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• THE PRINCIPLE OF METAL ARC WELDING CONSIST OF ESTABLISHING AN ELECTRIC ARC BETWEEN A METAL ELECTRODE AND THE WORK PIECE TO BE WELDED.
• ARC CAN BE DESCRIBED AS A STREAM OF INCANDESCENT VAPOR WHICH ACTS AS A CONDUCTING MEDIUM FOR ELECTRIC CURRENT FROM ONE TERMINAL TO ANOTHER TO COMPLETE THE CIRCUIT.
• THE ELECTRIC CURRENT HAS FAIRLY HIGH VOLTAGE TO OVERCOME THE EXTRA RESISTANCE OFFERED BY THE VAPOR.

FUNCTION OF FLUX :- 

1.  TO PRODUCE A GAS WHICH PROVIDES A SHIELD AROUND ARC TO PROTECT IT FROM ATMOSPHERE.
2.  FORMS SLUG BY MIXING WITH THE IMPURITIES OF MOLTEN METAL AND THUS REFINING THE METAL.
3.  SLAG FLOATS OVER THE MOLTEN METAL AND SOLIDIFICATION FORMS A THIN LAYER WHICH HELP IN GRADUAL COOLING OF WELD AND PREVENT ITS OXIDATION DURING COOLING
4. CONTROL BEAD SHAPE BY PROVIDING NECESSARY MATERIALS FOR THIS PURPOSE.

ARC BLOW :- 

IT IS THE DISTORTION OF ARC WHICH LEAD TO BENDING OF THE ARC AWAY FROM ITS INTENDED PATH IS CALLED ARC BLOW. MAINLY FOUND IN DC WELDING AS THERE IS FIXED POLARITY, THE INDUCED MAGNETIC FIELD ARE CONSTANT IN DIRECTION. MAINLY FOUND DURING STARTING AND END OF WELD.

USE OF AC WELDING :-

• IN AC WELDING, STEP DOWN TRANSFORMER REDUCES VOLTAGE FROM 440 VOLTS TO 80-100 VOLTS.
• THIS VOLTAGE IS REQUIRED ONLY TO START THE ARC AND FOR MAINTAINING THE SAME WE REQUIRE 30-40 VOLTS.
• THIS IS ACCOMPLISHED BY CURRENT REGULATOR, THROUGH WHCIH WE CAN ADJUST FLOW OF CURRENT AND ALSO RESISTANCE AND HENCE CAN OBTAIN THE DESIRED VOLTAGE.
• SINCE IN AC, IT PASSES 0, TWICE IN EVERY CYCLE HENCE VOLTAGE IS 0 AT THIS POINT, HENCE HIGHER VOLTAGE IS REQUIRED.

USE OF DC WELDING :-

• VOLTAGE IS 60-80 VOLTS TO STRICK THE ARC AND 15-25 VOLTS TO MAINTAIN IT.
• POSITIVE TERMINAL- HEAT DEVELOPED SHOULD BE 2/3
• NEGATIVE TERMINAL- HEAT DEVELOPED SHOULD BE 1/3
• STRAIGHT POLARITY – ELECTRODE IS NEGATIVE AND WORK PIECE IS POSITIVE
• REVERSED POLARITY – ELECTRODE IS POSITIVE AND WORK PIECE IS NEGETIVE.
• DUE TO THESE POLARITIES , ALMOST ALL METALS CAN BE WELDED BY USING DC WELDING AS MANY METALS REQUIRE MORE HEAT TO ACQUIRE THE FUSION STATE THAN THE ELECTRODE USED. EXAMPLE :- COPPER.

DIFFERENCE BETWEEN AC AND DC WELDING  :-

1.  AC IS CHEAPER AND EASILY MAINTAINED THAN DC.
2.  PREFERRED FOR WELDING AT LARGE DISTANCE FROM SUPPLY BECAUSE VOLTAGE DROP IS LESS WHILE DC WELDING IS USED FOR SHORTER DISTANCE.
3. STRIKING ARC IS DIFFICULT IN AC.
4. AC IS NOT PREFERRED FOR WELDING SHEETS WHILE DC IS PREFERRED.
5. NO ARC BLOW IN AC .
6. AC IS NOT SUITABLE FOR NON FERROUS METALS WHILE WE CAN WELD NON FERROUS METALS WITH DC.