Transmission Welding Research Articles

Laser transmission welding of PMMA using IR semiconductor laser complemented by the Taguchi method and grey relational analysis

Abstract Laser transmission welding involves localized heating and non-contact techniques at the interface of two polymer pieces to be joined. The welding quality is very sensitive to speed welding, thickness and welding line width and depth. This work presents the laser transmission welding of Poly(Methyl Meta acrylate) PMMA polymer transparent and oblique slabs. A semiconductor laser with 808 nm wavelength, 2 W output power and a 2 mm beam diameter was used. Three different transparent slabs with dimensions 40 × 60 mm and thickness (2.6, 3.7, 4.3 mm) were fixed tightly onto oblique slab (dark color). Three different selected speeds (44, 113, 150 mm/min) of the welding process were applied to the prepared samples. Results indicate the welding line width and depth were inversely proportional to the welding speed while the transparent slab thickness have little effect. Nine experiments were performed for the three different thickness at different speeds. Taguchi method and grey relational analysis were used to determine the optimum values for welding speed, width, depth and thickness during these nine experiments. The results exhibit good agreement with those of the Taguchi method and grey relational analysis.

Optimization of Laser Transmission Welding Process Parameters Using Single Objective and Multi Objective Optimization Technique

Optimization of Laser Transmission Welding Process Parameters Using Single Objective and Multi Objective Optimization Technique

Fabrication of bi-layer PMMA and aluminum 6061-T6 laminates by laser transmission welding: Performance prediction and optimization

Laser transmission welding can be effectively used in fabrication of laminated parts. In the present work an attempt was made to fabricate a polymeric laminate structure made of polymathic methacrylate (PMMA) and aluminum 6061-T6 by laser transmission welding process. A series of laser welding experiment has been carried out to analyze effect of laser power, welding speed, focal position and axial pressure on lap shear force and weld seam width. Response surface methodology including regression analysis, analysis of variances and desirability approach function has been utilized here to analyze the processing data. It is found from the results that focal position has greatest influence on both the lap shear force and weld bead width. Optimization is also carried out in two different criteria for achieving desired welding performance. The optimization results obtained by this approach are consistent well with the results measured through confirmatory experiments.

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.

Modeling of the thermally induced residual stresses during laser transmission welding of thermoplastics

Laser transmission welding is one of the various welding techniques used to join thermoplastics. Low heat introduction into the welded parts and a high welding speed are the reasons why laser transmission welding established itself in the plastic processing industry. In all welding processes for thermoplastics, residual stresses occur due to the thermal expansion during heating and its resetting during subsequent cooling. These thermally induced residual stresses impair the mechanical properties of the welded components. To estimate the residual stresses and therefore to improve the process setup and understanding of the welding process, a thermo-mechanical simulation model for the laser transmission welding is developed at the IKV. In a linked simulation, the temperature distribution and mechanical stresses are calculated parallelly for each time step. The material, a polyamide 6.6, is modeled with elastic, elastoplastic, and linear viscoelastic behavior. After cooling, residual stresses of a realistic magnitude are calculated. However, as the yield point of the polyamide is exceeded in the welding process, the tensile/compression ratio in the welded component is only mapped correctly for the elastic-plastic material model.

3D modeling of thermoplastic composites laser welding process – A ray tracing method coupled with finite element method

Laser Transmission Welding process (LTW) is a well know technic for polymer welding. This technic can also be applied to thermoplastic glass fiber composite (polymer + continuous glass fibers). But in this case two phenomenon occur and are able to make the welding in a poor quality: light absorption and light scattering. The two phenomenon occurring into the semi-transparent substrate of reinforced fiber thermoplastic, during the LTW, are numerically examined. A 3D transient thermal model of LTW is achieved with the FEM software COMSOL. A ray tracing software is developed to compute the heat source term used in the FEM simulation. First, the energy distribution coming from the laser irradiation is assessed. Ray tracing techniques allowed us to deal with both absorption and a strong light-scattering caused by the heterogeneity of composite: polymer matrix and glass fibers. Then, the energy balance equation was solved in order to study the heating stage. The present paper proposes an investigation on the influence of the absorption on the heat affected zone. The interest to consider absorption was shown for process optimization purposes and for the use of reinforced composites colored or filled with additives.

Evaluation of material based influences at laser based heat conduction welding of carbon fiber reinforced plastics

Today, fiber reinforced materials are present in a wide field of industrial applications. Short glass fiber reinforced composites are used in automotive, aerospace, and medical sectors. In the last couple of years, fiber reinforced thermoplastics have gained importance as a construction material, especially for lightweight assembly. There are different methods of joining thermoplastic materials such as vibration, resistance, and induction welding. A more advanced process is the laser transmission welding, which can be characterized by its excellent reproducibility, high flexibility, and potential for automation. Laser transmission welding can be applied for joining unreinforced or glass fiber reinforced thermoplastic parts. This welding process was transferred to heat conduction welding for joining thermoplastic carbon fiber reinforced plastics to itself. The goal of these investigations was to determine the influence of the material thickness on the weld seam quality.

