Fundamentals and Process Simulation (LiM 2017)

Experimental and numerical investigation of the capillary front and side walls during laser beam welding
Fetzer, Florian; Hu, Haoyue; Weber, Rudolf; Graf, Thomas

In deep penetration laser beam welding the distribution of absorbed laser intensity on the walls of the capillary determines local recoil pressure which is a driver for melt flow. This again influences the distribution of absorbed laser power and pressure at the capillary. High speed imaging of melting and evaporation caused by laser irradiation of edges of pure iron sheets is performed with the aim to visualize these phenomena at the capillary walls. High speed recordings with a frame rate of up to 100 kHz are acquired. These experiments are used to validate transient simulations of the capillary front wall in laser beam welding by applying an approach where ray-tracing calculations are coupled to a Smoothed Particle Hydrodynamics simulation of the fluid dynamics.

Keywords: laser beam welding; simulations; Smoothed Particle Hydrodynamics; evaporation


Dynamic method for determination of coupling efficiencies in laser material processing
Hipp, Dominik; Mahrle, Achim; Beyer, Eckhard

The knowledge about the amount of power transferred to a workpiece in relation to the applied total power of a
particular heat source is crucial for every kind of thermal material processing. It allows the evaluation of process
efficiencies for competing technologies and enables predictions of the thermal load of a workpiece. In case of laser
material processing, in which the workpiece is heated up by laser radiation, the coupling efficiency is often strongly
affected by or even corresponds to the absorptivity of the material being processed. However, documented values in
literature are only valid for perfect and smooth surfaces, mostly incomplete in terms of different laser wavelengths,
inconsistent or not available for relevant technical alloys. Therefore, a new method for determining absorptivity values
and energy coupling efficiencies for almost arbitrary materials and processing conditions was developed. This technique
relies on an adjustment of data achieved by experimental thermographic imaging and numerically computed
temperature fields and provides as a result the energy coupling efficiency of the considered process. The potential of the
proposed method was evaluated by performing experiments on standard type 304 stainless steel with the purpose to
determine its absorptivity for solid-state laser radiation with a wavelength of 1.07 μm. The results show the absorptivity
as a function of both the angle of incidence and the polarization state of the laser light.

Keywords: Absorptivity; Thermography; Simulation


Thermal & fluid ffield modelling for laser aided additive manufacturing
Chew, Youxiang; Song, Jie; Bi, Guijun; Chen, Hui-chi; Yao, Xiling; Zhang, Baicheng; Bai, jiaming; Guo, Zhaoqin; Moon, Seung Ki

A numerical model is developed which integrates laser-powder interaction into overall heat flux input for computing the
fully coupled melt-pool fluid and thermal field for clad geometries and transient thermal evolution predictions. Particle
velocities and the focused powder jet diameter were measured experimentally and used for computing laser power
attenuation effects under different powder feeding rates. This gives a more accurate modelling of the heat-flux incident
upon the curved advancing melt-pool surface. The Marangoni effect will be considered to capture the enhanced
convective heat transfer as well as the effects of melt-pool fluid flow on the clad bead profile. The clad bead shape is
determined by the balancing of the external ambient pressure with the tangential and normal surface gradient. Mass
addition into the melt-pool from the powder jets will be accounted for by using the moving mesh method, whereby the
mesh velocity within the melt-pool is defined in accordance with local powder intensity distribution. The numerical
results give good agreement when compared with experimentally obtained clad geometries & melt-pool depths under
different laser power, laser scanning speed & powder feeding rates.

Keywords: Additive-Manufacturing ; Marangoni; Meltpool dynamics; laser cladding


Interaction of powder jet and laser beam in blown powder laser deposition processes: Measurement and simulation methods
Wirth, Florian; Freihse, Sebastian; Eisenbarth, Daniel; Wegener, Konrad

The coating process by laser cladding as well as additive manufacturing by direct metal deposition are both influenced
by the powder jet characteristics and by the interaction between powder jet and laser beam. This is especially true in the
newly developed high-speed laser cladding process where this interaction is of great importance as the melting of the
powder particles even before reaching the melt pool is desired. A simulation model is presented, which predicts the
characteristics of the powder jet between powder nozzle and working plane and also how influential the powder jet will
be on the laser beam in terms of attenuation. A new measurement method has been elaborated to analyze the powder
particle density distribution in the working plane under conditions that replicate the actual process. This is in order to
characterize the powder jet and validate the simulation results. Moreover, a new measurement method which may
reveal the powder particle absorption coefficient is proposed. All this allows for the optimal alignment of the powder jet
with the laser beam in high-speed laser cladding as well as general predictions of the laser cladding process results by

