United States

Through experimental observation and auxiliary numerical simulation, this investigation studies the different types of grain refinement of 5754 aluminum alloy laser beam welding by applying a transverse oscillating magnetic field. Scanning electron microscope results have proved that the application of a magnetic field can reduce the average crystal branch width and increase its number. The interaction between the induced eddy current generated by the Seebeck effect and the applied external magnetic field produces a Lorentz force, which is important for the increase in the number of crystal branches. Based on the theory of dendrite fragmentation and the magnetic field-induced branches increment, the grain size reduction caused by the magnetic field is studied. Furthermore, the effects of the magnetic field are analyzed by combining a phase field method model and simulations of nucleation and grain growth. The grain distribution and average grain size after welding verify the reliability of the model. In addition, the introduction of a magnetic field can increase the number of periodic three-dimensional solidification patterns. In the intersection of two periods of solidification patterns, the metal can be re-melted and then re-solidified, which prevents the grains, that have been solidified and formed previously, from further growth and generates some small cellular grains in the new fusion line. The magnetic field increases the building frequency of these solidification structures and thus promotes this kind of grain refinement.

ICALEO 2023

Experimental and Numerical Study on Grain Refinement in Electromagnetic Assisted Laser Beam Welding of 5754 Al Alloy

periodic solidification pattern

crystal branch development

grain refinement

laser beam welding

magnetic field

technical paper

**General Chair Welcome**

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**Klaus Löffler **  
ICALEO General Chair   
Managing Director, Precitec GmbH

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Laser welding in transmission manufacturing opens up completely new kinds of product solutions with excellent properties in terms of wear, corrosion resistance and service life. Current welding designs are characterized in particular by difficult-to-weld material combinations (e.g., steel vs. cast iron) and a high component stiffness for load transfer, that is typically correlated with high residual welding stresses. The major challenge for these welded structural components remains both their crack-free weldability and their complex cyclic load capacity. 
In this frame, the subject of this contribution is the presentation of a simulation-based strategy for process- and load-compatible laser welded transmission components. Specifically, a systematic study is being conducted to understand and qualitatively evaluate effective methods for reducing residual weld stresses in circumferential welds. The recommendations developed as part of this study take particular account of the influence of process modifications, material conditions and geometric aspects on weldability and component distortion. In this context, the residual stresses can be reduced by up to 30 % using suitable modifications. In particular, structural welding simulations are performed and verified by experimental welding trials including metallographic examinations. To ensure the required component fatigue strength, a practical concept for determining Wöhler curves is presented. 
The simulation-based development methods enable the production of&nbsp;load-transmitting components in the fields of e-mobility, aerospace and industrial engineering.&nbsp;In particular, time-consuming and cost-intensive iterations of laser welding tests can be significantly reduced. Furthermore, the systematic investigations provide effective recommendations for phenomenological understanding and solving typical welding challenges in practice.

New Possibilities for Laser Welding of Highly Loaded Transmission Components by Strategic Use of Simulation Methods

Effective monitoring technologies are crucial for optimizing the efficiency of laser processes and manufactured parts. The integration of a monitoring system is especially important in industrial environments where the production of micro- and submicrometer features requires high levels of precision and reproducibility to enhance the surface functionalities. 
In this frame, to ensure the quality of the machined surfaces, various characterization methods such as scanning electron microscopy (SEM), atomic force microscopy (AFM), or confocal microscopy (CM) are commonly used to analyze the micro- and nanostructured surfaces ex situ. However, it is not possible to integrate these characterization methods as real-time monitoring systems. 
A possible solution is to use optical monitoring systems as well as infrared or other camera systems. In addition, technologies based on acoustic emission can also provide various insights into the laser process by analyzing the noise produced by the laser treatment. 
This presentation summarizes different approaches used recently for monitoring laser-based microstructuring processes, including LIPSS and DLIP surface treatments. A diffractive-based approached is used to determine in real-time topographical information (height and period) about produced LIPSS and DLIP features using ps-laser sources. In addition, infrared cameras are also implemented, allowing correlations between the detected signal with topographical features of the produced microstructures. In addition, general information regarding the homogeneity of surface features can be retrieved. Finally, the implementation of acoustical microphones is discussed, allowing to determining relevant information about the process.

Sensing Sound and Light for Monitoring Laser Microfabrication Processes

Ablation using ultrafast lasers is a commonly used method for creating structures and geometries in various materials, ranging in size from a few micrometers to millimeters. The most widely used system architecture involves the combination of a galvanometer scanner with ultrafast lasers that have pulse durations ranging from a few picoseconds down to a few hundred femtoseconds. In principal, almost any known material can be processed using a laser in this pulse duration regime. However, it is crucial to understand how to machine a specific material to achieve the desired quality, particularly when key parameters, such as the initial surface geometry or material composition, are unknown or may change during the machining process. One way to simplify the process planning and minimize the required knowledge is to use a sensor that can directly measure the surface topography. In recent years, the use of optical coherence tomography for laser ablation has gained significant attention in the field of laser material processing. This technique allows for precise measurement of surface topography and can help with a smart adjustment of the processing parameters to improve quality and to reduced processing time. This paper describes recent advancements in the use of optical coherence tomography to control laser ablation when machining difficult to process parts and materials from additive manufacturing or materials like carbon fiber-reinforced polymer. To achieve this, the OCT was superimposed with the processing laser beam, enabling real-time measurement and closed-loop adjustment of laser and scan parameters with developed software and special hardware.

