Thomas Frick

Optical properties of plastics and their role for the modelling of the laser transmission welding process

Laser transmission welding of plastics is a joining technique which permits the welding of plastic parts with low process inherent thermal and mechanical stresses. In order to compute the temperature fields during the welding process a simulation model of the process is established. Especially the optical and thermal properties of the welded materials are necessary as input parameters in order to model the process properly. While the thermal properties of polymers are well-investigated, there is little knowledge about the optical properties of plastics. The optical properties of different amorphous and semi-crystalline thermoplastics are therefore investigated in dependence on material temperature and additive concentration. Based on these data it is possible to analytically describe the absorption of laser energy in plastics. The computation of temperature fields during welding is carried out for polypropylene and the results are then compared with experimentally accomplished welding tests. The calculated results show good accordance with the experimentally determined melt pool geometries of welded specimens and prove the compatibility of the computation model with the experimental analysis.

Process control in laser manufacturing–dream or reality?

Looking at the status of control techniques in laser manufacturing processes one has to focus on the sensor technique, the computing hardware and the algorithms available. Sensor technology has evolved in the last years. Now fast two-dimensional sensors like CMOS cameras can be applied whereas five year ago mostly one dimensional sensors like photodiodes were common. Even three dimensional sensors are being developed and might have a good potential in future applications. The use of such sensors has shown varying success in different processes. Thus this article deals with materials processing like welding and brazing of metals and plastics welding as well as with medical applications like cutting human tissue.Looking at the status of control techniques in laser manufacturing processes one has to focus on the sensor technique, the computing hardware and the algorithms available. Sensor technology has evolved in the last years. Now fast two-dimensional sensors like CMOS cameras can be applied whereas five year ago mostly one dimensional sensors like photodiodes were common. Even three dimensional sensors are being developed and might have a good potential in future applications. The use of such sensors has shown varying success in different processes. Thus this article deals with materials processing like welding and brazing of metals and plastics welding as well as with medical applications like cutting human tissue.

Laser welding of dissimilar copper-aluminum connections by means of roll-cladded inserts

Thermal welding processes of copper and aluminum often lead to insufficient connections due to an embrittlement and an unsatisfactory ductility of the melting zone. This embrittlement is caused by an extensive intermixture of the dissimilar base materials during the molten state which effectuates a formation of intermetallic phases. While it is possible to increase the tensile strength of copper-aluminum joints using micro alloying methods and a lateral beam displacement, the ductility of such connections is still limited to a certain extent.A promising approach to reach a higher ductility of dissimilar copper-aluminum connections is the use of roll-cladded copper-aluminum sheet metal for the laser-welding process. With the standardized roll-cladding technique it is possible to reach excellent copper-aluminum connections by cold-pressure welding. Such roll-cladded elements can be used to split the joining sequence of copper and aluminum into two separate joining steps, which is on the one side an aluminum-aluminum welding and on the other side a copper-copper welding.Indeed the most important challenge for the guiding of the joining process is the prevention of intermetallic phases within the roll-cladded inserts, as the thermal laser-welding influences the contact layer of copper and aluminum inside the roll-cladded sheet metal.Thermal welding processes of copper and aluminum often lead to insufficient connections due to an embrittlement and an unsatisfactory ductility of the melting zone. This embrittlement is caused by an extensive intermixture of the dissimilar base materials during the molten state which effectuates a formation of intermetallic phases. While it is possible to increase the tensile strength of copper-aluminum joints using micro alloying methods and a lateral beam displacement, the ductility of such connections is still limited to a certain extent.A promising approach to reach a higher ductility of dissimilar copper-aluminum connections is the use of roll-cladded copper-aluminum sheet metal for the laser-welding process. With the standardized roll-cladding technique it is possible to reach excellent copper-aluminum connections by cold-pressure welding. Such roll-cladded elements can be used to split the joining sequence of copper and aluminum into two separate joining steps, which is on the one side an aluminum...

