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WELDING
– Welding is a materials joining process which produces coalescence of materials by heating them to suitable temperatures with or without the application of pressure or by the application of pressure alone, and with or without the use of filler material.
– Welding is used for making permanent joints.
– It is used in the manufacture of automobile bodies, aircraft frames, railway wagons, machine frames, structural works, tanks, furniture, boilers, general repair work and ship building.
TYPES
• Plastic Welding or Pressure Welding
The piece of metal to be joined are heated to a plastic state and forced together by external pressure
(Ex) Resistance welding
• Fusion Welding or Non-Pressure Welding
The material at the joint is heated to a molten state and allowed to solidify
(Ex) Gas welding, Arc welding
Classification of welding processes:
(i). Arc welding
• Carbon arc
• Metal arc
• Metal inert gas
• Tungsten inert gas
• Plasma arc
• Submerged arc
• Electro-slag
(ii). Gas Welding
• Oxy-acetylene
• Air-acetylene
• Oxy-hydrogen
(iii). Resistance Welding
• Butt
• Spot
• Seam
• Projection
• Percussion
• Arc welding
• Equipments:
• A welding generator (D.C.) or Transformer (A.C.)
• Two cables- one for work and one for electrode
• Electrode holder
• Electrode
• Protective shield
• Gloves
• Wire brush
• Chipping hammer
• Goggles
• Arc Welding Equipments
• Metal arc welding
• Arc Welding
• Carbon Arc Welding
• Arc welding
Advantages
– Most efficient way to join metals
– Lowest-cost joining method
– Affords lighter weight through better utilization of materials
– Joins all commercial metals
– Provides design flexibility
Limitations
• Manually applied, therefore high labor cost.
• Need high energy causing danger
• Not convenient for disassembly.
• Defects are hard to detect at joints.
• Comparison of A.C. and D.C. arc welding
Alternating Current (from Transformer)
More efficiency
Power consumption less
Cost of equipment is less
Higher voltage – hence not safe
Not suitable for welding non ferrous metals
Not preferred for welding thin sections
Any terminal can be connected to the work or electrode
• Comparison of A.C. and D.C. arc welding
Direct Current (from Generator)
Less efficiency
Power consumption more
Cost of equipment is more
Low voltage – safer operation
suitable for both ferrous non ferrous metals preferred for welding thin sections Positive terminal connected to the work Negative terminal connected to the electrode
GAS WELDING
• Sound weld is obtained by selecting proper size of flame, filler material and method of moving torch
• The temperature generated during the process is 33000c
• When the metal is fused, oxygen from the atmosphere and the torch combines with molten metal and forms oxides, results defective weld
• Fluxes are added to the welded metal to remove oxides
• Common fluxes used are made of sodium, potassium. Lithium and borax.
• Flux can be applied as paste, powder,liquid.solid coating or gas.
GAS WELDING EQUIPMENT...
1. Gas Cylinders
Pressure
Oxygen – 125 kg/cm2
Acetylene – 16 kg/cm2
2. Regulators
Working pressure of oxygen 1 kg/cm2
Working pressure of acetylene 0.15 kg/cm2
Working pressure varies depends upon the thickness of the work pieces welded.
3. Pressure Gauges
4. Hoses
5. Welding torch
6. Check valve
7. Non return valve
Oxy-Acetylene welding
TYPES OF FLAMES…
• Oxygen is turned on, flame immediately changes into a long white inner area (Feather) surrounded by a transparent blue envelope is called Carburizing flame (30000c)
• Addition of little more oxygen give a bright whitish cone surrounded by the transparent blue envelope is called Neutral flame (It has a balance of fuel gas and oxygen) (32000c)
• Used for welding steels, aluminium, copper and cast iron
• If more oxygen is added, the cone becomes darker and more pointed, while the envelope becomes shorter and more fierce is called Oxidizing flame
• Has the highest temperature about 34000c
• Used for welding brass and brazing operation
• Three basic types of oxyacetylene flames used in oxyfuel-gas welding and cutting operations: (a) neutral flame; (b) oxidizing flame; © carburizing, or reducing flame.
GAS CUTTING
• Ferrous metal is heated in to red hot condition and a jet of pure oxygen is projected onto the surface, which rapidly oxidizes
• Oxides having lower melting point than the metal, melt and are blown away by the force of the jet, to make a cut
• Fast and efficient method of cutting steel to a high degree of accuracy
• Torch is different from welding
• Cutting torch has preheat orifice and one central orifice for oxygen jet
• PIERCING and GOUGING are two important operations
• Piercing, used to cut a hole at the centre of the plate or away from the edge of the plate
• Gouging, to cut a groove into the steel surface
GAS CUTTING…
Weld joints
Brazing and Soldering
Brazing
It is a low temperature joining process. It is performed at temperatures above 840º F and it generally affords strengths comparable to those of the metal which it joins. It is low temperature in that it is done below the melting point of the base metal. It is achieved by diffusion without fusion (melting) of the base
Brazing
Advantages& Disadvantages
Advantages
• Dissimilar metals which canot be welded can be joined by brazing
• Very thin metals can be joined
• Metals with different thickness can be joined easily
• In brazing thermal stresses are not produced in the work piece. Hence there is no distortion
• Using this process, carbides tips are brazed on the steel tool holders
Disadvantages
• Brazed joints have lesser strength compared to welding
• Joint preparation cost is more
• Can be used for thin sheet metal sections
Soldering
• It is a low temperature joining process. It is performed at temperatures below 840ºF for joining.
• Soldering is used for,
• Sealing, as in automotive radiators or tin cans
• Electrical Connections
• Joining thermally sensitive components
• Joining dissimilar metals