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AC vs. DC Welding: Differences Compared (with Pros & Cons)

TJsmud
11 Min Read
AC vs. DC Welding: Differences Compared (with Pros & Cons)
AC vs. DC Welding: Differences Compared (with Pros & Cons)

AC and DC welding are forms of arc welding that use different currents to produce an electric arc. These types of welding involve the creation of an electric arc between an electrode and the metal being welded. The electric arc provides heat to fuse the metals together. A power supply is used to generate the arc, which can either use an alternating current (AC) or a direct current (DC). The choice of power source (AC or DC) decides the polarity of the electrical current that runs through the electrode. The power source used also impacts the weld produced.

AC vs. DC Welding

Welding Town

Selecting the correct electrode polarity impacts the strength and quality of your weld. Commonly known as ‘straight’ or ‘reverse,’ the two types of current flow are also called ‘electrode negative’ and ‘electrode positive.’  The DC polarity is constant while the AC polarity flows in one direction for half of the time and half of the time in the other, reverse, direction.

Read on to learn the differences more in-depth.

ACDC
Weld SpatterMoreLess
Arc StabilityWorseBetter
Filler Metal Deposition RatesModerateHigh
PenetrationModerateHigh
Voltage Drops Using Long LeadsNoYes
Arc Blow Occurs When Welding Magnetized MetalNoYes
Welds Ferrous Metal Like SteelSMAW onlyAll arc welding processes
Welds AluminumAC TIGDC MIG
TIG Welding Equipment CostHighLow
Stick Welding Equipment CostLowMedium to High
AC Vs. DC Welding Differences

What is DC Welding?

Direct current is an electric current that has a constant polarity flow in a single direction. This current can be positive or negative. With DC welding, since the magnetic field and current of the arc are constant, stable arcs are produced.

Advantages

The advantages of DC welding are:

  • A smoother welding output than with AC
  • A more stable arc
  • Less spatter
  • DC negative offers faster deposition rates when welding thin sheet metals
  • DC positive provides greater penetration into the weld metal

Disadvantages

The disadvantages of DC welding are:

  • DC welding is unable to fix arc blow problems
  • Equipment is more expensive as DC currents require an internal transformer to switch the current

Applications

DC welding is ideal for joining thinner metals as well as being used in most stick welding applications, including TIG welding of – steels. This form of welding is also good for overhead and vertical applications.

What is AC Welding?

An alternating current is an electric current that reverses its direction many times per second. A 60-hertz current will change its polarity 120 times per second. With AC welding, because the magnetic field and current rapidly reverse -direction, there is no net deflection of the arc.

Advantages

The advantages of AC welding are:

  • The alternating current between positive polarity and negative polarity allows for a steadier arc for welding magnetic parts
  • Fixes problems with arc blow
  • Enables effective aluminium welding
  • AC welding machines are cheaper than DC equipment

Disadvantages

The disadvantages of AC welding are:

  • More spatter
  • Weld quality is not as smooth as with DC welding
  • Less reliable and therefore more difficult to handle than DC welding

What is DC Welding?

Direct current flows in only one direction. This welding method has a higher deposition rate. The deposition rate is the amount of filler metal that is melted into the weld joint. The higher deposition rate makes DC welding good for a weld that needs a build-up of deposits. It also has less spatter than AC welding, which makes the weld bead more uniform and smoother.

DC is easier to work with since the electrical arc is more stable than with AC welding.

Advantages

  • A more stable arc.
  • A smoother weld.
  • Faster deposition rates.
  • Greater penetration into the weld metal.
  • Ideal for joining thinner metals.
  • It is also used for stick welding and is good for welding all types of steel.
  • Good for overhead and vertical applications. Vertical welds are required in different industries, primarily construction areas such as for buildings, power plant construction, oil and gas pipelines and fittings, and in shipyards where welders are required to weld huge metal structures that are in a vertical position.

Disadvantages

  • The machines are more expensive
  • It is unable to fix problems with arc blow.

