Mitigating the fire and explosion risks of laser metal processing
Proper extraction system maintenance and design is vital, says BOFA International’s Joshua Evans, to reduce fire and explosion risks associated with laser metal processing
Fibre laser technology looks set to expand rapidly in the next few years, as coding and cutting companies switch from CO2 lasers to take advantage of the reduced running costs and increased productivity gains associated with the technology.
An Allied Market Research report has predicted that the fibre laser market will be worth around $4.4bn by 2025 – compared to $1.8bn just over a year ago – thanks to processing speed improvements unlocked by the high beam quality that they offer.
This has the potential to bring significant productivity benefits to the laser coding processes used on metals (typically applying sell-by dates, product and tracking information to canned goods), in addition to laser cutting, engraving, machining and AM applications. To set the potential productivity gains into context, it is not unusual for fibre laser coding lines to run at 120,000 cans per hour.
Central to optimising the return on investment in this new laser era is effective fume extraction during metal processing. Fibre lasers seem to penetrate deeper into the base metal, where previously CO2 lasers only removed the surface oxide. This has implications on the design of the extraction systems used in metal processing, particularly when it comes to managing fire risks during the laser processing of aluminium and titanium, for example in the coding industry.
The risk of fire in fibre laser applications is guided by four simple principles:
- Lasers generate a large quantity of micron and sub-micron-sized particles
- The smaller a particle, the larger its surface area to mass ratio
- The larger the surface area, the faster the rate of reaction
- Aluminium and titanium possess highly exothermic oxidation reactions (fire)
While these principles provide the conditions for combustion, a properly specified, installed and maintained extraction system will mitigate the risk. Without effective extraction, the laser can generate a large number of un-oxidised particulates that can gather around the laser area – in either hoses/ducting, or in filters – or the laser can act as a source of ignition for an object trapped in front of it.
While this type of event is rare, aside from the risk of fire to human health, it will inevitably lead to a disruption in production and costly investigations, let alone causing damage to expensive equipment.
That is why working with reputable equipment manufacturers in specifying and maintaining extraction systems is so important. BOFA, for example, offers solutions for fibre laser systems that incorporate fire-resistant materials for casings and filters and that extend to thermal cut-out protection, should overheating occur.
Lasers processing generates a large quantity of micron and sub-micron-sized particles that can lead to combustion if not removed from the processing area. (Image: Guryanov Andrey)
The objective is to design-out any fire risks associated with thermal industrial processes, including lasering, particularly where a combustible dust can be generated by the process. This might also include protection through an inline fire-suppression system, which incorporates a fire extinguisher discharge mechanism, automated air flow shut down and isolation valves which cut off the oxygen supply when a fire is detected, thereby protecting the fume extractor and other areas nearby.
Correct extraction system design will help keep equipment, including hoses/ducting and filters, clear of particulate build-up. Regularly changing filters and cleaning or replacing ductwork will also reduce fire risk and hazard.
Thermite mixtures should also be avoided. The most common of these is iron oxide and aluminium – these do not need a source of ignition, burn very hot and don’t need a supply of oxygen. Therefore, aluminium and steel must not be processed on the same laser platform without thorough cleaning and the use of dedicated filter systems.
Protective coatings and lacquers applied to the metals can also aide ignition, or prevent oxidation when the lasered particulate is generated by forming a protective coating that can make it more flammable.
Figure 1: Particle size distribution of aluminium laser fume in a laser coding application
Whatever the risk, good machine maintenance and hygiene is central to risk mitigation, because a foreign object lodged in front of the laser can be an ignition source. At the same time, users need to prevent ignition sources reaching areas of particulate build up – this can be as simple as the use of a spark trap, spark arrestor or inerting powders.
And, of course, appropriate health and safety arrangements will require the presence of fire detection and extinguishing units. Extinguishers should be Class D – metal fire – as water extinguishers can react with aluminium or titanium to form hydrogen, which itself is flammable.
Managing inert atmospheres safely
Extra care should be taken when laser processing aluminium or titanium under inert atmospheres (eg. argon or nitrogen), as in the case of AM.
Inert atmospheres mean that none of the metal is oxidised and therefore, once the process is finished and exposed to air, the laser fume undergoes rapid oxidation and can catch fire. The answer is a passivation process for the fume, through the controlled reintroduction of air at a rate that manages thermal reaction.
Beware explosive conditions
Where there is a fire risk there can also be an explosion risk, should there be a dust cloud of the right concentration and particle size distribution. This is manageable through a disposable filter system where a dust cloud is not generated, or proper configuration of a self-cleaning filter system.
The key message for companies investing in fibre laser systems is to engage with their technology equipment partners and fume extraction specialists, such as BOFA, as soon as possible in the planning process. The engineers of these specialists add value when working alongside manufacturers and end-user teams, and can contribute to the design and commissioning of a system that will not only safeguard the working environment, but also optimise the value of the productivity gains available through using fibre laser technology. l
Joshua Evans, MEng (Hons) IChemE, is an applications engineer at BOFA International, a manufacturer and specialist supplier of fume and dust extraction systems