Laser Generated Fume Particulate and Fume Analysis For Health and Safety

 

Fumes produced from metal cutting applications using CO2 lasers can be a potential health hazard. The laser radiation combined with the assist gas (O2) causes atomisation of the process metal producing a fume containing small particles of the metal.The particulate forms through atomisation of melt by the assist gas jet, recondensation and droplets from the molten pool. Recent work carried out at the Department of Mechanical Engineering, Loughborough University has looked at the effects of changing the cutting conditions for different types of mild steel both on the health of the cutting process itself and the human health and safety implications of the fumes produced. Factors that have been monitored include the fume particles' shape and size distributions, and the laser process operating conditions including the laser stability, power and focus, assist gas flow and weld temperature as well as the cut quality. 

Non-invasive monitoring of the CO2 laser stability is provided using the beam monitor BM10.6, a novel laser beam sampling system from Precision-Optical Engineering. The BM10.6, is based on the use of single point diamond turning to produce a very weak diffraction grating on an OFHC copper substrate (usually a fold mirror). The BM10.6 replaces the final bending mirror in the optical train. The proportion of the beam sampled, and even the angle of the outcoupled sample can be varied to allow for customising to particular optical systems. Typical sampling is of the order of 0.0005 of the input beam.  The use of this sampling system does not affect the primary function of the beam and can be used continuously. Laser power is monitored from a current signal readout on two pyrometers that have previously been calibrated against a power meter, and a small laser diode is used to check focus.  

Both the forward and backscattered CO2 radiation can be monitored. The fraction of the main laser beam (forward scattered) provides an indication of the health of the optical system. The laser power at the lens (just after the BM10.6) can be compared with the value measured at the internal shutter dump (calorimeter type). The fraction sampled from the reflection off the surface of the material to be processed indicates process stability. Variation in the back-scattered power relate to fluctuations in the melt zone. It is also very useful for monitoring the drilling process (especially blind hole drilling). The backscattered power drops sharply as the beam breaks through the material. 

Particulate size distributions are measured using laser diffraction instrumentation. The distribution changes with processing parameters and cut quality of the product. This enables the process ´health═ to be monitored. Much of the particulate has been shown to have a mean particle size below 30 Ám - and these are respirable. The smallest particulates are formed when the process is optimised.

Scanning electron microscopy (SEM) of the particulate provides an indication of the shape, size and surface structure. SEM images show that the particulate formed during steel cutting is almost spherical. Some of the particles formed are hollow - as has been seen in atomised metal powders for thermal spray applications. From a health and safety point of view , the smallest particles are formed when the process is optimised, giving the highest health hazard.

 

Figure: SEM image of particulate formed during steel cutting, showing spherical shapes.  

 

Authors: Robin Addison, Precision-Optical Engineering, 42 Wilbury Way, Hitchin, Herts, UK, SG4 0TP, UK
and Dr. John Tyrer, Dr. Karen Williams and Leon Lobo, Department of Mechanical Engineering, Loughborough University, Loughborough, Leicestershire, LE11 3TU, UK 

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