Guide to the MMA (Stick) Welding Process

Guide to the MMA (Stick) Welding Process

The MMA (Manual Metal Arc) welding method originated in Russia. Early developments involved a bare metal welding rod until the introduction of the coated electrode thanks to the Kjellberg process in Sweden during the early 20th century. In the United Kingdom, the Quasi arc method further advanced the technology. Though the initial implementation of coated electrodes faced challenges, their benefits became more apparent over time, leading to their widespread use driven by the demand for stronger and more reliable welds.

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The welding process begins with the creation of an electric arc between the electrode and the workpiece. This arc melts both the electrode and the material being welded, generating a molten weld pool. The electrode itself is coated with a substance known as flux, which also melts to provide a protective atmosphere surrounding the weld pool, helping to prevent contamination while simultaneously facilitating arc stability.

As the weld pool cools, it forms a hard slag that must be removed after the welding operation or before subsequent welds are initiated. Due to the length of the electrodes, only short welds can be produced before a new electrode needs to be installed. The overall quality of the welds is heavily reliant on the welder's skill set. The power source is designed to deliver a constant current (CC) and can operate on either AC (alternating current) or DC (direct current).

MMA welding inverters are designed such that varying the arc length affects the welding current. When the arc is extended, the welding current decreases, and conversely, shortening the arc length increases the current. Thus, welders manipulate the electrode to achieve optimal results. In this context, voltage influences the height and width of the weld bead, while the current dictates penetration. Achieving a satisfactory weld also requires precise control of the electrode movement.

The power in the MMA welding circuit is a function of the arc voltage and current output.
Voltage (V) is influenced by factors such as the diameter of the electrode and the gap between it and the workpiece. The current in the circuit also depends on the electrode size, the thickness of the materials being joined, and the position of the weld being performed. Most electrode specifications will detail the compatible current types and optimal ranges for effective application.

MMA welding power sources that are capable of TIG welding may be referred to as "droopers" due to their characteristic output. Typically, these are robust, either simple selector-type units or magnetic amplifier-controlled machines, designed for use in demanding conditions.
Moreover, modern inverter designs have significantly improved performance and control, allowing for various outputs with enhanced efficiency compared to older models.

While smaller and more affordable AC units are commonly used for DIY projects, larger AC sets may find their use in more industrial settings, but the prevalence of DC output units is becoming increasingly standard.

Not all DC electrodes operate on AC sources, while AC electrodes can function on both. The DC mode is generally the most frequently utilized, with controls ranging from moving iron core to advanced inverter technology ensuring control over a variety of materials.

Inverter designs provide notable advantages, including:

• Lightweight and portable compared to traditional systems
  
• High energy efficiency and reduced operational costs
  
• Enhanced output capability for minimal input
  
• Superior control and performance levels

Ideally, MMA welding should occur in a flat or horizontal orientation. For vertical or overhead tasks, reducing current relative to horizontal settings can be advantageous. Consistent movement, travel speed, and uniform electrode feeding are vital to attaining optimal weld quality across all positions.

Components of the MMA (Stick) System

Welding Inverter Power Source

The selected MMA welding inverter power source should have sufficient capacity to melt both the electrode and the work material while maintaining the necessary arc voltage.

The MMA (Stick) welding process generally mandates high currents (ranging from 50 to 350 Amps) with low voltage (10–50 Volts). Specific MMA electrodes are tailored to function under various output power and voltage conditions, thus it's crucial to consult manufacturer specifications.

All welding electrodes function under DC but not all can operate with AC. Certain AC electrodes possess specific voltage stipulations. When using DC, ensure the polarity aligns with the electrode manufacturer’s recommendation—typically, this is electrode positive polarity, but exceptions exist.

When no arc is present, the power source exhibits "no load" or "open circuit voltage." Ratings are defined according to EN standards, taking into account the welding environment and electrical shock risk. A voltage reduction device (VRD) may be included in the power source.

Electrode Holder and Welding Cables

Electrode Holder and Welding Cables

The electrode holder secures the electrode's end using built-in conductive clamps that operate through either a twist action or a spring-loaded mechanism. This design facilitates the expeditious release of the remaining electrode stubs.

For optimal welding efficiency, the electrode must be securely fastened within the holder – improper contact can result in unstable arcs through voltage fluctuations and overheating. Additionally, the welding cable connects either mechanically, with crimped fittings, or via solder.

Electrode holders should comply with IEC standards.

Welding Cable Considerations

Welding cable diameters are generally chosen based on the current level; higher current levels require correspondingly larger cables to prevent overheating (as per relevant standards). If welding is conducted at a distance from the power supply, increasing the cable diameter can also mitigate voltage drops.

The Welding Electrode

The welding electrode consists of a core material aligned with the required material type (e.g. steel or stainless steel) that provides the necessary weld filler metal, encased in a protective flux coating that assists in arc formation and shields against contamination.

