Laser Microwelding Research Articles

Laser microwelding of NiTi/PtIr alloys with laser beam offset

The NiTi and PtIr alloy joint has been employed in biomedical devices to combine the superelasticity of NiTi alloy with the X-ray visibility of PtIr alloy. Laser microwelding is usually used for the joints, but there is a risk of forming brittle intermetallic compounds (e.g., Ni3Ti, Ti2Ni, and Ti3Pt) in the fusion zone (FZ), which could deteriorate joint strength. In this study, laser beam offset (laser offset) was implemented for a butt joint of Ni-49.8 at.% Ti and Pt-10.0 at.% Ir alloy wires to control the intermetallic compound formation in the FZ. Welding with 300 μm laser offset on the NiTi side achieved 2.3 times higher joint breaking stress and 13.0 times higher joint breaking strain than welding without laser offset. The joint breaking stress and strain were enhanced from 221 MPa and 0.9% to 502 MPa and 11.7% by 300 μm laser offset on the NiTi side, respectively. In the absence of laser offset, the dissolution of Pt and Ir into the FZ facilitated the M3Ti (M = Ni, Pt, Ir) formation in the FZ, resulting in crack propagation within the M3Ti. In contrast, the 300 μm offset on the NiTi side inhibited the M3Ti formation by mitigating Pt and Ir dissolution into the FZ. Laser offset on the NiTi side can be an attractive option to enhance the strength and ductility of NiTi and PtIr butt joints.

Investigation on Luminescence near Focal Point during Ultrafast Laser Microwelding of Glass Substrates

Investigation on Luminescence near Focal Point during Ultrafast Laser Microwelding of Glass Substrates

High-strength and impermeable sapphire/aluminum joints fabricated by ultrafast laser microwelding: Microstructures and joining mechanism

In this work, robust joints of sapphire and aluminum were successfully achieved by direct ultrafast laser transmission microwelding. Effects of laser parameters on the microstructures and mechanical properties were comprehensively studied. The welding process highly depended on the laser ablated metal nanoparticles within the heterojunction, as well as the thermal accumulation under multiple laser pulses irradiation. It showed that lack of fusion at the sapphire/aluminum interface can occur due to the incomplete melting of ablated nanoparticles at low laser fluence (<0.48 J/cm2). While serious cavities and cracks inside the brittle sapphire were formed with the intensified stress concentration after melting-solidification process at excessive laser fluence (>0.64 J/cm2). A nearly defect-free joint was obtained with max. shear strength of 128.1 ± 8.2 MPa at room temperature, when the laser fluence was 0.56 J/cm2. The high shear strength can be attributed to the tortuous interface with wide elemental transition layer. Meanwhile, the welded sapphire/aluminum joints exhibited high reliability under extreme service temperatures, which the shear strength can reach 159.8 ± 8.96 MPa at −50 °C and 80.6 ± 11.1 MPa at 200 °C. The sealing tests also showed the excellent impermeability of laser welded sapphire/aluminum structure, confirming the micro-crack free joint formation. This work demonstrates the feasibility of using ultrafast laser to achieve high-quality sapphire/aluminum joint, which is promising in heat-dissipation substrate manufacturing in power semiconductor devices.

Investigation on the modeling and simulation of hydrodynamics in asymmetric conduction laser micro-welding of austenitic stainless steel and its process optimization

Laser micro-welding is a joining technology utilized across various high-value industries, like medical, automotive, e-mobility, and aerospace. A trial-and-error process to identify welding parameters does not necessarily lead to optimized quality levels. Furthermore, offline non-destructive examination methods often launched to verify welding quality may inadvertently trigger excessive costs and time delays, ultimately failing to guarantee defect-free welds. In response to these challenges, this article introduces an advanced multiscale model designed to unravel the intricate dynamics of hydrodynamics and the overarching physics within laser micro-welding melting pools. Developed using the COMSOL software package, the model adeptly demonstrates how surface tension gradients shape the geometry of welds, thus influencing their quality. This knowledge allows the mapping of welding defects. One of the novelties of the article is to introduce geometric dissimilar welding conditions by simulating an asymmetric edge joint. It shows a study on a new, unstudied way to laser weld with many applications in the field. The model further establishes its utility in design experiments to determine parameter, tolerance, and system design. Moreover, the insights garnered from understanding and controlling these drivers have far-reaching implications for the advancement of subsequent methodological research and the development of in-situ quality control practices by characterizing the welding defects. Finally, the results shows that the discouragingly high computational costs restrict its potential application to support a Digital Twin.

