SECTION TITLE
ADDITIVE MANUFACTURING
MATERIAL DEVELOPMENTS
Metal additive manufacturing for high-temperature applications: Will Dick-Cleland assesses the use of a new alloy
T
he aerospace sector has long been recognised as an early adopter when it comes to cutting-edge technology inventions, and as such it is unsurprising that the use of additive manufacturing (AM) as a production tool is relatively familiar in many of the leading aerospace OEMS and their supply chains. Te sector is typically not a mass
production industry but is instead characterised by complex low-volume manufacture that plays directly to the strengths of metal AM (and specifically the laser powder bed fusion (L-PBF) process) when compared with traditional metal manufacturing processes. In an area that is highly competitive, where conventional manufacturing processes are costly – especially for metal components produced in relatively low volumes – and with supply chains constantly under pressure to conform to ever more stringent environmental performance restrictions, the aerospace sector needs to become more agile. Metal AM processes, with the ability to manufacture innovative, geometrically complex parts in a timely, local and cost- effective way, have a huge part to play. AM’s disruptive for potential for the
aerospace sector is widely acknowledged, holding out the opportunity to break the typical compromise between scale and scope in manufacturing first by reducing the capital expenditure required to achieve economies of scale, and second by increasing the variety of designs that a given amount of capital expenditure can produce. One AM platform can at the same time build a number of complicated parts with variable designs, and this means that large centralised factories with assembly lines are not always required, and with AM complexity, production change overs, and customisation are cheaper. As AM technologies continue to
develop and secure a stronger foothold 14
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in production scenarios, there is simultaneous development ongoing across the sector to refine associated parts of the AM ecosystem, with a great deal of emphasis being placed on in-process monitoring for traceability and validation purposes, as well as automated post-processing technologies. A further, vital area of development is materials, particularly metal powders for AM that meet – and exceed – the functionality and performance of existing production materials as well as meeting the requirements of specific applications.
ALLOYS FOR HIGH TEMPERATURE APPLICATIONS It is fair to say that, to date, metal AM processes have not performed well with the highest temperature materials – that is to say, materials for components that operate at the top of the temperature range within jet turbines. In the aerospace sector such applications would include (but are not limited to) the critical areas of a turbine engine such as the back-end of the compressor, the combustor, the high and low pressure turbine areas, and the exhaust. All parts in these areas of the engine are exposed to high temperatures and higher levels of oxidation. For such high-temperature applications,
the aerospace sector typically uses nickel (Ni) alloys such as IN738, IN713 and MarM247. However, these alloys are incompatible with welding techniques due to their chemistry, as they do not respond well to rapid thermal gradient changes and it is virtually impossible to control the amount of cracking during the weld process. Terefore, these alloys have historically been processed using casting methods where the cooling rates are relatively low.
A rocket combustion chamber
Tis is the conundrum facing users of metal AM within the aerospace sector looking to capitalise on the advantages that the process has to offer for high- temperature applications. It is this that has driven further exploration of alternative materials that could replace traditional high temperature alloys without compromising structural integrity while at the same time being compatible with AM. Te advantages of AM-compatible alloys for high-temperature applications derive from the fact that the components assembled in
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