Laser Microwelding Research Articles

Laser Micro-welding Applied to Target Manufacturing

In this work, samples of ultrapure aluminum (content 99.999%) and AA 5083 aluminum alloy (Al-Mg) have been welded under conduction regime, with a pulsed Nd:YAG laser. Due to the very specific final application (use of aluminum assemblies in cryogenic conditions), stringent specifications are imposed for the weld in terms of thermal conductivity and gas tightness at a temperature of 18°K. Process conditions were refined and were validated according to our cryogenic criteria

A microsphere assembly method with laser microwelding for fabrication of three-dimensional periodic structures

The orderly build-up of monosized microspheres with sizes of hundreds of micrometres enabled us to develop three-dimensional (3D) photonic crystal devices for terahertz electromagnetic waves. We designed and manufactured an original 3D particle assembly system capable of fabricating arbitrary periodic structures from these spherical particles. This method employs a pick-and-place assembling approach with robotic manipulation and interparticle laser microwelding in order to incorporate a contrivance for highly accurate arraying: an operation that compensates the size deviation of raw monosized particles. Pre-examination of particles of various materials revealed that interparticle laser welding must be achieved with local melting by suppressing heat diffusion from the welding area. By optimizing the assembly conditions, we succeeded in fabricating an accurate periodic structure with a diamond lattice from 400 µm polyethylene composite particles. This structure demonstrated a photonic bandgap in the terahertz frequency range.

Laser micro-welding of aluminum alloys: experimental studies and numerical modeling

Experimental and numerical studies were conducted on the effects of the laser beam pulse shaping in the time domain on the quality of the welding seam in laser micro-welded AlMg3 with a thickness of 0.2 mm and 1 mm thick AlMg4.5 Mg foils, respectively. The pulse shaping was realized by a time sequence of three different rectangular pulses with different duration and power level. The first pulse was used to pre-heat the sample, welding occurred with the second pulse and the third pulse controlled the melt pool behavior. The power level and the duration of the single pulses were varied systematically and the resulting microstructure was analyzed by scanning electron microscope. The experiments were accompanied by numerical simulations based on a finite volume model which considers the transient heat flow, melt convection and the evolution of a gas capillary during the deep penetration welding process.

Recent progress in micro and nano-joining

Micro and nano-joining has been identified as a key enabling technology in the construction of micromechanical and microelectronic devices. The current article reviews recent progress in micro and nano-joining. In particular, laser micro-welding (LMW) of crossed 316 LVM stainless steel (SS) wire was compared to conventional resistance micro-welding (RMW) and was successfully employed in welding a Pt-Ir /SS dissimilar combination. Welding of Au nanoparticles was realized using femtosecond laser irradiation and its application in the surface enhanced Raman spectroscopy was investigated. Brazing between carbon nanotube (CNT) bundles and Ni electrodes was attained in vacuum, resulting in the development of a novel CNT filament of incandescent lamps.

In-process monitoring and feedback control for stable production of full-penetration weld in continuous wave fibre laser welding

Laser micro-welding has been applied for device sealing in electronics and automobile industries. Welding of corners in goods and products is a problem owing to easier formation of a weld with burn-through, shallow penetration or a non-bonded part when a drastic change in the welding speed or laser power occurs. This research was therefore undertaken with the objective of obtaining a fundamental knowledge of in-process monitoring and feedback control for the stable production of a full-penetration weld with a constant bead width on the bottom surface irrespective of the changes in the laser power and the welding speed. Variation in weld penetration geometry was investigated by rapid deceleration and acceleration in the welding speed during lap welding of pure titanium thin sheets with a continuous wave (CW) single-mode fibre laser beam. The rapid deceleration in the welding speed led to a considerable change in the full-penetration weld geometry or a partially penetrated weld (if the power was accordingly reduced), resulting in the difficulty in the stable production of a full-penetration weld bead. The heat radiation intensity measured from the laser-irradiated area was useful as an in-process monitoring signal for detecting the molten pool size on the laser-irradiated surface. However, the utilization of monitoring of heat radiation was difficult for predicting the weld bead width on the bottom surface due to the formation of partial penetration or the change in the penetration shape. The laser power was controlled at a 4 ms interval according to the heat radiation signal in order to adjust the weld bead width on the laser-irradiated surface to the target weld penetration geometry affected by thermal storage. Consequently, the feedback-controlled laser power produced a stable full-penetration weld with the designed bead width on the bottom surface irrespective of the rapid deceleration of the welding speed and the corresponding decrease in laser power. Furthermore, the developed feedback control algorithm was effective in rapid acceleration of the welding speed. From these results, it was confirmed that the consideration of the feedback control algorithm including a thermophysical property such as thermal storage was essential for the suppression of the effect due to rapid deceleration and acceleration of the welding speed and the laser power in lap seam micro-welding with a CW fibre laser.

