Welding - Why Do Some Manufacturers Prefer DC Welding?

The job dictates the correct process.

Likely because for higher production welding MIG (Wire Feed) welding is far faster and cost effective and those welding systems are primary DC output.

AC welders produce zero power either 100 or 120 times per second depending on power supply frequency. (60 Hz being favored in N. America) The power looks basicallty like a sine wave so it is at zero that many times during polarity changes of AC power.

DC power does not cycle, so there is never a dead spot in the welding power.

Dc welders can be set to either DC+ or DC- depending on the type of wire and sheilding used.

Dear Mr. lyn,

You are right that AC supply touches zero 100 or 120 times for 50 or 60 Cycles/Sec. supply.

But it is a tiny fraction of one second. I am of the view that the Thermal Inertia will take care of this zero effect of supply.

DHAYANANDHAN.S

Should that not be "thermal momentum"?

No. Take a look at the chart to the right. You will note that full welding power is is only achieved during a small portion of each cycle. DC power does not fluctuate.

Text and chart below from: http://www.lincolnelectric.com/en-us/support/welding-solutions/Pages/polarity-for-smaw.aspx posted by pantaz.

To overcome this problem, some electrodes are designed specifically to operate on AC. They have certain elements in their coating which help keep the arc ignited as the output goes through periods of low and no output (loosely represented by the red zone on the figure 2 graph). However, the resulting arc still tends to have more fluctuation or flutter than it does on DC polarity.

Figure 2: Graph of Welding Output on AC Polarity

I think portability is another big factor. If a person is doing his welding out of the back of a truck, DC is the simplest way to go.

A good weld depends on having sufficient (enough but not too much) penetration. With steel penetration is usually accommodated by using a 6010 rod, a DC rod, which gives excellent penetration. Other rods usable with AC do not penetrate as far and do not provide as good a weld, ie. 6011 is the AC version of the 6010 rod but doesn't do as good a job when all things are considered. Other rods that are usable with AC such as the 6012, 6013, 7014, 7018, and 7024 do not provide deep penetration.

Since time is money the rod that provides the deepest penetration and requires the least time when other requirements are considered is the best rod to use. Such things as chamfering, grinding before the welding and grinding, chipping, needle scaling, removing splatter and flux after the weld consume time and therefore money.

Welding a pipe (other than high pressure pipelines) is usually done by grinding a chamfer on the pipe ends, welding with a 6010 rod for penetration that go completely through to the inside surface of the pipe, chipping and/or needle scaling to remove the flux, grinding to prepare the surface for another pass with a filler rod and then welding with a 7018 rod as a filler rod. The weld then is again cleaned and ground if necessary. Depending on the wall thikness of the pipe numerous multiple passes may be require. If the penetration weld (the first pass) is not done competently then the whole weld must be ground out with a grinder and done all over again.

Therefore the best rod to use is the one which does the best job. For most stick welding that is the 6010 rod, a DC rod!

Good Luck, Old Salt