Quasi-simultaneous Laser Transmission Welding Research Articles

Effect of the meltdown on thermoplastic joint produced by quasi-simultaneous laser transmission welding

This study aims to investigate the effect of the polymer pair meltdown process and mechanical properties using three different laser transparencies 20%, 30% and modulated, ranging from a minimum 27% to a maximum 47%. The cross-sections of joints were analysed with an optical microscope. Burst pressure was performed to evaluate the strength of welded joints produced with different laser welding process parameters. Welding parameters, scanning speed and meltdown were varied and their influence on the quality of the polymer joint was recorded and evaluated. With the increase of meltdown, a more molten volume of polymer can be seen in the cross-section area. It was shown that with the increase of meltdown, the formation of pores occurred due to the high energy concentration obtained within each laser scan passing across the weld seam. The decrease of burst pressure in all polymer pairs was observed with 0.33 mm and 0.34 mm targeted meltdown. Burn marks and burned lines were observed in joints with 20% laser transparency. Modulated laser transparency influenced non-homogeneous temperature distribution inside the joint, which influenced the formation of heat-affected zones (HAZ) and pores. A study of the influence of cooling time on joint formation showed that it is possible to variate total meltdown while changing cooling time until the joint is solidified. • Within the increase of meltdown, bigger weld flash occurred inside weld seam. • Peak weld strength observed at specific meltdown values. • The decrease of weld strength is influenced by defect formation inside seam. • Non-homogeneous melting, HAZ observed with modulated laser transparency polymers. • Total meltdown can be controlled by modulating cooling time.

Laser beam positioning in quasi-simultaneous laser transmission welding of polymers

Laser beam positioning in quasi-simultaneous laser transmission welding of polymers

Scanning techniques for optimized damage tolerance in quasi-simultaneous laser transmission welding of plastics

Fiber lasers are increasingly replacing the commonly used diode lasers in laser transmission welding of plastics for challenging applications due to their superior beam quality and the ability to use long working distances and small spot sizes. At the same time, these smaller spot sizes increase the risk of thermal degradation. In this work, we investigate different scanning strategies that allow controlling the weld seam widths. These strategies are characterized regarding the demands on the performance of the scanner system, the risk of thermal degradation and the mechanical properties of the resulting welds.

Simulation of quasi-simultaneous laser transmission welding of plastics: optimization of material parameters in broad temperature range

Thermo-mechanical simulation offers great opportunities to optimize welding processes of plastics. For realistic simulation, the temperature dependent mechanical properties need to be implemented from ambient temperature to temperatures above the flow temperature. Standard test methods are insufficient for characterization in the entire temperature range because close to the flow temperature the material is too soft for tensile tests and too stiff for rheometry. Therefore, an optimization strategy is developed, that determines unknown material parameters by testing in welding simulations. The unknown parameters are iteratively adjusted to minimize the mismatch between computed and measured set-paths. Thus, important process characteristics are calculated realistically, enabling the computer aided assessment of the weld quality.

Measurement of core temperature through semi-transparent polyamide 6 using scanner-integrated pyrometer in laser welding

Predicting the core temperature during welding is an ambitious aim in many research works. In this work, a 3D-scanner with integrated pyrometer is characterized and used to measure the temperature during quasi-simultaneous laser transmission welding of polyamide 6. However, due to welding in an overlap configuration, the heat radiation emitted from the joining zone of a laser transmission weld has to pass through the upper polymer, which is itself a semi-transparent emitter. Therefore, the spectral filtering of the heat radiation in the upper polymer is taken into account by calibrating the pyrometer for the measurement task. Thermal process simulations are performed to compare the temperature field with the measured temperature signal. The absorption coefficients of the polymers are measured, in order to get precise results from the computation. The temperature signals during welding are in good agreement with the computed mean temperature inside the detection spot, located in the joining area. This is also true for varying laser power, laser beam diameter and the carbon black content in the lower polymer. Both, the computed mean temperature and the temperature signal are representing the core temperature. In order to evaluate the spatial sensitivity of the measurement system, the emitted heat radiation from both polymers is calculated on basis of the computed temperature field. Hereby it is found, that more than 90 percent of the detected heat radiation comes from the joining area, which is a crucial information for contact-free temperature measurement tasks on semi-transparent polymers.

