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Thermal effect is inevitable during laser processing and is easy to induce cracks and damage on the hard and brittle materials, especially. The crack generation mechanism during laser ablation of single hole and groups of holes on alumina ceramic has been investigated. A heat conduction model of the nanosecond laser processing of a group of holes has been developed. The temperature field over a AL2O3 sample was modeled and simulated using COMSOL multi-physics. The temperature distribution on the AL2O3 sample was experimentally verified using an infrared thermometer. The present research provides guidance for the high-quality laser machining of group holes over large areas.

Aluminum nitride (AlN) is essential material to electronics industry. Compared to traditional machining methods, laser trepanning has become one of the most popular options for hole machining on AlN. Due to involvement of multiple parameters, such as scanning number, laser beam jump direction, scanning mode, and filling circle interval, the optimization of hole quality is complex. To tackle this problem, this paper systematically studied hole trepanning on AlN using nanosecond pulse laser in order to examine the relationship among trepanning parameters and hole dimensions, quality, and machining efficiency. Thereby, the optimized combination of parameters could be obtained to improve the quality and efficiency of hole machining. Moreover, possible effect mechanisms like effect of laser fluence on hole diameters, effects of jump directions and scanning modes on hole machining quality were discussed. Most importantly, it is the first time that the relationship among trepanning parameters, hole quality, and machining efficiency is presented.

The comparative study is presented of grooves ablation for the single- and multi-scan processing technology in Ni alloy using a 10 ps pulse laser with the wavelength of 532 nm and the repetition rate of 1 kHz. Results show, when the N (the effective ablation pulse number per unit area), for the single-scan processing of grooves, have been divided into multiple groups and combined with multiple times ablation, namely the multi-scan processing, the groove depth and morphology quality could be enhanced effectively. In addition, the temporary saturation of the groove depth and the bending on the deep groove bottom are reported. Especially for the bending phenomenon which hinders the further deepening of the groove, the related reasons and the solutions are discussed.

Comparing with the trepanning technology, cooling hole could be processed based on the percussion drilling with higher processing efficiency. However, it is widely believed that the ablating precision of hole is lower for percussion drilling than for trepanning, wherein, the melting spatter materials around the hole surface and the recast layer inside the hole are the two main issues for reducing the ablating precision of hole, especially for the recast layer, it can't be eliminated completely even through the trepanning technology. In this paper, the self-cleaning effect which is a particular property just for percussion ablating of holes has been presented in detail. In addition, the reasons inducing the self-cleaning effect have been discussed. At last, based on the self-cleaning effect of percussion drilling, high quality cooling hole without the melting spatter materials around the hole surface and recast layer inside the hole could be ablated in nickel-based superalloy by picosecond ultra-short pulse laser.

A theoretical study, based on the split as well as experiments, was conducted to investigate the formation of high-spatial-frequency laser-induced periodic surface structures (HSFLs) on rough indium-tin-oxide (ITO) films under 10-ps 532-nm-wavelength laser irradiation. At a peak laser fluence of 0.472 J/cm2, the theoretical periods of HSFLs (130–190 nm) matched the experimental values (128–200 nm). Both the theoretical and experimental results demonstrated that the transformation mechanism of laser-induced periodic surface structures (LIPSSs) from low-spatial-frequency LIPSSs (LSFLs) to HSFLs was attributed to split and the irregular period difference of HSFLs and LSFLs was attributed to the surface roughness. Deeper ablation occurred for LIPSSs with a larger period, and the difference at the ablated depth increased with increasing spot number. Therefore, the LIPSSs with the larger period were clearer demarcated and the initial pits in the convex portion of LIPSSs disappeared due to the laser-induced melting. Consequently, sub-100-nm-perioded HSFLs were invisible in spite of the theoretical minimum period of ~88.5 nm. Then, for pits of different depths, the difference of the ablated depth induced by a subsequent pulse can be narrowed by reducing the laser fluence. On this method, 83-nm-perioded HSFLs were obtained by reducing the peak laser fluence to 0.432 J/cm2. Keywords: Ripples, Split, Indium-tin-oxide film, Picosecond laser, Surface roughness, Drude-Lorentz model

An experimental study is presented of the effect of wavelength on the shape and morphology evolution of micro holes ablated on stainless steel surface by a 10 ps Q-switched Nd:VAN pulsed laser. Two routes of hole development are associated with the visible (532 nm) and near-infrared (1064 nm) laser beams, respectively. The evolution of various geometric shapes and morphological characteristics of the micro holes ablated with the two different wavelengths is comparatively studied for other given processing conditions such as a laser power levels and the number of pulses applied. Plausible explanations, based on the light-materials interaction associated with laser micromachining, are also provided for the discernable paths of geometric and morphological development of holes under laser ablation.

