Laser Percussion Research Articles

Multi Criteria Optimization of Laser Percussion Drilling Process Using Artificial Neural Network Model Combined with Genetic Algorithm

ABSTRACT This is a study of laser percussion drilling optimization by combining the neural network method with the genetic algorithm. First, optimum input parameters of the process were obtained in order to optimize every single output parameter (response) of the zprocess regardless of their effect on each other (single criterion optimization). Then, optimum input parameters were obtained in order to optimize the effect of all output parameters in a multicriteria manner. Artificial neural network (ANN) method was employed to develop an experimental model of the process according to the experimental results. Then optimum input parameters (peak power, pulse width, pulse frequency, number of pulses, assist gas pressure, and focal plane position) were specified by using the genetic algorithm (GA). The output parameters include the hole entrance diameter, circularity of hole entrance and hole exit, and hole taper. The tests were carried out on mild steel EN3 sheets, with 2.5 mm thickness. The sheets were drilled by a 400 w pulsed Nd:YAG laser emitting at 1.06 µm wave length. Oxygen was employed as the assist gas. Considering the accuracy of the optimum numerical results and the high capability of the neural network in modeling, this method is reliable and precise and confirms the qualitative results in the previous studies. As a result, one can use this method to optimally adjust input parameters of the process in multicriteria optimization mode, which indicates substitute application of the method for optimizing the laser percussion drilling process.

Investigation of laser percussion hole drilling by use of speckle correlation

In this paper speckle correlation is introduced as a tool to investigate the heat-influenced area during material processing with laser light. Two materials were investigated, a pure silver sheet and a sheet of SiC-diamond composite. The processing laser used in the experiments was a diode-pumped acousto-optical Q-switched Nd:YAG laser that allowed percussion hole drilling to be performed using green light through a second-harmonic crystal. The measurements were performed using a continuous-wave He-Ne laser and a digital camera. The experimental results show that the heat-influenced area is approximately 5000 times larger than the actual hole being drilled and that it reaches a steady-state condition toward the end of the processing cycle.

Statistical analysis of repeatability in laser percussion drilling

This study investigates the effect of six parameters in the repeatability of drilled holes in laser percussion drilling process by means of statistical techniques. Peak power, pulse width, pulse frequency, number of pulses, gas pressure and focal plane position were considered as independent process parameters. Experiments were designed with the aim of reducing the number of required experiments. The response surface method was used to develop the models for required responses. The significant factors in the process were selected based on the analysis of the variance (ANOVA). The experiments were conducted in mild steel sheet with a thickness of 2 mm. Each experiment was repeated 35 times in order to investigate the repeatability of the process. The equivalent entrance diameter, percentage of standard deviation of entrance diameter (%STD Eq Dia), circularity (ratio of minimum to maximum Feret’s diameter) and its standard deviation (STD circularity) were selected as process characteristics. The %STD Eq Dia and STD circularity, respectively, show the repeatability of equivalent diameter and circularity in the process.

A review of electrochemical macro- to micro-hole drilling processes

Electrochemical machining processes provide a viable alternative for drilling macro- and micro-holes with exceptionally smooth surface and reasonably acceptable taper in numerous industrial applications particularly in aerospace, electronic, computer and micro-mechanics industries. Advanced hole-drilling processes like jet-electrochemical drilling have found acceptance in producing large number of quality holes in difficult-to-machine materials. This paper highlights the recent developments, new trends and the effect of key factors influencing the quality of the holes produced by these processes. A comparative study of electro jet drilling with another non-traditional hole-drilling process (laser percussion drilling) has been presented which shows the potential and versatility of the electrochemical hole drilling processes.

