Welding Processes

Despite the wide availability of literature on welding processes, a need exists to regularly update the engineering community on advancements in joining techniques of similar and dissimilar materials, in their numerical modeling, as well as in their sensing and control. In response to InTech's request to provide undergraduate and graduate students, welding engineers, and researchers with updates on recent achievements in welding, a group of 34 authors and co-authors from 14 countries representing five continents have joined to co-author this book on welding processes, free of charge to the reader. This book is divided into four sections: Laser Welding; Numerical Modeling of Welding Processes; Sensing of Welding Processes; and General Topics in Welding

Developments in Nd :YAG laser welding

Laser welding in keyhole (KH) mode using solid-state laser emitting in near infrared at nearly 1 micron wavelength is discussed in this chapter. The main physical processes involved in laser welding are presented and the reasons for using this laser wavelength are shown. Section 3.2 describes the KH geometry and related physical mechanisms controlling its stability. The role of the main operating parameters is also presented. Section 3.3 shows examples of the evolution of keyhole and weld pool behaviour for various welding speeds illustrating the mechanisms discussed in Section 3.2. Finally in the conclusion, expected diagnostics improvements necessary for supporting adapted numerical simulations of this laser welding process are discussed

Simulation-Oriented Methodology for Distortion Minimisation during Laser Beam Welding

Distortion is one of the drawbacks of any welding process, most of the time needed to be suppressed. One doubtful factor that could affect welding deformation is the shape of the liquid melt pool, which can be modified via variation of process parameters. The aim of this work was to numerically study the dynamics of the weld pool and its geometrical influence on welding distortion during laser beam welding. To achieve such a goal, a promising novel process simulation model, employed in investigating the keyhole and weld pool dynamics, has successfully been invented. The model incorporated all distinctive behaviours of the laser beam welding process. Moreover, identification of the correlation between the weld pool geometry and welding distortion as well as, eventually, weld pool shapes that favour distortion minimisation has also been simulatively demonstrated

Simulation-Oriented Methodology for Distortion Minimisation during Laser Beam Welding

Distortion is one of the drawbacks of any welding process, most of the time needed to be suppressed. One doubtful factor that could affect welding deformation is the shape of the liquid melt pool, which can be modified via variation of process parameters. The aim of this work was to numerically study the dynamics of the weld pool and its geometrical influence on welding distortion during laser beam welding. To achieve such a goal, a promising novel process simulation model, employed in investigating the keyhole and weld pool dynamics, has successfully been invented. The model incorporated all distinctive behaviours of the laser beam welding process. Moreover, identification of the correlation between the weld pool geometry and welding distortion as well as, eventually, weld pool shapes that favour distortion minimisation has also been simulatively demonstrated

Experimental Study and Numerical Simulation of Heat Transfer and Fluid Flow in Laser Welded and Brazed Joints

Laser joining with advantages of high power density and high processing speed is becoming a dominant process for joining parts of the body in white (BIW) in automotive manufacturing. Aluminum alloys and new generations of advanced high strength steels (AHSS) are of great value for the automotive industry to build light weight, environmentally-friendly, high-quality, and durable vehicles. Their usage in body structure is increasing due to high strength-to-weight ratio and good formability. Lap and coach-peel joints are the most commonly used type of joints in assembly of the body components manufactured with each of these two alloys. Laser brazing is a widely used process for joining closure panels in automotive manufacturing exemplified by joints such as the upper to lower panels of a liftgate or the roof to body side outer panels. Laser brazed seams are in visible areas and require a high quality surface and seam characteristics. Therefore, in this study novel techniques were studied to develop a robust welding and brazing processes of similar and dissimilar materials. Experimental studies as well as the numerical modeling and high-speed imaging approaches were used to gain a deeper understanding of the laser welding-brazing process, determine the effect of process parameters on weld dimensions, and analyze the dynamics of possible imperfections during the process. In dissimilar application, a feasibility study was conducted on laser joining of aluminum alloy to galvanized steel by means of twin-spot laser beams. Twin-spot mode was introduced to heat up a large surface area with less reduction in energy density for coach-peel joints with a wider geometry. The filler material was brazed on the steel side and partially fusion-welded on the aluminum side. The brazed results were investigated from the perspectives of microstructure evolution, tensile strength, surface roughness, edge straightness, and fracture mechanism. The generation of intermetallic compound (IMC) at the steel/seam interface was optimized by introducing a validated finite element thermal model to obtain the temperature history during the process and predict the thickness of a possible IMC layer. A multi-response optimization approach based on response surface methodology (RSM) was developed to find the fit model that correlated the main process parameters (laser power, wire feed speed, and scanning speed) and their interactions to surface roughness and mechanical strength. Under optimum processing condition the effects of alloying elements were also investigated on the performance of resultant joints. Different percentages of Si, Mn and Zn were introduced into the weld through three Al-based (AlSi12, AlSi5, and AlSi3Mn) and one Zn-based (ZnAl15) filler wires. Joint mechanical properties were examined in terms of monotonic loading circumstance. Microstructural properties were evaluated in terms of the IMC layer thickness and composition. In laser brazing of galvanized steels, the effect of laser beam inclination angle was investigated on process stability and spatter occurrence. Steel outer panels in automotive application are zinc coated for improved corrosion protection; however, the existence of low boiling element in coating has made the laser brazing process more challenging. Zinc has a boiling point of 907 °C which is lower than the melting range of copper-based filler wire, 965 – 1032 °C and as such is the predominant cause of laser brazing process instability and spattering for zinc coated steels. Therefore, experimental and numerical methods were applied to investigate the effect of laser beam inclination angle on laser braze quality of galvanized steels. High-speed videography revealed that spatter mostly occurred at the wetting line and melt pool front where the escaping zinc vapor came into interaction with the melt material. Application of a developed thermo-fluid simulation model considering laser-material interaction, wetting dynamics, material melting, and solidification, resulted in temperature profiles during the brazing processes for given beam angles as well as both the positions of the zinc evaporation front and wetting front. It was found that negative travel angles helped to move the zinc evaporation front ahead of the wetting front and reduce the interaction between the zinc vapor and melt pool. Experimental observations confirmed that partially removing and/or evaporating the zinc layer ahead of the wetting zone contributed to a stable process and good braze quality

