Q&A: Binder Jet Materials Development

April 06, 2021
GE Additive Binder Jet

Ahead of the latest webinar on our Binder Jet technology, we catch up with Arunkumar Natarajan, principal engineer at GE Additive, and Srikanth Kottilingam, consulting engineer at GE Power, to discuss the role of materials development for this exciting modality.

Arun Natarajan - GE Additive & Sri Kottilingam - GE Power

 

 

 

 

 

 

Arunkumar Natarajan (left) and Srikanth Kottilingam

 

How is the Binder Jet launch coming along and how have you approached materials development?

The launch is still on track for the second half of this year. We see our materials and process offering as a key differentiator and are approaching binder development from four angles. 

First and foremost, we develop our binders with environment, health and safety (EHS) in mind. Safer binders protect the health of the operators who work with these chemicals and reduce the need for complex engineering controls. This is important for industrialization and factory acceptance and reduces environmental emission risks.

Second, we wanted to create binders with robust green strength. High green strength is key for producing large parts and parts with thin walls, sharp corners and other fine features. Green strength is also critical for depowdering and part handling and enables automated production.

Third, we wanted our binders to have good brown strength. This minimizes deformation during sintering and results in better geometric control. Parts can’t just be printable. They also have to be sinterable.

Fourth, we wanted to develop clean burning properties. Superalloys and other high-performance materials have very specific chemistries. It is important that we don’t leave behind impurities and residues that could affect the microstructure and mechanical properties of the final part.

Our current binder offerings can process a broad set of alloys used by the aviation, energy, automotive and industrial sectors. We are constantly developing new material-choice and material-shape solutions to address our customers’ application needs. 

Did you leverage materials sciences resources across GE?

There are decades of materials science and chemistry expertise embedded in each GE business and deep expertise at the GE Global Research Center. Our materials teams understand the complex requirements of binder jetting and can develop new material systems and binders as needed.

Our first binders were developed for our own internal applications and were focused on high-end nickel alloys. This allowed the team at GE Additive to collaborate with GE Power on power generation gas turbines, with GE Aviation on some of their components and with other GE business on specific applications.

To take our work to the next level, we began to design and build our own machines and develop our own processes. Other GE businesses have played a pivotal role in helping GE Additive to connect the dots with their in-depth manufacturing experience and part-design and applications expertise. 

The result is that our Binder Jet technology brings additional value and performance to the market.

Any lessons learned from your other additive modalities?

Laser- and EBM-based technologies melt and resolidify metal powder using a high-energy beam. Unless you understand the material and have suitable process parameters, you can get cracking in your final part. 
In binder jet, we never melt the raw material. We glue it together using a liquid binder and then sinter the bound (or green) part. This makes Binder Jet very suitable for superalloys (which typically suffer from weldability issues).

If we can print the shape and sinter it, we will be able to create the superalloy part.

Metal binder jetting does require post-processing steps. The printed part is initially in a green state with 60-65% density. It must then be sintered in an industrial furnace to achieve its final state (up to 99+% density). This sintering can be done through a batch furnace process or with a high-throughput continuous furnace.  Both have been proven to meet density and burnout requirements.

You recently co-authored a paper in Additive Manufacturing Journal on hard-to-weld nickel-based superalloys.  What were the key takeaways?

Binder Jet offers a significant benefit for materials with poor weldability, such as nickel-based superalloys. One of those alloys is RENÉ 108 (which is similar to MAR-M247). We wanted to use this material to make a component for GE Aviation, so we first needed to work out the factors that affected the process and metallurgy of RENÉ 108. 

In our paper, we analyzed microstructure, purity and particle boundaries – all of which are critical to the process parameters and resulting quality – of RENÉ 108 parts. Our goal was to enable customers to make shapes with complex internal features and good performance. We came to two major conclusions.

The first is that a clean-burning binder produces a better final product with better properties. It is important that there are no reactive residues left behind in the final part.

