Hybrid Laser-waterjet Research Articles

Optimization of energy consumption in coating removal for recycling scrap coated cemented carbide tools using hybrid laser-waterjet

In order to purified recovery of scrap cemented carbide, stripping of coatings was mandatory as engineering tools for improving service life were almost modified with anti-wear coatings. However, the low efficiency of current coating removal technologies leaded to the overmuch energy consumption, it was of great importance to achieve energy conservation to relieve the environment impact because of sustainable development. Reducing the energy consumption during processing could significantly improve the environmental performance of manufacturing systems. According to the energy consumption characteristics of hybrid laser-waterjet machine, a multi-objective optimization model for energy prediction with key parameters under actual constraints is constructed to trade off two responses: the cutting specific energy consumption (CSEC) and the processing time, which is transformed into a single-objective model and then solved by genetic algorithm. Simultaneously, the orthogonal experiment is proposed to identify the parametric influence on the multi-objective optimization and to validate the feasibility of the proposed model with the comparisons between single-objective and multi-objective optimization. Finally, the comparisons about energy consumption, carbon emission and the mechanical properties between high temperature oxidation method and hybrid laser-waterjet technology are carried out. The results indicate that larger feed speed and larger laser offset distance can benefit the energy conservation due to the shortened processing time. The multi-objective optimization model realizes about 80% of energy conservation compared to minimum CSEC, and 40% of which compared to minimum processing time. Contrasted to the energy conservation and substrate quality after processing, the hybrid laser-waterjet technology is much better than the high-temperature oxidation method.

Surface quality evaluation of single crystal 4H-SiC wafer machined by hybrid laser-waterjet: Comparing with laser machining

Single crystal silicon carbide (SiC), as a 3rd generation semiconductor material, has a wide application range and development prospect, especially in some key fields such as defense and aerospace industries. It's a typical difficult-to-machine and hard-brittle material due to its high hardness and brittleness. A hybrid laser-waterjet micromachining technology was employed to implement near damage-free and high-efficient micromachining of single crystal SiC. The workpiece material is heated by laser to a temperature near but below its melting point. The material is under an extremely “softened” but still solid status. A waterjet is applied off-axially to expel the heating-softened material and cool the material to eliminate thermal damage. It combines the advantages of laser processing with those of waterjet cutting. Moreover, it's a green machining approach, which is pollution-free and recyclable. In the present study, the surface quality of single crystal 4H-SiC machined by the hybrid laser-waterjet is evaluated and compared with that machined by laser. Among the wafers respectively machined by laser, under-water laser, waterjet-assisted laser and the hybrid laser-waterjet, the one machined by the hybrid laser-waterjet is with the best surface quality. Its cut boundaries between the machined area and the unmachined area are clear, the cut edges are straight, also the cut sides are clean. On the contrary, there is severe thermal damage along the cut edges and cut sides of laser machining. The microgrooves obtained from the hybrid laser-waterjet micromachining are with V-shaped cross-sections and the transition between the cut edge and the unmachined surface is flat and smooth. The cross-section profile of the microgroove machined by laser is M-shaped and there are obvious humps along the two sides of the microgroove. The microgroove bottom is almost at the same height with the unmachined surface. Besides, there are HAZs with a width of 50–100 µm and scaly recast layers on the sides of the cut machined by laser. In contrast, there is no recast layer and HAZ on the cut side of the hybrid laser-waterjet micromachining. The texture of the cut sidewall is clear and with the typical features of the material removed under the plastic mode. The EDS results show that the oxidation behavior happens during both the hybrid laser-waterjet micromachining and laser machining. However, the content of silicon dioxide generated in the hybrid laser-waterjet micromachining process (the oxygen content is about 5%) is obviously lower than that generated in laser machining (the oxygen content is more than 30%).

Effects of a low-pressure water jet assisting the laser etching of polycrystalline silicon

Laser micromachining has proven to be a useful tool for the precision processing of semiconductors. In the hybrid laser–water jet micromachining process investigated in this research, a laser is used for heating and softening the work material (polycrystalline silicon, in this case), while a water jet is employed for shearing the softened material and subsequently removing it due to the high-velocity of the liquid jet impingement, as well as for cooling the material. Specifically, the use of a low-pressure water jet to assist with the laser etching of polycrystalline silicon was investigated in this study. The energy efficiency of the processing as well as the width, depth, and surface microstructure of the groove etched in the polycrystalline silicon using the composite process were considered. With increasing jet velocity, the integrated energy initially decreased before increasing and the overall groove depth decreased, then increased, and finally decreased again. When the incidence angle was 30°, the groove width in the polypolycrystalline silicon increased as the jet velocity increased from 8 and 28 m s−1, and the use of a low-pressure jet to assist with the laser etching resulted in etched surfaces smoother than those obtained in air and hydrostatic conditions. The experimental results prove that the low-pressure water-jet-assisted laser etching technique can overcome the problems of micro-crack, slag, and recast layer formation that affect traditional laser etching and consequently improve the quality of machined parts.

