Selection of a welding process is determined primarily by the characteristics of the joint, the materials involved, their shape and thickness, and joint design. Additionally, production requirements, such as rate and quality, must also be considered. Only after the process has been determined can the proper power supply and accessory equipment be chosen. The process is the primary factor in their selection.
It is our purpose in this article to provide a guide to the selection of power supplies for some of the welding processes, which have come into maturity during the past decade. Once thought of primarily as special processes or processes designed primarily for mass production operations, they are now found in virtually every area of metal fabrication. The increase in the fabrication of once difficult-to-weld metals and the economic and qualitative advantages of the more advanced welding processes are today recognized by even the smallest metal fabricating shops.
Power and control requirements for these processes are somewhat more sophisticated than for conventional shielded metal arc or stick electrode welding. But since the average metallurgist or welding engineer isn’t an electrical expert, the selection of proper power supply and the reasons for it can be confusing. The purpose of this article is to eliminate some of this mystery through the examination of the power requirements of production welding processes.
The basic types of power supplies are:
- constant current and
- constant voltage.
Constant Current Power Supply
The conventional stick electrode welder is sometimes called a ’constant current machine’. It is also called a ’dropper’ because its voltage drops as welding current increases, thus its volt- ampere output curve ’droops’.
With the machine turned on but with no arc, and hence no current flowing, it has a relatively high open circuit voltage of 70-80 volts. Generally speaking welding is done at the steeper portions of the curve and this is ideal for manual stick electrode welding.
Arc voltage depends upon the physical length of the arc between the electrode and the work and this can never be held completely constant in manual welding. But, since rate of burn off of filler metal is determined by the amount of current, burn off stays substantially constant if current doesn’t vary.
There are many variations of this type of machine based upon power input (single or three phase), output (ac, dc, or ac/dc), and the type of output control (mechanical or electrical).
Constant Voltage Power Supply
The other basic type of arc welding power supply produces a constant voltage. Thus at any voltage setting current may vary from zero to an extremely high short circuit current. Such a machine is designed specifically for gas shielded metal arc welding and is not generally suitable for stick electrode welding.
Actually, no welding machine can produce a truly constant voltage. In practice, voltage drops at least 1 volt for each 100 amp output. Nevertheless, short circuit currents may be as high as several thousand amperes.
Normally, constant voltage machines have lower open circuit voltages than the constant current machines, about 50 volts maximum as compared to 80 volts. As a means of obtaining the desired arc voltage, the operator sets open circuit voltage, rather than current, at the machine. Settings may range from 10 to 48 volts.
Welding current can reach several thousand amperes at short circuit. Current adjusts itself to burn off filler metal at a rate sufficient to maintain the arc length required by the present voltage and is thus determined by the rate of electrode feed.
All welding machines are rated according to their duty cycle. Understanding this term is of utmost importance and it is often misunderstood. Duty cycle is based upon a ten minute time period. At rated voltage, a power supply with a 100% duty cycle rating can operate continuously at or below its rated current.
A 60% duty cycle does not mean that it can operate 60% of an indeterminate time at rated current and voltage. It means that the welder should operate only 6 out of every 10 minutes at that current and voltage. It should be allowed to idle 4 out of every 10 minutes for cooling. Machines rated for less than 100% duty cycle can be used continuously by decreasing their current rating.
Tungsten Arc Welding
Tungsten arc welding, using inert shielding gases, is hardly a new process. Nevertheless, it should be covered in any survey of advanced welding processes because of its particular applicability to difficult welding problems. This includes joining hard-to-weld and exotic materials such as the stainless steels, aluminum, magnesium, copper, beryllium copper, Hastelloy, Inconel, Invar and Kovar, especially in very thin cross sections.
Essentially, TIG welding calls for the same type of power source as shielded metal arc, or stick electrode welding, that is one with a drooping volt-ampere output curve. However, the process does present some problems which make a machine especially designed for the process much more suitable.
It should be noted that a conventional AC power source not specifically designed for TIG welding must be derated for AC TIG service. This is because partial rectification occurring at the arc introduces a DC component, which causes overheating of the main transformer. Other problems associated with TIG welding which are more acute than in conventional metallic arc work include arc starting, arc stabilization, and, of course, as the work becomes more delicate, control of all welding variables.
Gas Metal Arc Processes
Gas metal arc welding, in which a consumable wire electrode is fed continuously to a gas shielded arc zone, has replaced non-consumable tungsten inert gas welding and conventional shielded metal arc (stick electrode) welding in many applications. The main reason is its speed. Where it can be used, it is usually several times faster than other processes.
Other advantages include: cleaner welds, because the shielding gas greatly reduces and often prevents oxide formation; electrode savings through the virtual elimination of stub losses; excellent weld metallurgical and physical characteristics: and simplicity of operation which increases weld quality and reproducibility and generally reduces the human variable.
Arc Spot Welding
Arc spot welding has become very popular in recent decades. Its chief advantage is the ability to spot weld from one side of the work. In addition, it is a fast method of producing multiple spot welds with a high degree of reproducibility.
TIG Spot Welding
Arc spot welding may be performed either by tungsten inert gas or gas shielded metal arc processes. For tungsten spot welding the same type of power supply used for regular TIG welding may be employed. However, it requires a special spot welding gun and controls.
TIG spot welding involves the fusion of the parent metal only. Filler wire is not used. Generally TIG spot welding is performed on cold rolled and stainless steels.
MIG Spot Welding
Gas shielded metal arc spot welding is characterized by high amperages. Machines designed for this purpose normally have higher volts. Welding currents with smaller diameter wires up to about 1/16 in. may reach 500 amp. Wires 1/16 in. diameter and larger call for welding currents up to 750 amp. As in TIG spot welding, special controls are required. These are normally incorporated in a special wire drive and control unit.
Plasma Arc Welding
An extension of the commercially accepted plasma cutting technique and similar in some respects to the tungsten inert gas process, it has fewer limitations than such methods as electron beam, laser and ultrasonic welding and at far less initial cost.
Like tungsten inert gas welding, plasma welding is normally a fusion process although cold filler wire may also be employed, depending on the job. The electrode is tungsten coated and water cooled because of the high temperatures involved. The process differs from TIG welding in that, in addition to a shielding gas, a plasma forming gas is also involved. The plasma ’flame’, sometimes in combination with an electric arc can produce extremely high temperatures. The flow of the plasma can be focused by the design of the torch and as a result can be highly concentrated to produce deep, narrow penetration.