Hole Taper Characterisation and Control in Laser Percussion Drilling

Hole Taper Characterisation and Control in Laser Percussion Drilling L.Li, D.K.Y.Low and M.Ghoreshi Laser Processing Research Centre, UMIST, Manchester, UK Submitted by J.R.Crookall ( I ) , Cranfield University, UK

Abstract 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 interpulse shaping is used to control the hole-taper. Parallel holes are produced as a result of this new develop ment . Keywords: Laser, drilling, quality

1.

INTRODUCTION

In manufacturing industries, such as aerospace industry, small hole (0.1-0.8 mm diameter) drilling is carried out routinely using high power solid state lasers. Applications of laser drilling include manufacturing small holes in gas turbine components for effusion cooling [I-41, in aircraft wings to improve aerodynamic performance [5-61, in motor engine camshafts [7] and in surgical needles [7]. One of the important issues in laser hole drilling is the hole-taper control. In multiple pulse laser drilling (percussion drilling), the entry hole diameter is normally larger than the exit hole diameter and thus the holes drilled are generally positively tapered. In certain engineering applications, parallel holes (zero taper) and negatively tapered (entrance hole diameter is smaller than the exit hole diameter) are needed. Laser trepanning, i.e. laser hole cutting using multiple axis CNC/Robotic facilities is currently used for taper control. The process is, however, much slower than laser percussion drilling. If the hole-taper can be controlled in laser percussion drilling, much higher production rate can be achieved compared to laser trepanning. To date, very little has been reported on the understanding and control of hole taper in laser percussion (mu Itip le puIse) d riIIing . Ex perimental investigations on taper characteristics have been carried out by Yilbas et al [8-91 to investigate the effects of single pulse laser drilling parameters on the hole geometry and hole quality including recast, taper, barrelling, inlet cone, exit cone, surface debris and mean hole diameter. Factorial design and statistical modelling were used in the investigations. Hamoudi and Rasheed found that, materials with higher thermal diffusivities tend to produce holes with a larger taper [lo]. Drilling parameter optimisations have been used to control the hole-taper [lo-121. 1520% hole-taper (the percentage difference between the exit and entrance diameters) have been achieved.

The aim of the present work is to further understand multiple pulse laser drilling hole-taper formation characteristics through statistical modelling and experimental study and to develop a viable technique to control the hole taper. 2.

HOLE TAPER CHARCTERISTION

2.1. Taper formation mechanisms

It is known that, melt ejection is the dominating mechanism for material removal in laser percussion drilling with 0.1-10 ms pulse width range [13]. As illustrated in Figure 1, the hole wall close to the hole entrance (point A) is repeatedly eroded by multiple groups of hot flowing melt as the hole deepens with successive laser pulses, whereas point C would be eroded to a less extent. Therefore, more erosion takes place close to the entrance end of the hole. This is also verified by experiments as shown in Figure 2, where erosion at the entrance end is seen to be more severe. It is possible to say that melt erosion plays an important role in hole taper formation. Pulsed Laser Beam

a

:.

Melt interaction time: tA

i

I .........! i.i:..

_&

> ts > tc

I

c

i i

-2:-::j

+

i i i

_____

1" pulse

Tdpulse

I

i i i

3rd pulse

Figure 1: Illustration of melt erosion effect.

Figure 2: Typical hole-taper characteristic during laser percussion drilling. (a) Cross section of a hole. (b) A close-up view of the entrance wall cross section showing extensive erosion. On the other hand, as a diverging laser beam (after focal point) propagates through the hole, wall absorption will take place. This will reduce the laser power as the hole depth increases. Based on a simplified geometric configuration of the laser intensity variation with hole depth, as shown in Figure 3, the laser propagation along the hole depth can be modelled as:

Figure 4: Variation of laser power intensity over the depth of hole. Based on this model, the laser power variation over 2.6 mm drilling depth on Nimonic 263 alloy can be estimated as shown in Figure 4. It is apparent that the laser power density reduces as the beam propagates through the hole wall. This would be another important factor influencing the hole taper formation.

2.2. Statistical Modelling

where labs is the laser power density at the bottom of the hole for each laser pulse, lo is the incident laser intensity and R,,, is the reflectivity of the material which was measured to be 35.3% at 1.06 pn Nd:YAG wavelength for Nimonic 263 material. The first term on the right hand side of equation (1) is the laser intensity within D(O), which is the laser beam diameter at hole depth z =0, with respect to the actual beam diameter D(z) at depth z, the second term represents the intensity reflected from the hole walls at the reflection point (i.e. z=0.5hd) and the third term takes into account of the reflection of the beam at the hole bottom. This model is based on the following assumptions: a) the blind-hole is cylindrical in shape; b) the central core of the laser beam within the minimum laser beam spot size, d,, is absorbed at the hole bottom whilst the laser energy outside d, is partly absorbed by the hole walls at z=0.5hd and reflected once to the hole bottom. (note: when the laser is focused on the material surface, d,= D(0)). Incident laser beam

Figure 3: Geometric configuration of laser intensity variation analysis.

