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Parametric study to improve laser hole drilling process
ELSEVIER
Journal of Materials Processing Technology 70 (1997) 264273
Parametric study to improve laser hole drilling process B.S. Yilbas
’
Mechanical EngineeringDeparrment, King Fahd University of Perro/eumand Minerah, Dl~alwan3126l. Suudi Arabia
Received 21 January 1996
Abstracl
In the laser driYling process, the quality of the drilled holes is the main task. A method of studying the influence on the quality of the main process variables needs to be developed, which seeks to improve the quality and explains the drilling mechanism. In the presentstudy, the effectof the laser parametersand the material properties on the hole quality when drilling is examined. A statistical approach, referred to as factorial design, is employed to test the significance level of the factors that affect the hole quality. Three materials, stainless steel, nickel and titanium, are considered. The experimental study yields tables of significance of each factor on the aspects that determine the quality of the holes. The hole geometry is evaluated by assigning marks for each geometric feature, the tnarking scheme being conducted relevant to the importance of the hole feature. 0 1997 Else&r Sciena S.A. Key~vords:Laser drilling; Stainless steel; Nickel; Titanium; Streak photography
1. Introduction
from a vapor-solid heating model to considerations that attempt to throw light on liquid ejection. Consequently, mechanisms describing laser machining require a knowledge of the interaction mechanisms as well as the physical properties of the substance being drilled, such as thermal diffusivity, surface reflectivity, absorption coefficient, and etc. These properties are often unknown
at the elevated
temperatures
resulting
from the
laser beam heating. Therefore, a model describing the drilling process has not yet been fully established. Laser drilling is a common cotntn~rcially-developed process for metal working. There have been several research investigations conducted in the past on reducing the drilling time, the input energy, and the intensity of the laser irradiation [6-S]. Laser drilling into electronic components was studied by Taneko et al. [9], who showed that the use of a high peak pulsed CO,
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laser improved the drilling quality. Laser drilling of microvias in epoxy-glass printed circuit boards was investigated by Kestenbaum et al. [lo]. Based on their findings, holes of 0. I mm diameter could be drilled with a drilling rate of several hundred per second. Laser drilling of very fine electronic via holes in common circuit board materials was studied by Kenney and
265
Daily [ii]. They concluded that the energy delivered was the most critical parameter, but also suggesred that further improven,ent in the resulting hole quality was necessary. From the application viewpoint, it is the end product
from the laser system,
for instance
a drilled
hole, that matters. To apply the laser beam to micromachining, various factors must be considered and evaluated. These factors include the laser pulse length, the pulse energy, the focus settings of the focusing lens, and the workpiece thickness and thermal properties. Effective utilization of the laser depends very much upon the proper selection and optimization of these factors. Limited information on how these factors affect the laser-drilled hole quality have seriously hindered the growth of laser applications in micromachining, because the effects of some of these factors have been underestimated previously [l2]. Therefore, a systematic study to establish and recognize the machining standards to produce holes wltb good quality is necessary. The initial step in this regard is a stdtistical study of the parametric effects on the drilling pro-
cess. Two different techniques, namely multiple correlation and the factorial experiment, have the capability of achieving this aim. Factorial design is appropriate in this aspect as its assumptions are general and make no restriction as to linearity [i3]. Traditionally, experimentation in laser hole drilling has been performed using a one-variable-at-a-time approach, in this case all but one variable being held constant as the effect or relationship between that one variable and the response is determined by running a number of tests over its operability region. However, the one-variable-at-a-time approach is inefficient, can lead to misleading results, and is limited, since interactions (inter-dependencies) amongst variables are difficult to determine and are often overlooked [i3]. In the present study, a design of factorial experiment including four factors at four levels is considered. This design covers all the possible combinations of the factors over the operating regions. The response
for the factorial design is obtained by the evaluation of the features of the resulting hole geometry. Prior to the present experiment, a statistical study of the factors that are most significant to the drilling speed has been examined [14]. The results indicated that all the main effects of factors considered were highly signiticant in the decreasing order of Ls, material properties, focus settings, workpiece thickness, energy and
pulse lengths. Cocsequentiy, the results of previousiyconducted drilling-speed experiment are complementary to the present experiment in that the parameters for the latter are determined from them. Therefore, three workpiece materials and four factors at four levels are selected to conduct the factorial design experiment.
2. Experimental An experiment is designed to give insight into how and why variation in laser parameters and materials affecting the resultant hole quality when drilling. This may be achieved by an evaluation of holes so drilled by a subsequent statistical approach, referred to as hctoriai design. Factorial analysis is performed for each material selected as forming a block, thus having quantitative variables ia each block of experiments. Material effects could bc recognized as the effects between the blocks. The statistical analysis of the holes deals with four-way analysis of variance. 2.1.
Mathetnuticul
model
The mathematical model for the present experiment may be presented as given in Eq. (1): X,:,,,= /1+ P, + F, + J%+ T, + (PF), + (PE),, + VT),, + (PF),, + G=‘O,,+(ET),, f (PFE),,k+ U’FV,,, + (PET),,, + (FET),,, +
c,,tct
(1)
where ,I&, is the value given to the hole feature as a measure variable; p is a common effect in ail observations; P. F, E, T are the main effects of parameters of pulse length, focus setting, energy and workpiece thickness respectively; PF,..., ET are the first-order interactions of parameters; PEF,... FET are the secondorder interactions of the main effects; Q, is the random error in the experiment; and i, j, k, I are the lcl/eis of parameters P, F, E and T respectively. As there is only one treatment per combination, the third-or&r interaction is within experimental and random error, thereiore it is relegated to the error term by making the assumption that the third-order interaction does not exist and the hole is repeatable: even if it were present, it would be difficult to explain in practical terms. The mathematical model of this experiment is a fixed model each factor having four fixed levels. The calculation of the sum of squares of main effects, first-order and second-order interactions, and the error are not given here due to their lengthy arguments, but refer to [151. The significance of each effect and interaction is tested by making the initial hypothesis that the mean squares of the effects or interactions is from the same population as the error mean squares. As the mathematicai n~tidc! pms~ted hy Eq. (I) is a fixed and not random model, therefore the expected mean squares for the main effects and interactions are all estimated by the error mean variance. Hence, the mean squares for the main effects and interactions can be compared to the error mean square to test the respective hypothesis by means of an F test. The definition of variance :atio is given by:
266
B.S. Yilbas/Journal of Materials Promming Technology70 (1997) 264-273
F = Mean Sum of Squares p”q Error Mean Sum of Squares
(2)
If the calculated value of the variance ratios of a given measured variable exceeds the tabulated value for the number of degrees for the item and the error at a particular probability point, the item is significant at that level. It should be noted here that the appropriate level of significance will depend on the particular problem under consideratton. In this regard, 10, 5 and 1% are adopted as giving results that are significant, very significant and most significant, respectively.
Table 2 Coefficients
A
B
Linear Quadratic
-3
-I -I
Cubic
-I
-3A-B+C+3D
(3)
the quadratic effect by:
-A+3D-3C+D
(5)
Therefore, the sum of squares is: Quantitative
F: = (no. of observations in each level of factor F) x 20 (6)
20
= (No.
of observations
Quantitative
F2 in eich level of factorF) x 4
F, F2
=-
of observations in eaih level of factorF) x 20 (7) The variance ratios are: QuantitativeF Variance,, = Sum of mean squares of error The no. of dearees of freedom of the linear, auadratic and cubic effect is one. By comparing the values of variance ratios with the values in the F-table, the level of significance may be obtained. A computer program has been developed to calculate the variance ratios of the main effects, and the first- and second-order inter(no.
actions of the parameters.
