- Email: [email protected]
Laser modification of metal surfaces
Optics and Lasers in Engineering
18 (1993) 1-13
Laser Modification D. A. Scott, CSIRO
Division
of Metal Surfaces
M. Brandt,
B. Dorien-Brown
of Manufacturing Technology, Industrial Laser Centre, Lindfield, New South Wales 2070, Australia
B. Valentine Royal Australian
Mint, Canberra,
(Received
& P. De
Australian
6 March
PO Box 218,
Capital
1992; accepted
Territory
2601, Australia
10 April 1992)
ABSTRACT A series of experiments
has been undertaken to investigate the feasibility of using a Nd: YAG laser to produce a highly random surface on steel, which could be used as a finished, textured surface itself, or as a master for a stamping process. The primary aim of the experiments was to duplicate the appearance of a lightly sandblasted surface, but with more control of the surface properties. Although the two techniques are very different, sandblasting being a mechanical process and laser dulling being a thermal process, it was found that a surface of similar roughness and reflective qualities could be achieved. The technique was employed to laser dull a die used in the manufacture of proof coins and the results of trial stampings with this die are discussed.
INTRODUCTION Lasers have been used to directly dull metal sheet in the automotive industry in order to improve paint adhesion and finish quality.‘v2 This generally involves the production of shallow (-5 pm) indentations 20-200 ,um in diameter in the metal surface at regular intervals (30-300 ,um apart). The present study was undertaken in order to produce a highly random laser-dulled surface and to investigate whether its optical and other physical properties could duplicate those of a sandblasted surface. Opticsand Lasers in Engineering 0143-8166/93/$0600 Ltd, England.
Printed
in Northern
Ireland
0 1993 Elsevier Science Publishers
2
D. A. Scott et al.
The work was initiated following a request by the Royal Australian Mint who were seeking to improve the productivity of one of their coin manufacture processes. The conventional technique involves sandblasting a coin die blank in certain well-defined areas. This process is labour-intensive and time-consuming. Direct laser modification of the surface was seen as a possible way of producing a dulled surface in a controllable and single-step process. It was also seen as a method by which different textured and patterned coins may be produced. The proposal was to heat a metal surface using a laser to just over its melting point and then allow it to cool rapidly by thermal conduction into the bulk of the material thereby changing its surface properties, especially its topography and reflective properties. Initial experiments concentrated on the determination of the optimum laser conditions required to produce a surface with similar optical characteristics to that of a lightly sandblasted surface (grit size 100 mesh). These conditions were then used to dull a 14 x 14 mm area on the surface of a die. The die was then employed to stamp 10 blank coins in order to investigate the performance of the dulled surface under typical coin manufacturing conditions.
EXPERIMENTS The experiments were performed on tool steel dies (1% C, 1.5% Cr) commonly used in the manufacture of proof coins. The dies were 50 mm in diameter and were mounted on the CNC table of an industrial 500-W average power Nd : YAG laser machine equipped with a lOO-mm focal length processing lens. Two types of die surface were treated: one a flat, hand-polished surface (average roughness K;, = 28 nm), on which most of the experiments were performed, and the other a convex (radius of curvature 250 mm) highly polished surface (R, = 4.6 nm). The specular reflectance of both surfaces at 1.06 pm was about 70%, and the diffuse reflectance 20% for the hand-polished die and about 0% for the highly polished die. A range of laser processing parameters and scanning patterns was examined. The laser pulse duration and pulse energy could be varied independently. The range of energies used in the experiments was from 30 mJ to 300 mJ per pulse and the range of pulse durations (full width at half-maximum, FWHM) was from 50 ps to 500 ps. The pulse repetition rate was varied from 20 Hz to 300 Hz. Three different shielding gases (oxygen, nitrogen and argon) were used to examine the effects different gases have on the dulling process.