In order to understand the sensitivities of hole taper to various processing parameters, a statistical model was established by means of multiple linear regression analysis. Six independent input parameters (laser power, pulse width, pulse frequency, focal position, number of pulses and assist gas flow rate) were considered. The model assumes a quadratic relationship (including interaction terms) between the input parameters and hole taper that is the average angle of hole tapering over the depth of the hole and can thus be calculated from:

where T, is hole taper in degrees, dentis the hole entrance diameter, dex,t is the hole exit diameter and f h is the material thickness. The second order model is presented as:

Ta

=Po

R

R

j=1

j=1

+CPjXj+cPjjx:

+ccPvxlxj R

R

(3)

1=1 j=1

where k is the number of independent laser processing variables, x, and xj. In the present case, k = 6. The coefficient, PO. represents the response at the centre of the experiment when all of the variables are zero. p,, p,, ,and p,, represent the linear, quadratic and interaction effects of the variables respectively. The significant terms in the model were found by analysis of variance (ANOVA) at 1% and 7% [14]. Least square method was used to estimate the regression coefficients in a multiple regression model [15]. The accuracy of the model was examined by residual analysis including checking the normal probability line of residuals, outliers, Cook's distance and leverage [15]. Power transformation of the model was chosen according to the Box-Cox plots [ I 51 in order to obtain minimum residual sum of squares in the transformed model. The model was then verified by experiments. Statistical experimental design was employed to develop the model. Central Composite Design (CCD) is one of the most effective second order designs which can handle linear, quadratic and interaction terms in process modelling [14]. By taking account of the experimental limitations and characteristics of CCD, a % fractional CCD

with six factors, each one at five levels, was chosen for this work. For six factorial fractional CCD, 52 experimental points are required and the interval of levels for each factor should be -2.37, -1, 0, + I and +2.37 in coded form in order to made the design rotatable [14]. Coding of the factors allows the calculations to be perfumed independent of the units for each factor. Processing parameter sensitivity to taper control was obtained through this study for an Nd:YAG laser drilling process as shown in Figure 5. From this result it can be seen that taper is most sensitive to focal position variations (in a quadratic relationship) as the focal position controls the beam spot sizes at various locations in the hole. This is followed by a linear relationship with peak power, pulse width and number of pulses which together control the energy input to the material. Assist gas and pulse frequency do not have significant influences on the taper formation. In addition, the work shows by varying process parameters alone may not be sufficient to achieve zero taper.

5.0

4.3

i: 1

Laser Peak Power

Figure 6: Schematic diagram of the inter-pulse shaping using a linearly increasing scheme.

Middle Peakpoint PoweflkW] ( 0 ): = 5

~

~

B: Pulse Width[rns] = 1.2 C: Pulse Freq.[Hz] = 30 D: No of pulse = 12 E: Assist Gas Pressure[bar] = 4 F: Focal Plane Position[rnrn] = 0

h

(u

where En and € 1 are the final and initial pulse energies of the pulse train, n is the number of pulses used in the pulse train and f i s the laser pulse frequency. Subsequently, the total laser energy, ET,delivered by the entire pulse train can be expressed as:

?!?

D

5

3.5

-

ba

2

2.8

2.0

~

~

7 -2.378

-1.189

0.000

1.189

2.378

Factor Range in Coded Values

Figure 5: Statistical processing parameter relationship with hole taper in laser drilling. 3.

HOLE TAPER CONTROL

In laser percussion drilling, a series of identical laser pulses are delivered to the drilling point until the hole is drilled through. Despite efforts to modify the melt ejection process through ultrasonic vibration, zero hole -taper cannot be achieved [16-171. Because of the non-uniform wall erosion by the ejected molten material and laser beam intensity variation over the depth of the hole, taper formation is unavoidable using identical pulse drilling. To compensate for these effects, an inter-pulse laser beam shaping technique was developed. A feasible method found was to linearly increase the laser pulse energies throughout the pulse train (by increasing the peak power) as shown in Figure 6. With reference to the linearly increasing peak power pulses in Figure 6, where Ppnand Ppl are the final and initial peak powers of the pulse train, the rate of energy deposition can be described by:

The rate of power density increase can be determined according to equation (5) in order to compensate for the power losses. To verify the control strategy, experiments were carried out using both linearly increasing and linearly decreasing inter-pulse pulse shaping. The laser used to perform the drilling experiments was an Electrox Scorpion fibre-optic delivered Nd:YAG laser. The laser beam was focussed with a fused silica focussing lens with a focal length of 120 mm. The focal plane of the laser beam was placed on the material surface, giving a spot size of approximately 0.52 mm. A coaxial 0 2 assist gas (40 I/min) was employed through a 44 mm long conical nozzle. The nozzle-workpiece distance was kept constant at 2.2 mm. The material investigated was Nimonic 263 alloy (2.65 mm thick). A typical result is shown in Figure 7 where holes with zero and negative tapers (Figure 7 a,b) were drilled with linearly increasing pulses and the hole with positive taper (Figure 7c) was drilled with linearly decreasing puIses. The work has shown that with variable pulse energies for percussion drilling, not only the laser power at different depths can be controlled but also the amount of melt ejection from the hole entrance and hole exit can be controlled. With the initial lower power input in the pulse train a smaller hole was penetrated followed by the hole enlargement and melt flushing through the hole exit at the higher powers towards the end of the pulse train. A measurement of melt ejection showed 32-38% melt ejection from the exit end of the hole for the linearly increased pulse training drilling as compared to 204'5% melt ejection from the exit end for the holes drilled with constant pulses under the same total deposited energy. Drilling holes using constant pulses resulted in hole tapers of 11-18% (or 1.4-2.2 degrees) for the materials tested, whilst zero taper holes were produced using linearly increasing pulse trains with the same total energy input.

