What is the MMA Process?

Terms Used

MMA

Manual Metal Arc

SMAW

Shielded Metal Arc Welding

Stick Welding

Common Industry Term

The Process

The manual metal arc process was first developed in Russia in 1888 and comprised a bare metal welding rod. In the early 1900s, 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 initially slow due to high production costs, but the demand for higher integrity welds led to the process becoming increasingly adopted.

The material is joined when an arc is created between the electrode and workpiece, melting both the workpiece and the electrode to form a weld pool. Simultaneously, 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 must 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.

Power Source Characteristics: 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 welder is such that when the operator extends the arc length, it will reduce the welding current, and shortening the arc length (reducing the arc voltage) will have the opposite effect, 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.

Power Source Characteristics

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 workpiece. 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 that can TIG weld are often referred to as droopers 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".

Voltage-Current Characteristics

Drooping Characteristic: The shape of the characteristic allowed the operator to control heat input and weld bead shape by electrode manipulation.

Steep Characteristic: However, the TIG welder that can MMA weld had a much steeper characteristic curve, which gave more problems to the welder as the arc current remained constant despite large variations in arc voltage, reducing the control by manipulation.

Modern Inverter Technology

Modern inverter power supplies, however, can overcome these problems and provide excellent characteristics and performance as the curve can be controlled electronically for each process.

The small, relatively inexpensive AC sets are generally used in DIY or small maintenance functions, and some larger AC sets, often oil-cooled, may be used in heavier industry. However, DC output sets are now the most common in use.

Important: 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:

Lightweight & Portable

Very lightweight and portable compared to their predecessors

Energy Efficient

Very energy-efficient power supply offering energy cost savings

Higher Output

Able to provide higher outputs for lower inputs

Superior Control

High levels of control and performance

Best Practices: 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.

Knowledge Center

Guide to MMA Welding