Laser welding comes of age

(Photo: TRUMPF GmbH)

To stay successful in business one has to embrace emerging technology and wrest it to perform both to specification and to the benefit of your organization. Laser welding is an impressive technology that promises so much, and costs a bundle, but the big question is whether it will enhance your business model.

“Electron beam welding has been around since the 1950s,” said Paul Denney, Senior Laser Applications Engineer at Lincoln Electric.

“It is good for aerospace applications where you are very concerned about weld quality. Electron beam welding has primarily been done in a vacuum but there have been efforts to accomplish it in partial vacuum.” There has not been much change in electron beam welding for some time primarily because it is not a very good process for high volume applications. “Trying to get high volume of parts through a vacuum system is problematic because of the pump down times and the maintenance related to the vacuum systems,” he said.

Stripping away the laser welding science-fiction aura and looking at this welding process as another tool that may enhance your business is a healthy way to approach laser welding.

“In the case of welding, it is rarely a case of ‘I’ll just choose laser over another process such as TIG or MIG’ without some forethought to what your process objectives are,” said Bob Lewinski, Vice President, Marketing at Wayne Trail.

“If you have precision work to do, that either has certain cosmetic requirements, or where cycle time is important, and a small, heat-affected zone is beneficial so that with minimized metallurgical impact at the weld interface, you can more easily form these parts afterwards, I would say that then you start to consider laser as a good candidate for your application. You should always have a good application driver to justify the investment.”

Even with the prices of laser power supplies falling, a complete, properly executed laser processing cell is still generally a more expensive initial capital investment than a traditional non-laser based welding cell. But, the cost for laser has fallen dramatically in the last few years. A comparable laser that sold for over half a million dollars a decade ago might cost half that much today.

“There are safety interlock and compliance issues that must be properly addressed by the integrator or the end-user. All of these challenges and requirements are do-able. But you have to have the need or desire for laser processing as the ‘driver’ so that these costs are understood and accepted as part of the investment,” said Lewinski.

“People choose laser welding because of the reduction in the cost per part, not because of the cost of capital cost,” said David Havrilla, Manager, Products and Applications for TRUMPF Inc. Laser Technology Center. “Laser welding is becoming more accessible. For people who never would have considered laser welding a decade ago. The price is coming down to where even job shops might consider it, even though the capital cost is still higher than MIG or TIG."

Schematic illustration of keyhole welding, such as electron beam welding, laser welding and plasma welding. (1. Object, 2. Energy Ray, 3. Keyhole, 4. Weld)

"Because of the aesthetics of laser welding, you can reduce or eliminate secondary processes, like manual grinding, on parts and scrappage. This greatly increases throughput and reduces the cost per part.”

"A good rule of thumb for when to consider laser welding is to use it on very high volume applications, for example automotive or consumer products, or when the part has a very high value, like medical components,” said Denney. As an example, he noted that all Gillette Sensor razor blades are laser spot-welded.

"You are talking about a billion parts per year that are being produced.” However, resistance, MIG or TIG will have advantages over laser in some instances. In other applications laser will have superiority over those processes.

“Today, many more cost and process justifications lean towards viewing laser as an enabling technology,” said Lewinski. “If a customer says, ‘I can eliminate one or more process steps, or perhaps I can completely eliminate certain reinforcing components, or can use thinner or less material, or makes welds in materials not weldable using traditional techniques,’ now you have some tangible drivers towards the laser side of the process world."

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The high-speed, high-throughput automated requirements is in part tied into the capital investment required and the need, “to get the part price down to the level that is attractive to the customer. You end up applying the technology to highly automated, high throughput and high-volume applications,” said Havrilla.

High-volume (better process rates), and precision work (where access is difficult or weld placement is critical) is where laser welding really stands out.

“You generally have much less heat mark or deformation, or distortion because of the lower heat input,” said Lewinski. “You can also access the weld joint from a single side when using a laser process. You can weld a wide variety of metals that sometimes are not kind or may not react well to traditional high heat weld processes.

We have more customers who come to us who want to use lasers for one or more of the attributes I mentioned earlier, and need assistance as to how to properly apply the laser and the process into an industrial application, than we do who will arbitrarily ask, ‘I am MIG or TIG welding today ... is my particular job a good candidate for laser?’ We somewhat view this as an incomplete question. They should say that I would like to improve my product because I want to have less distortion; or I want to have less heat marking. Or I’m having a hard time joining these metals, or I don’t have access to get a traditional weld torch down in there. Now you start to open up some potentially great reasons for considering lasers.”

