Metal additive manufacturing for high-temperature applications: Will Dick-Cleland assesses the use of a new alloy
The 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.
The 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 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. Therefore, these alloys have historically been processed using casting methods where the cooling rates are relatively low.
This 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. The advantages of AM-compatible alloys for high-temperature applications derive from the fact that the components assembled in hot sections of engines are usually some of the most expensive, and due to the harsh environment in which they work, have short life-cycles. Using the L-PBF process with high-temperature alloys offers manufacturers the opportunity to benefit from better inventory management, reducing late penalties due to tooling delays, and creating more intricate shapes, often allowing the integration of several components in a sub-assembly into one.
It was to address this requirement that Alloyed worked with leading aerospace OEM Honeywell to test a new high-temperature nickel alloy, ABD 900AM, with the L-PBF process, to determine if it could serve as a replacement for existing high temperature casting alloys. The purpose of this effort was to discover at a high level if ABD 900AM showed promise and warranted further development.
After an initial introduction to Alloyed by a third party, Honeywell recognised the potential of the Alloys By Design (ABD) system brand developed by highly respected metallurgists from Oxford University and this stimulated further interest in the ABD-900AM material and its potential for high-temperature aerospace applications as it was not prone to cracking.
The result of this initial collaboration was a dedicated project that would see Honeywell build a limited number of test specimens in its additive manufacturing laboratory located in Bangalore, India. Honeywell would build and test the material to determine if the results merited continued investment and development.
ABD-900AM is an age-hardenable, nickel-based superalloy designed specifically for use as feedstock in the L-PBF process. It is optimised for environmental resistance and high-temperature tensile strength, with a working temperature range up to 900°C in its age-hardened state. It not only offers a higher operating temperature but also significant long term stability.
Exhibiting excellent creep strength, the material has also demonstrated superior resistance to cracking during manufacture and heat treatment, enabling complex part design. It is designed to be free of solidification, liquidation and strain-age cracks, and showcases exceptional printability for a 40% γ’-phase strengthened alloy. The alloy also shows high as-printed part density of >99.9%.
The scope of the collaborative project between Alloyed and Honeywell was the assessment/qualification of the ABD-900AM Ni-alloy powder for the next generation of additively manufactured high temperature applications. Honeywell has undertaken a series of tests to confirm and optimise the properties of the alloy with some extremely positive results.
From a broad perspective, the project provided a manufacturing assessment and a mechanical property assessment of the alloy.
Additive manufacturability. It was important to assess how the alloy behaved when processed on a L-PBF machine, and whether the welding operation involved – with all this implies in terms of rapid thermal gradients – would deleteriously effect the integrity of the final part. Various test geometries were trialled on an EOS metal system, and component scale testing was undertaken. It was found that there was no part distortion on cracking in the “as-printed” condition, there was good surface finish, and a wide processing window.
Powder recyclability. The ability to be able to re-use unsintered material from build-to-build is obviously an important factor when looking at the economics of using metal AM processes. Because of this, it was decided to manufacture and test parts made with “used” powder, and it was found that there was no significant change of properties in the parts made with used powder compared with virgin ABD-900AM powder.
Post processability. It is often the case that parts must undergo a variety of post-processing activities after the AM build, which can affect the mechanical properties of the material and therefore, the functionality of the part. To assess how the alloy behaved parts were subjected to vacuum heat treatment and hot isostatic pressing (HIP) often used on metal AM parts to eliminate internal voids. There was no “post-weld” cracking detected on parts made from the alloy.
The results of the work between Alloyed and Honeywell show that ABD-900AM is an alloy that demonstrates great potential for high temperature applications. The material welds and fuses extremely well, particularly when compared with other high temperature nickel alloys. Although it is not a replacement for CM Mar-247 in most cases due to the oxidation capabilities, it does exhibit very good mechanical properties at high temperatures when compared with Mar 247 or IN792 or IN713 or IN738.
Will Dick-Cleland is with the process technology team at Alloyed