Typical applications for MMA welds are many and varied, including general fabrication, structural steelwork, power and process plants, pressure vessels, cryogenic plants, pipelines, shipbuilding, bridge building and offshore fabrication. Its versatility makes it a popular choice for on-site maintenance and repair work across a range of industries.
The main drawback with stick welding is that only a limited amount of weld metal can be deposited from one electrode, so stick welders have to replace electrodes quite frequently. This can have an adverse impact on productivity. Many operators thus prefer semi-automatic and automatic welding processes such as metal inert gas (MIG ) welding.
Successful stick welding results with MMA hinge on the following factors and parameters:
Getting all of these variables right typically calls for a higher level of training and operator skill than that required for other electric arc welding methods.
MMA is a fusion welding process, using the heat generated by an electric arc to fuse metal in the joint area. The arc is struck between a covered consumable electrode and the workpiece. The process consists of a welding power source, which may provide either an AC, DC or DC plus AC electric current. The electrode is placed in the electrode holder connected to this power supply. The circuit is completed with an earth return cable fixed between the power source and the workpiece.
The MMA consumable electrode consists of a metal core wire around which a concentric clay-like mixture is extruded. The parts are held together with silicate binders. One end of the electrode is left bare, and this end is the one inserted into the electrode holder. At the other end, the core wire is left free of the coating, which is chamfered to aid the electric arc striking. A liquid graphite solution may also be applied to the electrode to make it easier to strike the arc. The proper positioning of the electrode in the electrode holder is crucial for optimal welding performance.
When the electric arc is struck between the tip of the electrode and the workpiece, the core wire begins to melt and material from the core wire is transferred across the arc to form the weld. The coating provides a protective gas and slag covering to the weld. The operator must maintain a constant electric arc length (i.e. the distance between the end of the electrode and the workpiece) to prevent the arc going out. Parent metal in the immediate area of the arc is also melted, and this combines with molten metal from the electrode to form the MMA weld pool.
Both backhand welding and forehand welding are used by stick welders. In backhand stick welding - also known as pulling, the electrode is angled away from the direction of progress of the weld so that the striking end of the electrode points back towards the deposited weld metal. This technique tends to be more popular as it prevents the slag being pushed in front of the advancing weld pool and causing inclusions.
With forehand stick welding, the electrode is angled toward the direction of progress of the weld so that the striking end of the electrode points away from the deposited weld metal and is aimed at unwelded material in the joint. This technique - also referred to as pushing - can be advantageous for MMA welds in the vertical-up position but can result in entrapment of slag and porosity.
The MMA process produces its own gas shield, created as the flux coating components decompose. This protects the weld pool and so the process does not require any auxiliary shielding gas cover. We can, however, support your MMA welding tasks with an impressive range of welding wires and electrodes in all shapes and sizes offering alloy compositions to suit your individual needs. We also offer welding equipment in some regions.
Contact your local Linde representative to check availability in your region.
The MMA process is often compared to metal inert gas (MIG) welding as both of these general-purpose processes are widely used and capable of welding steel. However, there are significant differences in terms of usability and productivity.
Key Differences between MMA and MIG
MMA stands for manual metal arc. The MMA welding process uses the heat produced by an electric arc generated between a non-consumable electrode and the workpiece. This welding process is also known as shielded metal arc welding (SMAW), manual metal arc welding (MMAW) and - perhaps more commonly - as 'stick-welding' or 'electrode welding'.
Welder productivity tends to be higher with MIG than MMA as MMA electrodes need to be replaced at regular intervals. However, MMA is a more portable method as the stick welder does not need to transport a shielding gas cylinder. This flexibility makes it ideal for work in the field. A greater level of skill is required for arc control in MMA welding and the welder may have to remove slag.
The MMA process does not require an auxiliary shielding gas. This is because the flux coating components produce their own gas shield as they decompose. This shield protects the weld pool.
MMA welding requires a power supply capable of supplying a controllable welding current of up to about 500 amps. Electrodes can be operated with AC or 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. The welding current level depends on the electrode size and should follow manufacturer specifications.
MMA welding electrodes are generally categorized by their flux coating. Arc stability, penetration, metal deposition rate and positional flexibility are largely determined by the chemical composition of the flux coating on the electrode. Electrodes can be divided into three main groups: rutile, basic and cellulosic. The selection of the correct type and size of electrode for any application is dependent on many factors, including the material composition and thickness, the mechanical properties required and the welding position.
Rutile electrode coatings contain about 50% rutile sand (titanium dioxide), making them easy to use. They can operate on both AC and DC currents. Easy striking, stable electric arc, low spatter, a good bead profile and relatively easy slag removal characteristics on the electrode make these general-purpose electrodes a firm favorite for mild and low-carbon steels in particular.
Also suited to AC and DC polarities, basic electrodes contain a high proportion of calcium carbonate and calcium fluoride, which tends to make them less easy to strike than rutile types. They have a more convex bead profile, and the slag is more difficult to remove, but they do give better weld metal properties than rutile types.
Cellulosic electrodes contain a high proportion of organic material, which gives them a fierce, deeply penetrating arc and a faster burn-off rate, but makes them more prone to spatter.
With metal powder electrodes, metal powder is added to the flux coating to increase the maximum permissible welding current level.
Arc blow happens when a direct current (DC) welding arc wanders - or strays - as a result of magnetic forces to favor one side of the workpiece or joint. This results in an uneven weld bead being deposited either all along the length of the weld or at the places where the arc blow occurred. It can also leave a long slag inclusion down the center of the joint. In most instances, arc blow is caused by poor earthing of the workpiece. It can be resolved by adjusting the work return clamp or switching to alternating current (AC).
Like all welding processes, MMA welding can present a number of hazards - some specific to the MMA welding process itself and others of a more general nature. Fume is a case in point as it is an unavoidable by-product of MMA welding. The amount and composition of particulate fume produced by MMA welding depends mainly on the composition of the consumable, the type of coating on the electrode and - to a lesser extent - on the composition of the material(s) being welded. Generally speaking, the higher the current for a particular diameter of electrode, the greater the amount of fume generated. Cellulosic and basic coated electrodes tend to generate more particulate fume than rutile types.
The more problematic pollutants encountered in MMA welding include chromium in its hexavalent form, nickel and fluorides. Exposure to hazards such as these can be mitigated through a combination of measures, including fume extraction equipment, good welding practices and posture, suitable electrode sizes, the correct arc length and the appropriate welding current.
In addition to a wide range of electrodes, we also offer personal protective equipment (PPE) in many regions, including respiratory protection, welding helmets with appropriate lens shades and safety glasses.