Coaxial monitoring of keyhole during Yb:YAG laser welding

In laser remote welding using a scanner, high-speed welding can be achieved by using a 6-axial robot and a galvanometric mirror. In this system, because the laser projection point changes depending on the mirror's position, coaxial monitoring is required to track welding phenomena.This paper presents coaxial monitoring of the keyhole generated by an Yb:YAG laser beam during laser lap welding of steel and Al sheets. A coaxial image camera and a coaxial illumination laser are integrated into the proposed monitoring system. The areas of the keyhole and the full penetration hole were calculated by image processing, and their behaviours were investigated under various welding conditions. The keyhole was monitored using various band-pass filters and a coaxial illumination laser. Adequate filters were suggested for steel and Al alloy welding.     

CO2 laser beam modulating for surface texturing machining

Laser texturing is a novel technique that may be used to texture a cold roller in the process of manufacturing high quality steel sheets. With the aim of improving the quality of the textured roller by using a CO₂ laser, a new laser beam modulating device is proposed. An optical beam expander with a fast rotating chopper system is designed. The laser pulse is split into two parts by the chopper blades; one is the preheating pulse that is reflected onto optical loop mirrors; the other is the directly transmitted pulse that creates the craters at the preheated spots. The preheating beam focus spot size and position can be adjusted. The focusing characteristics and optical parameter compensation for the flying optics are investigated. The heat transfer and melt process of laser texturing are numerically simulated. The effects of the double pulses on the texturing are analyzed. The effect of preheating the sample ahead of the laser texturing pulse is examined. The surface profile and bump hardness show improvements by using this approach.     

The use of Taguchi method to optimize the laser welding of sealing neuro-stimulator

Titanium has high strength-to-weight ratio, corrosion resistance, excellent weldability and well biocompatibility. It is applied in various fields, such as medical and aerospace industry. Laser welding is major joint technology for titanium sealing in medical field. It is difficult for sealing thin titanium shell of the neuro-stimulator by laser lap welding. The key point for welding quality is the combination of laser welding parameters. In this paper, the effects of the Nd: YAG laser welding parameters is discussed and analyzed at first, and then a novel application of Taguchi's matrix method is proposed to optimize the selection of laser seal welding thin titanium shell, including the main parameters such as laser power, welding speed, defocusing amount and shield gas, finally the manufacture process for sealing neuro-stimulator is confirmed. The results show that Taguchi method has optimized process parameters of the laser welding on sealing thin titanium shell of the neuro-stimulator, and the device is equipped properly in medical treatments.     

Laser welding of low carbon steel and thermal stress analysis

Laser welding of mild steel sheets is carried out under nitrogen assisting gas ambient. Temperature and stress fields are computed in the welding region through the finite element method. The residual stress developed in the welding region is measured using the XRD technique and the results are compared with the predictions. Optical microscopy and the SEM are used for the metallurgical examination of the welding sites. It is found that von Mises stress attains high values in the cooling cycle after the solidification of the molten regions. The residual stress predicted agreed well with the XRD results.     

Real-time monitoring of laser welding of galvanized high strength steel in lap joint configuration

Two different cases regarding the zinc coating at the lap joint faying surface are selected for studying the influence of zinc vapor on the keyhole dynamics of the weld pool and the final welding quality. One case has the zinc coating fully removed at the faying surface; while the other case retains the zinc coating on the faying surface. It is found that removal of the zinc coating at the faying surface produces a significantly better weld quality as exemplified by a lack of spatters whereas intense spatters are present when the zinc coating is present at the faying surface. Spectroscopy is used to detect the optical spectra emitted from a laser generated plasma plume during the laser welding of galvanized high strength DP980 steel in a lap-joint configuration. A correlation between the electron temperature and defects within the weld bead is identified by using the Boltzmann plot method. The laser weld pool keyhole dynamic behavior affected by a high-pressure zinc vapor generated at the faying surface of galvanized steel lap-joint is monitored in real-time by a high speed charge-coupled device (CCD) camera assisted with a green laser as an illumination source.     

