Characteristics of CO2 and Nd:YAG laser welded high carbon steels

Abstract

Greater understanding of the basic phenomena of laser welding and better control of the process is leading to improved weld quality. Because of the complexities of accurate modelling it is beneficial to examine the interaction and effects of parameters associated with the laser, processed material, and weld geometry by experiment. The results of a detailed investigation are presented that show the influence of: translation velocity, pulse length, pulse repetition frequency (PRF) and different geometries on the weld quality for Nd:YAG and CO2 laser welding of high carbon steels. An advantage of Nd:YAG laser processing is its shorter wavelength; consequently, because of the dependency of the material’s emissivity on the wavelength, energy is absorbed by the material more readily than for the CO2 laser and a lower energy can be used for welding, allowing greater control of the heat input. This is particularly useful when working with thin materials. Compared to arc or gas welding the energy input to a laser weld is extremely small; this results in an untempered martensitic structure in the samples due to the rapid quenching rate of the surrounding material. The two lasers used to weld the high carbon steels were: Ferranti’s MFKP continuous wave 1 kW CO2 device and Lumonic’s pulsed, 400W Nd:YAG laser. The effect of the translation velocity and weld geometry on the sample’s mechanical properties in the welded region was investigated for the CO2 laser. For the Nd:YAG laser, the effect of the pulse length and pulse repetition frequency on the weld quality was investigated. The mechanical properties of the weld were analyzed in the fusion and heat affected zones. Both lasers were used to weld different thicknesses of gauge plate that were then sectioned, moulded, etched and photographed, allowing examination of the sample’s microstructure. The parameters that were effective in controlling the sample’s phase transition properties to achieve the desired weld characteristics were found. Both lasers produced a fairly inhomogenous structure for the butt welding geometry; this was revealed by the mechanical properties of the weld joints and the microstructure near the fusion zone. Also, the hardness characteristics were dependent on the translation velocity. For the Nd:YAG laser, the sample’s hardness was greatly reduced by increasing the pulse length and pulse repetition frequency. For the CO2 laser, the sample’s hardness was greatly reduced by increasing the translation velocity.Greater understanding of the basic phenomena of laser welding and better control of the process is leading to improved weld quality. Because of the complexities of accurate modelling it is beneficial to examine the interaction and effects of parameters associated with the laser, processed material, and weld geometry by experiment. The results of a detailed investigation are presented that show the influence of: translation velocity, pulse length, pulse repetition frequency (PRF) and different geometries on the weld quality for Nd:YAG and CO2 laser welding of high carbon steels. An advantage of Nd:YAG laser processing is its shorter wavelength; consequently, because of the dependency of the material’s emissivity on the wavelength, energy is absorbed by the material more readily than for the CO2 laser and a lower energy can be used for welding, allowing greater control of the heat input. This is particularly useful when working with thin materials. Compared to arc or gas welding the energy input to a laser ...