Material removal of single crystal 4H-SiC wafers in hybrid laser-waterjet micromachining process

Yield stress, as well as ultimate stress, decreases with an increased temperature. A much smaller cutting force can be applied to perform the machining at high temperatures. Material is considered “softened” when its yield and ultimate stresses decrease due to the increase of temperature. During the hybrid laser-waterjet micromachining process, the material is heated by laser. Once the critical resolved shear stress (CRSS), related to the yield stress, decreases to be equal to the applied resolved shear stress induced by the pressurized waterjet impingement, the material begins to be deformed and then removed plastically. The hybrid laser-waterjet micromachining technology shows a satisfying performance on almost damage-free and high-efficient micromachining of thermal-sensitive and hard-brittle materials at their “softened” status. The present study is focused on the material removal mechanism of single crystal 4H-SiC in the hybrid laser-waterjet micromachining process. The temperature-dependent CRSS on the primary slip system of 4H-SiC is formulated. It is shown that the CRSS of 4H-SiC is less than 5 MPa at 1650 K. The shear stress induced by pressurized waterjet impingement, laser heating effect under the laser-water interference and the waterjet cooling effect are respectively studied. The range of water pressure used in the machining process is from 4 MPa to 13 MPa. A 2-D numerical model is developed using the finite difference method (FDM). The simulations show that the material begins to be removed before the laser intensity reaches its maximum in a laser pulse cycle. The depth and width of the microgroove increase gradually in the following several cycles. The increments become gradually less until this increase stops. The material is removed before it’s heated to its melting temperature and at a softened but still solid status. Heat accumulation between laser pulses is not expected in the process. The machined surface quality and material removal mechanisms of the hybrid laser-waterjet micromachining and laser dry ablation are compared by 3D profiles and SEM photographs of the microgrooves obtained from these two machining methods. The surface quality obtained from hybrid laser-waterjet micromachining is better than that obtained from laser dry ablation. The protrusion strips along the cut edges of laser dry ablation are scaly and similar to welded junction. It indicates that the material is melted or vaporized by laser heating and re-solidified at cut edges to form a recast layer during laser dry ablation. There aren’t recast layers on the machined surface of hybrid laser-waterjet micromachining. Its surface texture is characterized as a typical feature of plastic slip.

Investigation and modelling of hybrid laser-waterjet micromachining of single crystal SiC wafers using response surface methodology

A hybrid laser-waterjet micromachining technology was proposed to implement near damage-free and high-efficient micromachining of thermal-sensitive hard and brittle materials. A new material removal concept was used where a waterjet is applied off-axially to expel the “softened” elemental material by laser radiation and cool the material to eliminate thermal damages during the material removal process. The present study investigates the effect of process parameters and their interaction on the cutting depth, cutting width and the material removal rate in the hybrid laser-waterjet micromachining of single crystal SiC wafers. The analysis of variance (ANOVA) indicates that waterjet inclination angle, waterjet offset distance, nozzle stand-off distance, traverse speed of hybrid cutting head, average laser power and waterjet pressure are the significant terms on cutting depth, cutting width as well as material removal rate. The quadratic backward-eliminated regression models are developed using response surface methodology (RSM). The models show that the material removal ability and the material removal rate increase with the increased average laser power and decrease with the increased nozzle stand-off distance and waterjet pressure. Waterjet offset distance has a knee-point value on the machining results. Cutting depth and width decrease with the increased traverse speed of hybrid cutting head but material removal rate increases with the increased traverse speed when the traverse speed is smaller than a certain value. The turbulent water breaks down the laser optically and weakens its heating ability. The critical waterjet offset distance can also be changed since the interactive effect of the process parameters. The verification results show the models can perform predictions with acceptable errors. Moreover, as compared to laser dry ablation, the photographs of the machined surface and its 3D profile illustrate that hybrid laser-waterjet micromachining can obtain much deeper and wider groove with V-sharp cross-sectional profile. It can significantly reduce or even eliminate thermal damages such as heat-affected zone (HAZ) and re-solidified layer by using the hybrid laser-waterjet micromachining technology.