Analysis of a fibre laser welding case study, utilising a matrix flow chart

For fibre laser welding of an eccentric corner joint, the quality of the resulting weld cross section was studied with respect to the dependence on process parameters like lateral laser beam alignment, beam inclination, focal plane position or welding speed. The complex load situation of the support beamer was simplified to bending of one corner. Due to fatigue load, the weld properties causing the peak stress are essential, in particular the top and root shape of the weld cross section. For the parameters varied, the resulting shapes were categorized into different top and root classes, determined by certain key dimensions, considering also welding defects like undercuts. The shapes are boundary conditions for Finite Element Analysis of the joint under load for quantitative comparative analysis of the maximum stress. As two high strength steel grades were joined, the hardness transition across the weld was of interest, too. High speed imaging of the weld pool surface shape provided additional information on the relation between the parameter input and quality output. The different trends identified were discussed and guidelines were derived. As the systematic documentation of results is unsatisfactory in welding, a new method was developed and applied for the first time, called the Matrix Flow Chart. It enables an illustrative view on the resulting welding trends in a combined manner and is extendable by other researchers.     

Dissimilar material joining using laser (aluminum to steel using zinc-based filler wire)

Joining steel with aluminum involving the fusion of one or both materials is possible by laser beam welding technique. This paper describes a method, called laser braze welding, which is a suitable process to realize this structure. The main problem with thermal joining of steel/aluminum assembly with processes such as TIG or MIG is the formation of fragile intermetallic phases, which are detrimental to the mechanical performances of such joints. Braze welding permits a localized fusion of the materials resulting in a limitation on the growth of fragile phases. This article presents the results of a statistical approach for an overlap assembly configuration using a filler wire composed of 85% Zn and 15% Al. Tensile tests carried on these assemblies demonstrate a good performance of the joints. The fracture mechanisms of the joints are analyzed by a detailed characterization of the seams.     

Modelling the spot shape influence on high-speed transmission lap welding of thermoplastics films

Modelling high-speed laser lap welding of thermoplastic films has been accomplished and the influence of laser beam spot shape, dimensions, and position relative to sample displacement was analysed.Engineering parameters predicted by the model were applied to lap weld of high- and low-density polyethylene transparent samples with thickness between 10 and 100 μm, and experimentally validated. Experimental set-up allowed reaching welding constant linear velocities up to 10 m/s.Theoretical and experimental data show coincidence. Weld strength increases for larger beam spot diameters, and elliptical beam spots increase weld efficiency, allowing higher processing speeds or decreasing required laser power. An angular deviation of elliptical beam spot with regard to the sample's movement direction causes an increase of weld strength and a decrease of welding speed.     

Spectroscopic, energetic and metallographic investigations of the laser lap welding of AISI 304 using the response surface methodology

Spectroscopic signals originated by the laser-induced plasma optical emission have been simultaneously investigated together with energetic and metallographic analyses of CO₂ laser welded stainless steel lap joint, using the Response Surface Methodology. This statistical approach allowed us to study the influence of the laser beam power and the laser welding speed on the following response parameters: plasma plume electron temperature, joint penetration depth and melted area. A clear correlation has been found between all the investigated response parameters. The results have been shown to be consistent with quantitative considerations on the energy supplied to the workpiece as far as the laser power and travel speed were varied. The regression model obtained in this way could be a valuable starting point to develop a closed loop control of the weld penetration depth and the melted area in the investigated process window.     

Prediction of weld strength and seam width for laser transmission welding of thermoplastic using response surface methodology

In the present work, a study is made to investigate the effects of process parameters, namely, laser power, welding speed, size of the laser beam and clamp pressure, on the lap-shear strength and weld-seam width for laser transmission welding of acrylic (polymethyl methacrylate), using a diode laser system. Response surface methodology (RSM) is employed to develop mathematical relationships between the welding process parameters and the output variables of the weld joint to determine the welding input parameters that lead to the desired weld quality. In addition, using response surface plots, the interaction effects of process parameters on the responses are analyzed and discussed. The statistical software Design-Expert v7 is used to establish the design matrix and to obtain the regression equations. The developed mathematical models are tested by analysis-of-variance (ANOVA) method to check their adequacy. Finally, a comparison is made between measured and calculated results, which are in good agreement. This indicates that the developed models can predict the responses adequately within the limits of welding parameters being used.     

Optimization of laser welding of DP/TRIP steel sheets using statistical approach

Generally, the quality of a weld joint is directly influenced by the welding input parameter settings. Selection of proper process parameters is important to obtain the desired weld bead profile and quality. In this research work, numerical and graphical optimization techniques of the CO₂ laser beam welding of dual phase (DP600)/transformation induced plasticity (TRIP700) steel sheets were carried out using response surface methodology (RSM) based on Box–Behnken design. The procedure was established to improve the weld quality, increase the productivity and minimize the total operation cost by considering the welding parameters range of laser power (2–2.2 kW), welding speed (40–50 mm/s) and focus position (?1 to 0 mm). It was found that, RSM can be considered as a powerful tool in experimental welding optimization, even when the experimenter does not have a model for the process. Strong, efficient and low cost weld joints could be achieved using the optimum welding conditions.     

