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.     

Investigation of the effects of process parameters on the glass-to-mold sticking force during precision glass molding

Compression glass molding is a promising manufacturing process for high precision, low cost glass optical elements. However, the conditions during glass molding are known to cause sticking between the glass workpiece and the mold surface. Materials for glass molds need to have high hardness and heat resistance, therefore cemented carbide or ceramics are often used. To mitigate the adhesion problem, a thin coating layer of inert materials is deposited on the mold surface. In this research, two different coatings, Pt-Ir and TiAlN, were applied to both tungsten carbide and silicon wafer substrates and then tested in a real molding environment. Experiments based on the design of experiment or DOE revealed factors which significantly affect the sticking force under the described forming conditions including compression hold time, cooling time and compression force. The morphology of the coatings in this research was also carefully investigated using white light interferometry and scanning electron microscopy. The methodology developed in this research can be readily applied to quantitatively test the effectiveness of different thin film coatings thus providing the optics industry a solution for high performance molding process design. Results on both WC-Co and silicon substrates also support the notion that different engineering materials can be used for mold fabrication if proper coating is applied.     

Fabrication of hexagonally arrayed micro-structures with axially symmetrical surface profile by tri-axial excimer laser scanning

This paper presents an excimer laser contour scanning method for achieving hexagonally arrayed micro-structures with an axially symmetrical surface profile. It utilizes a specially designed contour mask for projected laser scanning along three different directions separated by 120° on the surface of a sample. Through rigorous analysis, a new contour mask design method is proposed which guarantees not only the profile accuracy but also the axial-symmetry of machined micro-structures. Experiments have been carried out using a 248 nm KrF excimer laser micromachining system. Hexagonal arrays of micro-lenses with an aperture size of 100 μm in diameter and a maximum sag height from 15 to 30 μm are successfully produced using designed contour masks. The machined profile accuracy is better than ±0.5 μm within most of the central area of lens aperture. The flexibility on machined surface profile and its machining accuracy allow new types of micro-lens arrays with specific optical functions and new applications in the future.     

Pin-plate micro assembly by integrating micro-EDM and Nd-YAG laser

This paper presents a novel technology that applies the principle of Molten-Separation Joint (MSJ) from the micro-part back to perform precise micro assembly. This has been realized by using a hybrid process that integrates micro-EDM and Nd-YAG laser welding on a single machine to fabricate micro parts and complete precise micro assembly. To demonstrate this novel technology, we used this hybrid process to assemble a product, which is referred as a pin-plate. It consists of a micro pin with down to 50 μm in diameter jointed to a thin plate with 200 μm thick, and is made of SUS304.A tensile mechanism has been designed to measure the strength of the pin-plate after micro assembly. The results on SUS304 show that the joint strength is higher than that of the substrate for a micro pin with 200 μm in diameter. In addition, the pin-plate perpendicularity can be measured with a micro probe and a short sense discharge circuit specially installed in the micro-EDM. The detected results were quite satisfactory. This was further assured by observing the assembled joint cross section through a microscope.     

镍基金属玻璃电化学特性与线切割实验研究

金属玻璃具有独特的原子结构和优异的材料性能,在产品微小型化、精密化方面已展现出重要而广阔的应用前景。但金属玻璃对微加工过程中温度敏感,且现有的金属玻璃微加工技术还不成熟,在一定程度上阻碍其广泛应用。提出了金属玻璃微细电解线切割加工方法,基于电化学阳极溶解原理对材料进行加工,加工温度低、无加工应力、材料不会晶化,是制造金属玻璃微结构零件理想的技术手段。研究了镍基金属玻璃Ni_(72)Cr_(19)Si_7B_2在不同溶液中的电化学特性,并优化微细电解线切割实验参数,加工出高质量的金属玻璃微结构。     

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.     

