Dun Liu

Effect of Cr/C Ratio on Microstructure and Corrosion Performance of Cr3C2-NiCr Composite Fabricated by Laser Processing

The present study focuses on the effect of different Cr/C ratios on the microstructure, microhardness, and corrosion resistance of Ni-based laser clad hardfacings, reinforced by in situ synthesized chromium carbide particles. Cr3C2-NiCr composites have been laser processed with graphite/Cr/Ni powder blends with varying Cr/C ratios. Following phase analysis (x-ray diffraction) and microstructure investigation (scanning electron microscopy; energy dispersive x-ray analysis; transmission electron microscopy), the solidification of laser melt pool is discussed, and the corrosion resistances are examined. Several different zones (planar, dendritic, eutectic and re-melt zone) were formed in these samples, and the thicknesses and shapes of these zones vary with the change of Cr/C ratio. The sizes and types of carbides and the content of reserved graphite in the composites change as the Cr/C ratio varies. With the content of carbides (especially Cr3C2) grows, the microhardness is improved. The corrosive resistance of the composites to 0.2M H2SO4 aqueous solution decreases as the Cr/C ratio reduces owing to not only the decreasing Cr content in the NiCr matrix but also the galvanic corrosion formed within the carbide and graphite containing Ni matrix.

Imaging-based amplitude laser beam shaping for material processing by 2D reflectivity tuning of a spatial light modulator

We have demonstrated an imaging-based amplitude laser-beam-shaping technique for material processing by 2D reflectivity tuning of a spatial light modulator. Intensity masks with 256 gray levels were designed to shape the input laser beam in the outline profile and inside intensity distribution. Squared and circular flattop beam shapes were obtained at the diffractive near-field and then reconstructed at an image plane of an f-theta lens (f∼100  mm). The observed intensity distribution inside the beam-shaping geometry was much more even than using binary masks. The ablation footprint well matches the desired beam shape.

Finite element analysis of the temperature field in laser cladding of Ni-based powders on teeth surfaces of the helical gear

The temperature field formed in the process of laser cladding of worn teeth surfaces of the gear shaft is simulated by the finite element analysis software. Isothermal lines inside the tooth are obtained. Simulated results are compared with experimental data. Recommendations are given on improvement of the cladding technology to provide a durable continuous coating.

Synchronization control for ultrafast laser parallel microdrilling system

Ultrafast lasers, emitting ultra-short pulses of light, generally of the order of femtoseconds to ten picoseconds, are widely used in micro-processing with the advantage of very little thermal damage. Parallel micro-processing is seen significant developments in laser fabrication, thanking to the spatial light modulator (SLM) which can concert single beam to multiple beams through computer generate holograms (CGHs). However, without synchronization control, on the conditions of changing different holograms or processing on large area beyond scanning galvo’s ability, the fabrication will be interrupted constantly for changing holograms and moving the stages. Therefore, synchronization control is very important to improve the convenience and application of parallel micro-processing. A synchronization control method, carried out through two application software: SAMLight (or WaveRunner) and Labview, is presented in this paper. SAMLight is used to control the laser and the scanning galvo to implement microprocessing, and the developed program with Labview is used to control the SLM and motion stages. The synchronization signals, transmitted between the two software, are utilized by a National Instruments (NI) device USB-6008. Using optimal control methods, the synchronized system can easily and automatically accomplish complicated fabrications with minimum time. A multi-drilling application is provided to verify the affectivity of the synchronized control method. It uses multiple annular beams, generated by superimposing multi-beam CGH onto a diffractive axicon CGH, to drill multiple holes at one time, and it can automatically finish different patterns based on synchronization control. This drilling way is an optical trepanning and it avoids huge laser energy waste with attenuation. The multi-beam CGHs, generated by the Grating and Lens algorithm, are different for different patterns. The processing is over 200 times faster than traditional mechanical trepanning, moving a small laser spot in a larger orbit.

