International Journal of Machine Tools and Manufacture
Spatter-free laser percussion drilling of closely spaced array holes
Introduction
In recent years, laser drilling has emerged as the prominent choice for the production of a large quantity of holes. The aerospace industry has been employing this hole-making technique to produce millions of small effusion cooling holes in turbine engine components such as combustion chambers and nozzle guide vanes [1], [2]. However, laser drilled holes are inherently associated with spatter deposition due to the incomplete expulsion of ejected material from the drilling site, which subsequently resolidifies and adheres on the material surface around the hole periphery. Indeed, spatter deposition is undesirable since laser drilled holes are generally employed in fluid flow applications. For example, the flow characteristic and efficiency of the cooling air over the effusion cooling holes in turbine engines are crucially dependent on the geometry and surface quality of the holes. Moreover, the removal of spatter through additional finishing processes (e.g. abrasive blasting) can consequently create additional production time and costs. Furthermore, two undesirable consequences could result during the drilling of closely spaced array holes. Firstly, pre-existing spatter at the drilling site may cause non-uniform absorption of the incident laser beam when drilling subsequent adjacent holes and as a result this could reduce the consistency/repeatability of the process. Secondly, Low et al. [3] found that the laser drilling of array holes could lead to an increase in the spatter bonding strength due to the overlapping of spatter between adjacent holes. When using inert assist gases such as N2 and Ar, complete removal of the overlapped spatter may not be possible without causing undesired workpiece surface modification [3]. Although the reduction in spatter deposition area could be achieved with the proper selection of process parameters [4], complete spatter prevention without the incorporation of specific techniques is unlikely to be feasible.
A number of techniques based on either chemical and physical anti-spatter mechanisms have been developed to either reduce or prevent spatter. Chemical techniques such as the spraying of selected solutions have been explored. Hamilton and James [5] produced a clean hole in bulk copper by spraying CCl4 into the interaction zone. However, the technique was reported not to be applicable to all metals. Orita [6] reported that the spraying of solutions of carbonates, such as Na2CO3 or NaHCO3, against the cut surface during laser cutting of high chromium steel could cause the carbonates to react chemically with materials such as highly oxidised chromium, and as a result prevent the formation of highly viscous oxidised chromium. Conversely, some investigators approached the problem using physical based techniques. Using ultrasonic vibration, Mori and Kumehara [7], [8] increased the fluidity of the spatter such that the resulting hole geometry was also inevitably affected by the amplitude of vibration. Williams [9] drilled two layers of metal in tandem so that the top layer acted as a mask for the underlying one. However, such a method may not be viable for components with surface curvatures. Otstat et al. [10] masked the material surface with paraffin wax or silicon grease such that the material expelled from the workpiece was cast onto the sacrificial coating. Sharp et al. [11] and Kamalu et al. [12] employed surfactant fluids to prevent the wetting of molten material onto the material surface. Nevertheless, surfactant fluids have the disadvantage of being blown away from the surface when an assist gas is employed. They also restrict the workpiece from being placed in an inclined manner during the drilling process as the fluid may simply flow downwards. In addition, surfactant fluids have the potential of being evaporated by the laser and hence reducing the effectiveness of spatter prevention [11].
In this present work a spatter prevention technique based on the application of a specially developed anti-spatter composite coating (ASCC) on the substrate material surface, prior to laser drilling, was investigated using a fibre-optic delivered 400 W Nd:YAG laser. The substrate material employed in the experiments was Nimonic 263 alloy sheet (2.65 mm thick); a material commonly used in gas turbine rings and casings [13]. Generally, there are three different approaches to laser drilling, namely, single pulse, percussion and trepanning drilling [14]. Laser percussion drilling has been investigated in the present study, where a series of laser pulses with specified properties are directed onto the same spot to form a through hole. Two main process characteristics associated when using the ASCC were examined. Firstly, the effectiveness of the ASCC in preventing spatter during closely spaced array drilling at 2 mm hole pitch was studied with the use of four different coaxial assist gases (i.e. O2, air, N2 and Ar). The extent of spatter prevention was compared with an array of holes obtained with drilling uncoated Nimonic 263 alloy. Secondly, comparative studies were made between the surface characteristics of the uncoated Nimonic 263 material and the ASCC after laser drilling, using SEM and EDX techniques. In addition, work has been conducted to determine the characteristics of spatter adhesion (wetting) and the material ejection spark during laser drilling, with and without the application of the ASCC, with particular emphasis on understanding the fundamental behaviour and spatter prevention mechanism governing the workability of the ASCC.