Characterization and prediction of the heat-affected zone in a laser-assisted mechanical micromachining process

Laser-assisted mechanical micromachining (LAMM) is a micro-cutting method that employs highly localized thermal softening of the material by continuous wave laser irradiation focused in front of a miniature cutting tool. However, since it is a heat-assisted process, it can induce a detrimental heat-affected zone (HAZ) in the part. This paper focuses on characterization and prediction of the HAZ produced in a LAMM-based micro-grooving process. The heat-affected zone generated by laser heating of H-13 mold steel (42 HRC) at different laser scanning speeds is analyzed for changes in microstructure and microhardness. A 3-D transient finite element model for a moving Gaussian laser heat source is developed to predict the temperature distribution in the workpiece material. The model prediction error is found to be in the 5–15% range with most values falling within 10% of the measured temperatures. The predicted temperature distribution is correlated with the HAZ and a critical temperature range (840–890 °C) corresponding to the maximum depth of the HAZ is identified using a combination of metallography, hardness testing, and thermal modeling.     

Increase of process reliability in the micro-machining processes EDM-milling and laser ablation using on-machine sensors

In micro production micro electric discharge machining (μEDM) and laser ablation are two established manufacturing methods. Due to the thermal material removal mechanism there are some challenges to provide the needed process reliability. In both cases it is important to ensure a precise manufacturing process by the correct positioning of the tool, e.g. the laser spot, relative to the work piece and the measurement and elimination of geometrical deviations.Using the EDM-milling process to manufacture small cavities a deviation of the manufactured depth and a waviness of the surface may appear if the compensation of the tool wear does not work correctly. A confocal white-light sensor can be used to measure the surface of the machined cavity after the EDM process. An on-machine system provides the possibility to rework the work piece without unclamping it. Combining these two procedures in an automatic working quality loop the accuracy of the manufactured depth can be improved down to a maximum deviation of less than 2 μm. Regarding the laser ablation process using ultra short pulsed Nd:YAG lasers problems arise if the focal plane is not on the surface of the work piece. Due to the manufacturing in layers occurring deviations in the surface remain until the end of the machining process. In this work an acoustical sensor is used to provide a correctly adjusted focal plane at the beginning of the first manufacturing step. Acoustic emissions are detected during the ablation process. A signal analysis of the airborne sound spectrum emitted by the process enables specific conclusions about the focal position of the laser beam. Based on this correlation an acoustic focus positioning is built up and tested on a ceramic work piece. The focal plane can then be adjusted automatically before the ablation.     

Glass forming ability and crystallization of Zr-Cu-Ag-Al-Be bulk metallic glasses

The glass forming ability of Zr₄₆Cu_37.64−xAg_8.36Al₈Be_x (x = 0, 6 and 10 at.%) bulk metallic glasses (BMGs) were significantly improved by Be addition. The critical size of amorphous rods can be over 35 mm diameter. The high GFA achieved is mainly due to the decrease of melting point and liquidus temperature, and suppression of the formation of crystalline phases during solidification from liquid state. The high stabilization with supercooled liquid regime of 115 K was found for the BMG with x = 10 at.%. Two independent exothermic events happen in x = 0 and 6 at.% BMGs, corresponding to the formation of primary crystalline phases Cu₁₀Zr₇ and AgZr, then transforming to final stable crystalline phases Zr₂Cu and AlCu₂Zr. However, in the x = 10 at.% BMG, the precipitation of primary phases and transformation to final stable phases are within the first exothermic event and the AlCu₂Zr phase is totally suppressed.     

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.     

