Manual metal arc welding (MMA or MMAW), also known as shielded metal arc welding (SMAW), flux shielded arc welding or stick welding, is a process where the arc is struck between an electrode flux coated metal rod and the work piece. Both the rod and the surface of the work piece melt to create a weld.
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Manual metal arc welding was first invented in Russia in 1888. It involved a bare metal rod with no flux coating to give a protective gas shield. The development of coated electrodes did not occur until the early 1900s when the Kjellberg process was invented in Sweden and the Quasi-arc method was introduced in the UK. It is worth noting that coated electrodes were slow to be adopted because of their high cost. However, it was inevitable that as the demand for sound welds grew, manual metal arc became synonymous with coated electrodes. When an arc is struck between the metal rod (electrode) and the workpiece, both the rod and workpiece surface melt to form a weld pool of molten metal. Simultaneous melting of the flux coating on the rod will form gas and slag which protects the weld pool from the surrounding atmosphere. The slag will solidify and cool and must be chipped off the weld bead once the weld run is complete (or before the next weld pass is deposited).
The process allows only short lengths of weld to be produced before a new electrode needs to be inserted in the welding electrode holder. Weld penetration is low and the quality of the finished weld deposit is highly dependent on the skill of the welder.
To strike an arc between the electrode and base metal, such as carbon steel, and to produce a good quality weld, a stick welder must ensure that their welding machines are fitted with suitable electrodes. Arc stability, depth of penetration, metal deposition rate and positional capability are greatly influenced by the chemical composition of the flux coating on the electrode. Electrodes can be divided into three main groups:
Cellulosic electrodes contain a high proportion of cellulose in the coating and are characterised by a deeply penetrating arc and a rapid burn-off rate giving high welding speeds. Weld deposit can be coarse and with fluid slag, deslagging can be difficult. These electrodes are easy to use in any position and are noted for their use in the 'stovepipe' welding technique.
Features:
Rutile electrodes contain a high proportion of titanium oxide (rutile) in the coating. Titanium oxide promotes easy arc ignition, smooth arc operation and low spatter. These electrodes are general purpose electrodes with good welding properties. They can be used with AC and DC power sources and in all positions. The electrodes are especially suitable for welding fillet joints in the horizontal/vertical (H/V) position.
Features:
Basic electrodes contain a high proportion of calcium carbonate (limestone) and calcium fluoride (fluorspar) in the coating. This makes their slag coating more fluid than rutile coatings - this is also fast-freezing which assists welding in the vertical and overhead position. These electrodes are used for welding medium and heavy section fabrications where higher weld quality, good mechanical properties and resistance to cracking (due to high restraint) are required.
Features:
Metal powder electrodes contain an addition of metal powder to the flux coating to increase the maximum permissible welding current level. Thus, for a given electrode size, the metal deposition rate and efficiency (percentage of the metal deposited) are increased compared with an electrode containing no iron powder in the coating. The slag is normally easily removed. Iron powder electrodes are mainly used in the flat and H/V positions to take advantage of the higher deposition rates. Efficiencies as high as 130 to 140% can be achieved for rutile and basic electrodes without marked deterioration of the arcing characteristics but the arc tends to be less forceful which reduces bead penetration.
Electrodes can be operated with AC and DC power supplies. Not all DC electrodes can be operated on AC power sources, however AC electrodes may be used on either AC or DC.
Mention AC/DC to most people, and they will immediately think of a popular Australian hard-rock band. On the other hand, welding professionals recognize it as polarity, referring to the negative and positive poles formed in the electrical circuit as soon as you turn on a welding machine.
What makes polarity so important in welding? Choosing the correct polarity can ensure the strength and quality of the weld, while the wrong one can cause poor penetration, excessive spatter, and less control of the welding arc.
Welding professionals often talk about the pros and cons of AC vs. DC welding. The primary difference is polarity: AC (alternating current) welding alternates rapidly between DC negative and DC positive, while DC (direct current) uses a straight polarity current to supply electricity to the electrode. Which of these you choose will depend on specific factors, so here’s what you should know:
AC welding means you are using an alternating current in the welding process. The alternating current reverses its direction many times per second. For instance, a 60-hertz current will alter its polarity 120 times per second. Since the magnetic field and current reverse direction, it allows for a steadier arc on magnetic parts.
DC welding employs a direct current with constant polarity flow in one direction. The current can be positive or negative, with positive polarity flowing from the welder to the workpiece to the electrode and back to the welder. The current flows from the welder to the electrode to the workpiece and then back to the welder with negative polarity. Electrode positive welding offers deeper penetration, while electrode negative provides faster deposition rates.
Even though DC welding is preferred for most welding applications, AC welding works well in some specific applications. TIG welding aluminum, welding magnetized metals, downhand welding on heavy plates, with fast-fill electrodes, and welding where the only electrical outlets are 110 volts all lend themselves to AC welding. Unlike DC welding, arc blow is far less noticeable when welding with AC.
On the other hand, direct current maintains a constant and stable arc and is more reliable than AC. It typically produces a smoother weld with less spatter because of the continuous direction of the current. DC also welds thinner materials better than AC, working best with stick welding, stainless steel TIG welding, and vertical or overhead welding.
As mentioned earlier, the DC current can be positive or negative. But how does that affect the welding process?
Sometimes called straight polarity, electrode-negative polarity is preferred for most welding jobs. It produces a hotter arc and faster electrode melting, allowing operators to be more productive and lay down a bead faster. Straight polarity DC will almost always be a welder’s first choice unless a specific need for something else comes up.
Electrode-positive polarity, also referred to as reverse polarity, generates deeper penetration. Even though it is slower than straight polarity, it is usually the choice for welding thick material. The advantage of reverse polarity becomes evident when working with metal more than half an inch thick.
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AC Welding
DC Welding
Polarity
Alternating polarity
A single polarity that could be positive or negative
Applications
Magnetized metals, aluminum, heavy plates
Stainless steel, overhead, vertical, and thinner plate welding (Most applications)
Weld strength
Weaker with more spatter
Stronger and smoother
Arc blow
Fixes arc blow issues
Can cause arc blow
Arc stability
Arc can be hard to work with
Stable arc
Welding machines
Less expensive, portable
More expensive and heavier
The answer is: it depends on your situation. While DC welding will probably be your first choice, each type of welding has some distinct advantages in specific circumstances. If you’re starting your welding career, DC welding is easier to work with and results in cleaner welds. However, DC welding machines are typically more expensive than AC, prompting beginners to lean toward the more affordable welders.
Since each has its strengths and weaknesses, you might be better served with a welding machine that can do both!
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