Laser modification of metal surfaces
3
For every set of laser parameters investigated, the laser spot size was varied by changing the distance between the focusing lens and the die surface. As the power density near the focus was sufficient to drill the surface, the beam was considerably defocused to achieve the desired Typically the laser beam was focused at about surface appearance. 8-10 mm above the surface, producing a spot of about 1.5-2.0 mm in diameter depending on other laser parameters. Four types of scanning were tried: simple raster scan, chequerboard raster, and ‘on-the-fly’ versions of these two. In the case of raster scan the die was moved horizontally under the laser beam (in the xdirection) for a given distance, usually 2 mm, then the laser was moved a short distance in the y-direction, usually 0.05 mm, and the die moved in the reverse x-direction by 2 mm and again incremented in the y-direction. This process was repeated until a square of dimension 2 mm x 2 mm was marked. In a chequerboard scan the die was scanned twice, first in the standard raster pattern, then in a raster pattern at 90” to the first, and over the same area. In all of these types of scan, the relative speed of the die and the laser beam is not constant. The table that holds the die has significant inertia which necessitates a deceleration and acceleration of the scanning beam relative to the die at each end of the raster scan. To assess the effects of this, an ‘on-the-fly’ version of the above types of scan was tried. Also, on-the-fly dulling is the preferred option for die dulling in a production environment. To achieve this the laser was raster scanned over a much larger area of 6 mm x 2 mm, with the laser only allowed to impinge on the die surface on the central 2 mm x 2 mm, by using a shutter to block the beam on the two end sections. This allowed the table holding the die to decelerate, increment in the y-direction, and accelerate back to the set scan speed before the shutter was opened to dull the required area. Both raster and chequerboard (with an overlapping scan at 9W) versions of this type of scan were tried. The optical characteristics of the dulled surface were measured using a CARY spectrophotometer equipped with a Halon D76 reference source. Hardness measurements were made using a standard Vickers microhardness tester (up to loads of 5000 mN) and a UMIS 2000 ultramicrohardness machine (up to 500 mN). After obtaining a surface with suitable optical characteristics, two hand-polished dies were laser dulled for evaluation by the Royal Australian Mint. A 14 mm x 14 mm square was dulled on each of the dies, with the focusing conditions for each square being slightly different. One die was marked with the surface 9.8 mm from the focal
D. A. Scott et al.
4
point of the laser (die A), the other at 10.0 mm from the focus (die B) in order to investigate the effects different surfaces have on the stamping process. The total marking time for this area using a single raster scan was about 7 min. The dies and the trial blanks made from them were examined under a scanning electron microscope and an optical microscope to assess the quality and reproducibility of the stamped blanks and the wear on the dies themselves. Ten blanks were stamped with each of the dies. The dies were examined in the same position before and after the trials to assess wear on particular features. Surface roughness measurements were made on the dies and blanks using a Talysurf 5M diamond stylus machine.
RESULTS Laser dulled surface It was possible to generate a wide range of surfaces using this technique. In general, as the power density at the die’s surface was varied from 500 W mm-* to 240 W mm-*, the resulting surface exhibited the following characteristics: (i) (ii) i’“,’ (:‘,
large gl o b u 1es with a thick, uneven resolidified background, smaller globules, with large, smooth tessellations, similar globules, smaller tessellations, no globules, smaller tessellations, the appearance of crumpled paper.
These five types of surfaces are illustrated in Fig. I. Figure 2 shows the standard sandblasted surface whose properties these experiments attempted to duplicate. It was found that very similar marks could be made under conditions of constant peak power, while the pulse durations were varied over a wide range (100-500 ps). It was the peak power per unit area (power density) which largely determined the type of surface generated-not the pulse energy and pulse duration independently. A critical parameter for achieving the desired surface properties was the laser spot size on the target. By using a series of different sized apertures it was found that only the central 0.4 mm of the beam (called the laser affected zone or LAZ) affected the surface in a significant manner. It was found that only the central 0.10-0.15 mm of the beam actually melts the surface.
Laser modification
(4
(e)
Fig. 1. Scanning electron microscope photographs of a die’s surface after laser dulling using incident power densities of (a) 500, (b) 425, (c) 340, (d) 280 and (e) 240 W mm-’ respectively.
It was observed passes (about 4 to brightness of the intensity. The number of
that at the beginning of a raster scan it took several 8 depending on laser parameters) before the visible laser spot on the die surface reached a steady passes
(separated
by 0.05 mm)
required
to reach
D. A. Scott et al.
Fig. 2.
Photograph
of ‘standard’
sandblasted surface Australian Mint.
presently
in use at the Royal
‘equilibrium’ in the spot brightness, and therefore the dulling action itself, supports the view that the laser affected zone, while not melting the surface, has an effect on it. The authors believe that this laser affected zone ‘prepares’ the surface prior to being exposed to the full power density of the laser at the centre of the beam. The nature of this preparation is not clear. This effective spot size, however, has important implications for the resolution with which dulling can be performed, as discussed below. With the other laser parameters fixed, increasing the repetition rate increased the average power of the laser and therefore the energy delivered to the die surface in a given amount of time. This had an effect similar to that of increasing the energy per pulse, namely globule and crack formation. At a given repetition rate of the laser there was a maximum scan speed above which no significant dulling of the surface occurred. For example, at a repetition rate of 100Hz the maximum scan speed to obtain dulling was about TO mm/s; at 12.5 mm/s only discolouration of the surface occurred. Experiments were performed to determine whether the same surface could be obtained at higher repetition rates and commensurately higher scan speeds. It was found that although the same amount of energy was delivered to the surface over the same area and in the same number of pulses, the surface so produced was different. For example, dulling the die with 100 pulses per second at a scan rate of 10 mm/s gave different results to dulling a die with 300 pulses per second at a scan rate of 30 mm/s. The authors believe this is due to the thermal conduction of the die itself. As it takes a finite amount of time for the volume of the die heated by the laser pulse to