Keywords: Laser cladding; direct metal deposition; laser powder interaction; powder jet characteristics; attenuation


Pulsed Nd:YAG laser drilling of alumina ceramics and silicon wafers
Chmelickova, Hana; Havelkova, Martina; Hiklova, Helena; Rihakova, Lenka

This paper reports about drilling of alumina ceramics samples and silicon slabs with thickness about 2.3 mm, 7.5 mm and
8 mm using a stable and two types of non-stable resonator setups in pulsed Nd:YAG laser KLS 246-102 with different
theoretical focus diameter and beam quality. Pulse energy and peak power were increased by setting of charging voltage
from 280 V to 370 V with 10 V step for each type of resonator, their maximal values always depend on resonator power
Constant burst of pulses is applied on thinner samples to find holes dimensions dependence on peak power, than
number of pulses needed to drill complete holes in thicker samples was investigated. Effect of the laser energy input and
laser beam diameter on the holes depth and diameters was evaluated for all samples using both laser scanning confocal
and optical stereo microscopes. It was observed, that achieved holes depth has tendency to increase with increasing
value of the energy in case of blind holes, just as entrance/exit diameter of complete holes. Exit diameter of cone-
shaped holes is always smaller than that of the entrance. Maximal material removal was achieved with the most
efficient stable resonator, but the best holes quality was obtain with non-stable resonator for drilling. Numerical model
of the vaporized and melted area in the holes cross section was made using finite differences method to display
temperature distribution dependence on increasing pulse energy.

Keywords: Laser drilling; alumina; silicon; beam quality; numerical model


Simulation of the buttonhole formation during laser welding with wire feeding and beam oscillation
Cho, Won-Ik; Schultz, Villads; Vollertsen, Frank

The gap bridge ability in laser welding of butt joints can be enhanced by wire feeding and beam oscillation. However,
effects of them on the molten pool like the surface improvement by the so-called buttonhole discovered and named in
previous experimental research by the authors are not understood. Thus, comprehensive models to simulate wire
feeding and melting behavior in combination with beam oscillation are newly suggested in this work. A
three-dimensional transient simulation has been conducted for laser welding of butt joints with large gaps. As result,
realistic solid wire feeding and its melting behavior as well as the formation of the buttonhole can be shown by
simulations. It was found that the buttonhole is induced by the keyhole in the molten pool and the formation is affected
by the shape of wire tip.

Keywords: numerical simulation; laser welding; wire feeding; beam oscillation


Nucleate boiling in laser beam welding of aluminum alloys
Hugger, Florian; Holzer, Matthias; Roth, Stephan; Schmidt, Michael

Aluminum alloys with high magnesium and zinc content are hard to weld due to severe spatter formation and welding
defects. To understand the physical mechanism of spatter formation a spot welding process is observed by high speed
imaging. The analysis of the videos with a frame rate of 240,000 fps shows a new phenomenon. Bubbles are generated
below the surface which expand and erupt. When collapsing the thin melt film which formed the shell of the bubble
disintegrates to spatters. The dynamics of bubble movement and the collapse of the melt film into spatter droplets are
Moreover a mechanism for the bubble generation is proposed. Due to selective evaporation of volatile alloying
elements, the concentration of alloying elements at the evaporating surface is reduced. In consequence of diffusion of
alloying elements towards the evaporating surface, these elements are reduced in the upper surface layers. Due to the
change of element concentration, the evaporation temperature of the surface and the subjacent melt layers can
increase. When the evaporation temperature is lower than the real temperature in the melt layers below the surface, a
vapor nucleus is generated. The depth of nucleus formation and the pressure inside a bubble are calculated.

Keywords: Welding, boiling, bubbles


Recent advances in the multiphysical simulation of laser assisted manufacturing processes
Gómez Vázquez, Rodrigo; Otto, Andreas; Peternel, Jaka

The industrial implementation of laser assisted processing technologies is nowadays well-established due to the unique
combination of accuracy, productivity and adaptability that laser sources permit. As laser assisted techniques allow for
precisely controlling the thermal heat input it is possible e.g. to produce high-end components with a minimum
influence on the functional characteristics. On the other hand, finding optimal process parameters is often a difficult task
which involves the use of experimental methods that not always provide enough relevant process information. In this
regard numerical simulations can help to fill this gap with the use of multiphysical models.
In this paper we present both recent achievements as well as current ongoing work lines of a multiphysical model
designed to accomplish the simulation of laser assisted manufacturing processes. The general design of the model allows
for the simulation of different kinds of laser processes such as welding, cutting or more recently even ultra-short pulse
ablation [1]. The model is able to perform the simulation of the complete process, thus providing useful information
such as the influence of keyhole front inclination or of surface tension on the flow of the molten material and its
consequences after the solidification. New developments presented are aimed to extend the physical capabilities of the
current model as well as to improve the calculation performance in massively parallel scenarios such as HPC-clusters or
cloud environments.

