The Science Of Control: How To Create The Surface Finish You Need For Aerospace Parts
| In the Spotlight
The global Vapormatt Group was established in 1978 and the Head Office is based in Taunton, Somerset in the UK. The Company is owned by the third generation of the Ashworth family. As the originators of the pressure recirculatory wet blast process, the Ashworth family has accumulated experience and expertise in this field for over 50 years. The Company designs and manufactures manual and automatic surface cleaning, peening and finishing machines, using the Vapormatt Wet Blast and High Pressure Water Systems.
The aerospace industry has incredibly high standards, and with good reason: safety is paramount. As a result, there are many specifications governing the manufacture and servicing of components. These components are made from a varied range of materials, from steel through to titanium – and, increasingly, polymeric materials such as carbon fibre composites.
Materials must be tough and resilient, but also practical for use in aerospace manufacturing – so be receptive to techniques like adhesion. One key to maximising their properties is correct surface treatment, and there are many ways of achieving this.
One technique that is rapidly gaining momentum within the aerospace sector is wet blasting. Very simply, it uses a suspension of solid particles in a carrier liquid to treat the surface. The liquid-borne abrasive is mixed with pressurised gas, and the resultant slurry is pumped through a nozzle and over the material’s surface. By varying each parameter – through careful control – this single technique can achieve a wide variety of aerospace surface finishing tasks, from mild washing to full surface peening.
Although the principle of wet blasting is straightforward, its implementation is more complex. To harness its full effect, wet blasting requires close control of around 15 separate parameters – such as the nature and size of the abrasive particle, the angle of each nozzle to the material surface, and the speed of the slurry exiting the nozzle.
Vapormatt’s ability to automate and control this entire process has helped to make wet blasting a key technique in the aerospace sector.
A longer-established technique for surface treatment is dry blasting. Here, abrasive particles are fired through the air and across the surface of the part. While this is an effective method, it has its drawbacks – in that it creates dust, static and noise. It is also more likely to introduce surface contamination – which is where cracks or corrosion can start. Wet blasting can solve these problems and – when appropriate – replace dry blasting in a number of applications.
Peening is a critical technique within the aerospace industry, as it can impart extra strength and longevity to parts such as turbine blades. It involves putting the surface under compression by bombarding it with hard, spherical particles. This makes it resistant to cracking, and thus extends service life.
Historically, these hard particles have been fired at the surface using dry blasting. This offers plenty of force, and is still widely used, but there are drawbacks – most notably the creation of dust.
Dusty parts must undergo post-process cleaning. There are also health and safety issues with a dusty environment – which, in extreme cases, can create an explosive atmosphere. While dry blasting is effective, these problems could be overcome by using wet blasting instead.
It should be stressed that aerospace manufacturers cannot simply choose to replace dry blasting with wet blasting. Some written specifications – particularly older ones – may insist on dry blasting. Others, while allowing a choice between techniques, still require skilful application of wet blasting in order to do the job properly. Vapormatt can advise on the best ways to apply wet blasting to this critical and important process – and the most critical element is control.
Peening is carried out in order to bring about a specified change in the surface of a metal (such as titanium). If sufficient abrasive particles are not spherical, i.e. they have broken down too much, they will have a lesser and detrimental effect– potentially leading to early (or even catastrophic) failure of a part like a turbine blade.
To combat this, Vapormatt has developed a sophisticated feedback technique that not only monitors the size of the particles (broken particles are smaller than unbroken), but also maintains the volume in process so that the slurry continues to have its full effect.
The system separates out the "spent" particles using its patented "elutriation towers". They work on the principle of Stokes’ Law – which identifies that the fall rate of a particle through a fluid is related to its size and density. As the slurry is recycled, it is fed into a column of water that is pumped upwards: heavier particles fall quickly to the bottom, while lighter ones rise with the water column and flow out of the top to be removed by conventional filters.
At the same time, the system measures how many "good" particles are left, through constant monitoring of slurry concentration by use of a density meter built specially for the task. By detecting when there are insufficient abrasive particles, new ones can be added automatically. This ability to maintain slurry condition and concentration within fine tolerances is a key factor behind the commercialisation of Vapormatt’s wet blasting technique for peening.