Laser modification
of metal surfaces
7
then if the scan speed of the laser is increased (along with the frequency) each laser pulse sees a surface slightly hotter than it would at the slower speed. (Note that the pulses overlap by about 75%.) As the temperature and cooling rate of the surface affects its absorption at the laser wavelength, therefore the amount of energy it will absorb and the maximum temperature it will reach are critically affected. The two different ways of scanning, raster and on-the-fly, produced surface finishes very similar to each other under the same operating conditions. The only slight difference between them was the edge quality between the dulled and non-dulled areas. When the normal raster scan was used, the edge showed evidence of a slightly higher energy input because of the longer dwell time, and the spatial definition of the edge is slightly better than in the case of on-the-fly scanning. However, these differences are small and demonstrate that it is feasible to use either scanning technique to dull the surface. Experiments using different shroud gases showed that oxygen was unsatisfactory as it promoted combustion and resulted in an uneven, oxidised surface. Little visible difference could be seen between the use of argon and nitrogen. As nitrogen is much less expensive it was used in most of the experiments. Also it may aid the formation of nitrides during the dulling process, thereby enhancing the hardness of the dulled surface. Attempts at measuring the hardness of the laser-dulled or sandblasted surfaces did not yield meaningful results due to the surface roughness. Achieving a regular indentor impression using either the UMIS-2000 (up to loads of 500 mN) or the Vickers microhardness tester (up to 5000 mN) was not possible. This is not a suitable technique for measuring the surface hardness: to obtain a regular and measurable indent, a high force is required (greater than 5000 mN) resulting in a measure of the bulk hardness of the die rather than the surface hardness. However, even from the highly scattered data, it appears that the hardness has certainly not decreased as a result of the dulling process. Experiments were performed on highly polished convex dies using the best operating conditions for laser dulling hand-polished dies. However, it was found that these conditions were not appropriate for highly polished dies. This may be attributed to the different reflective qualities of the die surfaces at the laser wavelength, causing a weaker dulling effect on the highly polished surface. Additionally, it was found that the die was very sensitive to small changes in the laser power density at the surface. A change in the distance between the focusing lens and the die surface of only O-1 mm was sufficient to change the character of the dulling visibly. cool,
D. A. Scott et al.
Fig. 3.
Photograph
of the surface
structure of die A, showing 10 Mm diameter.
globules
approximately
Stamping trials Stamping following (i) (ii) (iii)
trials were carried laser parameters:
out with dies A and B dulled
using the
pulse repetition rate = 100 Hz pulse duration = O-3 ms pulse energy = 150 mJ
Their surfaces are illustrated in Figs 3 and 4 respectively. The major differences between these two surfaces are the presence of resolidified globules and deeper cracks between tessellations on the die A. The
Fig. 4.
Photograph
of the surface
structure
of die B, showing
no globules.
9
Laser modification of metal surfaces
possible movement of the globules during stamping was one of the main reasons for producing these two surfaces. Measurements of the surface reflectivity of the dulled areas indicated that, firstly there was very little difference between the diffuse and specular reflectance for all three surfaces tested: die A, die B and the sandblasted surface currently used by the Royal Australian Mint. Secondly, the reflectances of all the surfaces were approximately independent of wavelength over the range 400-750 nm. Thirdly, reflectances of the sandblasted surface, die A and die B were about 24%, 51% and 59% respectively. As expected, the laser dulled surfaces are ‘shinier’ than the sandblasted surface, and die B (which had no surface globules) was more reflective than die A (with surface globules). Both dies exhibited wear after the trial stampings. In the case of die B this wear was exhibited as the formation of small particles, less than 10 ,um in diameter, especially noticeable near the edges of the dulled area. This appears to be the only form of wear shown by this die, except for particularly protuberant features which were partially destroyed after stamping. Figure 5(a) and (b) show this die’s surface before and after stamping respectively. Less wear is exhibited by the die in the central portion of the dulled area. In the case of die A, a marked change in surface appearance was observed under the SEM. The surface globules, evident in the die before stamping (Fig. 6(a)) are removed during stamping (Fig. 6(b)) and, similar to die B, more particles of about micron size were evident near the edges of the dulled area. The nodules that form the tessellated pattern appear damaged, either crushed or exfoliated.
(4 Fig. 5.
Surface
@I of die B (a) before
stamping
trials and (b) after stamping
trials.
Fig. 6.
Surface
of die A (a) before
stamping
trials and (b) after stamping
trials.
Inspection of the blanks stamped from these dies revealed that the stamping process could not fully duplicate the laser dulled surface. This is to be expected and is probably also the case when sandblasted surfaces are used. As the blank metal is not perfectly plastic, the cracks in the die’s surface between tessellations will not be replicated below a certain width. The surface roughness profiles of the standard sandblasted die surface and the laser dulled surface (die B) are shown in Fig. 7. The
DIE B
SANDBLAST
Fig. 7.
