WO2016100361A1 - System and method for ultrasonic vibration assisted continuous wave laser surface drilling - Google Patents

Abstract

According to an embodiment, a new laser drilling process entitled "ultrasonic vibration assisted continuous wave laser drilling" of materials is disclosed. An embodiment offers significant advantages in terms of efficiency of laser drilling and metallurgical and geometric quality of the laser drilled holes in materials. An embodiment utilizes a continuous wave C0₂ laser drilling with the simultaneous application of ultrasonic vibrations. An embodiment utilizes ultrasonic vibrations at a frequency of about 20 kHz and amplitudes in the range of about 20-50 μm to facilitate the material removal/melt expulsion by droplet ejection and vertical flow of melt from the laser melted region, creating holes. Simultaneous application of ultrasonic vibrations to the workpiece during continuous wave laser surface melting results in efficient melt expulsion, creating deep craters and micro-holes. An embodiment has been determined useful for laser drilling of stainless steels.

Description

SYSTEM AND METHOD FOR ULTRASONIC VIBRATION ASSISTED CONTINUOUS WAVE LASER SURFACE DRILLING

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Application serial number 62/092,015 filed on December 15, 2014, and incorporates said provisional application by reference into this document as if fully set out at this point.

STATEMENT REGARDING FEDERAL SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with U.S. Government support under NSF Grant No. CMMI-1 149079 awarded by the National Science Foundation. The Government has certain rights in this invention.

TECHNICAL FIELD

This disclosure relates generally to drilling operations and, more particularly, to the use of pulsed lasers for drilling holes in metal and other solid materials.

BACKGROUND

Laser drilling is one of the most commonly used micromachining processes for creating high quality and high aspect-ratio holes in a range of materials including high performance alloys, ceramics, and composites [1-2]. The laser drilling process offers several advantages such as non-contact processing, excellent reproducibility, and high production rates (up to 100 holes/s). Most of the laser drilling approaches such as single pulse drilling and percussion drilling involve pulsed laser irradiation of the material substrates. The laser irradiation causes substrate melting and evaporation at the melt surface [3]. The resultant evaporation-induced recoil pressure expels the melt radially outside the hole. The melt expulsion is recognized as one of the most efficient material removal mechanisms at the lower laser powers [4-5].

In most laser drilling applications, coaxial assist gases are used to facilitate the melt expulsion, protect the surface from oxidation, and shield the focusing lens [6].

Contrary to intuition, however, the increase in assist gas pressures actually increases the laser drilling time at all laser power levels [7-8]. It has been reported that the high gas pressures form density gradient fields and change the refractive index of the medium, resulting in defocusing of the laser beam (and hence lower energy density). In addition the high efficiency of materials removal (via ablation or melt expulsion), it is important that the laser drilled holes have excellent metallurgical and geometric quality. For example, the laser drilled holes are often associated with taper, and the taper angle depends on the thickness of the work-piece [9].

Significant efforts have been made to optimize the laser focusing conditions to minimize taper of the drilled holes. It has been reported that best quality holes in terms of straightness can be obtained by positioning the laser beam waist just below the surface of the workpiece [10]. However, the defocusing of the laser beam decreases the energy density and efficiency of laser melting/melt expulsion. Also, the laser drilling is often associated with formation of resolidified droplets (spatter) or redeposition of ablated particles on the machined substrate surfaces. Recently, Zheng and Huang reported ultrasonic vibrations-assisted femtosecond machining of microholes in Nitinol substrates with an improvement in hole wall surface quality and higher hole aspect ratio [1 1]. They used femtosecond pulsed laser (Ti-Sapphire) in combination with ultrasonic vibrations (frequency: 40 kHz; amplitude: 2.5 μιη) and reported that ultrasonic vibrations facilitates the removal of ablated particles by enhancing the heat transfer of the particles (i.e. better cooling of the particles, and hence, reduced tendency of the particles to bond to the hole wall and substrate surface) [1 1].

Thus, what is needed is a system and method of continuous wave laser drilling which does not suffer from the disadvantages of prior art approaches.

Before proceeding to a description of the present invention, however, it should be noted and remembered that the description of the invention which follows, together with the accompanying drawings, should not be construed as limiting the invention to the examples (or embodiments) shown and described. This is so because those skilled in the art to which the invention pertains will be able to devise other forms of this invention within the ambit of the appended claims.

SUMMARY OF THE INVENTION

According to an embodiment, a new laser drilling process entitled "ultrasonic vibration assisted continuous wave laser drilling" of materials is disclosed. An

embodiment offers significant advantages in terms of efficiency of laser drilling and metallurgical and geometric quality of the laser drilled holes in materials. An embodiment utilizes a continuous wave C0₂ laser drilling with the simultaneous application of ultrasonic vibrations. In an embodiment, ultrasonic vibration-assisted laser surface melting of austenitic stainless steel (AISI 316) is performed. While the application of ultrasonic vibrations during laser processing delays the laser interaction with material due to enhancement of surface convection, it resulted in expulsion of melt from the irradiated region (forming craters) and transition from columnar to equiaxed dendritic grain structure in the resolidified melt films. Systematic investigations on the effect of ultrasonic vibrations (with vibrations frequency of 20 kHz and power output in the range of 20-40%) on the development of microstructure during laser surface melting (with laser power of 900 W and irradiation time in the range of 0.30-0.45 s) are disclosed herein. The results indicate that the proposed ultrasonic vibration-assisted laser processing can be designed for efficient material removal (laser machining) and improved equiaxed microstructure (laser surface modifications) during materials processing.