An Nd YAG laser delivering output energies of 5-21 J within 1.2-2.7 ms pulses was employed, a 51 mm nominal focal length lens being used to focus the laser beam onto the workoiece surface. Three workoiece materials were selected due to their range of thermal these being
titanium,
nickel -and
stainless
experimental conditions and the parameters and their levels are given in Table 2. The focai positioi and laser output energy were set to the lowest levels. The focal oosition was then varied from the lowest to the highest level, requiring four holes to be drilled for each material at one thickness level. Sbsequently, drilling progressed and 1024 holes were drilled. After the drilling, the damage diameter of each hole was measured under a microscope. The holes were
Table I Parametersand their levels
Thickness (mm)
20 4
I
Quantitative
properties, steel. The
FL
Energy(J)
-I
-3
(4)
and the cubic effect by:
Pulse length (ms) Focus setting (mm)
3
E
3 I
2.3. Experimental apparatus
F,=A-B+C+D
Parameters
D I
F,
If the levels of a quantitative factor considered are equi-spaced it is possible to fit a polynomial equation of up to order n - 1 through n points, where n is the number of levels of the factor. This permits the obtaining of more infomration on how the response may vary with the changing levels of the quantitative factor than just the analysis of variance. For four factors at four levels, linear, quadratic and cubic effects can be extracted by the proper choice of coefficients for the response totals, the coefficients for of the response total being given in Table 1. As an example, if the measured response totals for the factor F (focus setting) at levels I, 2, 3 and 4 are A. B, C and 0, then the total for the linear effect is given by:
F,=
I
C
and:
2.2. Quantitative anulysis
FL=
in the quantitative analysis
Coefficients
Lexls I
2
3
4
1.5 50.5 I5 0.5
I.8 51 I7 0.75
2.0 51.5 19 I
2.5 52 21 1.25
cut as close as possible to the edge of the holes, leaving the holes unaffected, mounted-in perspex and then ground down to a diameter. Finally,.in order to eliminate scratches, caused by grinding, the samples were polished with diamond paste. These polished samples were examined under the microscope and scores were assigned
to the hole features.
3. Hole evaluation The score assigned was undertaken considering the importance of the hole feature as an overall quality [IS]: the hole feature is shown in Fig. 1.The procedure in evaluating the hole feature is given as follows. Radidifid ntuteriul: This was the measure of the amount of material that had vaporized or melted during drilling, but had not escaped from the hole and so
had resolidified on the internal surface. Marks were given out of 10 according to the fraction of resolidified material to the hole size. Thus a mark of 10 meant that there was no resolidified material and a mark of one meant that the hole was absolutelv full. Taoer: This was a measure of the overall taper of the hole sides, but did not include inlet and exit cones. The actual taper was measured using a microscope and a mark was assigned out of 10 for the ratio of the taper angle to 90’: a mark of 10 indicates a parallel-sided hole and one of 10 a taper of 90”. Burrellin~: The amount of barrellina in the sides of a
hole was r&en a mark between 0 and-4. i.e. 4 indicated a straightrsided hole. Inlet cone: The inlet cone was measured using a microscope. A mark out of 10 was given for the ratio of the cone angle to 180”: a mark of 10 indicates no inlet Exif cone: The exit cone is small relative to the size of
the hole, and it was impractical to measure it. A mark of 0 to 3 was given by inspection, i.e. 3 indicated no exit Surfuce D&is: This was an assessment of the amount of resolidified material appearing on the surface of the hole. A mark of either 0 or I was given. A mark of I meant no surface debris.
MCNIIHo/c Dimeter: This was measured using a microscope and the value assigned was the mean hole diameter in mm. This feature has no bearing on quality, but is an important factor in hole drilling and so is included in the analysis. Ocwull qrrdiry: This is the
sum of hole features. As some feature are considered to be’more important than others, weights are assigned to a particular feature. Resolidified material and taper are considered to have equal importance, but have twice that of barrclling and inlet cone. Therefore, the overall quality is: Qq=2R,+2T.,+B.,+E,+S,+I, where Qq, R,. T,, E,, EC, S,. i, are the overall quality, resolidified material, taper, barreling, exit cone. surface debris and inlet cone, respectively.
4. Results and discussions The results obtained for each hole feature are now discussed. taking each material separately. Some suggestions are put forward to explain the results for the
main effects.
The qualitative and quantitative analysis results are given in Table 3(a) and 3(b), respectively. Resoliclifid Muferid: Table 3(a) gives the focus setting and thickness as being the only significant parame-
ters. It is evident from Table 3(b) that the relationship for first-order interactions is mainly either linear and/or quadratic, with the amount of resolidified material decreasing as the focus setting increases. This suggests that there may be an optimum focus setting at which the highest power intensity occurs, giving a hole with least or no resolidified material. Moving further away from either side of this position, the resolidified c:aterial will increase. In this case. the power intensity reduces once the focal position changes.
The effect of thickness on resolidified material is less significant than that for the focus settings. The effect of interaction between thickness and pulse length is significant. Fig. 2 shows the amount of resolidified material
B - Resolidified material C - Exiteone D - Surface debris
a - Inlet cone
0 -Taper MD - Mean hole dkmeter
Fig.
I, Features of laser-drilled h&s
obtained at various pulse length thickness combinations for four materials. The figure shows strong interaction. as change in one factor produces a different change in the response at another level of this factor. Increasing the pulse length at thickness 1.25 mm, the amount of
resolidified material increases, consistent with the results of earlier work [IS]. Tqer: All of the parameters have a significant effect on taper, with thickness having the greatest effect. As the thickness increases, the amount of taper decreases. this being true for ail of the materials examined. This
268
B.S. Yilbas/JoumalofMaterials Processing Technology70 (1997)264-273
Table 3 (a) Level of significance of the affecting parameters &cts respectively)
P F E L PE PT FE FT ET PFE PFT PET FET
and their interactions
for stainless slecl(O.99.
0.95. 0.90 indicate the most, very and significant
Resol.metal
Taper
Barreling
Inlet cone
Exit tune
Surface debris
Mean
0.90 0.99
0.95 0.99 0.99 0.99 0.90 -
0.90 0.99 0.99 0.90 0.99
0.99 0.99 0.99 0.99 0.90
0.90
0.90 0.90 0.99 -
0.99 0.99 0.99 0.99 0.95 0.9 0.99 -_
0.99 0.99 0.99 0.90 -
0.99 0.99 0.99 0.99 -
0.90 0.90 0.90 0.90 -
0.95 _0.99 0.90 0.99 0.99 0.99
-
0.99 0.95 0.99 0.99 0.95 0.99 .0.95
0.99 0.90
0.95
0.99
0.90
Es 0.95
-
-
-
-
-
(b) Quantitative analysis results for the level of significance of affecting and significant effects respectively)
parameters
for
hole diameter
stainless steel (0.99.