Laser transmission welding with part adapted temperature fields

Thermoplastics are present in a wide variety of industrial applications. Different welding techniques like vibration, resistance and laser welding are used to join these materials. Laser transmission welding is known for high flexibility and extraordinary possibilities of process automation. Today, there is rising interest in joining large thermoplastic parts with three-dimensional formed weld seams. To successfully obtain these seams, the parts have to be clamped in such a way that is nearly gap-free. Gaps could result in processing issues such as welding failures, poor achievable process speed and low weld seam strengths. To overcome these problems, a new laser transmission welding technique was developed to achieve part adapted temperature fields based on a combination of contour and quasi-simultaneous welding. In addition, the authors are investigating the effectiveness of this strategy with a thermal finite element model. This model focuses on the heat affected zone geometries, temperature gradients and simulated melt pool geometries depending on the parameters creating part adapted temperature fields dynamically. The results from the model were verified by experiments with a diode laser. Furthermore, process control strategies were developed, which can later be used to ensure a constant weld seam quality. The process control is carried out by an on-axis pyrometer, which allows online temperature detection within the weld seam.Thermoplastics are present in a wide variety of industrial applications. Different welding techniques like vibration, resistance and laser welding are used to join these materials. Laser transmission welding is known for high flexibility and extraordinary possibilities of process automation. Today, there is rising interest in joining large thermoplastic parts with three-dimensional formed weld seams. To successfully obtain these seams, the parts have to be clamped in such a way that is nearly gap-free. Gaps could result in processing issues such as welding failures, poor achievable process speed and low weld seam strengths. To overcome these problems, a new laser transmission welding technique was developed to achieve part adapted temperature fields based on a combination of contour and quasi-simultaneous welding. In addition, the authors are investigating the effectiveness of this strategy with a thermal finite element model. This model focuses on the heat affected zone geometries, temperature gradients ...

Analysis of the interaction between the temperature field and the weld seam morphology in laser transmission welding by using two different discrete laser wavelengths

Laser transmission welding is a non-contact and efficient process technology for joining thermoplastic polymers. In the conventional process, laser sources in the wavelength range of 1 μm are usually used. Therefore, most of the laser radiation is transmitted through the upper joining partner and absorbed only in the lower joining partner. As a result, the possibilities to influence the temperature field especially in the upper joining partner are limited. To overcome these limitations, an additional thulium fiber-laser with a wavelength of 1.94 μm is used in this study and coaxially aligned with a diode laser. The use of an additional thulium fiber-laser leads to a significant absorption in the upper joining partner. Through this approach, it is shown that the temperature field and the weld seam geometry can be influenced by using these two different discrete laser wavelengths. Depending on the intensity distribution of both lasers, an increase of the size of the heat affected zone in the upper joining partner can be observed. In order to develop a better process understanding, a thermal finite element model is built up and verified by comparing the calculated size of the heat affected zone for different process parameters with the experimental data. The model is able to represent the influence of both laser sources on the temperature field and is used to calculate characteristics of the temperature field, such as maximum temperatures or cooling rates. The characteristics are then used to explain the weld seam morphology, such as occurrence and size of spherulitic structures in the weld seam.Laser transmission welding is a non-contact and efficient process technology for joining thermoplastic polymers. In the conventional process, laser sources in the wavelength range of 1 μm are usually used. Therefore, most of the laser radiation is transmitted through the upper joining partner and absorbed only in the lower joining partner. As a result, the possibilities to influence the temperature field especially in the upper joining partner are limited. To overcome these limitations, an additional thulium fiber-laser with a wavelength of 1.94 μm is used in this study and coaxially aligned with a diode laser. The use of an additional thulium fiber-laser leads to a significant absorption in the upper joining partner. Through this approach, it is shown that the temperature field and the weld seam geometry can be influenced by using these two different discrete laser wavelengths. Depending on the intensity distribution of both lasers, an increase of the size of the heat affected zone in the upper joining p...