Laser Welding,Microwelding,Nanowelding and Nanoprocessing(Invited Paper)

Laser Welding,Microwelding,Nanowelding and Nanoprocessing(Invited Paper)

Numerical Analysis on the Feasibility of Laser Microwelding of Metals by Femtosecond Laser Pulses Using ABAQUS

Ultrafast lasers of subpicosecond pulse duration have the potential for laser microwelding of micronscale fusion zone. Due to the extremely short pulse duration, laser-metal interaction involving ultrafast laser pulses should be analyzed using the two-temperature model. In this study, the two-temperature model is analyzed using ABAQUS to study the feasibility of laser microwelding with ultrafast laser. A material model is constructed using material properties and the subsurface boiling model. The model is validated using experimental results from the literature. Laser processing parameters of repetition rate, pulse duration, and focal radius are then investigated, in terms of molten pool generated in the material and requirements on those parameters for laser microwelding using ultrafast lasers are discussed.

Semiempirical, squeeze flow, and intermolecular diffusion model. II. Model verification using laser microwelding

AbstractThis article reviews the application of a coupled squeeze flow and intermolecular diffusion model, which was used to predict the quality and size of microwelds in plastics. Weld widths predictions were compared with previously presented experimental results using moving heat source models and temperature fields. The motivation for this work was to develop and verify a model based on fundamental principles that could accurately predict weld size and strength for conventional plastic welding techniques as well as novel techniques such as laser microwelding. It is envisioned that the resulting model could be used to predict proper welding parameters, including laser power and travel speed, to produce welds of varying size. Although insight into weld quality can be derived from this model, it was not the goal of this work to accurately predict weld strength for laser microwelding because of the difficulty in measuring weld strength on the micron scale. However, as reported in Part 1, weld strength for impulse welds were accurately predicted. In this model it was found that variable temperature histories, rather than a single value of maximum weld temperature, allows more accurate modeling of the welding process. In this work (Part 2), microwelds as small as 11 μm in width were produced with transmission infrared welding. In addition, welds over 150‐μm wide were also generated and the model was able to predict the range of weld widths that were found experimentally. It was found that the predictions were in very good agreement with the experimental results. There was some deviation between the experimental data and the model at the extreme parameters and it is believed that this was due to the temperature‐dependent material properties as well as optical aberrations. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers

Semi‐empirical, squeeze flow and intermolecular diffusion model. I. Determination of model parameters

AbstractThis article reviews the development of molecular healing models that couple squeeze flow and intermolecular diffusion. Historically there are a few studies that combine these processes for fusion bonding of thermoplastic composites. The motivation of this work was to develop similar models for welding of polymer films and films to substrates. These models are theoretically developed and experimentally verified. It was found that the time dependence for squeeze flow is of the same order of magnitude as that for intermolecular diffusion, making models for these processes indistinguishable experimentally, and therefore, they were assumed to occur simultaneously. Furthermore, it was found that healing of the weld can be better defined as a function of time and temperature instead of temperature alone, as historically done for welding applications. Comparison of the weld strength predictions with experimental data for impulse welds showed that the developed models were able to predict weld strength over a wide range of parameters. In Part 2, moving heat source heat transfer models together with the coupled squeeze flow and intermolecular diffusion model are used to predict healing during laser microwelding. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers

Creep and tensile behaviors of Fe–Cr–Al foils and laser microwelds at high temperature

We examine a Fe–Cr–Al foil-based material and a continuous-wave laser weld generated in ultra-fine “keyhole” mode undergoing tensile and creep-tension tests over a temperature range of 25–1000 °C. At all temperatures, the bead exhibits superior tensile resistance than the base material due to a homogenous reprecipitation of fine aluminum nitrides, AlN, but creeps faster at 900 °C, because of a finer-grained microstructure scarcely undergoing secondary recrystallization. Under tensile loading, the base material ductility is higher than that of the weld and increases with increasing temperature, but drops above 900 °C due to faster grain growth and chromium carbide precipitation. The base material stress–strain curves exhibit concomitant decrease of the yield point effect magnitude and increase of the strain hardening rate with the temperature, but only up to a critical value, which decreases with increasing the strain rate. After vanishing at this critical temperature, the yield point effect reappears upon the onset of sample necking responsible for the ensuing continuous decrease of the engineering stress. The strain–stress curves of laser welds show no yield point effect and the work hardening persists at all temperatures. Under creep-tension, the weld shows a strong anisotropic behavior, and the highest flow rate is recorded for welds oriented parallel to the loading direction, because of a more important cavity nucleation.