A multi-physics CFD study to investigate the impact of laser beam shaping on metal mixing and molten pool dynamics during laser welding of copper to steel for battery terminal-to-casing connections

This study aims to investigate the impact of laser beam shaping on metal mixing and molten pool dynamics during laser beam welding of Cu-to-steel for battery terminal-to-casing connections. Four beam shapes were tested during LBW of 300 µm Cu to 300 µm nickel-plated steel. Both experiments and simulations were used to study the underlying physics. A CFD model was firstly calibrated against experiments and then deployed to explore the effect of the increasing ring-to-core diameter, as well as a tandem laser spot configuration. The study showed that metal mixing is influenced by the keyhole dynamics and collapse events, but also there is an intricate interplay between keyhole geometry, fluid dynamics via Marangoni forces and buoyancy forces. Notably, the buoyance forces due to the different densities of steel and Cu, along with the recoil pressure contribute to the upward flow of steel towards Cu, and hence impact meaningfully the material mixing. The study pointed-out that the selection of a custom ring-to-core diameter and ring-to-core power is a decision with a trade-off between the need of stabilising the keyhole dynamics and the need to reduce the mixing. Findings indicated that 350 µm ring and 90 µm core with 30% of ring power (weld configuration C3) resulted in more stable dynamics of the keyhole, with significant reduction of collapse events, and ultimately controlled migration of steel towards Cu. Additionally, the pre-heating approach with the tandem beam only led to local fusion of Cu and no significant improvement in keyhole stability was observed

A physics-driven model for the closed-loop quality control of remote laser welding

