Single Pulse Processing Research Articles

Nanoengineered Laser Shock Processing Via Pulse Shaping for Nanostructuring in Metals: Multiscale Simulations and Experiments

AbstractModulating the heating and cooling during plastic deformation has been critical to control the microstructure and phase change in metals. During laser shock peening under optimal elevated temperatures, high-density dislocations and nanoprecipitates can be generated to greatly enhance material strength and fatigue life in metals. Currently, heating control during laser shock is limited to steady-state heat transfer, such as hot plate, irradiative heating, or far-infrared heating, which is slow for practical treatment and does not provide the transient conditions for generating nanostructures during shock processing. In this paper, we propose a general methodology to modulate the heating and cooling during laser shock processing via temporal pulse shaping, namely dual pulse laser shock peening (DP-LSP), which combines both ultrafast-heating and laser shock peening in one operation to generate desired microstructure and mechanical property. By modulating the duration of pulses as well as the spacing between pulses, different processing temperatures can be achieved. To test the feasibility of this novel process, DP-LSP has been applied to an Al matrix nanocomposite. Single pulse laser shock peening was able to remelt large second phase precipitates due to fast cooling, resulting in smaller grains (500 nm), while using DP-LSP with the appropriate pulse duration, dynamic precipitation effects can generate nanosized (30 nm) intermetallic phase Al3Ti with high density. By generation of grain size refinement, high-density nanoscale precipitates, and dislocations after DP-LSP, the yield strength increases by 18% and 102% compared with single pulse processing, and original sample respectively. Finite element method modeling was used to simulate the temperature profile in the alloy during the temporal modulated dual laser pulsing. A phase-field model and multiscale dislocation dynamics were applied to study dislocation dynamics and nanoprecipitation generation during DP-LSP, and their interactions at elevated temperatures. The work provides the basis for controlling microstructure in DP-LSP to enhance mechanical properties in metals.

Effect of Tempering Process on Microstructure and Properties of Resistance Spot-Welded Joints of δ-TRIP Steel

In this paper, a medium-frequency inverter spot welder was used for resistance spot-welding experiments on 980 MPa grade cold-rolled δ-TRIP(Transformation-induced Plasticity) steel. The effects of the tempering process on the morphology, microstructure, element distribution, and properties of spot-welded joints were studied by Scanning Electron Microscope (SEM), Transmission Electron Microscopy (TEM), and Electron-Probe MicroAnalysis (EPMA). The microstructure of the nugget zone obtained by single-pulse process was δ ferrite, lath martensite, and twin martensite. After adding tempering under the single-pulse process, the microstructure was δ ferrite and lath martensite. However, the morphology of the microstructure was still dendritic, which remained unchanged. The tensile shear failure of spot-welded joints under the two processes was an interface failure, and the fractures were cleavage fractures. After adding tempering, the interface fracture surface presents two kinds of fracture characteristics: the outer cracks’ growth direction was consistent with the columnar crystal growth direction, and the inner crystal cracks occurred in the nugget core and finally grew along the columnar grain boundary. Due to the significant hardness difference between δ ferrite (283 HV) and martensite (533 HV), the low-strength δ ferrite phase was the main channel of crack propagation. After adding tempering, the hardness distribution of the spot-welded joints was more uniform and the tensile shear force increased (7.4 kN→8.5 kN).

Spatially modulated femtosecond laser direct ablation-based preparation of ultra-flexible multifunctional copper mesh electrodes and its application.

Multifunctional electrodes possess superior properties such as high photoelectric properties and high stability. Laser manufacturing process is one of the widely used method for electrode fabrication. However, the current multifunctional electrode laser manufacturing process suffers from low fabrication speed. Here, we report a high-efficiency laser digital patterning process to fabricate copper-based flexible transparent conducting electrodes. By using a spatially modulated, one single laser spot is modulated into an array of spots with equal intensity, and the fabrication speed can be improved by more than 20 times over the traditional single pulse processing. In addition, copper mesh electrodes with a high photoelectric property have been fabricated. A transparent touch screen panel and multifunctional windows are fabricated with transparent electrodes to demonstrate their use in vehicle defogging, portable heating, and wearable devices.

