Eng S. Ng

Characterization of CO2 and diode laser welding of high carbon steels

Deep penetration welding with a high power CO2 and diode laser may offer an attractive means to join metal for certain applications, for instance: welding car drive shafts and gear plant. The rapid cooling rate of laser welding results in high hardness discontinuities across the welded joint; however, this leads to brittle weld and fatigue failure. To avoid this critical problem, it is useful to optimize the laser operating parameters in order to improve the mechanical properties of the weld. In this study, an experimental analysis was used to predict the significant effect of the weld quality using CO2 and high power diode laser (HPDL) laser welding, operating at 10.6 μm and 810 nm, respectively. Investigations into the weld quality were done to quantify the effect of different welding velocities by examining the hardness profiles, tensile strength, aspect, weld volume formation rate, and microstructure formation. In all cases, the results showed that for the HPDL weld configurations, cracking was observed in the fusion zone, whereas, for a CO2 laser weld, a greater weld strength and wider weld width were observed. For HPDL welding, center-line cracking was found along the fusion zone at higher welding velocities.Deep penetration welding with a high power CO2 and diode laser may offer an attractive means to join metal for certain applications, for instance: welding car drive shafts and gear plant. The rapid cooling rate of laser welding results in high hardness discontinuities across the welded joint; however, this leads to brittle weld and fatigue failure. To avoid this critical problem, it is useful to optimize the laser operating parameters in order to improve the mechanical properties of the weld. In this study, an experimental analysis was used to predict the significant effect of the weld quality using CO2 and high power diode laser (HPDL) laser welding, operating at 10.6 μm and 810 nm, respectively. Investigations into the weld quality were done to quantify the effect of different welding velocities by examining the hardness profiles, tensile strength, aspect, weld volume formation rate, and microstructure formation. In all cases, the results showed that for the HPDL weld configurations, cracking was observ...

Characteristics of Nd:YAG laser welded high carbon steels

A quantitative study of the relationship between the laser process parameters and the mechanical properties of welded high carbon steels was performed utilizing a Lumonics Nd:YAG pulsed laser, operating at 1.06μm, and a robotically manipulated fiber optic beam delivery system. A gage plate (0.88 mm thick) was butt welded with a constant power of 200 W and a He shielding gas was used at a pressure of 5 × 104 Pa. The welding performance of the Nd:YAG laser was strongly affected by the translation velocity, pulse length and pulse repetition frequency (PRF). The effects of varying these process parameters were quantified by measuring the samples hardness profile, weld width, weld penetration, and tensile strength. Furthermore, micrographic examinations were conducted at the welded joints. It was shown that by increasing the pulse length and pulse repetition frequency a deeper weld penetration and a wider bead width were achieved; moreover, the weld region became tougher. Hardness profiles transverse across t...

Characteristics of CO2 laser welded high carbon steel gauge plate

The welding performance of CO2 lasers is strongly affected by the clamping geometry and welding speed. Moreover, the rapid cooling rate associated with laser welding results in an untempered martensitic structure and a transverse variation in the hardness profile. An untempered martensitic structure can lead to brittle welds, and particularly for samples that are subject to cyclic loading, component fatigue and failure. To avoid this problem it is useful to optimize the laser operating parameters to reduce hardness discontinuities throughout the workpiece. An experimental investigation into the weld quality was performed to quantify the effects of clamping and translation velocity by examining the hardness characteristics, weld width and weld depth. A gauge plate, 2 mm thick, was welded with a 1 kW, CW, CO2 laser for a range of translation velocities between 800 and 1500 mm per min, a He shielding was used at a pressure of 5 × 104 Pa. Two weld geometries, namely clamped and unclamped, were considered. The clamped geometry gave improved hardness characteristics and a coarse grain structure. Whereas for the same operating conditions, the unclamped geometry gave a deeper weld penetration and wider weld width.

Multi-factorial Analysis of Nd:YAG Laser Welding of High Carbon Steels: Effect of Laser Parameters And Weld Geometry

The combination of laser processing, different welding geometries and techniques introduces a large number of factors which potentially influence the weld quality. A multi-factorially experiment was designed to assess the effect and interactions of a number of different laser parameters and different beam delivery systems. This included: pulse width, pulse repetition frequency (PRF), translation speed, and normal, pre-heating and post-heating of the weld. A general linear model was used to find the laser parameters which had a significant effect on the weld quality. This resulted in a faster, more efficient optimisation process. The weld quality was quantified by measuring the aspect ratio, weld volume formation rate and tensile strength. The welding was done with Lumonic’s MS830 Nd:YAG laser, operating at 1.06 μm, and with a maximum output power capacity of 400 watts. A dual-beam fibre optic delivery system was used to achieve pre-heat and post-heat treatments with a total power of 285 Watts. For the pre-heated geometry, a significant triple-interaction effect was observed for the measurement of weld volume formation rate. Interestingly, for all of the range of laser parameters investigated for the measurements of the tensile strength, there was no significant effect for the double or triple parameter interactions with the normal and post-heated weld geometries. For the pre-heated geometry, however, the interaction between the welding velocity and pulse width was significant. This investigation indicates the significant laser parameters, their interactions, and weld geometries that effect the weld’s: aspect ratio, tensile strength and weld volume formation rate.The combination of laser processing, different welding geometries and techniques introduces a large number of factors which potentially influence the weld quality. A multi-factorially experiment was designed to assess the effect and interactions of a number of different laser parameters and different beam delivery systems. This included: pulse width, pulse repetition frequency (PRF), translation speed, and normal, pre-heating and post-heating of the weld. A general linear model was used to find the laser parameters which had a significant effect on the weld quality. This resulted in a faster, more efficient optimisation process. The weld quality was quantified by measuring the aspect ratio, weld volume formation rate and tensile strength. The welding was done with Lumonic’s MS830 Nd:YAG laser, operating at 1.06 μm, and with a maximum output power capacity of 400 watts. A dual-beam fibre optic delivery system was used to achieve pre-heat and post-heat treatments with a total power of 285 Watts. For the pre...

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

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 ...