Sculpting surfaces - lasers for surface manipulation
Improved surface manipulation could have a range of benefits for industry. Paul Hilton and Jonathon Blackburn from TWI Ltd, Cambridge, UK, outline a laser beam induced approach that enables controlled modification.
A power beam that enables controlled surface features to be produced on a range of substrates, such as metals, polymers and ceramics, is now available. In application, a rapidly scanned power beam melts the material. The molten material subsequently moves, in part, due to the surface tension generated by a temperature gradient created across its surface. No material, such as wire or powder, is added, the process being entirely autogenous in nature. How the material moves, and ultimately the shape of the features produced, can be determined by precisely controlling the beam path and its speed over the surface.
The process can manufacture features, which may be identical or different. In particular, the scale of surface features available offers performance benefits for a number of applications, including orthopaedic implants, composite to metal joining, heat exchangers, and ultra-thick coating deposition.
A key characteristic is that the beam makes multiple returns to the same point and multiple swipes to sequentially build individual surface features. This differentiates the system from conventional surface texturing. Both electron beam and laser beams can be used to produce features by this method, which is called Surfi-Sculpt.
Coming into focus
In the laser variant, optical power is used to melt and displace material. Two recent developments in laser materials processing have been exploited – high brightness lasers to focus small spots of high power density, while still using a beam focusing lens of long focal length. This large distance between the focusing lens and the focused spot is needed as rapid manipulation of the laser beam is achieved by orthogonally mounted and galvanometer driven beam scanning mirrors.
Recent developments in this field have produced a new generation of laser beam scanning systems capable of operating with kilowatts of laser beam power. Both disc and fibre lasers have been used at modest power levels between 200-1,000W, in conjunction with two different scanning systems. See image (left) for the type of features that can be produced using this technique.
The diagram (right) shows how a linear feature is developed and how the beam’s successive movement is focused on the material’s surface (note the beam’s direction of motion). Material melted by the beam on its first pass flows back along the beam path before it solidifies, producing a slight protrusion at the initial point of contact and a slight depression at the end of the beam swipe. If heat input is correctly managed, successive swipes magnify this effect to the point where protrusions several millimetres high can be formed.
The images (left and right) show scanning electron microscope (SEM) image and photograph of such a linear sculpted feature. Each feature was built up from 400 repetitions over a four-millimetre swipe length. The laser beam scanning speed was 600mm/sec. The SEM images, in particular, show clear evidence of cyclic movement and material solidification. The features were made using a 200W single-mode fibre laser and a conventional laser marking scanner and software.
All shapes and sizes
More complicated shapes can be formed by more complex scan patterns. In the initial series of experiments, these patterns were limited by the scanning software packages, which were not written with this approach in mind. However, several options were still possible, such as the cones shown in (see images top). These were made by successively scanning the beam away from a central position, while rotating the swipe angle during the beam off time. The fine features shown (see images top) were produced with a laser spot size about 40μm in diameter, using the single-mode fibre laser.
It was also possible to produce similar effects using higher power multi-mode fibre and disc lasers. All the features shown in the images (see images top) were made on titanium plate, using argon shielding gas. The technique works in a similar way on Inconel, C-Mn steel, 304 stainless steel and even 7000 series aluminium, however, for the same processing conditions and programmed scan path, each shape is slightly different.
These differences are believed to be due to the physical properties of the different materials and how these relate to the physics of the technology. Viscosity of the molten material, its surface tension, thermal conductivity and reactivity to atmospheric oxygen will all play a part.
Shown in the images left (picture 1) is a series of protrusions produced on a 7000 series aluminium plate made in air. In this case, the laser power was one kilowatt from a disc laser and the sweep speed was 26mm/sec. The height of each feature is about 2.5mm. For comparison, picture 2 shows features produced using a similar scan pattern on a titanium plate (using an argon shield). This time a laser power of 750W was used and 16 legs form the basis of the feature. The domes produced were approx 2.5mm in height.
Picture 3 displays the construction of a ‘wall’ feature in Inconel alloy, using the 200W single-mode fibre laser. In this case, the scan patterns all terminated at the same point. The feature height is about two millimetres. This type of scan pattern and others have been combined to produce the array of surface structures shown in the image (picture 4).
The experimental work has shown that the process works best if the intervals between swipes in the feature are arranged so that the temperature distribution in the workpiece is carefully managed. Without attention to this point, treatments may not produce high aspect ratio builds of any kind, or may give features which self-limit in height or even simply melt. The software packages available with current laser scanning systems cannot accommodate this capability easily, because they were never designed to do so. Therefore, it is believed that, to fully optimise the capability of laser beam induced surface modification, changes to the scanning software are needed.
The research indicates that a variety of surface features can be reliably produced in a range of metallic materials using a relatively low power laser beam. Furthermore, the cost of such a system, including a small fibre laser and a conventional laser marking scanner, is reasonably cheap.
There are a number of emerging applications for fine-feature surface manufacture, which include, for example, biomedical devices and implants, where the integration of implants into the body is highly dependant on the surface. Specific small-scale features are required to promote bone growth, and these features are difficult and expensive to produce by other methods. The method here is relatively fast, allowing close control of feature shape and ensuring surface features are firmly bonded
In addition, from electronic devices to aircraft engines, available heat transfer surfaces and structures limit product performance. New and bespoke surface modifications are needed to provide additional freedom of design that help reduce size, weight and improve efficiency.
There is a demand for sophisticated, costeffective, volume surface manipulation of heat exchanging surfaces, such as fins, to increase the surface area for heat transfer, and introduce vortices into the airflow over the surface, both of which enhance microthermal management. More effective heat transfer would allow components to be reduced in size for a given thermal performance requirement, reducing material consumption.
In the area of lubricating surfaces and surface tribology, it is component design that dictates the wear rate and time between servicing of many products, for example, bearings, mechanical seals and vehicle engine parts. Designers require intelligently tailored bearing surfaces, leading to reduced friction and wear, oil and fuel consumption, and emissions. This is particularly relevant to the automotive and aircraft industries drive to meet proposed European mandates for nitrogen oxide and carbon dioxide emissions.
Bearing performance is also pertinent for many other sectors, including process plant, aerospace and domestic products. Designed surface textures of micro-features are needed to serve as micro-hydrodynamic bearings in cases of full or mixed lubrication, micro-reservoirs for retention of lubricants, or micro-traps for wear debris in lubricated or dry sliding.
The application and performance of thermal barrier coatings for high temperature devices, deposited by process technology, including plasma spray and physical vapour deposition, is limited by the relatively low strength of the coating to substrate bond, and the impact of thermal expansion mismatch at elevated temperatures. Enhanced surface structures can grade coatings and provide enhanced mechanical locking, which would be beneficial in emerging power generation technologies, such as biomass combustion waste incineration, where combustion products are particularly aggressive to structural materials.
Paul Hilton and Jonathon Blackburn, TWI Ltd, Granta Park, Great Abington, Cambridge, CB21 6AL, UK. Tel: +44 (0)1223 899000. Email: firstname.lastname@example.org or email@example.com Website: www.twi.co.uk