Efficiency enhancement by transient electron dynamic control in shaped femtosecond laser fabrication of metals

Ultrafast laser pulse train processing has been widely used to improve the efficiency and quality of nonmetallic material processing. However, it is commonly believed that the split of one pulse into sub-pulses for metallic material will suppress the ablation with near-threshold laser fluence. In this article, a high-efficiency metal processing by femtosecond laser double pulses train with different energy ratio is performed in the aspects of theory simulation and experiment. In theory, a Femto-picosecond and Nano-micrometre multiscale framework combining photoelectric effect and electron-phonon-coupled heat transfer proved that the pulse train can effectively control the electron properties, so as to enhance processing efficiency. Theory-guided experiments indicate that the ablation crater reaches maximum ablation depth with a sub-pulse energy ratio optimized pulse train, which is 40%–100% deeper than that of single pulse processing. This method can be used for femtosecond laser processing of various metal materials with high efficiency and high quality, and is helpful to solve the manufacturing bottleneck challenges in the fields of aviation, mechanics, electronics and materials engineering.

MHz burst mode processing as a tool for achieving removal rates scalability in ultrashort laser micro-machining

The average power of ultrashort laser source has been increasing continuously and, therefore, solutions are required to employ fully these technology advances for improving the ablation efficiency in laser micro-processing. The use of burst mode processing is one of the solutions that has attracted a significant research and industrial interest in the past decade. A novel empirical methodology is proposed and implemented in this research to assess the MHz burst mode impact on the specific removal rate (SRR) and processing efficiency in ultrashort laser micro-machining. Especially, the capability of the MHz burst mode processing is investigated to scale up SRRs achievable on copper and stainless steel while utilising fully the available maximum pulse energy and average laser power. The results showed that the MHz burst mode offer a significant SRR scalability potential that can be attributed to beneficial near optimum fluence level and other side effects such as heat accumulation. Also, it is evidenced from the obtained results that the surface quality attained with the burst mode processing was comparable to that achieved with the single-pulse processing and even better at some specific process settings. Thus, the obtained SRR improvements were not in expense of the surface quality and the MHz bust mode processing represents a promising solution to employ fully the constantly increasing average power in ultrashort laser processing operations.

Percussion drilling in glasses and process dynamics with femtosecond laser GHz-bursts.

We report for the first time to our knowledge on top-down percussion drilling of high-quality deep holes in different glasses with femtosecond laser pulses in GHz-burst mode. We reveal the dynamics of the percussion drilling process by pump-probe shadowgraphy and thermal camera imaging demonstrating that the drilling process in GHz-burst mode is fundamentally different from single-pulse processing and confirming the presence of thermal accumulation. Moreover, we show a comparison to drilling by femtosecond single-pulses containing an equal laser fluence in sodalime, alkali-free alumina-borosilicate, fused silica, and sapphire.

Chemical etching of fused silica after modification with two-pulse bursts of femtosecond laser.

Bursts of femtosecond laser pulses were used to record internal modifications inside fused silica for selective chemical etching. Two-pulse bursts with a variable energy ratio between those pulses at a fixed inter-pulse duration of 14.5 ns were applied for the first time. The selective chemical etching rate of the laser-modified material with the burst of two pulses was compared to the single-pulse regime when etching in HF and KOH etchants. The advantage of the burst-mode processing was demonstrated when etching was performed in the KOH solution. More regular nanogratings were formed, and the etching initiation was more stable when burst pulses were applied for fused silica modification. The vertical planar structures were obtained using the two-pulse bursts with an energy ratio of 1:2, increasing the etching rate by more than 35% compared to the single-pulse processing. The highest ever reported selectivity of 1:2000 was demonstrated by introducing the two-pulse burst mode.

Sequences of Sub-Microsecond Laser Pulses for Material Processing: Modeling of Coupled Gas Dynamics and Heat Transfer

Multipulse laser processing of materials is promising because of the additional possibilities to control the thickness of the treated and the heat-affected zones and the energy efficiency. To study the physics of mutual interaction of pulses at high repetition rate, a model is proposed where heat transfer in the target and gas-dynamics of vapor and ambient gas are coupled by the gas-dynamic boundary conditions of evaporation/condensation. Numerical calculations are accomplished for a substrate of an austenitic steel subjected to a 300 ns single pulse of CO2 laser and a sequence of the similar pulses with lower intensity and 10 μs inter-pulse separation assuring approximately the same thermal impact on the target. It is revealed that the pulses of the sequence interact due to heat accumulation in the target but they cannot interact through the gas phase. Evaporation is considerably more intensive at the single-pulse processing. The vapor is slightly ionized and absorbs the infrared laser radiation by inverse bremsstrahlung. The estimated absorption coefficient and the optical thickness of the vapor domain are considerably greater for the single-pulse regime. The absorption initiates optical breakdown and the ignition of plasma shielding the target from laser radiation. The multipulse laser processing can be applied to avoid plasma ignition.

