What is Cold Metal Transfer Welding? Understanding It Thoroughly
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Anyone who’s ever welded knows that the work can get hot. “Hot” doesn’t mean a cast iron skillet on the grill in the middle of the desert. It means that the arc of the weld can be upwards of 11,000° Fahrenheit. So why is a fabrication process, notorious for its capability to melt or liquefy metal, styled ‘Cold’ Metal Transfer (CMT)? We will see that this name makes much more sense in relation to other welding processes.
How Does Cold Metal Transfer Welding Work?
The first thing we need to understand in cold metal transfer welding is that ‘cold’ is a relative term. Compared to regular MIG short-circuit welding, CMT emits about 90% less heat into the workpiece. This makes it an ideal process for working with thin gauge metal, which is prone to warping from high heat input. The motivation behind the invention of the process was the need for efficient cladding, or the bonding of two different types of metals (e.g., aluminum with steel).
Cold Metal Transfer Welding uses much of the same equipment as MIG welding (welding gun, wire feeder, ground clamp, etc.). The essential difference is in the machine itself in how it operates. Whereas short-circuit MIG deposits metal by short-circuiting at the contact point between the electrode wire and the workpiece, CMT uses a background current phase in combination with a foreground current at a much higher heat input.
The foreground current phase ‘pulses’ and subsequently retracts the wire, aiding the deposition of the filler metal into the weld pool. This occurs about 70 times per second. The machine detects that a short-circuit has taken place and then defaults to the background current phase. The innovator of this technology, the Austrian manufacturer Fronius International GMbH, has described the resulting current as a ‘hot-cold-hot-cold’ sequence.
This process, since it does not require the usage of short-circuiting to deposit the filler metal, means that it saves the welder or operator from the dreaded task of cleaning up spatter. This makes it an ideal process for applications where clean-up is either difficult due to hard-to-reach places, or damage to surrounding components cannot be afforded.
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Different Types of Cold Metal Transfer Welding
Fronius has set forth four basic cold metal transfer welding processes.
Cold Metal Transfer Pulse
CMT Pulse uses direct current electrode positive (DCEP) polarity throughout. This means that approximately 70% of the heat in the arc will be taken by the electrode side and 30% will be taken by the workpiece.
Cold Metal Transfer Advanced
CMT Advanced welding process uses a type of alternating current. This effectively means that the welding arc’s heat distribution changes directions because the polarity of the current is constantly alternating. This allows for even cooler welding than the original CMT Pulse process. Since the alternating current keeps the weld and the workpiece much cooler, the user can afford to turn up the wire feed speed thus allowing for higher deposition rates, which usually means faster welding.
Cold Metal Transfer Pulse Advanced
This uses an alternating current just as CMT Advanced does. The primary difference between these two processes is that only the pulses of the sequence have a positive polarity. Since most of the heat is then concentrated toward the electrode, the filler metal is deposited much easier to bridge gaps in joints, thus keeping the workpiece cooler during the deposition. This process is specifically tailored to fix bridge gaping.
Cold Metal Transfer Dynamic
This process is designed to weld thicker plate. It increases the rate of the wire extension-retraction cycle thus allowing for higher wire feed speeds and consequently, higher deposition rates. This allows for greater penetration into the weld joint, which is ideal for thicker plate. This process stands outside of the original intention of cold metal transfer in general, in that it is not designed for cladding.
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Where Is Cold Metal Transfer Welding Used?
Cladding
Fronius originally developed this process for cladding. Cladding is the joining of two dissimilar metals. This is not achieved easily with other welding processes that produce more heat. Due to the extreme precision that Cold Metal Transfer Welding provides, neither of the joint members will overheat, and consequently, the bonding between the two metals is much more controllable.
Electronics
CMT, because of its extreme precision as compared to regular MIG welding, is one of the best choices for electronic enclosures, especially when the components are particularly sensitive. The low heat input is less likely to warp the enclosure during its construction than regular MIG welding. On the other hand, TIG welding, while adept to handle such a sensitive task, comes short in the efficiency department. This makes CMT an ideal process for this application.
Automobile
CMT has become a favorite application for many automobile manufacturers, including Tesla. This is especially the case when aluminum is primarily used to create the structure of the vehicle. The Tesla Model S is made from aluminum. After creating the main panels for the body of the vehicle using aluminum sheets put through die presses, they join the panels using fasteners, epoxy, and also cold metal transfer welding.
Advantages of Cold Metal Transfer Welding
Precision
One of the biggest advantages that CMT welding has is its accuracy. Since it works with a foreground phase and a background phase in the current, the pulse’s heat input and precision can be controlled electronically.
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Efficiency
Though all welding requires some level of skill from the welder and operator, electronic processes have much more consistency. Since most of the process is controlled electronically as opposed to manually, the margin for human error is narrowed.
Cleanliness
Since the short-circuit of the arc signals the machine to retract the wire, there is far less spatter. In other types of welding processes, especially Shielded Metal Arc and Flux Core, cleaning up spatter is routine. Even in TIG welding, because of human error, there can be burn-through or oxidization. But even these are all but completely factored out with CMT.
Disadvantages of Cold Metal Transfer Welding
Price
Since the equipment for CMT was designed for industrial applications primarily, it can be hard to acquire without an industrial-sized budget. Chances are that the average homeowner does not have cladding to do on the weekend. CMT Welding machines are tailored for specific industrial tasks. The cost tends to be on the higher end.
Lack of Resources and Availability
While the technology is touted as requiring little of the operator, troubleshooting any problems the operator has may prove difficult. This welding process is relatively new and constantly under development by teams of engineers. The average shop welder or foreman is not likely to be able to resolve major mechanical or electrical problems without an outside consultant.
Frequently Asked Questions
Does CMT welding need gas?
Yes. Part of the confusion for the neophyte when they hear the term ‘cold welding’ is they may assume that this process is related to the product JB Weld, which is in fact an epoxy adhesive that after curing can be as strong as some welds.
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Cold Metal Transfer welding works like MIG welding in that it requires shielding gas to protect the weld from outside contaminants such as oxygen. Also, different shielding gases and mixes can help create different arc characteristics.
What kind of shielding gas does CMT use?
With MIG welding, the type and mix of shielding gas can vary depending on the process and material. Short arc typically works well with a 75% carbon dioxide and 25% argon mix. Some welding processes such as MIG Spray may require a higher argon content. There is no simple answer to what kind of shielding gas works for CMT. Since CMT is a varied process, it will depend on the metals and alloys of the material as well as the specific application.
Is it possible to manually weld using CMT?
Yes. Though the process is often utilized by robotics for automation, the essential equipment is the same as MIG welding. This means that it can be used with a standard MIG welding gun.
Final Thoughts
The future of this technology looks bright. This is especially true as more manufacturers, especially in the automobile industry, are moving toward using metals other than mild steel. More sensitive metals require more sensitive manufacturing processes. But this does not mean steel is out of play. The Swiss company Embru Werke AG, which manufactures modular office equipment made from steel, has been lowering production costs by using CMT technology since there is relatively little cleanup and refinishing of material required.
Featured image credit: Studio 72, Shutterstock
