MMA welding
As a versatile and simple process, MMA welding has become established in many steel processing sectors. Due to its minimal equipment requirements, the process speaks for itself thanks to the fact that it is highly portable, especially on building sites. That wind-sensitive shielding gas is not required, is a further point in its favour. Despite its low-cost applicability, high-quality results can be achieved. Innovations in power source technology mean a stable arc, even in unfavourable conditions or when the mains cable is long.
As with MIG/MAG welding, so too in MMA welding the electrode fulfils a dual function of arc carrier and consumable filler material. In MMA welding, the electrode is referred to as ?filler rod? or ?rod electrode?. The heat from the arc melts the core wire of the filler rod and the base metal. At the same time, the filler rod casing acts as bell jar and layer of slag to protect the heated workpiece surface from chemical reactions with the surrounding air. This maintains the strength and durability of the weld metal. The filler rod is connected to one pole on the power source via the welding cable and electrode holder. The earth connection runs via the workpiece terminal and earthing cable to the other pole on the power source. The pole that represents the welding potential depends on the type of rod electrode being used. Rutile electrodes are mostly welded at the power source?s negative pole, whereas basic electrodes are mainly used at the positive pole. Under certain conditions, rutile electrodes are also suitable for alternate current welding with simple welding transformers and no current rectifier. Other characteristics of basic electrodes include easy weldability, even weld seam and spray transfer. In addition to large drop transfer, basic electrodes on the other appear to incorporate moisture, thereby causing pores in the weld metal in its undried state. Advantages include weldability in several positions and good mechanical properties of the weld. A further type of electrode is the cellulose electrode. In addition to spray transfer, cellulose electrodes have a very deep fusion penetration, good mechanical strength, and are suited to all welding positions, including vertical down seam. Disadvantages include difficult weldability and the generation of a substantial amount of smoke. Furthermore, these electrodes are not suitable for all types of power source.
With MMA power sources, it is essential to keep the set welding current constant, regardless of the presently prevailing arc length. The welding voltage varies according to the arc length at that moment. Modern power sources can maintain this regulation even during extreme fluctuations in the mains voltage or where mains cables are very long. The simplest power sources are transformers with no rectifier, with the desired welding current obtained by variable spreading of the magnetic field via an adjustable transformer yoke. These power sources are very inexpensive yet not suitable for all electrodes thanks to their being limited to alternate current. Other disadvantages include their heaviness and significant bulk. Thyristor power sources have a rectifier that generates the rectified welding current from the alternate current. The current is controlled by thyristors. These are the controllable switching elements in the rectifier. An induction coil smooths out unwanted current peaks, thereby reducing the tendency to produce welding spatter. These power sources are already direct current compatible and easy to control. On the down side, however, remains increased space requirements and heavy weight, as well as a slow control process and susceptibility to fluctuating mains voltages. Inverter power sources represent the latest technology. They produce a pulsed voltage with high frequency out of the mains voltage. This voltage arrives at the welding transformer which, thanks to the high frequency, can be designed in a much lighter, compact and efficient way than previously mentioned power sources. Inverter power sources also have a rectifier. The low current ripple of the transformer output current means a substantially more compact design, or no need for the output choke. The rectifier simply consists of uncontrolled diodes. The latest generation of MMA inverters has a so-called resonance inverter. The interaction of the welding transformer with a special arrangement of capacitors acting as energy reserves is fundamental. In addition, the transformer takes on this reserve role, while in turn obtaining electricity from the magnetism generated by itself while discharging. If transformers and capacitors are coordinated in such a way that they charge each other, this is referred to as resonance. The cleverly devised combination of resonance and storage function creates valuable power reserves that are available if required by the arc. The result is an ideal characteristic for ever-reproducible, perfect welding results and optimum process safety. Even mains cables of more than 100 m, fluctuations in the mains voltage, or generator-powered operation, have no negative effect on the welding result. Problem-free welding of all electrode types therefore becomes a matter of course.
With the exception of aluminium, MMA welding is suited to practically all metals. The process is not limited to workshops, but makes its presence felt outdoors, on construction sites and even under water. As a counterweight to relatively low welding speed and a lack of mechanisation of the process are the low equipment costs, easy handling and low noise of direct current welding. When welding is finished, there may well be a layer of slag to remove, yet this provides optimum protection for the join. For problem-free welding of the various materials, the latest generation of power sources have numerous extra functions. The basic principle of these extra functions is explained in the following. The ignition process itself should be quiet, precise and spatter-free. This requirement is met by the SoftStart function, making working with basic electrodes significantly easier. SoftStart is triggered by placing the bare electrode end onto the workpiece. As soon as the filler rod is raised, the arc ignites at approx. 30 A. By raising the electrode still further, the welding current increases continuously until it reaches the set value. The happy result is a soft, precise and spatter-free ignition. Precise, spatter-free and soft ignition with low current. Raising the filler rod increases the welding current continuously until it reaches the set value. During welding of large drop filler rods, there is a risk of sticking. Even before it gets to that stage, the welding current increases for a fraction of a second and breaks the electrode free. This is called dynamic (arc-force control). If the electrode should however become stuck, the anti-stick function is activated, stopping the welding current immediately. In this way, the electrode remains undamaged.
Moderate welding speed and a lack of mechanisation does of course limit MMA welding with regard to productivity. From a technological and metallurgical perspective, this process provides very good conditions for optimum welding results, all the more so as the latest generation of inverter power sources provides a very quiet, stable arc. An essential prerequisite for optimum welding results. MMA welding comes into its own for portable use on construction sites, as well as in the manufacture of components that require minimal welding.
