What is the Status Quo of utilising AM for Heat Exchanger production?

Firstly, I believe that kudos must be given to the entire metal AM community for the progress that has been made in the past 10 years that I have personally been working in the industry. I believe that the key accelerator for this increase in maturity can be attributed to the continued identification and development of applications that fully leverage the benefits of AM. This was especially evident at Formnext last year where there was an increased focus on applications, when compared to previous years.

A new hype cycle

My opinion, at least pre Covid -19 anyway, is that a new hype cycle is in existence, this being the common talk of using metal AM for serial production. This topic was ever present in many conversations that I took part in at conferences last year and continues to be a point of interest for our customers. Whilst examples of serial production are ever-growing, mainly in the aerospace and medical sectors. The truth is that AM is still a somewhat nascent technology when it comes to producing heat exchangers that can truly compete with their traditionally manufactured high performance counterparts.

Suitability of AM for heat exchangers

I always smile when I hear or read that heat exchangers are described ‘as one of the best suited applications for AM’. The main reason being that, I’ve also lost count of the number of times that we’ve had new customers coming to us saying that ‘We’ve previously tried to use AM for heat exchangers but we were not happy with the results and it did not meet our expectations’. I can relate closely to both opinions as I have been at the forefront of the manufacturing development efforts at Conflux from the beginning. I am pleased to say that Conflux has grown into the experts that we are through working hard to overcome the associated challenges and we continue to provide successful solutions to our customers’ requirements.

“Heat exchangers are extremely well suited to AM. However, our experience at Conflux proves that a deep understanding and combination of Design for AM (DfAM), fundamental principles of heat transfer/fluid mechanics, thermo-fluid simulation and AM process know how are required to achieve competitive results. An attractive well designed part for AM is not enough on its own”.

But why are they both challenging and well suited?

Heat exchangers are ubiquitous in our day to day lives and often appear to be basic in form and function? Yes, that can be the case, however some fundamental considerations exist when we couple AM and heat exchangers.

Fundamental considerations that exist when we couple AM and heat exchangers:

1. Importance of Surface Area Density
This remains the first order for performance of heat exchangers and a law of physics that even AM cannot change (sarcasm intended)!! Therefore, from first principles we naturally aim to maximise the surface area that is packaged into a given volume, without of course compromising/increasing the weight of the part. This can be challenging when considering that the wall thickness of traditionally manufactured heat exchange features/structures can be 2 to 3 times smaller than what is currently achievable with the most suited and best in class medium and large format LPBF systems. At Conflux, our heat exchangers are not often compared against other AM heat exchangers and it is more common for our parts to be benchmarked against an existing, traditionally manufactured heat exchanger with known performance. Therefore, the fundamental physics of the LPBF process mean that we are always on the back foot when considering the fundamental principles of heat transfer. This puts an even larger emphasis on leveraging the other benefits of AM in order to remain competitive in the heat exchanger space.

2. Large CAD and Build data
The importance of surface area density means that AM heat exchangers inherently contain large and dense arrays of complex features. This naturally results in very large native CAD files, which in turn results in very large STL and AM build files. This can make the creation and manipulation of the CAD geometry and build data extremely time consuming. Taking the click and wait process to another level. Also, making it difficult to comply with the fail fast mindset, which can be useful when considering a key benefit of AM is in the ability to perform rapid development cycles!!

There is no shortage of choice for medium and large format metal LPBF systems in the current marketplace. Hardware developments have been the focus for most equipment suppliers over recent years with the introduction of more ‘production ready’ platforms that are equipped with multiple lasers, process monitoring tools, integrated powder recycling and unpacking stations etc. However, in contrast to this, the practical considerations of being able to produce large and very complex parts from a software/data preparation perspective have not been fully addressed.

We have been successful at Conflux in developing innovative approaches to overcome the problems surrounding this topic. However, this remains a real concern as we are observing a market pull to produce larger format heat exchangers with AM. Meaning that we are being pushed to produce larger designs with increased complexity. It’s great to see companies such as nTopology making great strides forward in reducing the computational complexities that are associated with the design of complex parts and I look forward to future developments in this space, especially on the efficient generation of large AM build files.

3. Qualification and validation
This is one of the hot topics of conversation at Conflux, in the broader metal AM community and explains why I have written more on this point. Heat exchangers pose unique problems in this area as a single critical defect/pin hole in a single thin wall gas tight structure can be the difference between success or failure. This poses challenges in being able to produce parts that possess the required integrity whilst still achieving the end use performance targets in heat rejection and pressure drop. Especially given that we are operating right at the edge as to what is achievable with LPBF.

