As recently as a few years ago, when it came to quality assurance, two-dimensional X-ray inspection still prevailed in the aviation industry. Due to the increasing complexity of turbine blades and other parts to be inspected, three-dimensional computed tomography has gained more importance.
CT is increasingly replacing conventional inspection procedures because it has become distinctly faster and more economical. Rapid technical advances over the last few years, above all in fully automatic inline CT inspection, allows the part to be loaded, X-rayed, and analyzed automatically. As Stefan Neuhäusler, project manager for development of special inspection techniques at MTU Aero Engines, says: “For manufacturing processes of complex jet engine parts (parts that must withstand high stress loads), inspection via inline computed tomography is indispensable.”
The demand for increasingly higher combustion temperatures, to achieve lower fuel consumption and decrease pollutant emissions, places particular stress on the turbine blades in the first stage of a high-pressure turbine. In the area behind the combustion chamber, temperatures of up to 1200°C at a pressure of up to 36 bar prevail, with the leading edge of a turbine blade subjected to the highest thermal stress.
A very high level of precision is therefore necessary when producing the cooling channels to counteract the heat. The alloys from which high-end turbine blades are manufactured are very difficult to process mechanically. Cooling channels, leading from the turbine blade’s exterior surface to internal cavities, are cut using a laser. Care must be taken that the boreholes for the cooling channels do not overlap (merged holes) and that the opposing walls of the cavities are not damaged by laser bores through over deep penetration (over- shots).
Fuel consumption and pollutant emissions can be diminished additionally by reducing turbine-blade weight. The mechanical stresses being placed on a turbine blade, 11 cm high, with a weight of 275 grams and rotating at 15,000 to 18,000 revolutions per minute, lead to a weight equivalent of several tons dragging on the suspension.
Consequently, the manufacturing challenge is to produce turbine blades that are extremely stable and able to withstand high temperatures whilst, at the same time, reduce their weight. Reducing blade weight allows the rotor disc weight to be reduced and delivers a reduction in the overall weight. This is the only way to achieve the goal of lowering fuel consumption and pollutant emissions. Yet at the same time, this complex production process increases the danger of potential flaws and defects. These can be detected with certainty using inline CT inspection.
Inline CT inspection in practice
As a rule, customers’ specifications and manufacturers’ own quality standards require multiple inspections of turbine blades within the production process. The next step occurs after the finishing phase. For older or less sophisticated turbine blades, conventional 2D X-ray technology supplies sufficient results. In contrast, inspecting complex inner structures in high-end turbine blades for the presence of flaws and defects can only be performed with assurance using 3D computed tomography.
In inline CT inspection, the conveyor system is adapted to the cycle time for the production line. The pallet loaded with turbine blades is automatically channelled into the X-ray cabinet, where a robot sets the individual turbine blades in the positions required for inspection. Following acquisition of the individual X-ray projections, a 3D volume for the turbine blade is reconstructed from the raw data obtained. The inspection items are now examined for the presence of flaws and defects with the help of automatic defect recognition (ADR software). The main focus of attention is directed to inspecting hole patterns in the cooling channels.
Diameters here can be as small as a 300 µm. The correct position and length of these channels are extremely important for cooling off the turbine blades. A total of several dozen quantitative and qualitative parameters are registered per turbine blade and assessed fully automatically in compliance with predefined quality assurance stipulations. After inspection the robot sorts out defective inspection items.
With regard to time and cost factors, a fully automatic, 100% inline CT inspection is profitable only when the cycle time remains lower than 10 minutes per turbine blade. This specification is achieved by systems from the Y.CT Solution line because the X-ray source, detector, loading and fully automatic analysis are coordinated precisely with one another.
The role of the components
The system’s individual components such as X-ray tube, detector, and analysis software play an essential role in successfully deploying a computed tomography solution.
The X-ray tube
An X-ray source with a very high output is necessary for computed tomography because, with all other parameters remaining equal, the measurement time is inversely proportional to the output (at half the output the measurement time is doubled). Until ‘high-power’ technology for X-ray tubes was launched in 2006, high-resolution X-ray tubes had been comparatively ‘low-performance’, meaning that the acquisition time for capturing X-ray projections was too long to carry out a 100% inspection in series production. Through the introduction of high-output technology, X-ray tubes displayed a distinctly greater performance while additionally offering a significantly smaller focal spot. This, in turn, guarantees a higher resolution, and thus greater detail detectability.
