Nicholas Goffin, research associate at Loughborough University, investigates where energy savings can be made in laser processing
Climate change is increasingly on peoples’ minds. We’ve all seen footage of melting ice caps, increasing global temperatures, and protesters promoting use of public transport by gluing themselves to subway trains. Misguided though some of the protest efforts may be, the issues they draw attention to are very real and are an increasing threat to life on Earth.
How is this relevant to laser processing? It is now a legal requirement that the UK achieve net-zero carbon emissions by 20501, which includes manufacturing. Since manufacturing consumes 21 per cent of total delivered energy and accounts for 29 per cent of CO2 emissions (as of 20162), UK manufacturing industries face the need to significantly improve the energy efficiency of their processes.
With laser manufacturing processes, energy efficiency research has generally focused on the beam-material interaction specifically (with some exceptions, notably Kellens et al for laser cutting3 and Paul and Anand for laser powder-bed additive manufacturing4). Over time, the industry has moved to shorter wavelengths with higher power absorption, as well as different laser beam shapes such as ring or top-hat beams. This approach, while highly beneficial, ignores a significant factor – the laser itself is only one part of a complete laser processing system. This means that potential energy savings elsewhere in the system are ignored.
A general categorisation of a laser system, encompassing the device/unit level all the way up to the facility level, was first carried out in the field of laser safety by John O’Hagan5. By using his categories (shown in table 1), we can see that laser systems have a common set of sub-systems, such as power supplies, cooling, extraction, beam delivery, material feed, fume extraction and motion – which all use energy.
Table 1: Common laser sub-systems
The purpose of this project was to take a whole-system approach to laser welding energy use, to identify potential energy savings that might otherwise be missed. At Loughborough University (UK), in combination with project partners from the University of Manchester, Chongqing University (China) and Huazhong University of Science and Technology (HUST, China), the first steps have been taken in the development of an analytical process which can capture all the energy used in laser welding, from the plug socket at one end to the melt pool at the other.
Methodology
The purpose of this work was to characterise the energy use of the laser welding process, not to introduce any kind of process novelty. As such, standard components were used where possible. The beam itself was supplied with a JK300FL fibre laser (wavelength: 1,070nm, maximum power: 300W) and delivered using a standard JK welding head with a f = 76mm focussing lens. This was set to give a gaussian beam profile with a 1/e2 diameter of 1mm. Autogenous welding was carried out on 0.8mm thick stainless steel 316L substrates, at a range of different traverse speeds (2, 3, 4, 5 and 6mm/s) and laser output powers (80, 120, 160, 200 and 240W), for 25 test conditions in total. Laser beam power was characterised using a Coherent air-cooled power meter. This meant that the laser beam power measurement used was the true beam power, not the power according to the laser’s internal power meter.
The novel part was the use of an energy monitor to measure the electrical energy drawn by the system. This measured the electrical draw of the various sub-systems, which could then be related to processing parameters. Three types of electrical measurement were relevant to the investigation:
- Apparent power (kVA): This is the direct multiplication of voltage and current, measured in kW but defined as kVA to avoid confusion with the load. Apparent power is the electrical power supplied out of the plug socket. In direct current (DC) electrical systems, the apparent power and the load are the same.
- Load (kW): This is the electrical power actually used by the system, defined as the multiplication of apparent power and power factor.
- Power factor (non-dimensional): This is the ratio between the apparent power and load, and is analogous to efficiency in mechanical systems.
The power draws of the various sub-systems were then characterised according to their role in the welding process. This categorisation happened in a similar way to the Embodied Product Energy (EPE) model created by Seow et al6. Modified laser-specific versions of those categories were defined as follows:
- Processing power (PP): A laser-specific modification of the concept of direct energy (DE), which is the sum of all of the power inputs that directly contribute to the melting process. For this investigation, this was only the laser beam, but it is easy to imagine a scenario with a hybrid laser-arc torch or preheating, which would also be included. Saving energy in this area is where the majority of laser welding research has tended to focus its efforts.
- Auxiliary power (AP): This is the sum of all the process inputs that make it function, but do not directly contribute to the melting itself. This covers subsystems such as cooling, extraction and motion.
- Electrical supply waste (ESW): This category incorporates the sum of all the electrical losses, as calculated from the apparent power and the power factor. Short of complete redesign of subsystem equipment, there are no energy savings available here.
For the purposes of this investigation, five subsystems were defined – laser, cooling, extraction, motion, and support systems (computer, lights, interlocks etc).
Results
As desired, welding results showed an utterly predictable process with nothing unexpected occurring that could trouble the energy readings. Examples from these results are shown in figure 1.
Figure 1: Selected welding results.
When traverse speed is increased at constant power (160W), the specific energy decreases from 80J/mm to 26.7J/mm, thus the weld track gets smaller. When power increases at a given traverse speed (4mm/s) specific energy increases from 20J/mm to 60J/mm, thus the weld track gets larger.
When the energy was measured for all the laser subsystems, it was found that the laser subsystem represented a minority portion of the total power drawn by the welding system. Figure 2 shows these energy flows in the form of Sankey diagrams at different laser beam power levels. ESW occupied a significant proportion of the electrical power draw as well, but the vast majority of power draw was actually taken by auxiliary systems (auxiliary power), not by the laser itself.
Figure 2: Laser welding energy flows at different beam power levels.
It was also found that laser processing parameters affect overall system power draw, as illustrated in figure 3. As the laser power increases, its electrical power draw also increases. Other subsystems, such as the extraction, stay constant, with the result that the laser subsystem takes up an increasing proportion of total power as the laser beam power increases.
Figure 3: Laser beam and sub-system power draws as a percentage of overall system power draw.
Conclusion
In this investigation, the various types of energy used by the laser welding system have been categorised and quantified, so as to assist decision-making for energy saving. We have shown that the laser beam and subsystem itself actually represent a minority portion of the total electricity used by a laser welding system.
To maximise energy saving, it would therefore be wise for operators and system designers to look beyond the laser, at the wider system – for example, in the cooling, extraction and motion subsystems – because energy savings made in those areas can potentially eclipse those achieved via the laser itself. The proportion of total power taken by the laser also changes depending on parameters, which means that energy saving measures will have varying levels of effectiveness, depending on what parameters the particular system is using.
