Fundamentals and Process Simulation (LiM 2021)

Numerical study of the bulging effect in deep penetration laser beam welding
Marcel Bachmann, Antoni Artinov, Xiangmeng Meng, Michael Rethmeier

This article is devoted to the study of the bulging effect in deep penetration laser beam welding. The numerical results of the investigations are based upon experimental results from previous studies to reveal the relationship between the bulging effect and the hot cracking formation, as well as the mixing of alloying elements in the weld pool. The widening of the molten pool in its center area can be observed in full penetration as well as in partial penetration welds on 8 mm and 12 mm thick structural steel plates, respectively. The weld pool shape is extracted from the simulations to evaluate the extent of the necking of the solidification line as well as the bulging phenomena and its influence on the hot cracking phenomena. Relying on an earlier numerical study utilizing a fixed keyhole, simulation models considering a dynamic keyhole are developed thereto. Additionally, the mixing behavior of alloying elements during partial penetration is investigated. The link between the bulge and the studied phenomena is found to be significant.

Keywords: deep penetration laser beam welding ; welding simulation ; solidification cracking ; bulging effect


Numerical simulation of power control in laser-assisted metal-polymer joining
Klaus Schricker, Jean Pierre Bergmann

Laser-assisted joining enables a direct connection between polymers and metals without using additional elements (e.g. screws, rivets) or adhesives. The process is well known in terms of surface pretreatment, achievable mechanical properties and materials. However, the quality of the joint is affected by varying manufacturing conditions, e.g. heat accumulation at edges, heating of the clamping device or different material batches. The article is dedicated to power control in laser-based joining of polymers with metals for this reason. A PID controller was integrated to control the beam power of a diode laser as a function of temperature based on a transient thermal model. The investigations were carried out on polypropylene in combination with high-alloy steel AISI 304. A comparison of surface/interface temperatures, controlled/uncontrolled processes and the introduction of disturbances allow conclusions on process control and on implementation in real production processes.

Keywords: Joining; System Technology and Process Control; Fundamentals and Process Simulation; Metal-Polymer Joining; Metal-Plastic joining


Estimating heat accumulation upon ultrafast laser irradiation
Liliana Cangueiro, Thomas Kiedrowski, Nikolai Schroeder, David Bruneel, Andrés Fabián Lasagni, J. A. Ramos-de-Campos

Ultrafast lasers micromachining results depend on both the processing parameters and the material properties. The obtained thermal effects are negligible if a good combination of processing parameters is chosen. However, optimizing the processing parameters leading to the required surface quality on a given material can be quite complex and time consuming. Within the framework of the European project LAMpAS, we developed a model to estimate the heat accumulation on a surface as a function of the laser fluence, scanning speed and line pitch. The simulation results were correlated with experimental ones on different materials. The predictions of the model allow evaluating the heat distribution on the surface, as well as optimizing the ultrafast laser micromachining strategy yielding negligible thermal damage.

Keywords: Ultrafast laser ablation; thermal effects; simulation


Experimental setup for determination of absorption coefficient of laser radiation in molten metals as a function of temperature and angle
Tjorben Bokelmann, Marius Lammers, Jörg Hermsdorf, Sobhan Emadmostoufi, Oleg Mokrov, Rahul Sharma, Uwe Reisgen, Stefan Kaierle

For the process development of laser assisted double wire welding with nontransferred arc (LDNA), the simulation of the molten pool and its interaction with the laser radiation is of great importance. Therefore, an experimental setup for the determination of the temperature and angle dependent absorption coefficient of laser radiation in molten metals such as stainless steel will be presented. An Yb:YAG disc laser with 1030 nm is used as laser beam source. The stationary molten metal is inductively warmed and superheated by the laser beam with approximately 300-1000 W, whose radiation is shaped by homogenizing optics and ensures equal intensity when angle is adjusted.

Keywords: molten metal; absorption coefficient; laser beam; LDNA; temperature; angle


Determination of the beam position in laser deep penetration welding using coaxially acquired images of the keyhole front and machine learning
Pablo Dilger, Carola Forster, Elias Klein, Silvana Burger, Eric Eschner, Michael Schmidt

The joining technology of laser beam welding offers high flexibility and productivity. However, the small laser beam fo-cus demands dependable quality assurance to ensure a sufficient connection of the parts. In keyhole welding of metal sheets in butt joint configuration, a gap is visible at the keyhole front, which correlates with the leading joint position. This process feature can be used for quality control by arranging a high-speed camera coaxially to the laser beam to monitor the keyhole. Here, we present a machine learning approach for a robust determination of the beam position relative to the joint based on the keyhole front morphology. For this purpose, we conducted a series of experiments to produce a set of labeled images, which are used to train a convolutional neural network. After training on the data, the network can predict the position of the keyhole front gap, setting the foundation for a quality control system.

Keywords: laser deep penetration welding; machine learning; image processing; quality assurance


Corrosion resistant blackmarking via numerical modeling and simulation
Urs Eppelt, Jörg Ziegler, Daniel Seitz

Classical Laser Blackmarking with USP laser sources is a well-established process that is commercially available nowadays since a few years. This is especially true for its wide use on stainless steel supplies of the medical industry. Surprisingly, other industries (like household and consumer products industries) have even higher requirements on wear behavior (like corrosion resistance) than the medical industry with its strict approval procedures. With the help of mathematical modeling and numerical simulation we wanted to understand the in-depth reasons for the limitations of classical blackmarking and develop a new laser marking process that could also fulfill the demands for acid resistance on products like white goods, sanitary fittings or automotive accessories which have not yet been opened up for this kind of laser process. We describe our success in modeling and simulation as well as process development in that field which is then evaluated by corrosion testing procedures.