Spatter prevention during the laser drilling of selected aerospace materials

The aerospace industry has been employing laser drilling to produce high quality closely spaced holes in turbine engine components for effusion cooling. However, laser drilled holes are inherently associated with the deposition of spatter as the ejected material resolidifies and adheres around the periphery of the holes. Work has been conducted to investigate the effectiveness of an anti-spatter composite coating (ASCC) for the drilling of closely spaced array (2 mm hole pitch) holes in three aerospace materials (IN 100, Nimonic PK 33 and 263 alloys) with an optically delivered 400 W Nd:YAG laser. The ASCC comprised of a mixture of ceramic filler particles embedded in a silicone elastomer matrix was applied on the surfaces of the alloy sheets before the process of laser percussion drilling. The work shows that the ASCC effectively prevents the deposition of spatter such that laser drilled through-holes are produced whilst maintaining the as-received surface characteristic for the various assist gases (O 2, air, N 2 and Ar) tested. It was observed that wetting contact angles for the ASCC and one of alloys (Nimonic 263) were 118° and 46°, respectively. Such a difference was identified to be primarily due to the low surface tension of the silicone elastomer matrix and as a result, the ejected molten material would not wet the surface of the ASCC. The spatter prevention mechanism is based on the provision of a physical constriction (i.e. ASCC hole) at the drilling site (i.e. around the to-be drilled alloy substrate hole). Due to this constriction and the sufficient thermal resistance of the ASCC, the free space on the surfaces of the alloy substrates is eliminated such that the spatter is continually ejected out of the drilling site by the high-pressure forces from the assist gas jet and the recoil pressure.

Comparative statistical analysis of hole taper and circularity in laser percussion drilling

The present study investigates the relationships and parameter interactions between six controllable variables on the hole taper and circularity in laser percussion drilling. Experiments have been conducted on stainless steel workpieces and a comparison was made between stainless steel and mild steel (IMechE Part B: J. Eng. Manufact. (in press)). Laser peak power, laser pulse width, pulse frequency, number of pulses, assist gas pressure and focal plane position were selected as independent process variables. The central composite design (CCD) was employed to plan the experiments in order to achieve required information with reduced number of experiments. The process performance was evaluated in terms of equivalent entrance diameter, hole taper and hole entrance circularity. The ratio of minimum to maximum Feret’s diameter was considered as circularity characteristic of the hole. The models of these three process characteristics were developed by linear multiple regression technique. The initial models were computed according to the least squares procedure and were finalised by stepwise regression method. The significant coefficients were obtained by performing analysis of variance (ANOVA) at 1, 5 and 7% levels of significance. The final models were checked by complete residual analysis and finally were experimentally verified. It was found that the pulse frequency had a significant effect on the hole entrance diameter and hole circularity in drilling stainless steel unlike the drilling of mild steel where the pulse frequency had no significant effect on the hole characteristics.

Geometrical features and metallurgical characteristics of Nd:YAG laser drilled holes in thick IN718 and Ti–6Al–4V sheets

Laser drilling is increasingly becoming the method of choice for precision drilling of a variety of components, particularly in the aircraft industry. Notwithstanding the current level of acceptance of laser drilling in the aerospace industry, a number of defects such as spatter, recast and taper are associated with laser drilled holes and elimination of these defects is the subject of intense research. The present paper deals with Nd:YAG laser drilling of 4 and 8 mm thick sections of IN718 and Ti–6Al–4V materials. The influence of type of material and its thickness, as well as parametric impact of key process variables like pulse frequency and pulse energy, have been determined. In the course of this study, relevant geometrical features of the drilled holes, like hole diameter and taper angle, have been comprehensively investigated. In addition, all metallurgical characteristics of interest, viz extent and nature of spatter, recast and heat-affected zone, have been evaluated. Effort has also been made to obtain some insights into the evolution of a through-thickness hole during laser percussion drilling of thick sections by careful experimentation involving monitoring the progression of the drilled hole with increasing number of laser pulses. Issues pertaining to variation of taper with depth of hole, change in crater depth with progressive drilling and specific energy consumption are also discussed.

Repeatability characteristics of laser percussion drilling of stainless-steel sheets

An analysis on the repeatability of a laser percussion drilling process is conducted using a flash lamp pumped Nd:YAG laser on 2 mm thick stainless-steel sheets. Laser drilling process is finding increasingly widespread application in the industry and has continually attracted new interests to the industry in recent years. However, the inherent problem of hole geometry repeatability associated with laser percussion drilling is likely to limit the extent of industrial applications of the process. The characteristic of melt ejection is found to be dependent on the parameter setting and is shown to have a significant influence on entrance hole geometry and hence repeatability. The relationship between the percentage standard deviation of entrance hole diameter and the operating parameters is established, and varies between 1.8% and 5.6% in the operating range under this study.