ESTIMATION OF THERMAL CONTACT CONDUCTANCE DURING RESISTANCE SPOT WELDING

This article describes an experimental procedure conducted to estimate and investigate the transient thermal contact conductance (or thermal contact resistance) between the electrodes and workpieces during resistance spot welding. A fine thermal metrology was developed to collect thermal histories near the welding region. Indeed, the electrode tip was instrumented with several interior microthermocouples for measuring the transient temperature response during the welding process. A simple mathematical model, using an inverse heat transfer method, was built for the estimation of the transient heat transfer coefficient from interior transient temperature measurements. A simple resistance welding case of two steel sheets was investigated. The initial transient values of thermal contact conductance were found to be in agreement with those observed in the dry copper–steel solid contact case. At the end of the process, the transient heat transfer coefficient reaches a high value corresponding to the best heat transfer phenomenon at the interface during the welding process. When the metal is melted, the contact quality increases due to the high-applied electrode force. Higher electrode force and heating temperatures produce lower thermal contact resistance. The results obtained show the capabilities and the power of the coupled thermal metrology and transient inverse technique developed to investigate thermal history of resistance spot welding.     

A study of the shielding gas influence on the laser beam welding of AA5083 aluminium alloys by in-process spectroscopic investigation

In laser welding, the shielding gas is commonly used to stabilize the welding process, to improve welded joints features and to protect the welded seam against oxidization. Besides the type of shielding gas used, the nozzle parameters play an essential role. In fact, the chemical composition of the shielding gas and the flow geometry are key factors limiting the size of the plasma plume and its contamination by the surrounding atmosphere, and affecting the final quality of the welded joints.In this work, we present an experimental study on the complex physical phenomena occurring during the interaction between the plasma plume, the laser beam and the shielding gas by using an in-process spectroscopic investigation of the plasma plume characteristics under different operating conditions. A correlation was found between the spectral features and the formation of oxide layers on the surface of the welding seam, caused by defective gas shielding and by the vaporization of alloying elements. Experimental results have given useful indications for the development of innovative welding nozzle for application in laser welding of aluminium alloys.     

Statistical modeling of laser welding of DP/TRIP steel sheets

In this research work, a statistical analysis of the CO₂ laser beam welding of dual phase (DP600)/transformation induced plasticity (TRIP700) steel sheets was done using response surface methodology. The analysis considered the effect of laser power (2–2.2 kW), welding speed (40–50 mm/s) and focus position (?1 to 0 mm) on the heat input, the weld bead geometry, uniaxial tensile strength, formability limited dome height and welding operation cost. The experimental design was based on Box–Behnken design using linear and quadratic polynomial equations for predicting the mathematical models. The results indicate that the proposed models predict the responses adequately within the limits of welding parameters being used and the welding speed is the most significant parameter during the welding process.     

Power semiconductor laser diode arrays characterization

Nowadays, power semiconductor laser diode arrays are becoming a widespread source for a large variety of industrial applications. In particular, the availability of low-cost high-power laser diode arrays makes their use possible in the industrial context for material cutting, welding, diagnostics and processing. In the above applications, the exact control of the beam quality plays a very important role because it directly affects the reliability of the final result. In this paper, we present two different approaches useful for the characterization of the beam quality in laser diode arrays. The first one, starting from total intensity measurements on planes orthogonal to the beam propagation path, is able to deduce the working conditions of each laser setting up the array. The second one is aimed at the measurement of a global quality factor of the array itself; to this end, the empirical extension of the M² concept to composite beams is presented along with some experimental results. As the first technique is especially intended for the non-destructive detection of design problems in the array itself and in the bias circuitry, the second one represents a powerful tool for the rapid on-line diagnostics of the laser beam during its use.     

