United States

In today’s manufacturing, additive manufacturing processes enable the production of complicated three dimensional structures that are hard to be manufactured with traditional manufacturing processes. Due to its high build rate, the Directed Energy Deposition (DED) process is an attractive and versatile process to manufacture these kind of components. In addition to the production of components, DED is used for repair or coating applications. The DED process consists of a multitude of complex thermal mechanisms with high heating and cooling rates of 10&lt;sup&gt;3&lt;/sup&gt; up to 10&lt;sup&gt;5&lt;/sup&gt; K/s. For materials with a high hardness or a low thermal conductivity like tool steels, cast iron or tungsten carbide these high cooling rates can lead to defects in the microstructure like cracks, pores or delamination between the substrate and the deposited structures. By preheating the substrate, the cooling rates can be reduced, and defects can be eliminated. In this paper a preheating cycle was developed, which uses the laser of an DMG MORI LT 65 DED hybrid machine as a moving heat source for the substrate preheating. For this cycle, process parameters, a tool path strategy and a temperature control system were developed. The impact of the elaborated concept was shown by welding tungsten carbide in nickel matrix on S235 steel substrate.

ICALEO 2023

Development of a Laser Preheating Concept for Directed Energy Deposition

substrate preheating

directed energy deposition

additive manufacturing

In today’s manufacturing, additive manufacturing processes enable the production of complicated three dimensional structures that are hard to be manufactured with traditional manufacturing processes. Due to its high build rate, the Directed Energy Deposition (DED) process is an attractive and versatile process to manufacture these kind of components. In addition to the production of components, DED is used for repair or coating applications. The DED process consists of a multitude of complex thermal mechanisms with high heating and cooling rates of 103 up to 105 K/s. For materials with a high hardness or a low thermal conductivity like tool steels, cast iron or tungsten carbide these high cooling rates can lead to defects in the microstructure like cracks, pores or delamination between the substrate and the deposited structures. By preheating the substrate, the cooling rates can be reduced, and defects can be eliminated. In this paper a preheating cycle was developed, which uses the laser of an DMG MORI LT 65 DED hybrid machine as a moving heat source for the substrate preheating. For this cycle, process parameters, a tool path strategy and a temperature control system were developed. The impact of the elaborated concept was shown by welding tungsten carbide in nickel matrix on S235 steel substrate.

technical paper

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This paper provides quantitative information about the paths taken by powder particles which do not enter the laser generated melt pool during laser cladding (DED). These wasted powder particles bounce off the melt pool and the surrounding solid areas. This wastage reduces the productivity of the process. In this paper specially developed software was used to analyze High Speed Imaging videos of the cladding process, to monitor the directions of powder particle flight away from the melt pool area. This information has been correlated to the geometry of the melt pool zone for three different cladding techniques; single track cladding (A tracks), standard overlapping track cladding (AAA cladding) and a recently developed technique called ABA cladding. The results show that the melt pool geometry has a strong influence on powder catchment efficiency and the trajectories of powder particles which bounce away from the laser-material interaction area. ABA cladding was found to have considerably better powder catchment efficiency than standard AAA cladding and this improvement can explained by consideration of the geometries of the melt pools and surrounding solid material in each case. As powder costs are an important factor in industrial laser cladding, the adaption of the ABA technique could have a beneficial effect on the profitability of the process

Laser Cladding (DED); A High-Speed-Imaging Examination of Powder Catchment Efficiency as a Function of Melt Pool Geometry

With recent technological advances in additive manufacturing, laser-directed energy deposition (L-DED) is frequently chosen as a manufacturing process of Inconel 718 (IN718), which is a precipitation hardening alloy that exhibits excellent mechanical properties at elevated temperature after various heat treatments. However, undesirable recrystallization occurs during the post-heat treatment, resulting in the growth of equiaxed grains, which offset the advantage of columnar grain structure from the layer-by-layer deposition. Induction heating-assisted L-DED, which uses induction heating during the deposition, has the potential to skip the first step of the heat treatment by maintaining the deposit temperature at the heat treatment temperature. In this study, three cube-shaped IN718 were deposited using the induction heating-assisted L-DED system at the holding temperature of homogenization, solution, aging heat treatment, respectively. The temperature conditions were selected according to the standard heat treatment specification of IN718 product. In the induction heating-assisted L-DED, microstructure of the deposits strongly depends on the holding temperature of induction heating during deposition. For each deposit,&nbsp;the secondary phases were analyzed by using a scanning electron microscope and the grain structures were studied using electron backscattered diffraction. Additionally, the mechanical properties of each deposit were evaluated. The results of this study can provide the guide for selecting the proper holding temperature during the induction heating-assisted L-DED to achieve the desired microstructure and mechanical properties.

