Structured surfaces appear virtually everywhere in daily life and have a vast distribution in nearly every field of application. In many cases the structures are tailored specially for one application, and its properties and functions dominate the visual appearance. Especially polymer components are used in all kinds of modern products and one common production technology for multiplying structured plastics is the injection molding process. For this process a tool is structured usually by milling, turning, photochemical etching, laser ablation or other conventional technologies and replicated several hundred thousand times.
2. Basic process principle
In this new process, the surface structure and the micro roughness result from a laser-controlled self-organization of the melt pool due to surface tension. Based on this remelting principle an innovative structuring technique is investigated were three laser beams (two cw and one pulsed) are superposed. A melt pool is generated by two cw laser beams with constant feed rate. A pulsed laser is superposed and evaporates a small amount of molten material and, therefore, generates vapour pressure. This pressure influences the melt pool surface and the solidification follows this newly shaped surface [Fig. 1
Fig. 1 Schematic of assumed active principle for structuring due to vapour pressure.
3. N. Pirch, S. Höges, and K. Wissenbach, “Mechanisms of surface rippling during laser polishing,” Proceedings of the 8. International Seminar on Numerical Analysis of Weldability. Graz-Seggau, 25–27 (2006)
]. Hence a structuring of the surface is achieved with almost no ablation of material. With a fast control (≤ 1 ms) of the position (Δ xfast
) of the pulsed laser in comparison to the cw beams raised or lowered structures can be produced. Slow (> 1 s) deflection of one cw beam relatively to the other (Δ xslow
) facilitates the control of the temperature gradients around the working zone due to preheating of the material, which improves the process quality and extend the capabilities for parallel processing. The advantages expected by this method of controlling the melt pool surface are significantly higher aspect ratios (ratio of height to width) for smaller structures.
In order to investigate the feasibility for structuring due to laser-induced vapour pressure, two different strategies are investigated [Fig. 2
Fig. 2 Scheme of process strategies to achieve structuring by remelting via vapour pressure resulting from localized vapourisation of molten material (onsight).
]. In both strategies a melt pool is generated by the cw laser beams while moving over the surface with constant feed rate and changing scanning directions. For reasons of depiction the relative deflection of the second cw laser beam is not figured out. In both strategies the pulsed laser beam diameter is significant (up to five times) smaller than the melt pool created by the cw laser beams.
One process strategy is to vaporize molten material from the middle of the melt pool at defined time distances [Fig. 2
(left)]. In order to achieve that, the pulsed laser beam is moved over the surface with the same scanning velocity as the cw laser beams. Every five milliseconds a single laser pulse is emitted which vaporizes a small amount of molten material from the middle of the melt pool.
The second process strategy includes a movement of the pulsed laser beam relatively to the melting pool [Fig. 2
(right)]. The movement of the pulsed laser beam starts near to the melt front. The direction of movement is diametrical relative to the scanning direction of the cw laser beams and ends near to the solidification front. While moving the pulsed laser beam relatively to the melt pool, material is evaporated with a constant repetition frequency and pulse energy. Therefore multiple pressure waves are induced in the melt pool due to the local evaporation of molten material. The pressure waves interfere with each other on their way to the solidification front which leads to even higher structures.
The capabilities and limitations (e.g. structure dimensions, quality and process velocity) of structuring of metals by vapour pressure while generating molten material by laser radiation are unknown yet. Understanding the interdependencies between the process parameters, the dimensions and dynamics of the melt pool, the resulting structures and the microstructure and functional properties of these structures is of highest priority. The technical challenge is the precise control of small melt pools (< 10−3 mm3) to create defined and reproducible structures with high processing speeds. Therefore an optical set-up was designed and built up which allows for the combination of two cw and one pulsed laser beam along with some other features, which are unique in their combination.