Small levels of porosity are an acceptable given for most LPBF applications. This remains true for heat exchangers as post processing methods such as Hot Isostatic Pressing (HIP) are not deemed suitable for AM heat exchangers due to the risk of deforming the filigree structures that are packaged inside the part. Therefore, at Conflux we largely rely on our in depth AM process know how to achieve successful results in a consistent manner.

Traditionally manufactured heat exchangers are commonly designed and sized using look up tables as data sets and standards have been generated over time. This does not mean that there are no existing pain points with such devices, but rightly or wrongly it does provide the end user with confidence knowing that they are not going into the unknown. The same cannot be said about utilising AM for heat exchangers where you have a ‘clean sheet’ approach to the problem space. Whilst many embrace the ‘clean sheet’ methodology, there are still equally as many people, if not more who are cautiously optimistic. These concerns are mainly centred around the geometric and material characteristics/properties. Therefore, what can we do to speed up and simplify the qualification and validation process and provide customers with more confidence in AM heat exchangers? Here are a few topics for consideration:

a. Understanding the implications of defects is critical instead of trying to eliminate them. A lot of emphasis can sometimes be wrongly placed on the fact that a part possesses a certain amount of porosity without considering the end use implications. Therefore, let’s make informed and logical decisions that can assist in achieving quicker development and qualification.

b. In depth and robust development programmes. Process monitoring, Computed Tomography, performance and durability/fatigue testing are integral and continue to be performed during a typical Conflux development cycle, as we strive for more statistical confidence.

c. There is a still a certain level of ambiguity involved in the AM process flow, especially in relation to the calibration, fine tuning and set up of LPBF machines themselves. Do we achieve the same results if we take a package of identical build data (material, geometry, support structures and process/build parameters) and build it on the same make and model of AM system in four different companies/locations? No, is often the answer from the first round of trials in my experience. Often, this can be turned into a positive outcome with further investigation, but still provides food for thought in that the ‘human(s) in the loop’ still have a significant influence in the world of digital manufacturing.

“I am pleased to say at Conflux we are now leveraging the full benefits of AM to produce complex and conformable heat exchangers that exceed the performance and efficiency of the traditionally manufactured parts that we are often being judged against”.

Are we happy with where we are at as a company?
Yes, because we continue to receive positive customer feedback, which is of paramount importance to us. Do we want to improve our offering and capabilities? Absolutely, as Abraham Maslow once stated: “One can choose to go back toward safety or forward into growth”.

So where to next then??

The global heat exchanger market was valued at USD 18 billion in 2018 and forecast to grow to approximately USD 30 billion by 2026 [1]. The market size that AM can currently address is somewhat smaller than this due to the natural fit, maturity, and economics of the technology. However, below are what I believe to be some key areas of focus that will increase the ‘sweet spot’ and enable AM to be more of a mainstream technology for producing heat exchangers:

Technology Accelerators for Mainstream Heat Exchanger Production

1. Increased productivity rates of AM platforms – how many times have you heard that LPBF is too slow and too expensive? Fundamentally, this is still the case for heat exchangers, as more productive machines come at a cost unfortunately. Whilst the benefits of AM can justify an increased part cost in many instances. The reality is that the number of suitable heat exchanger applications would increase if a higher productivity could be achieved. Given this, I am looking forward to the next step change in productivity that will bring a 10 times increase in productivity without compromising the cost per part through increased capital expenditure.

2. Improved detail resolution – the importance of surface area density was discussed previously. Therefore, it goes without saying that a 2 to 3 times reduction in the minimum achievable wall thickness on medium to large format AM systems would certainly open the door for new heat exchange applications.

3. New materials – we are currently observing an increased level of activity in this space and it is impressive to see the work that companies such as Elementum3D are performing in the development of alternative Aluminium and Copper alloys that can be utilised for heat exchange applications. However, I am sure that I am not alone in waiting for ‘Unobtanium’ to arrive in the marketplace i.e. a super lightweight, high strength, thermally conductive, corrosion resistant material that can operate consistently at elevated temperatures.

4. Standards and certification – the design and implementation of specific standards can only help and assist in the qualification and validation of AM heat exchangers.

5. Education – we need to continue to collectively educate the market and provide the correct information in order to assist decision making processes.

Taking time to reflect and write this article has reinforced two main things. Firstly, that we have collectively achieved a lot over the past two and a half years at Conflux, and secondly that there is still plenty of work ahead of us in order to grow the market and address the challenges that we face. I look forward to working together with the broader AM community in increasing the adoption of AM for heat exchanger manufacturing and opening new applications.

Please send me an email at kevin.hazlehurst@confluxtechnology.com if you would like to discuss these topics and how Conflux can assist in helping to address your thermal management needs.

[1] Fortune Business Insights Report, accessed at: https://www.fortunebusinessinsights.com/industry-reports/heat-exchangers-market-100919

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