May 16th 2010 marked the 50th anniversary of the laser and the application of laser technology to material processing is undergoing yet another transformation.
Meanwhile the latest generation plasma cutters deserve a second look. Lasers power a suite of material processing applications known as Laser Machining.
It should be noted that these innovations are not limited to use in metal processing, but find applications in the fabrication of a wide range of materials including plastics, vinyl, glass, marble, graphite, nylon, ceramics, carbon fibre and rubber.
When electrical energy is applied to certain gases they enter an ionized state called plasma. Plasma arc cutting was developed in the 1950s for cutting of metals that could not be flame cut, such as stainless steel, aluminium and copper.
It may be said that plasma cutting technology emerged before it’s time, because when first offered to industry many operators were unprepared for the Computer Numeric Control (CNC) that was required to maximise the benefit. CNC devices are computers that control the motion of an instrument like a welding device or plasma cutter. Over time several classes of plasma cutting system have been developed that cover a wide range of cost-performance levels.
Hobby-class air plasma are available complete with CNC at very low cost, but these should be recognised for what they are. Unfair comparisons to laser or water jet systems of far higher performance specification and cost, merely because they share similar physical dimensions has encouraged the perception that plasma technology is inferior. In fact, modern plasma technology presents a technically viable and cost competitive alternative for many cutting applications where the ultimate precision is not required and Heat Affected Zone metallurgy is less critical.
Plasma cutters and CNC devices had trouble working together in the past because to create an arc to excite the gas power surges would frequently reboot the CNC computer. This has been overcome by installing a pilot starter in the nozzle of the higher end plasma cutters. The other big advantage of this pilot system is previously the plasma torch would need to be placed close to the metal surface, which would be charged with electricity and complete the circuit to create an arc. Internally arced plasma torches can generate heat near contact instead of on contact, making them more versatile and removing the surge issues.
The plasma arc cutting process uses electrically conductive gas to transfer energy from an electrical power source through a plasma cutting torch to the material being cut. The plasma gases include argon, hydrogen, nitrogen and mixtures, plus air and oxygen.
The mating of solid state laser devices with optical fibre technology first commercialised in the telecommunications industry, have produced laser cutting systems that offer greater flexibility and less maintenance than the CO2 lasers they are replacing. Known as Fibre Lasers, their extremely small beam width and superior focus enable the cutting of reflective metals like brass with ease and also boast power efficiency in the 26-30% range.
Energy efficiency is one topic that proponents of laser technology deliberately avoid because the gas lasers widely used by the fabrication industry today are grossly inefficient, typically in the 3-5% range. The worldwide escalation of energy prices demands that any system employing massive chillers to remove up to 95% of the power consumed as unavoidable waste heat, must come under scrutiny.
Unquestionably laser technology occupies the high ground in terms of cutting accuracy, operating features and capabilities. It may surprise many that alternative systems, when correctly matched to the job parameters, can offer far higher capital productivity and output cost per unit with acceptable tolerances and high output rate. Recent innovations such as the hybrid use of laser and water-jet cutting have injected fresh life into non-abrasive cutting methods in recent years.
Laser machining embraces a range of techniques that employ high intensity laser beams of various widths for drilling holes, applied to drilling, slotting, scoring, scribing, and surface ablation.
Laser drilling finds new applications as miniature-sized components are required for use in electronics, medical devices, computers and avionics. Laser drilling and hole patterning outperform chemical etching, mechanical machining/cutting, electroforming, and other processes in the manufacture of complex shapes, because the process is non-contact and flexible.
Laser scoring allows material to be easily removed to a desired depth to form a precise crease in any material, therefore to facilitate bending or tearing it easily.
Excimer laser micro machining allows for the creation of three-dimensional structures in a wide range of materials by using image projection technique. The consistent area of the image allows for the controlled removal of a given material to a required depth.
Laser ablation uses a laser beam to selectively remove one or more layers of material from the surface of coated or composite materials. Manipulation of the laser wavelength, pulse energy, and the amount of pulses enable the efficient removal of surface coating while preserving the properties of the base material. Coating removal with a laser is claimed to be more environmentally friendly than conventional etching methods.
Laser heat treating
In addition, lasers are also invading the field of metal heat treatment and achieving remarkable results. Conventional heat treating applies many different techniques in order to improve the performance of metal components by modifying or enhancing the crystal or chemical structure of the treated material. Common objectives of heat treatment are increased wear resistance and improved fatigue strength.
Laser techniques have been successfully applied in only three new heat treatment applications:
Laser Transformation Hardening applies a laser’s heat through a beam integrator to produce a rectangular or square-shaped beam uniformly across the surface of the part being treated. Carbon is an element in steel that greatly influences it’s hardness and tensile strength, but is often unevenly distributed within the metal. Raising the metal temperature to a shallow depth allows the carbon atoms to diffuse through the crystal structure and become more evenly distributed.
Rapid cooling of the surface as heat is conducted into the deeper layers of metal then locks the carbon atoms into a metastable crystal structure.
The result is a hard, wear-resistant surface that requires no further surface treatment. First applied in the 1970s, this technique is popular today for cylinder liners in diesel engines, piston ring grooves, photocopier rollers and power steering components.
Annealing has not been as widely practised using laser energy, but it has been successfully applied to steel wires for the production of steel belted radial tyres to improve the fatigue strength by a factor of 3-4 times that of wire in the ‘as drawn’ condition. After heating by the laser beam, the steel is allowed to cool slowly, while allowing the crystal structure that is severely distorted in the drawing process to normalise.
Laser Surface Melting takes the transformation process a step further by melting the surface and achieving a greater degree of surface homogeneity. While further processing may be required to remove resulting surface ripples, the improvement in properties justifies its application to certain components.
Cast-iron is notorious for containing carbon or carbide blocks or inclusions while adjacent zones suffer carbon depletion. Cast iron cam shafts are a typical example where the high carbon material is rendered extremely wear-resistant. Another interesting one is the application of lasers to melt the surface of cast nickel-aluminium-bronze in order to improve corrosion resistance. Marine propellers are treated in this way to reduce the risk of cavitation erosion.
Fibre-laser spot welding is an exciting development challenging the conventional resistance spot welding that has dominated the assembly of steel automobile bodies for decades. By almost completely eliminating idle times, cycle times are significantly reduced and laser welded seams offer mechanical advantages. Safety issues, however, will complicate their use in open production facilities requiring fast reaction safety devices.
Typical applications for this system are sheet metal assemblies in the body-in-white production lines which up to now have been joined with many resistance welding spots. Typically two welding spots spaced 30mm apart would be replaced by one laser step seam of approximately 30mm and the cycle time to complete a body seam would be reduced from 75-37 seconds. Considerable saving in floor space could also be achieved.
Gas demand implications
Laser cutting has generated new demands on the industrial gases industry that challenged their traditional modes of supply and spawned several technical innovations.
Ferrous metals are normally cut using an oxygen-assist gas that reacts chemically with the substrate being cut, requiring minimal pressures and flows. Cutting stainless steel, on the other hand, requires high-pressure nitrogen of up to 30 bar to remove material during the cut. Non-ferrous metal cutting requires flow rates exceeding 1400 litre/min and higher purity levels than ferrous metal cutting. Stainless steel cutting requires nitrogen purity at levels above 99.9%.
The reliable operation of modern laser cutters depends critically on consistent gas supply and the importance of a correctly designed storage and supply system cannot be over-emphasised.