Parameters controlling weld bead profile in conduction laser welding

In laser welding and other processes, such as cladding and additive manufacturing, the weld bead geometry (depth of penetration and weld width) can be controlled with different parameters. A common practice is to develop process parameters for a particular application based on an engineering approach using the system parameters i.e. laser power and travel speed. However, in such a case the process is optimised for a particular system only. This study is focused on understanding of the phenomena controlling the weld profile in conduction welding for a wide range of beam diameters from 0.07 mm to 5.50 mm. It has been shown that the weld bead geometry can be controlled by the spatial and temporal distribution of laser energy on the surface of workpiece, such as power density, interaction time and energy density. This means that similar depths of penetration can be achieved with various optical set-ups. It has been also found that it is more difficult to achieve pure conduction welds with small beam diameters, which are typically used in powder bed additive manufacturing, due to high conduction losses and low vaporisation threshold

Effect of beam shape and spatial energy distribution on weld bead geometry in conduction welding

The size of a projected beam onto a workpiece and its intensity distribution profile defines the response of the material to the applied laser heat. This means that not only the processing parameters, but also the optical set-up and process tools define the process and the resulting weld profile. In high power laser delivery systems the beam propagation characteristics of the laser beam can vary during processing. A change of the focal distance, for instance, alters the spot size projected on the workpiece as well as its intensity distribution. Some dynamic optical systems can also change the shape of the projected beam. Galvo-scanners induce a small distortion to the projected beam from circular to elliptical when the mirrors deflect the beam across the working domain. This continuous change of the spatial energy distribution may affect the process stability and material response locally. This work examines the influence of changing the shape of the projected beam and its energy distribution on the weld bead profile in conduction laser welding, which is also relevant to laser cladding and additive manufacture. It has been found that for the same optical set-up and system parameters, different bead profiles can be obtained with different degree of distortion of the beam profile. In addition, different intensity distribution profiles led to different penetration depths for the same nominal beam diameter and energy density due to the difference in peak intensity

Root stability in hybrid laser welding

Hybrid laser welding offers promising advantages over the traditional arc-based welding processes. The high penetration ability of lasers and the filler wire delivery of gas metal arc welding (GMAW) enable joining of thick section materials without the need of multi-pass. The output power of modern solid state lasers provides enough energy to penetrate thicknesses exceeding 20 mm in steel. However, the high aspect ratio fusion zone with the rapid solidification does not always provide beneficial conditions for achievement of good weld profiles. Distribution of the liquid metal between the top and root sides of a joint, and hence the weld profile, are determined by a complex balance between the vaporization pressure of a laser, the electromagnetic force of an arc and the surface tension of a meltpool. In this work, the stability of root profile and all aspects related to the achievement of acceptable roots in pipeline welding have been investigated. It has been found that in order to achieve a smooth root profile in deep penetration hybrid laser welding, not only a sufficient penetration force, but also a certain amount of energy need to be provided. This is required to maintain the keyhole fully developed with a steady state pressure balance throughout the thickness. It is also important to achieve sufficient temperature in the root and to provide appropriate wetting between the liquid metal and the back surface of the material. Depending on the power density and energy used, different regimes were identified with sagging of the root in the initial stage, followed by good quality root profiles and then ending on excessive melt expulsion with further increase of power density. The results suggest that if operated in the right regime, the process is very tolerant, in terms of energy and power density required for acceptable root profiles and good quality joints can be achieved

The application of specific point energy analysis to laser cutting with 1 μm laser radiation

Specific point energy (SPE) is a concept that has been successfully used in laser welding where SPE and power density determine penetration depth. This type of analysis allows the welding characteristics of different laser systems to be directly compared. This paper investigates if the SPE concept can usefully be applied to laser cutting. In order to provide data for the analysis laser cutting of various thicknesses of mild steel with a 2kW fibre laser was carried out over a wide range of parameter combinations. It was found that the SPE concept is applicable to laser cutting within the range of parameters investigated here

The application of specific point energy analysis to laser cutting with 1 μm laser radiation

Specific point energy (SPE) is a concept that has been successfully used in laser welding where SPE and power density determine penetration depth. This type of analysis allows the welding characteristics of different laser systems to be directly compared. This paper investigates if the SPE concept can usefully be applied to laser cutting. In order to provide data for the analysis laser cutting of various thicknesses of mild steel with a 2kW fibre laser was carried out over a wide range of parameter combinations. It was found that the SPE concept is applicable to laser cutting within the range of parameters investigated here

The application of specific point energy analysis to laser cutting with 1 μm laser radiation

Specific point energy (SPE) is a concept that has been successfully used in laser welding where SPE and power density determine penetration depth. This type of analysis allows the welding characteristics of different laser systems to be directly compared. This paper investigates if the SPE concept can usefully be applied to laser cutting. In order to provide data for the analysis laser cutting of various thicknesses of mild steel with a 2kW fibre laser was carried out over a wide range of parameter combinations. It was found that the SPE concept is applicable to laser cutting within the range of parameters investigated here

