High surface quality micro machining of monocrystalline diamond by picosecond pulsed laser

In micro machining of monocrystalline diamond by pulsed laser, unique processing characteristics appeared only under a few ten picosecond pulse duration and a certain overlap rate of laser shot. Cracks mostly propagate in parallel direction to top surface of workpiece, although the laser beam axis is perpendicular to the surface. This processed area can keep diamond structure, and its surface roughness is smaller than R_a = 0.2 μm. New laser micro machining method to keep diamond structure and small surface roughness is proposed. This method can contribute to reduce the polishing process in micro machining of diamond.     

Ultrashort pulsed laser drilling and surface structuring of microholes in stainless steels

Microholes for the production of gasoline direct injection nozzles were obtained by ultrashort pulsed laser machining in martensitic stainless steels. The inner surface analysis was carried out by a specifically conceived Scanning Probe Microscopy instrument and revealed the presence of periodic surface microstructures whose formation was studied as a function of process parameters (pulse energy, repetition rate, rotational speed, drilling strategy). Experiments demonstrated that geometrical features of the microstructures can be varied by a proper parameter selection and open the perspective for an optimized process enabling a reduction of coking accumulation during service life and a more effective atomization of the fuel jet.     

Efficient and damage-free ultrashort pulsed laser cutting of polymer intraocular lens implants

Ophthalmic intraocular lenses are conventionally machined by diamond tools. A promising alternative approach is contour cutting by ultrashort pulsed laser micromachining. To improve process knowledge, a parametric study of picosecond and femtosecond laser machining of medical grade hydrophilic copolymers and PMMA is carried out. Material removal rates and machining quality with respect to main process parameters are determined. Reasons for chipping and formation of heat affected zones are identified and an optimized process strategy is derived. By choosing a defined pulse overlap, heat accumulation is kept minimal while increasing absorptivity through incubation avoids chipping.     

Complex Process Chains for Manufacturing of Invisible Displays Integrated in Bulk Metal Panels

Information displays that are invisible in the inactive state, and concealed in transparent or semi-transparent materials such as plastic or glass, are commercially available. However, if displays were to be invisibly integrated in bulk metal panels, very few known solutions exist. This has to do with the fact that very thin (20-50 nm) metal layers are required for the light to penetrate. This paper describes the realisation of displays integrated in bulk metal panels through the development and optimisation of complex process chains involving laser micromachining, micro electrochemical machining and selective anodising. An extensive experimental investigation of the single processes as well as their sequential combination is reported, and the capabilities of the different process chains are demonstrated.     

Laser milling of ceramic components

Conventional methods of producing ceramic components are based on sintering technology which requires expensive tooling making it uneconomic for small batch fabrication. Laser milling provides a new method of producing parts in a wide range of materials, including ceramics, directly from CAD data. This paper considers the technical capabilities of laser milling when applied to the machining of microcomponents from alumina and silicon nitride ceramics. The main parameters affecting the material removal characteristics of laser milling are reviewed. A new technique for machining alumina components is proposed emphasising the importance of correct set-up design in achieving a high level of accuracy. Process parameters influencing part quality are analysed and guidelines for machine set-ups are formulated. The paper concludes with an assessment of the accuracy of the laser milling process.     

Sequential Laser and EDM Micro-drilling for Next Generation Fuel Injection Nozzle Manufacture

High quality holes of diameters less than 145 μm are required for the manufacture of next generation diesel fuel injection nozzles for improved combustion efficiency and reduction of emission to the environment. The current practice of using electro-discharge machining (EDM) drilling of fuel injection nozzles is limited in terms of the hole size it can produce effectively and the length of time needed to drill. In addition, the tooling cost is high. This paper reports on an investigation into a sequential laser and EDM micro-drilling technique for the manufacture of next generation fuel injection nozzles. A laser-drilled pilot hole is rimmed out by EDM drilling. It was found that this hybrid process has eliminated the problems of recast and heat affected zones typically associated with the laser drilling process. The new process has enabled a 70% reduction in total drilling time compared to standard EDM drilling as less material is removed by the EDM. The quality of the holes is as good as direct EDM drilling, thus eliminating the need for re-certification of the drilling process. Various combinations of laser/EDM drilling conditions have been examined. Optimum diameters for the pilot hole and the EDM electrode have been identified for a particular diameter of fuel injection nozzle, giving the minimum total drilling time and the best quality holes. A special system was designed to enable the alignment of nozzles to be controlled to within ± 20 μm. The technique has enabled valuable cost savings and increase in production capacity for next generation fuel injection nozzle manufacture.     