3. Optical set-up
Structuring by laser remelting takes full advantage of highly dynamic intensity distributions in the interaction zone. The optical system has to provide great flexibility with respect to different superposition strategies and must be highly automated to facilitate convenient and clean operation. To meet the requirements of the process, a huge amount of degrees of freedom has to be maintained. The general requirements that have to be covered are listed as follows:
-Superposition of two cw as well as a pulsed laser beam
-Variable laser beam diameters of 100 µm < dL < 1000 µm
-Continuous attenuation of cw laser power PL < 500 W
-Offset dxy between laser beams max. 1 mm
-Fast movement (approx. 1 mm at 2 kHz) and control of pulsed laser beam
-Relative, high speed, high accuracy deflection of both cw and pulsed laser beams
Such an optical system has not been realized in that configuration before and is being discussed in the following.
Fig. 3 Scheme of the optics for superposing three laser beams and a fast two-dimensional deflection for one laser beam relatively to the other (on-sight).
depicts the scheme of the optical concept, containing the different laser sources alongside with beam shaping and deflection units.
Central to the optical system are two separate fiber-coupled laser sources, a pulsed source and a continuous (cw) emitting at 1064 nm and 1030 nm, respectively.
After its collimation and expansion the cw laser beam is polarized elliptically by a λ/4-plate. Afterwards the beam is splitted into two beams by separation due to polarization. The control of laser power is based on depolarization of light by using λ/2-retardation-plates, which are operated in motorized mounts of continuous rotation. The consolidation of the two beams by polarization coupling facilitates continuous variation of the amount of reflected or transmitted energy, depending on the separated beams´ polarization directions. The lost energy is dissipated by a beam dump.
A slow beam deflection unit (< 1 Hz), comprising a motorized 2D-tipping mirror, allows for moving one cw-laser spot relatively to the other so that after their coupling by polarization, two lateral displaced spots can be superimposed on the work piece. Furthermore, the optical paths for the two cw beams comprise zoom-telescopes to vary the spot diameters as well as apertures to prevent obscurations due to the following optical components´ apertures. To minimize the beam divergence the last polarizer-plate is followed by a relay-telescope that images the apertures in the entrance aperture of the 3D-scanning system.
Since a fiber laser became the first choice for the pulsed laser source, the coupling of the pulsed laser source is realized in the same way as the coupling of the cw-laser. The collimated laser beam is guided through a telescope with fixed magnitude, which provides an appropriate beam diameter for the following optical devices. A motorized zoom-telescope maintains the spot diameter. For the two-dimensional transversal deflection of the pulsed laser beam, two one-dimensional piezo actuators are implemented, placed orthogonally to each other. The piezo-driven mirrors allow for a small but very accurate and fast (2 kHz) manipulation of the direction of the beam. These small changes in the direction of the pulsed beam then lead to an offset (Δx, Δy) between the pulsed and the cw laser beams in the focal plane.
The fast scanning unit is projected by a telecentric relay-telescope into the entrance aperture of the scanning unit to minimize the beam divergence analogously to the cw beam paths. Directly behind their imaging-optics the pulsed and cw beams are superimposed by wavelength multiplexing and build a “virtual” source, whose focal length and deflection are finally controlled by the 3D-scanning unit and the f-theta lens.
The combination element of the two laser beam sources is positioned before the main scanning unit. This allows maximum freedom in changing the diameter of each beam individually. A possibility to realize process monitoring is given by a dichroic, high-reflective mirror placed in front of the scanning unit. Using a CCD camera the attenuated laser beams intensity profile can be monitored without disturbing influences of the scanning unit. On the one hand this configuration supports the adjustment of the pulsed laser beams position relatively to the cw-beams position and on the other hand it allows the measurement of the piezo actuators maximum deflection.
Table 1. Postulated variables in the performance of the optical system
summarizes the postulated variables in the optical performance and the parameter space within their change is required.
