WELDING PROCESS 
WELDING PROCESS
CAN ALUMINUM BE WELDED WITH THE MIG/MAG PROCESS?
The answer is very simple: yes, it is possible. In fact, since the early development of the GMAW…
Welding machines or equipment are classified according to the way they supply energy; various criteria exist. For the analysis of the following article, the following classification will be adopted:
Static machines
• Transformers.
• Rectifiers.
• Transformer-Rectifiers.
• Inverters.
Rotating machines (converters)
• Electric motor-driven.
• Internal combustion engine-driven, which may be:
1. Gasoline.
2. (Diesel).
Static machines are those that have no continuously moving parts; exceptionally, some include a cooling fan.
Rotating machines are those that do have constantly rotating parts.
Static machines are in turn classified into the following types:
- Transformer-type machines. – They supply alternating current for welding.
- Rectifier-type machines. – These are transformer machines that, by means of rectifiers, convert alternating current to direct current for welding.
- Transformer-rectifier units. – These machines supply both direct current and alternating current for welding. Their special electrical design allows switching from one type of current to the other simply by toggling a changeover switch.
- Technological advances have led electronics to enter many areas of industry; in welding, units that operate using electronic circuit boards improve arc efficiency and energy consumption. This family of equipment is known as inverters.
Rotating machines or converters consist basically of a motor, which drives a generator at a specific rotational speed to produce the appropriate welding current. The motor may be:
- Electric, operating on current supplied from a general power grid.
- Combustion engine-driven, whether gasoline or diesel. Engine-driven welders are machines used preferentially in locations that lack access to a general power grid.
Static and dynamic characteristics
The primary objective that a welding power source must meet is to deliver a controllable current at the voltage demanded by the process in question. Depending on the Voltage-Amperage characteristics, power sources may be classified as:
• Constant current power sources.
• Constant voltage power sources.
The NEMA (National Electrical Manufacturers Association) standard defines the former as: “Those having a drooping Volt-Ampere characteristic, delivering a relatively constant current for moderate changes in load voltage.”
Constant voltage power sources are, in turn, defined as: “Those in which the Volt-Ampere characteristic is essentially flat, producing a relatively constant voltage for moderate changes in load current.”
Nevertheless, a welding arc is, by its very nature, unstable. Therefore, the “dynamic” characteristics of a power source — that is, the machine’s response capability to rapid variations in current or voltage in the load circuit — have a decisive influence on arc stability and, consequently, on the welding process itself.
Specific voltages and amperages are required to strike and maintain the arc. Their magnitudes behave in the arc in a manner inversely opposite to what Ohm’s Law would predict. What is known as the arc characteristic drops along a very steep slope.
The sharply dropping characteristic means, in practice, that striking the arc necessarily requires a higher open-circuit voltage Uo than that needed for welding itself. As befits the particular nature of welding, three distinct stages must be considered: open-circuit operation, arc striking, and welding. In the first case, an open-circuit voltage (Uo) of approximately 75 volts exists between the terminal of the welding unit and the workpiece, with zero current. Upon striking (short circuit), the voltage drops practically to zero (0) and the current reaches a well-defined maximum that is often above the welding current value. As this occurs, the voltage rises to between 15 and 45 volts (arc voltage) and the current stabilizes at a value corresponding to the welding current. From this it follows that the power source must adapt, as instantaneously as possible, to the rapidly changing arc conditions. For all load changes that occur slowly, the “static” characteristic applies; when they occur rapidly, however, the “dynamic” characteristic is decisive.
Duty cycle
It is clear that not all welding processes will impose the same demand on a power source. For example, in automatic processes the arc-on time (actual welding time) will be much greater than in manual processes, in which operator fatigue, the need to change electrodes, and other factors make frequent interruptions necessary. For this reason, it is customary to define a “DUTY CYCLE” as the percentage of time during which the machine must supply rated current to the load. This duty cycle is determined over a 10-minute period, such that, for example, a power source with a 60% duty cycle must be capable of delivering rated current for 6 out of every 10 minutes. For automatic processes, the duty cycle is normally specified at 100%.