Laser induced quasi-periodical microstructures with external field modulation for efficiency gain in photovoltaics

The overall reflectivity of silicon is decreased by 10% altering the surface topology by ultra-short pulsed laser ablation and resulting in an efficiency increase of solar cells. The size of quasi-periodical μm-structures on the surface can be defined by the applied laser parameters. The topology is additionally adapted in size and distance of the microstructures at constant laser parameters with a specifically applied external electrical field leading to a cone-like microstructure with an adjustable light-trapping geometry. On large scale multicrystalline silicon solar wafers with a laser generated μm-scale surface topology were processed into cells with an absolute efficiency gain.     

Sequential laser and mechanical micro-drilling of Ni superalloy for aerospace application

Laser percussion drilling is inherently associated with poor geometry and thermal defects. While mechanical micro-drilling produces good quality holes, premature drill breakage often occurs and it is difficult to drill holes at acute angles. This paper presents the feasibility and basic characteristics of a new approach for micro-drilling In718 alloy sheets at an acute angle, using sequential laser and mechanical drilling. The results demonstrate that sequential laser-mechanical micro-drilling alleviates the defects associated with laser-drilled holes, reduces burr size and machining time and increases the tool life compared with mechanical drilling.     

Investigation of Femtosecond Laser-assisted Micromachining of Lithium Niobate

Lithium Niobate has a potential for applications in electronics and communication industries due to its unique electro-optical, piezoelectric and nonlinear properties. Femtosecond laser machining offers the best alternative to machine the mechanically fragile and optically delicate lithium niobate crystal. This paper reports a study of the effect of femtosecond laser machining on the surface integrity of lithium niobate. The transmission electron microscopy reveals a 100nm thin amorphous region and a void. The chemical analysis shows a loss of lithium and oxygen from the surface and sub-surface. Optical illumination facilitates the selective readout of the written spots of 2 microns size.     

Precision grinding of optical glass with laser micro-structured coarse-grained diamond wheels

This paper presents a series of micro-structured coarse-grained diamond wheels for optical glass surface grinding aiming to improve the grinding performance, especially subsurface damage. The 150 μm grit size, single layer electroplated diamond grinding wheels with different interval micro-groove arrays were manufactured by nanosecond pulsed laser, successfully. The influence of micro-structures on surface roughness and subsurface damage was investigated. Compared with conventional coarse-grained diamond wheel, the subsurface damage depth was reduced effectually from 5 to 1.5 μm, although the better surface roughness was not obtained by the micro-structured coarse-grained diamond wheel. In addition, the surface roughness and subsurface damage depth were both reduced with the decreasing interval of micro-groove arrays.     

Manufacturing by combining Selective Laser Melting and Selective Laser Erosion/laser re-melting

This study presents an experimental investigation to improve Selective Laser Melting (SLM) regarding aspects such as surface roughness, density, precision and micro machining capability by employing secondary processes such as Selective Laser Erosion (SLE) and laser re-melting. SLM is a layered additive manufacturing technique for the direct fabrication of functional parts by fusing together metal powder particles. Laser re-melting, applied after each layer or only on the top surfaces, is used to improve the roughness and density while SLE, a subtractive process, is combined with SLM to improve the precision and micro machining capability.     

Disturbance of material removal in laser-chemical machining by emerging gas

Using laser-chemical machining allows a localized and precise processing of metallic work pieces. The temperature distribution on the surface is the primary factor of this selective and gentle machining method. Investigations regarding temperature and material removal related surface effects like locally induced gas bubbles and reduced material removal are shown. It is shown that the processing feed rates only have a negligible impact on the resulting temperature field and thus the width of the cavity, while laser intensity appears to be the dominant parameter. Furthermore, it is shown that emerging gas bubbles caused reduced material removal resulting in irregular cavities.     