Application of Closed-Loop Controlled Laser Ablation for High Quality Processing

Laser chemical machining (LCM) is a method of laser processing based on a gentle material removal by means of thermal induced chemical dissolution. Since LCM depends predominantly on the surface temperature of the workpiece, the process window is restricted by the appearance of gas bubbles at higher laser powers and their associated shielding effect. In order to extend the process understanding, the influence of the laser power modulation on the removal behavior is investigated in the present work. The experiments were conducted on titanium grade 1 and with phosphoric acid. Based on the response time in experiments with a single step function of the laser power, a threshold special frequency was determined above which a constant removal depth could be expected. To proof this hypothesis, the laser power was modulated rectangularly in time resulting in combination with the process velocity in different spatial frequencies varying from 1 mm-1 to 20 mm-1 . The investigations showed that the removal cavity exhibited sin-shaped variation in depth along the line with a spatial frequency corresponding to the spatial frequency of the laser power. When the spatial frequency exceeded the determined threshold frequency, the cavity depths is constant. This established the basis for generating complex removal profiles by varying the power in the range below the threshold frequency.

Material Removal in Laser Chemical Processing With Modulated Laser Power

Ultrashort pulsed lasers have been proven to foster micro-manufacturing processes such as drilling, cutting, structuring and even welding, respectively. Using ultrashort pulsed laser, these processes are associated to a significantly reduced heat-affected zone, high precision and the possibility to process various materials via single or multi-photon absorption. However, in typical laser machine setups using galvanometer scanners or conventional processing optics, the processing area is limited to a 2 - 2.5 D micromachining with limited workpiece dimension. To expand the potential of ultrashort pulsed lasers for micromachining to a more flexible and large-area 3D micro processing tool, we report for the first time on the design, realization and application of an ultrashort pulsed laser equipped to a 6-axis robot. In addition, we show and discuss a sophisticated beam guidance and focus positioning concept, that are required for such a combination of laser-robot. For the fundamental characterization of the laser robot system, the dynamic laser beam position is evaluated under movement of the robot by CMOS cameras. As a particular result, for the compensation of occurring position deviations of the laser beam during and after the movement of the robot axes, the development of an innovative and high dynamic beam stabilization concept is necessary. In addition, differently pronounced influences of individual axes must be considered on beam positioning. Moreover, first laser cutting and structuring applications are demonstrated to highlight the enormous potential of the laser robot system for a flexible and large 2D and 3D laser micromachining using ultrashort laser pulses.

Realization of an Ultrashort Pulsed Laser Robot System: Characterization, Limits and Application

In laser metal deposition (LMD), the powder is fed into the laser-induced melt pool using different powder nozzles for the purpose of additive manufacturing and the generation of wear and corrosion protection coatings. So far, there are no industrially established in-process monitoring systems for the powder stream, but mainly measuring systems that examine the powder stream propagation offline and without the processing laser. A challenge in implementing an image-based in-process monitoring system is the process illumination, for the segmentation of the powder particles from the background radiation caused by the processing laser and the melt pool. To overcome this challenge, filtering is needed to attenuate the process emissions and simultaneously brighten the powder stream. Therefore, this work focuses on generating a continuous high contrast between the powder and the background. The powder particles are illuminated by a light source mounted laterally to the powder stream in the horizontal plane below the nozzle opening to make the reflecting powder particles visible to the camera. The optical process emissions were characterized during LMD with respect to the influence of an increasing laser power which was presented in correlation to the increasing process emissions. The evaluation of the spectrograms has made it possible, due to the adapted illumination and filtering, to ensure a constantly high contrast between the process emissions and the powder, so that online monitoring of the powder stream was implemented successfully during the LMD process despite the active processing laser.

