames:-

(1) Neutral Flames

When oxygen and acetylene are mixed in same proportion, the flame generated is a neutral flame. The additional oxygen required to combust is obtained from the atmosphere. The neutral flame has clear abd luminous cone indicating that the combustion is complete.

Neutral welding falmes are generally used to weld Mild steel, aluminium, copper, cast iron, stainless steel etc. It is also used in preheating of metals before cutting.

Fig is shown below

(2) Reducing flames:-

Carbuirizing or reducing flame is achieved by the combustion of excess acetylene with oxygen. It is identified by it's inner feathery cone extending beyond its edge.

It is used in back hand welding for welding high carbon steel and non ferrous alloys such as nickel and monel. It is also used in silver solder or soft solder operations.

(3) oxidizing flame:-

An oxidizing flame is achieved when more than one volume of oxygen is mixed with one volume of acetylene. To obtain this flame the welding tourch is started at neutral and then the amount of oxygen is increased. It produces a hissing sound.

It is used in welding of metals such as zinc, copper, magnesium steel, cast iron, etc.

(C) MIG and TIG welding

Refer to following images

The difference between MIG TIG welding and Submerged arc welding is that submerged arc welding is an automated process which uses flux over the area to be welded and electrode is continuously fed.

Whereas the other processes generally are manual and uses generally sheilding gas above the metal and the electrode maybor may not continuously be fed.