Effects of Holding Temperature during Induction Heating-Assisted Laser-Directed Energy Deposition of Inconel 718

Current rapid growth of the e-mobility sector is driving demand and innovation in the manufacture of batteries. This is particularly evident in the development of laser-based processes for battery cell manufacturing. Whether its cylindrical, pouch or prismatic all battery cells require foils to be cut. Industrial demands are quite demanding and varied due to complexity of the foil materials as well as the electrode designs, making it a challenge to identify the most appropriate laser technology. Requirements range from the slitting of thin bare metal foils to profiling of coated materials where both bare foil and coated material need to be cut. The materials break into anodes and cathodes each posing different challenges as manufacturers require highest possible processing speeds with the highest possible edge quality. Pulsed lasers are becoming the tool of choice, while many foils can be adequately cut with ns lasers some material combinations benefit from shorter ps pulses. This contribution gives a broad overview of the challenges of laser cutting of battery foils and explores the pros and cons of ns vs ps for different materials. The analysis focuses on achievable processing speeds as well as cut edge quality and looks at a variety of laser parameters such as pulse duration, pulse energy and peak power. The influence of laser parameters such as spot size, working fields and processing strategies are discussed. This work shoes that choices are not always straight forward, and users are left to balance the need between Cost – Quality – Productivity.

Battery Foil Cutting – Should You Use ns or ps Laser Sources?

Laser-based manufacturing has become a key enabling technology in the production of batteries and battery cells for e-mobility applications. Several applications, in fact, have already been industrialized for welding, cutting and stripping with lasers. Cutting, in particular, has to fulfil many very stringent constraints, such as: highly reflective materials (aluminum, copper), very low thickness (6-12 µm), on-the-fly processing, high quality of the cutting surface. According to those considerations, the present paper deals with the application of remote cutting of 8 µm thick aluminum and copper foils by means of a galvo scanner and two different fiber laser sources: single mode CW and nanosecond pulsed ones. The experimental activity is devoted to understanding the feasibility of the process and to point out pros and cons of the two different lasers involved. The cutting edges are analyzed by means of optical and SEM microscopy and confocal profilometry, in order to characterize the quality of the cutting, the presence of burr and the extension of the heat affected zone. The process is also characterized in terms of maximum achievable speed in order to understand the limits of both laser and galvo scanning system and some considerations are drawn on the role of the typical fine tuning parameters of the galvo scanner on the possibility to achieve high cutting speeds in an on-the-fly process.

High Speed Laser Cutting of Ultra-Thin Metal Foils for Battery Cell Production

Temporal beam-shaping is a powerful tool for micromachining of various materials with femtosecond lasers. In this contribution, we present our advances of dielectrics microprocessing with an ultrafast laser source which can operate either in the MHz- or in the GHz-burst regime. We compare top-down percussion drilling in glasses with a Gaussian beam in both burst regimes. The GHz-burst mode allows for obtaining crack-free holes with an outstanding surface quality of the inner walls and featuring an almost cylindrical shape without shifting the beam focus nor the sample. The deepest hole is measuring 3.7 mm length and 25 µm entrance diameter, corresponding to an aspect ratio of 150, which is the highest reported so far with femtosecond GHz-burst drilling to the best of our knowledge. Moreover, we show a combination of temporal and spatial beam shaping for cutting of transparent dielectric materials. A Bessel beam is formed using an axicon, and we compare the cutting results in both burst operation regimes. We illustrate the influence of the laser beam parameters such as the burst energy and the pitch between consecutive Bessel beams on the machining quality of the cutting plane and provide processing windows for both burst regimes. In both regimes, dust- and chipping-free cutting has been achieved for glass samples of up to 1 mm thickness. However, the best cutting plane quality in terms of surface roughness is obtained in the GHz-burst regime.

Temporal and Spatial Beam-Shaping for Innovative Femtosecond Laser Microprocessing of Dielectrics

In the field of laser material processing, laser beam cutting is still the commercially most important application. Several 10,000 new 2D flatbed cutting systems are built every year. At the same time, laser cutting of larger sheet thicknesses is increasingly coming into focus of potential customers. Thus, laser powers &gt;20 kW are becoming more and more important to significantly increase productivity in laser cutting, especially for sheet thicknesses &gt;10 mm. Furthermore, laser powers of &gt;40 kW have the potential to compete with plasma cutting, providing better edge quality, less material consumption due to smaller cutting kerfs at higher energy efficiency, thus also lowering CO2 emissions. This market demand is challenged by the intrinsic thermal focus shift of the individual optical elements within the cutting system. Today, optical coatings for up to 20 kW of laser power are often optimized to balance performance vs. economic aspects. Minimizing focus shift can be done by, first, reducing energy deposition within AR-coatings, second, pre-correcting actual shifts by means of approximation, and third, measuring the shift and correcting it in a closed loop control. A detailed analysis of the in-situ thermal focus shift in real applications will be presented, providing an optimal combination of listed measures.