Glass capillary optics for producing nanometer sized beams and its applications

We have developed a method to produce micro/nano meter sized beams of keV energy highly charged ions (HCIs) and MeV energy protons/He ions with tapered glass capillary optics for application of surface modifications and a biological tool called “cell surgery”, respectively. The transmission through the tapered glass capillaries with inlet diameter of 0.8 mm?, outlet diameter from 900 nm? to several tens of microns and length of about 50 mm was performed using 8/64 keV Ar⁸⁺ beams. The transmitted beams had a density enhancement of about 10 and were guided through a capillary tilted by as large as ±100 mrad. The charge state of the beams was kept during the transmission. The combination of MeV proton/He ion beams and the capillary with a thin end window at its outlet can realize pinpoint energy deposition and three-dimensional selection of the bombarding point in an arbitrary position of a living cell or in any liquid object. We demonstrated that a real biological cell, HeLa cell with the nucleus labeled by green fluorescent protein (GFP), was irradiated with the microbeam, which was prepared by 4 MeV He²⁺ entering a capillary with an end window of 7.3 μm in thickness and outlet diameter of 9.6 μm?. The transmitted MeV ion beams had density enhancement up to 1000 according to the capillary outlet sizes, which are applicable to various material analyses employing microbeams.     

Fabrication and characterization of micro dent arrays produced by laser shock peening on titanium Ti-6Al-4V surfaces

Surface micro dents may act as lubricant reservoirs to reduce friction and wear in sliding and rolling contact applications. Surface patterning has become a valuable technique for fabricating micro dents. Alternative methods such as micromachining present obvious limitations in comparison with laser shock peening (LSP). In this paper, the use of micro LSP along with an automatic XY table proves to be an attractive and reliable method for producing micro dent arrays with enhanced surface integrity. Surface topography, residual stress, and microhardness of the fabricated micro dent arrays on polished Ti-6Al-4V surfaces have been characterized. It has been shown that LSP is capable of efficiently fabricating mass micro dent arrays with controllable size via adjusting laser power. The center area of the peened dents has the highest hardness. In addition, high compressive residual stress can also be produced.     

Voltage pulse frequency and duty ratio effects in an electrochemical discharge microdrilling process of Pyrex glass

Electrochemical discharge machining (ECDM) is used to microdrill glass wafers. One of the drawbacks of ECDM is the heat-affected zone (HAZ) left on the microdrilled hole surface. To reduce the HAZ, a series of rectangular voltage pulses, were applied in this study instead of the rectified or full-wave DC voltages. The effect of the frequency and duty ratio of the voltage pulse on the ECDM of Pyrex glass was experimentally investigated. The experimental results show that the thermal damage of the microdrilled hole decreases as the frequency increases and as the duty ratio decreases. It was also found that the clearance increases as the tool diameter decreases. This experimental investigation provides a new method that exhibits several advantages over conventional ECDM.     

In situ synthesis and fabrication of tricalcium phosphate bioceramic coating on commercially pure titanium by laser rapid forming

Titanium possesses good mechanical property, but a lack of bioactivity. Bioceramic is usually used to coat the surface of a titanium implant to enhance its bioactivity. Therefore, it is important for the coating to have a high bonding strength to the titanium substrate and contain bioactive phases. In this work, CaCO₃ and CaHPO₄·2H₂O powders were used to fabricate a bioceramic coating on commercially pure titanium (cp-Ti) by the laser rapid forming (LRF) technique. The phase composition of the coating contained 95 wt.% of β-TCP and 5 wt.% of α-TCP. Three layers were found in the coating: a ceramic layer, a transitional layer, and the substrate layer. In the transitional layer, interpenetration of phases was observed. The bonding strength between the coating and the cp-Ti substrate was in excess of 40.17 MPa. In addition, the elastic modulus and the micro-hardness of the coating were 117.61 GPa and 431.2 HV_0.1, respectively. Furthermore, the static immersion test has confirmed that the coating not only prevented the corrosion of cp-Ti, but also induced the redeposition of β-TCP in synthetic saliva.     

Fabrication of concave micro lens array using laser patterning and isotropic etching

Micro lens arrays are widely used in optical communication and laser-fiber coupling applications. In this paper, a technique to fabricate concave micro lens arrays on glass substrate using a third harmonic Nd:YAG laser direct patterning and followed by chemical wet isotropic etching is presented. The patterning process was done on gold film, which was coated on a glass substrate by using a NC controlled laser ablation tool paths. The glass substrate is then etched by using hydrofluoric acid (HF) solutions whereby the exposed area will be dissolved away by chemical reaction with HF. The type of etching process is an isotropic etching which the etching rate is equal at all direction thus produce hemispherical concave profile on glass. The optimum laser patterning parameters is obtained and the effect of different types of HF solutions on etching efficiency is studied. The surface morphology, 2D and 3D profiles are also measured. Various micro lens diameters are fabricated with different values of lens sag.     

