ARC WELDING - MMA

ARC WELDING - MMA
The term 'arc welding' covers a large number of welding processes. What they have in common is the fact that they all use electric current to melt the filler metal and the parent metals.
The term 'arc welding' is commonly used only about manual arc welding with a coated electrode. That's why the process is called 'Manual Metal Arc welding' - (MMA).
When referring to other arc welding processes, welders use other terms, such as TIG, MIG or CO2 welding, for example. In these processes a gas is used to shield - to protect - the weld pool, and that's why these processes are often referred to as 'gas-shielded processes'. Gas-shielded arc welding can also be carried out manually, but it is more widely used for automatic or semi-automatic processes.
MMA
Manual metal arc (MMA) welding is often referred to as shielded arc welding (SMAW) in the USA. Arc welding with a coated electrode is usually carried out manually. It is the most widely used welding process and accounts for approximately 50% of all the welding carried out in the world today, although this percentage is decreasing because of the increased use of gas-shielded processes.
There's a great variety of electrodes that are used in MMA. And the choice of electrode depends on the type of metal that's to be welded, its thickness and preparation, and of cause the quality of the welded joint. What they have in common is that the electrodes used in manual arc welding are 'coated'; they consist of a core surrounded by a layer, a coating.
The core functions as an electrode: it conducts the electric current that is taken from the welding machines. When the electrode touches the metal plates, there's a short circuiting of the current. This short circuiting creates an eletric arc that's like a hot flame that melts the core and the metals that the electrode touches.
The coating contains various substances and its purpose is to protect the weld pool from the oxygen and impurities in the air. During welding the coating melts and creates a gas shield around the pool. Apart from protecting the weld pool, the coating also has another important function: to add alloying elements to the weld so as to improve its quality. The coating also improves arc initiation and arc stability. Some of the minerals or substances in the coating will settle on the finished weld as slag, but can be removed easily.
Welding Equipment
All kinds of arc welding are carried out by means of electric current supplied from the mains to the welding machine. This is an alternating current commonly of 230V. The welding machine is often referred to as a transformer or a rectifier depending on the current that it supplies for the welding process. If the current is direct current - dc -, the welding machines is referred to as a rectifier because it changes - rectifies - the alternating current from the mains into direct current.
This voltage level of the current taken from the mains is dangerous in case the welder touches the uninsulated parts of the electric wires. As a safety measure, regulations state that for manual arc welding the voltage level should be less than 80V ac or 100V dc. This voltage level is commonly called 'the open circuit voltage' or 'the no load voltage'. Welders use the abbreviation OCV for 'open circuit voltage'. This voltage level is not the same as the voltage level that's used during welding. The latter voltage ranges between 20 and 25 volts for most types of electrodes. Most welding machines are able to step down and rectify the current to different levels, and they are equipped with switches and dials for this purpose.
Polarity
When welding with direct current - dc -, the electrode can be connected to either the positive pole or the negative pole of the terminal; 'positive polarity' or ' negative polarity' are the terms that are most commonly used. If the electrode is connected to positive polarity, the workpiece must be connected to negative polarity. This connection is called 'positive polarity' or 'electrode positive'. In America, the term 'reverse polarity' is used about such a connection.
If the electrode is connected to negative polarity, the term used is 'electrode negative'. In the US, the term 'straight polarity' is preferred.
The most common parameters for the correct setting of the electric power are ac or dc, voltage and ampere. The levels of these welding parameters depend to a large extent on the type of metal to be welded, the welding position and the specifications as to the quality of the welded joint. Information concerning parameters and welding procedure is stated in the drawings and the job specifications.
In arc welding there are two cables, leads, connected to the welding set. One lead is connected to the output terminal, the supply terminal of the welding set. The output terminal can be either ac or dc, the polarity is usually positive, but may be negative in some cases. The other end of the output cable is connected to the electrode holder. The second lead is the return cable which is connected to the welding machine at one end and to an earth clamp at the other end. During welding, the earth clamp is clamped to the workpiece.
Welding is started by bringing the electrode in contact with the workpiece. When contact is made, the flow of electricity is short-circuited. The electrode is then lifted some millimetres above the workpiece, and an electric arc is created between the workpiece and the electrode. The electric arc is actually a spark between the two terminals of the electric circuit: the workpiece and the electrode. When the arc is made, the fusion temperature of the metals and the electrode is quickly reached.
Gas-shielded arc welding (GMAW)
In gas shielded arc welding a separate gas is used to protect the weld pool. The filler wire that is used may either be consumable electrode, as in MMA or MIG welding. In these processes the filler wire - which is identical with the electrode - melts during welding. But the electrode can also be non-consumable, as in TIG welding. In TIG welding therefore a separate filler wire is used together with an electrode.
The core of the filler wire contains the same material as the metals. It may consist of carbon steel or an alloy steel. During welding the core conducts the welding current and is melted and fused with the base metal to form one continuous weld.
TASKS
CC (constant current) - CV (constant voltage)
Explain the abbreviations "CC" (constant current) and "CV" (constant voltage).