In this study, a two-dimensional thermo-chemical reaction model with temperature-dependent thermo-physical parameters on Si3N4 with 10 ns laser was developed to investigate the ablated size, volume and surface morphology after single pulse. For model parameters, thermal conductivity and heat capacity of β-Si3N4 were obtained from first-principles calculations. Thermal-chemical reaction rate was fitted by collision theory, and then, reaction element length was deduced using the relationship between reaction rate and temperature distribution. Furthermore, plasma absorption related to energy loss was approximated as a function of electron concentration in Si3N4. It turned out that theoretical ablated volume and radius increased and then remained constant with increasing laser energy, and the maximum ablated depth was not in the center of the ablated zone. Moreover, the surface maximum temperature of Si3N4 was verified to be above 3000 K within pulse duration, and it was much higher than its thermal decomposition temperature of 1800 K, which indicated that Si3N4 was not ablated directly above the thermal decomposition temperature. Meanwhile, the single pulse ablation of Si3N4 was performed at different powers using a TEM00 10 ns pulse Nd:YAG laser to validate the model. The model showed a satisfactory consistence between the experimental data and numerical predictions, presenting a new modeling technology that may significantly increase the accuracy of the predicated results for laser ablation of materials undergoing thermo-chemical reactions.

Laser fusion welding technique was used for lap joining of Q235 steel to AA5052 aluminum alloy using a flux-cored Zn-22Al filler wire. The influence of laser power on the microstructure and mechanical properties of laser Al/steel joints was investigated. A fusion welded region and a brazed region were formed at the fusion zone/steel interface: Fe2Al5−xZnx and FeZn10 were formed at the both regions, while a new FeAl IMCs with much lower hardness and brittleness was observed in the welded region. The phase constitutions at the both regions were unvaried with the change of laser power, while the phase morphologies were slightly changed. The tensile-shear testing results showed that the joint fracture load first increased then decreased with the rising laser power and the maximum value reached 1225 N at laser power of 2800 W. The joint fractured at the fusion zone/steel interface when the laser power was less than 2800 W and it changed into fusion zone when the laser power reached 3000 W. The change of joint fracture load and fracture behavior was mainly attributed to the change of morphology of IMC and interfacial bonding length.

High repetition rate can produce high laser ablation efficiency. However, there is the issue of shortened pulses interval in the case of high repetition rate. Moreover, both the ejecting time of the ablated materials and the cooling time of the substrate material have been decreased, due to the high repetition rate. These all make some significant influences on micro-hole drilling using ultra-short pulse laser. In this paper, the effect of the repetition rate, on the dimension and morphology of micro- shallow and deep holes, has been systematically studied in metals ablated by picosecond ultra-short pulse laser. The relationship between the repetition rate and micro-hole dimensions, such as diameter and depth, is described. Most importantly, the effect of the repetition rate on micro-hole surface and side-wall morphology is presented for the first time. The possible influencing mechanisms are discussed.

Investigations on the morphological features of holes and grooves ablated on the surface of stainless steel using the picosecond dual-wavelength laser system with different powers combinations are presented based on the scarce researches on morphology of dual-wavelength laser ablation. The experimental results show the profiles of holes ablated by the visible beam appear V-shaped while those for the near-infrared have large openings and display U-shaped, which are independent of the ablation mechanism of ultrafast laser. For the dual-wavelength beam (a combination of visible beam and near-infrared), the holes resemble sunflower-like structures and have smoother ring patterns on the bottom. In general, the holes ablated by the dual-wavelength beam appear to have much flatter bottoms, linearly sloped side-walls and spinodal structures between the bottoms of the holes and the side-walls. Furthermore, through judiciously combining the powers of the dual-wavelength beam, high-quality grooves could be obtained with a flat worm-like structure at the bottom surface and less resolidified melt ejection edges. This study provides insight into optimizing ultrafast laser micromachining in order to obtain desired morphology.