Electrophotographic multi-material powder deposition for additive manufacturing

In this paper, the use of electrophotographic polymer powder transfer for the preparation of multi-material layers is discussed with respect to the application in powder bed-based additive manufacturing technologies as selective laser sintering (SLS). Therefore, the challenges of this task are considered verifying the critical process steps in order to develop a concept for an electrophotograhic laser sintering machine. On that basis, an experimental setup with a two-chamber design is realized which enables the investigation of the electrophotographic powder transfer at typical process conditions of SLS. Using this setup, transfer tests of polypropylene powder patterns were performed and qualitatively analyzed by photographic imaging. The results confirm the high potential of the application of electrophotography for multi-material powder deposition and show how a residual electrophotographic powder deposition can be achieved in general, which is independent from the already produced part height, in order to build up three-dimensional multi-material components.In this paper, the use of electrophotographic polymer powder transfer for the preparation of multi-material layers is discussed with respect to the application in powder bed-based additive manufacturing technologies as selective laser sintering (SLS). Therefore, the challenges of this task are considered verifying the critical process steps in order to develop a concept for an electrophotograhic laser sintering machine. On that basis, an experimental setup with a two-chamber design is realized which enables the investigation of the electrophotographic powder transfer at typical process conditions of SLS. Using this setup, transfer tests of polypropylene powder patterns were performed and qualitatively analyzed by photographic imaging. The results confirm the high potential of the application of electrophotography for multi-material powder deposition and show how a residual electrophotographic powder deposition can be achieved in general, which is independent from the already produced part height, in order...

Fast and flexible production of mechatronic integrated devices by means of additive manufacturing

Due to continuous improvements in process technology additively generated components are increasingly used for prototypes and small series devices. In comparison to conventional manufacturing process chain, additive manufacturing enables in principle embedding of conductive circuits and electronic components during the part building process. However, technologies like Selective Laser Sintering (SLS) or stereolithography (SLA) imply disadvantages concerning high process temperatures or time consuming cleaning steps. In contrast, Fused Deposition Modeling (FDM) seems to be well suited for production of mechatronic integrated devices due to the use of thermoplastic materials for electronic productions (e.g. ABS, PC/ABS), a low thermal load of electronic parts and a simple process management. In this paper, additive manufacturing of mechatronic devices by means of FDM in combination with direct printing of silver ink for generating conductive circuits is investigated. The silver ink is deposited in the matrix or applied on the surface of the part by a dispensing system during the building process. The in situ sintering process of the ink is carried out by a subsequently infrared laser (IR) irradiation. In order to generate components with a high mechanical stability the adhesion of the extruded material strings is optimized considering processing temperature and flow properties of the extruded thermoplastics (ABS, PC/ABS). Furthermore, depending on the flow behavior important FDM process parameters (e.g. volume flow, hatch distance) are adjusted to minimize cavities and generate media-tight components. The conductive circuits are characterized with respect to their electrical conductivity and mechanical stability.

Laser-based hot-melt bonding of CFRP and GFRP - A novel joining technology for multi-material-design

Due to increasing ecological awareness lightweight design became a focus in the past few years. Fiber reinforced plastics (FRP) with thermosetting matrix are commonly used for lightweight constructions. Their strength-to-weight ratio is significantly higher in comparison to metallic construction materials. However in many cases product-specific requirements can be only achieved by multi-material-systems. Therefore it is necessary to reach an integration of different materials by means of suitable joining technologies. According to the current state of the art joining of thermosetting composite to thermoplastic is limited by available joining techniques. A solution for this problem offers laser-based hot-melt bonding. This paper presents experimental results of joining carbon fiber reinforced plastics (CFRP) as well as glass fiber reinforced plastics (GFRP) to thermoplastics by means of laser-based hot-melt bonding. Thereby the influence of surface treatment and used thermoplastic on the joint connection is investigated. The specimens are characterized by microscopy and tensile shear tests. Additionally climate tests are performed to analyze the long-term durability of the joint connections. Besides a thermal simulation of the joining process is developed that predicts temperature fields during the joining process. Finally a comparison between laser-based hot-melt bonding and conventional adhesive bonding is given.Due to increasing ecological awareness lightweight design became a focus in the past few years. Fiber reinforced plastics (FRP) with thermosetting matrix are commonly used for lightweight constructions. Their strength-to-weight ratio is significantly higher in comparison to metallic construction materials. However in many cases product-specific requirements can be only achieved by multi-material-systems. Therefore it is necessary to reach an integration of different materials by means of suitable joining technologies. According to the current state of the art joining of thermosetting composite to thermoplastic is limited by available joining techniques. A solution for this problem offers laser-based hot-melt bonding. This paper presents experimental results of joining carbon fiber reinforced plastics (CFRP) as well as glass fiber reinforced plastics (GFRP) to thermoplastics by means of laser-based hot-melt bonding. Thereby the influence of surface treatment and used thermoplastic on the joint connection i...