Bead shape control in wire based plasma arc and laser hybrid additive manufacture of Ti-6Al-4V

Wire based plasma transferred arc (PTA)-laser hybrid additive manufacture has the potential to build large-scale metal components with high deposition rate and near-net shape. In this process, a single bead is the fundamental building block of each deposited component, and thus the bead shape control is essential for the deposition of different geometries. However, how to control the bead shape by manipulating various process parameters is still not understood. In this study, the effect of different process parameters, including laser power, energy distribution between the PTA and laser, wire feed speed, travel speed, and laser beam size on the deposition process and bead shape was investigated systematically. The results show that the optimum operating regime for the hybrid process is with the wire being fully melted by the PTA and the melt pool being controlled by the laser, which gives a good bead shape as well as a stable deposition process. The bead shape is significantly affected by the laser power and travel speed due to the large variation in energy input. The effect of wire feed speed is more complex with the bead width initially increasing to a maximum and then decreasing as the wire feed speed increases. The laser beam size has a minor effect on the bead shape, but a small beam size will result in an irregular bead appearance due to the unstable process caused by the high power density. In addition, a procedure for controlling the bead shape in the hybrid process was proposed, which provided a reference for selection of different process parameters to achieve required bead shapes. The feasibility of this proposed procedure was demonstrated by the two deposited multi-layer single-pass walls

Measurement and modelling of the residual stresses in autogenous and narrow gap laser welded AISI grade 316L stainless steel plates

Thick-section austenitic stainless steels have widespread industrial applications, where stress-corrosion cracking is often of major concern. Problems tend to arise in the vicinity of welds, where substantial residual stresses often reside. This paper describes an investigation into the residual stresses in autogenous high power laser welds and narrow gap laser welds (NGLW) in 10 mm thick AISI grade 316L steel plates, using both neutron diffraction and the contour method. The influences of laser power, welding speed and the time interval between weld passes on residual stress were analysed. For the NGLW process, finite element modelling was employed to understand the influence of thermal history on residual stress. The results for the NGLW technique show that the laser power has a significant effect on the peak value of residual stress, while the welding speed has a more significant influence on the width of the region sustaining tensile stresses

Wire laser arc additive manufacture of aluminium zinc alloys

Aluminium zinc alloys are widely used in the aerospace industry due to their high strength. However, only a few studies have been reported on the additive manufacture of aluminium zinc alloys. This rarity is due to the difficulties occurring during the fusion processing of these alloys and to the lack of available raw material. This paper presents an alternative process used for the deposition of aluminium zinc alloys. In this study, a Wire Laser Arc Additive Manufacture (WLAAM) system was used. This consisted of a gas metal arc power source, used to generate the melt pool, and a laser beam applied to control the melt pool size. By using this approach, it was possible to produce an elongated melt pool and feed zinc into it with a cold wire without compromising the process stability. A welding camera along with a system measuring the arc voltage and current was used to monitor the process. Different process parameters and configurations were investigated along with their effect on process stability and deposited material microstructure. A very high zinc concentration was achieved in the deposited material without macro-segregation

Penetration and mixing of filler wire in hybrid laser welding

Modern lasers allow achievement of full penetration single pass welds in steel plates with thicknesses exceeding 20 mm, at welding speeds much greater than any traditional arc-based process. However, the addition of filler wire, which in most structural welds is required to ensure good mechanical properties, is more challenging. Most welds from laser and hybrid welding with filler feeding start to exhibit inhomogeneous fusion zones above a particular joint thickness. The filler consumable segregates near the top section of the joint, while the bottom forms by the re-melted parent metal, which negatively affects mechanical properties. In this work, the homogeneity of laser-arc hybrid welds was investigated experimentally, using a filler wire with a signature element, whose distribution was measured. Three different bevels with different geometry were used to study the flow of liquid filler wire across the joint. The laser and arc parameters were varied to establish the dominant forces responsible for the transport of filler wire and weld homogeneity. The results indicate that hybrid welds are susceptible to form inhomogeneous fusion zones and to achieve acceptable welds, two aspects need to be controlled. The first one is the average content of an alloying element in the meltpool, which is mainly controlled by the wire composition, its deposition rate and dilution with the parent metal. Whilst the second aspect being the weld homogeneity. It has been found that the laser power density is predominantly responsible for the transport of the consumable metal across the material. Furthermore, most processing parameters, such as the arc power or laser power, play contradicting roles, improving one aspect and simultaneously hindering another. The best way of achieving fully homogenous welds with known composition is by applying sufficient wire deposition, to satisfy the compositional requirement, and then provide enough laser power density to transport it across the full thickness