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Enhanced fatigue resistance, improved performance, increased efficiency
Aerospace Shot peening – the process
Shot peening is a cold working process used widely to enhance the fatigue resistance of highly stressed metallic components such as aero engine compressor blades, aircraft structural parts and automotive transmission systems. Any metallic component that is subjected to cyclic stresses within its elastic limit can benefit from the aerospace shot peening process.
The basic peening principal has been around for centuries and today the process is applied using highly controllable, usually programmable, machines. It is achieved by propelling a stream of shot peening media (precisely manufactured round steel shot, glass bead peening or ceramic beads) at aircraft components, at high velocity under fully controlled conditions utilizing a compressed air stream or, centrifugally, by means of a vaned wheel.
Compressive stress helps prevent crack formation
The shot peening process works by introducing residual compressive stress in the surface of the component. The compressive stress helps to prevent crack initiation, as cracks cannot propagate in the compressive environment generated by peening.
Compressive stresses are generated when the impact of each particle of shot on the component produces a small indentation. It follows that if the surface has been dented then the material beneath the dent has been compressed. Peening generates not just one dent but many thousands over the surface. Eventually the component becomes encased in a compressively stressed layer.
Measuring effectiveness
Shot Peening is often used to improve the performance of safety-critical components, so it is important to ensure that the correct intensity of stress is being created. This is achieved with the proven ‘Almen Strip’ testing procedure. The ‘Almen Strip’ – manufactured from spring steel to strict tolerance of hardness, size and flatness – is peened on one side only. The effect of the induced compressive stress on the strip results in bowing or curving.
The extent of the curve is proportional to the energy imparted by the shot and is measured on an ‘Almen Gauge’. The Almen Strip arc height varies according to both the velocity and mass of the shot i.e. the amount of energy imparted by the stream of shot and absorbed by the strip. X-Ray Diffraction techniques also provide an accurate method of measuring the actual stresses within the component and quantifying the actual effect of the peening process. Results are achieved by measuring the angle of reflection in relation to the angle of incidence that varies dependent on material composition and residual stresses.
Reduced friction, less corrosion, wear resistant
In addition to preventing premature failure caused by fatigue, there are other important benefits from the process.
Fretting and galling – components which are moving in relation to each other, for example as bolted or riveted assemblies, or within bearings as sliding or rolling members, can wear and fail as a result of microscopic transfer of material from one surface to the other. The surface finish produced by the peening process provides pockets for lubricant retention; it also reduces the surface
area in contact under rolling or sliding conditions, thus reducing friction. Peening is a surface treatment which also has a surface hardening effect and helps the ‘skin’ of the material to resist wear. These characteristics, when combined, provide excellent anti-galling properties making your components more resistant, harder working and providing increased efficiency. Corrosion – intergranular and stress corrosion cracking can be inhibited by shot peening which modifies the properties of the metal at a metallurgical level. The process advantageously alters the granular structure at and near the surface, producing a condition less prone to corrosion.
Typical applications
The benefits of peening have been well proven, both with components operating in a highly-stressed but relatively short-lived environment, such as motor racing, and for critical parts with a much longer and predictable operating life in aeroengines and aircraft structures.
Process selection is critical to performance
Equipment and process selection, whether to employ a compressed air system with blast nozzles or a wheel blasting technique, are of paramount importance when developing solutions for new components. The selection of the correct equipment and process parameters is complex. Component variables such as throughput, size, shape, material, hardness, application and operating environment all have to be considered. The topography of the component is also an important factor. For instance, if a radius is smaller than that of the shot, then a non-peened area will result. Sharp edges, blind holes or hidden areas also need special attention to ensure good coverage and to avoid damage.
Peen forming – size doesn’t matter, it can still be made stronger!
Peen forming is a variation of shot peening. The forming of wing skins by pressing or rolling can introduce the tensile stresses that are responsible for the initiation and propagation of cracks. Because shot peening introduces compressive stresses, it is possible to form thin components like aircraft wing skins into shape, simply by using the peening technique. The results are a component of the correct form but with inherent compressive stresses present on both sides, thus preventing crack initiation.
