Dr Peter Heath explains the benefits of using water as a coolant in laser machining.
Compared to dry machining, mechanical material removal processes are almost always enhanced by the use of coolant – the fluid cools the cutting zone, lubricates the process and washes away the debris.
In contrast, most precision laser machining is carried out completely dry or with a gas atmosphere. Under such dry conditions, heat can build in the work piece, thermal expansion can result in distortions, brittle materials may crack, a large heat affected zone (HAZ) may form and oxidation may also result. An inert gas may provide a better atmosphere, but a gas does not have sufficient thermal capacity to give adequate cooling. Last but not least, the cut produced by a focused dry laser beam is V-shaped – as the cut gets deeper, the kerf becomes wider at the entry side of the cut.
In recent years, water has been used to try to reduce these negative effects.
Fire and water combined
In a wet laser, the beam is coupled with a hair-thin jet of water just 50 microns or less in diameter. The laser is focused not onto the work piece, but through a water chamber onto the bore of a water-jet nozzle. As a result, the laser pulses couple with the water and they travel down the water column by total internal reflection. The jet acts like a liquid optic fibre.
By repeatedly scanning the laser water jet over a programmed cutting path, material is removed by ablation and the trench produced gets deeper and deeper. Importantly, the water jet continues to carry the laser energy to the bottom of the cut, giving a parallel-sided cut instead of one that is V shaped.
The useable, laminar flow portion of the jet can be maximised up to 1,000 times the jet diameter – a 50-micron jet can give a 50mm working zone length. A work piece placed anywhere in this working zone will be cut and, because the laser pulses are now being guided to the bottom of the cut by the water jet, there is no need to refocus the wet laser as the cut deepens. Nor is it necessary for the work piece to be flat – a round bar, for example, can be cut by simply traversing the bar back and forth within the working zone of the laser water jet.
The laser most often employed is a Nd:YAG laser at a wavelength of 532nm. Water is particularly transparent to light of this wavelength and energy absorption by the water column is therefore minimal. Fire and water have been combined and with each laser pulse (typically 6,000/second) a repeating cycle of heating, ablation, cooling and washing is achieved.
Applications of the wet laser
The wet laser can process a broad spectrum of materials, including all common metals and alloys, most ceramics and some composites. It is particularly useful for machining difficult metals such as titanium, molybdenum and the heat-resistant superalloys.
The luxury watch industry, for example, uses the wet laser to quickly manufacture functional and decorative parts from foil or sheet materials such as brass, copper-beryllium, stainless steel and titanium. The wet laser allows design changes to be made quickly – prototypes can be assessed and production batches manufactured without the need for delays associated with stamping dies. The parts are clean and show no oxidation, and there is no hanging burr produced on the exit side of the cut (see left).
Cutting a swathe
Over the past two years, the wet laser has made significant in-roads in cutting both gem and industrial-quality diamond. The parallel sided cut means higher yields and the flat, non-tapered surfaces require minimal polishing. Natural diamonds up to 25mm in diameter have been cut from one side of the stone rather than double sawing from both sides and, because the water cools as it cuts, the breakage of highly stressed stones has been reduced by 50% compared to dry laser cutting.
Even in diamond, it is possible to drill deep, parallel-sided holes (see bottom right) or trepan small diameter rods within a few minutes. A 1mm diameter, 7mm long diamond rod can be cut from a natural diamond stone with ±2µm accuracy in less than eight minutes.
In the industrial diamond field, synthetic materials such as cemented carbide-backed, polycrystalline diamond or its ultra-hard sister material, polycrystalline cubic-born nitride can be cut or shaped as required. Mother discs can be diced into blanks for tooling or the tools themselves shaped to the required cutting geometry. Effective cutting speeds are 5–6mm/minute and the surface finishes produced by the wet laser are similar to or better than that produced by wire cutting.
In gas turbines and jet engines, the hollow super-alloy blades are drilled with numerous cooling holes to reduce blade temperatures. Traditionally, these holes have been drilled by deep-hole electrical discharge machining (EDM). However, the blades nearest the combustion chamber have an additional ceramic thermal barrier coating to protect the blades from the extremely corrosive and high temperature exhaust gases. After the holes in the bare super-alloy blades have been drilled by EDM, the blades are given their ceramic coating and a secondary operation with a (dry) laser must be used to pierce the non-electrically conducting coating in exactly the same places as the underlying holes in the blade body.
Wet lasers mounted on large five-axis machines are now being used to drill through the ceramic coating and continue through the underlying superalloy wall of gas turbine blades (see top right). Early tests suggest that the reduced heat affected zone in the bore of each hole is such that blade’s fatigue resistance is improved. Shaped holes are also possible.
Combining laser ‘fire’ with a hair-thin jet of water may seem counterintuitive, but it is possible and it significantly improves laser processing. As with the use of a coolant in mechanical machining, precision laser machining is also ‘better when it’s wetter’.