Manual Metal Arc Welding
The oldest of the arc welding processes, Manual Metal Arc (MMA) welding, has a filler rod coated with flux. When an arc is struck between the rod and the work piece the filler rod and flux melt. As the flux melts it creates a gas shield around the molten weld pool to prevent oxidisation.
Immediately after fusion the flux forms a hard slag over the weld to prevent oxidisation during cooling. When welding is complete this slag is chipped off. MMA welding is a versatile process as it can be performed with basic equipment, used in most conditions and for joining most weld able metals with the exception of aluminium, which can be MMA welded but not very satisfactorily.
Types of Electrode:
Rutile Flux Coated
The most common type of coating, rutile coated electrodes are easy to use, offer good levels of mechanical strength and produce a weld of good visual appearance.
Basic Flux Coated, Basic or Low Hydrogen, coatings are used for higher strength joints and are recommended for structural applications and materials thicker than 10mm. Basic coatings are more difficult to use than rutile and do not offer such a good weld appearance, they are therefore best chosen by those with experience.
Cellulosic Flux Coated
Best known for deep penetration and rapid burn speed, cellulosic coated rods are commonly used for professional root welding of plate and pipe.
Welding current is largely determined by the type of electrode, the type of job and the material to be welded. Most manufacturers will recommend a current range for a given product, however as a general guide allow 35 amps of welding current for every millimetre of electrode diameter (plus or minus 15% depending on the job) i.e.: 2.5mm electrode = 88 amps (or a range of 75 - 102 amps).
Metal Inert Gas (MIG)/ Metal Active Gas (MAG) welding is the most popular of the arc welding processes; this is mainly because it is easy to use and versatile. With an Inert Gas, such as Argon, MIG welding can be used to weld aluminium. With an Active Gas, such as an Argon/Co2 mixture (typically 5-20% Co2), MAG welding can be used to weld most steels, including stainless.
The process strikes an arc between the continuously fed filler wire and the work piece, whilst protecting the arc from oxidisation by pouring a shielding gas over the weld pool.
Two methods of metal transfer from the filler wire to the work piece are used:
This is achieved at lower voltage settings. The wire is fed into the weld pool creating a "short circuit" the resulting heat burns the wire off. This happens many times per second and is responsible for the characteristic crackling sound of Mig/Mag welding and the associated spatter. Dip transfer offers the advantage of being able to weld in position, ie vertical.
This is achieved at higher voltage settings and is where the filler wire is melted and transfers in globules to the weld pool. Spray transfer offers smoother welds with little spatter but cannot be used in position.
Where gas is either unavailable or impractical (such as outside in windy conditions) flux cored wires can be used. Welding with flux cored wire is like MMA welding but with a continuous electrode.
In TIG (Tungsten Inert Gas) welding an arc is struck between a tungsten electrode and the work piece. A separate filler wire can be added to the weld pool as necessary. The weld pool is protected from oxidisation by pouring an inert gas (usually Argon) over the weld pool. Tig welding is fairly easy to learn and is probably the most versatile of the arc welding processes, the down side is that it is slow and equipment can be prohibitively expensive for all but professional welders.
Tig welding processes fall into two basic categories, DC Output Current is used for most applications including the welding of Steel, Stainless Steel and Copper. AC Output Current is used for Aluminium and Aluminium Alloys.
Tig welding equipment fall into three main types:
Scratch/Touch Start - this is the least expensive and most basic form of equipment, relying on the tungsten electrode being touched down onto the job then lifted off to form an arc (much the same as MMA welding).
High Frequency Start - this type of equipment uses a burst of high frequency to establish the arc. This is easier and more precise than Scratch starting, but HF start machines are significantly more expensive.
High Frequency Start - these are the most sophisticated, and expensive, of machines and allow the Tig welding of most materials from Steel to Aluminium.
Two types of Tungsten Tig Electrode are available, Thoriated (red tip) for DC applications and Zirconiated (white tip) for AC applications.
When AC welding the arc will cause the tungsten to "ball" at the end.
The diameter of this ball should not exceed the diameter of the tungsten, if it does a larger tungsten should be used. When DC welding the tungsten should be ground to a point. This point should be as sharp as possible with the grinding lines running from the point, down the length, NEVER around the point.
Five key hazards should be considered when Arc welding.
This condition is caused when the Ultra Violet radiation the arc welding processes emit burns the retina of the eye. Arc-eye is extremely painful and can lead to permanent blindness. It is essential that suitable eye protection is used and that the welding workstation is shielded from others.
Ultra Violet radiation can also burn skin, in exactly the same way as the sun can, only much quicker. Like sunburn, long term implications can include skin cancer. It is therefore essential that suitable workwear is worn and that others are protected.
Welding gives off a variety of particle fumes and gases. As most of the fumes and gases are harmful it is essential that operators protect themselves. This can be done in several ways. Fume extraction offers the best method of protection providing the fume is extracted "at source", that is to say before the fumes can reach the welders breathing zone. Where fume extraction is unavailable or impractical, a suitable respirator should be used to filter the air that the welder breathes. The respirator may be of the disposable, replaceable cartridge or air fed welding helmet type. Where fumes are particularly hazardous, such as when welding galvanised steel and painted materials, both fume extraction and a respirator should be used.
Never use arc welding equipment in damp conditions and always ensure the equipment is in good condition. If in doubt, consult a qualified welding equipment engineer.
Spatter and sparks from welding can set fire to materials that are left lying around the work station, i.e. paper, rags etc. Always work in a clean well-organised area.