AC Vs. DC Stick Welding (SMAW)

The first stick welders, also called buzz boxes, were AC output machines. The polarity-switching action caused the device to sound like a bumblebee — hence the buzz box name.

While these AC welders got the job done, the arc wasn’t nearly as stable as DC output power sources. So, stick welding is primarily performed with DC machines.

However, the polarity choice plays a crucial role in achieving quality SMAW welds. DCEP provides the best penetration, even if the electricity is focused on the stick welding electrode.

That sounds counterintuitive, I know. But try to visualize the following…

The heat is concentrated on the electrode causing it to melt, and the liquid droplet at the rod’s tip is propelled into the welded metal, shooting it deep into the weld joint. 

So, DCEP is the most commonly used polarity when stick welding. However, DCEN also has its uses. For example, you don’t need maximum penetration when welding thin material or performing surfacing welds. 

There are tens of SMAW electrodes available for welding mild and stainless steel. Some can be used with all polarities, while others work best with AC, DCEN, or DCEP. It depends on the flux coating composition. 

AC Vs. DC TIG Welding (GTAW)

Depending on the welded material, the TIG welding process uses DC or AC output. Mild steel, stainless steel, and carbon steel are usually welded using DC, while aluminum and magnesium require an AC output.

DC TIG

Standard DC TIG welding polarities are the opposite to stick welding. So, DCEN is the preferred polarity because the current and heat are focused on the welded metal. 

DCEP focuses too much heat on the tungsten electrode, which leads to tungsten erosion and the electrode tip melting. If you need to ball up the tungsten tip, you can briefly use DCEP. But, other than that, DCEP is rarely used for TIG welding.

AC TIG

Aluminum develops an oxide film on its surface when exposed to air, and this oxide layer melts at a higher temperature (3200º+F) than the pure aluminum underneath it (1200ºF). So, to successfully weld aluminum, you must remove the oxide layer before welding.

However, removing this layer is impossible, especially since it immediately starts forming after you clean it. A micro oxide film forms in less than a second after cleaning, and these oxides lead to weld contamination.

Luckily, the DCEP TIG welding polarity breaks this aluminum oxide residue efficiently. As the electrons from the workpiece ( the negative pole) arc onto the tungsten electrode (the positive pole), the aluminum oxide on the surface is broken up and burns away.

However, DCEP offers poor penetration, and to prevent the tungsten electrode from burning, the electrode diameter must be large.

So, DCEN plays a critical role in penetrating the aluminum underneath the oxides after the DCEP cleans the surface. AC TIG employs both electrode positive and electrode negative by switching the polarity many times per second.

The TIG AC welding frequency can be preset at 50-60Hz or range from 20 to 500Hz if the welder controls allow the frequency to be set by the user.

AC Vs. DC MIG Welding (GMAW)

The MIG welding process requires DCEP polarity, just like stick welding. It is impossible to use DCEN to MIG weld because the welds will be globby, and the arc will be horrible.

And before you ask…

MIG welding aluminum also requires DCEN. So, there is no change in polarity regardless of the welding material when using the MIG process.

AC MIG welding is used in industrial processes like cold metal transfer welding and AC aluminum pulse. But these shouldn’t concern you unless you are a welding engineer working on enhancing automatic welding productivity.

AC Vs. DC Flux-Cored Welding (FCAW)

Gasless or self-shielded flux-cored wires require a DCEN polarity. So, for typical FCAW welding jobs, you have to switch polarity if using a MIG welder.

However, gas-shielded flux-cored wires need the same polarity as the MIG welding process (i.e., DCEP). 

Conclusion

DC output is an all-around better choice for welding, except for aluminum. But, certain applications require using an AC output, too. So, professional welders like to have both options available.

Some high-quality TIG/Stick multi-process welding machines allow you to choose between AC and DC outputs for both processes, not just for TIG.

The DC polarities can get confusing at first. But, with some experience, you’ll learn the differences between DCEN and DCEP by heart and it will become second nature.

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