Classification of Flux/Electrodes

Factors such as arc stability, penetration depth, deposition rates, and positional capabilities are greatly influenced by the flux coating’s chemical composition. Electrodes can be primarily categorized into three main types:

• Basic
  
• Cellulosic
  
• Rutile

Basic Welding Electrodes

Basic welding electrodes primarily consist of calcium carbonate and calcium fluoride in their flux coating, making their slag more fluid and quick-freezing, thus facilitating vertical and overhead welding. They are ideal for medium to heavy-scale fabrications demanding high weld quality and crack resistance.

When exposed to moisture, these electrodes quickly absorb it, necessitating thorough drying if they are to be used effectively—this typically takes around one hour at 150-300°C, but always check manufacturer guidelines.

Post-drying, basic electrodes must be stored at a temperature between 100°C and 150°C to prevent moisture re-absorption. This can be achieved using a holding oven or heated container.

Metal Powder Electrodes

These electrodes include metal powder in their flux coating, improving current handling capabilities, leading to enhanced deposition rates and efficiencies. They are frequently utilized in flat and H/V welding positions due to their high deposition rates with the added benefit of easy slag removal.

N.B.: Consistent electrode performance is critical to weld quality. Flux coatings should remain intact without chipping or dampness. Various coatings necessitate specific handling protocols.

Cellulosic Welding Electrodes

Cellulosic electrodes offer deep penetration and rapid burn-off, enhancing their welding speed, although their slag can be challenging to remove. They are versatile for all positions, particularly effective when employing the 'stovepipe' welding technique.

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' Deep penetration achievable across all positions
' Well-suited for vertical down welding
' Satisfactory mechanical properties
' High hydrogen generation risk, increasing crack potential in the heat-affected zone (HAZ)

Rutile Welding Electrodes

Rutile electrodes incorporate titanium oxide for easier arc ignition, stable operation, and minimized spatter. They are versatile general-purpose electrodes suitable for various welding scenarios and positions, favorable particularly in welding fillet joints.

These electrodes yield moderate mechanical properties with good bead profiles facilitated by a viscous slag, which is also easily removable. However, excessive moisture can deteriorate their performance, necessitating adherence to manufacturer guidance regarding storage and handling.

Hard Wear/Facing Welding Electrodes

'Hard-facing electrodes are specifically intended to create a tough surface on softer base materials. They are widely utilized in areas subject to wear, such as mining and construction machinery repairs.

DC Copper Coated Welding Electrodes

' As the most prevalent type, these electrodes boast extended life spans. They are made from a mix of carbon and graphite, bound together and coated with copper, ensuring stable arc characteristics with consistency in usage.

DC Plain Electrodes

Similar to copper-coated electrodes, but without the copper coating, these are consumed more quickly in typical operations.

AC Coated Electrodes

These electrodes are crafted with elements specifically added to improve stable arc characteristics, using a binding agent mixed with rare earth materials, and they are typically copper coated.
Utilizing compressed air at 80-100 psi at the electrode holder does not improve the efficiency of metal removal.

Welding Electrode Storage

Electrodes should be kept in a dry and well-ventilated space, preferably stored on wooden pallets, clear of the floor. Unused electrodes must also be stored upright to mitigate moisture absorption.

Optimal storage conditions should be maintained at a minimum of 10°C above external temperature to avoid moisture condensation. When stored adequately, electrode longevity becomes nearly unlimited, facilitated further by modern hermetically sealed packaging that reduces the need for immediate drying. However, if opened or damaged, electrodes must be re-dried per manufacturer instructions.

Electrode Drying Procedures

' Effective drying must follow the manufacturer's guidelines tailored to electrode types. Contemporary electrode options often come in sealed containers, reducing immediate drying needs before use. If the container is compromised, re-drying becomes essential.

Electrode Selection Criteria

Choosing electrode diameters should be based on various factors, including the thickness of the workpiece, the welding position, and joint formation.

Selection of welding current is influenced by electrode size, with manufacturers providing recommended operating ranges for different electrode sizes. Ultimately:

• Electrodes must remain dry and utilized according to guidelines, effectively minimizing the risk of hydrogen presence in the molten weld pool, thereby preventing blowholes and cold cracks.

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• The arc length should be managed appropriately; excessive length may lead to unstable arcs and spatter, while too short a length raises the risk of electrode stick.

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• In MMA welding, optimal arc length typically measures between 0.5 to 1.0 times the diameter of the electrode, with basic electrodes recommended to stay shorter than their diameter and short arc techniques advised for acids.

Inverter Controls in MMA (Stick) Welding

Welding Current Control (A)

Current controls manage the output levels from the welding inverter, directly affecting the deposition rate based on electrode specifications.

Modern electronic welding inverters often feature remote control capabilities for current adjustment.

Hot Start Functionality

' The hot start function provides an initial surge of current for easier arc initiation without electrode sticking. Some equipment includes automatic settings, while others allow variable operator control.

Arc Force Control

During operation, the typical arc voltage hovers around 20V. Nonetheless, shorter arcs can lead to lower voltages and increased probability of electrode sticking. Arc force control mechanisms help counter this by boosting current during voltage drops to ensure effective metal transfer and minimize electrode adherence.

Common Issues in MMA (Stick) Welding