Statistical modelling of laser percussion drilling for hole taper and circularity control

The aim of the present work is to investigate the effects of different variables and their interactions in laser percussion drilling and to understand the possibility of controlling simultaneously the two important characteristics of a laser drilled hole: taper and circularity. Six variables, namely the peak power, pulse width, pulse frequency, number of pulses, assist gas pressure and focal plane position, were selected as independent controllable factors. The response surface method was used to statistically analyse the process. For each variable, five levels were considered and the experiments were performed based on a central composite design. A fibre-optic delivered neodymium-doped yttrium aluminium garnet laser was used to laser percussion drill mild steel sheets with a thickness of 2.5mm. The entrance diameter, the taper and the ratio of maximum to minimum Feret diameter for the hole entrance (circularity) were considered as responses/outputs in order to evaluate the process performance in terms of hole taper and circularity. Statistical modelling was carried out to develop mathematical models to relate the responses (outputs) to the six independent variables through multiple regression. A complete analysis of variance (ANOVA) was performed to test the significance of the obtained coefficients at 1, 5 and 7 per cent levels of significance. The developed models were verified by experiments. The work has shown that the pulse width and peak power have significant effects on the hole diameter, hole taper and hole circularity. However, the pulse frequency has no effect on these three hole characteristics. The number of pulses has no significant effect on hole diameter and circularity but has a significant effect on hole taper.

Comparison of intra- and interpulse modulation in laser percussion drilling

The effects of intra- and interpulse temporal modulation (shaping) during laser percussion drilling are investigated in terms of hole geometry characteristics and control. While intrapulse shaping involves temporally shaping within an individual laser pulse and repeatedly using the shaped pulse throughout the pulse train, interpulse shaping is carried out by temporal pulse shaping the overall delivery pattern of a pulse train required to percussion drill a hole. A fibre optic delivered neodymium-doped yttrium aluminium garnet (Nd:YAG) laser was employed to drill through-holes in stainless steel 304 plates with various combinations of intra- and interpulse shapes. The fundamental effects of intra- and interpulse shaping are also compared with percussion drilling using unshaped rectangular intrapulses and the normal delivery pattern interpulse shape. The work has shown that interpulse shaping is effective in reducing the hole taper and improving the overall hole wall parallelism during laser percussion drilling. This was chiefly attributed to the optimized laser pulse distribution as the beam propagates into the hole.

Models for Solidification and Splashing in Laser Percussion Drilling

This paper studies systems of partial differential equations modelling laser percussion drilling. The particular phenomenon considered in detail is the ejection of the thin layer of molten material. This thin layer is modelled as an inviscid flow between the fluid surface and fluid/solid interface, both of which are unknown moving boundaries. Through a regular asymptotic expansion, the governing equations are reduced to a combination of the shallow water equations in the zero gravity limit and a two-phase Stefan problem; the key small parameter is the square of the aspect ratio. These leading-order problems exhibit shocks which represent a possible mechanism for the previously unexplained fluid clumping. Approximate formulas and a parameter grouping are derived to predict the rate of melt solidification during ejection. Finally, weak formulations of the convection-diffusion equation for energy conservation are presented. These weak formulations are novel because the fluid is moving across a solid surface....

Hole Taper Characterisation and Control in Laser Percussion Drilling

Small hole (< 1 mm diameter) drilling by lasers is widely applied in various manufacturing processes. Hole tapering is one of the inherent manufacturing problems associated with laser percussion drilling (multiple pulse drilling) whereby material is ejected in the form of molten droplets when a series of laser pulses are delivered to a point on a workpiece. This paper reports an investigation into the mechanisms of hole taper formation, its characteristics and the development of a taper control technique. A statistical modelling technique is used to characterise the parameter relationships during hole-taper formation. Laser beam inter-pulse shaping is used to control the hole-taper. Parallel holes are produced as a result of this new development.