Second, we found that performance is not just dependent on the binder. For example, the machine-material interaction can cause damage and limit fatigue life.

This helped us to understand the different process and material issues associated with Binder Jet and what we needed to focus on as a business so we can apply metal Binder Jet processes to critical industry applications.

Why is GE Power interested in this technology?

We recognized that that materials such as RENÉ 108 – with their unique creep strength and oxidation resistance – can be used to improve the efficiency of gas turbines. In a simple or combined cycle power plant utilizing gas turbines, the overall efficiency of the plant depends on the efficiency of the gas turbine. If you increase the efficiency of the power plant, you use less fuel, and this can ultimately lead to higher profit margins.

Gas turbine efficiency can be improved by increasing the firing temperature and reducing the amount of air required to cool the hot section components.  To cool the parts efficiently, you need internal passages and intricate designs so that the air can be used very efficiently to cool the system.

Advanced materials that can withstand high temperatures and processes that allow manufacturing of components with intricate internal cooling features are required to address these requirements.

In our turbines, nickel-based superalloys with high volume percent gamma prime are the material of choice for the hot section components. The solid-state and thermal design aspects of binder jet help us to create more intricate, efficient parts with fewer limitations.

Printing of Green & Sintered Parts
 
Why are high-temperature performance and elongation both important for Binder Jet materials?

For high-temperature applications, creep strength and oxidation resistance of the material is of the utmost importance. Also, the fatigue and cycle loading are important for gas turbines.

The creep strength is related to grain size, so a material with larger grains has a higher creep strength. Therefore castings, with their inherent larger grains, have typically dominated high-temperature applications. The small size of the powdered feedstock material in Binder Jet enables a high final density, and as a result, finer grain size to be achieved.  While beneficial for improved fatigue capability, it can lead to debited creep behavior versus cast material. Thus, a balance must be targeted to establish a microstructure for the intended application.

Another important property for consideration when developing a material is ductility. Anytime there are impurities in a part, the ductility properties can be affected, reducing fatigue strength and the life of the components. To overcome this, you need to maintain metallurgical cleanliness in the part. Therefore, the feedstock cleanliness and the burn-out characteristics of the binder are critical.

Ductility Improvement Through Microstructure Control
 
What factors can impact performance? Do impurities and residues play a part?

The binders that glue the metal layers together are oligomeric or polymeric species, so we need to make sure that they fragment and vaporize during the debinding and sintering step. Otherwise, you will end up with organic residues in your part. If you leave behind residues, they will react at high sintering temperatures, so the carbon becomes metal carbide, the oxygen becomes metal oxides, and so forth.

These complexes tend to decorate the particle boundaries in certain alloys and prevent the sintering mechanism from happening. This affects the ductility properties (as many of these complexes are brittle), and in turn, the microstructure and mechanical properties of the part. If large amounts of these residues build up in certain areas, then micro- or macro-cracks can form. The presence of these complexes can also prevent you from achieving full density.

Another issue is that the small size of the powder feedstock material increases the powder surface area. This is another means by which oxygen can enter the powder matrix.

To stop these issues from occurring, you need to understand what you’re putting in; the sintering and debinding conditions; the reactivity relationship between the powder, the binder and the alloy; and the signature of the polymer burn out. 

If you want to obtain a production-quality part, you need to be confident that you can burn these residues out without unacceptable distortion of the part geometry.

Summary

Binder Jet additive manufacturing offers many advantages over other modalities. Because you never melt the metal, superalloys and other hard-to-weld materials can be used to create complex and intricate parts. The speed and supportless nature of metal binder jetting also offer time and cost savings.

To make parts for critical applications, you need to understand all the steps very clearly – especially from a chemical perspective – so that you don’t suffer any issues from impurities.

If you can understand all the different aspects of Binder Jet, you will be able to use it to manufacture a wide range of parts.  

 

If you would like to find out more about how you can use Binder Jet to your advantage, you can catch Arun and Sri talking about these areas in a GE Additive webinar, or reach out to them directly.
 

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