Laser micro-welding of transparent materials by a localized heat accumulation effect using a femtosecond fiber laser at 1558 nm

We report on laser micro-welding of materials based on a localized heat accumulation effect using an amplified femtosecond Er-fiber laser with a wavelength of 1558 nm and a repetition rate of 500 kHz. We demonstrated the welding of non-alkali alumino silicate glass substrates, resulting in a joint strength of 9.87 MPa. We also welded a non-alkali glass substrate and a silicon substrate using the 1558-nm laser pulses, resulting in a joint strength of 3.74 MPa. Our laser micro-welding technique can be extended to welding of semiconductor materials and has potential for various applications, such as three-dimensional stacks and assembly of electronic devices and microelectromechanical system devices.

Heat transfer and fluid flow in laser microwelding

The evolution of temperature and velocity fields during linear and spot Nd-yttrium aluminum garnet laser microwelding of 304 stainless steel was simulated using a well-tested, three-dimensional, numerical heat transfer and fluid flow model. Dimensional analysis was used to understand both the importance of heat transfer by conduction and convection as well as the roles of various driving forces for convection in the weld pool. Compared with large welds, smaller weld pool size for laser microwelding restricts the liquid velocities, but convection still remains an important mechanism of heat transfer. On the other hand, the allowable range of laser power for laser microwelding is much narrower than that for macrowelding in order to avoid formation of a keyhole and significant contamination of the workpiece by metal vapors and particles. The computed weld dimensions agreed well with the corresponding independent experimental data. It was found that a particular weld attribute, such as the peak temperature or weld penetration, could be obtained via multiple paths involving different sets of welding variables. Linear and spot laser microwelds were compared, showing differences in the temperature and velocity fields, thermal cycles, temperature gradients, solidification rates, and cooling rates. It is shown that the temperature gradient in the liquid adjacent to the mushy zone and average cooling rate between 800 and 500 °C for laser spot microwelding are much higher than those in linear laser microwelding. The results demonstrate that the application of numerical transport phenomena can significantly improve current understanding of both spot and linear laser microwelding.

Comparison of high power diode laser and Nd:YAG laser microwelding of k-type thermocouples

k-Type thermocouple (NiCr–NiAl) joints are currently manufactured by argon arc welding. One of the weaknesses of this joining method is that the weld bead diameters are about three times of the thermocouple wires. This slows the response time of these thermocouples. Furthermore, porosity exists in the joints affecting the conductivity and repeatability for temperature measurement. In this paper, the feasibility and the characteristics of welding 0.2 and 0.5 mm diameter k-type thermocouples using a 120 W diode laser and a 400 W Nd:YAG laser are reported. The energy density requirement and the effects of shroud gas are described and analysed quantitatively. The performances of the laser welded thermocouples are evaluated against the commercial ones. Reductions of 35 and 15% in response time have been achieved compared to the commercial and the Nd:YAG laser welded 0.2 mm diameter thermocouples, respectively, whereas for the 0.5 mm diameter thermocouples, the HPDL welded ones performed 5% better and the Nd:YAG welded ones performed 9% better. The tensile strength of the diode laser welded thermocouples was found to be around 1620 N mm −2 at 0.2 mm diameter and 480 N mm −2 at 0.5 mm diameter. This is well above the industrial standard of 400 N mm −2. The work shows that diode laser welding can generate beadless and porosity-free butt joints in the thermocouples superior to the ones produced by argon arc welding.

Mechanical Characterizations of Laser Microwelds for MEMS Packaging

ABSTRACTLaser micromachining has proven to be a very powerful and successful tool for precision machining and microfabrication with applications in electronics, MEMS, medical, and biomedical fields. For MEMS fabrication and packaging, several approaches based on localized heating and bonding have been proposed such as localized eutectic bonding, fusion bonding, solder bonding, and chemical vapor deposition (CVD) bonding. However, these approaches are based on resistive heating which requires intimate contact at the electrodes, and this is not preferred for MEMS packaging applications. As a good alternative, laser microwelding has advantages of non contact, low heat distortion, high speed, high precision, and consistent weld integrity. Therefore, it is finding increasing use in MEMS fabrication and packaging applications. In laser microwelding, the material in the HAZ of the workpiece experiences heating, melting, and re-solidifying stages, and the final characterizations of the laser microwelds can be influenced by various factors such as the laser beam properties, the system cooling condition, and the surface roughness/reflectivity of the component. This paper presents characterization of laser microwelds, with emphasis on study of deformation and strength of the microwelds made by a Nd:YAG pulsed laser. Optical interferometry is used to nondestructively record fringe patterns of the shape and deformation of the laser microwelded workpiece before, during, and after the microwelding process, and tensile tests are performed to study strength of the laser microwelds as a function of different welding parameters. Results indicate that quality of the laser microwelds depends on selection of an appropriate set of parameters controlling the microwelding process.

