The main objective of this project is to explore, develop and demonstrate the technological and economic benefits of dual wavelength (ultraviolet and infrared (IR) laser processing for manufacturing technology.
Task 1 - Process Analysis. The enhanced absorption effect was characterised and analysed which allowed a fundamental understanding of the processes operating to be gained. It also allowed the process to be accurately controlled and it was found possible to change the surface reflectivity of aerospace alloys to any value between the bulk highly polished reflectivity and very highly absorbing. The surfaces produced are stable and reproducible. A reflectometer was produced which allowed the accurate measurement of the effective absorption coefficient at two different wavelengths (Both BAe).
Task 2 - Off Axis Beam Delivery Systems. To enable on line processing to be carried out, off axis beam delivery systems were built. They consisted of, a) a simple mirror system (CRF) and b) a fibre optic based unit which provides a practical ruggedised system (BAe/OPL).
Task 3 - Coaxial Beam Delivery System. A coaxial beam delivery system was built to provide true DUWALP processing with coaxial delivery of the separate laser beams. In addition it utilised an active control system to improve the processing control for DUWALP welding (CL).
Task 4 - Processing of Fibre Reinforced Composites (FRCs). Single wavelength processing of these materials was fully characterised and a major discovery made, this was that it was possible to achieve the same very high quality as obtained with the excimer laser using CO2 laser processing provided that the pulse duration was short enough. This was fully investigated and proved the best route to high quality, high efficiency, low cost processing of these materials (BAe).
DUWALP processing of FRCs only provided small benefits and the process rate was still limited to that of the excimer laser. A process model was developed which allows the prediction of process rates for pulsed laser cutting or drilling of materials based upon simple measurements of the material properties. In addition it allows prediction of optimum processing parameters and laser systems for specific applications (BAe).
Task 5 - Machining and Joining of Metals. Use of enhanced absorption enabled the process map for welding of aluminium to be greatly extended but with the penalty of an increase in porosity for some alloys. It was in fact possible to produce conduction welds at relatively high velocity in 1.6mm 8090 alloy with only 1300W of laser power. This regime also showed an interesting result in that increasing penetration was obtained with decreasing intensity at constant power input (BAe).
Precleaning of the CO2 laser weld using the excimer laser enabled control of the porosity to be obtained and this shows high promise for welding techniques where the reflectivity is not a problem (eg Nd-YAG, TIG etc.) (BAe).
A sensor was built that allowed the measurement of the plasma electronic temperature in the welding of aluminium, this was used to provide process control (CL).
For cutting 2014 alloy an increase in cutting speed by a factor of 6 was achieved by using enhanced absorption. It was found that the degree of improvement in cutting speed depended on the initial reflectivity of the alloy in question (BAe).
Task 6 - Surface Modification of Metals. It was demonstrated that the excimer laser could be used to increase the percentage area of graphitisation on the surface of cast iron for both original material and CO2 laser case hardened material (CRF). Surface treatment of aluminium showed that DUWALP processing provided control of the depth of the rapidly solidified layer as expected and that the effects of DUWALP processing were additive in the case of the two alloys studied. Also it was shown that similar rapidly solidified layers were produced by TEA CO2 laser treatment as for excimer laser treatment (all BAe).
From these results it can be seen that the main area of application for DUWALP processing is in welding and surface treatment of aluminium alloys where major benefits have been demonstrated. For processing FRCs the best route is to utilise short pulse CO2 laser processing which can provide the quality and process rates required.
On metals preprocessing by an excimer laser significantly increases the processing efficiency and control of IR lasers due to an increase in the effective IR absorption coefficient. This effect will be studied in detail and utilised in applications including welding of aluminium alloys, cutting of Nimonic alloys and surface treatment of both aluminium and ferrous alloys. DUWALP processing will also be applied to fibre reinforced composites (FRC) where the benefits of the photoablative effects of the UV laser will be combined with thermal processing of the IR lasers to achieve fast, efficient processing along with high edge quality. Specific objectives include :
- cutting of FRCs with process speeds of >1m/min, an edge roughness of <0.1 mm and HAZ restricted to 0.5 mm
- cutting of Nimonic alloy at 2 m/min with a reduced HAZ from CO2 laser cutting alone
- welding of aluminium alloys with low porosity and tensile strengths of the welds 75 % of the parent material
- improvement of the corrosion resistance of surface treated aluminium alloy by a factor of 1.5 with areduction in fatigue strength of no more than 5 %
- improvement sistance of cast iron alloys by 20 % with an increase in the friction coefficient with lubricant by a factor of 2.
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