Keywords: multiphysics; simulation; laser welding; laser cutting


Numerical simulation of WC particles distribution in laser melt injection with external electromagnetic field support
Wang, Liang; Yao, Jianhua; Hu, Yong; Zhang, Qunli; Liu, Rong

An advanced particle distribution controlling approach is proposed for laser melt injection process, which applies an
-magnetic compound field to assist the laser melt injection process. The electric-magnetic synergistic effect on the
reinforcement particle distribution in laser melt injection is investigated using numerical and experimental methods.
Spherical WC particles are used as the reinforcement and their distribution in the longitudinal sections of the laser melt
injection layers is examined with SEM and studied with computer graphics processing. The distributions of fluid
temperature, fluid velocity and reinforcement particles in the molten pool are simulated using a 2D multi
-physics model
coupled with the equations of heat transfer, fluid dynamics, drag force, Lorentz force and phase transition. The results
show that, the directional Lorentz force due to an electric
-magnetic compound field, as a sort of volume force, can change
the equivalent buoyancy acting on the particles. When the Lorentz force and gravity force are in same direction, majority
of particles are trapped in the upper region of laser melt injection layer, whereas most particles are concentrated in the
bottom region. As a result, the distribution gradient of WC particles can be controlled by the electric
-magnetic compound
field, instead of the time
-consuming adjustment of process parameters.

Keywords: Laser melt injection; Lorentz force; Distribution; Electric-magnetic compound field; Reinforcement particle;


Numerical modeling of laser welding process of NiTi shape memory alloy
Mehrpouya, Mehrshad; Gisario, Annamaria; Elahinia, Mohammad

Shape memory alloys (SMAs) are currently used in several applications due to their unique characteristics such as shape
memory effect and superelasticity. The usage of NiTi shape memory in complex structures is limited due to the
inadequacy of laser joining processes in minimizing the thermal effect on the SME and SE behaviors. In fact, laser
welding of SMAs is a challenging process based on the specific behavior of the material in different temperatures and
stresses conditions. The aim of the present study is to investigate the reliability and the consistency of a finite element
model to predict thermo-mechanical behavior induced by the laser welding. Effects of the heat source distribution, laser
power and scan speed on the temperature changes are investigated. Accordingly, the transient temperature
distributions and bead dimensions of the welded NiTi plates during welding will be predicted. The numerical model is
utilized to predict both the fusion and the heat affected zone. Obviously, achieving the optimum laser parameters has a
significant influence on the quality of welded parts so that the HAZ dimension will be reduced, consequently improving
the weldability. Simulation results demonstrate a good agreement with experimental temperature distributions, which
were recorded by a thermocouple during the welding process. The thermal effect on the weld geometries was also
comparable with the experimental results. Indeed, the control of laser parameters can effectively improve the
mechanical and functional behavior of the NiTi welded component.

Keywords: Laser welding; Shape memory alloy; NiTi; Finite element method


Marangoni force and numerical instability when modelling keyhole laser welding
Svenungsson, Josefine; Choquet, Isabelle; Kaplan, Alexander

This work is a first step in a study aiming at predicting defects such as porosity in keyhole laser welded titanium alloy
Ti 6Al4V in order to understand their cause and improve the process. For this amulti -phase thermo -fluid model for
keyhole laser welding was implemented in the open source softwa re OpenFOAM®. In order to model porosity, free
surface tracking and the gas phase are included in the model. Surface tracking is done with a Volume of Fluid method
improved to reduce numerical diffusivity. Solid, liquid and gas phases are taken into account and vaporization is
modelled through its effect on pressure and enthalpy. This paper treats the validation of the model by comparing the
model to published numerical test cases. A test case revealed stability problems related to the implementation of
Marangoni force. It wa s shown that the implementation method used for the Marangoni force can lead to numerical
instabilities. This unphysical behavior, which could affect the prediction of pore formation, might not be detected by the
usual weld geometry validation test case . This resulted in improvement of the numerical model. Another test case is a
comparison between different surface tracking methods and wa s done in order to evaluate the model ability to capture
formation of porosity, vapor bubbles. The results show a significant difference between VOF and CLSVOF.

