Microstructural and mechanical characterisation of laser-welded high-carbon and stainless steel

Laser welding is becoming an important joining technique for welding of stainless steel to carbon steel and is extensively used across various sectors, including aerospace, transportation, power plants, electronics and other industries. However, welding of stainless steel to high-carbon steel is still at its early stage, predominantly due to the formation of hard brittle phases, which undermine the mechanical strength of the joint. This study reports a scientific investigation on controlling the brittle phase formation during laser dissimilar welding of high-carbon steel to stainless steel. Attempts have been made to tailor the microstructure and phase composition of the fusion zone through influencing the alloying composition and the cooling rate. Results show that the heat-affected zone (HAZ) within the high-carbon steel has significantly higher hardness than the weld area, which severely undermines the weld quality. To reduce the hardness of the HAZ, a new heat treatment strategy was proposed and evaluated using a finite element analysis-based numerical simulation model. A series of experiments has been performed to verify the developed thermo-metallurgical finite element analysis (FEA) model, and a qualitative agreement of predicted martensitic phase distribution is shown to exist

Numerical simulation of alloy composition in dissimilar laser welding

A three-dimensional multiphase computational fluid dynamic model was developed to investigate the meltpool fluid dynamics, dilution and alloy composition in laser welding of low carbon steel and stainless steel. Using the developed model, independent predictions on weld properties are made for a range of laser parameters, and in all cases the results of the numerical model were found to be in close agreement with experimental observations. The investigation revealed that above certain specific point energy the materials within the melt pool are predominantly homogenous. A minimum meltpool convention is required in dissimilar laser welding to obtain weld bead properties suitable for industrial applications. The present model provides a simple yet effective method to predicting the weld bead alloying concentration and homogeneity encompassing wide range of materials

Laser surface modification of carbon fiber reinforced composites

The removal of top resin layer is an essential task prior to adhesive bonding of carbon fiber reinforced polymer (CFRP) composites. This paper investigates the technical feasibility of using a low power continuous wave carbon dioxide laser for removing the top resin layer of CFRP without damaging the underlying fiber. The operating window and damaging threshold were experimentally determined. Irradiating the CFRP surface at a power of 14 W, scanning speed of 880 mm/sec, and a beam overlap of 25% provides an optimal thermal condition for removal of top resin layer. A finite element model was used to explain the removal mechanisms

Laser polishing of selective laser melted components

The shape complexities of aerospace components are continuously increasing, which encourages industries to refine their manufacturing processes. Among such processes, the selective laser melting (SLM) process is becoming an economical and energy efficient alternative to conventional manufacturing processes. However, dependant on the component shape, the high surface roughness observed with SLM parts can affect the surface integrity and geometric tolerances of the manufactured components. To account for this, laser polishing of SLM components is emerging as a viable process to achieve high-quality surfaces. This report details an investigation carried out to understand the basic fundamentals of continuous wave laser polishing of SLM samples. A numerical model, based on a computational fluid dynamic formulation, was used to assist the understanding of melt pool dynamics, which significantly controls the final surface roughness. The investigation identified the input thermal energy as the key parameter that significantly affect the melt pool convection, and essentially controls the surface quality. Minimum meltpool velocity is essential to achieve wider laser polished track width with good surface finish. Experimental results showed a reduction of surface roughness from 10.2 μm to 2.4 μm after laser polishing with optimised parameters. Strategies to control the surface topology during laser polishing of SLM components are discussed

Laser net shape welding

Over the last 40 years of laser welding practice, weld bead geometry always experiences a section of the weld bead slightly above or below the parent material surface. In this paper, a new concept – net shape welding is introduced, whereby the weld joint fusion zone is flat to the parent material surface. Experimental work was carried out to demonstrate net shape laser square butt welded mild steels sheets. Tensile test results show that the net-shape welds well outperform those with traditional weld bead geometry. Computational fluid dynamic and finite element models have been used to assist in the understanding of net-shape weld geometry formation and the superior mechanical properties

Modelling of the melt pool geometry in the laser deposition of nickel alloys using the anisotropic enhanced thermal conductivity approach

Use of appropriate modes of heat transfer in finite element modelling simulations of laser deposition is important for enhancing the reliability of the predicted results. An important contributory mode is melt pool convection, which is the focus of this work. Using the anisotropic enhanced thermal conductivity approach, this study examines the strategies relating to the choice of appropriate values for the thermal conductivity enhancement factors in the orthogonal axial directions x, y, and z. In order to investigate different combinations of values for these factors in the laser deposition of one track of Inconel 718 powder on an EN-43A mild steel substrate, finite element models were prepared and results from these were compared with the corresponding experimental results. The results of the study suggested that no thermal conductivity enhancement should be enforced in the direction of the depth of the sample. Thermal enhancement factors in the two orthogonal directions are required, but the factor in the direction parallel to the direction of beam scanning should be of greater magnitude. Analysis of the thermal gradients from the model also showed that failure to incorporate any allowance for the melt pool convection effect with appropriate choice of thermal conductivity enhancement factors in the finite element modelling of the laser deposition can result in overprediction of thermal stress, which can lead to undue threats of various forms of distortion during the deposition process