A particular embodiment utilizes ultrasonic vibrations of a frequency of 20 kHz and amplitudes in the range of about 20-50 μιη to facilitate the material removal/melt expulsion by droplet ejection and vertical flow of melt from the laser melted region, creating holes. Simultaneous application of ultrasonic vibrations to the workpiece during continuous wave laser surface melting results in efficient melt expulsion, creating deep craters and micro-holes. An embodiment has been determined useful for laser drilling of stainless steels.

According to an embodiment there is provided an apparatus for laser drilling into a sample, comprising: a laser positionable to drill into the sample when activated; and, an ultrasonic generator positionable to be in mechanical communication with the sample during activation of said laser, said ultrasonic generator vibrating the sample at an ultrasonic frequency when so activated.

According to another embodiment there is provided a method of laser drilling into a sample, comprising the steps of: placing the sample in mechanical communication with an ultrasonic generator, said ultrasonic generator at least for vibrating the sample at an ultrasonic frequency when activated; positioning a laser to drill into the sample when activated; activating said laser to initiate the drilling into the sample; and, while said laser is activated, activating said ultrasonic generator, thereby vibrating the sample at an ultrasonic frequency while the sample is being drilled. According to still another embodiment, there is provided an apparatus for laser drilling into a sample, comprising: a sample support member at least for supporting a sample; an ultrasonic generator in mechanical communication with said sample support member, said ultrasonic generator at least for providing an ultrasonic vibration to the sample; and a laser drill operable to drill into the sample while the ultrasonic generator vibrates the sample at an ultrasonic frequency.

The foregoing has outlined in broad terms some of the more important features of the invention disclosed herein so that the detailed description that follows may be more clearly understood, and so that the contribution of the instant inventors to the art may be better appreciated. The instant invention is not to be limited in its application to the details of the construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. Rather, the invention is capable of other embodiments and of being practiced and carried out in various other ways not specifically enumerated herein. Finally, it should be understood that the pliraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting, unless the specification specifically so limits the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and further aspects of the invention are described in detail in the following examples and accompanying drawings.

Figure 1 contains a schematic illustration of the set-up for ultrasonic vibration- assisted continuous wave laser drilling of materials of an embodiment.

Figure 2 contains an illustration of resolidified melt film thickness with ultrasonic power output (laser irradiation time of 0.45 s) for an embodiment.

Figure 3 contains schematics showing stages leading to formation of a crater during ultrasonic vibration-assisted laser surface melting for various embodiments. .

Figure 4 contains a chart of variation of hole diameter, depth, and aspect ratio with ultrasonic power output (laser power: 900 W; laser irradiation time: 0.45 s; laser head to sample distance: 1.5 cm) according to an embodiment.

Figure 5 contains another schematic illustration of an embodiment.

Figure 6 contains plots of vibration displacements for different ultrasonic power outputs for an embodiment. Figure 7 contains XRD (x-ray diffraction) patterns of untreated/base AISI 316 steel sample, and samples laser melted without and with ultrasonic vibrations for power outputs of 20 %, 30%, and 40% (laser irradiation time of 0.45 s).

Figure 8 contains variations of (a) diameter and (b) depth of craters for ultrasonic vibration-assisted laser surface melting with ultrasonic power output for irradiation times of 0.35 and 0.45 s. according to one embodiment.

Figure 9 contains schematic representations of an embodiment showing: (a) evolution of depth of laser melted pool without ultrasonic vibrations for shorter irradiation time; (b) laser surface heating with ultrasonic vibrations for shorter irradiation time; (c) laser surface melting and melt expulsion with ultrasonic vibrations for longer irradiation time; and (d) laser surface melting and melt expulsion with ultrasonic vibrations of higher amplitude for longer irradiation time.

DETAILED DESCRIPTION

While this invention is susceptible of embodiment in many different forms, there is shown in the drawings, and will herein be described hereinafter in detail, some specific embodiments of the instant invention. It should be understood, however, that the present disclosure is to be considered an exemplification of the principles of the invention and is not intended to limit the invention to the specific embodiments or algorithms so described.

According to an embodiment, there is provided a novel ultrasonic vibration assisted continuous wave C0₂ laser surface drilling of austenitic stainless steel. Of course, a C0₂ laser is just one example of the sort of laser that could be used in connection with various embodiments. Other laser types suitable for use with an aspect include, without limitation, solid-state lasers, gas lasers, and fiber lasers. Similarly, those of ordinary skill in the art will recognize that, although an embodiment discussed herein is illustrated as drilling into a stainless steel substrate material, most of the metallic substrates would be suitable including, without limitation, other steels, aluminum alloys, titanium alloys, magnesium alloys, etc.

