by Edgar Willenborg and Roman Ostholt
Re-melting with laser radiation is a new method for the automated polishing of 3D surfaces in the tooling industry and medical engineering
The surface roughness of a part or product strongly influences its properties and functions including abrasion and corrosion resistance, tribological properties, and optical properties as well as the visual impression the customer desires. Therefore, in industrial manufacturing grinding and polishing techniques are widely used to reduce the roughness of surfaces. Based on large-area abrasion, automated polishing techniques—for example, electropolishing, electro-chemical polishing, or slide grinding—are predominantly processing edges and overhanging surfaces and lead to edge rounding. Furthermore deeper shafts are not processed. Therefore, current automated polishing techniques are often not applicable on parts with freeform surfaces and function-relevant edges. Thus, polishing in the tooling industry is mostly done manually.
FIGURE 1. Basic principle of polishing with laser radiation.
The quality of manual polishing strongly depends on the worker's skill and experience. Skilled workers are a scarce resource because manual polishing is demanding, monotonous work, and companies have problems recruiting suitable employees. Due to the low processing speed (typically in the range of 10 to 30min/cm²) and the sequential nature of the workflow, production of molds and dies with manual polishing is time consuming and cost intensive. For the manufacturing of injection and die casting molds 12 to 15 percent of the manufacturing costs1 and 30 to 50 percent of the manufacturing time2 is associated with polishing.
A new method for automated polishing of metals is polishing with laser radiation where a thin surface layer is melted with surface tension leading to material flow from the peaks to the valleys (see FIGURE 1). Material is not removed, rather it is relocated while molten. The laser beam is guided over the surface in contour-aligned patterns. A surface roughness of Ra ˜ 0.05 µm is achievable with laser polishing, depending on the material and the initial roughness. This surface quality is sufficient in a broad field of applications.
FIGURE 2. Laser polished tool steel.
The main characteristics and advantages of laser polishing are: a high level of automation which leads to short machining times especially in comparison to manual polishing; the ability to polish grained and micro-structured surfaces without damaging the structures; and small micro roughness as the surface results from the liquid phase. There is no pollution impact from grinding and polishing wastes and chemicals and there is no change in the form of the workpieces. Deviations of the desired form are not corrected but otherwise an already perfect form is not damaged.
FIGURE 3. TiAl6V4 surface milled (top) and micro polished (bottom).
When laser polishing metals, two sub-variants exist: macro and micro polishing. Macro polishing is carried out with CW laser radiation. Milled, turned, or EDM-processed surfaces with a roughness up to several micrometers can be polished as seen in FIGURE 2.3,4,5 Remelting depths between 20 to 200 µm are used. Beam diameter and remelting depth have to be chosen depending on the material and the initial surface roughness. Normally, fiber-coupled Nd:YAG lasers are used with laser powers of 70 to 300W. The processing time is between 10 and 200s/cm² depending on the initial surface roughness, the material, and the desired roughness after laser polishing. The achievable roughness depends on several influencing variables3. Segregations and inclusions can downgrade the surface quality. The best result achieved so far is the reduction of the roughness of a turned tool steel from Ra = 5.0 µm down to Ra = 0.05 µm.
In contrast to macro polishing, micro polishing is carried out with pulsed laser radiation.4, 6,7,8,9,10 The pulse duration normally is in the range of 20 to 1000 ns and the remelting depth in the range of 0.5 to 5 µm. With the micro polishing process variant, only fine pre-processed surfaces (e.g. ground, micro milled) can be polished. Due to the small remelting depth, larger surface structures remain unaffected and can, therefore, not be eliminated. The most important process parameters are pulse duration and intensity. Longer pulses can eliminate laterally larger surface structures. A top-hat intensity distribution is preferable to generate a homogenous remelting depth. Fiber-coupled Nd:YAG and excimer lasers are used. Processing times less than 3s/cm² are achievable. FIGURE 3 shows a TiAl6V4 surface before and after micro polishing. Due to the fine micro roughness, micro polishing is especially suitable for tribological and medical applications.
FIGURE 4. Technology specific CAM-NC process chain.11
The micro polishing process is limited to smooth surface structures with lateral dimensions of up to 20 to 100 µm depending on the material. Larger surface structures can only be removed with the macro polishing process. But, in contrast to the macro polishing process, the micro polishing process often leads to a finer micro roughness and, therefore, to a higher gloss. As a consequence, for some applications a combination of both variants is used: first macro polishing to eliminate the tracks from milling or turning, then micro polishing to enhance the gloss level.
Examples for polishing results are shown in the Table. Copper, gold, and aluminum alloys show hitherto predominately unsatisfying results, but otherwise laser polishing of these metals has not yet been investigated as closely as the polishing of steels and titanium alloys has.
The main field of application for laser polishing is the substitution of the time and cost-consuming manual polishing, such as in tool and mold manufacturing or in medical engineering. The results shown in the Table are suitable for many applications but the task now is to achieve these results on complex shaped freeform surfaces.
For freeform applications a technology specific CAM-NC process chain for laser polishing is indispensable. The starting point for the CAM-NC process chain that is under development for laser polishing is a 3D CAD model of the workpiece11 shown in FIGURE 4. Using conventional CAM software for five-axis milling applications the tool paths are generated. The geometrical information of the tool paths are exported via the machine neutral APT (automatically programmed tools) interface in newly developed technology-specific post-processor software. This technology module comprises an application database which contains all necessary process parameters for laser polishing and carries out technology-specific corrective actions on the tool path necessary to gain good polishing results. In the final step of the CAM-NC process chain the data is exported in a machine-specific NC code and the workpiece is laser polished using an 8-axis laser polishing machine.
Examples for laser polished freeform surfaces are shown in FIGURE 5. The material of the mold in the left picture is GGG40 and the initial roughness is Ra=1.7µm. The required roughness of Ra<0.4µm is achieved in a processing time of less than 1min/cm². The material of the mold in the right picture is tool steel, the roughness after laser polishing is Ra<0.2µm. This mold is used for the manufacturing of shafts and stems of wine glasses.
The processing of 3D surfaces is still under investigation, but these examples show that laser polishing is applicable even for freeform surfaces. Laser-polished molds have been applied in production and have shown at least equal wear resistance properties in first sampling tests than manually polished molds.
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Dr.-Ing. Edgar Willenborg (email@example.com) is with the Fraunhofer Institute for Laser Technology, Aachen, Germany, and Dipl.-Ing. Roman Ostholt (firstname.lastname@example.org) is with RWTH Aachen University - Chair for Laser Technology.