Ng, Eric Eng Seng
Laser welding of high carbon steels.
PhD thesis, University of Glasgow.
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Laser welding, unlike conventional arc or gas welding, can be effectively utilised to produce high quality, clean and tough welds in high carbon steels. Results of welding high carbon steel are presented. The weld characteristics related to the fast cooling rate were critically evaluated and methods to reduce the rate of cooling were developed. The grain size produced in the fusion and narrow heat affected zones significantly affected the mechanical properties of the welded joint. Three lasers were used: Nd:YAG, CO2 and a high power laser diode (HPDL).
The investigations were carried out using a pulsed, 400 W, Nd:YAG laser, a CW, 1.2 kW, CO2 laser and a CW, 1.4 kW high power diode laser. For the Nd:YAG laser, the dual beam delivery system was achieved with a step index fibre to produce in-line process heat-treatment during welding. The spatial and temporal temperature distribution was controlled in the weld region to generate the desired mechanical properties, without losing the benefits of this low distortion joining process. For the CO2 laser system, a dual beam system was successfully designed, fabricated and the performance of the multiple beam system was evaluated. The welding quality was characterised by quantifying the effect of different laser parameters and welding geometry, including flat, angular, clamped and unclamped.
The welding performance of the Nd-YAG laser was dependent on the welding speed, pulse width and pulse repetition frequency (PRF). The effect of varying the laser parameters was quantified by measuring the hardness profiles, tensile strength, weld width, weld penetration and the rare of weld volume formation. Furthermore, microscopic examination was conducted at the welded joint. The quality of the welds was improved by increasing the pulse width and pulse repetition frequency (PRF), achieving a deeper penetration, wider weld width and greater weld volume formation rate and a tougher weld. At a slower welding speed, and for the higher pulse width and PRF, the hardness profiles were greatly reduced due to the greater spatial overlap of laser beam on the workpiece.
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