The MMA (Manual Metal Arc welding process was first developed in Russia in 1888 and comprised a bare metal welding rod. In the early 1900’s the coated electrode was introduced when the Kjellberg process was invented in Sweden. In the UK the Quasi arc method was introduced. The use of the coated electrode was slow due to the high production costs but the demand for higher integrity welds led to the process becoming increasingly used.
The material is joined when an arc is created between the electrode and work piece melting the work piece and the electrode to form a weld pool. At the same time the electrode has an outer coating sometimes called electrode flux which also melts and creates a shield over the weld pool to prevent contamination of the molten pool and assist in establishing the arc.
This cools and forms a hard slag over the weld which then needs to be chipped away from the weld bead upon completion or before another weld bead is added. The process allows only short lengths of weld to be produced due to the electrode length before a new electrode needs to be inserted
in the holder. The quality of the weld deposit is highly dependent on the skill of the welder.
The power source provides a constant current (CC) output and can be either AC (alternating current) or DC (direct current).
The design of the MMA welding inverter is such that the operator extending arc length will reduce the welding current and shortening the arc length (reducing the arc voltage) will do the opposite i.e. increase the current. As a guide the voltage controls the height and width of the weld bead whilst the current controls penetration, therefore the welder manipulates the electrode to achieve a satisfactory weld.
The power used in the welding circuit is determined by the arc voltage and current.
The voltage (V) is determined by the electrode diameter and the distance between the electrode and work piece. The current within the circuit is dependent on the electrode diameter, the thickness of the materials to be welded and the position of the weld. Most electrode information will show details of current types to be used and optimum current range.
MMA welding power sources which can TIG weld are often referred to as drooper’s or drooping characteristic power sources. They are typically basic selector type, magnetic amplifier control or engine driven units with a robust design as they are often required to work in extreme conditions.
The characteristic of the output shape gave rise to the term “drooper”.
Modern welding inverter power supplies however can overcome these problems and provide excellent characteristic and performance as the curve can be controlled electronically for each process.
The small relatively cheap AC sets are generally used in the DIY or small maintenance functions and some larger AC sets often oil cooled may be used in heavier industry but the DC output set are now the most common in use.
Electrode manufacture means that not all DC electrodes can operate on AC power sources but AC electrodes can operate on both AC and DC. Direct current (DC) is the most commonly used mode. Control of AC units tends to be moving iron core or switched transformers.
DC output power sources can be used on many material types and can be obtained in wide current ranges. Controls of these units vary from moving iron core control to the latest inverter designs. Inverter design has brought many advantages as they are:
• Very lightweight and portable compared to their predecessors
• Very energy efficient power supply and offer energy cost savings
• Able to provide higher outputs for lower inputs
• High levels of control and performance
In general it is preferable to weld in the flat or horizontal position. When welding in position is required such as vertical or overhead it is useful to reduce the welding current compared to the horizontal position. For best results in all positions maintaining a short arc, uniform movement and travel speed in addition to consistent feeding of the electrode are required.
What makes up the MMA (Stick) System?
The Welding Inverter Power Source
The welding inverter power source selected should have sufficient power to melt the electrode and weld material with enough capacity to maintain the arc voltage.
The MMA (Stick) welding process typically requires high current (50-350 Amps) at relatively low voltage (10-50 Volts). The MMA welding electrodes are designed to operate on different types of output power and voltage and you should always read the manufacturers data.
All welding electrodes can be used on direct current (DC) but not all on alternating current (AC). Some AC electrodes also have certain voltage requirements. When used in the DC mode the electrode lead should be connected to the polarity recommended by the electrode manufacturer, in most cases this will be electrode positive polarity but there are electrodes that use electrode negative polarity. The power source operates with a “no load” or “open circuit voltage” present when no welding arc is struck. This no load voltage rating is defined in the standard EN 60974-12012 (EN 60974) in accordance with the welding environment or risk of electrical shock.The power source may have a voltage reduction device (VRD) fitted either internally or externally.
The Electrode Holder and Welding Cables
The Electrode Holder and Welding Cables
The electrode holder clamps the end of the electrode with conductive clamps built into its head. These clamps operate either by a twist action or spring-loaded clamp action (crocodile type).
The clamping mechanism allows for the quick release of the remaining unused electrode end (stub end).
To ensure the maximum welding efficiency the electrode has to be firmly clamped into the holder, otherwise poor electrical contact may cause arc instability through voltage fluctuations and overheating of the holder.
The welding cable is connected to the holder either mechanically, crimped or soldered.
Electrode holders should conform to IEC 60974-11.
Welding cable diameter is generally selected on the basis of welding current level. The higher the current and duty cycle,
the larger the diameter of the cable to ensure that it does not overheat (see relevant standard). If welding is carried out some distance from the power source, it may be necessary to increase cable diameter to reduce voltage drop.
The Welding Electrode
The welding electrode consists of a core material of the material type i.e. steel or stainless steel etc. which provides the weld filler metal. This is covered by an outer coating called a flux which helps in creating the arc and shields the arc from contamination with what is called slag.
Types of Flux/Electrodes
The stability of the arc, depth of penetration, metal deposition rate and positional features are significantly influenced by the chemical composition of the flux coating on the electrode. Electrodes can be divided into three main types:
Basic Welding Electrodes
Basic welding 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.
