What is the MMA Process?
MMA - Manual metal arc
SMAW -Shielded metal arc welding Stick welding
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 arc welders 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 Welding 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 arc welder 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 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.
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