Arc welding is all about producing heat: an arc is generated to produce that heat; the arc melts the parent material; it melts the filler material, and that has to be fed into the weld pool in a controlled manner. Developments in arc welding involve how the heat is controlled and how we feed the wire into the weld pool to make a good weld.
Recent developments in equipment have given improved quality, improved productivity, improved tolerance to fit-up, and have extended the capability of the process, whether that be welding thicker materials or thinner materials, by controlling the heat and the filler material.
Traditionally, TIG (tungsten inert gas/GTAW) welding is seen as very high-quality, particularly used in areas of the aerospace industry, while the MIG/MAG (GMAW) process is a higher-productivity process. In a recent trend, process improvements have raised the productivity of TIG welding and the quality of MIG/MAG welding too.
When I first started in the welding industry – and this is going back into the 1980s – arc welding equipment was pretty basic. It was based on big, heavy, copper-wound transformers, and this type of equipment responds very slowly to changes in the arc, so it’s very difficult to control arc length and spatter because the welding current simply cannot respond as quickly as would be desirable to control the process.
Then inverter technology came along. Initially inverters were low-frequency, but they’re now very high frequency. The current can be changed very quickly, so if anything happens in the arc, the equipment can rapidly respond to that and restore the stability of the process. Of course, inverters have other attributes as well. They’re much smaller than a conventional transformer and they take up much less space, and also they are far more energy efficient. So, improvements in welding processes have been led by improvements in the inverter power sources.
TIG welding process developments have improved how the wire is fed into the weld pool. Hotwire TIG is nothing new; it’s been around a long time, but there continue to be new ways of controlling the wire. One example is TIPTIG, which is a way of mechanically feeding the wire for manual TIG welding. A vibration is superimposed on the wire which is claimed to improve productivity and reduce porosity in the weld.
Other ways to improve the TIG process involve producing a hotter arc to increase penetration in thicker materials. Examples include K-TIG, CF-TIG and FocusTIG. This is achieved by cooling the electrode right down to the tip, so the arc is more focused with a higher current density, which gives deeper penetration.
In TOPTIG process, which we’ve done quite a bit of work on at TWI, the wire is fed into the arc itself rather than the weld pool. This has certain advantages, one of which is greater productivity; it also lends itself to robotic welding because the torch is easier to manipulate.
Pulsing the current in TIG welding at a high frequency (in the kHz region, up to 15 kHz) has been found to increase the pressure of the arc. This creates a much narrower, more confined arc, which gives greater stability at low currents. At very low currents, of the order of 10A, welders find the arc is more stable and controllable, and at higher currents, higher penetration can be achieved in thicker materials.
MIG WELDING
There are many process variants of MIG welding. One of the earliest is Lincoln STT, which has been around for many years and has gone through a number of iterations. Then there is Fronius CMT, which is quite widely known; more recent are Fronius LSC or PMC. These are all process variants of dip (short arc) transfer, diagrammed at left. Sometimes it can be difficult to decide which process to use.
At higher welding current, the mode of metal transfer is known as spray transfer, and there are a number of process variants, such as EWM ForceArc, Migatronic PowerArc and OTC D Arc. In all of these, better control of the arc gives improved performance. There are also variants of pulsed welding such as ESAB SuperPulse and Lincoln RapidArc. Using the technology that has emerged, we have much better control over process to give us an improvement in making a weld.
The key aims of the developments in the dip transfer short-arc process for MIG/MAG welding are to be able to weld open roots, to weld thin sheet materials, to control the heat, to bridge gaps and to reduce the spatter.
The metal transfer mode in the MIG/MAG process varies with welding current.
DIP TRANSFER
At low currents, the wire shorts into the weld pool at regular intervals, and so this mode of metal transfer is known as ‘dip transfer’ or ‘short arc’.When the wire shorts into the weld pool, the current rises until the arc re-ignites, which can cause a lot of spatter. At higher currents (see graph above), the arc is longer and metal is transferred as a stream of small droplets. In the intermediate region between the two, large globules of molten metal form on the wire tip, so this is known as ‘globular transfer’. Globular transfer is unstable and usually avoided. At very high currents, the arc on the end of the wire will rotate, and this can become unstable unless well controlled.
Pulsed welding can be used to overcome the instabilities in dip and globular transfer. Pulse parameters are chosen to ensure a small droplet of molten metal is transferred per pulse in a stable manner at all welding currents. So pulse welding bridges the gap between dip and spray transfer without the irregular globular transfer mode.
One particular process variation that improves the dip transfer process is the CMT (cold metal transfer) process by Fronius. A similar process is Synchro-feed by OTC. With both processes, the wire is withdrawn and the current reduced at the moment of a short circuit, so this reduces spatter and the amount of heat going into the weld.
There are many other techniques that control the dip transfer mode in MIG/MAG welding, but without retracting the wire. Due to the fast response time of inverter welding power sources, the moment just before the arc re-establishes is sensed by measuring the voltage and the current is reduced to a low value. This reduces spatter and the arc is more consistent.
MIG VARIANTS
The fast response of inverter power sources has also enabled the development of other MIG/MAG process variants, including pulsing at high and low frequencies and AC MIG welding.
AC MIG is quite an interesting process actually; it hasn’t really gained wide acceptance, but what AC MIG enables the process to do is to weld thinner materials. This fits into an industrial trend that sees MIG welding moving from higher productivity and lower quality to being able to weld thin materials approaching that what we can do with the TIG (GTAW) process through controlling the welding parameters, mainly the current, both in terms of shape and the polarity balance.
Another process variant is multi-wire, and new variants are coming out all the time. The Fronius TimeTwin is well-known and been around a few years. It has been developed further with waveform technology, including CMT Twin, AdvancedTwin and so forth. This process has two power sources and two contact tips in the torch nozzle. The two power sources work together as a master and slave.Another process variant is the recently-developed Lincoln Electric Hyperfill, where there is only one power source and two reels of wire are fed through holes in the same contact tip. That is a different approach to solve the same problem: having more than one wire improves the deposition rate and the size of the welds possible with the process.
As an example, switching from a single wire at 300mm/min to a tandem wire at twice the welding speed offers much higher deposition rates, something like about three times as high. The general rule is that with two wires you get three times the effect; from 7kg/hr deposition to 17kg/hr with the tandem system. That allows us to go from making a small fillet in one pass, or a multi-pass fillet, to a larger single fillet with the tandem process, with very good penetration.
Another example is the EWM ForceArc. This is a typical example of work TWI has done to validate the process; we carried out welding trials to demonstrate that with power over the welding arc by high-frequency control of the voltage and the current, it is possible to increase productivity. The preparation angle on the butt weld was reduced, but showed good penetration in the sidewall and reduced the number of passes required on this weld from 11 to five with ForceArc (pictured, top left). So this reduced the number of passes and reduced the time to make that weld to about half (351sec vs 679sec), which yields a significant increase in productivity.
A recent development is the similar, but perhaps more dramatic, D-Arc process by OTC, which can weld up to about 20mm-thick plate. The process produces a very deep penetrating arc which is buried in the weld preparation and is capable of making a single pass in 15mm plate at 300mm/min.
This article is an edited version of the presentation ‘Technology developments in Arc Welding,’ given in June. The entire event is available to watch online; see www.is.gd/peniqu.TWI is a world-leading research and technology organisation. More than 700 staff give impartial technical support in welding, joining, material science, structural integrity, NDT, surfacing and packaging. Services include generic research, confidential R&D, technical information, technology transfer, training and qualification.
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