Roughness
profiles,
Ra = 0.46 pm
Ra = 0.33 pm
as measured by the Talysurf 5M machine. ‘standard’ sandblasted surface.
of die B and the
Laser modification of metal surfaces
11
sandblasted surface usually used by the Royal Australian Mint has a roughness of about 0.33 pm = R,.Before use in the stamping trials the values for the two dies were 1.17 pm and O-49 pm for the dies with and without globules respectively. After use in the trials these values were 0.90 pm and 0.46 pm respectively. The measured roughness of the blanks was the same as that of the die after stamping both for the first and the tenth blanks. This indicates that the smoother of the dies (B) suffered very little damage during stamping whereas the rougher of the dies (A) had many of the high points, such as the globules on the surface, removed or reduced. This must happen during the first stamping as the first and tenth blanks were the same in terms of the roughness parameter. It also indicates that this single roughness index is inadequate for the task of measuring the detailed surface parameters. This conclusion is supported by the fact that although the sandblasted surface had a lower roughness index than the two laser-dulled dies, its reflectance was less than half the reflectance of the laser-dulled dies. Clearly the Talysurf diamond stylus itself, which has an included angle of 90”, cannot give a true measure of the surface imperfections which have a depth to width ratio of greater than O-5.
DISCUSSION
OF LASER
DULLING
APPLICATION
As described earlier, it was determined that the laser spot size during marking was between 1.5 and 2-O mm in diameter, with only the central 0.4 mm approximately contributing to marking the die surface. As a consequence every laser dulled area has a zone around it of dimension about O-2 mm, which is not uniformly dulled. This effective resolution of the laser dulling experiments is unacceptable to the present application of the Royal Australian Mint because the edge quality needs to be more defined and moreover there are sections of some dies of only 0.05 mm in width which would require dulling. For this particular application the dulling process would have to be modified. Firstly, a uniform intensity across the beam diameter would be desirable. This may be achieved by passing the laser beam through a stepped-index optical fibre which ‘scrambles’ the beam and results in a relatively uniform intensity beam profile. This technique would reduce the problem of non-uniform heating and marking of the die surface, but would limit the minimum size of the laser spot that could be used. Secondly, the beam diameter at the die’s surface should be reduced. This can be done by performing the dulling with the die surface near
12
D. A. Scott et al.
the focal position of the laser, at which point the beam size would be about 10 times smaller, i.e. about 0.15 mm. Consequently the effective resolution of the dulling system could be reduced to about 0.02 mm; still not small enough for the present application. The beam intensity would also need to be reduced by a factor of about 100 to keep the intensity of the laser beam at the low level required for surface dulling. However, if the beam area is reduced by a factor of 100 (or preferably more) the time required to dull a given area would be increased by the same factor. A high repetition rate laser could be used but, as the experiments have shown, increasing the repetition rate and the scan speed by the same factor do not generate the same dulling effect. It is by no means clear that appropriate parameters could be found for dulling at this speed and it is likely that the thermal properties of the die metal may limit the speed at which the die can be dulled. These experiments have also shown the sensitive nature of the laser beam interaction with a highly polished surface compared with a hand-polished surface, and in this application a mixture of these surfaces would be encountered on a single die. If the laser dulling process is to be uniform, the laser beam must interact with the surface in exactly the same way, regardless of the surface’s pre-dulled state. To make both the highly-polished and hand-polished dies appear the same to the laser, the whole die would have to be treated in some way, perhaps coated or laser pre-treated. This is challenging as it is well known’ that the damage threshold, i.e. the laser intensity above which the surface is modified, is proportional to a-“’ where o is the RMS surface roughness. This means that it is primarily the physical roughness that has to be uniform before the laser can interact consistently with the surface.
CONCLUSIONS These experiments have demonstrated that laser dulling is capable of generating a wide variety of surfaces. Randomisation of the surface roughness has been achieved without beam scrambling as a direct consequence of the type of heating, melting and resolidification caused by the laser pulses. The slight appearance of the scan pattern when a single raster is used is not considered a problem and could be eliminated using a chequerboard scan if a uniform roughness is required. Although problems were found in applying this technique to the the dulling process outlined in Royal Australian Mint’s application,
Laser modification of metal surfaces
13
these experiments has potential applications in areas such as small-area dulling of metallic components, or in the dulling of dies for both metallic and plastic mouldings. It offers significant advantages to conventional techniques, including a high degree of controllability, no need for masking, no mess (such as in sandblasting), small and/or complex parts or areas can be processed as well as the production of not only patterned surfaces, but also ones of varying texture and roughness.
REFERENCES 1. Minamida, K., Suehiro, J., Toshimitu, T. & Kawamoto, T., Laser system for dulling work roll by Q-switched Nd: YAG laser, J. of Laser Applications, l(4) (1989) G-20. 2. Snaith, B., Probert, S. D. & Pearce, R., Characterization cold-rolled steel sheets, Wear, 109 (1986) 87-97.
of laser-textured
3. House, R. A., Bettis, J. R. & Guenther, A. H., Surface roughness and laser damage threshold, IEEE J. of Quantum Electronics, QE-13(5) (1977) 361-3.