Laser microwelding for fiber-chip coupling modules with tapered standard monomode fiber ends for optical communication systems

We design a welding machine for fabricating butterfly (BFY) modules with an optical coupling setup that is well suited for rapid prototyping and small-scale production of InP and GaAs-based waveguide-fed photonic components such as diode lasers, semiconductor amplifiers, optical modulators, and optoelectrical ICs (OEICs). The light of these devices is transferred very efficiently into single-mode fibers using integrated tapered lensed ends. In our setup the mechanical adjustment of the fiber-chip coupling is done semiautomatically by a newly designed coupling machine while welding performs the fixing of the fiber-chip connection. We found a yield of 47% within a shift of 1.0 μm at the initial welding spot. With additional weld adjustment, it was possible to extend the yield of 67%. We introduce, to our knowledge for the first time, an adjustable welding technique, which we call strain-reducing welding, which has several benefits in comparison to the well-known laser hammering technique.

A study on heat source equations for the prediction of weld shape and thermal deformation in laser microwelding

In the area of laser welding, numerous studies have been performed in the past decades using either analytical or numerical approaches, or both combined. However, most of the previous studies were process oriented and modeled conduction and keyhold welding differently. In this research, various heat source equations that have been proposed in previous studies were calculated and compared with a new model. This is to address the problem of predicting, by numerical means, the thermomechanical behavior of laser spot welding for thin stainless steel plates. A finite-element model (FEM) code, ABAQUS, is used for the heat transfer and mechanical analysis with a three-dimensional plane assumption. Experimental studies of laser spot welding and measurement of thermal deformation have also been conducted to validate the numerical models presented. The results suggest that temperature profiels and weld deformation vary according to the heat source equation of the laser beam. For this reason, it is essential to incorporate an accurate model of the heat source.

Investigation on the effect of laser pulse shape during Nd:YAG laser microwelding of thin Al sheet by numerical simulation

In the welding of thin A3003 Al sheet by a Nd:YAG laser beam, the laser pulse shape plays an important role in enhancing the welding penetration stability. In order to evaluate the effect of laser pulse shape during Nd:YAG laser welding of a thin Al sheet and to predict the welding performance by numerical simulation, a three-dimensional finite differential method (FDM) analysis is presented for heating with different laser pulse shapes and related welding parameters. The simulated results give good agreement with experimental results, where a sound weld shape and crack-free weld pool are obtained. The simulation results show that the welding stability is greatly affected by the modulation of laser pulse shape for the same laser energy and welding parameters. As a rectangular laser pulse is modulated to have three stages with high, medium, and low power levels for the first, second, and third stages, respectively, more energy is absorbed in the melt pool, and the cooling rate is reduced. While a high power level at the first stage increases the laser beam absorption, the thermal energy of the third stage prevents fast cooling of the melt pool. Also, evaporation is prevented by proper modulation of the laser pulse. If the laser pulse is modulated properly, the optimum melt-pool size and cooling rate can be obtained; also, the desired weld depth and welding stability are achieved for the conduction welding mode. The numerical simulation method can be used to determine the proper conditions for good welding performance.

A study on the prediction of the laser weld shape with varying heat source equations and the thermal distortion of a small structure in micro-joining

In the area of laser welding, numerous studies have been performed the past decades using either analytical or numerical approaches, or both combined. However, most of previous studies were process oriented and modeled differently in conduction and keyhole welding. In this research, the results of calculation using various heat source equations that have been proposed in previous studies were compared to the predictions of a new model. This model treats the problem of predicting, by numerical means, the thermo-mechanical behavior of laser spot welding for thin stainless steel plates. A finite element code, ABAQUS, is used for the heat-transfer analysis with a 3D model. Experimental studies of the laser spot welding have also been conducted to validate the numerical models presented. The results suggest that the predicted temperature profiles and weld dimensions vary according to the heat source equation of the laser beam. For this reason, it is essential to incorporate an accurate description of the heat source. Thermal and mechanical analyses of the laser micro-welding of a small structure are performed from these results.

Mathematical model of penetration in laser microwelding of dissimilar materials

Mathematical model of penetration in laser microwelding of dissimilar materials

Thermal effects in laser microwelding

In this paper the melting process in elements subjected to laser microwelding is analysed. In the model presented we take into account a change of laser beam intensity in time and in cross-section. Moreover, the range for which we can use the surface absorption model is determined. This nonlinear phenomenon is solved using the finite element method. Numerical examples complete the presented analysis.