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2019 Vol. 1, No. 4

A review on glass welding by ultra-short laser pulses
Kristian Cvecek, Sarah Dehmel, Isamu Miyamoto, Michael Schmidt
2019, 1(4) doi: 10.1088/2631-7990/ab55f6
Due to its physical properties and availability, glass is a highly appealing material for a variety of optical and technological systems. However, in order to realize complex glass geometries orhermetically sealed enclosures, applicable joining technologies are required. For the production of high performance parts, classical joining technologies, introducing additional material within the joint area, are not ideal since the joint affects the local physical properties. Alternatives require high processing time and a global thermal cycling of the material itself. Within the present paper, we provide an overview on the current state of research of ultra-short pulsed (USP) laser-based joining of glass, a joining technology both eliminating the need for additional material and long processing times with a global thermal cycling of the part. The joints are typically formed in glass that is transparent to the laser by exploiting nonlinear absorption effects that occur under extreme conditions. Though the temperature reached during the process is on the order of a few 1000 °C, the heat affected zone (HAZ) is confined to only tens of micrometers. It is this controlled confinement of the HAZ during the joining process that makes this technology so appealing to a multitude of applications because it allows the foregoing of a subsequent tempering step that is typically essential in other glass joining techniques, thus making it possible to effectively join highly heat sensitive components.
The interaction between grain boundary and tool geometry in nanocutting of a bi-crystal copper
Zhanfeng Wang, Tao Sun, Haijun Zhang, Guo Li, Zengqiang Li, Junjie Zhang, Yongda Yan, Alexander Hartmaier
2019, 1(4) doi: 10.1088/2631-7990/ab4b68
Anisotropy is one central influencing factor on achievable ultimate machined surface integrity of metallic materials. Specifically, grain boundary has a strong impact on the deformation behaviour of polycrystalline materials and correlated material removal at the microscale. In the present work, we perform molecular dynamics simulations and experiments to elucidate the underlying grain boundary-associated mechanisms and their correlations with machining results of a bi-crystal Cu under nanocutting using a Berkovich tool. Specifically, crystallographic orientations of simulated bi-crystal Cu with a misorientation angle of 44.1° are derived from electron backscatter diffraction characterization of utilized polycrystalline copper specimen. Simulation results reveal that blocking of dislocation motion at grain boundaries, absorption of dislocations by grain boundaries and dislocation nucleation from grain boundaries are operating deformation modes in nanocutting of the bi-crystal Cu. Furthermore, heterogeneous grain boundary-associated mechanisms in neighbouring grains lead to strong anisotropic machining behaviour in the vicinity of the grain boundary. Simulated machined surface morphology and machining force evolution in the vicinity of grain boundary qualitatively agree well with experimental results. It is also found that the geometry of Berkovich tool has a strong impact on grain boundary-associated mechanisms and resultant ploughing induced surface pile-up phenomenon.
Femtosecond laser additive and subtractive micro-processing: enabling a high-channel-density silica interposer for multicore fibre to silicon-photonic packaging
Gligor Djogo, Jianzhao Li, Stephen Ho, Moez Haque, Erden Ertorer, Jun Liu, Xiaolu Song, Jing Suo, Peter R Herman
2019, 1(4) doi: 10.1088/2631-7990/ab4d51
Today, the ultrafast laser has become a powerful and robust tool for both science and commercial applications, opening new discoveries in high intensity interaction with materials through to enabling new medical procedures or manufacturing of novel product concepts. One of the most rewarding areas has been the novel means for inducing absorption inside of normally transparent materials. In Prof. Herman’s group, these interactions are studied in the context of strong nonlinear optical effects and the unfolding of remarkably energetic processes that begin in the non-equilibrium domain of thermodynamics. By tuning such processes, our group has sought out the technical benefits in three-dimensional (3D) structuring of optical materials to nano-scale dimensions, aiming to invent new forms of 3D additive and subtractive manufacturing. The research often leads to invention of new processes and product concepts touching the wide areas of photonics, optical communications, optical packaging, fibre optics, data storage, security marking, optical sensing, heads-up display, biosensing, lab-in-a-chip, and lab-in-fibre. The present paper builds on a myriad of additive and subtractive laser techniques that have culminated in transparent glasses to address a significant challenge in packaging of photonics devices. On the level of laser processing, femtosecond laser interactions were exploited for full-scale 3D structuring of the internal dimensions of glass wafers to define optical circuits and to guide etching selectively along laser tracks. Following chemical etching, the optical glass circuits become precisely arranged with alignment slots for enabling a facile assembly and packaging with optical fibers or other optical devices, with the focus here on the silicon photonic chip and multi-core optical fiber. The alignment slots provide exception self-guidance in optical-to-optical interconnections to sub-micron dimensions. The optical interposer presented herein demonstrates the potential for scaling up of laser 3D writing to enable high-density optical packaging, specifically addressing the major bottleneck for efficiently connecting optical fibers to silicon photonic processors as required in telecom and data centers. Moreover, such 3D additive and subtractive processing promises higher scale integration and rapid photonic assembly and packaging of micro-optic components for broader-based applications from integrated biophotonic chips to wearable displays.
