Jens Bliedtner

Investigations of welding instabilities and weld seam formation during laser microwelding of ultrathin metal sheets

During laser microwelding, the melt pool behavior and the formation of the weld seam depend on various process parameters. In this paper, the authors performed tests to clarify the influence of laser power P, the feeding rate vf, the focal diameter df, the foil thickness s, and the thermophysical material properties. Ultrathin metal foils such as stainless steel, aluminum, and titanium in thicknesses of 50 and 100 μm were welded in bead-on-plate welds in order to generate a full penetration weld. For this purpose, focal diameters between 25 and 78 μm were applied. By means of high-speed videography and micro-cross-sections, the observations were analyzed depending on the feeding rate. Imperfections such as root defects, surface structures, and humps were described and evaluated. Moreover, the influence of melt pool behavior prior to the appearance of humping is illustrated for full penetration microwelding in contrast to insufficient welds. The Rayleigh theory of the instability of a free suspended liquid...

Remote micro welding with multi-mode and single-mode fiber lasers – A comparison

Both multi-mode and single-mode lasers are well established in welding applications. In micro welding, single-mode lasers are often used while multi-mode lasers are more common when it comes to welding tasks in the macro range.In this work, a 500 W multi-mode and a 1,000 W single-mode fiber laser were compared concerning their practicability in terms of micro welding. For this reason, stainless steel foils in thicknesses of 25 µm and 50 µm were overlap welded with focal diameters between 22 µm and 204 µm using 2D scanning systems. The process boundaries were described and process behavior was determined by examining welding regime, melt flow-induced seam imperfections, and specific energy demand while welding.Additionally, measurements of hardness and tensile tests illustrate usage properties and constraints of both fiber laser concepts in micro welding.Both multi-mode and single-mode lasers are well established in welding applications. In micro welding, single-mode lasers are often used while multi-mode lasers are more common when it comes to welding tasks in the macro range.In this work, a 500 W multi-mode and a 1,000 W single-mode fiber laser were compared concerning their practicability in terms of micro welding. For this reason, stainless steel foils in thicknesses of 25 µm and 50 µm were overlap welded with focal diameters between 22 µm and 204 µm using 2D scanning systems. The process boundaries were described and process behavior was determined by examining welding regime, melt flow-induced seam imperfections, and specific energy demand while welding.Additionally, measurements of hardness and tensile tests illustrate usage properties and constraints of both fiber laser concepts in micro welding.

Manufacturing of three dimensional silicate moldings by selective laser beam sintering

The efficient production of complex glass components is often not possible with classical manufacturing methods. In order to obtain glazed three-dimensional quartz glass moldings, the alternative method of selective laser beam sintering is investigated. Using synthetic and natural silica powders enables an additive production by high-temperature selective laser sintering (HT-SLS). Particle-diameters in the range of 19...78 μm and spheroidal and vitrified particle-shapes allow to manufacture green bodies. For this purpose, an experimental set up as well as material-specific scan and parameter concepts are developed. Component densities of ρR = 65 % and surface roughness of Ra = 32.21 μm are achieved. Subsequently a glassy, opaque molded body is produced by temperature pressure sintering. The component density increases to ρR = 96 % with a shrinkage of 16%. In order to use the glazed molded body as glass fiber preform, polishing of the shell surface is necessary. Surface roughness of Ra = 10.4 nm can be realized by laser beam polishing. Basically, HT-SLS is an alternative method to the classical isostatic pressing of glass powder. In particular, an increase in efficiency with regard to the producible component geometry of the green bodies can be achieved.

Investigations of surface processing of functional ceramics applying ultrashort laser pulses