Overall
quality
0.95, 0.90 indicate the most.
very
P
Linear Quad. Cubic F Linear Quad. Cubic E Linear Quad. Cubic T Linear Quad. Cubic
0.90
0.99 -
0.99 -
0.99 -
0.95
0.99 0.90 0.90
0.99 -
0.99 -. -
0.99
0.99 0.99 -
0.w 0.99 0.95
0.99 0.90
0.95 0.99 0.99 -
-_
-
_
0.99 -
-
0.99
0.90
0.99 0.99
-
0.95
-
-
0.95
0.99 0.90 0.95
0.99 0.90
-
0.95
0.99 0.95
0.99 0.90 0.95
0.99 0.99
0.99 0.95 -
0.90 0.99 -
that the evaporation of the metal takes place at first within a large solid angle (2~). As the crater depth increases, the vaporized metal develops sufficient pressure in the crater to eject the liquid material [16]. The
suggests
0.99
liquid metal ejection angle will he decreased in proportion to the diameter/depth ratio of the crater. Therefore, a large angle (measured from the normal to the workpiece surface) of liquid ejection is associated with a relatively shallow hole. Energy is the second significant parameter, in this case the amount of taper increasing with increasing energy. The behavior of the inlet cone, barrelling and the mean hole diameter with energy are the same as for taper, i.e. they increase linearly with energy. Barrding: The effect of energy on barrelling is simi-
Fig. 2. Variation of resolidified pulu? lengths for stainless steel.
with thickness at diNerent
lar to that for taper. The effect of thickness is, however, opposite to that of taper. As thickness increases, more energy is trapped inside the workpiece to form a wider cavity, then the barrel so formed will tend to guide the ejected material as it passes up the hole, forcing the molten material around the hole to come away from the sides. Therefore, some erosion occurs at the top of the hole, which in turn causes taper and an inlet cone to develop.
B.S. :‘dws /Journd OJ Marerids ProcessrngTeckro/ogy 70 (1997)X4-273
269
4.2. Nickel Table 4(a) and 4(b) give the qualitative and quantitative analysis results, respectively. Resolidt$ed material: Nickel behaves differently from
the other two metals and pulse length is the significant factor (Table 4(a)). Maximum resolidified material occurs at a pulse length of 2.5 ms. Interaction between the pulse length and thickness is the most significant, showing that the resolidified material would be reduced by reducing the pulse length at a given thickness. HowFig. 3. Variationof exit COWwah thickness for the materialsselected.
Inlet cofle: All the parameters are found to be significant, with the thickness being the most significant parameter: the inlet cone decreases linearly- as the thickness decreases. which mav be due to the similar situation that was explained for the case of taper and barrelling. Stainless steel happened to have thk largest inlet cone as compared to the other materials examined. In addition. the inlet cone increases linearlv as the energy increases and it decreases as the focus setting moves further away from the nominal focal I
__
Exit cone: The only significant parameter is the thickness, the variation being quadratic. Fig. 3 shows the existence of the maximum imount of cone. It has
been shown previously that the exit diameter becomes smaller for thick workpieces, therefore, most of the holes formed are conical shape and, because of the barrelline effect. a small exit cone forms. SlNfG debris: The only significant parameter is the thickness. Its relationship is cubic, which is similar to that affecting the amount of resolidified material, since the surface bebris is the resolidified material that is deposited on the workpiece surface around the hole diameter. Mean hole diameter: All the four factors are signifi-
cant, the most significant being the focus setting followed by thickness. The linear effect of energy __is significant, increasing energy resulting in increase in the rear relatively intensity Overull effect on
hole diameter. Shorter pulse lengths gives larger holes, which suggests that high power produces a large mean hole diameter. quality: Thickness has the most significant the overall quality. This is due to the taper
and the inlet cone improving with thicker workpieces. Energy has a significant effect, in this case, high input energy producing better holes, and larger values of focus settings give improved holes, upholding the suggestion that the focus setting for the best hole is slightly away from the nominal focal plane. The effect of pulse length is found to be less significant.
ever, difficulty has been encountered, in practice, in controlling the amount of resolidified materiai when drilling nickel. Toper: The effect of thickness is very significant as in the case of stainless steel. Taper decreases linearly with
increasing thickness, i.e. a larger angle of liquid ejection is associated with a relatively shallow hole. The other significant parameter is the focus setting. The best focus setting probably occurs somewhere away from the nominal focal plane. An inspection of the inlet cone, barrelling and the mean hole diameter indicates that these hole features increase linearly with focus settings.
Pulse length is also significant, a longer pulse length giving less taper, i.e. high power intensity is needed to reduce the taper. Barrelling: Thickness is a very significant parameter, barrelling increasing linearly with increasing thickness. Focus setting is also found to be a very significant parameter, the amount of barrelling increasing linearly with focus settings. Inlet cone: The focus setting is the most significant parameter and linear, quadratic effects of the focus setting are important. The inlet cone increases as the
focus setting increases. Thickness has the only significant effect on the inlet cone, the inlet cone decreasing as the thickness increases. Pulse. length also has a very most significant affect, longer pulse lengths resulting in less inlet cone. The first-order interactions of pulse length-thickness, focus setting-energy and focus settingthickness are very significant, and so is the second-order interaction of pulse length-focus setting-thickness. Exit cone: Thickness and focus setting are the signilicant parameters, the amount of exit cone increasing with increasing thickness. Mean ho/e diameter: Increase in energy results in high
material removal rate and so a large mean hole diameter results. Overall qualify: Thickness is the most significant parameter, since a large thickness gives less taper, inlet cone and resolidified material. The linear effect of focus setting is very significant, i.e. a decrease in focus setting results in improved hole geometry. The effect of energy is significant, the variation of overall quality with energy being in quadratic form.
270 Table 4 (a) Level of significanceof the alfecting parametersand their interactionsfor nickel (0.99. 0.95. 0.90 indicate the most. very and significantefl&cts respectively)
Exit cone
Resol. metal
Taper
Barreling
Inlet
0.9 -
0.90 0.99 0.99 0.9 -
0.95 0.99 0.9 0.99
0.99
: -
0.99 0.99 0.9 0.99 0.99 -
-
-
0.99
0.99
-
0.99
g ET PFE
1 -
0.95 0.95
-0.95 -
0.95 0.99 0.95
0.99 0.99
PFT PET FET
0.9 -
0.99 -
_
0.99
0.99
0.95
-
_
-
P F E T PF PE FT
(b) Quantitative analysis resultsfor the level of significant effects respectively)
0.99
-
0.99 0.99
0.95
-
0.95 _-
0.90 0.95
Cubic
0.95
0.99 _
0.99 _
-
0.90
0.99 0.99 0.99
-
0.95 0.90
_ --
0.95 -
0.95 -
0.99 0.99 0.90
0.99 -
0.99
1 -
Mean hole diameter
Ove~ll qualily
0.99 0.99 0.99 0.99 _ -
0.99 0.99 0.99 0.90
0.99
-
_ -
0.99 _ 0.99 _
0.99 0.95 0.99
0.99 0.90
__ _
0.99 0.99 0.90
:
0.99 ._ _
0.99 GO _
_
-
_
0.99
0.95
4.3. Titaniutn Quantitative and qualitative analysis results are given in Table S(a) and 5(b), respectively. ResolidiJed nrateriul: Focus setting, pulse length and interaction of energy with focus setting are significant. The amount of resolidified material decreases as the focus and pulse length increase. Taper: Energy is the most significant parameter, with thickness also being a significant parameter. Small thickness results in a larger taper. The first-order interactions of factors are negligible and insignificant. Burrelling: Thickness is the most significant parameter. First-order interactions of pulse length-focus setting and pulse length-thickness are significant. with pulse
length alone being found to be an insignificant parameter.