Keywords: process simulation; CFD; keyhole laser welding; OpenFOAM; validation; Marangoni convection


Enhancement of the area rate for laser macro polishing
Kumstel, Judith

Look at the state of the art for polishing of metals in industry most of the parts are polished manually, because current
automated polishing techniques often cannot be used on parts with freeform surfaces and function relevant edges.
One possible solution to automate the polishing of metallic freeform surfaces is the 3D laser polishing with continuous
wave laser radiation. The achievable roughness is limited to Ra values between 0.05 and 0.5 μm depending on the
material and its homogeneity. Depending on the used circular shaped intensity distribution with beam diameters up to
600 μm the area rate for laser polishing is limited to 1 cm²/min. Take into account the machine costs and the running
costs the costs for laser polishing are approx. 70 €/hour.
Based on the limited roughness and/or on the costs currently only few industrial implementations exist for laser
polishing. To bring laser polishing in industry there are two possibilities: on the one hand the achievable roughness has
to be decreased and on the other hand the laser polishing process has to be speeded up to reduce the costs.
This paper will focus on the increase of the area rate and will present two approaches for the increase.

Keywords: laser polishing; intensity distribution; area rate


Hybrid joining of high reflective and thin metal substrates with polymers by laser micro-structuration with short and ultra-short pulsing lasers
Henrottin, Anne; Patars, Jérôme; Ramos-de-Campos

The research of new assembly technologies for dissimilar materials is growing up continuously due mainly to the high
demand for transport application of obtaining lighter structures reducing as a result energy consumption. In other
sectors as medical and micro-electronics, the benefit of laser hybrid welding is in the possibility to create new
components with complex geometries, new properties while ensuring the functionality of the product. The present
study proposes a method, based on short and/or ultrashort pulsed lasers, for welding metal having complex properties
for the joining process with a polymer. High reflective, high thermal conductivity and thin metal plates, as aluminum,
stainless steel, copper and titanium are studied in this present work.

Keywords: Laser hybrid joining; reflective metal; thin metallic substrates; polymer; ultra-short pulsing laser; micro-machining.


High speed and high power laser material processing: first determination of process limits
Hildenhagen, Jens; Bant, Paul; Dickmann, Klaus

The increasing output power and brilliance of laser sources allows in principle faster material processes but needs novel
handling technologies, e.g. beam guiding systems. Current developments like polygon scanner are dissolving consisting
restrictions and finally physical properties (e.g. primary thermal conduction and heat capacity) will be the remaining
limitations for process speed. However, depending on the scope of application and technological progress it will still take
years to reach industrial requirements. First investigations should give an outlook what might be possible in the field of
high speed laser material processing when the above listed technical limitations have been overcome. Therefor different
samples (e.g. prevail coated materials like corroded steel or anodized aluminium), were mounted on a fast rotating
cylinder (circumferential speed up to 120 m/s) and treated with a 30 kW (cw) fiber laser. At this speed the applied laser
spot diameter of 200 μm lead to an interaction time of 1.7 μs and intensity of 108 W/cm². These specifications are known
from pulsed laser systems and enabled distinct surface modification or even ablation by a single laser pulse. The high
speed - high power setup allows to transfer such laser parameters in a continuous process with comparable raise of
ablation rate. Thus it was possible to remove oxide layer or other resistant coatings with an output of several square
meter per minute.

Keywords: high speed, high power, surface cleaning


Layer-selective laser-lift off and removal mechanism in a TCO/Si thin film system by nano- to femtosecond pulses
Krause, Stephan; Miclea, Paul-Tiberiu; Kaufmann, Kai; Hagendorf, Christian; Bulgakova, Nadezhda M.

Narrow craters and trenches have been scribed into Si/TCO thin film systems on glass by selective laser lift-off using
-, pico- and femtosecond pulses of ~ 0.5 μm wavelength. A high-resolution microstructural material investigation
shows a significant reduction of heat affected zone (HAZ) by changing from ns to ps and fs pulse durations. Thus, ablation
efficiency and selectivity of ps
- and fs-lift-off thin film processing in comparison to thermal ablation by ns pulses is
strongly increased. Difference in film removal mechanisms depending on pulse duration is discussed based on multi
thermal modeling of the laser
-induced spatio-temporal temperature fields. For ultrashort laser pulses, a continuum
model has been applied, aimed at describing the dynamics of electronic excitation, heating, and melting at TCO/Si
interface region under femto
- and picosecond laser irradiation.

Keywords: Laser lift-off; ultra-short pulses; transparent conductive oxide; Si micro processing; laser-interaction modelling