Challenges and opportunities in laser welding of 6xxx high strength aluminium extrusions in automotive battery tray construction

Laser welding has been increasingly adopted into the automotive sector due to its competitive processing speed, less restrictive single-sided access requirements and improved process flexibility. These benefits notwithstanding, laser welding still remains susceptible to the weld cracking especially for 6xxx high strength aluminium extrusions, which are widely used in the automotive industry for body-in-white and battery tray manufacturing. This paper reviews current challenges and opportunities for construction of battery tray using aluminium alloys with laser welding process. It aims to provide a view on the selection of welding equipment in terms of beam oscillation, power modulation, beam shaping, filler wire and shielding gas, and analyze their impact on joint integrity for 6xxx grades aluminium extrusions. The driving idea is to control the thermal history in and around the molten pool and to modify the chemical composition in the fusion zone to reduce the formation of solidification cracks. Results of the study have shown that the modification of chemical composition by the use of filler wire is currently the most efficient approach to improve joint strength. Further results evidences also showed that beam shaping with adjustable ring mode laser helps to stabilize the keyhole and achieve a wider molten pool and weld interface width. Manufacturing implications are reviewed and discussed throughout the paper

Effect of focal position offset on joint integrity of AA1050 battery busbar assembly during remote laser welding

This paper aims at studying the impact of focal position offset on joint integrity of AA1050 battery busbar during remote laser welding. Welding experiments were conducted at different focal position offsets in the AA1050 L-joint with the integration of laser beam oscillation. The impact of focal position offset was evaluated by multiple joint indicators including weld geometry, thermal profile, weld porosity and mechanical strength. Results showed that defocusing of the laser beam tends to be favourable for the reduction of peak temperatures and weld porosity. However, a defocusing more than 3 mm can lead to 60% drop in bonding integrity and 40% loss of joint strength. A tolerance window of [-2 1] mm was determined for the focal position offset, corresponding to 70% and 30% of the Rayleigh length. This result is a useful reference to inform precise focal position control with respect to the accumulated variations of part tolerance, part-to-part gap and part positioning errors. This study also indicated that the thermal measurement can be a potential approach for in-process monitoring of joint integrity. Indeed, the peak temperature near the weld zone showed good linear correlation with off-line detectable joint indicators including weld depth, weld width at interface, weld tensile strength and weld porosity

Applying optical coherence tomography for weld depth monitoring in remote laser welding of automotive battery tab connectors

This paper addresses in-process monitoring of weld penetration depth (WPD) during remote laser welding of battery tab connectors using optical coherence tomography (OCT). The research aims at studying the impact of welding process parameters on the accuracy of WPD measurements. In general, the highest measurement accuracy is achievable by positioning the OCT measuring beam toward the bottom of the keyhole. However, finding and maintaining the alignment between the OCT measuring beam and the bottom of the keyhole is a challenging task because of the dynamic changes in the size and shape of the keyhole itself. The paper addresses the above challenge by (1) developing welding process parameters for the Al-Cu thin foil lap joint (Al 1050 foil 450 μm and Ni-plated Cu foil 300 μm) using a novel adjustable ring mode (ARM) laser and (2) integrating OCT technology with two beams: one targeting the bottom of the keyhole and another as a reference to the part surface (TwinTec technology). The methodology is underpinned by the “Keyhole Mapping” approach, which helps one to identify the optimal placement of the OCT measuring beam with considerations to both measurement accuracy and stability of the keyhole. Findings indicated that welding with the ARM laser results in a more stable process, reduces fluctuations of the keyhole opening, and, therefore, helps one to improve the measurement accuracy by a factor of 50% (from the average error of 0.22 mm to 0.11 mm). Results further identified that the feasible operating window of the OCT measuring beam, corresponding to the highest measurement accuracy, is below 20 μm in length

A novel methodology for investigating the through-thickness molten pool shape during remote laser beam welding

Molten pool shape can retrieve information on thermal gradients and solidification rates which provide substantial process signatures on cracking and weld quality. However, monitoring the molten pool shape remains a challenging task due to the complex nature of the laser welding process. The current study employs a novel monitoring methodology to examine the role of through-thickness molten pool shape in solidification cracking in partially penetrated welds. A Coherent ARM (Adjustable Ring mode) fibre laser, featuring independent control of the core and ring beam with the ratio of respective beam power specified by power ratio, was employed to study different molten pool shapes. Experimental investigations further include welding of aluminium alloy and quartz glass in butt configuration with a high-speed imaging system facing the longitudinal cross-section of the weld. The molten pool shape was evaluated using the developed image processing algorithm and the curvature analysis. Furthermore, ad-hoc Digital Image Correlation (DIC) was used to examine the thermal strain development in the through-thickness direction. Results showed that the proposed methodology could provide an accurate detection of the through-thickness molten pool shape with an improved accuracy of 94.3 % compared to the advanced prior models available in the literature, with an accuracy of 30.23 % and 42.5 %. It also revealed a tail-like feature in the molten pool's rear end, which influences the crystallisation paths and facilitates premature solidification, leading to greater tensile strains during solidification. The increments in power ratio from 0.36 to 1.5 reduced the tail-like feature and reduced the shrinkage strains in the fusion zone

Effect of Micro Solidification Crack on Mechanical Performance of Remote Laser Welded AA6063 Fillet Lap Joint in Automotive Battery Tray Construction