Correcting distortion without compromising quality or efficiency
Since it is possible to form parts with peening, it follows that the process can also be used to correct distorted components, without including undesirable tensile stresses. Components machined from solid material and of thin section (certain large airframe parts like body segments or aircraft wing ribs, for example) are especially prone to distortion, no matter how good the machining or stable the original billet. Correction, or flattening, is achieved by introducing compressive stresses to counteract the existing tensile stresses that are causing the part to distort.
Customer Focused Aerospace Blasting Solutions
Our team of experts is on hand to work with you and answer any questions you may have regarding shot peening equipment or service. They are always delighted to discuss new and exciting applications you may have.
NADCAP accreditation assures highest standards of quality and service
Impact Finishers, the sub-contract division of Wheelabrator Group has passed an in-depth examination to receive the aerospace industry’s acclaimed NADCAP accreditation (NADCAP approved shot peening). The distinction applies to Impact Finishers sub-contract shot peening facilities in the UK at Slough and Coventry. Launched in 1990 by the Society of Automotive Engineers, Nadcap is a universal standard administered by the not for profit Performance Review Institute (PRI), to ensure manufactured aircraft and aero-engines are of the highest quality. It is now specified by the majority of prime manufacturers in these sectors.
The accreditation will allow aerospace equipment makers to use Impact Finishers, confident of receiving the highest standard of service, conforming to strict industry guidelines.
The site in Slough, which handles mainly large, pocketed airframe components, has been accredited for manual, automatic and peen forming processes. The plant in Coventry, which undertakes work on mainly rotational parts, has been accredited for automatic processes. Wheelabrator Group established a dedicated three-strong Nadcap committee at each Impact Finishers location, their eight-month bid to secure the accreditation culminating in a two day in-depth audit at both sites covering shot
peening, peen forming and glass bead peening. Christian Tyroll, quality manager based in Slough, headed the group’s campaign and regularly attended Nadcap task group meetings with experts involved in maintaining the scheme.
“Added reassurance of exceptional service”
He said: “We’re delighted to have secured this prestigious accreditation, which will promote and strengthen our relationships with original equipment manufacturers (OEMs) and fellow Nadcap accredited suppliers. While we already conform to AS9100 and ISO9001 standards, as well as holding customer approvals from several major OEMs, this new endorsement of our equipment and processes is an added reassurance of the exceptional service we provide.”
Seema Saleem, director of European operations for PRI, said “When a supplier achieves Nadcap accreditation, it is a testament to their dedication to special process quality.”
The Wheelabrator Group - Impact Finishers team provides services to the automotive, aerospace and medical industries. Subcontracting to them allows customers to concentrate on their core activities, reduce bottlenecks and handle erratic production runs.
Flexible Blast Solutions - can you afford to miss out?
A quality blast result is key to customer satisfaction, but it is not the only deciding factor for aerospace companies when making equipment purchasing decisions. Efficiency and cost are also high on the priority list. The double-frame technology pressure blasting systems from Wheelabrator Group offers you flexible blasting solutions without compromising quality, efficiency or operating costs. It also saves you time, allowing continuous production without time lost for set up in between component change over (even with workpieces of different sizes).
The double-frame technology is already being used in blast cleaning equipment and hardening integral frames, axle tubes and other complex workpieces. The concept uses two frames which carry the workpieces and allow the component change over and blasting process to occur simultaneously. The blasting process is mostly performed by blast robots, especially for complex components, with very low machine downtime.
Minimal machine downtime with maximum results
Outside the machine, one frame can be loaded and reloaded automatically or manually while the second frame is in the treatment process. This allows a continuous machine operation without the usual downtime and significantly reduces the processing costs and time for each workpiece. A further cost reduction is achieved by the operation and maintenance friendliness of the machine, its features include: walk-in blast rooms, ground level installation of the robots, good accessibility to the machine components, and easy menu control with integrated maintenance instructions. These features enable a high speed, effective blasting process to be carried out at a fraction of the usual cost, but with maximum results.
Flexible even for special components
The flexible nature of the double-frame technology also allows special components to be blasted without interfering with ‘normal’ production runs. Special workpiece carrying frames are used which are equipped with manually or automatically loadable workpiece supports and are easily entered into the blast rooms via inlet openings. This reduces the set up times and also solves the safety problems generated by manual loading in an automatic production line. After the special components have been blasted, ‘normal’ production can be resumed immediately.