4.1 Characterization & testing of optical set-up
Fig. 4 Different superpositions illustrating the systems parameter range.
demonstrates the flexibility of the experimental set-up showing the superposition of different laser beam sources mapped with a MicroSpotMonitor by Primes GmbH.
The Figs. 4(a)
illustrate both cw parts that are scaled in diameter utilizing the zoom-telescopes. The Fig. 4(d)
depicts the variable attenuation of one cw beam, resulting in decreased intensity. In Figs. 4(e)
finally the pulsed laser beam is added and laterally displaced.
Using the zoom telescopes a variation of the spot diameters by a factor of 11 can be obtained. The field of available focus diameters of the cw-beams can be extended to the range from 54 µm to 3 mm if optical fibers and/or f-theta lenses are swapped. For this project lenses with focal lengths of fT = 100 mm, fT = 163 mm and fT = 254 mm are available. A detailed analytical model that can be used to calculate the diameter and maximum deflection of the focal spot as a function of all optical components (Scanner, Piezo mirror, imaging optics, magnification of zoom telescope…) has been developed to design the optical system and predict spot geometries.
4.2 Structuring by vapor pressure
Fig. 5 Miscoloured plots in different resolutions mapped by WLI of a structured surface using the new vapour pressure process.
shows miscoloured plots of the surface structure achieved by using the first process strategy with non-deflected pulsed laser beam [Fig. 2
Considering a scanning velocity of vscan
= 50 mm/s, the emitted laser pulses lead to a periodic structure with a wavelength of approx. 250 µm. The vaporization of molten material induces presumably a local pressure wave in the middle of the melting pool. This pressure wave travels through the melting pool and solidifies at the solidification front as an elevation with a height of up to five microns. The wavelength of the pressure wave can be identified in a magnification of the surface structure [Fig. 5
(right)]. The local vaporization not only generates a macro structure with a wavelength of 250 µm but also an overlying microstructure with a wavelength of approximately 25 µm.
Fig. 6 Miscoloured plots of a structured surface mapped by WLI using strategy 2.
depicts surface structures achieved by utilizing the second process strategy, with deflection of the pulsed laser beam relatively to the cw beams [Fig. 2
(right)]. Material is evaporated with a constant repetition frequency (fP
= 30 kHz) and pulse energy (EP
≈0.5 mJ). A three-dimensional and an on sight plot of the structured surface are shown, each mapped by a white light interferometry and illustrated in false colors. In contrary to Fig. 5
this structure has at least twice the height (hλ
= 20 µm; λ = 0.2 mm; n = 1) than achieved with the static strategy. Additionally, there is no overlaying microstructure of the induced pressure waves left.
For the structuring process based on a local vaporization of molten material a new optical set-up was designed, built-up, tested and characterized. On the one hand the realized concept includes the combination of three individual laser beams (two continuous wave and one pulsed laser beam); on the other it also enables the variation of relative laser beam locations and diameters. The flexibility is increased by an independent control of process parameters for each laser beam such as laser power, laser beam diameter, and relative displacement to each other.
The systematical process development of the structuring by remelting process due to localized vaporization of molten material is a challenging task. On one side the process offers a large freedom for the generation of complete new structures, but on the other side an also large set of procedural parameters and their interaction have to be taken under investigation.
Main challenges and objectives in terms of process development, enhancement of the optical set-up and modeling of the process are:
-To develop and set-up an optical system for the laser beam with an anamorphic zoom telescope to generate elliptical intensity distributions to enable circular intensity distributions on the work piece surface also for a non-perpendicular angle of incidence. In addition the optical system must be equipped with an element (e.g. a dove prism) that enables rotating the elliptical intensity distribution around the optical axis.
-To broaden the knowledge of the mechanisms for the structuring by vapour pressure process, to elaborate physical limitations of the process and to identify the influence of procedural parameters on generated structures.
-To develop a FEM model for the vapor pressure process to identify physical limits and to enhance the knowledge of the process fundamentals. Therefore the existing FEM model must be expanded.