18 (1993) 1-13
Laser Modification D. A. Scott, CSIRO
Division
of Metal Surfaces
M. Brandt,
B. Dorien-Brown
of Manufacturing Technology, Industrial Laser Centre, Lindfield, New South Wales 2070, Australia
B. Valentine Royal Australian
Mint, Canberra,
(Received
& P. De
Australian
6 March
PO Box 218,
Capital
1992; accepted
Territory
2601, Australia
10 April 1992)
ABSTRACT A series of experiments
has been undertaken to investigate the feasibility of using a Nd: YAG laser to produce a highly random surface on steel, which could be used as a finished, textured surface itself, or as a master for a stamping process. The primary aim of the experiments was to duplicate the appearance of a lightly sandblasted surface, but with more control of the surface properties. Although the two techniques are very different, sandblasting being a mechanical process and laser dulling being a thermal process, it was found that a surface of similar roughness and reflective qualities could be achieved. The technique was employed to laser dull a die used in the manufacture of proof coins and the results of trial stampings with this die are discussed.
INTRODUCTION Lasers have been used to directly dull metal sheet in the automotive industry in order to improve paint adhesion and finish quality.‘v2 This generally involves the production of shallow (-5 pm) indentations 20-200 ,um in diameter in the metal surface at regular intervals (30-300 ,um apart). The present study was undertaken in order to produce a highly random laser-dulled surface and to investigate whether its optical and other physical properties could duplicate those of a sandblasted surface. Opticsand Lasers in Engineering 0143-8166/93/$0600 Ltd, England.
Printed
in Northern
Ireland
0 1993 Elsevier Science Publishers
2
D. A. Scott et al.
The work was initiated following a request by the Royal Australian Mint who were seeking to improve the productivity of one of their coin manufacture processes. The conventional technique involves sandblasting a coin die blank in certain well-defined areas. This process is labour-intensive and time-consuming. Direct laser modification of the surface was seen as a possible way of producing a dulled surface in a controllable and single-step process. It was also seen as a method by which different textured and patterned coins may be produced. The proposal was to heat a metal surface using a laser to just over its melting point and then allow it to cool rapidly by thermal conduction into the bulk of the material thereby changing its surface properties, especially its topography and reflective properties. Initial experiments concentrated on the determination of the optimum laser conditions required to produce a surface with similar optical characteristics to that of a lightly sandblasted surface (grit size 100 mesh). These conditions were then used to dull a 14 x 14 mm area on the surface of a die. The die was then employed to stamp 10 blank coins in order to investigate the performance of the dulled surface under typical coin manufacturing conditions.
EXPERIMENTS The experiments were performed on tool steel dies (1% C, 1.5% Cr) commonly used in the manufacture of proof coins. The dies were 50 mm in diameter and were mounted on the CNC table of an industrial 500-W average power Nd : YAG laser machine equipped with a lOO-mm focal length processing lens. Two types of die surface were treated: one a flat, hand-polished surface (average roughness K;, = 28 nm), on which most of the experiments were performed, and the other a convex (radius of curvature 250 mm) highly polished surface (R, = 4.6 nm). The specular reflectance of both surfaces at 1.06 pm was about 70%, and the diffuse reflectance 20% for the hand-polished die and about 0% for the highly polished die. A range of laser processing parameters and scanning patterns was examined. The laser pulse duration and pulse energy could be varied independently. The range of energies used in the experiments was from 30 mJ to 300 mJ per pulse and the range of pulse durations (full width at half-maximum, FWHM) was from 50 ps to 500 ps. The pulse repetition rate was varied from 20 Hz to 300 Hz. Three different shielding gases (oxygen, nitrogen and argon) were used to examine the effects different gases have on the dulling process.
Laser modification of metal surfaces
3
For every set of laser parameters investigated, the laser spot size was varied by changing the distance between the focusing lens and the die surface. As the power density near the focus was sufficient to drill the surface, the beam was considerably defocused to achieve the desired Typically the laser beam was focused at about surface appearance. 8-10 mm above the surface, producing a spot of about 1.5-2.0 mm in diameter depending on other laser parameters. Four types of scanning were tried: simple raster scan, chequerboard raster, and ‘on-the-fly’ versions of these two. In the case of raster scan the die was moved horizontally under the laser beam (in the xdirection) for a given distance, usually 2 mm, then the laser was moved a short distance in the y-direction, usually 0.05 mm, and the die moved in the reverse x-direction by 2 mm and again incremented in the y-direction. This process was repeated until a square of dimension 2 mm x 2 mm was marked. In a chequerboard scan the die was scanned twice, first in the standard raster pattern, then in a raster pattern at 90” to the first, and over the same area. In all of these types of scan, the relative speed of the die and the laser beam is not constant. The table that holds the die has significant inertia which necessitates a deceleration and acceleration of the scanning beam relative to the die at each end of the raster scan. To assess the effects of this, an ‘on-the-fly’ version of the above types of scan was tried. Also, on-the-fly dulling is the preferred option for die dulling in a production environment. To achieve this the laser was raster scanned over a much larger area of 6 mm x 2 mm, with the laser only allowed to impinge on the die surface on the central 2 mm x 2 mm, by using a shutter to block the beam on the two end sections. This allowed the table holding the die to decelerate, increment in the y-direction, and accelerate back to the set scan speed before the shutter was opened to dull the required area. Both raster and chequerboard (with an overlapping scan at 9W) versions of this type of scan were tried. The optical characteristics of the dulled surface were measured using a CARY spectrophotometer equipped with a Halon D76 reference source. Hardness measurements were made using a standard Vickers microhardness tester (up to loads of 5000 mN) and a UMIS 2000 ultramicrohardness machine (up to 500 mN). After obtaining a surface with suitable optical characteristics, two hand-polished dies were laser dulled for evaluation by the Royal Australian Mint. A 14 mm x 14 mm square was dulled on each of the dies, with the focusing conditions for each square being slightly different. One die was marked with the surface 9.8 mm from the focal