Keywords: modeling; simulation; blackmarking; corrosion-resistance


Experimental and numerical analysis of local gas supplies for spatter reduced high speed laser beam welding
Leander Schmidt, Klaus Schricker, Jean Pierre Bergmann

Spatter formation is a major issue in deep penetration welding with solid-state lasers at high welding speeds. The use of local gas supplied near to the keyhole proved to be very effective to avoid spatter formation. This publication examines the flow conditions and pressure field of a local supply of argon/helium by welding stainless steel (X5CrNi18-10/AISI304) at welding speeds beyond 8 m/min to get a deeper understanding of the acting mechanisms. By varying the flow rate, the flow field characteristics were visualized by Schlieren imaging and quantified by Schlieren image velocimetry (flow pattern, flow velocity). In order to specify the resulting pressure field, a computational fluid dynamics analysis have been performed based on a k-ω-SST multi component turbulence model. By combining the experimental und numerical findings, it was possible to derive a comprehensive model representation of the fundamental effect mechanisms.

Keywords: welding; macro processing; spatter formation; low-spatter welding; effect of local gas supply


Cavitation phenomena in BA-LIFT
Juan José Moreno Labella, Miguel Morales Furió, David Muñoz Martín, Carlos Molpeceres Álvarez

In Blister-Actuated Laser-Induced Forward Transfer (BA-LIFT) the direct interaction between the laser pulse and the transference fluid is removed interposing an intermediate polyimide layer that absorbs and pushes away the fluid. A study of the transference mechanisms has been made through a Phase Field model in COMSOL Multiphysics. The differences between the experimental images and the simulations led to the suggestion of a cavitation bubble. Contrary to the LIFT process, this bubble cannot be laser-induced, as there is no direct interaction owing to the polyimide layer, so it must be mechanically induced.
In this work, some shadowgraphy images of the cavitation bubble along with its effects on the Phase Field FEM-CFD model are shown. In the numerical model, the expansion of the main jet –including the observed secondary effects– can be reproduced. Including a second push 9 μs after the blister expansion makes the fluid take the secondary effects’ shapes. Considering a cavitation bubble like in other LIFT processes –in which the bubble has been generated directly by the laser pulse–, there are three possible causes of its appearance: absorption of the laser pulse in the fluid –through the polyimide intermediate layer–, thermal evaporation due to heat conduction, or pressure fall due to fluid velocity. Besides, a mechanical rebound of the elastically deformed blister has also been considered, as the effects in the model suggest. After the analysis, the only explanations that cannot be rejected –the depressurization area in front of the blister– led to the proposed hypothesis: the velocity field from the blister expansion causes a cavitation bubble in front of itself, whose effects are equivalent to the cavitation of the vapor bubble described in other LIFT techniques.

Keywords: Laser-Induced Forward Transfer (LIFT); Blister-Assisted Laser-Induced Forward Transfer (BA-LIFT); Laser process modeling; Fluid dynamics;


Developing process parameters through CFD simulations
Pareekshith Allu

Laser-material interaction is complex, and to accurately simulate it requires implementing the physics models that are relevant at these temporal and spatial scales. Process parameters such as laser power, scanning velocity, geometric scanning path, pre-heating temperature and powder size distribution influence the melt pool dynamics, which play a role in the stability of the additive manufacturing process. In this presentation, we will look at underlying mechanisms behind the formation of defects such as balling, porosity and spatter using computational thermal-fluid dynamics models built in FLOW-3D AM. While low energy densities can lead to lack of fusion defects, high energy densities result in strong recoil pressure and unstable keyholes that can lead to the formation of porosity and spatter. In addition to helping with process parameter development for both LPBF and DED processes, such models also output thermal gradient and cooling rate data that can be used to predict microstructure evolution.

Keywords: CFD simulations; laser powder bed fusion process; FLOW-3D; melt pool dynamics; direct energy deposition


Predicting qualitative and quantitative properties of metal powders for additive generative manufacturing using hyperspectral imaging and machine learning
Florian Gruber, Christoph Wilsnack, Axel Marquardt, Julius Hendl, Sebastian Witte, Martin Schäfer, Karol Kozak, Wulf Grählert, Lukas Stepien

Quality control of metal powders in powder bed-based additive manufacturing processes is currently very time-consuming and costly and is not possible inline, i.e. during the process. The powder properties are of crucial importance for the quality of the components produced. A fast and complete inline characterization of metal powders is therefore desirable. In this work, a method for powder quality control based on hyperspectral imaging and machine learning is presented. The aim is to qualitatively distinguish different powder types and powder charges based on hyperspectral measurements, and to quantitatively predict some powder properties (size distribution and sphericity). The determination of powder type and charge was possible for the investigated samples with an accuracy of up to 100 %, and good results were also achieved for the quantitative prediction of powder properties. The results show that hyperspectral imaging appears to be a promising method for inline powder characterization for additive manufacturing processes.

Keywords: additive manufacturing; powder characterization; hyperspectral imaging; machine learning