A laser welding test at LWT (Lindoe Welding Technology). The 32 kW laser system is in this test limited to 12 kW. The invisible laser beam is coming directly from above. Inert gas from the big nozzle flows around the weld bead and compressed air from the small nozzle removes fumes. (Photo: Wiki Commons)

The traditional arc processes, TIG and MIG, are two-dimensional heat sources in the sense that the heat transfers from the arc. “Whether it is an arc with a droplet coming off it like in GMAW or a tungsten electrode like you would have in a GTAW process,” explains Denny, “you basically have a heat source on the top surface and you are waiting for the fluid flow to take that heat down into the part. Laser and other high density power processes, such as electron beam or plasma, have the ability to give your heat a third dimension. For these high power density processes you actually melt, and then boil the metal, forming a ‘keyhole’ you can actually get down into the part - so processes like the laser can get the heat deeper and to the joint better than an arc process - meaning less heat and/or higher processing speeds.”

"Another advantage is the ability to weld in areas that are difficult to reach with other techniques. Laser welding is, “able to weld in spaces that are very difficult or impossible to reach with traditional techniques,” said Havrilla.

Laser quality:

The advantage of using a laser beam to weld is that heat input results in low distortion, small heat-affected zones. The narrow weld beam, low distortion and small heat-affected weld zone leads to good weld aesthetics, reducing secondary processes.

“If you have low distortion you don’t have to straighten the part after welding,” said Havrilla. “The welds that you get from laser welding are often high-strength welds because there is a fast cooling rate associated with laser welding. The energy density is quite high and the welding speed is quite high and as you can imagine the quench rate, the cooling rate, is quite high and that gives you two advantages.

"One, the weld itself is high strength and besides that, you will have a small heat-affected zone. And when we say small, we mean small in two ways. Geometrically it is small, much smaller than a heat-affected zone of a TIG or MIG weld and the second way it is smaller in terms of metallurgical effects.”

Typically if you have an arc weld process the heat-affected zone will be softer because of the slow quench rate and the annealing effect that you get in the area. In a laser weld you get a much quicker cooling rate and the heat affect zone is not impacted by that so you end up without softening of the heat-affected zone. The concentrated energy of the laser makes it more efficient.

“A laser beam of one inch diameter can focus to within one-ten-thousandth of an inch or even smaller. When you start putting four, five, six or even seven, eight kilowatts of laser power into that kind of spot diameter, all of a sudden you can get very fast welding speeds with very narrow welds,” said Havrilla.

“The heat is going more vertically down into the weld as opposed to going horizontally into the material. People who are used to arc welding are tentative when we say go and pick up the part right after we have laser welded it. They are used to parts that are smoking hot. They are used to parts that are very hot so you are not able to pick them up with your hands. They are very surprised that you can pick up the part right out of the fixture and it is still relatively cool.”

“You are able to weld faster than some of these conventional arc techniques,” he said.

The laser experience:

Most, if not all, industrial laser weld systems encompass a fully automated and enclosed work cell. In the cell, you might typically have a six axis robot with either a single, or in some cases, multiple laser process heads.

“For instance, using what is called a ‘beam switch’ that can direct the output of one laser power source to multiple focus devices, a modern work cell such as the Flex Lase system, can utilize a laser welding head, and a laser cutting head each connected by a fiber connected to a common laser source with the ability to quickly switch between processing with either of those heads on demand,” said Lewinski. The robot then can automatically couple or uncouple from the process head you desire.

For welding you can put the welding robot head in play, let it do its process and then park it in a holster. It can automatically uncouple from the head in seconds and recouple to the other process head, (to a laser cutting head for example), and then perform a laser cutting operation on the part much the same way a tool changer on a big machine tool would work.

“So it becomes a very flexible tool,” said Lewinski. “You can have a robot as the prime mover, or in some cases you can have a higher precision multi-axis gantry as the prime mover. You can locate the laser, chiller and motion control system in the cell, often on the backside, in what we call a power module.”

“Everything you need to have a successful laser process can be contained on a common platform and that platform starts with a structural and sturdy base on which you can mount the proper and safe rated laser enclosure that is typically rated as a Class 1 – when it comes to fiber or disk laser, due to the short wavelength,” said Lewinski.