A Review on Ultrafast Laser Microwelding of Transparent Materials and Transparent Material–Metals

Transparent hard and brittle (THB) materials have generated significant interest due to their excellent properties, such as wide spectral transmittance, heat resistance, chemical inactivity and high mechanical strength. To further explore the application of THB materials, it is inevitable to be confronted with a range of joining THB materials and THB material–metals. Ultrafast (UF) laser microwelding enables a new means of joining THB materials and THB material–metals, due to a localized energy deposition method, which is dominated by nonlinear absorption. This process can realize high-quality micro-zone direct joining of THB materials or THB material–metals without the assistance of a light-absorbing intermediate layer. In this paper, we review the advances in UF laser microwelding of THB materials and THB material–metals considering the last two decades, from the analysis of the interaction mechanism between UF laser and matter to the key influencing factors and practical applications of this technology. Finally, the existing problems and the future research focus of UF laser microwelding technology of THB materials and THB material–metals are discussed.

Micro-welding of sapphire and metal by femtosecond laser

A direct joining of sapphire and Fe–36Ni alloy was successfully realized via femtosecond laser micro-welding for the first time. A sound joint without any voids or microcracks was obtained with a narrow interface width less than 1 μm. There was no obvious element diffusion or metallurgical reactions at the interface. Sapphire and Fe–36Ni alloy were found chemically bonded and mechanically interlocked evidenced by jagged feature at the interface due to “cold” machining of femtosecond laser ablation. The highly localized femtosecond laser irradiation and smaller heat-affected zone contributed to the shear strength of the joint as high as 108.35 MPa. A higher laser scanning speed corresponded with less jagged feature and thermal stress due to the reduced thermal deposition at the interface. Proper micro-welding parameters were obtained for sapphire/Fe–36Ni alloy, and was verified in the direct joining of sapphire/steel and sapphire/silicon. It appears the femtosecond laser micro-welding technique is promising for direct joining of materials with large physical property disparities, and beneficial for manufacturing of optomechanical components at high precision, efficiency and performance.

Laser micro-welding of stainless steel foil: Welding mode, microstructure and corrosion properties

Laser micro-welding differs from macro-welding in that at least one dimension of the weld is less than 100 μm. Although process research on laser micro-welding has recently made some progress, the influence of welding mode on microstructure and corrosion resistance has remained unknown, which has been shown to have a significant influence on laser macro-welding. In this work, a single-mode fiber laser equipped with a scanning galvanometer is used to weld AISI304 stainless steel foils with a thickness of 100 μm. Similar to laser macro-welding, keyhole formation is used to describe two welding modes, namely thermal conduction welding and penetration welding. The laser-material interaction experiences a transient phase in which the welding mode alternates between conduction welding and penetration welding as reported by previous work. However, we show that gas protection eliminates the transient phase, proving that the transient phase develops as a result of the unsteadiness of the penetration welding caused by oxidation during the welding process. The crystallographic texture and phase constitution vary between conduction and penetration welds due to variations in heat transfer behavior during welding. The conduction weld has greater Σ3 CSL boundaries and a more uniform microstructure than the penetration weld, resulting in better corrosion resistance.