Grain refining in weld metal using short-pulsed laser ablation during CW laser welding of 2024-T3 aluminum alloy
Masaki Kasuga, Tomokazu Sano , Akio Hirose
2019, 1(4) doi: 10.1088/2631-7990/ab563a
The 2024 aluminum alloy is used extensively in the aircraft and aerospace industries because of its excellent mechanical properties. However, the weldability of 2024 aluminum alloy is generally low because it contains a high number of solutes, such as copper (Cu), magnesium (Mg), and manganese (Mn), causing solidification cracking. If high speed welding of 2024 aluminum alloy without the use of filler is achieved, the applicability of 2024 aluminum alloys will expand. Grain refining is one of the methods used to prevent solidification cracking in weld metal, although it has never been achieved for high-speed laser welding of 2024 aluminum alloy without filler. Here, we propose a short-pulsed, laser-induced, grain-refining method during continuous wave laser welding without filler. Bead-on-plate welding was performed on a 2024-T3 aluminum alloy at a welding speed of 1 m min−1 with a single mode fiber laser at a wavelength of 1070 nm and power of 1 kW. Areas in and around the molten pool were irradiated with nanosecond laser pulses at a wavelength of 1064 nm, pulse width of 10 ns, and pulse energy of 430 mJ. The grain-refinement effect was confirmed when laser pulses were irradiated on the molten pool. The grain-refinement region was formed in a semicircular shape along the solid–liquid interface. Results of the vertical section indicate that the grain-refinement region reached a depth of 1 mm along the solid–liquid interface. The Vickers hardness test results demonstrated that the hardness increased as a result of grain refinement and that the progress of solidification cracking was suppressed in the grain refinement region.
Impact characteristics and stagnation formation on a solid surface by a supersonic abrasive waterjet
Kunlapat Thongkaew, Jun Wang , Guan Heng Yeoh
2019, 1(4) doi: 10.1088/2631-7990/ab531c
A computational fluid dynamics (CFD) study of the impact characteristics and stagnation formation on a solid target surface by an abrasive waterjet at supersonic velocities is presented to understand the impact process. A CFD model is developed and verified by experimental water and particle velocities and then used to simulate the jet impact process. The trends of the stagnation formation and its effect on the jet flow with respect to the jetting and impacting parameters are amply discussed. It is found that stagnation formation at the impact site increases with an increase in the impact time, nozzle standoff distance and nozzle diameter, while the initial peak velocity at the nozzle exit has little effect on the size of the stagnation zone. It is shown that stagnation markedly changes the water and particle flow direction, so that the particle impact angle is varied and the jet impact area is enlarged. The jet structure may be classified to have a free jet flow region, a jet deflection region with a stagnation zone and a wall jet region. Furthermore, the stagnation affects significantly the waterjet and particle energy transferred to the target surface. The average particle velocity across the jet is reduced by approximately one third due to the damping effect of the stagnation under the conditions considered in this study.