To investigate laser surface melting with the application of ultrasonic vibrations, experiments were conducted with a number of different laser irradiation times. It was considered that, even with the enhanced surface convection, these laser irradiation times would be long enough to raise the surface temperature above melting point (~ 1375 X ) of the steel samples. Detailed discussion on the development of surface microstructure with and without application of ultrasonic vibrations for different laser irradiation times follow. Procedure

A schematic of the set-up for ultrasonic vibration-assisted laser surface processing according to certain embodiments are shown in Figures 1 and 5. The set-up consisted of a sample support member comprising threaded titanium alloy probe (horn) of 13 mm diameter and 139 mm length. The ultrasonic power supply delivered the power output of 750 W at a fixed frequency of 20 kHz (Sonics & Materials, Inc, Newtown, CT). AISI 316 austenitic stainless steel (17.45% Cr, 1 1.81% Ni, 2.5% Mo, 0.05% C, 1.35% Mn, 0.68% Si, 0.01 1 % S, 0.047% P, and balance Fe by weight) tips of 2 mm thickness were used as specimens for ultrasonic vibration-assisted laser surface processing. The surfaces of the steel specimens were polished with SiC paper (1200 grit) followed by cloth polishing with alumina powder (0.5 μιη particle sizes) to render mirror finish. To increase the absorption of laser radiation, the specimen surfaces were etched for 20 s using a common etchant for 300 series stainless steels (Carpenters stainless steel etch; 6 mL HN0₃, 122 mL HC1, 122 mL Ethanol, 8.5 g FeCl₃, and 2.4 g CuCl₂). The specimens were immediately washed and dried after etching. Each specimen tip was then screwed on the threaded end of the ultrasonic probe. For the ultrasonic-vibration assisted laser surface processing, a continuous wave C0₂ laser with laser power of 900 W was irradiated on the vibrating specimens. The laser beam diameter was about 100 μηι at the exit of the laser head. The laser processing was conducted with the defocused beam (about 7 mm in diameter) with the distance from the laser head to the surface of the specimen of about 5 cm. The laser beam energy profile was Gaussian, and the beam was irradiated perpendicular to the vibrating surface (i.e. vibration amplitude parallel to the beam as shown in Figure 1). The laser processing was conducted for three irradiation times: 0.30, 0.35, and 0.45 s. For each laser irradiation time, the processing was conducted with three ultrasonic vibration power outputs: 20, 30, and 40%. The power output controls the amplitude of vibration at the surface of the specimen tip. A 3D optical profilometer (Nanovea, Irvine, CA) was used to measure the vibration displacement at each power output. The optical profiler detects a step corresponding to vibration displacement in the surface profile when the ultrasonic system is turned on during the measurement. The surface profiles for the ultrasonic vibrations at the specified power outputs are shown in Figure 6. The vibration displacements of 23, 37, and 51 μηι were measured for the power outputs of 20, 30, and 40%, respectively. The laser irradiation times and ultrasonic power outputs used in this investigation were the optimum parameters that resulted in the laser-material interaction (heating/melting), and showed transition in the melt flow behavior for the case of surface melting. The laser processing and ultrasonic vibration parameters for the three sets of experiments are summarized in Table 1. The phase identification of the laser processed specimens was performed using an x-ray diffractometer (BRU ER AXS, Inc, Madison, WI) operating with Cu Ka radiation. The diffraction angle (2Θ) was varied between 20 and 100°. The surface profiles of the laser processed specimens were also recorded using 3D optical profilometer (Nanovea, Irvine, CA). A scanning electron microscope (JEOL Ltd, Tokyo, Japan) was used to characterize the microstructures at the surface and in the polished cross sections of the laser processed specimens. ImageJ software was used for the measurement of melt film thickness, and at least 9 measurements were taken for each sample on the crater walls from the cross-sectional SEM micrographs.

Laser surface processing for irradiation time of 0.30 s

Turning next to a discussion of various embodiments in more detail, steel samples were laser irradiated for 0.30 s with and without the simultaneous application of ultrasonic vibrations. The sample laser irradiated without application of ultrasonic vibrations exhibited a well-defined central melted region with surrounding heat affected zone (HAZ). From the surface micrographs, the diameter of the affected region in this sample is about 1.5 mm. The corresponding cross sectional image shows a well-defined melt pool characteristic of laser surface melting of materials. The width and depth of the melt pool was 988 and 157 μπι, respectively. However, for the similar laser processing parameters (laser power of 900 W and interaction time of 0.30 s), the laser interaction with the surface was very weak with the application of ultrasonic vibrations. The surfaces of the samples processed with ultrasonic-assisted laser surface processing showed only darker contrast due to surface heating, and no melting was observed for these samples. While the diameter of the heat affected zones remained at about 1.5 mm, the contrast of the laser irradiated regions became progressively weaker with increasing ultrasonic vibration output power (corresponding to increasing vibration amplitude). The corresponding cross sectional SEM micrographs also showed only HAZ without any surface melting. The depth of the HAZ decreased from 220 μΐΉ to 185 μιη with increasing ultrasonic power output from 20 to 30%. At about 40% power output, the contrast of the laser irradiated region was almost completely faded (not shown in the figure). The cross sectional SEM micrograph also did not show any contrast for this condition. Clearly, the ultrasonic vibrations weaken the laser interaction with the steel samples, eliminating the possibility of surface melting as observed for the samples laser irradiated without the application of vibrations. Also, the laser interaction (formation of HAZ at the irradiated regions) progressively diminishes with increasing ultrasonic power output. Deformation/fiow lines (wavy patterns) in the substrate regions were unaffected by laser irradiation. The appearance of such flow lines post deformation processing such as extrusion, forging, and rolling is very common.