The development of ultrashort pulse lasers has enabled many new process technologies in the past few years. The nonlinear absorption caused by high peak intensities and the nonthermal ablation are two of the most attractive benefits of pulse durations in the pico- and femtosecond regime, which allow for the processing of a wide variety of materials. Even dielectric, brittle-hard substrates can be processed precisely and gently without cracking or inducing stresses. For this reason, the technology is particularly suited to open up new processing possibilities in the field of functional ceramics. These materials offer many opportunities to set up complex microsystems and multisensor systems. However, the options to structure and shape functional ceramics were limited in the past and had to be solved by elaborate mechanical processes so far. By means of ultrashort pulse laser processing, new applications in the fields of precise shape formation and microstructuring of functional ceramics become accessible. In order to reveal and optimize the processes occurring during surface ablation, investigations with different laser systems have been executed and evaluated by applying various characterization techniques. The results show how the properties of the bulk material and the process parameters such as pulse energy, wavelength, and pulse overlap influence the removal rate as well as the material characteristics, for instance, roughness and morphology. Thereby, the attention is focused on the dependence of the process on the pulse duration. In contrast to the homogenous surface profile that is created during picosecond ablation, the femtosecond process exhibits material modifications in terms of melting patterns. This effect is strongly dependent on the pulse duration, the fluence, and the pulse overlap. It leads to an increase in roughness, which affects the precise material removal. Nevertheless, the investigations also show that the material melting can be utilized to achieve a smoothing effect of the surface if the parameters are well adjusted. The experimental investigations result in optimized process strategies to realize user-defined aims like high ablation rates, high accuracy, or low damage.The development of ultrashort pulse lasers has enabled many new process technologies in the past few years. The nonlinear absorption caused by high peak intensities and the nonthermal ablation are two of the most attractive benefits of pulse durations in the pico- and femtosecond regime, which allow for the processing of a wide variety of materials. Even dielectric, brittle-hard substrates can be processed precisely and gently without cracking or inducing stresses. For this reason, the technology is particularly suited to open up new processing possibilities in the field of functional ceramics. These materials offer many opportunities to set up complex microsystems and multisensor systems. However, the options to structure and shape functional ceramics were limited in the past and had to be solved by elaborate mechanical processes so far. By means of ultrashort pulse laser processing, new appl...

Experimental determination of influencing factors on the humping phenomenon during laser micro welding of thin metal sheets

Industrial applications such as joining pressure sensors or battery cells often demand short processing times for economic reasons. Thin metal sheets of thickness smaller than 100 μm are suitable for this purpose. The possible maximum feed rate for an efficient welding process is limited by weld defects, which occur at a certain threshold value of feed rate. Materials such as stainless steel, aluminum, and titanium were welded in bead-on-plate welds in order to generate a full penetration weld. Here, our attention is focused on understanding this instability. In this paper, we performed tests to clarify the influence of the thermophysical properties of the applied materials and the process factors laser power and focal diameter on the humping effect. Due to these attributes, the weld seam formation and hydrodynamic behavior of the melt change. By means of microscopical surface line scans and high-speed imaging, the observations were analyzed depending on the feed rate. The results from the line scans provide the possibility to analyze the surface topography of the weld seam. In particular, the distance, height, and axial frequency of the solidified humps can be categorized in order to get a deeper understanding of the solidified hump structure and the phenomenon in general. To avoid the occurrence of humping, a criterion is defined by the ratio of laser power to weld seam cross section for the applied materials.Industrial applications such as joining pressure sensors or battery cells often demand short processing times for economic reasons. Thin metal sheets of thickness smaller than 100 μm are suitable for this purpose. The possible maximum feed rate for an efficient welding process is limited by weld defects, which occur at a certain threshold value of feed rate. Materials such as stainless steel, aluminum, and titanium were welded in bead-on-plate welds in order to generate a full penetration weld. Here, our attention is focused on understanding this instability. In this paper, we performed tests to clarify the influence of the thermophysical properties of the applied materials and the process factors laser power and focal diameter on the humping effect. Due to these attributes, the weld seam formation and hydrodynamic behavior of the melt change. By means of microscopical surface line scans and high-speed imaging, the observations were analyzed depending on the feed rate. The results from the line scans prov...

Process characterization of powder based laser metal deposition on thin substrates