Inletcore:
Surface debris
affecting parametersfor nickel (0.99. 0.95. 0.90 inchcateIhe most, very and
P Linear Quad. F Linear Quad. Cubic E Linear Qund. Cubic T Linear Quad. Cubic
cone
Thickness is a very significant parameter, where inspection of the effect of thickness on barrel@, inlet cone, exit cone and mean hole diameter shows that
_ _
0.99 _
0.99 0.99 0.99
_ _ 0.90 0.99 _ _
0.99 _ o.q9 0.90 0.90
they are all at the lowest level (have a minimum value) at a thickness of I mm. The focus setting is the next very significant parameter, focus setting corresponding to high power intensity producing less inlet cone. It should be noted here that large inlet cones are usually
associated with refilled holes. Exit cone: Pulse length, energy and focus setting have little contrcl over the exit cone. The effect of thickness is most significant, i.e. a smaller thickness produces bigger exit cones. Surface drbris: The focus setting and first-order interactions of pulse length-focus setting and pulse lengththickness are very significant, long focus setting giving a large amount of surface debris. Mmn hole ciktnwter: Fig. 4 shows that titanium has the largest mean hole diameter amongst the materials examined. This may be due to titanium having the lowest thermal diffusivity amongst the three materials; the amount of heat lost to adjacent zones by thermal conductivity is comparatively small, so that sufficient
B.S. Yilhm , J 01,m,l o, Mur~nr,L Pr,,~,w,,~ T<~
271
Tdbtc 5 (a) Level of significanceof the affectmg parameters.md thew mterdctionsfor t~tnmum(0.9U. 0.95. 0.90 indicate the most, very and Ggnitic;mt effects respectively).
Resol. melal P F E T
PP PE Pr FE Fr ET PFE PIT
PET FET
-‘itper
Rdrrcb”b _--__
0.99 0.99
Inlet cone
debrib
0.95 0 99
0.95
0 99
0 YY
O,YY
0 Y9
O.YY
0 90
O.Y5
a.99 0.90 0.95 0.99
Mean hole diameter
0.9
0.99
0 99
0.95
0.90 _
_. _ _
0.99
0.9’)
0.90 0 90
(b) Quantitatwe analysts results for the level of b~@icdnce of dffectmg p*r.,meters for significant effect\ refpsctwely) P Linear
0.99
Quad. Cubic
0.90
F Linear
0.95 0 Y5 0 90 0.95 __ __
0.95 _
E Linear
0.90
-
Quad. Cubic
o.YO
--
0.99 0.99 _
~ 0.95 0.95
Quad. Cubic
Linear
Quad. Cubic
Ovemll quahty
095 0.99
0.99 _ -
the most. very and
0 90
0.99
_
0 99 0.90
0 YY
0.95
0.99
0.95
0 99
KY9 0.99 0.99
099 0 99
energy is available for mass removal from the cavity. Thickness has the most significant effect on the mean hole diameter, a large thickness resulting in small mean
Fig. 4. Variation of mean hole diumcter with thtcknebs for the materials selected.
0.99 0.99 0.90
0.99 0.99 0.95
hole diameters. The effect of focus setting is found to be signihcant parameter also. Owrrrll yurrliry: Thickness is the most significant parameter, a thickr,:ss of mm giving the best overall quality, since it produces least inlet cone, barrelling, resolidified material and exit cone. Pulse length is also a very significant factor, a short pulse length giving improved holes as a result of small inlet and exit cones. and less resolidified material. Focus setting has a significant effect on overall quality. Fig. 5 presents photographs of hole cross-sections for stainless steel. nickel and titanium. It is evident that stainless steel and nickel result in better hole geometries rhan does titanium. This may be due to the thermal properties of titanium which has the lowest thermal Jiffusivity and density. Barrelling, inlet and exit cones, and some resolidification are evident from the photographs. Holes drilled in stainless steel appear to be parallel-sided with less hole defects.
I
B.S. Yilbas /Journal of Morerids Processing Technology 70 (1997) 264-273
212
Laser Bela rarectbm
ElWrW’lSJ
2lJ
4
-A
all of the materials examined. For the second-order interactions, only pulse length-focus setting-thickness is found to be significant. The effect of each parameter may be concluded as follows. Pulse length: The improvement of hole quality by varying the pulse length can only be found significant on mean hole diameter and inlet cone. In the case of nickel, pulse length reduces the amount of inlet cone, taper and mean hole diameter. Resolidified material reduces with a reduction in the pulse length for titanium. Focus setting: The effect of focus setting is very significant and more critical than the pulse length. Hole geometry can be improved by increasing the focus setting above the workpiece surface within the focal range employed in the present study. However, the optimum focus setting for each material varies, which may be due to the thermal properties and the coupling effect of the focus setting and power intensity distribution on the workpiece surface. Energy: An increase in pulse energy increases the mean hole diameter. Inlet cone, barrelling and taper increase linearly with increasing energy for stainless steel. The effect of energy on the overall quality of the holes is significant for all of the materials examined. Thickness: The effect of thickness is found to be very significant in most cases. The common trend is that the amount of taper decreases as the thickness increases.
Nkket:l?Waeu-:85mm: FocawttLL-Sl.llr:RL~b-l.lllla
Ewgy-
15 J 4
21 J -
4
Fig. 5. Cross-sectionsof holes drilled in St, Ni and Ti at energy levels.
5. conel~ons
The parameter which is found to be very Bignificant in most cases is the workpiece thickness. The first-order interaction of pulse length-thickness is the most significant, whilst pulse length-focus setting is significant for
[I] B.S. Yilbas. Heating of metals at a free surface by laser xradialion: an electron kinetic theory approach, Int. J. Eng. Sci. 24 (8) (1986) 1325-1334. [2] H.S. Kim, Y. Domankevitz. H.S. Kwok, Effect of choppmg on laser penetration of metal targets, Appl. Phys. Lett. 55 (8) (1989) 726-728. [3] B.S. Yilbas, Analytical solution for heat conduction mechanism appropriate to the laser heating process, Int. Comman. Heat Mass Transf. 20 (1993) 543-555. (41 B.S. Yilbas, The study of laser produced plasma behavior using streak photography, Jpn. 1. i ppl. Phys. 24 (II) (1985) l4171420. [S] B.S. Yilbas. 2. Yilbas, Plasma transientsduring laser drilling in subatmosphericpressureatmospheresof air, Opt. Lasers Eng. 7 (1987) l-13. [6] B.S. Yilbas. R. Davies, Z. Yilbas. F. Begh, Study into the measurementand prediction of penetration time daring CO, laser catting process, Prof. Inst. Mech. Eng., Part B: J. Eng. Manuf. 204 (1990) 105-113. [7j B.S. Yilbas. A. Sahin, Law pulse optimization for practical laser drilling, Opt. Laws Eng. 20 (1994) 31 I-323. [S] B.S. Yilbas, A. Sahin, Laser heating mechanismincluding evaporation process initiating the laser drilling. Int. J. Mach. Tools Manuf. 35 (7) (1995) 1047-1062. [9] S. TanPko, M. Massahura. H. Seigo, Laser drilling by high peak pulsed CO, laser, ICALEO Laser Material Processing,Orlando, 1990.