When it comes to composites, wet blasting proves itself to be vastly superior to other methods of preparing the surface for adhesive or paint bonding.
Carbon fibre reinforced plastics (CFRPs) have steadily become more important in aerospace, in models such as the 787 Dreamliner and Airbus 380. Future Materials Group, a UK-based strategic advisory firm, believes that smaller planes – such as the Airbus 320 – might also adopt more carbon fibre in future. This would shift the industry further towards a ‘carbon future’.
The key to their use in aerospace is to combine them with traditional materials – and that means bonding the two together. Wet blasting is the best way of preparing composites surfaces for bonding thanks to its gentle, repeatable action – which ensures that the individual fibres are not destroyed. At the same time, its ability to treat every part of the surface gives it a huge advantage. It is the only technique that is delicate enough to remove some of the polymer matrix between fibres – and ensure a highly activated surface for bonding.
Established processes such as manual abrasion, dry blasting and peel ply are limited in their ability to provide precise, reproducible results. This means that they might not meet certification levels required for composite parts.
Wet blasting offers enhanced precision and reproducibility in composite part preparation. For high value composite parts such as fan blades or engine parts, reproducibility becomes even more important: dry blast and manual abrasion may lead to over-processing. Even if this only lasts for a few seconds, it can damage the underlying fibres and hence the part.
With controls in place for slurry as well as air pressures, a finely tuned buffer can be configured to ensure a gentler action that will leave the fibres undamaged. The water buffer relies on the ratio between air and water pressure, and has several effects: it lubricates the action of the abrasive media; it eliminates static build-up; it washes the surface; it prevents impregnation of hard particles into the soft composite resin surface; and it avoids fibre damage.
Peening and composite surface treatment are the main applications of wet blasting, but there are many others – from cleaning and paint stripping to testing and manufacturing.
When aircraft tyres are changed, for instance, the wheels are inspected for cracks using non-destructive testing. Firstly, though, surface dirt must be removed, and this can be done using wet blasting. With the correct abrasive, cracks are cleaned out rather than being disguised by being peened over. In similar fashion, ramping up the blast pressure – and maybe changing the abrasive – takes this a stage further, and can strip the paint from a part. Here, it is a safer alternative to a technique like chemical cleaning.
There are also ways to incorporate the benefits of wet blasting directly into manufacturing: other than its role in peening – a key step in the manufacture of parts like aerofoils on fan blades – it can be applied to manufacturing operations within foundries, such as when removing resist from delicate features on castings produced by the lost wax process.
One advantage of wet blasting is that a single technique can be applied to such a wide variety of industry tasks by varying the process conditions – and the nature of the abrasive.
The importance of control
By bringing control and automation to wet blasting, Vapormatt has made it a powerful – and widely applicable – technique within aerospace.
Wet blasting was traditionally a manual process, in which an operator used a blasting gun to treat large components – rather like using a pressure washer to clean your car. However, it has become more automated – especially in the highly regulated aerospace industry – in order to guarantee consistency.
A good example is when surface-treating an aerofoil, allowing a de-icing strip to be adhered to it. Automated wet-blasting ensures consistent cleaning of the surface, and guarantees an identical bond every time.
Automation also helps to prevent "over processing" of parts. One example, already noted, is in composites – where excessive treatment can damage fibres and reduce mechanical properties. However, the principle also applies to robust metal parts.
Combustion casing and discs, for example, are repaired and cleaned using wet blasting. However, they must never be over-processed. This is so critical that parts are weighed before and after processing, to ensure that no metal has been removed from the part.
Nobody wants to be responsible for scrapping a £1m disc. This, more than anything else, shows that simply blasting a surface is not enough – and that the process must be kept under tight control.
Overall, automated wet blasting is a flexible, adaptable process that offers many advantages over traditional surface treatment techniques. It is becoming a powerful tool in the aerospace sector – and this will continue to grow as composites become ever-more important in the industry.
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