When a laser beam is irradiated on a metallic surface, an absorbed energy results in the excitation of electrons generating heat. Various heat transfer processes such as conduction into the material and convection and radiation from the surface cause the dissipation of heat generated at the surface. Most of the laser-material interaction effects such as heating and melting are due to the conduction of heat into the material. An overall conduction flux in the material based on energy balance is given by:

where k is the thermal conductivity, A is the absorptivity, IQ is the incident laser intensity, ε is the emissivity, σ is the Stefan-Boltzmann constant, and h is the convective heat transfer coefficient. It appears that, during laser irradiation for 0.30 s, the ultrasonic vibrations accelerate the heat dissipation by convection from the surface of the sample. This causes the samples to heat relatively slowly, and the surface temperature does not reach the melting point (—1375 °C) of the steel within the laser irradiation time of 0.30 s. For the given ultrasonic vibration frequency (20 kHz), the convective heat dissipation becomes increasingly severe with increasing ultrasonic power output (corresponding to increasing vibration amplitude). This results in progressive disappearance of HAZ with increasing ultrasonic power output.

It appears that convection enhancement due to ultrasonic vibrations during laser surface irradiation slows the heating of the specimen, precluding the possibility of surface melting within the laser irradiation time of 0.30 s in this example. For the given frequency of 20 kHz, the convection enhancement is expected to be increasingly larger by increasing the vibration power output from 20% through 40%. The laser processing is often used for surface heat treatment of materials; the potential application of ultrasonic vibrations during such heat treatments must take into account the changes in overall heat transfer and resultant microstructural or phase transformations.

Laser surface processing for irradiation time of 0.45 s

Surface and cross sectional SEM micrographs for the laser irradiated steel sample without the application of ultrasonic vibrations were obtained. As with the previous case (iiTadiation time of 0.30 s), the sample irradiated with 0.45 s shows a distinct heat affected region with a diameter of about 2 mm with surface melting at the center and HAZ around the laser melted surface. The width and depth of the melting increased significantly with increasing laser irradiation time from 0.30 to 0.45 s. The width and depth of the laser melted region for the samples laser irradiated for 0.45 s were about 1700 μηι and 270 μη , respectively.

High magnification cross sectional SEM micrographs of the sample laser irradiated for 0.45 s without the application of ultrasonic vibrations were also obtained. The base material exhibited well defined austenite grains with average grain size of 7-9 μπι typical of AISI 316 stainless steel. The laser melted (fusion) region showed two distinct zones: the columnar dendritic grains growing perpendicular to the interface towards the center of the resolidified melt pool and the equiaxed dendritic grains at the center of the resolidified melt pool. The columnar dendritic grains appear to be growing from the partially melted austenite grains of the substrate. The packets of columnar dendrites growing in same direction form grains with characteristic substructure. The average dendritic arm spacing in these grains was about 2.2 μηι. The equiaxed dendritic grains at the center of the resolidified melt pool had the average grain size of about 3-5 μιη.

Several mechanisms such as grain detachment, dendrite fragmentation, and heterogeneous nucleation are cited for the formation of equiaxed grains during

solidification of castings. During rapid directional solidification encountered in stationary (not moving) laser processing of multi-component alloys, solute pile up ahead of the solid- liquid interface and results in sufficient constitutional undercooling. The observed equiaxed dendritic grains seems to have been formed by homogeneous nucleation in the undercooled melt, and these nuclei grow in different directions taking well defined grainlike morphology. Clearly, there was an indication of columnar dendritic to equiaxed dendritic transition towards the center of the resolidified melt pool. The evolution of dendritic grain morphology is governed by local solidification conditions defined by temperature gradient and solidification or growth rate. It has been observed that the region extending about 50 μιη into the base material near the fusion interface exhibited typical ditch structure indicative of sensitization of the steel in the HAZ. The ditch structure is characterized by the presence of darker grain boundaries due to intergranular corrosion associated with the precipitation of chromium-rich carbides at the grain boundaries and the depletion of chromium in the adjacent regions.

Figure 7 presents the XRD patterns for the base material and laser surface melted samples with and without the application of ultrasonic vibrations. The base material shows characteristic peaks of austenite phase. The sample laser melted without the application of ultrasonic vibrations also shows similar peaks, but the (220) peak emerges as the strongest peak indicating development of crystallographic texture consistent with the dendritic grain structure observed in the SEM micrographs.

Surface melting was observed for the laser irradiation time of 0.45 s with the simultaneous application of ultrasonic vibrations. However, the morphology and microstructure of the solidified surface layer was significantly modified when compared to the case where no vibrations were applied. SEM surface and cross-sectional SEM micrographs for the samples laser irradiated for 0.45 s with the application of ultrasonic vibrations of given power output levels were obtained. All the samples in this example showed deep craters (blind holes) of resolidified material with a build-up of material around the rims of the craters. It appears that once the surface is melted, the ultrasonic vibrations quickly displace the molten film forming the deep crater at the laser irradiated surface. As discussed earlier, the ultrasonic vibrations enhance the surface convection and delay surface heating due to laser irradiation. No melting was observed for the laser irradiation time of 0.30 s with the application of ultrasonic vibrations. Hence, it appears that in this embodiment the surface melting and rapid displacement of the melt happen in the last 0.15 s of 0.45 s irradiation time. It is important to note that the absorption of laser radiation after surface melting is significantly influenced by the dynamics of the melt film at the surface. In the case of laser melting with ultrasonic vibrations, the displacement of melt film creates a favorable situation for laser absorption. The enhanced absorption of laser radiation in the crater is likely the reason for rapid surface melting and displacement of molten film in the later stage of the irradiations time of 0.45 s. Surface micrographs also showed resolidified droplets (spatter) due to ejection of melt with the application of ultrasonic vibrations. For the given laser irradiation time (0.45 s), the diameter of the surface craters was not significantly varied with increasing ultrasonic power output and remained at about 1.3-1.5 mm. All the samples showed thin resolidified film in the craters. The variation of resolidified melt film thickness with ultrasonic power output is shown in Figure 4. It can be observed that the resolidified melt film thickness decreases with increasing ultrasonic power output. Clearly, the ultrasonic vibrations of higher amplitude for a given frequency are more effective in displacing the laser melted surface film.