D. A. Scott et al.
4
point of the laser (die A), the other at 10.0 mm from the focus (die B) in order to investigate the effects different surfaces have on the stamping process. The total marking time for this area using a single raster scan was about 7 min. The dies and the trial blanks made from them were examined under a scanning electron microscope and an optical microscope to assess the quality and reproducibility of the stamped blanks and the wear on the dies themselves. Ten blanks were stamped with each of the dies. The dies were examined in the same position before and after the trials to assess wear on particular features. Surface roughness measurements were made on the dies and blanks using a Talysurf 5M diamond stylus machine.
RESULTS Laser dulled surface It was possible to generate a wide range of surfaces using this technique. In general, as the power density at the die’s surface was varied from 500 W mm-* to 240 W mm-*, the resulting surface exhibited the following characteristics: (i) (ii) i’“,’ (:‘,
large gl o b u 1es with a thick, uneven resolidified background, smaller globules, with large, smooth tessellations, similar globules, smaller tessellations, no globules, smaller tessellations, the appearance of crumpled paper.
These five types of surfaces are illustrated in Fig. I. Figure 2 shows the standard sandblasted surface whose properties these experiments attempted to duplicate. It was found that very similar marks could be made under conditions of constant peak power, while the pulse durations were varied over a wide range (100-500 ps). It was the peak power per unit area (power density) which largely determined the type of surface generated-not the pulse energy and pulse duration independently. A critical parameter for achieving the desired surface properties was the laser spot size on the target. By using a series of different sized apertures it was found that only the central 0.4 mm of the beam (called the laser affected zone or LAZ) affected the surface in a significant manner. It was found that only the central 0.10-0.15 mm of the beam actually melts the surface.
Laser modification
(4
(e)
Fig. 1. Scanning electron microscope photographs of a die’s surface after laser dulling using incident power densities of (a) 500, (b) 425, (c) 340, (d) 280 and (e) 240 W mm-’ respectively.
It was observed passes (about 4 to brightness of the intensity. The number of
that at the beginning of a raster scan it took several 8 depending on laser parameters) before the visible laser spot on the die surface reached a steady passes
(separated
by 0.05 mm)
required
to reach
D. A. Scott et al.
Fig. 2.
Photograph
of ‘standard’
sandblasted surface Australian Mint.
presently
in use at the Royal
‘equilibrium’ in the spot brightness, and therefore the dulling action itself, supports the view that the laser affected zone, while not melting the surface, has an effect on it. The authors believe that this laser affected zone ‘prepares’ the surface prior to being exposed to the full power density of the laser at the centre of the beam. The nature of this preparation is not clear. This effective spot size, however, has important implications for the resolution with which dulling can be performed, as discussed below. With the other laser parameters fixed, increasing the repetition rate increased the average power of the laser and therefore the energy delivered to the die surface in a given amount of time. This had an effect similar to that of increasing the energy per pulse, namely globule and crack formation. At a given repetition rate of the laser there was a maximum scan speed above which no significant dulling of the surface occurred. For example, at a repetition rate of 100Hz the maximum scan speed to obtain dulling was about TO mm/s; at 12.5 mm/s only discolouration of the surface occurred. Experiments were performed to determine whether the same surface could be obtained at higher repetition rates and commensurately higher scan speeds. It was found that although the same amount of energy was delivered to the surface over the same area and in the same number of pulses, the surface so produced was different. For example, dulling the die with 100 pulses per second at a scan rate of 10 mm/s gave different results to dulling a die with 300 pulses per second at a scan rate of 30 mm/s. The authors believe this is due to the thermal conduction of the die itself. As it takes a finite amount of time for the volume of the die heated by the laser pulse to
Laser modification
of metal surfaces
7
