Comparing YAG / FSS fibre lasers for pulsed welding applications

The story

More than 30 years ago, the first welding lasers were fitted with microscopes, thus allowing the smallest defects to be repaired using a microscope. This was the start of laser technology moving into toolmaking and die and mould production. Around the same time, these devices found another larger group of users in dental laboratories and in the field of jewellery production.

They continued their success story with the first automated lasers in the fields of sensor technology and medical technology. Many thousands of Nd:YAG lasers were sold in the industries mentioned worldwide, thus establishing this technology as a permanent feature of the repair and manufacturing process.

For more than 20 years, it seemed that there was no alternative to the Nd:YAG laser. Even if semiconductor lasers already existed, generating the necessary laser pulses using these semiconductor lasers was unthinkable.

Even if it had been technically possible, these lasers would have been prohibitively expensive at the time. Then, in 2012, IPG came in with a game-changing new product. IPG was the first company in the world to launch laser modules that generated short pulses in the kW range and that could be used in the same setup as the Nd:YAG laser. The prices were significantly higher than those of the Nd:YAG lasers, but the benefits from a technical perspective were obvious.

As a technology-oriented company, we tested this technology extensively as early as 2013 so that we could use it in our devices later on if necessary.
The initial tests were very positive and our customers were also very excited after the initial tests. So, we decided to integrate these modules in our serial products, even if they would then become more expensive and our profit margins would decrease. It was clear to us that we would not be able to stop the success of this technology and that it would be better to develop new devices based on this technology from the start and thus be able to offer our customers Nd:YAG lasers as well as the new fibre lasers.

Technical comparison: Nd:YAG laser / FSS fibre laser

Efficiency: socket to laser output power

The efficiency of Nd:YAG lasers is around 3-4%. That’s why lasers with a higher output power need three-phase current, as well as an external water-cooling system. A 600W Nd:YAG laser would have an input power of approx. 15,000W and would need approx. 14,400W of cooling power.
By contrast, the fibre laser, which has an average output of 600W, would only need 2,200W, can be operated using a normal 230V wall socket and only needs a couple of integrated fans for air cooling.


Beam quality (Beam quality describes the ability to focus the laser beam)

Due to the thermal load in the laser crystal, the Nd:YAG laser becomes less focusable as the output increases. That means that the laser beam becomes bigger on the workpiece. The focusability of the laser beam is significantly worse in comparison to the fibre laser, even with individual laser pulses. The so-called beam parameter product (which describes the focusability) is approx. 20 for an Nd:YAG laser and becomes significantly worse as the output increases. With a fibre laser, this value is 2.5 and doesn’t change, even at maximum output. In practice, using the same lens (190 mm focal length) results a minimum spot diameter of 250 μm for a YAG and 100 μm for a fibre laser.


Stability of the laser energy

The laser output for an Nd:YAG laser is largely dependent on the condition and alignment of the lenses. Towards the end of the service life of the bulb, the output of the laser also decreases. The smallest deviations in alignment have serious effects on the laser’s output. When welding very thin wires with very low pulse energies, these fluctuations are very noticeable. If the Nd:YAG laser has a pulse-to-pulse energy deviation of +-10% at low energy, this is +-1% for a fibre laser. The diode current can also be adjusted extremely precisely, allowing for stable pulses of a few mJ. These pulses cannot be generated by normal Nd:YAG lasers. The use of a mode shutter helps here, but it also further reduces the efficiency to an extreme degree and still never reaches the stability of a fibre laser.


Calibration to factory standard

With fibre lasers, it is possible to calibrate a laser to factory standard. That means that all lasers with the same settings emit the same energy. This is only possible thanks to the high level of stability of a fibre laser and Vision’s calibration software. It is possible to return to the ISO standard using a suitable measuring device.


Maximum energy

This is where the Nd:YAG laser has a real advantage. While fibre lasers can only apply 10 times their average output, Nd:YAG lasers have a significantly higher peak pulse output, even at lower average output. This is particularly advantageous with materials like pure copper or silver. This is particularly advantageous with massive workpieces made from pure copper or silver. However, the fibre laser also works very well with thin wires or fine components.


Maintenance and wear and tear

With Nd:YAG lasers, the flashbulb is a real wear part and must be replaced regularly. Large systems even have two flashbulbs installed. Depending on the use of the machine, they will need to be replaced once or twice per year. Depending on the manufacturer, the costs for this will range from EUR 500 to EUR 1500 per replacement. The mirrors and even the crystal may need to be replaced over the years. It is recommended that the resonator is adjusted once a year when changing the flashbulb or if there are any signs of negative deviations in the output.

The fibre laser is a completely hermetically sealed system, which has no wearing parts and doesn’t even need to be adjusted. The lifespan of the diodes is stated as being 100,000 hours. Maintenance is limited to cleaning the ventilation openings.

Unlike with Nd:YAG lasers, there is no gradual loss of output with fibre lasers. At least, we have not yet noticed any such loss in any systems in nearly 10 years now. For this reason, we are offering an optional warranty extension of up to 60 months for all FSS lasers from Vision Lasertechnik.