Laser transmission welding and surface modification of graphene film for flexible supercapacitor applications

When a graphene film is coated onto a flexible polymer substrate, weak adhesion can cause delamination of the film under mechanical bending. Moreover, as each graphene layer restacks, the performance of the film as an electrode for a supercapacitor becomes limited. In this study, facile laser welding and surface modification processes are demonstrated to overcome these limitations. First, a continuous wave laser beam is applied to the interface between the coated graphene and the underlying transparent polycarbonate substrate. This welding process significantly improves their adhesion and enables excellent mechanical bendability. Second, surface modification of graphene is achieved under ambient conditions by irradiating the graphene film surface with a nanosecond pulsed laser. Sandwich-type supercapacitors are fabricated using these surface-modified graphene electrodes with a PVA-H3PO4 electrolyte. The effect of the laser fluence on the performance of the supercapacitor is investigated. At an optimal laser power, an areal capacitance of 4.7 mF/cm2 is achieved.

Investigations on the Interplay Between Focusing and Absorption in Absorber-Free Laser Transmission Welding of Plastics

Lab-on-a-chip devices, where small channels and reaction chambers have to be covered with a sealing lid, laser beam welding has already been proven to show considerable potential for standard material combinations. To avoid absorbing additives, the wavelength of the laser has to fit the absorption bands of the polymers. For it to achieve a localized weld seam, the laser beam has to be strongly focused. To achieve the desired temperature distribution, we establish and discuss the heat rate generated by a focused beam as a function of the absorption coefficient and the Rayleigh length. Thermal simulation of the weld process reveals the dependence of the temperature on Rayleigh length and absorption coefficient. Experiments were carried out with a tunable laser source emitting between 700 and 1000 nm on polycarbonate samples doped with Lumogen 788 dye. Microtome cuts of the welded samples reveal the dimensions of the heat-affected zone and, therefore, of the seam. Experimental and simulation results show that for typical welding conditions a localized seam inside a sample can be achieved with an appropriate focusing of the laser beam and tuning of the wavelength. The dimensions of the heat-affected zone decrease as absorption coefficient decreases and welding velocity increases. The discussion of the functional dependence of the heat rate on absorption coefficient and Rayleigh length shows that Rayleigh length and sample thickness have to be smaller than the optical penetration depth.

An artificial neural network model for laser transmission welding of biodegradable polyethylene terephthalate/polyethylene vinyl acetate (PET/PEVA) blends

Laser transmission welding is a quick, easy, and viable method to join plastic materials for several industrial domains. The main challenge for manufacturers is still on how to choose the input process parameters to achieve the best joint performance. Joining between PET (polyethylene terephthalate) films does not make an exception, with quality strictly depending on laser joining parameters. The purpose of the present study is to estimate the weldability of a polymeric material couple according to their thermal and optical properties. This paper investigates an experimental study of laser transmission welding of PET 100% and PET-PEVA (polyethylene vinyl acetate) 5%, 10%, and 15% sheets by a diode laser. In the present work, laser power and scan speed were considered as operational parameters, which have a significant influence on the quality of the joint zone. Then, the influence of PEVA aliquots in PET/PEVA blends, which altered the mechanical properties, such as joining behavior, mechanical characterization, and degradation level, was analyzed. In addition, an artificial neural network model is developed to achieve the optimal laser parameters. The obtained results proved the advantage of this model, as a prediction tool, for developing laser welding parameters.

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.

Welding Strength and Interface Characteristics in Laser Transmission Welding of Polyamide Resin

Welding Strength and Interface Characteristics in Laser Transmission Welding of Polyamide Resin

Improvement of Laser Transmission Welding of Glass with Titanium Alloy by Laser Surface Treatment.

Laser surface treatment of the titanium alloy was locally oxidized on the metal surface to improve the joint strength of laser transmission welding of high borosilicate glass with titanium alloy. The results find that the welding strength was increased 5 times. The welding mechanism was investigated by the morphology of the welded parts, the tensile-fracture failure mode, the diffusion of the interface elements, and the surface free energy. The results show that there are many adherents between the titanium alloy and high borosilicate glass after tensile fracture, the welding strength was higher when the laser voltage was 460 V, and the tensile–fracture failure mode is mainly ductile fracture. Element-line scanning analysis revealed that elemental diffusion occurred in the two materials, which is an important reason for the high welding strength. Surface free-energy analysis shows that laser surface treatment improves the surface free energy of titanium alloy, promotes the wettability and compatibility, and increases the welding strength of titanium alloy with glass.

Effects of Metal Absorber Thermal Conductivity on Clear Plastic Laser Transmission Welding**Supported by the National Key R&D Program of China under Grant No 2016YFA0401100, the National Natural Science Foundation of China under Grant No 61575129, and the National High-Technology Research and Development Program of China under Grant No 2015AA021102.