A cross sectional SEM image of a crater and high magnification images of the resolidified film in the crater observed for the ultrasonic power output of 20% was obtained. The cross sectional image shows the tapered crater with the depth of about 320 μιη. The build-up of the material around the rim of the crater is also visible in the cross sectional image. The high magnification images show that the thin film is also resolidified on the surface of the crater. The thickness of the resolidified film varies with the position on the crater. The film is thinnest (about 16 μπι) at the bottom surface (at highest depth) and thickest (about 25 μηι) at the tapered walls of the crater, indicating vertical flow of molten material during laser melting with the assistance of ultrasonic vibrations.

The flow of molten material modifies the grain structure in the resolidified film. The solidification appears to start with the partial melting of grains in substrate. However, the predominantly columnar dendritic structure similar to that as observed for laser surface melting without ultrasonic vibrations is absent in the resolidified melt film of the crater. The microstructure in the resolidified melt film consists of predominantly equiaxed dendritic or short fragmented grains with grain size of 1.6 μηι. It is important to note that ultrasonic vibrations and mechanical stirring have long been used in the casting technologies for the grain refinement in the fusion zone. It is often proposed that the mechanical forces fragment the tips of columnar dendritic grains and the convection brings them in undercooled melt where they grow as equiaxed dendritic grains. It appears that formation of predominantly equiaxed grains in the resolidified film during ultrasonic vibration assisted laser surface melting is most likely due to rapid dendritic fragmentation. The XRD patterns of the samples laser melted with simultaneous application of ultrasonic vibrations are also presented in Figure 7. These XRD patterns show stronger (220) peak compared to the base material, indicating the development of crystallographic texture due to formation of dendritic grains. However, this crystallographic texture is relatively weaker compared to the samples laser melted without the application of ultrasonic vibrations strengthening the argument that the ultrasonic vibrations cause the

fragmentation of dendritic grains. The samples laser surface melted with the application of ultrasonic vibrations also show the characteristic ditch structure in the HAZ observed for the case without ultrasonic vibration assistance. Similar observations of the formation of crater and resolidified film, and the development of equiaxed grain structure in the film were made for the ultrasonic vibration assisted laser surface melting with the irradiation time of 0.35 s.

The variations of diameter and depth of craters observed for ultrasonic vibration assisted laser surface melting with the ultrasonic power output for the laser irradiation times of 0.35 and 0.45 s are presented in Figure 8. For the irradiation time of 0.45 s, the diameter of crater was in the range of about 1.3-1.5 mm and not much change in the diameter was observed with increasing ultrasonic power output. A slight decrease in crater diameter was observed for the irradiation time of 0.35 s. The depth of crater also decreased with increasing ultrasonic power output; however, the effect was much more pronounced for the irradiation time of 0.35 s. The results of observed diameter and depth of craters for the different processing conditions are also summarized in Table 1 below.

Table 1. Processing parameters and observed crater dimensions for ultrasonic vibration-assisted laser surface processing.

As observed earlier, the ultrasonic vibrations of higher power outputs are more effective in expelling the melt film as observed from thinner resolidified surface layers (Figure 2). While the melt expulsion is effective at higher vibration amplitudes, the extended delay in surface heating and melting associated with the enhanced surface convection effects results in smaller depth of craters for the given laser irradiation time and ultrasonic vibration frequency. The schematic of the evolution of depth of melting (without ultrasonic vibrations) and crater (with ultrasonic vibrations) as a function of laser irradiation time is presented in Figure 9. The figure shows the formation of un-distorted melt pool even for short laser irradiation time during laser surface melting without ultrasonic vibrations (Figure 9(a)). For the similar irradiation time, the convection enhancement due to ultrasonic vibrations slows the surface heating and precludes the possibility of surface melting (Figure. 9(b)). The surface melting can be initiated in the ultrasonic vibrations case for longer laser irradiation time (Figure 9(c)). This laser irradiation time is sufficient to initiate surface melting even with slow heating rate. The molten layer is expelled due to ultrasonic vibrations forming a crater with a resolidified surface film. An increase in ultrasonic power output for these conditions (longer irradiation time) further delays surface heating and melting forming a smaller crater even though the melt expulsion is more effective at higher vibration amplitude.