Innovation giving you total control
The double-frame pressure blasting systems from Wheelabrator Group are fully flexible allowing total customer control. Their innovative concept and technology allows the treatment of workpieces of all different shapes and sizes, the workpiece supports in the carrying frames can be adapted to allow for serial production and special components, the frames are variable in length and rotatable, the number of blast robots and the nozzle movements can be adapted to fit the exact length of the workpiece carrying frame and to fit the workpiece complexity.
Customer Focused Blast Solutions
Wheelabrator can work with you to produce the appropriate blast programmes to suit your individual needs. This may include the complete integration of the blast cleaning equipment into your production processes including materials handling systems; you may require help with projects requiring high automation solutions or projects where space is a limitation. Whatever your surface preparation requirements are, Wheelabrator Group will work with you to deliver the right solution that meets your needs and ultimately the needs of your customers.
Wheelabrator launches a new generation of shot peening equipment for the treatment of
reactor disks.
The Wheelabrator Group Technology Centre in Charleville, France, specialises in the development and manufacturing of shot-peening equipment and ‘air blast cleaning equipment’ dedicated to the aerospace industry. The range, principally CNC machines and robotic systems, offers a set of successful and innovative solutions for the different types of surface treatment dedicated to the manufacture and maintenance of aerospace components (engine parts, landing gear, aircraft structural elements and aircraft wing panels). The new Sisson Lehmann MP 1800, developed in close cooperation with aerospace OEM specialists, is dedicated to the shot peening of jet engine disks. The operation, which crucially guarantees part reliability, consists of introducing high compressive stresses into the component surface layer, by high energy shot blasting, in order to improve fatigue life resistance.
The installation’s main features are:
9 accurate synchronized axes :
Enabling the simultaneous treatment of external and internal surfaces of jet engine rotors. The machine has a 6 axis poly articulated robot which handles 4 projection nozzles, and a 2 axis manipulator equipped with a bore peening lance and an accurate numerical turn table.
Projection parameters to control abrasives:
The shot peening media, continually calibrated in dimension and shape, is thrown by two automatic pressure generators. To guarantee the process, each nozzle feeding arrangement includes a set of high precision devices for shot flow regulation, air pressure regulation and air flow measurement. Besides these systems, an integrated control unit allows automatic flow calibration of each nozzle with continual precision.
User friendly interfaces and greatest traceability:
The programming of the robot trajectories is achieved off-line using 3D component & robot CAD drawings. This method allows the optimisation of machine immobilisation times and guarantees precision.
The machine programming has been developed especially for the application and offers a greater operation flexibility through specialised robotics (‘programmed stop’, ‘stop at the end of sequence’, ‘parametric sequence resuming’). The whole system is monitored by a PC supervision station ensuring the piloting functions and maintenance assistance, as well as a permanent audit of the process (registration of events, procedural follow up, stability statistics, quality report, production follow up).
Optimum ergonomics and easy maintenance blast rooms:
The whole installation is soundproofed not to exceed a 75 dbA sound level. The motorised front main entrance door provides complete clearance of the working zone, yet minimizes the layout space. For improved ergonomics, the main access zones for
operators and maintenance procedures are located at ground level. Additionally, a stairway and secured platform allow easy access to the higher levels.
After 12 months of production, this machine has successfully passed the most demanding qualification programme, and has been entitled to join the prestigious Sisson Lehmann MP series (Machine de Précontrainte in French , or…Mastering Peening!) from
Wheelabrator Group.
Wheelabrator Group teams up with Sonats to relieve stress in China
Stressed out aerospace companies can take to the skies after Wheelabrator Group announced it had signed an exclusive deal with high-tech French shot peening specialists Sonats, bringing a new range of machines to Asia. Sonats’ innovative technology is similar to traditional shot peening processes, using the impact of small spherical media to impart stresses on the surface of a component to increase fatigue life. However, instead of propelling the balls under pressure via nozzles, metallic elements form an acoustic block, vibrating with an ultrasound frequency to maintain momentum - named Stresssonic®.