From MDPI via Jisc Publications RouterHistory: accepted 2021-05-07, pub-electronic 2021-05-15Publication status: PublishedFunder: High Value Manufacturing Catapult; Grant(s): : Chamaeleon - New lightweight Materials and Processing Technologies for Common Lightweight Architecture of Electric and Hybrid Powertrain SystemsFunder: Innovate UK; Grant(s): LIBERATE: Lightweight Innovative Battery Enclosures using Recycled Aluminium TechnologiesRemote laser welding (RLW) has shown a number of benefits of joining 6xxx aluminium alloys such as high processing speed and process flexibility. However, the crack susceptibility of 6xxx aluminium alloys during RLW process is still an open problem. This paper experimentally assesses the impact of transverse micro cracks on joint strength and fatigue durability in remote laser welding of AA6063-T6 fillet lap joints. Distribution and morphology of transverse micro cracks were acquired by scanning electron microscope (SEM) on cross-sections. Grain morphology in the weld zone was determined by electron backscatter diffraction (EBSD) while static tensile and dynamic fatigue tests were carried out to evaluate weld mechanical performance. Results revealed that increasing welding speed from 2 m/min to 6 m/min did not introduce additional transverse micro cracks. Additionally, welding at 2 m/min resulted in tensile strength improvement by 30% compared to 6 m/min due to the expansion of fusion zone, measured by the throat thickness, and refinement of columnar grains near fusion lines. Furthermore, the weld fatigue durability is significantly higher when fracture occurs in weld root instead of fusion zone. This can be achieved by increasing weld root angle with optimum weld fatigue durability at around 55°

Effect of micro solidification crack on mechanical performance of remote laser welded AA6063-T6 fillet lap joint in automotive battery tray construction

Remote laser welding (RLW) has shown a number of benefits of joining 6xxx aluminium alloys such as high processing speed and process flexibility. However, the crack susceptibility of 6xxx aluminium alloys during RLW process is still an open problem. This paper experimentally assesses the impact of transverse micro cracks on joint strength and fatigue durability in remote laser welding of extruded AA6063-T6 fillet lap joints. Distribution and morphology of transverse micro cracks were acquired by scanning electron microscope (SEM) on cross-sections. Grain morphology in the weld zone was determined by electron backscatter diffraction (EBSD) while static tensile and dynamic fatigue tests were carried out to evaluate weld mechanical performance. Results revealed that increasing welding speed from 2 m/min to 6 m/min did not introduce additional transverse micro cracks. Additionally, welding at 2 m/min resulted in tensile strength improvement by 30% compared to 6 m/min due to the expansion of fusion zone, measured by the throat thickness, and refinement of columnar grains near fusion lines. Furthermore, the weld fatigue durability is significantly higher when fracture occurs in weld root instead of fusion zone. This can be achieved by increasing weld root angle with optimum weld fatigue durability at around 55°

Application of adjustable ring mode laser in remote laser welding of additive manufactured AlSi10Mg alloy

Additive manufacturing (AM) is an innovative manufacturing technology that offers the ability to build parts with both geometric and material complexities. However, limitations, including low build volume capability and production rate, yield its rapid application in high volume production. This paper presents the potential of remote laser welding (RLW) as a post-AM joining approach to scale up the AM components. The AM AlSi10Mg alloy was fabricated by direct metal laser sintering and subsequently joined by RLW without filler wire or shielding gas. A novel adjustable ring mode (ARM) laser beam was employed during the RLW process where the ring beam is designed to stabilize the keyhole by providing the preheating and postheating while the core beam guarantees a sufficient weld penetration. The impact of the ARM laser on weld porosity was evaluated in both fillet lap and bead-on-plate welding configurations, accompanied by the variation of core/ring beam power ratios. Crack-free welds with promising weld appearance were obtained among all welding trials, indicating that the ARM-RLW process can be employed for the robust connection of AM AlSi10Mg alloys. Optimizing the power ratio can substantially reduce the weld porosity area ratio from 24.3% to 13.5% in the fillet lap configuration and from 24.2% to 14.4% in the bead-on-plate configuration. Analysis of variance tests statistically confirmed the significant impact of the power ratio on the porosity area ratio. Future work has been suggested for the process maturation of RLW as a post-AM joining approach in industrial application

Deep learning enhanced digital twin for Closed-Loop In-Process quality improvement

A digital twin framework is presented for assembly systems with compliant parts fusing sensors with deep learning and CAE simulations. Its underlying concept, ‘process capability space,’ updates iteratively during evolving tasks of new product introduction with resulting model fidelity able to simulate dimensional, geometric and weld quality of parts and assemblies; isolate root causes of quality defects; and, suggest corrective actions for automatic defects mitigation; thereby, enabling ‘Closed-Loop In-Process (CLIP) quality improvement’ during assembly system development. Results, using the first fully digitally developed remote laser welding process for aluminium doors, yielded a right-first-time rate of >96% for door assembly cell development

Using photodiodes and supervised machine learning for automatic classification of weld defects in laser welding of thin foils copper-to-steel battery tabs

This paper has been designed to study whether photodiodes and supervised machine learning (ML) algorithms are sufficient to automatically classify weld defects caused by simultaneous variation of the part-to-part gap and laser power during remote laser welding (RLW) of thin foils, with applications in battery tabs. Photodiodes are used as the primary source of data and are collected in real-time during RLW of copper-to-steel thin foils in the lap joint. Experiments are carried out by the nLight Compact 3 kW fiber laser integrated with the Scout-200 2D scanner. The paper reviews and compares seven supervised ML algorithms (namely, k-nearest neighbors, decision tree, random forest, Naïve–Bayes, support vector machine, discriminant analysis, and discrete wavelet transform combined with the neural network) for automatic classification of weld defects. Up to 97% classification rate is obtained for scenarios with simultaneous variations of weld penetration depth and part-to-part gap. The main causes of misclassification are imputed to the interaction between welding parameters (part-to-part gap and laser power) and process instability at high part-to-part gap (high variation in the process not captured by the photodiodes). Arising opportunities for further development based on sensor fusion, integration with real-time multiphysical simulation, and semi-supervised ML are discussed throughout the paper