Paper presented at International conference on power beam processing technologies (ICPBPT2010) Oct. 25-29, 2010 Beijing, China
The functional properties of materials can be enhanced using surface features, for instance; to improve the thermal properties of heat exchangers. The development of the laser Surfi-Sculpt® process over the past decade has resulted in a technology capable of producing a wide variety of features in metallic substrates. Furthermore, the potential for producing similar features in polymeric and ceramic substrates also exists. Recently the potential for utilising high power focused laser beams to produce surface features using this process has been demonstrated. This paper details the results of recent trials performed with a 200 Yb-fibre laser and a galvanometer driven beam scanner for producing surface features using the Surfi-Sculpt process. A compact, cheap, easily robotically automated, laser Surfi-Sculpt machine, being able to operate at atmospheric pressure may have potential in a range of industry sectors.
Keywords: Laser, Surfacing, Yb-fibre, Surfi-Sculpt
The development of the laser Surfi-Sculpt® process over the past decade has resulted in a technology capable of producing a wide variety of features in metallic substrates[1-3]. Furthermore, the potential for producing similar features in polymeric and ceramic substrates also exists. Fundamentally, the surface features are be produced by inducing a movement of material from the bulk substrate to points on the surface, resulting in arrays of protrusions and corresponding arrays of intrusions. Since the power beam induces the material movement, it is a non-contact process and does not suffer from tool-wear difficulties. The majority of Surfi-Sculpt research has been performed using electron beams, which are highly focused and quickly deflected with computer controlled electromagnetic coils. However, recently the potential for utilising high power focused laser beams to produce surface features using this process has been assessed. This has been possible as a result of the development of solid-state laser sources with excellent beam qualities.
The beam quality of modern solid-state laser sources (such as Yb-fibre and Yb:YAG disc) are significantly better than those emitted from traditional Nd:YAG laser sources, whose beam quality was limited by thermal lensing and birefringence effects. As a result of their excellent beam qualities, the beams emitted from modern solid-state laser sources can be; (i) focused to particularly small spot diameters allowing power densities approaching MW/mm2 to be produced; (ii) be utilised with relatively long stand-off distances, allowing remote processing; or, (iii) be focused such that they have a long depth of focus. Consequently, solid-state laser sources can now be integrated with beam scanning systems (capable of moving the focused beam across the surface of the workpiece at particularly high travel speeds) as the beam quality gives the necessary focusing length for use with these systems, whilst retaining the power density required on the surface of the material.
In the research performed by Hilton and Nguyen, experimental trials were performed using two different modern solid-state laser sources, which were focused and deflected using galvanometer driven beam scanners. They reported that macroscopic surface features could be produced in a variety of metallic materials with this equipment combination. Ordinarily, the focused spot diameters were >300µm, and the scanning speeds were <400mm/s. In comparison with surface features produced with electron beams, the laser processing speeds are relatively low. Nevertheless, the research demonstrates that the Surfi-Sculpt process can also be performed using high-power focused laser beams. Significantly, this allows production of features at atmospheric pressure, which is not possible with electron beams. Furthermore, the fibre optic delivery of 1µm wavelength solid-state laser sources allows the beam scanner to be easily integrated with robotics manipulators.
Hilton and Nguyen, utilised multi-mode solid-state laser sources. Yb-fibre lasers with a beam parameter product (BPP) of ~0.3 are commercially available at powers exceeding 1kW6. This excellent beam quality allows focused spot sizes of <50µm to be obtained, whilst maintaining a useable working distance. Consequently, single-mode solid-state laser sources should be capable of producing features on a smaller scale than previously possible with laser beams. Furthermore, lower power (<500W) diffraction limited Yb-fibre lasers are significantly less expensive than the laser sources utilised by Hilton and Nguyen and commercially available electron beam processing equipment. A compact, cheap, easily robotically automated, laser Surfi-Sculpt machine, being able to operate at atmospheric pressure may have potential in a range of industry sectors. This paper details the results of recent trials performed with a 200 Yb-fibre laser and a galvanometer driven beam scanner for producing surface features using the Surfi-Sculpt process.
All the trials were performed with a single-mode Yb-fibre laser, manufactured by SPI lasers, which produced a rated maximum output power of 200W ± <0.5%. The beam scanning system utilised was a Nutfield Technology XLR8-15-1064 galvanometer driven scan head, which utilised a 125mm focal length Jenoptic F-theta lens for focusing. This combination of laser source and scanning system gave a beam width of 38µm diameter, and a working field of approximately 60mm x 60mm. Unless stated otherwise, the minimum beam waist was positioned on the top surface of the workpiece. Processing was performed with 200W requested power.
The materials used in this investigation were 50mm x 50mm coupons of grade 304 stainless steel, C-Mn steel, Inconel 718, and Ti-6Al-4V. Prior to processing, the surface of all materials was prepared using a 2500 grit abrasive paper and cleaned with acetone to remove any grease or dirt deposits. All processing was performed at atmospheric pressure, either in a chamber supplied with argon (99.998% purity) or in air. The typical experimental set-up is shown in Fig.1.
