"Why is DCEP generally used in consumable welding processes?
As per my understanding more heat is generated where the electrons collide (anode), so in consumable welding, we want deposition and also want penetration,".
Welding is about three things: the best process (SMAW, GMAW, SMAW or TIG) for what is to be accomplished, a balance of the parameters for the work, and knowledge of the exceptions applicable to the process and work.
It's never one or two things, penetration and deposition, unless an exception applies; for example, for extremely thin metal you wouldn't want penetration and deposition.
Source: "Difference Between DCEN Polarity and DCEP Polarity in Arc Welding":
"Direct Current Straight Polarity (DCSP) or Direct Current Electrode Negative (DCEN): It occurs when electrode is connected with the negative terminal of the power source and base metals are connected with the positive terminal.".
"Direct Current Reverse Polarity (DCRP) or Direct Current Electrode Positive (DCEP): Here the base metals are connected with the negative terminal of the power source, while the electrode is connected with the positive terminal.".
A consumable welding process is one where the electrode becomes a part of the finished weld, GTAW is an example of a non-consumable process.
DCEP has been used for GTAW welding of magnesium or aluminum sheet metal using larger electrodes and lower currents, DCEP provides the necessary cleaning action.
See this article on Miller's blog: "Pro Tips for Understanding TIG Welding Waveforms and Controls":
"If you weld aluminum with DC EN, a ‘skin’ of aluminum oxide will be floating on the puddle, hampering the flow of the metal you’re trying to weld.".
[In reference to their Dynasty welder, in asymmetrical squarewave mode they write:]
"For most work, 75% EN is preferred, since 25% EP is usually sufficient for cleaning, and any more than that limits the total heat input, which could decrease the weld penetration.
The relation between the EN ratio setting and welding result are shown in the table below:
| Negative Direction | EN Ratio Setting | Positive Direction |
| Decreasing | EN Ratio | Increasing |
| Slow | Wire Melting Speed | Quick |
| Small | Gap Tolerance | Large |
"So the question is why to use DCEP when we can get deeper penetration with the DCEN? (is melting of work piece is necessary or deposition of molten metal from the electrode?) Or if i am wrong what is the reason for getting more penetration in DCEP?".
DCEP tends to cause the electrode to melt (see exception above) which is necessary for consumable welding. Penetration can be obtained by increasing the amperage and/or decreasing the surface area of the electrode (example FCAW).
Increasing the wirespeed (amperage) also increases the deposition.
One exception is using 7018 to weld open root. (Sources: 1, 2). See: Hobart Brothers' article: "The Basics of Welding: 7018 Welding Rod Amperage & Beyond", 7018 is usually DCEP.