The aim is to introduce compressive residual stresses for fatigue life enhancement or to be able to reduce the distortion after other manufacturing processes.
The Stresssonic process is very repeatable which is a key point within the aerospace industry. There are only three parameters to be controlled: mass of balls, time and vibration amplitude of the sonotrode. All are continuously controlled during the treatment.
The Beijing Aeronautical Manufacturing Technology Research Institute (BAMTRI), was interested in this new process and after few demonstrations was convinced that this was the solution to their shot peening issue. Bamtri bought a Stressvoyager® unit and is operating it in their plant in Beijing for shot peening aircraft components after welding and also for the peen forming of wing panels.
Read more about the Stresssonic technology in our next issue of Aerospace News.
GKN sees clear improvements with new Wheelabrator air blast cleaning equipment
Leading manufacturer GKN Aerospace Transparency Systems is flying through the cleaning process at its UK site in Birmingham.
Thanks to Wheelabrator Group, it has seen a 66 per cent reduction in the time taken to remove oxides from tooling parts, at its factory in Kings Norton, England. GKN Aerospace purchased a Vacu-Blast Ventus 125PR air blast cabinet to clean black residue left by the coatings process, from aluminium parts housed inside its vacuum chambers.
Graham Workman, aircraft focus factory manager at GKN Aerospace, said: “Prior to installing the Wheelabrator machine, we used our wet blast cabinet to clean the tooling, but this was causing problems. This machine was used to perform surface preparation work on our products, but cleaning the dirty aluminium as well led to oxide residue contaminating the abrasive media.
“Dramatic reduction in time”
“By purchasing a machine specifically to remove the oxide, we’ve not only freed up the wet blast cabinet to be used for its original intended purpose, we’ve seen the dramatic reduction in the time taken to clean the parts.” Applied to the surface of windows for aircraft and locomotives in a sealed vacuum chamber, the coating acts as a de-icer, helping to keep the material clear at high altitudes and speeds. GKN Aerospace uses six machines, which together coat about 700 parts each week. Mr Workman said: “As our glass products are processed, the tooling inside the chamber inevitably picks up some of the coating and we replace it with duplicate parts after every production run of each type of part. The efficiency of the new Vacu-Blast machine means the tooling is ready to be refitted when it’s time for the duplicate parts to be cleaned, giving us a streamlined operation with no machine downtime.”
GKN Aerospace needed a compact machine, which could fit into its current factory, near to the vacuum chambers, as it had previously transported the parts for cleaning to another floor. Mr Workman said: “The Wheelabrator machine has enabled us to lower costs by recycling the media, as dry blasting allows the used abrasive to be separated from the oxide particles. It also means the parts are ready for use when they come out and we don’t have to rinse off any abrasive. The nozzle design, in particular, is also preferred by our operators as we can clean the varied and complex areas of the tooling easily.”
GKN Aerospace uses high tech transparency design processes to manufacture cockpit and cabin windows for a broad range of commercial and military aircraft.
Customer satisfaction leads to repeat purchase for Wheelabrator Group
In 2002 MTU Maintenance bought surface preparation equipment from Wheelabrator Group for their new plant in Zhuhai China. The package included one of Wheelabrator’s shot blasting machines, one shot peening machine, two wet blasting/peening machines and four hand blasting cabinets.
Three years later MTU came back to Wheelabrator Group, this time to help them expand and rebuild the MTU centre of excellence for airfoil repairs, Airfoil Services Sdn. Bhd., located near Kuala Lumpur in Malaysia. This project was a joint venture with partner Lufthansa Technik and included the purchase of an aluminium oxide blasting machine (MP 1200 Ti), a glass bead peening machine (MP 1500 TD), a steel shot peening machine (MP 1500 Ti Ts) and two hand blasting cabinets (Vacu-Blast 125PR), covering all Airfoil Services Sdn. Bhd’s needs for surface preparation equipment.
Mr Juergen Ripke, Project Manager for the new plant said “We were very happy to be able to work with Wheelabrator Group again. It was an easy decision to make and was driven by the excellent experience, service and results we received with the similar machines in our facilities in Munich, Hannover and Zhuhai. We really appreciate the strong team work, knowledge and expertise in designing and setting up this equipment, which resulted in a production start in time and without problems.”