Remote Laser Welding (RLW) has grown in importance over conventional joining methods such as Gas Metal Arc Welding (GMAW), Resistance Spot Welding (RSW), Self-Pierce Riveting (SPR) since it offers advantages, such as weight reduction, high processing speed, ability to weld a wide range of metals, and better weld quality. Despite such advantages, it also poses several challenges that have prevented its widespread implementation in the industry. The presented thesis deals with the RLW of galvanized steel (i.e. zinc-coated steel) since it is widely used in the automotive industry due to better resistance to corrosion and better adhesion of the paint to the surface. However, RLW of such steel is challenging because the zinc vapour disturbs the molten pool resulting in weld defects. Therefore, RLW of galvanized steel is performed in overlap configuration with a joining gap to ventilate the zinc vapour from the welding area. An important challenge faced during the laser welding of galvanized steels is to achieve a consistent joining gap between two metals. If the gap is too wide, two metals do not join together. If the gap is too narrow, welding takes places with defects such as explosions, spatters and porosities. The maximum joining gap is controlled by the welding fixture; whereas, the minimum joining gap is controlled by the laser dimpling process (i.e. an upstream process). In the literature, the following research gaps have been identified regarding the laser dimpling process. These gaps are as follows: (i) lack key performance indicators to determine the dimple quality, (ii) lack a comprehensive characterization of dimpling process considering multi-inputs (i.e. key control characteristics) and multi-outputs (i.e. key performance indicators), and (iii) an effective implementation in a real manufacturing system taking into consideration process variation. Overcoming the aforementioned limitations in the literature, the presented thesis introduces proposes methodologies to develop: (i) surrogate models for dimpling process characterization considering multi-inputs and multi-outputs system by conducting physical experimentation, (ii) process capability spaces based on the developed surrogate models that allows the estimation of a desired process fallout rate in the case of violation of process requirements, and (iii) the optimization of the process parameters based on the developed process capability spaces. The weld quality is measured by key performance indicators defined in industrial standards (EN ISO 13919-1, 1997; EN ISO 13919-2, 2001). The weld must be produced such that each key performance indicator meets its defined allowable limits and any deviation from these limits is considered as a weld defect. The weld profile is important because the weld should have a desired profile for achieving the maximum strength. In this thesis, the weld profile is determined by penetration, top width, interface width (i.e. fusion zone dimensions). It must be pointed out that the presented fusion zone dimensions are difficult to measure directly during the welding process unless production is stopped which is nearly unfeasible as it is economically unjustified; whereas, it can be monitored by process signals (e.g. autistic, optical, thermal). Today, in-process monitoring is often provided by photodiodes or cameras. Owing to the lack of understanding of the process, it is limited to empirical correlations between the appearance of a weld defect and signal changes. The lack of methods linking (i) in-process monitoring data (e.g. visual sensing, acoustic and optical emissions); with, (ii) multi fusion zone dimensions (e.g. penetration, interface width, etc.), and (iii) welding process parameters (e.g. laser power, welding speed, focal point position) underscores the limitations of current data-driven in-process monitoring methods. Furthermore, the current in-process monitoring methods is an indirect measurement of fusion zone dimensions. Therefore, an accurate model to perform non-destructive measurement of fusion zone dimension is essential for on-line monitoring of laser welding as a part of quality assurance. Based on this requirement, the occurring physics in the laser welding process are decoupled by sequential modelling. It consists of three steps as follows: (i) calculating the laser intensity acting on the material, (ii) calculating the keyhole profile in using an analytic method, and (iii) solving the heat equation using the FEM to calculate the temperature distribution. After obtaining the temperature distribution, the fusion zone profile is defined by selecting an isotherm. Then, the aforementioned fusion zone dimensions (i.e. Penetration, Top Width, Interface Width) are measured from the calculated the fusion zone profile according to the industrial standard

Laser welding of dissimilar carbon steel to stainless steel 316L

Laser welding of metals and alloys is extensively used in industry due to its advantages of controlled heating, narrow weld bead, low heat affected zone (HAZ) and its ability to weld a wide range of metals and dissimilar metals. Laser welding of dissimilar metals such as carbon steels and stainless steel is still a challenging task, particularly due to the formation of brittle phases in the weld, martensitic formation in the HAZ and solidification cracking in the fusion zone. These issues can significantly deteriorate the strength of the welded joint. The aim of this work is to investigate the fundamental phenomena that occur inside the dissimilar weld zone and their effect on weld quality. In order to establish the key process variables, an initial study concentrated on the effect of different laser process parameters on dissimilar weld quality. In the second part of the work, a comprehensive study was performed to understand and subsequently control the alloying composition in laser dissimilar welding of austenitic stainless steel and low carbon steel. A dissimilar weld that is predominantly austenitic and homogeneous was obtained by controlling the melt pool dynamics through specific point energy and beam alignment. The significance of dilution and alloying elements on joint strength was established. A coupled CFD and FEM numerical model was developed to assist in understanding the melt pool dynamics and transportation processes of alloying elements. The model has been validated by a series of laser welding experiments using various levels of specific point energy. The laser welding characteristics in terms of geometric dimensions, surface morphology, alloying concentration, and dilution, were compared, and it is concluded that the specific point energy and laser beam position are the key parameters that can be controlled to obtain a weld bead with characteristics most suitable for industrial applications. In the third part of the work, a comparative study was performed to understand the significance of cooling rate, and alloying composition on the microstructure and phase structure of the dissimilar weld zone. Results show that the HAZ within the high carbon steel has significantly higher hardness than the weld area, which severely undermines the weld quality. A new heat treatment strategy was proposed based on the results of the numerical simulation, and it is shown to control the brittle phase formation in HAZ of high carbon steel. A series of experiments was performed to verify the developed thermo-metallurgical FEA model and a good qualitative agreement of the predicted martensitic phase distribution is shown to exist. Although this work is presented in the context of dissimilar laser welding of mild steel to stainless steel, the concept is applicable to any dissimilar fusion welding process