The effect of laser peak power and pulse width on the hole geometry repeatability in laser percussion drilling

In this work, the two main factors that influence the repeatability of the laser percussion drilling process are identified. Experimental parametric analysis was carried out to correlate the laser parameters with the repeatability of a laser percussion drilling process. The experiment was conducted using a flash lamp pumped Nd:YAG laser to drill 2 mm thick mild steel sheets. The relationship between the percentage standard deviation (PSD) of entrance hole diameter, hole circularity and the operating parameters is established. Thirty-five holes were drilled and analysed for each set of identical laser parameters. The PSD of entrance hole diameter ranges between 1.47% and 4.78% for an operating window of 3.5– 7 kW peak power, and 1– 3 ms pulse width. The circularity of the entrance hole (defined as the ratio between the minimum and maximum diameters of the hole) ranges from 0.94 to 0.87, and is found to correlate with repeatability. The work shows that higher peak power, and shorter pulse width gives better hole geometry repeatability. The effect of melt ejection on hole geometry repeatability is also investigated. Melt ejection and spatter formation have been found to contribute to the poor repeatability of the process.

Spatter removal characteristics and spatter prevention during laser percussion drilling of aerospace alloys

The aerospace industry has been employing laser drilling to produce high quantity closely spaced holes in turbine engine components for effusion cooling. However, laser drilled holes are inherently associated with the deposition of spatter as the ejected material resolidifies and adheres around the periphery of the holes. This article presents an investigation on the effectiveness of an antispatter composite coating (ASCC) for the drilling of closely spaced array (2 mm hole pitch) holes in three aerospace materials (IN 100, Nimonic PK 33, and 263 alloys) with an optical fiber delivered 400 W Nd:YAG laser. The ASCC, comprised of a mixture of ceramic filler particles embedded in a silicone elastomer matrix, was applied on the surfaces of the alloy substrates before laser percussion drilling. The work shows that the ASCC effectively prevents the spatter deposition such that laser drilled through holes are produced while maintaining the as-received surface characteristic when using either O2 or Ar assist gas. The spatter prevention mechanism and the characteristic of the ASCC have been investigated. Contact angle analysis revealed that the contact angles for the ASCC and one of alloys (Nimonic 263) were 118° and 46°, respectively. Such a difference was identified to be primarily due to the low surface tension of the silicone elastomer matrix. The thermal resistance of the ASCC was increased due to the embedding of the ceramic particles, which absorb a significant proportion of the heat generated by the ejected molten material and prevented the coating from being burned away.

The influence of temporal pulse train modulation during laser percussion drilling

The temporal pulse train modulation during laser percussion drilling was found to effect significant changes to the material ejection processes. In particular, distinct differences in the material ejection processes have been observed between a temporal pulse train shaping technique termed as sequential pulse delivery pattern control (SPDPC) and the normal delivery pattern (NDP), wherein the parameters of successive laser pulses were constant. Due to the reduced upward material removal fractions in SPDPC drilling, the spatter deposition area was reduced from approximately 6.7 to 2.7 mm 2 . In addition, the melt layer thicknesses at the hole bottom were significantly increased from 11–61 to 18– 369 μm . Such changes were identified as being due to the low laser pulse intensities before beam breakthrough associated with the SPDPC method. It was observed that the use of the linearly increasing SPDPC method increased the downward material removal fractions, from 20% to 28% observed in NDP drilling, to 34%–39%. Such an increase in the downward material ejection mechanism in SPDPC drilling was identified as being primarily due to the pointed blind-hole profile generated before the onset of beam breakthrough. The work has shown that modulating the entire pulse train in laser percussion drilling could control the material ejection processes. Furthermore, the fundamental elements of the SPDPC technique are given in terms of the rate of energy deposition and total pulse train energy.