Hybrid CO2 laser waterjet heat (LWH) treatment of bindered boron nitride composites with hardness improvement

Boron nitride (BN) material is chemically and thermally stable which makes it desirable for high speed machining in demanding chemical and thermal environments. Although the hardness of BN material is well below that of single polycrystalline diamond (PCD), a laser waterjet heat (LWH) treatment process provides a new potential approach to achieve hardness values that are comparable to diamond hardness. This study investigates the hardness change of LWH-treated bindered cBN/TiN and cBN/AlN composites. Results indicate that measured hardness increase is dependent on the laser beam pass and the distance from the beam center.

A study of hybrid laser–waterjet micromachining of crystalline germanium

An experimental investigation of the cutting performance in hybrid laser–waterjet (or laser-assisted waterjet) micro-grooving of germanium wafers is presented, with a view to eliminate or minimize the laser-induced thermal damages to the workpiece. Various process parameters are considered, such as water pressure, laser pulse overlap, pulse energy and focal plane position. It is found that the hybrid laser–waterjet is a viable technology for micromachining of germanium with negligible thermal damage. A Raman spectroscopy study did not reveal any crystalline change in the material on the machined surfaces. The effects of process parameters on the heat-affected zone and groove characteristics are amply discussed. It is shown that good grooves of within 100 µm in top width and up to 300 µm in depth can be machined with high material removal rates, and the heat-affected zone size can be controlled to within 20 µm on each side of the grooves. Recommendations are also made on the appropriate process parameters that may be used in the process.

Microgrooving of Germanium Wafers Using Laser and Hybrid Laser-Waterjet Technologies

Lasers have the potential for the micromachining of germanium (Ge). However, the thermal damages associated with the laser machining process need to be properly controlled. To minimize the thermal damages, a hybrid laser-waterjet ablation technology has recently been developed for micromachining. This paper presents an experimental study to assess the machining performances in microgrooving of Ge by using a nanosecond laser and the hybrid laser-waterjet technology. The effects of laser pulse energy, pulse overlap and focal plane position on the groove geometry and heat affected zone (HAZ) size are analyzed and discussed. It is shown that the hybrid laser-waterjet technology can give rise to narrow and deep microgrooves with minimum HAZ.

Heating and material removal process in hybrid laser-waterjet ablation of silicon substrates

A hybrid laser-waterjet micromachining technology has recently been developed for near damage-free micro-ablation. It uses a laser to heat and soften the target material and a waterjet to expel the laser-softened elemental material to decrease thermal damages and increase the material removal. A computational model for the hybrid laser-waterjet micro-grooving process for single crystalline silicon is presented in this paper using an enthalpy-based finite difference method. Laser heating and waterjet cooling and expelling with the temperature-dependent silicon properties are considered in the model to predict the temperature profiles of silicon and groove characteristics under different machining conditions. The simulation results show that the introduction of a high pressure waterjet enables to remove material at its soft-solid status much below its melting temperature, while the waterjet cooling effect can reduce the workpiece temperature during the laser non-pulse period and eliminate the effect of heat accumulation, so that the thermal damage induced by laser heating is minimized. The temperature field model is also used to predict the groove depth and profile, and it is found that the model can reasonably represent the machined groove characteristics when comparing to the experimental data.

Optimization of Hybrid Laser-Waterjet Micromachining of Silicon

Micro/nanofabrication with less damage has been raised as a challenging issue in advanced micro/nanomanufacturing industries. Recently, a new hybrid laser-waterjet machining technology has been developed, in which material is removed by laser heating and softening and waterjet cooling and expelling with negligible thermal damage to the workpiece. An optimization of the process parameters, such as laser pulse energy, laser pulse overlap, focal plane position, and waterjet offset distance, in the machining of silicon using this hybrid technology is presented in this study. Grey relational analysis based on an orthogonal array is employed to optimize the multi-performance characteristics, where the groove width and heat-affected zone are minimized while the groove depth is maximized.

An investigation of hybrid laser–waterjet ablation of silicon substrates

A novel hybrid laser–waterjet ablation technology is developed to minimize the thermal damages, which occur during material melting and vaporization associated with the traditional laser machining processes. With this technology, the target material is heated and softened by a laser and the softened material is expelled by a waterjet. The waterjet also takes a cooling action. Using a single-crystalline silicon as the specimen material, the process parameters relevant to the hybrid process were investigated along with the interactions between the laser and waterjet. Further, the cut geometries and the surface characteristics after machining have been experimentally studied. It has been found that almost no heat-affected zone (HAZ) with a higher material removal rate can be achieved when using this hybrid laser–waterjet technology, as compared to the conventional laser dry micromachining process.