The TIG welding technique – also known as GTAW – makes use of a tungsten rod and an atmosphere of inert gas (argon in most cases) to achieve a clean, strong and impurity-free weld. One of the main parameters when welding is the type of current, which can be alternating or direct.
In this article we will analyze the difference between both types of current and their impact on welding, as well as the difference between welding machines direct current and those that allow both types of welding.
Alternating current and direct current
In this graph we can see the “shape” traced by the current as a function of time. In green it alternates it, in red it continues it.
Electric current —which allows us to weld— has two parameters that define its behavior: voltage and intensity. The variation of these two parameters over time defines the type of current. If there is no variation in voltage, the current is said to be direct; If the current fluctuates uniformly in a regular way, it is said to be alternating; if it fluctuates stochastically (irregularly), it is a variable current. Finally, if the current presents a regular but not uniform variation, it is said to be pulsed.
The electric current that we receive in our homes is alternate; the waveform, in a voltage-time function, is sinusoidal, and can have a frequency of 50 Hz or 60 Hz. This frequency is the number of cycles per second, and is expressed in Hertz (Hz).
To understand the concept a little better, let’s think that the alternating current (or alternating current) is generated by means of a machine very similar to an electric motor, but that applies the inverse principle: instead of using electrical energy to generate movement, it takes advantage of the movement to generate energy.
This machine produces electrical energy through an internal winding. In single-phase electric generators, one turn of the generator equals one complete cycle. If the generator (called alternator) rotates at a rate of 50 RPM, we will have an alternating electric current of 50 Hz at the output of the terminals. It is important to understand that what varies with time is the tension. In the first half of a cycle we will have a voltage whose value is positive, and in the second half, a negative voltage. This means that the voltage is reversed once per cycle.
On the contrary, direct current (or direct) is characterized by maintaining a constant voltage over time (hence its graphic representation is a line and not a sinusoidal wave). A battery, for example, is a direct current generator; so is the dynamo, present in many cars.
TIG AC / DC welders versus TIG DC welders
TIG welding machine
In order to be able to use AC / DC and DC machines, we must first take into account that they are designed to work specifically with the TIG technique; Sometimes it is possible that the possibility of working with arc welding, or MMA, is also incorporated.
Direct current welding with DC equipment
Direct current TIG welding responds to a need, which is to weld certain metals together. For example, to weld steel, nickel, titanium, and other metals, a negatively charged electrode (DCEN) is used. This technique is also used in automated welding of aluminum and magnesium, using helium as an inert atmosphere, which prevents the dreaded oxide layer from forming on the surface of these two metals.
The negatively charged electrode generates heat by emitting electrons, which travel through the arc towards the material to be welded; This causes the thermal ionization of the inert gas and increases the temperature of the material to be welded. Thus, the inert gas flows towards the electrode and not towards the welding area, making it possible for the seam to present oxide.
If reverse current is used (with the electrode receiving electrons), it is achieved that the inert gas is deposited on the material, but at the same time the heat is concentrated on the electrode, with which it is only possible to make seams in thin materials such as sheets and profiles, although this ensures that the seam is perfectly clean.
Welding in alternating current with AC / DC equipment
On the other hand, the use of alternating current is indicated for manually weld aluminum and magnesium– By reversing the polarity several times per second, the problems of electrode heating and exposure to rust at the seam are avoided. In advanced equipment it is possible to regulate the time of each hemicycle, which determines either a greater presence of inert gas or heat in the work area.
Art in Welding
The reason for the existence of direct current only TIG machines
At first it might seem strange that, with AC / DC welding equipment being more versatile than those with only direct current, there are both models on the market. The answer is that the exclusively direct current models are specialized in manual welding of iron, steel, nickel, titanium and others, while the alternating current models specialize in manual welding of aluminum and magnesium.
Teams solder AC / DCAlthough they are almost universal, they are more restricted in terms of the peculiarities of direct current welding: generally the diameter of the electrode is not variable (or if it is, it allows only a small variation) and there are not too many presets of welding with respect to the specialized machines in direct current.
Equipment specialized in direct current, on the other hand, offers greater control over welding and a greater range of electrodes.
When to choose to buy DC machines?
If you are welding only iron, steel, nickel, titanium, and other metals except aluminum and magnesium, purchase a DC machine.
When to choose to buy AC / DC machines?
If your intention is to work with aluminum or magnesium, purchase a machine capable of welding with both types of current. Remember that aluminum has a tendency to cover its surface with a thin layer of oxide almost instantly; To prevent the formation of this layer, you must work with an inert atmosphere that at the same time keeps the temperature on the part under control. This is much better accomplished with an alternating cycle than a continuous one.
The important thing when choosing which welding machine to buy is to properly consider the type and the tasks to be carried out more or less frequently, hence the answer.
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