“essing Tec/tno/ogy 70 (1997) 264-273 [lo] A. Kestenbaum,J.F. D’Amico, B.J. Bolmenstock. M.A. DeAngelo, Laser drilling of microvias in epoxy-glassprinted circuit boards, IEEE Trans. Components, Hybrids Manuf. Technol. I3 (4) (1990) 1055-1062. [I I] A.L. Kenney. J.W. Dally. Laser drilling of very small electronic via holes in common circuit board materials, Circuit World 14 (3) (1988) 31-36. [12] B.S. Yilbas, Parametersaffecting hole geometrym laser drilling of nimonecs75, SPIE Proc. 774 (1987) 87-31.
273
[I31 Chartield C., Statisticsfor Technology, Wiley, New York, 1978. [14] B.S. Yilbas. Z. Yilbas. Investigation into drilling speedduring laserdrilling of metals,Opt. Laser Tcchnol. 20 (I) (1988) 29-32. [IS] B.S. Yilbas, Study of affecting parametersin laser hole drilliog of sheet metals, Trans. ASME: 1. Eng. Mater. Technol. 109 (1987) 282-287. [I4 B.S. Yilbas. Z. Yilbas, Surface line and plug flow modelsgoveming laser produced vapor from metallic surfaces, Pramana J. Phys. 38 (24) (1992) 33-38.
Journal of Materials Processing Technology 70 (1997) 264273
Parametric study to improve laser hole drilling process B.S. Yilbas
’
Mechanical EngineeringDeparrment, King Fahd University of Perro/eumand Minerah, Dl~alwan3126l. Suudi Arabia
Received 21 January 1996
Abstracl
In the laser driYling process, the quality of the drilled holes is the main task. A method of studying the influence on the quality of the main process variables needs to be developed, which seeks to improve the quality and explains the drilling mechanism. In the presentstudy, the effectof the laser parametersand the material properties on the hole quality when drilling is examined. A statistical approach, referred to as factorial design, is employed to test the significance level of the factors that affect the hole quality. Three materials, stainless steel, nickel and titanium, are considered. The experimental study yields tables of significance of each factor on the aspects that determine the quality of the holes. The hole geometry is evaluated by assigning marks for each geometric feature, the tnarking scheme being conducted relevant to the importance of the hole feature. 0 1997 Else&r Sciena S.A. Key~vords:Laser drilling; Stainless steel; Nickel; Titanium; Streak photography
1. Introduction
from a vapor-solid heating model to considerations that attempt to throw light on liquid ejection. Consequently, mechanisms describing laser machining require a knowledge of the interaction mechanisms as well as the physical properties of the substance being drilled, such as thermal diffusivity, surface reflectivity, absorption coefficient, and etc. These properties are often unknown
at the elevated
temperatures
resulting
from the
laser beam heating. Therefore, a model describing the drilling process has not yet been fully established. Laser drilling is a common cotntn~rcially-developed process for metal working. There have been several research investigations conducted in the past on reducing the drilling time, the input energy, and the intensity of the laser irradiation [6-S]. Laser drilling into electronic components was studied by Taneko et al. [9], who showed that the use of a high peak pulsed CO,
’ Fax: + 966 3 8602944; e-mail: [email protected] 09240136/97/S17.00d 1997 Elsevier Science PI1 SO924-0136(97)00076-9
rights reserved.
laser improved the drilling quality. Laser drilling of microvias in epoxy-glass printed circuit boards was investigated by Kestenbaum et al. [lo]. Based on their findings, holes of 0. I mm diameter could be drilled with a drilling rate of several hundred per second. Laser drilling of very fine electronic via holes in common circuit board materials was studied by Kenney and
265
Daily [ii]. They concluded that the energy delivered was the most critical parameter, but also suggesred that further improven,ent in the resulting hole quality was necessary. From the application viewpoint, it is the end product
from the laser system,
for instance
a drilled
hole, that matters. To apply the laser beam to micromachining, various factors must be considered and evaluated. These factors include the laser pulse length, the pulse energy, the focus settings of the focusing lens, and the workpiece thickness and thermal properties. Effective utilization of the laser depends very much upon the proper selection and optimization of these factors. Limited information on how these factors affect the laser-drilled hole quality have seriously hindered the growth of laser applications in micromachining, because the effects of some of these factors have been underestimated previously [l2]. Therefore, a systematic study to establish and recognize the machining standards to produce holes wltb good quality is necessary. The initial step in this regard is a stdtistical study of the parametric effects on the drilling pro-
cess. Two different techniques, namely multiple correlation and the factorial experiment, have the capability of achieving this aim. Factorial design is appropriate in this aspect as its assumptions are general and make no restriction as to linearity [i3]. Traditionally, experimentation in laser hole drilling has been performed using a one-variable-at-a-time approach, in this case all but one variable being held constant as the effect or relationship between that one variable and the response is determined by running a number of tests over its operability region. However, the one-variable-at-a-time approach is inefficient, can lead to misleading results, and is limited, since interactions (inter-dependencies) amongst variables are difficult to determine and are often overlooked [i3]. In the present study, a design of factorial experiment including four factors at four levels is considered. This design covers all the possible combinations of the factors over the operating regions. The response
for the factorial design is obtained by the evaluation of the features of the resulting hole geometry. Prior to the present experiment, a statistical study of the factors that are most significant to the drilling speed has been examined [14]. The results indicated that all the main effects of factors considered were highly signiticant in the decreasing order of Ls, material properties, focus settings, workpiece thickness, energy and
pulse lengths. Cocsequentiy, the results of previousiyconducted drilling-speed experiment are complementary to the present experiment in that the parameters for the latter are determined from them. Therefore, three workpiece materials and four factors at four levels are selected to conduct the factorial design experiment.
2. Experimental An experiment is designed to give insight into how and why variation in laser parameters and materials affecting the resultant hole quality when drilling. This may be achieved by an evaluation of holes so drilled by a subsequent statistical approach, referred to as hctoriai design. Factorial analysis is performed for each material selected as forming a block, thus having quantitative variables ia each block of experiments. Material effects could bc recognized as the effects between the blocks. The statistical analysis of the holes deals with four-way analysis of variance. 2.1.
Mathetnuticul
model
The mathematical model for the present experiment may be presented as given in Eq. (1): X,:,,,= /1+ P, + F, + J%+ T, + (PF), + (PE),, + VT),, + (PF),, + G=‘O,,+(ET),, f (PFE),,k+ U’FV,,, + (PET),,, + (FET),,, +
c,,tct
(1)
where ,I&, is the value given to the hole feature as a measure variable; p is a common effect in ail observations; P. F, E, T are the main effects of parameters of pulse length, focus setting, energy and workpiece thickness respectively; PF,..., ET are the first-order interactions of parameters; PEF,... FET are the secondorder interactions of the main effects; Q, is the random error in the experiment; and i, j, k, I are the lcl/eis of parameters P, F, E and T respectively. As there is only one treatment per combination, the third-or&r interaction is within experimental and random error, thereiore it is relegated to the error term by making the assumption that the third-order interaction does not exist and the hole is repeatable: even if it were present, it would be difficult to explain in practical terms. The mathematical model of this experiment is a fixed model each factor having four fixed levels. The calculation of the sum of squares of main effects, first-order and second-order interactions, and the error are not given here due to their lengthy arguments, but refer to [151. The significance of each effect and interaction is tested by making the initial hypothesis that the mean squares of the effects or interactions is from the same population as the error mean squares. As the mathematicai n~tidc! pms~ted hy Eq. (I) is a fixed and not random model, therefore the expected mean squares for the main effects and interactions are all estimated by the error mean variance. Hence, the mean squares for the main effects and interactions can be compared to the error mean square to test the respective hypothesis by means of an F test. The definition of variance :atio is given by:
266
B.S. Yilbas/Journal of Materials Promming Technology70 (1997) 264-273
F = Mean Sum of Squares p”q Error Mean Sum of Squares
(2)
If the calculated value of the variance ratios of a given measured variable exceeds the tabulated value for the number of degrees for the item and the error at a particular probability point, the item is significant at that level. It should be noted here that the appropriate level of significance will depend on the particular problem under consideratton. In this regard, 10, 5 and 1% are adopted as giving results that are significant, very significant and most significant, respectively.