Strategies for Reducing the Thermally Induced Focus-shift in High Power Laser Cutting

The ISO 11146-1 is a well know standard for measuring the M-Square value of a laser or laser system. The multi-spot method of measuring the M-Square against the ISO 11146-1 standard has not, to date, been measured one to one against this standard using both the traditional scan method and the multi-spot method. This work validates the multi-spot method against the standard by using the identical optical setup and simply changing the software algorithm to accommodate both methods while staying within the ISO 11146-1 parameters.&nbsp; We then compare the results for the same laser within a very short span of time. This work covers the results of three (3) different ISO compliant measurement methods: full caustic multi-spot, scan method of one spot at a time and a smaller number of multi-spots within 3 different Rayleigh ranges of the beam caustic. The multi-spot method has the significant advantage of providing an ISO compliant measurement within a single laser pulse whether it be a femtosecond, picosecond, nanosecond, or millisecond pulse. The traditional scan method can only provide a time averaged measurement and is not possible in making a single pulse M2 measurement.&nbsp; The multi-spot, therefore, provides instant feedback of the laser performance and reduces the optimization time.

Advances in Laser Cutting Thick Sheet Metal with “Light Tunnel Generators” Materials Processing

In the naval applications, as well as for infrastructure and construction sectors, welding of thick plates (&gt;15 mm) has become of crucial importance. Nowadays, the common techniques involve several passes of traditional Gas-Metal Arc Welding (GMAW) to fill a gap with a precise geometry. These techniques are, inevitably, time consuming due to the necessity of multiple passes, material consuming because of the volume of the gap which needs to be filled and complicated due to the precise preparation of the joints. To overcome these drawbacks, we propose here a Hybrid Laser-Arc Welding (HLAW) technique in a configuration which allowed us to achieve a sound weld of two plates, 20 mm thick, of naval steel EH40 in a single pass. The goal has been obtained with a 16 kW fiber laser and a welding speed of 1500 mm/min. The most important result of this research is that the welds have been performed in a simple butt configuration, with straight edges of the plates and without any chamfer or additional preparation. Moreover, the length of the weld seam, 1 meter, guarantees the stability and the solidity of the process, its scalability over longer plates, and thus, an easy implementation on an industrial machine for massive production. 
This result opens the way to a faster, more reliable, and less costly process for the welding of thick plates. Furthermore, since the constant increase of the achievable laser power, there is the reasonable hope to push the single-pass maximum weldable thickness even further.

Welding of 20 MM Thick EH40 Steel by Means of a Single-Pass Hybrid Laser-Arc Welding Technique.

We present a framework for developing laser weld schedules featuring circular oscillation in the beam trajectory designed to reduce keyhole induced porosity in aluminum welds.&nbsp; Our approach focuses on minimizing the frequency of keyhole collapses during the welding process by stabilizing the keyhole with circular beam oscillation.&nbsp; The approach is guided by the hypotheses that if the oscillation period is less than the characteristic time of keyhole collapse, the keyhole can remain stable.&nbsp; The characteristic keyhole collapse time is governed by the melt density and surface tension, as well as the keyhole radius, which can be increased by utilizing a larger oscillation radius.&nbsp; By eliminating keyhole collapse, the creation of vapor bubbles in the melt flow can be avoided, thus eliminating a major source for weld porosity.&nbsp; &nbsp;To verify these hypotheses, experimental results from laser welding of 3 mm thick aluminum alloy (AA 5754) sheets are presented. The test results confirm that porosity is substantially reduced when the oscillation amplitudes are increased beyond predicted threshold values for different oscillation frequencies.&nbsp; These findings help to elucidate a new mechanism for improving keyhole stability in laser welding processes via laser beam oscillation and provide a framework for developing improved laser weld schedules.

Mechanism-Guided Approach for Porosity Reduction in Laser Welding of Thick Aluminum Sheets

Silicon heterojunction (SHJ) solar cells take advantage of the passivation properties of amorphous silicon: it can be easily doped, deposition temperatures are very low (below 200◦C), and it has very good passivation properties, but it has the drawback of very high sheet resistance for heterojunction emitters. An alternative to the use of a-Si:H in SHJ cells are transition metal oxide (TMO) films. These films have interesting properties as passivators and selective contacts for novel semiconductor devices, i.e. hole transport layers (HTL) or electron transport layers (ETL), and can be deposited on both electrodes of a solar cell. In this work, we present a complete study of the laser ablation of three transition metals that show good properties as p-type front contacts for n-type crystalline silicon heterojunction solar cells: WOx, VOx and MoOx. We characterize the ablation process of films deposited on crystalline silicon using laser sources of three different wavelengths (355 nm, 532 nm, and 1064 nm) and in two temporal regimes(picoseconds and nanoseconds). We measure the fluence threshold to remove the transition metal oxide (TMO) film and to induce the damage in the crystalline silicon substrate, determining the parametric window of the process. We also studied the morphology to analyze the relationships between crater dimensions and ablation parameters using confocal microscopy. Complete removal of the three different TMO films was achieved without damaging the silicon substrate. Results of diode isolation and their electrical characteristics are presented, showing the quality of the laser isolation processes.

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Laser Cladding (DED); A High-Speed-Imaging Examination of Powder Catchment Efficiency as a Function of Melt Pool Geometry

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Next from ICALEO 2023

Laser Cladding (DED); A High-Speed-Imaging Examination of Powder Catchment Efficiency as a Function of Melt Pool Geometry

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