While the exact mechanisms of melt expulsion during the ultrasonic vibration assisted laser surface melting are not clear, it appears that the mechanisms are similar to those observed during early stages of ultrasonic atomization. In ultrasonic atomization, a uniform thickness layer of low viscosity liquids (water, oil etc.) is vibrated on the surfaces of the ultrasonic probes. When sufficient ultrasonic energy is imparted to the fluid, capillary waves are formed on the liquid surface leading to eventual breaking (ejection) of droplets from the tips of these waves. Significant progress has been made towards predicting the wavelength of capillary waves and the droplet size based on thickness of liquid layer, liquid material thermo-physical properties, and ultrasonic vibration parameters. The shape of the melt pool in case of laser surface melting is semi-elliptical below the surface level of the substrate in contrast to the uniform liquid layer above the substrate surface in case of ultrasonic atomization.

2 1

Furthermore, the viscosity (0.005 N.s.m^" ) and surface tension (1.6 N.m^" ) (restoring force acting against capillary forces) of liquid steel are significantly greater than for the liquids typically atomized using ultrasonic methods. The highly constrained physical shape of the melt pool and the limited physical properties of the molten steel appear to make nearly complete/full ejection of liquid metal highly unlikely in the case of laser surface melting with moderate ultrasonic vibration frequency used in this investigation. However, the observation of distributed spatter (resolidified droplets) on the surface of the samples indicates the existence of such a mechanism, though very weak. Instead of melt ejection, the molten material is expelled vertically outside the laser irradiated region forming surface craters. In the case of ultrasonic atomization, the thickness of liquid film determines the minimum amount of ultrasonic energy required to cause the melt ejection. For a given liquid, higher ultrasonic energy is required to eject thinner liquid film. Similarly, the thickness of the resolidified film on the surfaces of the craters represents the critical thickness of the liquid metal below which it will not be possible to expel the material at the given ultrasonic power output.

The observed trend of decreasing resolidified film thickness with increasing ultrasonic output power thus indicates that thinner melt film is expelled at higher ultrasonic power output. Based on these observations, important stages in the development of crater can be outlined (Figure 9). During ultrasonic vibration assisted surface processing, the melting initiates at the surface of the material and extends deeper with continued irradiation. The melt film is likely to be stable until it reaches the critical thickness for vibration induced melt expulsion. Once the melt expulsion is initiated, the surface melting and melt expulsion are expected to continue with continued laser irradiation to maintain the melt film thickness corresponding to critical thickness for melt expulsion.

Clearly, the simultaneous application of ultrasonic vibrations during laser surface melting resulted in expulsion of melt (forming craters) and modification of microstructure of the resolidified surface film (columnar to equiaxed dendritic transition). The melt expulsion is the important mechanism during most of the laser machining processes, while the equiaxed microstructure is often a desirable microstructure for the laser surface modification processes. The results presented in this paper suggest that application of ultrasonic vibrations during laser processing presents a great potential for improving material removal during laser machining processes and for improving the microstructure during laser surface melting processes. However, the application of ultrasonic vibrations delays the interaction of laser with material due to enhancement of surface convection effects. The potential utilization of such ultrasonic vibration assisted laser processes will require longer laser irradiation time than that is needed for conventional laser processing. Also, further investigations are needed to design the ultrasonic systems for scalable processing (i.e. application of ultrasonic vibrations to the larger substrates) at the desired vibration frequency and amplitudes.

While the exact mechanism of melt expulsion during the ultrasonic vibration assisted laser surface melting is not entirely clear, it appears that the mechanisms are similar to those observed during early stages of ultrasonic atomization. In ultrasonic atomization, a uniform thickness layer of low viscosity liquids (water, oil etc.) is vibrated on the surfaces of the ultrasonic probes or other support surface. When sufficient ultrasonic energy is imparted to the fluid, capillary waves are formed on the liquid surface leading to eventual breaking (ejection) of droplets from the tips of these waves. Significant progress has been made towards predicting the wavelength of capillary waves and the droplet size based on thickness of liquid layer, liquid material thermo-physical properties, and ultrasonic vibration parameters.

The shape of the melt pool in case of laser surface melting is semi-elliptical below the surface level of the substrate in contrast to the uniform liquid layer above the substrate surface in case of ultrasonic atomization. Furthermore, the viscosity (0.005 N.s.m^* ) and surface tension (1.6 N.m^"1) (restoring force acting against capillary forces) of liquid steel are significantly greater than for the liquids typically atomized using ultrasonic methods. The highly constrained physical shape of the melt pool and the limited physical properties of the molten steel appear to make nearly complete/full ejection of liquid metal highly unlikely in the case of laser surface melting with moderate ultrasonic vibration frequency used in this investigation.

However, the observation of distributed spatter (resolidified droplets) on the surface of the samples indicates the existence of such a mechanism, though very weak. Instead of melt ejection, the molten material is expelled vertically outside the laser irradiated region forming surface craters. In the case of ultrasonic atomization, the thickness of liquid film determines the minimum amount of ultrasonic energy required to cause the melt ejection. For a given liquid, higher ultrasonic energy is required to eject thinner liquid film. Similarly, the thickness of the resolidified film on the surfaces of the craters represents the critical thickness of the liquid metal below which it will not be possible to expel the material at the given ultrasonic power output. The observed trend of decreasing resolidified film thickness with increasing ultrasonic output power thus indicates that thinner melt film is expelled at higher ultrasonic power output. Based on these observations, important stages in the development of crater can be outlined (Figure 3). During ultrasonic vibration assisted surface processing, the melting initiates at the surface of the material and extends deeper with continued irradiation. The melt film is likely to be stable until it reaches the critical thickness for vibration induced melt expulsion. Once the melt expulsion is initiated, the surface melting and melt expulsion are expected to continue with continued laser irradiation to maintain the melt film thickness corresponding to critical thickness for melt expulsion.