Powder-based laser metal deposition is a well-established generative manufacturing process in the field of tool and mould building to produce or repair components. Conventionally manufactured tools are often completely constructed of a high-alloyed, hardened-tempered, and expensive tool steel. An alternative way to manufacture tools and moulds is the combination of a cost-efficient, mild steel, and a functional coating. Such specific, locally adapted coating can be generated by laser metal deposition. The functional coating is just located in the area of the interaction zone of the tool. Consequently, costs and resources can be saved. Such coatings are created by positioning multiple individual weld tracks next to and on top of each other in an overlapping manner. It is therefore useful to characterize und optimize the individual weld track. Thermal processing methods, like a laser metal deposition, are always characterized by thermal distortion. The resistance against the thermal distortion of the work piece is decreasing with the reduction of the material thickness. Thus, there is a special process management for the laser based coating of thin-walled parts or tools with a small material thickness needed to reduce thermal distortion. The experimental approach in the present paper is to keep the energy and the mass per unit length constant by varying the laser power, the feed rate, and the powder mass flow. The typical seam parameters (such as width, height and depth, cross-sectional area, and angular distortion) are measured in order to characterize the cladding process, define process limits, and evaluate the process efficiency of an individual weld track. Ways to optimize dilution, angular distortion, and clad height are presented. After the characterization of individual weld tracks, optimized process parameters are deduced from the process window and used to create functional, two-dimensional coatings on thin substrates. Different scanning strategies are compared with each other to reduce processing time and thermal distortion.Powder-based laser metal deposition is a well-established generative manufacturing process in the field of tool and mould building to produce or repair components. Conventionally manufactured tools are often completely constructed of a high-alloyed, hardened-tempered, and expensive tool steel. An alternative way to manufacture tools and moulds is the combination of a cost-efficient, mild steel, and a functional coating. Such specific, locally adapted coating can be generated by laser metal deposition. The functional coating is just located in the area of the interaction zone of the tool. Consequently, costs and resources can be saved. Such coatings are created by positioning multiple individual weld tracks next to and on top of each other in an overlapping manner. It is therefore useful to characterize und optimize the individual weld track. Thermal processing methods, like a laser metal deposition, are always characterized by thermal distortion. The resistance against the thermal distortion of the work p...

Influencing factors on humping effect in laser welding with small aspect ratios

In the present work, the humping phenomenon is investigated regarding various influencing factors such as volume flow rate, power level, focal diameter, welding situation, material thickness, and thermophysical material properties by means of three-dimensional microscopy inspection, high-speed imaging, and micro-cross sections. Due to applied small focal diameters and shallow weld depths, the results are in particular suitable to welding with small aspect ratios and a predominant horizontal melt flow field. Differences in welding situations caused by two-dimensional and three-dimensional heat conduction are clarified using various material thicknesses. Additionally, influences on onset of humping effect in welds with root fusion and incomplete penetration are compared. Stainless steel, nickel, and titanium are used as specimens in order to point out the influence of thermophysical material properties. Using the example of stainless steel, a functional description of the humping threshold feed rate is introduced based on the volume flow rate and compared qualitatively to the other materials. Finally, the influence of power level, focal diameter, and material thickness on onset of humping is clarified.In the present work, the humping phenomenon is investigated regarding various influencing factors such as volume flow rate, power level, focal diameter, welding situation, material thickness, and thermophysical material properties by means of three-dimensional microscopy inspection, high-speed imaging, and micro-cross sections. Due to applied small focal diameters and shallow weld depths, the results are in particular suitable to welding with small aspect ratios and a predominant horizontal melt flow field. Differences in welding situations caused by two-dimensional and three-dimensional heat conduction are clarified using various material thicknesses. Additionally, influences on onset of humping effect in welds with root fusion and incomplete penetration are compared. Stainless steel, nickel, and titanium are used as specimens in order to point out the influence of thermophysical material properties. Using the example of stainless steel, a functional description of the humping threshold feed rate is intr...

Precision structuring and functionalization of ceramics with ultra-short laser pulses