In our previous study, metals have been used as absorbers in the clear plastic laser transmission welding. The effects of metal thermal conductivity on the welding quality are investigated in the present work. Four metals with distinctly different thermal conductivities, i.e., titanium, nickel, molybdenum, and copper, are selected as light absorbers. The lap welding is conducted with an 808nm diode laser and simulation experiments are also conducted. Nickel electroplating test is carried out to minimize the side-effects from different light absorptivities of different metals. The results show that the welding with an absorber of higher thermal conductivity can accommodate higher laser input power before smoking, which produces a wider and stronger welding seam. The positive role of the higher thermal conductivity can be attributed to the fact that a desirable thermal field distribution for the molecular diffusion and entanglement is produced from the case with a high thermal conductivity.

Connection Mechanism of Molten Pool during Laser Transmission Welding of T-Joint with Minor Gap Presence.

Laser welding of T-joint transmitting from the face plate to the core is commonly used in the sandwich structure preparation. Minor gaps between the face and the core plate are inevitably present after several beads on the sandwich structure welding due to the thermal deformation. The effects of gap presence on fluid flow from the face to the core plate are rather significant, where the gas can be easily entrapped into the pool and form the pores. To this end, three-dimensional transient simulations based on VOF (volume of fluid) method were conducted to explore and ascertain the effect of fluid flow inside the pool on the pore formation due to the gap presence. It was found that minor gap within 0.2 mm will not reduce the welding quality. Under the effects of gravity and surface tension, the fluid from the face sheet will drop down to the core, which removes all the air out of the gap and the laser goes through the fluid of the gap and then shines on the core, which prevents the air from being entrapped into the pool. While the laser goes though gap, the wall of keyhole opens and closes continuously. The vibrating time of keyhole is approximately 0.029 s. After finishing the vibration, the welding is stable, which is the same as common unfull penetration. Finally, the simulated results are well verified through observing the plasma oscillating frequency in the gap and comparing to the pore-free bead profile. This paper supplies evidence that minor gap presence during laser transmitting welding on sandwich structure has nothing to do with pore formation.

Analysis of the Weld Seam Morphology of Polypropylene in Laser Transmission Welding

Laser transmission welding is a well-known joining technology for welding thermoplastics. Although the process is already used industrially, fundamental process-structure-property relationships are not fully understood and are therefore the subject of current research. One aspect of these mentioned process-structure-property relationships is the interaction between the temperature field during the welding process, the weld seam morphology of semi-crystalline thermoplastics, and the weld seam strength. In this study, the influence of the line energy on the weld seam morphology of polypropylenes is analyzed. For this purpose, the size of spherulites in the weld seam is investigated, as well as different occurring phases of polypropylene (α- and β-phase). It is shown that both the spherulite size of the α-phase and the amount of β-phase increase with increasing line energy. For the explanation and discussion of the results, a temperature-dependent thermal simulation model is used to derive characteristic attributes of the temperature field (maximum temperatures, cooling rates, temperature gradients).

Application specific intensity distributions for laser materials processing: Tailoring the induced temperature profile

In laser materials processing the intensity distribution of the laser beam strongly influences the spatial and temporal temperature profile that is induced within the treated material and therefore the shape of the heat influenced zone. As a consequence, the processing quality and efficiency can be increased by using adapted intensity distributions that explicitly tailor the generated temperature profile. In this work, an efficient numerical method to calculate application specific intensity distributions is presented that induce prescribed spatial and temporal temperature profiles in the material. To this end, this task is described as an inverse heat conduction problem which is solved with the conjugate gradient method with adjoint problem. As temperature-dependent thermo-physical material properties, volumetric beam absorption and quasi-stationary distributions can be accounted for, this approach proves to be very general. The performance of the algorithm is then shown by presenting two different test cases. While the first one is taken from the application of laser softening, the second one simulates a time-dependent intensity distribution for laser transmission welding. Furthermore, ideas how to implement the obtained intensity distributions, which are very inhomogeneous, are provided using specialized beam shaping techniques. The results for both test cases are validated using commercial FEM solvers.

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.

Effect of processing parameters on meltdown in quasi-simultaneous laser transmission welding

Meltdown is the term used to describe the collapse of two thermoplastic parts during welding. It is a critical process parameter in the laser-transmission welding of thermoplastics. This study examines the effect of quasi-simultaneous (QS) laser transmission welding (LTW) processing parameters including input power (P), number of passes (N), scan speed (V), total scan length (LS) and weld pressure on the meltdown behavior of polycarbonate (PC) and polypropylene (PP), using a T-shaped test assembly. The total meltdown is shown to depend linearly on the total line energy (LET) defined as the product of P and N divided by V. Increasing LS increases the critical LET for meltdown to begin. A simple model is presented that captures the main elements of meltdown in this thermoplastic welding process.