Andreas Patschger

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.

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...

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...

Constraints and optimization of the laser microwelding process of thin metal foils

Laser microwelding of thin metal foils of various materials is established in several fields of application. Within all these application fields, a sufficient and reliable welding process is required to join thin metal foils successfully. One crucial function of the weld is often a gas-tight sealing. For instance, titanium foils provide hermetic and sterile packaging of medical implants, and vacuum insulation panels and pressure sensors are assembled from stainless steel foils. Even if gas-tightness is not the aim, weld defects lower the functionality of the product. In the case of a roll imprint process in order to structure optical surfaces on films and panels, where nano-structured nickel foils serve as masters, weld imperfections lead to failures in the optical structure. Furthermore, insufficiently welded cathode foils, which are made of aluminum, cause an increasing electric resistivity. Weld defects during laser microwelding are provoked by thermally induced distortion due to small foil thickness. ...

New approach to clamping in microwelding

For the purpose of manufacturing lithium ion cells, microwelding is a commonly used procedure. It is also applied to new products as vacuum insulation elements which are enclosed in stainless steel foils. Another new application can be found in welding of catalytic converters. Before, they were usually joined by furnace brazing. All these products consist of thin metal foils which can be considered two-dimensional work pieces. They have to be joined in a stable process in order to avoid rejects. One of the biggest challenges in thermal microwelding is the gap forming between the join partners due to thermally induced distortion. In microwelding, the possibility to bridge the gap between the join partners is ensured as long as the gap is smaller than approximately half of the material thickness. Beyond this point, weld seam imperfections occur and the process collapses. In order to minimize gap formation, a new approach to clamping is presented which directly applies the clamping force in the welding zone....

Micro welding - How far can we go?

“There is a plenty of room at the bottom.” stated Richard Feynman in 1959, considering micro- and nanoscale systems. For the purpose of assembling these micro- and nanoscale systems the proper joining technology is the key. Laser micro welding is commonly used in order to join down scaled components with small thicknesses around 100 µm. Applications can be found in medical technology, precision mechanics or energy storage. But the material thickness has to be lowered far beyond for proper usage in microscale systems. Material stiffness decreases exponentially und thermal induced distortion increases exponentially with decreasing material thickness. Therefore, welding below 100 µm is quite challenging.In this work the seam welding of 2 × 5 µm stainless steel foils is presented. The thermal induced distortion can be suppressed and the thermal contact between the joining partners is optimized by means of an adapted, novel clamping approach. Additionally, a customized welding process, fitted to the requirements of ultra-thin material thicknesses, lead to a successful joint.By developing an imperfection-free, customized welding process the challenges of down scaled material thicknesses can be overcome and the micro welding can be qualified for ongoing miniaturization of MEMS.“There is a plenty of room at the bottom.” stated Richard Feynman in 1959, considering micro- and nanoscale systems. For the purpose of assembling these micro- and nanoscale systems the proper joining technology is the key. Laser micro welding is commonly used in order to join down scaled components with small thicknesses around 100 µm. Applications can be found in medical technology, precision mechanics or energy storage. But the material thickness has to be lowered far beyond for proper usage in microscale systems. Material stiffness decreases exponentially und thermal induced distortion increases exponentially with decreasing material thickness. Therefore, welding below 100 µm is quite challenging.In this work the seam welding of 2 × 5 µm stainless steel foils is presented. The thermal induced distortion can be suppressed and the thermal contact between the joining partners is optimized by means of an adapted, novel clamping approach. Additionally, a customized welding process, fitted to the requiremen...

Flexible and efficient laser remote welding of ultra-thin metal foils

This paper discusses investigations regarding flexible and efficient strategies of laser remote welding of ultra-thin metal foils (≤ 50 µm) which are applied in the fields of electronics, packaging, and construction. A single-mode fiber laser was used, equipped with a scanner head and diverse objectives. Thus, different optical settings and material thicknesses could be tested and compared with regard to process stability, reliability and efficiency. The long-term stability of the optical setting was determined related to focus shift and beam shape in the working plane. The specimens were analyzed with methods of micro-sections, microhardness and tensile strength tests. In this way, it could be shown that laser remote welding of ultra-thin metal foils is a well-suited modern technology which is able to substitute former slower or inflexible techniques respectively compound materials with a high in-house production depth.This paper discusses investigations regarding flexible and efficient strategies of laser remote welding of ultra-thin metal foils (≤ 50 µm) which are applied in the fields of electronics, packaging, and construction. A single-mode fiber laser was used, equipped with a scanner head and diverse objectives. Thus, different optical settings and material thicknesses could be tested and compared with regard to process stability, reliability and efficiency. The long-term stability of the optical setting was determined related to focus shift and beam shape in the working plane. The specimens were analyzed with methods of micro-sections, microhardness and tensile strength tests. In this way, it could be shown that laser remote welding of ultra-thin metal foils is a well-suited modern technology which is able to substitute former slower or inflexible techniques respectively compound materials with a high in-house production depth.

Welding with high brilliance lasers and high power density

The increasing demand for efficient and powerful lasers causes the market for fiber-and disc lasers to grow. These systems include high brilliance and so the ability of realizing very small foci or long focal length.This research phenomenological analysis was performed to understand the correlation between small foci and welding depth. Besides, the stability of the laser process and the resulting strength of a double steel sheet joint are investigated.The power density in the laser beam increases as focusing increases and foci become smaller. But as the flank angle becomes steeper, the Rayleigh length decreases. This can also mean less welding depth. The investigations on bead on plate and beam shape measurements determine a focal diameter threshold below which the welding depth starts decreasing.Often root fusion occurs at multi sheet joints to maximize the resulting strength. But this can also lead to weld imperfections through holes or burn-ins. This research demonstrates that a controlled reduced welding depth can also provide nearly the same strength and does not impair the underside of the sheet or its coating.The increasing demand for efficient and powerful lasers causes the market for fiber-and disc lasers to grow. These systems include high brilliance and so the ability of realizing very small foci or long focal length.This research phenomenological analysis was performed to understand the correlation between small foci and welding depth. Besides, the stability of the laser process and the resulting strength of a double steel sheet joint are investigated.The power density in the laser beam increases as focusing increases and foci become smaller. But as the flank angle becomes steeper, the Rayleigh length decreases. This can also mean less welding depth. The investigations on bead on plate and beam shape measurements determine a focal diameter threshold below which the welding depth starts decreasing.Often root fusion occurs at multi sheet joints to maximize the resulting strength. But this can also lead to weld imperfections through holes or burn-ins. This research demonstrates that a controlled reduced weld...