Section snippets
Materials and laser process parameters
A composite can be defined as a homogenous material created by the synthetic assembly of two or more materials (a selected filler or reinforcing elements and compatible matrix binder) to obtain specific characteristics and properties [15]. In this study, the specially developed ASCC is comprised of ceramic filler particles (90 μm) embedded in a black-coloured silicone elastomer matrix. The mixed ASCC was coated onto the substrate surface (Nimonic 263 alloy) using a knife-edge to a coating
Effectiveness of ASCC during laser drilling with O2, air, N2 and Ar assist gases
Fig. 2(a)–(d) shows the array holes drilled at 2 mm hole pitch in uncoated Nimonic 263 alloy using O2, air, N2 and Ar assist gases respectively. As can be seen, the peripheries of the laser drilled holes were inherently encircled with spatter deposited onto the material surface. The average ferets' diameters [17] (distance between pairs of parallel tangents to the projected outline of the particle) of a single spatter was typically between 3.0 and 4.6 mm. Therefore, array drilling at 2 mm hole
Surface characteristics of uncoated and ASCC coated Nimonic 263 workpieces after laser drilling
The surface characteristics of laser drilled uncoated and ASCC coated Nimonic 263 workpieces were investigated. In particular, the composition of materials deposited on the top surfaces of the workpieces when using O2 and Ar assist gases were determined. The laser drilling of uncoated Nimonic 263 alloy is inherently associated with the deposition of spatter as shown in Fig. 6(a) and (b). However, the physical characteristics and chemical composition of the resulting spatter are largely
Spatter adhesion (wetting) characteristics of Nimonic 263 alloy substrate and ASCC
The formation of spatter occurs when the ejected material resolidifies around the periphery of the hole on the top surface of the workpiece. Whether or not the ejected material will adhere to the top workpiece surface is largely dependent on the wetting characteristics of the top workpiece material with the ejected material. Hence, contact angle analysis was carried out to examine the wetting characteristics of the Nimonic 263 substrate and ASCC surfaces with molten droplets of the Nimonic 263
Uncoated drilling
During the first laser pulse [Fig. 11(a)–(c)], the drilling region was clouded with an intense, volatile and explosive ejection spark. Such violent material ejection was due to the sudden thermal loading on the uncoated metallic Nimonic 263 alloy. When undergoing the second laser pulse [Fig. 11(d)–(f)], material was ejected mainly radially away from the drilling site, across the workpiece surface. The third laser pulse again produced some radial expulsion of material.
ASCC coated drilling
By contrast, the first
Spatter prevention mechanism
In most cases, laser drilling of metallic materials is dominated by melting followed by liquid ejection when the power density is of the order of 105−107 W/cm2 [35], [36]. Since the power density employed in the experiments was 3.56×106 W/cm2, and from previously reported work by the authors and other investigators [3], [18], [35], [36], [37], [38], it was known that the material removal mechanism in this study was dominated by liquid ejection. Therefore, the deposition of spatter was mainly
Conclusions
The use of the specially developed anti-spatter composite coating (ASCC) proved to be highly effective in preventing spatter during laser percussion drilling of the Nimonic 263 substrate alloy for all the assist gases tested (O2, air, N2 and Ar). The prevention of spatter formation by the ASCC eradicated many of the problems experienced during closely spaced array laser drilling, namely:
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The change in surface absorption by the pre-existing spatter during the drilling of subsequent adjacent holes.
Acknowledgements
The authors would like to express their gratitude to Rolls-Royce plc for their financial support towards this research. DKYL acknowledges the receipts of a UMIST Graduate School Scholarship, and an Overseas Research Scholarship awarded by the Committee of Vice-Chancellors and Principals of the Universities of the United Kingdom (CVCP). Many thanks also go to Dr Jonathan Lawrence and Mr Ian Brough of UMIST for sharing their time and expertise. The authors are also grateful for the loan of a
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