3-D FE heat transfer simulation of quasi-simultaneous laser transmission welding of thermoplastics

Laser transmission welding of thermoplastics is a non-contact welding process for producing aesthetically and qualitatively high-grade joints with low thermal and mechanical stresses. In this research work, a three-dimensional heat transfer model is developed to simulate the quasi-simultaneous variant of laser transmission welding of thermoplastics. The thermal analysis is implemented to acquire the temperature history in this multi-pass welding process. The transient temperature field is computed using ANSYS® finite element code. The multi-pass movement of laser beam is realized by implementing a subroutine in ANSYS® Parametric Design Language (APDL). The effect of process variables namely laser power, welding speed, number of welding passes and line energy on temperature field during welding is investigated. The predicted outcomes of the temperature profiles, weld pool and time–temperature history are useful for optimizing the process parameters in the quasi-simultaneous laser transmission welding process.

Simulation-based investigation on the temperature influence in laser transmission welding of thermoplastics

Because of several advantages, e.g., an exact and high energy input, laser transmission welding has become more and more important in the last few years. Due to the contactless energy input, a sufficient process control is a challenge. In industrial production, the process parameters for a good weld seam are qualified by the energy input, which describes the process parameters laser power, laser velocity and irradiation time. These process parameters lead to the welding temperature, which influence the weld seam quality. The question remaining is whether the energy input describes the weld strength sufficiently or whether the welding temperature has a higher influence on the weld quality. In this study, the influence of the energy input on the weld quality is determined for an industrially relevant material combination (PBT ASA-GF20 and PC) in experimental examinations for quasi-simultaneous laser transmission welding. The welding temperature for every design point is calculated and the influence of the temperature on the weld strength is analyzed in an FEM model. In order to compare the influences of the two factors, welding temperature and energy input, a correlation analysis is performed. The correlation analysis shows a higher influence of the welding temperature on the weld strength compared to the energy input. But the energy input is also able to describe the weld strength.

In-Situ process monitoring during laser transmission welding of PA6-GF30

Quasi-simultaneous laser transmission welding is preferably used for packaging sensors and electronics. In order to protect the components from moisture, a hermetic encapsulation is needed. However, local weld seam interruptions cannot be identified with the commonly used set-path monitoring. By using a pyrometer, coaxially integrated into a 3D-scanner, gaps between the joining partners can be allocated on basis of the measured temperature. However, the scattering of the heat radiation, especially caused by the fiber reinforcement of the plastics, leads to a reduction of the accessible heat radiation, which makes the identification of gaps considerably more difficult. The herein used experimental setup is characterized by a small detection spot and only by a slight weakening of the heat radiation inside the scanner. Hence, for welding PA6-GF30, the detection of small sized gaps is possible, even if a glass fiber content of 30 percent (wt.) and a weld seam width with approximately 1 mm are given.

Gap-bridging During Quasi-simultaneous Laser Transmission Welding

Tightness is often the main requirement for quasi-simultaneous laser transmission welds. However, remaining gaps cannot be detected by the used set-path monitoring. By using a pyrometer in combination with a 3D-scanner, weld seam interruptions can be localized precisely while welding, due to temperature deviations along the weld contour. To analyze the temperature signal in correlation to the progress of gap-bridging, T-joint samples with predefined gaps are welded. The set-path is measured synchronously. Additionally, the temperature distribution and the influence of the thermal expansion of the polymers are studied by a thermo-mechanical FEM-process simulation. On top of that, the melt blow-out of the welded samples is analyzed using μCT-measurements. The experiments have shown that closing of a gap can be identified reliably by the temperature signal and that the squeezed melt flow into the gap and the thermal expansion in the gap zone accelerates gap-bridging. Furthermore the inserted heat can be adapted in the fault zone, in order to avoid thermal damage.

Influence of Adapted Wavelengths on Temperature Fields and Melt Pool Geometry in Laser Transmission Welding

Laser transmission welding is an established joining technology for the creation of strong, hermetic and aesthetic weld seams between thermoplastic parts. However, weld seam properties are strongly dependent on the optical properties of the materials involved. This paper investigates the wavelength-dependent absorption properties of polymeric materials and carbon black, their influence on temperature field generation and the resulting melt pool geometry in laser transmission welding. A FE simulation model is developed to examine the possibilities of influencing the temperature fields during contour and quasi-simultaneous laser transmission welding by adapting the wavelengths under consideration of the absorption and scattering properties. The application of laser wavelengths in the spectral range of 1400nm to 2000nm leads to modified temperature fields and melt pool geometries, which are expected to feature a better load-bearing capacity and a much improved gap-bridging capability.

Process Monitoring at Laser Welding of Thermoplastics

AbstractA pyrometer, integrated into a 3D‐scanner, offers the possibility to measure the weld seam temperature at quasi‐simultaneous laser transmission welding. Experimental studies have shown that gaps located in the joining zone can be identified by a temperature rise even at a high scanning velocity. This enables the implementation of algorithms for observation and control strategies.