“The system also usually contains the motion control system, such as robot controller, the laser chiller, fume exhaust and filtration system, entry and exit automation if desired, for parts in process, and of course the tools and fixtures — all located on a common easy to connect, easy to deploy or redeploy platform. Our approach has everything you need on a common platform.”

The systems can be any size, from quite small to huge units with headstock/tailstock systems in place to rotate or reposition the components under the laser beam. Such systems are getting to be quite standardized.

This results in an ease of installation and setup – essentially a turnkey solution. “The customer needs only to physically locate the work cell at their destination and connect it to their power source, which is usually three-phase power and compressed air, and maybe, or maybe not, process gas,” he said.

“Everything else arrives and installs just at it were tested at our facility before shipment. So it is a very short time to install and bring back to life, so to speak. With today’s fiber laser technology, the laser is not something the end user will have to check on very much. There is very little maintenance required. So the customer can concentrate more on the process and the fixturing of the parts than the actual laser itself.”

Another attribute of the fiber laser, is it’s ability to work well in applications ranging from welding and cutting, to cladding, marking, heat-treating and more. Since all laser welding occurs in a sealed unit there is no special safety equipment needed when operating the units. This is akin to a CD or DVD player where the laser operates in a safe and sealed environment. Special eyewear and clothing is only needed when servicing or aligning the laser heads. The units also have a built in fume extraction system so any danger is contained.

One of the functions has been at the behest of the oil and gas and power generation companies and some other industries to look at coating material to give better performance. “Laser cladding is being considered when people of parts exposed to corrosive and/or abrasive environments and it is too costly to make the components out of stainless steel or nickel alloys. In this case cladding is a potential solution. As an example you could make the part out of carbon steel and then deposit material that best protects the structure,” said Denney.

“Because you can control the heat and power distribution with laser you can put the material down very thinly. Instead of using powders, we are developing the use of ‘hot wires’ taking advantage of Lincoln’s expertise in power supplies and consumables.”

Consumables:

Generally, many laser welding applications today are autogenous, meaning that no filler metal is needed. For they are highly focusable to a small spot size. They have low heat input when they are welding, and that low this though, the gap has to be quite minimal whether it be a butt weld, an overlap joint, a T-joint or a fillet. You need an intimate fit-up of the parts, so that the weld can use only the parent material. Proper fixturing is critical to successful laser welding,” said Lewinski.

However, the addition of filler metals can be quite popular and can solve some typical gap-related welding problems. This process is generally called hybrid laser welding, where you introduce wire, much like you would have in a MIG weld application, under the focused laser beam.

“That wire may be preheated through electrical resistance and a suitable weld power supply, so that the wire is already near, but not beyond the melt point. This wire can add some process tolerance to a less than perfect gap condition in some cases. It could add an element of alloying if it is desirable to have that in the weld joint,” he said.

The wire could alleviate certain issues either contaminants or metallurgical. Because of the characteristics of the base metal, you could introduce a filler metal that would smooth, ease or even speed-up the process. This capability further widens the ability of the laser to compete with other technologies such as MIG welding or TIG welding. In most cases, the wire used is the same wire as used in GMAW or GTAW welding.

“We have worked with some customers where they have needed to tweak, if you would, the chemistry because you don’t see the amount of melting you would in an arc process and the cooling rates are much higher,” said Denny.

“So the weld metal has to have a certain chemistry as a result to get the proper performance. So we have to tweak the chemistry. That is one of the capabilities we have here at Lincoln because we do produce metal-cored wires, we can make variations to the chemistry. The hybrid process can use solid wire or metal-core wire.”

Gas is typically the only consumable used in laser welding. “With the one micron wavelength you don’t even need the shielding gas per se,” said Havrilla. “With carbon dioxide lasers, because of the wavelength which is 10.6 microns, the shielding gas has two functions — plasma suppression, meaning that bright blue plasma that forms above the weld is suppressed, and to shield the molten material from oxidation.

With the 10.6 micron wavelength, too much of the laser wavelength itself would be absorbed into the plasma if you did not suppress or quench that plasma and you would not get the power at the part where you need it. You want all that power to get into the components that you are trying to join.”

With the one-micron wavelength, diode lasers, disk lasers, fiber lasers, pulse lasers, the laser wavelength does not see the plasma.