High-performance ceramics have been firmly established for the manufacturing of tools and components in the modern electronics industry and mechatronics. Various components such as circuit boards, bearings, and sensors benefit from their specific characteristics, such as wear resistance, stiffness, and electrical neutrality. Apart from these advantages, the brittleness and hardness of ceramics turn the mechanical processing into a challenging and difficult task. Against this background, modern laser technologies have already been used to process ceramics for many years, enabling a contactless and wear-free machining. However, regarding high precision applications, for instance, the drilling of micro-holes or the fabrication of well-defined cavities and three-dimensional structures, conventional laser processes reach their limits. Especially due to thermal influences of the laser radiation, brittle edges, stresses, and redeposited layers emerge. Ultrashort pulse lasers enable completely new processing qualities in these fields. The extremely short pulse durations within the pico- and femtosecond range lead to nonlinear absorption mechanisms and an almost athermal material removal. Thereby, dielectric materials can be processed precisely and gently. In the course of a comprehensive process study, the beam–material interactions of ultrashort pulses with ceramics have been investigated. Besides the material properties, the ablation process is influenced by a multitude of laser parameters, such as wavelength, pulse overlap, and fluence. In order to reveal the most important variables, the experiments have been conducted by applying modern statistical methods. Using alumina (Al2O3) as an example, it is shown how different parameter regimes lead to disparate process qualities and efficiencies. The generated models have been used to optimize industrially interesting applications, from the separation of ceramic printed circuit boards to the realization of precise design structures.High-performance ceramics have been firmly established for the manufacturing of tools and components in the modern electronics industry and mechatronics. Various components such as circuit boards, bearings, and sensors benefit from their specific characteristics, such as wear resistance, stiffness, and electrical neutrality. Apart from these advantages, the brittleness and hardness of ceramics turn the mechanical processing into a challenging and difficult task. Against this background, modern laser technologies have already been used to process ceramics for many years, enabling a contactless and wear-free machining. However, regarding high precision applications, for instance, the drilling of micro-holes or the fabrication of well-defined cavities and three-dimensional structures, conventional laser processes reach their limits. Especially due to thermal influences of the laser radiation, brittle edges, stresses, and redeposited layers emerge. Ultrashort pulse lasers enable completely new processing qual...

Production of glass filters by selective laser sintering

Glass filters are often used in the field of medical technology and chemical analysis to separate particles of a defined size out of liquids. Depending on the application, different pore width from 1.6 μm to 500 μm are necessary. Glass materials are particularly suitable, because a high purity, the chemical resistance and a high thermal resistance of the filter are necessary. Traditionally, these glass filters are produced by conventional sintering. In the new investigations, selective laser sintering is investigated as an alternative method. The conventional sintering process allows defined pore sizes to be adjusted by varying the sintering time. The high purity of the glass filters can be achieved by a binder-free production. The material properties of the glass material, such as the chemical resistance or thermal stability is maintained by the sintering process. Typically, fused silica or borosilicate glasses are used as basis materials. High-temperature selective laser sintering (HT-SLS) is an additive manufacturing process for the production of silicate and porous components. This manufacturing technology allows complex and unconventional geometrics to be realized efficiently and flexibly. For this purpose, the volume model to be produced is first separated into the desired layer geometry and number of layers. In the subsequent specific process cycle, the glass powder is distributed by a squeegee on a building platform in a defined manner. A solid material layer is created by means of scanning CO2 laser radiation. After lowering the building platform and transporting the powder again, the component can be generated layer by layer. For the production of the glass filters by HT-SLS, initial investigations are carried out with synthetic and natural fused silica glass powders with particle diameters in the range of 19... 78 μm in spheroidized and vitrified form. A laser sintering furnace has been specially designed for the HT-SLS, which achieves process temperatures up to T = 1000 °C as well as low contamination of the glass powder. In addition, material-specific scan and parameter concepts are developed. A high component quality can be achieved by combining a hull-and-core scan strategy with a 180° scan field rotation each sintered layer. Also a bidirectional beam guide and a material-specific parameter concept is needed. The absorption of CO2 laser radiation and the heat-conduction of the powder are supported by the process-dependent plasma and the preheating of the building platform. The generated porous components are investigated with regard to the density and the bending strength. Component densities of ρ = 65 % and bending strengths of σ = 13.6 MPa are achieved. Basically, HT-SLS is an alternative method to the classical sintering process of glass powder to produce glass filters. In particular, an increase in efficiency with regard to the producible component geometry of the porous components can be achieved. This new technology offers a high degree of innovation, while at the same time requiring a high level of research.

New surface smoothing technologies for manufacturing of complex shaped glass components

The production of complex glass components with 2.5D or 3D-structures involves great effort and the need for advanced CNC-technology. Especially the final surface treatment, for generation of transparent surfaces, represents a timeconsuming and costly process. The ultrasonic-assisted grinding procedure is used to generate arbitrary shaped components and freeform-surfaces. The special kinematic principle, containing a high-frequency tool oscillation, enables efficient manufacturing processes. Surfaces produced in this way allow for application of novel smoothing methods, providing considerable advantages compared to classic polishing. It is shown, that manufacturing of transparent glass surfaces with low roughness down to Rq = 10 nm is possible, using an ultra-fine grinding process. By adding a CO2-laser polishing process, roughness can be reduced even further with a very short polishing time.