Figure 7 presents the XRD patterns for the base material and laser surface melted samples with and without the application of ultrasonic vibrations according to one embodiment. The base material shows characteristic peaks of austenite phase. The sample laser melted without the application of ultrasonic vibrations also shows similar peaks, but the (220) peak emerges as the strongest peak indicating development of crystallographic texture consistent with the dendritic grain structure observed in the SEM micrographs.

Note that, although a 20 kHz frequency has proven to be useful for some of the materials tested, those of ordinary skill in the art may need to find the particular frequency that works best for a given laser type and/or material. The instant inventors have found that frequencies in the range of about 20 to 80 kHz are useful in some embodiments. Of course, it may be that, depending on the material drilled and other factors, frequency values greater or less than that range might prove to be optimum. Additionally, the amplitude of the vibration may need to be varied based on, for example, the laser type and/or material. Amplitudes up to 100 micrometers have been used in some applications, e.g., amplitudes in the range of about 1 to 100 micrometers are potentially useful.

However, it may be that amplitudes in excess of 100 micrometers would be useful in some cases. Those of ordinary skill in the art will be able to readily adjust the relevant parameter(s), potentially on a trial-and-error basis, to obtain a satisfactory result in a particular case.

Further the method of preparation of the sample discussed herein is only given as an example and those of ordinary skill in the art will be able to readily modify it according to the needs of a particular case.

Still further, although various embodiments illustrated herein have shown a probe as a support member, those of ordinary skill in the art will recognize that any structure capable of supporting the sample could be used and, in various embodiments, the member should also be capable of communicating the vibration of the ultrasonic generator to the sample via mechanical, fluid, hydraulic, etc., communication (collectively "mechanical communication" hereinafter) could be used.

It is to be understood that the terms "including", "comprising", "consisting" and grammatical variants thereof do not preclude the addition of one or more components, features, steps, or integers or groups thereof and that the terms are to be construed as specifying components, features, steps or integers.

If the specification or claims refer to "an additional" element, that does not preclude there being more than one of the additional element.

It is to be understood that where the claims or specification refer to "a" or "an" element, such reference is not be construed that there is only one of that element.

It is to be understood that where the specification states that a component, feature, structure, or characteristic "may", "might", "can" or "could" be included, that particular component, feature, structure, or characteristic is not required to be included.

Where applicable, although state diagrams, flow diagrams or both may be used to describe embodiments, the invention is not limited to those diagrams or to the

corresponding descriptions. For example, flow need not move through each illustrated box or state, or in exactly the same order as illustrated and described.

Methods of the present invention may be implemented by performing or completing manually, automatically, or a combination thereof, selected steps or tasks.

The term "method" may refer to manners, means, techniques and procedures for accomplishing a given task including, but not limited to, those manners, means, techniques and procedures either known to, or readily developed from known manners, means, techniques and procedures by practitioners of the art to which the invention belongs.

For purposes of the instant disclosure, the term "at least" followed by a number is used herein to denote the start of a range beginning with that number (which may be a ranger having an upper limit or no upper limit, depending on the variable being defined). For example, "at least 1" means 1 or more than 1. The term "at most" followed by a number is used herein to denote the end of a range ending with that number (which may be a range having 1 or 0 as its lower limit, or a range having no lower limit, depending upon the variable being defined). For example, "at most 4" means 4 or less than 4, and "at most 40%" means 40% or less than 40%. Terms of approximation (e.g., "about", "substantially", "approximately", etc.) should be interpreted according to their ordinary and customary meanings as used in the associated art unless indicated otherwise. Absent a specific definition and absent ordinary and customary usage in the associated art, such terms should be interpreted to be ± 10% of the base value.

When, in this document, a range is given as "(a first number) to (a second number)" or "(a first number) - (a second number)", this means a range whose lower limit is the first number and whose upper limit is the second number. For example, 25 to 100 should be interpreted to mean a range whose lower limit is 25 and whose upper limit is 100. Additionally, it should be noted that where a range is given, every possible subrange or interval within that range is also specifically intended unless the context indicates to the contrary. For example, if the specification indicates a range of 25 to 100 such range is also intended to include subranges such as 26 -100, 27-100, etc., 25-99, 25-98, etc., as well as any other possible combination of lower and upper values within the stated range, e.g., 33-47, 60-97, 41-45, 28-96, etc. Note that integer range values have been used in this paragraph for purposes of illustration only and decimal and fractional values (e.g., 46.7 - 91.3) should also be understood to be intended as possible subrange endpoints unless specifically excluded.

It should be noted that where reference is made herein to a method comprising two or more defined steps, the defined steps can be carried out in any order or simultaneously (except where context excludes that possibility), and the method can also include one or more other steps which are carried out before any of the defined steps, between two of the defined steps, or after all of the defined steps (except where context excludes that possibility).

Further, it should be noted that terms of approximation (e.g., "about",

"substantially", "approximately", etc.) are to be interpreted according to their ordinary and customary meanings as used in the associated art unless indicated otherwise herein.