Additive manufacturing of small geometrical structures - An analysis of the potential regarding the laser sintering process of a high temperature resistant thermoplastic

Additive manufacturing already addresses the sub millimeter region of small geometrical parts. Regarding laser sintering of polymers with small structures a basic restriction which is often named is the average particle size of the used powder material. But also material properties and the used system technology determine the limits of small geometrical structures. These restrictions often decide about the feasibility of a given task. In this paper the influence of material properties and system parameters are investigated and strategies for optimizing the quality of small geometrical part features are proposed. As an example of a material with a high temperature resistance above 200 °C, tensile strengths above 50 N/mm² and high durabilities of functional parts the high performance polymer polyetheretherketone (PEEK) is chosen. The investigations include the manufacturing of thin single layers perpendicular to the building direction, small pins parallel to the building direction, thin walls manufactured by single laser lines in each building layer and cylindrical cavities as well as vertical inner geometrical features. For these structures the minimum achievable sizes are presented in this paper and an outlook on the potential of the process for manufacturing even smaller structures is given.Additive manufacturing already addresses the sub millimeter region of small geometrical parts. Regarding laser sintering of polymers with small structures a basic restriction which is often named is the average particle size of the used powder material. But also material properties and the used system technology determine the limits of small geometrical structures. These restrictions often decide about the feasibility of a given task. In this paper the influence of material properties and system parameters are investigated and strategies for optimizing the quality of small geometrical part features are proposed. As an example of a material with a high temperature resistance above 200 °C, tensile strengths above 50 N/mm² and high durabilities of functional parts the high performance polymer polyetheretherketone (PEEK) is chosen. The investigations include the manufacturing of thin single layers perpendicular to the building direction, small pins parallel to the building direction, thin walls manufactured b...

Additive manufacturing of functional components – Recent developments and challenges for the future

Additive manufacturing techniques are becoming more and more widespread in industry during the last years due to their high flexibility regarding part geometry. But a true breakthrough does not take place due to the limited choice of applicable materials and consequently the restricted functions the generated parts can fulfill especially in case of polymer materials. Certain process limitations like a special melt and crystallization behavior or an exact defined UV-curability restrict the material selection. One strategy beside the qualification of new materials could be the use of different functionalization approaches. These approaches include not only the possibility to improve certain part properties by filling materials or coating parts but also to establish new part functions. Besides filling or coating a further strategy to enhance the functionality of additive manufactured parts can be the embedding of functional components.This paper presents the challenges in manufacturing functional parts by means of additive technologies and discusses results on new materials and techniques in order to achieve this aim. It especially focuses on the industrial relevant additive technologies of laser sintering and stereolithography. In the field of laser sintering functional additives or coatings allow an improvement of the biocompatibility of individual implants made from PEEK but also the integration of electrical functions on the surface of laser sintered PA 12 parts usable as electronic circuit boards in the near future. Further a new technology ‘embedded stereolithography’ (E SLA) is shown which allows the fabrication of complex mechatronical modules in one manufacturing system. Based on the presented findings an outlook will be given which shows the necessary work for the future.Additive manufacturing techniques are becoming more and more widespread in industry during the last years due to their high flexibility regarding part geometry. But a true breakthrough does not take place due to the limited choice of applicable materials and consequently the restricted functions the generated parts can fulfill especially in case of polymer materials. Certain process limitations like a special melt and crystallization behavior or an exact defined UV-curability restrict the material selection. One strategy beside the qualification of new materials could be the use of different functionalization approaches. These approaches include not only the possibility to improve certain part properties by filling materials or coating parts but also to establish new part functions. Besides filling or coating a further strategy to enhance the functionality of additive manufactured parts can be the embedding of functional components.This paper presents the challenges in manufacturing functional parts by me...