Droplet-assisted laser micromachining of hard ceramics

Hard ceramic materials like tungsten carbide (WC) are extensively used in high value manufacturing, and micromachining of these materials with sufficient quality is essential to exploit its full potential. A new micro-machining technique called droplet assisted laser micromachining (DALM) was proposed and demonstrated as an alternative to the existing nanosecond (ns) dry pulse laser ablation (PLA). DALM involves injecting liquid micro-droplets at specific frequency during the nanosecond laser micromachining to create impulse shock pressure inside the laser irradiation zone. The impulse shock pressure is generated due to the explosive vaporisation of the droplet, during its interaction with high temperature laser irradiation zone. In this paper, the DALM uses a nanosecond pulsed Nd:YAG laser to machine tungsten carbide substrate. The results suggest that the impulse shock pressure generated during the DALM process can transform the melt ejection mechanism of the ns laser micromachining process. The change in ejection mechanism results in a 75% increase in material removal rate and 71% reduction in the spatter redeposited compared to conventional dry ns laser micromachining

Performance evaluation of a three dimensional laser scanner for industrial applications

Laser scanners are nowadays extensively used in industries due to their high accuracy, resolution and robustness. However, the specifications provided with most laser scanners are debatable and without proper analysis the output results from the scanners cannot be trusted. The performance of laser based scanners depends on many aspects including ambient lighting condition, surface reflectivity, surface roughness and stand-off distance. In this paper, a set of performance evaluation tests for a three dimensional (3D) laser scanner is presented. An initial definition of the best strategy for testing prior to its use is proposed. The performance of the scanner was evaluated under different operating conditions such as different surface reflectivity, viewing angle, surface roughness and stand-off distance. The optimum working range of the laser scanner was established and the regions where the laser scanner produces inappropriate data was identified and quantified. A similar testing approach can be used for any industrial laser scanner prior to its application to minimize any unambiguity in measurements. This will also enable the users to have confidence in the measurements returned by laser scanners

Laser stripping of TiAlN coating to facilitate reuse of cutting tools

The potential of using the laser ablation process to perform controlled stripping of titanium aluminium nitrite (TiAlN) coating from tungsten carbide (WC) substrate is explored in this paper. TiAlN coatings are extensively used in cutting tools to improve machining capability, and to extend the life of the tools. However, if any error is detected on the coated tools, or when the tooling needs to be reused, it is mandatory to remove the existing coating to facilitate reshaping/recoating. The existing coating removal process uses chemical stripping methods, which are not environmentally friendly and not suitable for selective coating removal. In the present work, excimer laser removal of TiAlN from coated WC flat plates has been studied and demonstrated as a viable alternative to existing chemical stripping methods. The ablation thresholds of the TiAlN coating and WC substrate were identified as 1.85 J/cm2 and 2.3 J/cm2 respectively. The paper also presents experimental and theoretical evidence of the process mechanism responsible for laser stripping of TiAlN coating

Numerical simulation of laser machining of carbon-fibre-reinforced composites

The growing use of carbon-fibre-reinforced polymer (CFRP) composites as high-performance lightweight materials in aerospace and automotive industries demands efficient and low-cost machining technologies. The use of laser machining for cutting and drilling composites is attractive owing to its high speed, flexibility, and ease of automation. However, the anisotropic material properties of composites, and issues related to the heat-affected zone (HAZ), charring, and potential delamination during laser processing, are major obstacles in its industrial applications. In order to improve the quality and dimensional accuracy of CFRP laser machining, it is important to understand the mechanism of the transient thermal behaviour and its effect on material removal. Based on the ‘element death’ technique of the finite element (FE) method, a three-dimensional model for simulating the transient temperature field and subsequent material removal has been developed, for the first time, on a heterogeneous fibre—matrix mesh. In addition to the transient temperature field, the model also predicts the dimensions of the HAZ during the laser machining process. Experimental results obtained with same process variables using a 355 nm DPSS Nd:YVO4 laser were used to validate the model. Based on the investigation, the mechanism of material removal in laser composite machining is proposed. The results suggest that the employed FE approach can be used to simulate pulsed laser cutting of fibre-reinforced polymer composites