Laser welding replaces TIG

Czech companies market laser processing systems

Tomáš Mužík, Stanislav NEˇmeCˇek, and Tomáš Attl

In the Czech Republic, the towns of Plzenˇ (Pilsen) and Cˇeské Budeˇjovice (Budweis) are well known thanks to the production of beer. But these towns are also part of a country that is known for its reputation in the production of heavy machinery and automobiles. Car manufacturers Skoda, TPCA, Hyundai, and Tatra are located in the country, with KIA and VW in neighboring Slovakia. Tool shops and automotive subcontractors are located in almost every town and village. This manufacturing sector growth is promising, with the economic crisis definitely over and many companies now investing again. Generally, the economic situation is stable and much better than in southern EU countries that are still experiencing problems.

Laser welded mild steel tube after "cone test." During this test, a cone is pushed into the tube and it must withstand defined diameter increases. Results like this mean excellent quality.

Since 2008, Matex PM, the laser job shop and system integrator, has focused its services on the automotive and heavy machinery sectors, offering laser surface hardening, welding, and additive manufacturing capabilities. The company currently runs three robotic laser systems in two subsidiaries and operates its own R&D center for quality assurance and development of in-house process development.

Production line for laser welded stainless steel tubes. The input of the steel plate into the forming rolls is shown in the middle; laser source is on the left.

Together with ATTL s.r.o., a machinery builder from Prague, the companies offer large lines for the production of tubes and various profiles. Some of the profiles are mechanically formed, but most of them are longitudinally welded. Usually these production lines consist of a set of forming rolls that take steel from a coil and form it into a desired profile. In the middle, there is a welding system where the profile is precisely aligned, welded, and sometimes pre-heated or annealed. At the end, profiles are automatically cut in desired lengths and transported to storage. The tubes and profiles produced are used mainly in industries such as automotive, construction, or machinery.

Conventionally, welding is done by induction coil or tungsten inert gas (TIG). The joint efforts of Attl and Matex PM produced a new approach – laser welding, a process that offers many technological and economic advantages, but some challenges, too. Let’s take a look at them.

 

1. Speed of welding

Usually the most important parameter of such production lines is production speed, which is limited by the welding process and by the cutting at the end. Consider the most common weld seam configuration: butt joint with a thickness of 1 mm. With laser welding, it is easy to reach weld speeds of 10 m/min, which is much higher than that from conventional TIG welding, but lower than that from induction welding. However, the laser can reach these speeds even on thicker materials; it depends on laser power and the investment budget. Currently, on one line being installed, speed is over 20 m/min.

2. Heat input / power consumption

The biggest difference between conventional welding and laser welding is the much lower total heat input in the case of lasers. This results because the laser beam produces a narrow weld seam with fast energy transfer from the beam into the material. We estimated the energy input into the welded material at 1 mm thickness of high-strength steel. In the case of the laser, it is about 15 J/cm, while with TIG, it is at least 60 J/cm, and with metal active gas (MAG), it is 85 J/cm.

This estimate is important for the calculation of running costs (see opposite page) and has important consequences on the mechanical properties of the weld seam, which are usually much better in the case of lasers, but the fast cooling of materials or welding speeds that are too high can cause unexpected problems, too.

3. Weld seam properties

A weld seam made by any technology has three basic zones – molten zone, heat affected zone, and transition zone in the basic material. Laser welding makes the seams narrow because of fast cooling rates, an advantage because this produces low distortion and low heat degradation of the basic material. However, fast cooling can be a problem when welding high-carbon steels, especially when a subsequent cyclic load is expected. In this case, the heat affected zone can be too hard and it can be prone to cracking.

Laser welding station.

This problem can be solved by careful technology development — by the proper selection of the laser source and the setting of focus size and other welding parameters. In some cases, even pre- or post-heating is necessary. Therefore, laser welding is able to weld "problematic" steels better than any other technology.