Table 2 Coefficients
A
B
Linear Quadratic
-3
-I -I
Cubic
-I
-3A-B+C+3D
(3)
the quadratic effect by:
-A+3D-3C+D
(5)
Therefore, the sum of squares is: Quantitative
F: = (no. of observations in each level of factor F) x 20 (6)
20
= (No.
of observations
Quantitative
F2 in eich level of factorF) x 4
F, F2
=-
of observations in eaih level of factorF) x 20 (7) The variance ratios are: QuantitativeF Variance,, = Sum of mean squares of error The no. of dearees of freedom of the linear, auadratic and cubic effect is one. By comparing the values of variance ratios with the values in the F-table, the level of significance may be obtained. A computer program has been developed to calculate the variance ratios of the main effects, and the first- and second-order inter(no.
actions of the parameters.
An Nd YAG laser delivering output energies of 5-21 J within 1.2-2.7 ms pulses was employed, a 51 mm nominal focal length lens being used to focus the laser beam onto the workoiece surface. Three workoiece materials were selected due to their range of thermal these being
titanium,
nickel -and
stainless
experimental conditions and the parameters and their levels are given in Table 2. The focai positioi and laser output energy were set to the lowest levels. The focal oosition was then varied from the lowest to the highest level, requiring four holes to be drilled for each material at one thickness level. Sbsequently, drilling progressed and 1024 holes were drilled. After the drilling, the damage diameter of each hole was measured under a microscope. The holes were
Table I Parametersand their levels
Thickness (mm)
20 4
I
Quantitative
properties, steel. The
FL
Energy(J)
-I
-3
(4)
and the cubic effect by:
Pulse length (ms) Focus setting (mm)
3
E
3 I
2.3. Experimental apparatus
F,=A-B+C+D
Parameters
D I
F,
If the levels of a quantitative factor considered are equi-spaced it is possible to fit a polynomial equation of up to order n - 1 through n points, where n is the number of levels of the factor. This permits the obtaining of more infomration on how the response may vary with the changing levels of the quantitative factor than just the analysis of variance. For four factors at four levels, linear, quadratic and cubic effects can be extracted by the proper choice of coefficients for the response totals, the coefficients for of the response total being given in Table 1. As an example, if the measured response totals for the factor F (focus setting) at levels I, 2, 3 and 4 are A. B, C and 0, then the total for the linear effect is given by:
F,=
I
C
and:
2.2. Quantitative anulysis
FL=
in the quantitative analysis
Coefficients
Lexls I
2
3
4
1.5 50.5 I5 0.5
I.8 51 I7 0.75
2.0 51.5 19 I
2.5 52 21 1.25
cut as close as possible to the edge of the holes, leaving the holes unaffected, mounted-in perspex and then ground down to a diameter. Finally,.in order to eliminate scratches, caused by grinding, the samples were polished with diamond paste. These polished samples were examined under the microscope and scores were assigned
to the hole features.
3. Hole evaluation The score assigned was undertaken considering the importance of the hole feature as an overall quality [IS]: the hole feature is shown in Fig. 1.The procedure in evaluating the hole feature is given as follows. Radidifid ntuteriul: This was the measure of the amount of material that had vaporized or melted during drilling, but had not escaped from the hole and so
had resolidified on the internal surface. Marks were given out of 10 according to the fraction of resolidified material to the hole size. Thus a mark of 10 meant that there was no resolidified material and a mark of one meant that the hole was absolutelv full. Taoer: This was a measure of the overall taper of the hole sides, but did not include inlet and exit cones. The actual taper was measured using a microscope and a mark was assigned out of 10 for the ratio of the taper angle to 90’: a mark of 10 indicates a parallel-sided hole and one of 10 a taper of 90”. Burrellin~: The amount of barrellina in the sides of a
hole was r&en a mark between 0 and-4. i.e. 4 indicated a straightrsided hole. Inlet cone: The inlet cone was measured using a microscope. A mark out of 10 was given for the ratio of the cone angle to 180”: a mark of 10 indicates no inlet Exif cone: The exit cone is small relative to the size of
the hole, and it was impractical to measure it. A mark of 0 to 3 was given by inspection, i.e. 3 indicated no exit Surfuce D&is: This was an assessment of the amount of resolidified material appearing on the surface of the hole. A mark of either 0 or I was given. A mark of I meant no surface debris.
MCNIIHo/c Dimeter: This was measured using a microscope and the value assigned was the mean hole diameter in mm. This feature has no bearing on quality, but is an important factor in hole drilling and so is included in the analysis. Ocwull qrrdiry: This is the
sum of hole features. As some feature are considered to be’more important than others, weights are assigned to a particular feature. Resolidified material and taper are considered to have equal importance, but have twice that of barrclling and inlet cone. Therefore, the overall quality is: Qq=2R,+2T.,+B.,+E,+S,+I, where Qq, R,. T,, E,, EC, S,. i, are the overall quality, resolidified material, taper, barreling, exit cone. surface debris and inlet cone, respectively.
4. Results and discussions The results obtained for each hole feature are now discussed. taking each material separately. Some suggestions are put forward to explain the results for the
main effects.
The qualitative and quantitative analysis results are given in Table 3(a) and 3(b), respectively. Resoliclifid Muferid: Table 3(a) gives the focus setting and thickness as being the only significant parame-
ters. It is evident from Table 3(b) that the relationship for first-order interactions is mainly either linear and/or quadratic, with the amount of resolidified material decreasing as the focus setting increases. This suggests that there may be an optimum focus setting at which the highest power intensity occurs, giving a hole with least or no resolidified material. Moving further away from either side of this position, the resolidified c:aterial will increase. In this case. the power intensity reduces once the focal position changes.
The effect of thickness on resolidified material is less significant than that for the focus settings. The effect of interaction between thickness and pulse length is significant. Fig. 2 shows the amount of resolidified material
B - Resolidified material C - Exiteone D - Surface debris
a - Inlet cone
0 -Taper MD - Mean hole dkmeter
Fig.
I, Features of laser-drilled h&s
obtained at various pulse length thickness combinations for four materials. The figure shows strong interaction. as change in one factor produces a different change in the response at another level of this factor. Increasing the pulse length at thickness 1.25 mm, the amount of
resolidified material increases, consistent with the results of earlier work [IS]. Tqer: All of the parameters have a significant effect on taper, with thickness having the greatest effect. As the thickness increases, the amount of taper decreases. this being true for ail of the materials examined. This
268
B.S. Yilbas/JoumalofMaterials Processing Technology70 (1997)264-273
Table 3 (a) Level of significance of the affecting parameters &cts respectively)
P F E L PE PT FE FT ET PFE PFT PET FET
and their interactions
for stainless slecl(O.99.