then if the scan speed of the laser is increased (along with the frequency) each laser pulse sees a surface slightly hotter than it would at the slower speed. (Note that the pulses overlap by about 75%.) As the temperature and cooling rate of the surface affects its absorption at the laser wavelength, therefore the amount of energy it will absorb and the maximum temperature it will reach are critically affected. The two different ways of scanning, raster and on-the-fly, produced surface finishes very similar to each other under the same operating conditions. The only slight difference between them was the edge quality between the dulled and non-dulled areas. When the normal raster scan was used, the edge showed evidence of a slightly higher energy input because of the longer dwell time, and the spatial definition of the edge is slightly better than in the case of on-the-fly scanning. However, these differences are small and demonstrate that it is feasible to use either scanning technique to dull the surface. Experiments using different shroud gases showed that oxygen was unsatisfactory as it promoted combustion and resulted in an uneven, oxidised surface. Little visible difference could be seen between the use of argon and nitrogen. As nitrogen is much less expensive it was used in most of the experiments. Also it may aid the formation of nitrides during the dulling process, thereby enhancing the hardness of the dulled surface. Attempts at measuring the hardness of the laser-dulled or sandblasted surfaces did not yield meaningful results due to the surface roughness. Achieving a regular indentor impression using either the UMIS-2000 (up to loads of 500 mN) or the Vickers microhardness tester (up to 5000 mN) was not possible. This is not a suitable technique for measuring the surface hardness: to obtain a regular and measurable indent, a high force is required (greater than 5000 mN) resulting in a measure of the bulk hardness of the die rather than the surface hardness. However, even from the highly scattered data, it appears that the hardness has certainly not decreased as a result of the dulling process. Experiments were performed on highly polished convex dies using the best operating conditions for laser dulling hand-polished dies. However, it was found that these conditions were not appropriate for highly polished dies. This may be attributed to the different reflective qualities of the die surfaces at the laser wavelength, causing a weaker dulling effect on the highly polished surface. Additionally, it was found that the die was very sensitive to small changes in the laser power density at the surface. A change in the distance between the focusing lens and the die surface of only O-1 mm was sufficient to change the character of the dulling visibly. cool,
D. A. Scott et al.
Fig. 3.
Photograph
of the surface
structure of die A, showing 10 Mm diameter.
globules
approximately
Stamping trials Stamping following (i) (ii) (iii)
trials were carried laser parameters:
out with dies A and B dulled
using the
pulse repetition rate = 100 Hz pulse duration = O-3 ms pulse energy = 150 mJ
Their surfaces are illustrated in Figs 3 and 4 respectively. The major differences between these two surfaces are the presence of resolidified globules and deeper cracks between tessellations on the die A. The
Fig. 4.
Photograph
of the surface
structure
of die B, showing
no globules.
9
Laser modification of metal surfaces
possible movement of the globules during stamping was one of the main reasons for producing these two surfaces. Measurements of the surface reflectivity of the dulled areas indicated that, firstly there was very little difference between the diffuse and specular reflectance for all three surfaces tested: die A, die B and the sandblasted surface currently used by the Royal Australian Mint. Secondly, the reflectances of all the surfaces were approximately independent of wavelength over the range 400-750 nm. Thirdly, reflectances of the sandblasted surface, die A and die B were about 24%, 51% and 59% respectively. As expected, the laser dulled surfaces are ‘shinier’ than the sandblasted surface, and die B (which had no surface globules) was more reflective than die A (with surface globules). Both dies exhibited wear after the trial stampings. In the case of die B this wear was exhibited as the formation of small particles, less than 10 ,um in diameter, especially noticeable near the edges of the dulled area. This appears to be the only form of wear shown by this die, except for particularly protuberant features which were partially destroyed after stamping. Figure 5(a) and (b) show this die’s surface before and after stamping respectively. Less wear is exhibited by the die in the central portion of the dulled area. In the case of die A, a marked change in surface appearance was observed under the SEM. The surface globules, evident in the die before stamping (Fig. 6(a)) are removed during stamping (Fig. 6(b)) and, similar to die B, more particles of about micron size were evident near the edges of the dulled area. The nodules that form the tessellated pattern appear damaged, either crushed or exfoliated.
(4 Fig. 5.
Surface
@I of die B (a) before
stamping
trials and (b) after stamping
trials.
Fig. 6.
Surface
of die A (a) before
stamping
trials and (b) after stamping
trials.
Inspection of the blanks stamped from these dies revealed that the stamping process could not fully duplicate the laser dulled surface. This is to be expected and is probably also the case when sandblasted surfaces are used. As the blank metal is not perfectly plastic, the cracks in the die’s surface between tessellations will not be replicated below a certain width. The surface roughness profiles of the standard sandblasted die surface and the laser dulled surface (die B) are shown in Fig. 7. The
DIE B
SANDBLAST
Fig. 7.
