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

Physical mechanisms controlling keyhole and melt pool dynamics during laser welding

The aim of this chapter is to review our main recent understanding of physical mechanisms occurring during keyhole laser welding. The focus is on the analysis of melt pool dynamics showing that it is the interaction of the vapour plume emitted from the keyhole front with the melt pool that plays a dominant role in melt pool dynamics. Different specific regimes concerning the behaviour of the melt pool for a large range of welding speeds can be observed, and these regimes are precisely defined by the inclination of the keyhole front. a model taking into account these physical processes is proposed and allows us to describe the keyhole parameters and to understand these different observed results

Influence of ultrasound on pore and crack formation in laser beam welding of nickel-base alloy round bars

Welding by laser beam is a method for creating deep and narrow welds with low influence on the surrounding material. Nevertheless, the microstructure and mechanical properties change, and highly alloyed materials are prone to segregation. A new promising approach for minimizing segregation and its effects like hot cracks is introducing ultrasonic excitation into the specimen. The following investigations are about the effects of different ultrasonic amplitudes (2/4/6 µm) and different positions of the weld pool in the resonant vibration distribution (antinode, centered, and node position) for bead on plate welds on 2.4856 nickel alloy round bars (30 mm diameter) with a laser beam power of 6 kW. The weld is evaluated by visual inspection and metallographic cross sections. The experiments reveal specific mechanisms of interaction between melt and different positions regarding to the vibration shape, which influence weld shape, microstructure, segregation, cracks and pores. Welding with ultrasonic excitation in antinode position improves the welding results

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