Investigations in nanosecond laser micromachining on the Zr52.8Cu17.6Ni14.8Al9.9Ti4.9 bulk metallic glass: experimental and theoretical study

In addition to their attractive mechanical properties, the lack of grain boundaries and crystal defects of Bulk Metallic Glasses (BMGs) make them suitable materials for integration in micro-systems fabrication chains, either as final components or mould inserts. The application of short or ultra-short pulsed lasers has recently been demonstrated for the processing of BMGs in this context, particularly as a possible solution for machining micro-scale features on the surface of BMG workpieces. Therefore, complementary studies that consider both experimental and thermal simulation data at the same time in the context of multiple and moving pulses irradiating a BMG specimen are desirable to gain insight into micromachining phenomena. Although such models have been reported in the context of single pulse processing of BMG substrates, their extension to multiple pulse scenarios for micromachining is lacking. Thus, the overall objective of this paper is to combine both theoretical and experimental laser micromachining investigations using multiple pulses when irradiating a BMG workpiece. The specific focus is on conducting such laser operations in the nanosecond regime on a Zr-based BMG, with composition Zr52.5Cu17.9Ni14.6Al10Ti5, also known as Vitreloy 105. This combined experimental and theoretical approach enabled the identification of a suitable set of laser parameters with respect to the process efficiency over a range of pulse duration, fluence values, scanning speed and track distance between machined grooves. In addition, it was highlighted that laser micro-machining operations can lead to BMG materials experiencing thermal cycles, which are quite different from that taking place during the conventional synthesis of glassy alloys, thus complicating the further characterisation and understanding of crystallisation phenomena at play.

Simulation and Experimental Study of Nanosecond Laser Micromachining of Commercially Pure Titanium

Nanosecond laser machining of titanium has gained increased interest in recent years for a number of potential applications where part functionalities depend on features or surface structures with microscale dimensions. In particular, titanium is one of the materials of choice to sustain the demand for advanced and miniaturized components in the biomedical and aerospace sectors for instance. This is due to its inherent properties of high strength-to-weight ratio, corrosion resistance, and biocompatibility. However, in the nanosecond laser processing regime, the resolidification and deposition of material expelled from the generated craters can be detrimental to the achieved machined quality at such small scale. Thus, this paper focuses on the investigation of the laser–material interaction process in this pulse length regime as a function of both the delivered laser beam energy and the pulse duration in order to optimize machining quality and throughput. To achieve this, a simple theoretical model for simulating single pulse processing was developed and validated first. The model was then used to relate (1) the temperature evolution inside commercially pure titanium targets with (2) the morphology of the obtained craters. Using a single fiber laser system with a wavelength of 1064 nm, this analysis was conducted for pulse durations comprised between 25 ns and 220 ns and a range of fluence values from 14 J cm−2 and 56 J cm−2. One of the main conclusions from the study is that the generation of relatively clean single craters could be best achieved with a pulse length in the range of 85–140 ns when the delivered fluence leads to the maximum crater temperature being above but still relatively close to the vaporization threshold of the cpTi substrate. In addition, the lowest surface roughness in the case of laser milling operations could be obtained when the delivered single pulses did not lead to the vaporization threshold being reached.

Double-pulse nanosecond laser ablation of silicon in water

This work was conducted experimentally to investigate the material removal rate and its mechanisms during the single-pulse and double-pulse nanosecond laser ablation of a silicon wafer in distilled water. The laser ablation processes were performed under the same experimental conditions with the same total pulse energy (Esingle pulse = Edouble pulse). The amount of ablated material was estimated for all of the processes based on measuring the dimensions (depths and widths) and volumes of the laser-induced craters on the silicon wafer. The results indicate that double-pulsed laser processing can result in a higher material removal rate compared to the more common single-pulse process, when the inter-pulse delay time is less than the pulse duration. The higher ablation yield in the double-pulse process can be due to the higher coupling efficiency of the second laser pulse with the melted target induced by the laser pre-pulse, leading to the more efficient laser energy absorption and deposition within the irradiated region. The double-pulse nanosecond laser processing with delay time of ~5 ns not only results in a higher material removal rate, but also leads to preparation of silicon nanoparticles with a greater mean particle size compared to that of the more common single-pulse laser ablation process.