The influence of shielding gas in hybrid LASER-MIG welding

Hybrid LASER-GMAW welding technique has been recently studied and developed in order to meet the needs of modern welding industries. The two sources involved in this process play, in fact, a complementary role: fast welding speed, deep bead penetration and high energy concentration can be achieved through the LASER beam, while gap bridgeability and cost-effectiveness are typical of the GMAW process.Particularly interesting, in this context, is the CO₂ LASER-MIG welding which differs from the Nd:YAG LASER-MIG technique for the high powers that can be exploited and for the good power/cost ratio of the process.This paper is a part of a wide study on the hybrid CO₂ LASER-MIG welding and investigates the influence of the shielding gas both on the stability of the process and on the dimensional characteristics of the weld bead. Two different parameters have been taken into consideration in order to develop this analysis: the shielding gas composition and the shielding gas flow.The experiment, performed on AISI 304 stainless steel plates, has been planned exploiting design of experiment techniques. The results have been analyzed through a statistical approach in order to determine the real influence of each parameter on the overall process.     

Parametric optimisation and microstructural analysis on high power Yb-fibre laser welding of Ti–6Al–4V

In this work thin sheets of Ti–6Al–4V were full penetration welded using a 5 kW fibre laser in order to evaluate the effectiveness of high power fibre laser as a welding processing tool for welding Ti–6Al–4V with the requirements of the aircraft industry and to determine the effect of welding parameters including laser power, welding speed and beam focal position on the weld microstructure, bead profile and weld quality. It involved establishing an understanding of the influence of welding parameters on microstructural change, welding defects, and the characteristics of heat affected zone (HAZ) and weld metal (WM) of fibre laser welded joints. The optimum range of welding parameters which produced welds without cracking and porosity were identified. The influence of the welding parameters on the weld joint heterogeneity was characterised by conducting detailed microstructural analysis.     

New solid-state lasers and their application potentials

In recent years, Nd:YAG-lasers have found increasing interest in many fields of high-power applications that formerly had been the domain of CO₂-lasers. This was mainly due to several consequences of their wavelength, such as a higher absorptivity, lower sensitivity against laser-induced plasmas and, in particular, the use of flexible glass fibres for beam handling. Disadvantages like poor beam quality and low efficiency are being effectively reduced by recent developments of diode-pumped systems. Some promising concepts based on different pumping techniques and crystal geometries — rods, discs, fibres — will be discussed in view of attainable beam quality and means of power scaling. The second part of the paper will deal with investigations aimed at utilizing the beneficial properties of Nd:YAG-lasers, especially for welding. In particular, the advantages of the twin-focus technique are discussed in some detail with regard to power scaling, process improvements and flexibility increase. Based upon experience, the extension to a multi-focus technique is proposed by presenting experimental data obtained with lamp-pumped high-power lasers and results of numerical modelling. This evidence demonstrates the potential for industrial applications and provides an idea of what can be expected from the new generation of diode-pumped solid-state lasers with high beam quality.     

Pulsed Nd:YAG laser welding of AISI 304 to AISI 420 stainless steels

The technique to weld AISI 304 stainless steel to AISI 420 stainless steel with a pulsed Nd:YAG laser has been investigated. The main objective of this study was to determine the influence of the laser beam position, with respect to the joint, on weld characteristics. Specimens were welded with the laser beam incident on the joint and moved 0.1 and 0.2 mm on either side of the joint. The joints were examined in an optical microscope for cracks, pores and to determine the weld geometry. The microstructure of the weld and the heat affected zones were observed in a scanning electron microscope. An energy dispersive spectrometer, coupled to the scanning electron microscope, was used to determine variations in (weight %) the main chemical elements across the fillet weld. Vickers microhardness testing and tensile testing were carried out to determine the mechanical properties of the weld. The results of the various tests and examinations enabled definition of the best position for the incident laser beam with respect to the joint, for welding together the two stainless steels.     

Research on laser welding of high-strength galvanized automobile steel sheets

The paper presented some useful results of deep-penetration laser welding of high-strength galvanized steel sheets, which had been carried out by a self-made CO₂ laser unit with maximum power output of 1.5 kW. The workpieces of high-strength galvanized automobile steels with thickness of 1.5 mm were butt-welded with argon as the shielding gas. The effects of such factors as laser power, welding speed, focal position, shielding gas and zinc vaporization on the quality of welds are investigated. With the processing parameters optimized and the proper shielding gas used in both coaxial and side-blow direction, most of the defects, such as pores, cracks and softening in HAZ, can be avoided in laser welding joints. The microstructure, the hardness distribution and the elemental distribution in the welding joints can be changed due to laser heating and recrystallization. In order to determine the mechanical properties of the welding joints, the static tensile strength was tested. Experimental results indicated that both the strength and microhardness of welding joints were higher than those of the base metal. Consequently, the welding quality is reliable for manufacturing of automobile bodies.