Multi-objective optimization of actuator system design for laser micro adjustment

Complex design processes require a high level of expertise and are time consuming. By assisting the engineer with a computer aided design system the design process can be accelerated and be made more reliable. Actuator system design for laser micro adjustment is complex and its challenges may be a hindrance for the application of laser micro adjustment. To overcome this obstacle a computer aided design system was developed which utilizes a multi-objective optimization algorithm to automatically improve actuator design. In this paper, the system and its components are presented. A special focus will be upon the assessment functions which allow the efficient assessment of an actuator design. An application example will be given to demonstrate the functionality of the design system. The presented work was carried out within the research project 530/59-2 entitled “Algorithmen und Bewertungsmethoden zur Optimierung von Aktorsystemen für die Mikrosystemtechnik” funded by the German Research Foundation (DFG). The authors would also like to thank the Erlangen Graduate School in Advanced Optical Technologies (SAOT) for their support.     

Analysis of laser micromachining in silica glass with an absorbent slurry

The authors have investigated a method for machining a 3D microchannel in silica glass using a UV nanosecond pulsed laser and an absorbent slurry. 3D microstructures in glass materials are required for optical waveguides, microfluidic chips, etc. The depths of the grooves and holes produced in the silica glass were found to be proportional to the number of laser pulses. The material removal process in the proposed method was the melting of the glass by heat transfer from the absorbent particles, which were attached to the surface of the glass, providing for strong laser absorption.     

Droplet detachment regimes in annular laser beam droplet generation from a metal wire

Pendant droplet detachment regimes in the novel annular laser beam droplet generation from a metal wire are analyzed. In drop-on-demand generation, droplet detachment can be achieved via Rayleigh–Plateau instability based molten wire column break-up. Detachment dynamics are influenced by the distance between the annular laser beam focus and the pendant droplet neck. In the continuous generation of a droplet sequence, droplet detachment is governed by the laser pulse frequency, resulting in a spontaneous, resonant, or Rayleigh–Plateau instability based detachment regime. In addition to drop-on-demand generation, continuous droplet generation with spontaneous and mass-spring resonant detachment are suitable for metal droplet based engineering applications where accurate droplet diameter and deposition position are required.     

Prediction of hypo eutectoid steel softening due to tempering phenomena in laser surface hardening

The paper presents a mathematical model for predicting material mechanical property variation, in laser hardening of hypo eutectoid steel, when the softening effects due to the overlapping trajectories are considered. This generally occurs during laser hardening of industrial parts, especially when wide areas have to be treated, due to the tempering phenomena.An original tempering model for the prediction of the hardness reduction is presented in this paper. The proposed model is integrated in a Laser Hardening simulation package, previously developed by the authors. Experimental activities are also presented to validate the model.     

微小整体构件加工技术的现状和发展

介绍了微小整体构件制造技术的现状,对数控切削、特种加工、精密铸造等加工工艺方法做出了相应比较.重点论述了以数控电解、数控电火花为加工代表的特种加工技术对微小整体构件的加工,以此为基础,阐明了组合/复合加工技术有望在未来制造中发挥更重要的作用.最后对微小整体构件的发展趋势及应用前景进行了展望.     

Direct laser deposition of Cu alloy on forming tool surfaces—Process window and mechanical properties

Direct laser deposition offers a widespread spectrum of applications. Creating functional surfaces for forming tools is one of them where inexpensive material for the main tool body is complemented layer by layer with a second material to tune the desired properties and shape. Investigations on coating mild tool steel with copper alloy have been carried out to outline the chances and challenges in this cost effective way of producing forming tools for processing stainless steels, e.g. 1.4301. This paper reports on experimental investigations showing the influence of the absolute angle of the tool surface and the relative angle between surface and laser beam/powder nozzle on the process window and the mechanical properties that can be obtained.     

Chemical Assisted Laser Machining for The Minimisation of Recast and Heat Affected Zone

Laser processing techniques have been widely used for high speed, high accuracy subtractive manufacturing such as cutting, drilling, milling and micro-machining. Most of these processes are based on thermal mechanisms. For the machining of metallic materials, a layer of recast and heat affected zone is normally present on the laser-machined components. This paper reports a novel technique that aims to minimize such heat affects and at the same time to improve the material removal efficiency. A relatively environmentally friendly salt solution, in contact with the beam-material interaction point, was used in this study to enable material removal to be based on laser activated thermal-chemical mechanism. It has been shown that, not only the recast layer can be removed during the processing, the material removal rate can be increased up to 300% for 316 stainless steel work piece.