0.95. 0.90 indicate the most, very and significant
Resol.metal
Taper
Barreling
Inlet cone
Exit tune
Surface debris
Mean
0.90 0.99
0.95 0.99 0.99 0.99 0.90 -
0.90 0.99 0.99 0.90 0.99
0.99 0.99 0.99 0.99 0.90
0.90
0.90 0.90 0.99 -
0.99 0.99 0.99 0.99 0.95 0.9 0.99 -_
0.99 0.99 0.99 0.90 -
0.99 0.99 0.99 0.99 -
0.90 0.90 0.90 0.90 -
0.95 _0.99 0.90 0.99 0.99 0.99
-
0.99 0.95 0.99 0.99 0.95 0.99 .0.95
0.99 0.90
0.95
0.99
0.90
Es 0.95
-
-
-
-
-
(b) Quantitative analysis results for the level of significance of affecting and significant effects respectively)
parameters
for
hole diameter
stainless steel (0.99.
Overall
quality
0.95, 0.90 indicate the most.
very
P
Linear Quad. Cubic F Linear Quad. Cubic E Linear Quad. Cubic T Linear Quad. Cubic
0.90
0.99 -
0.99 -
0.99 -
0.95
0.99 0.90 0.90
0.99 -
0.99 -. -
0.99
0.99 0.99 -
0.w 0.99 0.95
0.99 0.90
0.95 0.99 0.99 -
-_
-
_
0.99 -
-
0.99
0.90
0.99 0.99
-
0.95
-
-
0.95
0.99 0.90 0.95
0.99 0.90
-
0.95
0.99 0.95
0.99 0.90 0.95
0.99 0.99
0.99 0.95 -
0.90 0.99 -
that the evaporation of the metal takes place at first within a large solid angle (2~). As the crater depth increases, the vaporized metal develops sufficient pressure in the crater to eject the liquid material [16]. The
suggests
0.99
liquid metal ejection angle will he decreased in proportion to the diameter/depth ratio of the crater. Therefore, a large angle (measured from the normal to the workpiece surface) of liquid ejection is associated with a relatively shallow hole. Energy is the second significant parameter, in this case the amount of taper increasing with increasing energy. The behavior of the inlet cone, barrelling and the mean hole diameter with energy are the same as for taper, i.e. they increase linearly with energy. Barrding: The effect of energy on barrelling is simi-
Fig. 2. Variation of resolidified pulu? lengths for stainless steel.
with thickness at diNerent
lar to that for taper. The effect of thickness is, however, opposite to that of taper. As thickness increases, more energy is trapped inside the workpiece to form a wider cavity, then the barrel so formed will tend to guide the ejected material as it passes up the hole, forcing the molten material around the hole to come away from the sides. Therefore, some erosion occurs at the top of the hole, which in turn causes taper and an inlet cone to develop.
B.S. :‘dws /Journd OJ Marerids ProcessrngTeckro/ogy 70 (1997)X4-273
269
4.2. Nickel Table 4(a) and 4(b) give the qualitative and quantitative analysis results, respectively. Resolidt$ed material: Nickel behaves differently from
the other two metals and pulse length is the significant factor (Table 4(a)). Maximum resolidified material occurs at a pulse length of 2.5 ms. Interaction between the pulse length and thickness is the most significant, showing that the resolidified material would be reduced by reducing the pulse length at a given thickness. HowFig. 3. Variationof exit COWwah thickness for the materialsselected.
Inlet cofle: All the parameters are found to be significant, with the thickness being the most significant parameter: the inlet cone decreases linearly- as the thickness decreases. which mav be due to the similar situation that was explained for the case of taper and barrelling. Stainless steel happened to have thk largest inlet cone as compared to the other materials examined. In addition. the inlet cone increases linearlv as the energy increases and it decreases as the focus setting moves further away from the nominal focal I
__
Exit cone: The only significant parameter is the thickness, the variation being quadratic. Fig. 3 shows the existence of the maximum imount of cone. It has
been shown previously that the exit diameter becomes smaller for thick workpieces, therefore, most of the holes formed are conical shape and, because of the barrelline effect. a small exit cone forms. SlNfG debris: The only significant parameter is the thickness. Its relationship is cubic, which is similar to that affecting the amount of resolidified material, since the surface bebris is the resolidified material that is deposited on the workpiece surface around the hole diameter. Mean hole diameter: All the four factors are signifi-
cant, the most significant being the focus setting followed by thickness. The linear effect of energy __is significant, increasing energy resulting in increase in the rear relatively intensity Overull effect on
hole diameter. Shorter pulse lengths gives larger holes, which suggests that high power produces a large mean hole diameter. quality: Thickness has the most significant the overall quality. This is due to the taper
and the inlet cone improving with thicker workpieces. Energy has a significant effect, in this case, high input energy producing better holes, and larger values of focus settings give improved holes, upholding the suggestion that the focus setting for the best hole is slightly away from the nominal focal plane. The effect of pulse length is found to be less significant.
ever, difficulty has been encountered, in practice, in controlling the amount of resolidified materiai when drilling nickel. Toper: The effect of thickness is very significant as in the case of stainless steel. Taper decreases linearly with
increasing thickness, i.e. a larger angle of liquid ejection is associated with a relatively shallow hole. The other significant parameter is the focus setting. The best focus setting probably occurs somewhere away from the nominal focal plane. An inspection of the inlet cone, barrelling and the mean hole diameter indicates that these hole features increase linearly with focus settings.
Pulse length is also significant, a longer pulse length giving less taper, i.e. high power intensity is needed to reduce the taper. Barrelling: Thickness is a very significant parameter, barrelling increasing linearly with increasing thickness. Focus setting is also found to be a very significant parameter, the amount of barrelling increasing linearly with focus settings. Inlet cone: The focus setting is the most significant parameter and linear, quadratic effects of the focus setting are important. The inlet cone increases as the
focus setting increases. Thickness has the only significant effect on the inlet cone, the inlet cone decreasing as the thickness increases. Pulse. length also has a very most significant affect, longer pulse lengths resulting in less inlet cone. The first-order interactions of pulse length-thickness, focus setting-energy and focus settingthickness are very significant, and so is the second-order interaction of pulse length-focus setting-thickness. Exit cone: Thickness and focus setting are the signilicant parameters, the amount of exit cone increasing with increasing thickness. Mean ho/e diameter: Increase in energy results in high
material removal rate and so a large mean hole diameter results. Overall qualify: Thickness is the most significant parameter, since a large thickness gives less taper, inlet cone and resolidified material. The linear effect of focus setting is very significant, i.e. a decrease in focus setting results in improved hole geometry. The effect of energy is significant, the variation of overall quality with energy being in quadratic form.