Measuring sealing degree of welded glass substrates after ultrafast laser microwelding

Measuring sealing degree of welded glass substrates after ultrafast laser microwelding

Direct microwelding of dissimilar glass and Kovar alloy without optical contact using femtosecond laser pulses

In the current microwelding process using femtosecond (fs) laser between dissimilar materials, surface polishing and pressure assistance, so-called optical contact, are believed necessary. In this paper, direct welding of soda lime glass and Kovar alloy using a fs laser is investigated to overcome the limit of optical contact. The processing of fs laser welding is comprehensively studied by varying the laser power, welding velocity and the number of welding. The shear joining strength is as high as 2 MPa. The cross-section of glass-Kovar alloy joints, the elemental diffusion and the fracture behavior of welded joints were studied. The results show that the fs laser irradiates the surface of Kovar alloy, micron/nanometer-sized metal particles are generated. These particles perform the role as an adhesive part in the welding process. It is believed that the Si atoms diffuses to Kovar alloy from the glass and partially replaces the Fe2+ ions on the surface of Kovar alloy, indicating that the mixing and interdiffusion of materials have occurred during the welding process. Finally, the welded sample was tested and has excellent water resistance and sealing property. Furthermore, to justify that this method can be applied to other stack ups, the glass-copper, the glass-Al6063 and sapphire-ceramic are also welded together. This work greatly simplifies the fs laser microwelding process and promotes its industrial applications, such as optoelectronic devices, medical devices and MEMS.

Modelling the impact of laser micro-joint shape and size on resistance and temperature for Electric Vehicle battery joining application

Laser micro-welding is increasingly being used to produce electrically conductive joints for automotive battery packs or energy storage devices to weld tabs to cylindrical cell terminals or pouch cell tabs to a busbar. There is little research currently existing in the literature reporting the joint characteristics in terms of electrical resistance and temperature rise due to charge-discharge current. This paper focuses on characterising and modelling electrical and thermal behaviours of laser micro-welds considering different shapes and sizes of the welding seam. A wide variety of laser joint shapes, including linear seam, multiple laser spots, circular seam, cross-seam and square seam were investigated with varying seam sizes considering 0.3 mm copper and Hilumin steel tab materials. Results showed that under-weld, good-weld and over-weld had a high impact on mechanical strength and fusion zone characteristics, however, the electrical resistance and temperature rise behaviours were not influenced by these weld categories. The linearized resistivity model was able to capture the effect of Joule heating and predicted resistance and temperature behaviours within ±6% error. The suitability of multiphysics electrical and thermal modelling for predicting the resistance and temperature behaviours at the early design stage has been demonstrated.

Picosecond laser microwelding of AlSi-YAG for laser system assembly.

We report the successful picosecond laser welding of AlSi and YAG. This material combination is of significant interest to the field of laser design and construction. Parameter maps are presented that demonstrate the impact of pulse energy and focal position on the resultant weld. Weld performance relevant to industrial applications is measured, i.e., shear strength, process yield, and absolute thermal resistance are presented.

Laser microwelding of stainless steel and pure aluminum foil

Joining of materials, with a strong bearing on the manufacturing industry, has remained an active area of research for decades. The demand for the fabrication of a variety of miniature components has placed more emphasis, in particular, on the welding of thin materials. Welding of thin materials is a challenging task, and more so if they are foils of dissimilar materials, as even a tiny weld flaw invariably can lead to a rapid blemishing of the job. Aluminum with its good thermal and electrical properties, low specific weight, and low cost is often considered a preferred material in many applications. In this communication, the authors present the result of a feasibility study of laser-assisted welding of stainless steel (AISI 304) and pure aluminum foils. A repetitive single-mode nanosecond fiber laser was used to carry out the weld in the lap joint configuration. Welding between the foils was done in the weld brazing mode. Electron microscopy, microhardness measurements, and tensile testing were carried on the weld to evaluate its microstructural and mechanical properties. In the course of welding, stainless steel remained in solid state, while aluminum underwent localized melting over a narrow zone at the interface. The use of very short duration repetitive laser pulses with lower heat input restricted the bulk diffusion of elements across the interface and thereby the generation of the intermetallic compound/second phase with minimum Heat Affected Zone and almost no distortion. This study establishes a nanosecond laser-assisted welding technique as an option for microwelding between stainless steel and aluminum foil.

Stress induced self-rollable smart-stent-based U-health platform for in-stent restenosis monitoring.

