What is MMA Welding?
One of the most common types of arc welding is manual metal arc welding (MMA) or stick welding. An electric current is used to strike an arc between the base material and a consumable electrode rod or “stick”. The electrode rod is made of a material that is compatible with the base material being welded and is covered with a flux that gives off gaseous vapours that serve as a shielding gas and providing a layer of slag, both of which protect the weld area from atmospheric contamination. The electrode core itself acts as filler material the residue from the flux that forms slag covering over the weld metal must be chipped away after welding.
The Metal Wire Core works as conductor of the current that maintains the arc. The core wire melts and is deposited into the welding pool.
The covering on a shielded metal arc welding electrode is called Flux. The flux on the electrode performs many different functions. These include:
producing a protective gas around the weld area
providing fluxing elements and deoxidizer
creating a protective slag coating over the weld as it cools
establishing arc characteristics
adding alloying elements
Covered electrodes serve many purposes in addition to filler metal to the molten pool. These additional functions are provided mainly by the covering on the electrode.
The arc is initiated by momentarily touching the electrode to the base metal.
The heat of the arc melts the surface of the base metal to form a molten pool at the end of the electrode.
The melted electrode metal is transferred across the arc into the molten pool and becomes the deposited weld metal.
The deposit is covered and protected by a slag which comes from the electrode coating.
The arc and the immediate area are enveloped by an atmosphere of protective gas.
Manual metal arc (stick) electrodes have a solid metal wire core and a flux coating. These electrodes are identified by the wire diameter and by a series of letters and numbers. The letters and numbers identify the metal alloy and the intended use of the electrode.
MMA Welding Fundamentals:
Electrode Selection
As a general rule, the selection of an electrode is straight forward, in that it is only a matter of selecting an electrode of similar composition to the parent metal. However, for some metals there is a choice of several electrodes, each of which has particular properties to suit specific classes of work. It is recommended to contact your welding supplier for the correct selection of electrode.
Electrode Size
The size of the electrode generally depends on the thickness of the section being welded, and the thicker the section the larger the electrode required. The table gives the maximum size of electrodes that may be used for various thicknesses of section base on using a general purpose type 6013 electrode.
Average Thickness of Material | Maximum Recommended Electrode Diameter |
|---|---|
1.0-2.0 mm | 2.5 mm |
2.0-5.0 mm | 3.2 mm |
5.0-8.0 mm | 4.0 mm |
>8.0 mm | 5.0 mm |
Arc Length
To strike the arc, the electrode should be gently scraped on the work until the arc is established. There is a simple rule for the proper arc length; it should be the shortest arc that gives a good surface to the weld. An arc too long reduces penetration, produces spatter and gives a rough surface finish to the weld. An excessively short arc will cause sticking of the electrode and result in poor quality welds. General rule of thumb for down hand welding is to have an arc length no greater than the diameter of the core wire.
Electrode Angle
The angle that the electrode makes with the work is important to ensure a smooth, even transfer of metal. When welding in down hand, fillet, horizontal or overhead the angle of the electrode is generally between 5and 15 degrees towards the direction of travel. When vertical up, welding the angle of the electrode should be between 80 and 90 degrees to the work piece.
Travel Speed
The electrode should be moved along in the direction of the joint being welded at a speed that will give the size of run required. At the same time, the electrode is fed downwards to keep the correct arc length at all times. Excessive travel speeds lead to poor fusion, lack of penetration etc. , while too slow a rate of travel will frequently lead to arc instability, slag inclusions and poor mechanical properties.
Welding Current (Amperage)
Correct current selection for a particular job is an important factor in arc welding. With the current set too low, difficulty is experienced in striking and maintaining a stable arc.
Electrode Size (ø mm) | Current Range (Amps) |
2.5 mm | 60-95 |
3.2 mm | 100-130 |
4.0 mm | 130-165 |
5.0 mm | 165-260 |
The electrode tends to stick to the work, penetration is poor and beads with a distinct rounded profile will be deposited. Too high current is accompanied by overheating of the electrode resulting undercut and burning through of the base metal and producing excessive spatter. Normal current for a particular job may be considered as the maximum, which can be used without burning through the work, over-heating the electrode or producing a rough spattered surface. The table shows current ranges generally recommended for a general-purpose type 6013 electrode.
Material and Joint Preparation
The material to be welded should be clean and free of any moisture, paint, oil, grease, mill scale, rust or any other material that will hinder the arc and contaminate the weld material. Joint preparation will depend on the method used include sawing, punching, shearing, machining, flame cutting and others. In all cases, edges should be clean and free of any contaminates. The type of joint will be determined by the chosen application.
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