Spatter-free laser percussion drilling of closely spaced array holes

Spatter is one of the inherent defects commonly associated with holes produced with laser drilling. This work reports on a method of spatter prevention, workable for a wide range of process parameters. The method is based on the application of a specially developed anti-spatter composite coating (ASCC), containing a mixture of ceramic filler particles embedded in a silicone elastomer matrix, on the workpiece surface prior to laser percussion drilling. Experiments were conducted using a fibre-optic delivered 400 W Nd:YAG laser for the drilling of closely spaced through holes (2 mm hole pitch) in Nimonic 263 alloy sheets. The work revealed that the ASCC effectively prevented the deposition of spatter such that laser drilled holes were produced whilst maintaining the as-received surface characteristics of the Nimonic 263 alloy for all the assist gases tested (O 2, air, N 2 and Ar). The process characteristics and spatter prevention mechanism associated with the use of the ASCC have been investigated using optical microscopy, scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis, contact angle analysis and high-speed photographic imaging. Comparative studies were also made with the case of uncoated laser drilling.

Optical fibre beam delivery of high-energy laser pulses: beam quality preservation and fibre end-preparation

This paper investigates the extension of optical fibre beam delivery to high-brightness applications, in particular laser percussion drilling, where both a good beam quality and high peak power are required. Beam quality preservation through a number of optical fibres is studied both experimentally and by using a ray propagation model. It is determined that in order to achieve the beam quality required for percussion drilling ( M 2<30) the largest fibre which can be used is 400 μm diameter. The laser-induced damage threshold is measured for a number of 400 μm fibres, and a CO 2 laser-annealing technique is shown to increase the damage threshold by a factor of 10, allowing 28 J, 1 ms pulses to be transmitted.

The effects of process parameters on spatter deposition in laser percussion drilling

This paper reports on the characterisation and analysis of spatter deposition during laser drilling in Nimonic 263 alloy for various laser processing parameters using a fibre-optic delivered 400 W Nd:YAG laser. The principal findings are a large proportion of the spatter (approx. > 70%) was deposited due to the initial laser pulses (before beam breakthrough) required to drill a through-hole. Short pulse widths, low peak powers and high pulse frequencies generated smaller spatter deposition areas. At high pulse frequencies, the spatter distribution/thickness can be altered as a result of laser-ejected material interaction. Focal plane positions between −0.5 and +1.5 mm produced relatively similar spatter areas of about 14 mm 2 . As a result of the reduction in the material removed per pulse, a longer focal length of 160 mm generated smaller areas of spatter deposition in comparison to a shorter focal length of 120 mm . In addition, a generic relationship between the spatter area and d entrance/ d exit with increasing total laser energy has been established.

The influence of assist gas on the mechanism of material ejection and removal during laser percussion drilling

This paper elucidates the effects of different gases on the dominant mechanisms of the material ejection and removal and the cooling rate during laser percussion drilling. In particular, work was conducted for the reactive O2 and inert Ar assist gases, in terms of their effects on (a) the behaviour and characteristics of spatter, (b) the recast layer characteristics and (c) the material ejection spark distribution for the drilling of Nimonic 263 alloy sheets using a Nd-doped yttrium aluminium garnet laser. It was shown that using O2 assist gas, the material ejection was characterized with increased liquid particle ejection, coupled with volatile burning, possibly similar to that of nucleation-induced explosions, while the employment of Ar assist gas influenced the material ejection to be dominated by bulk radial liquid ejection.

Effects of assist gas on the physical characteristics of spatter during laser percussion drilling of NIMONIC 263 alloy

The effects of different assist gases, O 2, Ar, N 2 and compressed air, during laser percussion drilling are investigated with regard to the physical features, i.e., thickness and surface geometry, of the resulting spatter and its bonding strength. Laser drilling was conducted on NIMONIC 263 alloy sheets, 2.6 mm thick, using a fibre-optic delivered 400-W Nd:YAG laser. The work has revealed that the spatter generated with O 2 assist gas is distinctly different from those produced with N 2, Ar and compressed air. The influence of the assist gas type on the mechanism of material ejection/removal is reported. The spatter bonding strength has been found to be associated with the ‘inertness’ of the assist gas employed, in which a progressive increase was observed in the order of O 2, compressed air, and N 2 and Ar. It appears that the spatter bonding strength is dependent on the assist gas type and the workpiece material composition. In addition, it was found that the overlapping of spatter between adjacent holes, in closely spaced array holes, possessed higher bonding strengths. Consequently, the removal of overlapped spatter produced with inert assist gases is difficult to achieve without causing undesired modification to the material surface and hole geometry.