To date, several smart stents have been proposed to continuously detect biological cues, which is essential for tracking patients' critical vital signs and therapy. However, the proposed smart stent fabrication techniques rely on conventional laser micro-cutting or 3D printing technologies. The sensors are then integrated into the stent structure using an adhesive, conductive epoxy, or laser micro-welding process. The sensor packaging method using additional fabrication processes can cause electrical noise, and there is a possibility of sensor detachment from the sent structure after implantation, which may pose a significant risk to patients. Herein, we are demonstrating for the first time a single-step fabrication method to develop a smart stent with an integrated sensor for detecting in-stent restenosis and assessing the functional dynamics of the heart. The smart stent is fabricated using a microelectromechanical system (MEMS)-based micromachining technology. The proposed smart stent can detect biological cues without additional power and wirelessly transmit the signal to the network analyzer. The cytocompatibility of the smart stent is confirmed through a cytotoxicity test by monitoring the cell growth, proliferation, and viability of the cultured cardiomyocytes. The capacitance of the smart stent exhibits an excellent linear relationship with the applied pressure. The exceptional sensitivity of the pressure sensor enabled the proposed smart stent to detect biological cues during in vivo analysis. The preliminary findings confirmed the proposed smart stent's higher level of structural integrity, durability and repeatability. Finally, the practical feasibility of the smart stent is demonstrated by monitoring diastole and systole at various beat rates using a phantom. The results of the phantom study showed a similar pattern to the human model, indicating the potential use of the proposed multifunctional smart stent for real-time applications.

Laser Microwelding Technology and Equipment for Cross-Scale Collimator Grid of Deep-Space Exploration Satellite

Laser Microwelding Technology and Equipment for Cross-Scale Collimator Grid of Deep-Space Exploration Satellite

Defect localization during laser microwelding of battery connectors using long exposure imaging and few-shot learning

In the production of battery systems for electromobility, even a single weak electrical connection of the battery cells can lead to a failure of the entire stack. During laser welding of dissimilar materials such as aluminum and copper, real-time information about the actual part quality is essential for cost-efficient production and high product quality. To address these needs, long exposure optical imaging is used to generate detailed images during the process based on local near-infrared process emission. The high-resolution images are subsequently processed by using a novel data analysis approach. Existing image classification methods based on supervised learning usually require thousands of labeled images for training. To overcome these shortcomings, a few-shot learning approach based on Siamese Neural Networks is investigated, which require only a few samples to detect critical welding defects. The approach is evaluated on industry-relevant experiments and demonstrates promising results in terms of high defect recognition accuracies.

Investigating sensitivity to process parameters in pulsed laser micro-welding of stainless steel foils

The laser micro-welding process plays a crucial role in advanced manufacturing industries due to a limited heat-affected zone and the ability to precisely join a wide range of materials. In this regard, this study aims to investigate the effects of pulsed laser micro-welding parameters on the thermo-mechanical specifications of 316L thin foils, a widely used industrial material, in the lap joint configuration. In this case, the influence of micro-welding speed, power, pulse duration, and frequency on the deformation, interface spot diameter, spot overlapping, and heat-affected zone (HAZ) is analyzed by the finite element method (FEM). The numerical results are validated by comparing with foil distortion measurements. The results show that foil distortion, interface spot diameter, and HAZ are the most sensitive to the pulse duration. However, spot overlapping is mainly influenced by welding speed. HAZ dimension, a critical quality criterion of steel welding, is predominantly affected by pulse duration, welding speed, laser beam power, and frequency, respectively. Moreover, the dimensions of fusion zone, HAZ, spot overlapping, and total distortion increase by increasing the laser beam power, pulse duration, and frequency as well as reduction of the welding speed. By selecting the process parameters based on numerical analysis, it is possible to achieve desirable thermo-mechanical specifications and productivity of pulsed laser micro-welded stainless steel foils.

