Crystal clear traceability for turbine blades

As a key limiting component in jet engine performance, gas turbine blade manufacturing represents the cutting edge of materials engineering. The blades, which sit in the flow of combusting gas and turn the engine's shaft, operate in temperatures of around 1500°C, significantly above the melting point of the high-tech alloys from which they're made. This is possible thanks to a combination of heat-resistant ceramic coatings and internal vents through which cooling air is blown to control the temperature of the metal. Modern blades are made from single-crystal nickel ‘super alloys' that can contain up to 10 other exotic alloying elements. These alloys are designed to give the highest possible melting point, and also to eliminate creep – the deformation of the metal that can occur when centrifugal force acts on the softened material over a prolonged period. This is obviously critical, as elongation of the blades could cause them to contact the casing and result in the failure of the blade. The complexity of the component and critical nature of the role it performs presents a unique traceability challenge for aero engine manufacturers. <The production process> Turbine blades are manufactured in an investment or ‘lost wax' casting, which allows the creation of very complex cast shapes, including the hollow cores required for cooling. David Ray, technical director at traceability specialist Pryor Marking Technology, explains the production process: “For each blade that is cast, the process begins with the creation of a disposable wax form in the shape of the final blade. This in itself is a complex process, with the wax passing through several machining processes and quality checks before it is ready for the next stage. “The final wax form is then coated in a heat-resistant ceramic material from which the wax is then melted away, leaving a perfectly formed shell into which the alloy is cast. To create the hollow core, a separate ceramic form is placed into the mould before the alloy is poured. Once cast, the blade will then go through a number of machining stages and quality control checks before it is ready to install.” As with any complex production line for critical parts, traceability through the whole process is a mandatory engineering requirement, so that any defects in quality in the final component can be traced back to the specific tool that caused the issue. This means monitoring and controlling every step in the process, maintaining a comprehensive historical record and ensuring that each blade coming off the production line is marked with a unique identifier code that allows a process engineer to say with certainty which processes it passed through and when. For investment moulding processes, this means being able to connect the code on the finished part with the individual wax mould that was used to cast it – but creating a mark that will survive the investment moulding process means multiple marking techniques are needed for different stages of production. <Automated, multi-stage solution> Pryor Marking Technology has recently installed a complete marking programme for a state-of-the-art turbine blade production facility run by a leading aero engine manufacturer. “The customer needed the solution to not only meet the challenge of providing traceability right through the process, but also to be automated to remove the need for constant attention by a production engineer,” explains Ray. “The multi-stage solution we designed incorporated three different marking methods along the line, combined with machine vision cameras to read marks, this ensures their legibility, and automatically applies the data to the component in the format required for the next stage.” The first stage was to identify a means of marking the wax moulds so they could be tracked through the initial screening process. The marking method usually favoured for production line traceability is the dot peen Data Matrix – a 2D machine-readable code applied by punching a pattern of shallow circles into the material. This approach isn't suitable for the wax moulds used in this process, as they are sensitive to impact and also because the size and spacing of the dots required to ensure readability in the soft wax would be too big for the small surface area available. The blades themselves vary in size from around 100mm to 150mm in length, but the mark cannot be placed on aerodynamically-designed gas-washed surfaces but rather to the much lower-stressed ‘fir tree' end faces at the base of the blade. As a result, any mark applied is limited to a maximum width of around 30mm, staying clear of edges and other features. To address these issues, an inkjet system was developed to apply a good quality, repetitive 2D code to the surface of the wax. Next came the challenge of applying a physical mark to the wax that would remain readable when it appeared on the alloy blade. Having already established that dot peen was not workable, Pryor developed a bespoke scribe-marking system that used a custom-designed tip with the right flexibility for the surface. Initially, a scribe-marked bar-code approach was considered, but the reduction in resolution that resulted from passing through the casting process resulted in less than 100% legibility during the testing phase. The solution was to apply an alphanumeric code with the scribe marker, which would then be read by an optical character recognition (OCR) system once the blade was cast. In tests, this approach delivered the 100% legibility required. In order to ensure the markings on the blade are consistent with those used across every single one of the manufacturer's components, the alphanumeric code is re-applied to the blade as a dot peen 2D matrix in a separate location, before the alphanumeric mark is machined away to leave a smooth surface. The end result is a permanent, machine-readable mark for the life of the part containing a unique serial number that will allow anyone investigating a manufacturing or in-service event to link the serial number to the detailed manufacturing and inspection history for the part. Of course, in the highly-regulated aerospace industry, a high level of traceability has been a central requirement for many years. However, in the past this has typically been a laborious manual data-transfer process where mistakes and errors can inevitably add risk and uncertainty to process data. Thanks to innovative automated marking solutions such as that outlined above, human error has been significantly removed from the process, saving time and increasing the reliability of manufacturing and traceability data. www.pryormarking.com