On-Demand Deposition of Functional Oxide Microdots by Double-Pulse Laser-Induced Dot Transfer

We have newly developed a double-pulse laser-induced dot transfer technique to realize ondemand deposition of functional oxide microdots under room-temperature atmospheric conditions. In our double-pulse system, the first pulse was irradiated to preheat an oxide source film, and then the second pulse was more tightly focused on the same position to deposit an oxide microdot onto a receiver substrate. As a model case, indium tin oxide microdots with much smaller lateral dimensions than the laser focal area were reproducibly arrayed on a silica glass substrate by the doublepulse process, while there were microdot vacancies at a rate of approximately 30 % in the case of a single-pulse process without preheating. In order to explore the effect of double-pulse, laser-induced temperature distribution was also investigated from a finite elemental approach.

Performance Analysis of OS-CFAR with Binary Integration for Weibull Background

The binary integration, also known as S out of M detection, is a popular technique by which the detection statistics of a radar can be greatly improved. In this work, the performance of the ordered-static constant false-alarm rate (OS-CFAR) with binary integration for M sweeps in Weibull background is investigated with an assumption of known shape parameter, not only for homogeneous environment, but also for nonhomogeneous background caused by interfering targets and clutter edges. It's shown that the detection performance of the OS-CFAR with binary integration can not only be greatly improved with respect to the OS-CFAR scheme in a single pulse processing situation, but it can also control the rise of the false alarm rate at clutter edges more effectively. Moreover, an analytic expression for the false alarm rate of the OS-CFAR with binary integration for M sweeps in Weibull background is derived.

Performance of clutter map with binary integration against Weibull background

In order to improve the detection performance of Nitzberg's clutter map in a more spiky clutter environment, a new clutter map detection scheme which combines binary integration with the classic clutter map is proposed and analyzed. It's shown that the detection performance of the clutter map with binary integration is greatly improved compared to the classic clutter map with a single pulse processing.

Performance evaluation of OSSO-CFAR with binary integration in Weibull background

The performance of the Ordered-Statistic Smallest Of (OSSO) Constant False Alarm Rate (CFAR) with binary integration in Weibull background with known shape parameter is analyzed, in the cases that the processor operates in homogeneous background and non-homogeneous situation caused by multiple targets and clutter edge. The analytical models of this scheme for the performance evaluation are given. It is shown that the OSSO-CFAR with binary integration can greatly improve the detection performance with respect to the single pulse processing case. As the clutter background becomes spiky, a high threshold S of binary integration (S/M) is required in order to obtain a good detection performance in homogeneous background. Moreover, the false alarm performance of the OSSO-CFAR with binary integration is more sensitive to the changes of shape parameter or power level of the clutter background.

Self-assembled organic monolayers as high-resolution resists in rapid nonlinear processing with single femtosecond laser pulses

Femtosecond laser patterning of alkanethiol monolayers on gold-coated silicon substrates at λ=800 nm, τ<30 fs and ambient conditions has been investigated. Single-pulse processing allows one to selectively remove the organic coating. Subsequently, pattern transfer into the gold film via wet etching in ferri-/ferrocyanide solution is achieved. As demonstrated, burr-free patterning can be carried out over an extremely wide range of laser pulse fluences from above 2 J/cm2 down to 0.5 J/cm2. Moreover, at low fluences, sub-wavelength processing down to λ/5 is feasible. In particular, at a 1/e laser spot diameter of about 1 μm, holes with diameters of 160 nm and step edges below 80 nm are fabricated. These results emphasize the prospects of organic monolayers as high-resolution resists in rapid nonlinear femtosecond laser processing.