Figure 7: Various tapered holes drilled using variable pulse percussion laser drilling technique. (a) "zero" taper, (b) negative taper, (c) positive taper.

4.

CONCLUSION

Non-uniform melt ejection erosion to the hole walls and laser power reduction as the beam propagates through the hole have been identified to contribute to the hole taper formation. Through statistical modelling, hole taper has been found to be most sensitive to focal plane position variations followed by pulse width, peak power and number of pulses. A new approach of drilling with variable pulses (inter-pulse shaping) rather than identical pulses has been demonstrated. The new process has been shown to be able to produce zero-tapered holes as well as positively and negatively tapered holes. 5.

AKNOWLEDGEMENT

The authors acknowledge the financial support of some of the work by Rolls-Royce PIC. The authors would like to thank Professors J.McGeough and S.Hinduja for their invitation and encouragement for the submission of this paper. The authors also appreciate useful suggestions by Professor J.R.Crookall. REFERENCES Bostanjoglo, G., Sarady, I., Beck, Th., and Webber, H., 1996, Processing of Ni-based Aero Engine components with Repetitive Q-switched Nd:YAGlasers, Proc of the SPIE, 2789: 145-157 VanderWert, T.-L., Litzer, S.-A,, Loh, W.-M., Laser Drilling Effusion Cooling Holes in Low Nox Turbine Engine Components, 1996, Proc. of ASME Turbo Asia Conf. Jakarta, Indonesia, 1-8. Corfe, A,-G., 1983, Laser Drilling of Aeroengine Components, Proc. of the 1" International Conf. On Lasers in Manufacturing, Brighton, 31-40 Toller, D.-F., Laser Drilling of Aeroengine Components, in Laser Welding, Cutting and Surface Treatment, The Welding Institute, 48-52 Beck, Th., Bostanjoglo, G., Kugler, N., Richter K., Weber, H., 1997, Laser Beam Drilling Applications in Novel Materials for the Aircraft Industry, Proc. of ICALE0'97, An Diego, CA, 83E: 93-102 Giering, A,, Beck, M., Bahnmuller, J., 1999, Laser Drilling Aerospace and Automotive Components, Proc. of ICALE0'99, San Diego, CA, 87C:80-87

Murphy, S., 1987, Laser Drilling: Capabilities and Trends, Laser & Applications, 6:59-62 Yilbas, B.-S., Yilbas, Z., 1987, Parameters Affecting Hole Geometry in Laser Drilling of Nimonic 75, Proc. SPIE, 744 87-91. Yilbas, B.-S., 1987, Study of Affecting Parameters in Laser Hole Drilling of Sheet Metals, Trans. ASME: J. Eng. Mat. & Tech., 109: 282-287. Hamoudi, W.-K., Rasheed, B.-G., 1995, Parameters Affecting Nd:YAG Laser Drilling of Metals, International Journal for the Joining of Materials, 7163-69 Govorkov, S.-V., Slobodtchikov, E.-V.,Wiessner, A,.O., 2000, Effect of the Wavelength on High-Aspect Ratio Microdrilling of Steel with and All-solid-state Laser, Conference on Lasers and electro-Optics (CLE0'2000), San Francisco, CA, USA: 580-581 Murray, A,-J., Tyrer, J.-R., 1998, Low Taper Laser Drilling of 8.26 mm Thick ZrOn, Lasers in Engineering, 6(4):273-289 Low, D.K.-Y., 2001, Spatter and Taper Control in Laser Percussion Drilling, PhD Thesis, University of Manchester Institute of Science and Technology (UMIST), Manchester, UK. Montgomery, D.-C., 1997, Design and Analysis of Experiments, 4'h Ed., John Wiley & Sons, New York. Myers, R.-H. and Montgomery, D.-C., 1995, Response Surface Methodology, John Wiley & Sons, New York. Lau, W . 3 , Yue, T.-M., Wang, M., 1994, Ultrasonicaided Laser Drilling of Aluminium-Based Metal Matrix Composites, Annals CIRP, 43(1):177-180 Yue, T.-M., Chan, T.-M., 1996, Analysis of Ultrasonic-aided Laser Drilling Using Finite Element Method, Annals CIRP, 45(1):169-172