Absent a specific definition within this disclosure, and absent ordinary and customary usage in the associated art, such terms should be interpreted to be plus or minus 10% of the base value.

Still further, additional aspects of the instant invention may be found in one or more appendices attached hereto and/or filed herewith, the disclosures of which are incorporated herein by reference as if fully set out at this point. Thus, the present invention is well adapted to carry out the objects and attain the ends and advantages mentioned above as well as those inherent therein. While the inventive device has been described and illustrated herein by reference to certain preferred embodiments in relation to the drawings attached thereto, various changes and further modifications, apart from those shown or suggested herein, may be made therein by those of ordinary skill in the art, without departing from the spirit of the inventive concept the scope of which is to be determined by the following claims.

References:

[I] Dahotre NB, Harimkar SP. Laser fabrication and machining of materials. 1st ed. New York: Springer; 2008.

[2] Bharatish A, Narasimha Murthy HN, Anand B, Madhusoodana CD, Praveena GS, Krishna M. Characterization of hole circularity and heat affected zone in pulsed C0₂ laser drilling of alumina ceramics. Opt Laser Technol 2013; 53:22-32.

[3] Chan CL, Mazumder J. One-dimensional steady-state model for damage by vaporization and liquid expulsion due to laser-material interaction. J Appl Phys 1987; 62:4579-4586.

[4] Verhoeven JC, Jansen JK, Mattheij RM. Modelling laser induced melting. Math Comput Model 2003; 37:419-437.

[5] Semak V, Matsunawa A. The role of recoil pressure in energy balance during laser materials processing. J Phys D: Appl Phys 1997; 30(18):2541.

[6] Rodden WSO, Kudesia SS, Hand DP, Jones JDC. Use of "assist" gas in the laser drilling of titanium. J Laser Appl 2001 ; 13(5):204-208.

[7] Patel RS, Brewster MQ. Gas-assisted laser-metal drilling: theoretical model. J Thermophys Heat Transfer 1991 ; 5:32-39.

[8] Patel RS, Brewster MQ. Gas-assisted laser-metal drilling: Experimental results. J Thermophys Heat Transfer 1991 ; 5:26-31.

[9] Bandyopadhyay S, Sundar JK, Sundararajan G, Joshi SV. Geometrical features and metallurgical characteristics of Nd:YAG laser drilled holes in thick ΓΝ718 and Ti-6A1-4V sheets.

J Mater Process Technol 2002; 127:83-95.

[10] Yeo CY, Tarn SC, Jana S, Lau MW. A technical review of laser drilling of aerospace materials. J Mater Process Technol 1994; 42: 15-49

[I I] Zheng HY, Huang H. Ultrasonic vibration-assisted femtosecond laser machining of microholes. J Micromech Microeng 2007; 17:N58-N61.

Claims

claimed is:

An apparatus for laser drilling into a sample, comprising:

(a) a laser positionable to drill into the sample when activated; and,

(b) an ultrasonic generator positionable to be in mechanical communication with the sample during activation of said laser, said ultrasonic generator vibrating the sample at an ultrasonic frequency when so activated.

An apparatus for laser drilling into a sample according to Claim 1, wherein said laser is selected from the group consisting of a C0₂ laser, a solid-state laser, a gas laser, and a fiber laser.

An apparatus for laser drilling into a sample according to Claim 1, wherein said ultrasonic generator vibrates the sample at an ultrasonic frequency of about 20kHz when activated.

An apparatus for laser drilling into a sample according to Claim 1, wherein said ultrasonic generator vibrates the sample at with an amplitude in a range of 1 μιτι to 100 μη when activated.

An apparatus for laser drilling into a sample according to Claim 1, wherein said ultrasonic generator vibrates the sample at a frequency between 20 kHz and 80 kHz when activated.

A method of laser drilling into a sample, comprising:

(a) placing the sample in mechanical communication with an ultrasonic

generator, said ultrasonic generator at least for vibrating the sample at an ultrasonic frequency when activated;

(b) positioning a laser to drill into the sample when activated;

(c) activating said laser to initiate the drilling into the sample; and, (d) while said laser is activated, activating said ultrasonic generator, thereby vibrating the sample at an ultrasonic frequency while the sample is being drilled.

7. A method of laser drilling into a sample according to Claim 6, wherein step (d) comprises the step of, while said laser is activated activating said ultrasonic generator, thereby vibrating the sample at an ultrasonic frequency between 20 kHz and 80 kHz.

8. An apparatus for laser drilling into a sample according to Claim 1, wherein said laser drill is selected from the group consisting of a C0₂ laser drill, a solid-state laser drill, a gas laser drill, and a fiber laser drill.

9. An apparatus for laser drilling into a sample according to Claim 1, wherein said ultrasonic generator vibrates the sample at an ultrasonic frequency of about 20kHz when activated.

10. An apparatus for laser drilling into a sample according to Claim 1, wherein said ultrasonic generator vibrates the sample at a frequency of about 20 kHz and at amplitudes in a range of 20 μηι to 50 μηι when activated.

11. An apparatus for laser drilling into a sample, comprising:

a. a sample support member at least for supporting a sample;

b. an ultrasonic generator in mechanical communication with said sample support member, said ultrasonic generator at least for providing an ultrasonic vibration to the sample; and

c. a laser drill operable to drill into the sample while the ultrasonic generator vibrates the sample at an ultrasonic frequency.