Development of an Intelligent Quality Management System for Micro Laser Welding: An Innovative Framework and Its Implementation Perspectives

Laser micro-welding manufacturers face substantial challenges in verifying weldment quality, as the industry and applications are requiring increasingly the miniaturization and compactness of products. The problem is compounded by new stringent demands for personalized products at competitive, low costs and the highest quality levels. High-pressure equipment manufacturers, in particular, rely on ISO 3834:2021 to assure and demonstrate best welding practices but also to manage risks associated with liability issues. ISO 3834:2021, like all conventional quality management systems, offers a one-dimensional, quasi-static overview of welding quality that may fail to deal with these new challenges and underlying complexities required to deal effectively with process variability. This paper presents a framework for welding companies to integrate horizontally their suppliers and customers with their processes and products, which are also integrated vertically in the context of Smart Manufacturing or Industry 4.0. It is focused on the development of a smart quality management system for intelligent digitization of all company manufacturing and business processes. Furthermore, an innovative data-based welding quality management framework is described for laser micro-welding applications and their implementation perspectives. The research is driven by an inductive methodology and based on a seamless integration of engineering-oriented heuristic and empirical approaches that is appropriate for intelligent and autonomous quality management, given the lack of research in this niche, but increasingly important topic area.

Analysis of Acoustic Emission (AE) Signals for Quality Monitoring of Laser Lap Microwelding

In this study, the correlation between welding quality and features of acoustic emission (AE) signals collected during laser microwelding of stainless-steel sheets was analyzed. The performance of selected AE features for detecting low joint bonding strength was tested using a developed monitoring system. To obtain the AE signal for analysis and develop the monitoring system, lap welding experiments were conducted on a laser microwelding platform with an attached AE sensor. A gap between the two layers of stainless-steel sheets was simulated using clamp force, a pressing bar, and a thin piece of paper. After the collection of raw signals from the AE sensor, the correlations of welding quality with the time and frequency domain features of the AE signals were analyzed by segmenting the signals into ten 1 ms intervals. After selection of appropriate AE signal features based on a scatter index, a hidden Markov model (HMM) classifier was employed to evaluate the performance of the selected features. Three AE signal features, namely the root mean square (RMS) of the AE signal, gradient of the first 1 ms of AE signals, and 300 kHz frequency feature, were closely related to the quality variation caused by the gap between the two layers of stainless-steel sheets. Classification accuracy of 100% was obtained using the HMM classifier with the gradient of the signal from the first 1 ms interval and with the combination of the 300 kHz frequency domain signal and the RMS of the signal from the first 1 ms interval.

Analysis and prevention of weld crater cracking in circumferential laser microwelding of automotive pressure sensors

An analysis presents a weld leakage problems of automotive pressure sensor, caused by weld crater cracking. A two-beam laser welding (TBLW) was used to weld a circumferential weld, which undesirably increases the probability of weld leakage by creating two weld end craters on a single weld. Microstructural analysis showed that microsegregation of alloying elements combined with imposed strains causes solidification cracking at the weld end crater. A novel “zigzag” laser power ramp-down was used and the results showed a limited crack propagation by producing significantly shorter discontinuous cracks. In such weld crater endings the leakage is no longer an issue.

Macro-Modelling of Laser Micro-Joints for Understanding Joint Strength in Electric Vehicle Battery Interconnects.

Laser micro-welding is increasingly being used to produce electrically conductive joints within a battery module of an automotive battery pack. To understand the joint strength of these laser welds at an early design stage, micro-joints are required to be modelled. Additionally, structural modelling of the battery module along with the electrical interconnects is important for understanding the crash safety of electric vehicles. Fusion zone based micro-modelling of laser welding is not a suitable approach for structural modelling due to the computational inefficiency and the difficulty of integrating with the module model. Instead, a macro-model which computationally efficient and easy to integrate with the structural model can be useful to replicate the behaviour of the laser weld. A macro-modelling approach was adopted in this paper to model the mechanical behaviour of laser micro-weld. The simulations were based on 5 mm diameter circular laser weld and developed from the experimental data for both the lap shear and T-peel tests. This modelling approach was extended to obtain the joint strengths for 3 mm diameter circular seams, 5 mm and 10 mm linear seams. The predicted load–displacement curves showed a close agreement with the test data.