Roughness
profiles,
Ra = 0.46 pm
Ra = 0.33 pm
as measured by the Talysurf 5M machine. ‘standard’ sandblasted surface.
of die B and the
Laser modification of metal surfaces
11
sandblasted surface usually used by the Royal Australian Mint has a roughness of about 0.33 pm = R,.Before use in the stamping trials the values for the two dies were 1.17 pm and O-49 pm for the dies with and without globules respectively. After use in the trials these values were 0.90 pm and 0.46 pm respectively. The measured roughness of the blanks was the same as that of the die after stamping both for the first and the tenth blanks. This indicates that the smoother of the dies (B) suffered very little damage during stamping whereas the rougher of the dies (A) had many of the high points, such as the globules on the surface, removed or reduced. This must happen during the first stamping as the first and tenth blanks were the same in terms of the roughness parameter. It also indicates that this single roughness index is inadequate for the task of measuring the detailed surface parameters. This conclusion is supported by the fact that although the sandblasted surface had a lower roughness index than the two laser-dulled dies, its reflectance was less than half the reflectance of the laser-dulled dies. Clearly the Talysurf diamond stylus itself, which has an included angle of 90”, cannot give a true measure of the surface imperfections which have a depth to width ratio of greater than O-5.
DISCUSSION
OF LASER
DULLING
APPLICATION
As described earlier, it was determined that the laser spot size during marking was between 1.5 and 2-O mm in diameter, with only the central 0.4 mm approximately contributing to marking the die surface. As a consequence every laser dulled area has a zone around it of dimension about O-2 mm, which is not uniformly dulled. This effective resolution of the laser dulling experiments is unacceptable to the present application of the Royal Australian Mint because the edge quality needs to be more defined and moreover there are sections of some dies of only 0.05 mm in width which would require dulling. For this particular application the dulling process would have to be modified. Firstly, a uniform intensity across the beam diameter would be desirable. This may be achieved by passing the laser beam through a stepped-index optical fibre which ‘scrambles’ the beam and results in a relatively uniform intensity beam profile. This technique would reduce the problem of non-uniform heating and marking of the die surface, but would limit the minimum size of the laser spot that could be used. Secondly, the beam diameter at the die’s surface should be reduced. This can be done by performing the dulling with the die surface near
12
D. A. Scott et al.
the focal position of the laser, at which point the beam size would be about 10 times smaller, i.e. about 0.15 mm. Consequently the effective resolution of the dulling system could be reduced to about 0.02 mm; still not small enough for the present application. The beam intensity would also need to be reduced by a factor of about 100 to keep the intensity of the laser beam at the low level required for surface dulling. However, if the beam area is reduced by a factor of 100 (or preferably more) the time required to dull a given area would be increased by the same factor. A high repetition rate laser could be used but, as the experiments have shown, increasing the repetition rate and the scan speed by the same factor do not generate the same dulling effect. It is by no means clear that appropriate parameters could be found for dulling at this speed and it is likely that the thermal properties of the die metal may limit the speed at which the die can be dulled. These experiments have also shown the sensitive nature of the laser beam interaction with a highly polished surface compared with a hand-polished surface, and in this application a mixture of these surfaces would be encountered on a single die. If the laser dulling process is to be uniform, the laser beam must interact with the surface in exactly the same way, regardless of the surface’s pre-dulled state. To make both the highly-polished and hand-polished dies appear the same to the laser, the whole die would have to be treated in some way, perhaps coated or laser pre-treated. This is challenging as it is well known’ that the damage threshold, i.e. the laser intensity above which the surface is modified, is proportional to a-“’ where o is the RMS surface roughness. This means that it is primarily the physical roughness that has to be uniform before the laser can interact consistently with the surface.
CONCLUSIONS These experiments have demonstrated that laser dulling is capable of generating a wide variety of surfaces. Randomisation of the surface roughness has been achieved without beam scrambling as a direct consequence of the type of heating, melting and resolidification caused by the laser pulses. The slight appearance of the scan pattern when a single raster is used is not considered a problem and could be eliminated using a chequerboard scan if a uniform roughness is required. Although problems were found in applying this technique to the the dulling process outlined in Royal Australian Mint’s application,
Laser modification of metal surfaces
13
these experiments has potential applications in areas such as small-area dulling of metallic components, or in the dulling of dies for both metallic and plastic mouldings. It offers significant advantages to conventional techniques, including a high degree of controllability, no need for masking, no mess (such as in sandblasting), small and/or complex parts or areas can be processed as well as the production of not only patterned surfaces, but also ones of varying texture and roughness.
REFERENCES 1. Minamida, K., Suehiro, J., Toshimitu, T. & Kawamoto, T., Laser system for dulling work roll by Q-switched Nd: YAG laser, J. of Laser Applications, l(4) (1989) G-20. 2. Snaith, B., Probert, S. D. & Pearce, R., Characterization cold-rolled steel sheets, Wear, 109 (1986) 87-97.
of laser-textured
3. House, R. A., Bettis, J. R. & Guenther, A. H., Surface roughness and laser damage threshold, IEEE J. of Quantum Electronics, QE-13(5) (1977) 361-3.