The Y.TU225-D04 X-ray tube with high-power technology is used in the Y.CT Solution line. With its 800 watts it exhibits an output that is more than twice as high as the model preceding it. The relevant focal-spot size has been reduced from 500 µm to 400 µm (in accordance with the EN12543 standard). The reverse conclusion is that the measurement time can be halved as long as the resolution remains constant. When the newest technology comes into play, the Y.TU225-V01 variofocus X-ray tube, resolution can be doubled in comparison to the above-mentioned technology. What’s more, this X-ray tube offers optimum coordination of the focal spot to the application involved, and, as a result, opens up even more possibilities toward reducing cycle times.
In computed tomography, the object is rotated in order to X-ray it from every angle. Sufficient radiographic intensity must be able to be measured so that the information about the object is complete: that also includes the object’s thickest points. At the same time, the X-rays passing by the object at its sides cannot be allowed to overexpose the detector. For these reasons, digital detector arrays (DDA) - as opposed to image intensifiers - have asserted themselves in recent years. Flat-panel detectors have a considerably larger scope of dynamics, up to 65,000 shades of gray. As a result, the lightest areas are not overexposed, while at the same time, in the darkest areas, image information remains intact.
The analysis software
Optimised program code, along with several computers calculating simultaneously, allows a 3D volume to be generated and evaluated (using ADR software) in the same cycle time it takes to irradiate one turbine blade. A cycle time of about 10 minutes per turbine blade is thus achieved.
It is not only the turbine blades which must meet high standards to be licensed by the air safety authorities (FAA, EASA): the quality assurance process itself must be certified, too. Non-destructive inline CT inspection is not only an extremely certain method of ensuring quality assurance, it also supplies important indications regarding workflows and courses of action within the production process.
Traditionally, samples are typically taken from the production process on a spot-check basis. In the course of this quality inspection, the turbine blades are sliced apart in order to test the turbine blades’ highly complex inner structures. Each modification made in a turbine blade, or in the production process, requires a renewed verification to confirm that the spot-check sample procedure being applied can guarantee sufficient process assurance and fulfils the requirements in accordance with the QA process ? all of which is costly and time-consuming. If a deviance from the production process is established during this testing, the production process must be stopped and corrected. In addition, all turbine blades produced between this last spot check and the one preceding it must be re-inspected retroactively.
By contrast, using inline computed tomography for the non-destructive, 100% inspection of turbine blades poses a more economical and faster alternative. Here, all of the turbine blades are inspected, allowing process deviations to be noticed immediately. This, in turn, ensures that all parts produced comply with the predefined specifications.
Innovative aerospace companies like MTU profit first and foremost from this technology because, by using it, improvements and modifications made on the complex-shaped turbine blades during the production process can be enacted on a timely basis. The inline CT solution ensures the necessary quality, but without incurring the long delays that occur due to statistical, cost-intensive and time-consuming destructive methods of inspection. A high degree of process stability can thus be achieved in the shortest time span, and innovations make their way onto the market faster. If an automatic analysis of volume data - with the help of ADR software - is utilized as well, the inspection decision itself can be made objective when compared with subjective inspection decisions. Inspection quality that remains constant lowers costs effectively and increases the rate of throughput. As Stefan Neuhäusler comments: “A production process that runs stably can be made distinctly faster by deploying inline computed tomography.”
What is likely the future demand for 3D X-ray CT systems? Since both the complexity in the topology of turbine blades and the number of high-end turbine blades are definitely going to increase in the future, a steadily rising need for modern CT technologies in this sector is predicted. In terms of the coming decade, growth forecasts for the high-end turbine blade market lie in the double-digit region, and is clearly above those for conventional turbine blades. In light of the increasing demands being placed on jet engine efficiency, the requirements set for inspection technology are going to increase, too. Companies are going to progress toward measuring additional parameters, such as complete hole patterns, hole cross-sections and wall strengths. And while doing so, an ever-greater measurement precision in computed tomography is going to replace other technologies, such as ultrasonics.
The Author, Dr. Malte Kurfiß, Product Manager Computed Tomography Systems, YXLON International GmbH