Nonlinear femtosecond laser processing of alkylsiloxane monolayers on surface-oxidized silicon substrates

Femtosecond laser patterning of octadecylsiloxane monolayers on surface-oxidized silicon substrates via single-pulse processing at λ=800nm, τ&amp;lt;30fs, and ambient conditions has been investigated. Depending on the laser pulse fluence, local irradiation results in circular spots of distinct size and morphology. At high fluences, a particular rich complexity of distinct surface morphologies is observed including hole, rim, and ripple formation, and a faint boundary area where monolayer decomposition sets in. At low fluences, subwavelength patterning of the organic monolayer is feasible. In particular, at a 1∕e laser spot diameter of 1.3μm, surface spots with a width down to 300nm are fabricated. Selective processing of the organic monolayer, though, is restricted to a very narrow range of fluences between 1.1 and 1.2J∕cm2. A significantly larger parameter range for selective processing is anticipated in the case of functional monolayers that incorporate aromatic groups. Promising perspectives in femtosecond laser processing of organic monolayers are discussed.

Performance analysis of ordered-statistic greatest of-constant false alarm rate with binary integration for M-sweeps

The performance of the ordered-statistic greatest of (OSGO) constant false alarm rate (CFAR) scheme with binary integration for M non-coherent sweeps in Weibull background was investigated for homogeneous and non-homogeneous backgrounds, with an assumption of known shape parameter. This kind of processing is based on the fact that the clutter can be segmented in regions in many real radar scenarios where a time-varying two-parameter distribution function family can be fitted, but the clutter power may vary locally inside the region. Under the assumption of known shape parameter, the authors examined the changes of the false alarm rate and detection probability of the OSGO-CFAR with binary integration when the shape parameter differs from the nominal one, and compared them to those of the OSGO-CFAR with single pulse processing. The authors have derived analytic expressions of the detection probability and false alarm rate during clutter power transitions for the OSGO-CFAR with binary integration in Weibull background. It is shown that the OSGO-CFAR with binary integration can not only improve the detection performance significantly, but it also control the rise of the false alarm rate at clutter edges more effectively compared to the OSGO-CFAR with single pulse processing. Moreover, it exhibits a good immunity to the variation of the shape parameter.

Time-varying thermal effect of laser crystal in pulsed diode laser side-pumped Nd∶YAG laser

In pulsed diode laser side-pumped solid-state laser, time-varying distribution of temperature in laser crystal is numerically calculated. Three-direction pumped beam intensity distribution in laser crystal is analyzed. Based on the solution, the temperature distribution and its influence in single-pulse process of pulsed diode laser array side-pumped Nd∶YAG laser is obtained by finite element method. The results show that the process of temperature rise is influenced by pump condition and cooling method, but the main influencing factor are the pump conditions such as pumping power and beam waist. Cooling process is influenced by crystal thermo-physical parameters, radius and cooling condition. After the transient temperature distribution has reached dynamic equilibrium, because of radius temperature gradient periodically changing with time, the optical path difference between center and edge beam is also periodically changing with time.

The effects of DNIC-type inhibition on temporal summation compared to single pulse processing: Does sex matter?

A few experimental observations have suggested that diffuse noxious inhibitory control (DNIC)-type inhibition acts preferentially on the pain system if this is in a sensitised state, e.g. after slow temporal summation (wind-up). However, firm evidence is still missing. Furthermore, sex-related factors, which seem to affect temporal summation as well as DNIC effects, might thus also modulate the interaction of these two processes. To answer these questions, we investigated 40 young and pain-free subjects (20 female and 20 male). The conditioning stimulus in our DNIC paradigm was realized by immersion of the hand into a water tub containing either 42 °C (non-painful heat) or 46 °C (painful heat) hot water. The test stimuli were either single pulses or series of five pulses (0.5 Hz repetition frequency) produced by a pressure algometer. The VAS ratings for the last stimulus in the series were significantly higher than for the single pulse (temporal summation). The ratings were significantly reduced by the 42 °C conditioning stimulus and even more by the 46 °C conditioning stimulus, suggesting DNIC-like inhibition. This was equally true both for the single pulse and for the series of pulses. Sex differences were not observed for temporal summation, DNIC inhibition or for the interaction of the two processes, although women exhibited significantly lower pressure pain thresholds and higher ratings for the tonic heat stimuli. In conclusion, DNIC-type inhibition apparently does not preferentially act on a sensitised pain system after slow temporal summation. Considering the sex of the subjects does not change this insight.