270 Table 4 (a) Level of significanceof the alfecting parametersand their interactionsfor nickel (0.99. 0.95. 0.90 indicate the most. very and significantefl&cts respectively)
Exit cone
Resol. metal
Taper
Barreling
Inlet
0.9 -
0.90 0.99 0.99 0.9 -
0.95 0.99 0.9 0.99
0.99
: -
0.99 0.99 0.9 0.99 0.99 -
-
-
0.99
0.99
-
0.99
g ET PFE
1 -
0.95 0.95
-0.95 -
0.95 0.99 0.95
0.99 0.99
PFT PET FET
0.9 -
0.99 -
_
0.99
0.99
0.95
-
_
-
P F E T PF PE FT
(b) Quantitative analysis resultsfor the level of significant effects respectively)
0.99
-
0.99 0.99
0.95
-
0.95 _-
0.90 0.95
Cubic
0.95
0.99 _
0.99 _
-
0.90
0.99 0.99 0.99
-
0.95 0.90
_ --
0.95 -
0.95 -
0.99 0.99 0.90
0.99 -
0.99
1 -
Mean hole diameter
Ove~ll qualily
0.99 0.99 0.99 0.99 _ -
0.99 0.99 0.99 0.90
0.99
-
_ -
0.99 _ 0.99 _
0.99 0.95 0.99
0.99 0.90
__ _
0.99 0.99 0.90
:
0.99 ._ _
0.99 GO _
_
-
_
0.99
0.95
4.3. Titaniutn Quantitative and qualitative analysis results are given in Table S(a) and 5(b), respectively. ResolidiJed nrateriul: Focus setting, pulse length and interaction of energy with focus setting are significant. The amount of resolidified material decreases as the focus and pulse length increase. Taper: Energy is the most significant parameter, with thickness also being a significant parameter. Small thickness results in a larger taper. The first-order interactions of factors are negligible and insignificant. Burrelling: Thickness is the most significant parameter. First-order interactions of pulse length-focus setting and pulse length-thickness are significant. with pulse
length alone being found to be an insignificant parameter.
Inletcore:
Surface debris
affecting parametersfor nickel (0.99. 0.95. 0.90 inchcateIhe most, very and
P Linear Quad. F Linear Quad. Cubic E Linear Qund. Cubic T Linear Quad. Cubic
cone
Thickness is a very significant parameter, where inspection of the effect of thickness on barrel@, inlet cone, exit cone and mean hole diameter shows that
_ _
0.99 _
0.99 0.99 0.99
_ _ 0.90 0.99 _ _
0.99 _ o.q9 0.90 0.90
they are all at the lowest level (have a minimum value) at a thickness of I mm. The focus setting is the next very significant parameter, focus setting corresponding to high power intensity producing less inlet cone. It should be noted here that large inlet cones are usually
associated with refilled holes. Exit cone: Pulse length, energy and focus setting have little contrcl over the exit cone. The effect of thickness is most significant, i.e. a smaller thickness produces bigger exit cones. Surface drbris: The focus setting and first-order interactions of pulse length-focus setting and pulse lengththickness are very significant, long focus setting giving a large amount of surface debris. Mmn hole ciktnwter: Fig. 4 shows that titanium has the largest mean hole diameter amongst the materials examined. This may be due to titanium having the lowest thermal diffusivity amongst the three materials; the amount of heat lost to adjacent zones by thermal conductivity is comparatively small, so that sufficient
B.S. Yilhm , J 01,m,l o, Mur~nr,L Pr,,~,w,,~ T<~
271
Tdbtc 5 (a) Level of significanceof the affectmg parameters.md thew mterdctionsfor t~tnmum(0.9U. 0.95. 0.90 indicate the most, very and Ggnitic;mt effects respectively).
Resol. melal P F E T
PP PE Pr FE Fr ET PFE PIT
PET FET
-‘itper
Rdrrcb”b _--__
0.99 0.99
Inlet cone
debrib
0.95 0 99
0.95
0 99
0 YY
O,YY
0 Y9
O.YY
0 90
O.Y5
a.99 0.90 0.95 0.99
Mean hole diameter
0.9
0.99
0 99
0.95
0.90 _
_. _ _
0.99
0.9’)
0.90 0 90
(b) Quantitatwe analysts results for the level of b~@icdnce of dffectmg p*r.,meters for significant effect\ refpsctwely) P Linear
0.99
Quad. Cubic
0.90
F Linear
0.95 0 Y5 0 90 0.95 __ __
0.95 _
E Linear
0.90
-
Quad. Cubic
o.YO
--
0.99 0.99 _
~ 0.95 0.95
Quad. Cubic
Linear
Quad. Cubic
Ovemll quahty
095 0.99
0.99 _ -
the most. very and
0 90
0.99
_
0 99 0.90
0 YY
0.95
0.99
0.95
0 99
KY9 0.99 0.99
099 0 99
energy is available for mass removal from the cavity. Thickness has the most significant effect on the mean hole diameter, a large thickness resulting in small mean
Fig. 4. Variation of mean hole diumcter with thtcknebs for the materials selected.
0.99 0.99 0.90
0.99 0.99 0.95
hole diameters. The effect of focus setting is found to be signihcant parameter also. Owrrrll yurrliry: Thickness is the most significant parameter, a thickr,:ss of mm giving the best overall quality, since it produces least inlet cone, barrelling, resolidified material and exit cone. Pulse length is also a very significant factor, a short pulse length giving improved holes as a result of small inlet and exit cones. and less resolidified material. Focus setting has a significant effect on overall quality. Fig. 5 presents photographs of hole cross-sections for stainless steel. nickel and titanium. It is evident that stainless steel and nickel result in better hole geometries rhan does titanium. This may be due to the thermal properties of titanium which has the lowest thermal Jiffusivity and density. Barrelling, inlet and exit cones, and some resolidification are evident from the photographs. Holes drilled in stainless steel appear to be parallel-sided with less hole defects.
I
B.S. Yilbas /Journal of Morerids Processing Technology 70 (1997) 264-273
212
Laser Bela rarectbm
ElWrW’lSJ
2lJ
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all of the materials examined. For the second-order interactions, only pulse length-focus setting-thickness is found to be significant. The effect of each parameter may be concluded as follows. Pulse length: The improvement of hole quality by varying the pulse length can only be found significant on mean hole diameter and inlet cone. In the case of nickel, pulse length reduces the amount of inlet cone, taper and mean hole diameter. Resolidified material reduces with a reduction in the pulse length for titanium. Focus setting: The effect of focus setting is very significant and more critical than the pulse length. Hole geometry can be improved by increasing the focus setting above the workpiece surface within the focal range employed in the present study. However, the optimum focus setting for each material varies, which may be due to the thermal properties and the coupling effect of the focus setting and power intensity distribution on the workpiece surface. Energy: An increase in pulse energy increases the mean hole diameter. Inlet cone, barrelling and taper increase linearly with increasing energy for stainless steel. The effect of energy on the overall quality of the holes is significant for all of the materials examined. Thickness: The effect of thickness is found to be very significant in most cases. The common trend is that the amount of taper decreases as the thickness increases.
Nkket:l?Waeu-:85mm: FocawttLL-Sl.llr:RL~b-l.lllla
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15 J 4
21 J -
4
Fig. 5. Cross-sectionsof holes drilled in St, Ni and Ti at energy levels.
5. conel~ons
The parameter which is found to be very Bignificant in most cases is the workpiece thickness. The first-order interaction of pulse length-thickness is the most significant, whilst pulse length-focus setting is significant for
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[I31 Chartield C., Statisticsfor Technology, Wiley, New York, 1978. [14] B.S. Yilbas. Z. Yilbas. Investigation into drilling speedduring laserdrilling of metals,Opt. Laser Tcchnol. 20 (I) (1988) 29-32. [IS] B.S. Yilbas, Study of affecting parametersin laser hole drilliog of sheet metals, Trans. ASME: 1. Eng. Mater. Technol. 109 (1987) 282-287. [I4 B.S. Yilbas. Z. Yilbas, Surface line and plug flow modelsgoveming laser produced vapor from metallic surfaces, Pramana J. Phys. 38 (24) (1992) 33-38.