- Low hydrogen weld metal
- Requires high welding currents/speeds
- Poor bead profile (convex and coarse surface profile)
- Slag removal difficult
When these electrodes are exposed to air moisture pick-up is rapid. Because of the need for hydrogen control these electrodes should be thoroughly dried in a controlled temperature drying oven.
Typical drying time is one hour at a temperature of approximately 150oC to 300oC but you should always consult the manufacturer data before use.
After controlled drying, basic and basic/rutile electrodes must be held at a temperature between 100oC and 150oC to help protect them from re-absorbing moisture into the coating. These conditions can be obtained by transferring the electrodes from the main drying oven to a holding oven or a heated quiver at the workplace.
Metal Powder Electrodes
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 130-140% 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.
NOTE: The quality of weld relies upon consistent performance of the electrode. The flux coating should not be chipped, cracked or more importantly, allowed to become damp. Electrodes are made with different types of coating and require different handling.
Cellulosic Welding Electrodes
Cellulosic welding 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, de slagging can be difficult. These electrodes are easy to use in any position and are noted for their use in the ‘stovepipe’ welding technique.
• Deep penetration in all positions
• Suitability for vertical down welding
• Reasonably good mechanical properties
• High level of hydrogen generated – risk of cracking in the heat affected zone (HAZ)
These electrode coatings are designed to operate with a definite amount of moisture in the coating. The coating is less sensitive to moisture pick-up and does not generally require a drying operation. However, drying may be necessary in cases where ambient relative humidity in which the electrodes have been stored in, has been very high.
Rutile Welding Electrodes
Rutile welding 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.
• Moderate weld metal mechanical properties
• Good bead profile produced through the viscous slag
• Positional welding possible with a fluid slag (containing fluoride)
• Easily removable slag
The rutile coatings can tolerate a limited amount of moisture and coatings may deteriorate if they are over dried. Always consult the manufacturer data before use.
Hard Wear/Facing Welding Electrodes
Hard facing or wear electrodes are used primarily to put a hard surface over a softer base material. There is an extensive range of these types of products and a common use area is the repair of wearing surfaces such as teeth on earth moving and mining equipment.
DC Copper Coated Welding Electrodes
This is the most common type because of its comparatively long electrode life. These electrodes are constructed by mixing and baking carbon, graphite and a binding agent and coating them with copper. They provide stable arc characteristics and uniform grooves.
DC Plain Electrodes
These are constructed in the same way as the DC copper coated electrode but without the copper coating. They are more rapidly consumed compared to copper coated in use
AC Coated Electrodes
These electrodes are constructed by mixing and baking carbon, graphite and a special binding agent with added rare earth materials to help with arc stabilization.
They are copper coated.
The process uses compressed air at between 80-100 psi at the electrode holder.
Increasing air pressure will not remove metal more efficiently.
Welding Electrode Storage
Electrodes should always be kept in a dry and well-ventilated store. It is good practice to stack packets of electrodes on wooden pallets or racks well clear of the floor. Also, all unused electrodes which are to be returned should be stored so they are not exposed to damp conditions to regain moisture.
Good storage conditions are 10°C above external air temperature. As the storage conditions are to prevent moisture from condensing on the electrodes, the electrode stores should be dry.
Under these conditions and in original packaging, electrode storage time is practically unlimited. Modern electrodes are now available in hermetically sealed packs which remove the need for drying. However, if necessary, any unused electrodes must be redried according to manufacturer’s instructions
Drying of Electrodes
Drying is usually carried out in accordance with the manufacturer’s recommendations and requirements will be determined by the type of electrode.
Many electrodes are now available in hermetically sealed containers. These vacuum packs obviate the need for drying the electrodes immediately prior to use. If however the container has been opened or damaged, it is essential that the electrodes are re dried according to the manufacturer’s instructions.
The electrode diameter selection is based on the work piece thickness, welding position, joint form, welding layer etc.
Welding current level is determined by the size of electrode – the normal operating range and current are recommended by manufacturers. Typical operating ranges for a selection of welding electrode sizes are illustrated in the table.
- Electrodes should be dry and used according to the instructions.
This will reduce the hydrogen in the molten pool and welding seam, avoiding the blowholes and cold cracking.
- In the welding process, the arc must not be too long; otherwise, it will cause unstable arc burning, large amounts of spatter, light penetration, undercut, blowholes etc. If the arc is too short, it will cause electrode sticking to the work piece.
- In MMA welding the arc length is usually equal to 0.5~1.0 times the diameter of the electrode. The basic electrode’s arc length is not more than the electrode diameter and short arc welding is preferred. When using acid electrodes the arc length is equal to the electrode diameter.
Inverter Controls Used in MMA (Stick) Welding
Welding Current Control (A)
The current control regulates the amount of current output from the welding inverter and hence the deposition rate dependent on the electrode diameter.
Often current can be controlled via remote controls on more modern electronic welding inverters.
At the start of welding the hot start provides an increased amount of current to enable the electrode to strike the arc without sticking to the work piece. Some machines have an automatic hot start current with a set time and level others have variable hot start control for an operator to select.
During welding the arc voltage is normally in the region of 20V. Often the situation may demand a shorter arc which results in a lower voltage and the electrode is prone to “sticking to the work piece” as the arc has actually extinguished. The arc force control will overcome this problem by increasing the current when the arc voltage falls to ensure the metal transfer of the electrode and prevent the electrode sticking. Some machines have an automatic arc force current others have variable arc force control for an operator to select the required level.