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BrightLine Weld enables low spatter laser welding at feed rates, only achievable today with CO2 lasers. Partial penetration welds for powertrain applications or full penetration welds for pipes and tubes applications – BrightLine Weld has the potential to revolutionise laser welding. Article by TRUMPF.

The technology allows for vastly improved productivity and energy efficiency. High quality weld seams result in high mechanical strengths of components produced. Minimised spatter behaviour reduces contamination of workpieces, clamping devices and optics, as well. That results in reduced machine downtime, less rework of parts, long cover slide lifetime and hence significantly reduced costs.

Introduction And Motivation

Reduction of cycle time and improved productivity play an ever increasing role in current industrial manufacturing. Especially within the automotive industry, where the total length of laser welded seams can add up to 60 metres per car, it is important to minimise processing time by means of high welding speeds. Perfect basis are fibre guided solid state lasers, eg, disk lasers with high beam quality at laser powers in the multi-kW range. Yet, the use of modern solid state lasers comes not for free; obstacles need to be overcome, ie, heavy spatters and contamination of workpieces and clamping devices.

Laser Welding

Compared to conventional welding, laser welding allows for heat conduction welding and deep penetration welding, as well. Thin and deep weld seams can be produced contact free and at high feed rates. A small heat affected zone (HAZ) minimises thermal distortion of parts. Welding depth can be as 10 times larger than the welding width and can reach up to 25 mm.

Yet, feed rates are limited for laser welding. One important factor is spatter behaviour and resulting mass loss of the weld seam. In general, both of these aspects increase with feed rate and laser power used. Solid state laser welding of mild steel typically results in increased mass loss starting from a feed rate of 5 m/min.

Limitations Of Welding With Solid State Lasers

Increased spatter behaviour at higher feed rates:

• Risk of mass loss at high feed rates leads to side kerfs at the seam front side which results in low mechanical strength and quality of the weld seam.
• Clamping devices are being contaminated and need to be cleaned. That leads to unproductive machine down-times.
• Cover slide glasses need to be re-placed often which results in increased costs.

So far, acceptable spatter behaviour could only be achieved at feed rates of up to 5 m/min hence resulting in low productivity.

That is a contradiction to the current demand for reduced cycle time within industrial production facilities. With the new welding technology BrightLine Weld, TRUMPF offers for the first time a solution meeting these requirements.

BrightLine Weld: Low Spatter Welding

BrightLine Weld is a new technology which allows for an almost spatter free welding process during deep penetration welding, even at high feed rates.

Figure 1: Welding depth depending on welding speed. The state of the art is compared with the BrightLine Weld technology.

Slim and deep weld seams produced with BrightLine Weld are of high quality. The low spatter formation results in the process regime extending to significantly higher feed rates. Figure 3 illustrates the welding depth depending on the feed rate at a laser power of 5 kW in mild steel both for the state of the art laser welding and for BrightLine Weld. The colour of the data points in the diagram is an indicator of the quality of the weld seam achieved:

• Green: weld seam of high quality, which meets current requirements.
• Yellow: weld seam of medium quality, which does not meet all requirements, but is acceptable for various applications.
• Red: weld seam of poor quality, which is not acceptable anymore.
• Violet: from this welding speed humping occurs. The resulting weld seam quality is insufficient.

The data points of the BrightLine Weld curve are green up to a speed of 20 m/min. Up to this range, the weld seam is of high quality. In the state of the art, the data points are yellow at a welding speed of 5 m/min. For even higher welding speeds, they are red or violet. Thus the quality of the weld seam at 5 m/min is only medium and poor or insufficient at higher speeds. This means that with BrightLine Weld, the maximum feed rate in mild steel could be increased by approximately +300 percent up to 20 m/min at a comparable welding depth. In stainless steel, the tests showed an increase in the maximum feed rate by +100 percent to 10 m/min.

Figure 2: Mass loss of the weld seam depending on welding speed for conventional laser welding with solid-state lasers and BrightLine Weld.

Figure 4 shows the mass loss of the partial penetration welds in stainless steel produced with BrightLine Weld in detail. To classify the results, the mass loss measured for conventional laser welding with solid-state lasers is also shown. Red data points again indicate an insufficient weld quality of the test welds. The conventional laser welding process shows an increased mass loss from a feed rate of 5 m/min. The mass loss of the weld seams produced with BrightLine Weld, on the other hand, is up to a feed rate of 20 m/min in a range which can be described as almost spatter free (< 0.4 mg/mm). At the same time, all weld seams made with BrightLine Weld show a high quality. Moreover, the seams have no humping up to a feed rate of at least 20 m/min.

Advantages Of Laser Welding With BrightLine Weld

The use of BrightLine Weld results in the following main advantages for the user:

• Significantly higher feed rates at a constant seam quality increase productivity. In mild steel the maximum feed rate can be increased without difficulties by +300 percent and in stainless steel by +100 percent.
• Minimal spatter formation and less contamination reduce cost of ownership. This results in a lower machine downtime, less rework of parts and lower consumption of cover slide glasses at the same time.
• A lower laser power is required for the same welding depth. The high efficiency allows up to 50 percent energy saving at the same welding depth and at the same quality.
• BrightLine Weld produces high quality weld seams. In favourable cases, weld seams do neither show undercuts nor end craters. Due to the reduced energy input the part deformation is very low.

How does BrightLine Weld work on real parts? This question is answered in the next section using a powertrain part as application example.

BrightLine Weld In Powertrain

A typical powertrain application is the welding of gear wheels. Depending on type, gear wheels are, eg, welded with a feed rate of 5 m/min and a laser power of 3.4 kW. Spatters which are generated during welding have to be exhausted.

For this application, the BrightLine Weld technology provides a significant improvement. Thereby BrightLine Weld can be used flexibly: either for optimising energy efficiency or for optimising machine productivity. If BrightLine Weld is used to optimise energy efficiency, the identical part can be welded at the same feed rate with a 40 percent lower laser power of 2 kW. With BrightLine Weld slightly slimmer weld seams are produced, which is why less laser power is needed to achieve the same welding depth. At the same time, the spatter formation is reduced, so even no exhaustion is required, hence reducing costs.

For those, who are more focused on improving productivity, they can also increase the feed rate at a higher laser power than in the state of art.

With BrightLine Weld the feed rate could be increased, eg, by up to 220 percent from 5 m/min to 16 m/min while still keeping the mass loss minimal. The result is a high quality weld seam at the welding depth desired.

Outlook

In the future, BrightLine Weld should not only be used in powertrain, but also in other industries such as tubes and profiles.

Tubes and profiles are typically bent and welded from very long sheets (so-called continuous process). In contrast to powertrain applications, these welds are full penetration welds. Commonly, very high feed rates of, eg., 30 m/min are used, which cannot be achieved with solid-state lasers today.

These requirements increase the complexity but promising approaches could already be found. With BrightLine Weld, it is possible to produce full penetration weld seams at high feed rates, which meet the requirements.

Summary

The new technology BrightLine Weld has the potential to revolutionize laser welding with solid state lasers. BrightLine Weld allows for constantly high weld seam quality – independent of the welding speed. The user has the choice between optimising, ie, minimising energy consumption or optimising, ie, maximising productivity of his machine. In addition, the feed rate with BrightLine Weld is no longer a parameter which must be optimised. This makes parameter optimisation easier and accelerates process development.

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