Multiple NUV beam internal structuring of materials using a spatial light modulator

Liquid crystal Spatial Light Modulator (SLM) can suffer irreparable damage, when exposed at wavelengths shorter than 400 nm combined with high peak intensity, due to photodegradation of the liquid crystal or the polymer alignment layer. By placing a thin BBO nonlinear crystal immediately after an SLM addressed with Computer Generated Holograms (CGHs), the first order diffracted NIR components at 775 nm can be converted to parallel second harmonic NUV beams at 387 nm, avoiding the potential damage while simultaneously reducing the order of non-linear absorption for refractive index modification. This procedure requires attention to phase matching of multiple beams and opens up parallel processing at UV wavelengths. Multiple NUV femtosecond beam direct writing of volume Bragg gratings inside poly(methyl methacrylate) and fused silica is demonstrated. First order diffraction efficiency over 70% is observed. By changing CGH, grating parameters such as period and thickness can be easily adjusted. This technique provides good flexibility and shows great potentials in rapid fabrication of volume gratings. The limitations of this technique are also discussed.Liquid crystal Spatial Light Modulator (SLM) can suffer irreparable damage, when exposed at wavelengths shorter than 400 nm combined with high peak intensity, due to photodegradation of the liquid crystal or the polymer alignment layer. By placing a thin BBO nonlinear crystal immediately after an SLM addressed with Computer Generated Holograms (CGHs), the first order diffracted NIR components at 775 nm can be converted to parallel second harmonic NUV beams at 387 nm, avoiding the potential damage while simultaneously reducing the order of non-linear absorption for refractive index modification. This procedure requires attention to phase matching of multiple beams and opens up parallel processing at UV wavelengths. Multiple NUV femtosecond beam direct writing of volume Bragg gratings inside poly(methyl methacrylate) and fused silica is demonstrated. First order diffraction efficiency over 70% is observed. By changing CGH, grating parameters such as period and thickness can be easily adjusted. This techniqu...

Multiple beam ultrashort pulse laser microprocessing

A problem identified in the use of ultra short pulse lasers for the processing of materials has been that, to work close to the ablation threshold and hence avoid thermal damage, a large percentage of the available laser energy must be attenuated. The surface ablation of materials, using multiple diffracted beams generated by a Spatial Light Modulator (SLM), driven by Computer Generated Holograms (CGH) and synchronised with a scanning galvanometer system, has been shown here to result in flexible and high throughput parallel processing. By using multiple low energy beams, derived from a single higher energy beam, more efficient use of the available laser energy is made and process time is significantly reduced. The results demonstrate high precision microprocessing, showing the potential for ultra short pulse laser parallel processing in real industrial application. A review of the background of the use of SLMs in parallel processing is given, followed by examples of applications investigated.A problem identified in the use of ultra short pulse lasers for the processing of materials has been that, to work close to the ablation threshold and hence avoid thermal damage, a large percentage of the available laser energy must be attenuated. The surface ablation of materials, using multiple diffracted beams generated by a Spatial Light Modulator (SLM), driven by Computer Generated Holograms (CGH) and synchronised with a scanning galvanometer system, has been shown here to result in flexible and high throughput parallel processing. By using multiple low energy beams, derived from a single higher energy beam, more efficient use of the available laser energy is made and process time is significantly reduced. The results demonstrate high precision microprocessing, showing the potential for ultra short pulse laser parallel processing in real industrial application. A review of the background of the use of SLMs in parallel processing is given, followed by examples of applications investigated.

Nano-Silicon Sol-Gel Film Refraction Index Modulation with Femtosecond Laser

Patterned structures were created by exposing SiO2 sol-gel films containing nano-silicon particles to a Clark MXR CPA-2010 fs laser (387 nm). A refractive index variation of 0.2 was obtained, similar to that of polymer films, however in an entirely superior stability class (structural, chemical, thermal, radiation, etc). The useful optical range of refractive index modulation is beyond 800 nm, respectively near-IR. Material characteristics were investigated with atomic force microscopy (AFM), Raman spectroscopy and spectro-ellipsometry measurements. Material properties were also investigated on different substrates in order to determine the influence of substrate type in laser processing.