Porous structures fabrication by continuous and pulsed laser metal deposition for biomedical applications; modelling and experimental investigation

The use of porous surface structures is gaining popularity in biomedical implant manufacture due to its ability to promote increased osseointegration and cell proliferation. Laser direct metal deposition (LDMD) is a rapid manufacturing technique capable of producing such a structure. In this work LDMD with a diode laser in continuous mode and with a CO₂ laser in pulsed modes are used to produce multi-layer porous structures. Gas-atomized Ti-6Al-4V and 316L stainless steel powders are used as the deposition material. The porous structures are compared with respect to their internal geometry, pore size, and part density using a range of techniques including micro-tomography. Results show that the two methods produce radically different internal structures, but in both cases a range of part densities can be produced by varying process parameters such as laser power and powder mass flow rate. Prudent selection of these parameters allows the interconnected pores that are considered most suitable for promoting osseointegration to be obtained. Analytical models of the processes are also developed by using Wolfram Mathematica software to solve interacting, transient heat, temperature and mass flow models. Measured and modelled results are compared and show good agreement.     

Ti6A14V laser alloying process control by using a self-organizing fuzzy controller

The quality of laser surface alloying process depends on many process parameters, such as power density and beam velocity. Unacceptable results may occur due to the parameters across a certain specified range. Hence, during the alloying process, a feedback control system is required to monitor the variation of operating parameters for yielding a good quality control and tackle the process disturbance. Since the laser surface alloying process has non-linear coupling and time varying characteristics, it is difficult to establish an appropriate process model for this multi-input and multi-output (MIMO) system. Hence a model-free self-organizing fuzzy control (SOFC) method is employed in this study. It has on-line learning ability, the rule tables can be modified automatically and continuously. Two SOFC controllers are designed for tuning the laser power and the traverse velocity simultaneously to tackle the geometrical and absorptivity variations of the work piece. The simulation results show that this control method can achieve good control performance. The control performance is compared with that of a traditional PID controller.     

Glass transition kinetics of La55Al25Ni10Cu10 bulk metallic glass by electrical resistivity measurement

The glass transition behavior of La55A125-Ni10CU10 bulk metallic glass was investigated using elec-trical resistivity and differential-scanning calorimetry measurements. There are good agreements of the onset temperature （Tg-onset） and the end temperature （Tg-end） of the glass transition between the DSC curve and tempera-ture dependent electrical resistivity （TER） curve in the same heating rate. Kinetics of the glass transition is studied with the electrical resistivity data. The apparent activation energy of the glass transition （△Eg） is found to be 306.4 kJ.mol^-1. Fragility parameter D^＊ and VFT temper-ature To are 9.8 and 324.9 K, respectively.     

Repair welding of ductile cast iron by laser cladding process: microstructure and mechanical properties

Abstract

The aim of this research was to determine the feasibility of a newly developed process in the repair of cracked gas turbine casings made of ductile cast iron. This study investigated the microstructural characteristics, metallurgy and mechanical properties of the repair weldments produced using fibre laser cladding. Optical microscopy, scanning electron microscopy and element probe microanalysis were used to investigate the microstructure at the cladding weld interface. The mechanical properties of the cladded specimens were evaluated after laser cladding. Our results revealed that the weldability of ductile cast iron can be enhanced by performing laser surface pretreatment to sublimate graphite nodules. Microhardness at the interface of the laser cladded weldments depended largely on the range of the heat affected zone and the degree of phase complexity. Under tensile loading, failures were limited to the base metal region of the weldments. Test results demonstrate that the impact toughness of the interface between the fusion zone and the base metal can be enhanced through the application of post-cladding heat treatment.