From experience with electron beam and early laser Surfi-Sculpt processing, it is known the simplest features which can be created are those produced by repeating a swipe a number of times, in the same position on the workpiece (Fig.2a). Features more complex in nature can be produced by incorporating several swipes into a motif (for examples those shown in Fig.2b and 2c). The motif is then repeated a number of times depending upon the required feature height. It was envisaged that by understanding and optimising the single protrusion process through empirical investigation, then larger and more complex features could be more easily produced.
a) single swipe, b) multiple single swipes integrated into a simple motif, c) multiple single swipes integrated into a complex motif
Table 1 details the process parameters investigated. In order to allow a large number of parameter permutations to be assessed, arrays of features were produced.
Table 1: Process parameters investigated
|Swipe speed, mm/s
|Number of swipe repeats
|Swipe length, mm
|Focal plane position
||-1.5 - +2.5
Single protrusions up to ~3mm in height could be produced with the correct combination of process parameters. A macro image and a scanning electron micrographs of a typical protrusion, produced with optimised parameters, in Ti-6Al-4V is shown in Fig.3. Swipe speeds of 400-1000mm/s and swipe lengths of 2.5-6.0mm were examined with 160 swipe repeats and a constant swipe delay when processing Inconel 718. Fig.4 details the effects of different swipe speeds on the resultant protrusion height. The general trend indicates that taller features were produced with longer swipe lengths and higher swipe speeds. At the swipe speed producing the highest protrusions (i.e. 600mm/s), and a constant swipe delay, protrusions with varying swipe lengths were produced using a different number of swipe repeats. Increasing the number of swipe repeats, and hence the work performed per protrusion, increased the height of the protrusion, although diminishing returns were seen at higher swipe repeat rates.
The depth of focus (characterised here as the length over which the focused beam diameter increases by 5% from its beam width) of the laser beam used in this investigation is ±0.61mm, whereas its Rayleigh length was ±0.95.
Parameter combinations which gave the quickest build rate in Inconel 718 were repeated on the other materials to assess their transferability. Processing was performed both in air and argon environments. The resultant protrusion heights are detailed in Fig.5. All protrusions produced in the argon environment were free of oxidation. From Fig.5 it is apparent that the argon environment has little effect on the build rate of the protrusions produced in steel, whereas it has a negative and a positive effect on the protrusions produced in Inconel 718 and Ti-6Al-4V respectively.
Taking the results from the production of simple protrusions, a variety of more complex features were produced using a motif arrangement, such as those in Fig.2a and Fig.2b. Fig.6a shows an example of the linear wall features which can be produced by reducing the separation between different swipes in the motif. Although the results are not specifically detailed here, management of the heat input between successive swipes was critical in optimising the build rate.
Further experimental trials were performed with the swipes arranged in a circular motif, as indicated in Fig.2c. Primarily, the eight swipe motif (Fig.2c) was adopted and similar investigations to those with the simple protrusions were performed to assess the influence of the process parameters on the build rate. Further experimental trials were performed with an increasing number of swipes in the motif (up to 64 swipes was examined). Fig.7 details typical conical Surfi-Sculpt features produced using this method. Through correct management of the heat input to the workpiece, a large array of Surfi-Sculpt features were produced on a 32.0mm diameter stainless steel round bar, as detailed in Fig.8.
The production of large arrays of surface features is not currently feasible at high laser utilisation rates. This is as a result of necessary delays built into the processing sequence in order to prevent over heating of the individual features. Ideally, the laser beam would be processing at another position/feature on the workpiece and would return after the required delay. However, this is not possible with the current software, since it was designed primarily for marking applications. Consequently, the laser duty cycle was approximately 10% for the array of features produced in Fig.8. This corresponds to a build time of 40s per feature. Specific programs for the laser beam path could be written to increase the laser utilisation rate to nearly 100%, which would allow the entire array of 170 features shown in Fig.8. to be produced in approximately 700s, if time is also allocated for positioning of the laser beam and part handling etc.
In order to make a direct comparison with electron beam Surfi-Sculpt, a similar array of features to those shown in Fig.8 were also produced using a modified Hawker-Siddeley 150kV 6kW rated electron beam machine, fitted with a 5-axis beam deflection system. The proprietary software utilised allowed a ~98-99% beam utilisation rate which resulted in a process time, including all beam and workpiece manipulation, of 60s per part. It should remembered that a simple vacuum handling strategy would also be required and would add about 15s per part. This time is an order of magnitude less than achieved with the low power laser system, although it is worth noting that the cost of such an EB system is at least an order of magnitude greater.
The potential for using low power Yb-fibre lasers for producing features using the Surfi-Sculpt process has been assessed. The following conclusions are made:
- A 200W diffraction limited Yb-fibre laser, focused into a 38µm diameter beam width, is sufficient to produce surface features, at swipe speeds up to 800mm/s, in a range of metallic materials.
- Successful management of the temperature distribution in the workpiece is critical to integrating single protrusions in to large surface features.
- In order to increase the productivity of the laser Surfi-Sculpt process, software is required (similar to that run on electron beam machines) to increase the beam utilisation rate.
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