High-strength steels that are interesting for lightweight car construction are welded by laser, preserving their strength and plasticity, contrary to conventional welding, which destroys the special microstructure of these steels and downgrades their mechanical properties to levels of common mild steel.

4. Weld seam quality

Another question is weld seam quality and its stability during long-term production. Generally, laser welded seams have much higher quality, low surface distortion, low oxidation, etc. Weld spatter on the root side is limited even without the use of protecting gas from below. Weld root quality is one of the most important arguments for laser welding.

However, it is much more complicated to set the laser process correctly, as this process runs quite fast in a small spot so there is no chance to manually react to process fluctuations or imperfections. The equipment suppliers have to make the welding system as robust as possible so that it is stable and able to keep the right setting for long periods, even in a harsh industrial environment.

Profile for separating glass plates in windows, welded by a single-mode fiber laser.

Installing a seam quality checking system is a great advantage as it can check the weld seam geometry and/or the welding process stability. Usually such systems have to be taught how the weld looks when everything is OK, which takes some time. But then, the operator has 100% on-line quality checking, which is a great advantage and sometimes a "must-have".

5. Protecting gas

The laser welding systems described use common argon gas from the upside only. Helium or a mixture of gases have some advantages, but with much higher costs. This may look like a minor issue, but gas prices have a high impact on total running costs of different technologies and even of different laser systems.

Root protecting gas is not necessary on common mild or stainless steels.

6. Running costs

When comparing welding of round tubes of mild steel at 1.5 mm thickness, summarizing the costs for energy and protecting gas and not including the costs of amortizations, staff, and service costs, the following are obtained:

High frequency (HF) coil welding is a standard technology for production of common construction tubes. It has quite low running costs and very high welding speed of about 80 m/min. But the weld seam quality is limited, heat input is high, and a complicated power source with an extremely heavy power line is needed.

TIG and plasma welding are used where a higher quality of weld seams is needed, usually on stainless steel. TIG technology is limited mainly by speed, especially on thicker materials. Consumption of protecting gas is an important factor, doubled when root protection is necessary.

As can be seen, there is a substantial difference between CO2 and fiber or diode laser, mainly because the latter two do not need "laser gas," and the fiber-guided lasers are about two times more electrically efficient. The fiber and diode laser beams are more efficiently absorbed by metals; consequently, lower laser power is needed for the same productivity with much lower service costs. There is one reason why CO2 lasers are used – they have, in some special cases, somewhat better weld seam quality.

7. Problems

There are also some problems and limitations for the line producers and for the operator — they have to understand laser safety. They cannot observe and align the process directly like a conventional system.

The laser process is sensitive to proper alignment and servicing of the laser optics and precision machinery. Improper settings or a variation can cause poor weld seam quality, which might not be found during production.

Building a completely new production line takes a long time, with high investment, where the price of a laser welding cell is not the most important cost. There are a lot of older systems based on TIG welding – could they be changed to laser welding, too? The answer is in most cases, yes, it is possible to change the conventional TIG welding technology in the middle of the line and replace it with a laser system. Then, much higher production rates can be achieved, limited probably only by the speed of cutting at the end of the line. Typically, a conventional system with a speed of 0.5 m/min can be upgraded to 3 m/min with no problems. No pre- or post-heating is necessary and neither is a root protecting gas. Weld seam quality is even better than by TIG, and mechanical properties of the seam are greatly enhanced.

Laser welding of tubes and profiles has a promising future because there is a high demand from the automotive industry for these products, where the laser is better than conventional welding of tubes welded from zinc-coated sheets, profiles from high-strength steels, tubes to be processed by hydroforming, etc. Manufacturing lines producing outstanding quality, reasonable prices, and very low running costs are now available to users. ✺


Tomáš Mužík and Stanislav Neˇmecˇek are with Matex PM (www.matexpm.com) in Pilsen and Tomáš Attl is with Attl www.attl.cz/en/index.html) in Prague, Czech Republic.

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