Characterization of Optical Emissions During Laser Metal Deposition for the Implementation of an In-Process Powder Stream Monitoring

Controlling heat transfer in casting tools is a key quality aspect. It can be improved by selectively applying volumetric aluminium bronze (CuAl9.5Fe1.2) sections in the core of the tools and subsequently depositing these cores with hard-facing H13 tool steel. Directed energy deposition (DED) can be used for both additive manufacturing of aluminium bronze and hard-facing by depositing the filler-material onto a substrate-surface or previously manufactured bodies. A sufficient metallurgical bonding of the deposited filler-material and the underlying layer must be ensured. However, high dilution of the underlying layer and the filler-material negatively affects the desired properties and must be monitored. Optical emission spectroscopy of the DED process-emissions, is investigated by comparing the emission-lines of the individual elements comprising the base and the filler-materials. 
Multiple single tracks using aluminium bronze as the filler-material are laser cladded with varying power, onto the two different types of substrates i.e. mild steel (S355) and hot working tool steel (1.2343). Additionally, single tracks of H13 are deposited with varying laser powers onto an additively manufactured core of aluminium bronze. Both resulting in deposition-tracks with varying dilution values. Multiple emission-lines of Cr, Fe, Cu, Al and Mn are detected and measured (line-intensity). Line‑intensity‑ratios using the element-emission-lines are calculated and correlated with the respective metallographic results of the deposition-tracks (dilution and chemical composition). Deposition-tracks with a higher dilution (CuAl9.5Fe1.2 onto S355/1.2343 as well as H13 onto CuAl9.5Fe1.2) showed an increased line‑intensity-ratio of the underlying-material to the filler‑material. Moreover, this technology was transferred in a multilayer industrial application.

Monitoring the Degree of Dilution During Directed Energy Deposition of Aluminium Bronze and H13 Tool Steel Using Optical Emission Spectroscopy

Directed Energy Deposition is an additive manufacturing process that allows the production of near net shape structures. Moreover, the process can also be applied for the repair of high value components. To obtain structures with consistent good characteristics, the Directed Energy Deposition process requires the implementation of a control system. The current applied approaches for control that are discussed in literature have specifically focus on melt pool temperature control. Pyrometers have been used for such purpose; however, they provide only a single scalar value without any spatial information. In this paper the implementation of a high-speed hyperspectral camera-based system is discussed with a high spatial resolution unlike the pyrometers. Different calibration and temperature estimation procedures for this camera-based system are evaluated and analyzed. The number of effective wavelengths needed for temperature estimation will be discussed in detail and provide an outlook to the potential of this hyperspectral camera-based system. In addition to the number of wavelengths, another important aspect of the temperature estimation methods is the stability with respect to disturbances. Within this paper the impact of different parameters such as: the scanning strategy, the nominal laser power will be evaluated on the stability of the temperature signals for a control system.

Comparison and Analysis of Hyperspectral Temperature Data in Directed Energy Deposition

The rapid growth of electric vehicle industry requires reliable high-speed and high-quality manufacturing processes. One of the important components of the electric vehicles is the motor which incorporates a large quantity of welds to join wires. These wires are insulated with a variety of polymers to prevent shorts in the motor. Insulation removal and stripping are essential to achieve welds with acceptable electrical and mechanical properties. However, conventional mechanical stripping of insulated hairpin wires is inadequate for high-speed mass production due to the slow process speed, poor surface quality, long-term tool wear, and production line interruptions. The use of a CO2 laser for stripping the polymer followed by laser welding as an alternative can address speed limitations of conventional mechanical methods. However, single-step CO2 laser stripping is known to leave a thin polymeric residue on the copper surface that may result in welding defects such as excessive porosity formation, brittleness, and excessive spatter and therefore not acceptable. Herein, a two-step stripping and cleaning process using CO2 and UV lasers is introduced that enables effective insulator removal to achieve low-defect low-spatter welds. Analytical techniques are utilized to assess the surface quality and correlate that with weld quality. Results will be presented.

Novel Laser Stripping and Cleaning Approach to Enhance Laser Welding Quality of Electric Motor Hairpins

High power lasers have become an industry standard for electric drive manufacturing as has the common design of such motors, called ‘hairpin motors’. The term refers to the structure of the windings and&nbsp;has been adapted by all relevant automotive OEM and suppliers. TRUMPF is by far technology leader in this specific manufacturing area and a vital part to all new developments. 
Highly integrated lasersystems are utilized, including high power cw lasers with beam shaping and high beam qualities, scanning optics and camera based as well as OCT sensors for weld joint and seam tracking. 
In this work we are presenting the latest advancements meant for moving away from static processing strategies and parameter sets towards adaptive and automated processing taking into account the challenges specifically evolving from parts clamping and tolerances. By adjusting the processing strategy based on acquired measurement data the welding results were successfully stabilized under the presence misalignments. Typical misalignment types ‘gap between pins’, ‘lateral offsets’ and ‘height differences’ were simulated and welded in different variations. To prove the effectiveness of adaption, a comparison of static welding results to the ones of adapted welding has been carried out. The process of compiling the adapted strategies to level misalignments will be demonstrated. Evaluations have been carried out looking into joint strength, weld sections, porosity formation and weld seam shape and will be shown in this presentation. Finally, a generalizable systematic on adapted processing strategies will be derived based on results of further, different wire types.

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Experimental and Numerical Study on Grain Refinement in Electromagnetic Assisted Laser Beam Welding of 5754 Al Alloy

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