The future of additive manufacturing

Since its inception in the early 1980s, additive manufacturing (AM) has evolved into a beneficial and flexible process. However, from a mass production standpoint, complete acceptance has been hindered due to high costs tempering expectations for how 3D printing (3DP) can be optimized. Today, the question remains: Can AM produce enough return on investment (ROI) soon, or will it slog along at a pace disadvantageous for those invested in the process?

There are reasons to think either reality could play out, and the next few years could be pivotal, especially for the aerospace industry. Some early patents are expiring, and opportunities with open source could grow AM, if the relatively slow progression of the science can keep up with the logistics and costs involved with machinery and technology.

Interesting, complicated evolution

AM experienced a big change during the past 20 years, particularly in producing large structural components (such as wing spars and panels). However, some of these components are now transitioning to carbon fiber composites. The major area for metal AM in the aerospace sector is its use in engines. Several alloys in use decrease weight and increase engine life and fuel efficiency. There’s also a big drive to replace traditional cast and forged components with those produced by AM.

This represents a marked transformation from 40 years ago, when Dr. Hideo Kodama developed a layer-by-layer approach for manufacturing using a photosensitive resin polymerized by UV light. Credited with inventing AM, Kodama was attempting to find a way to develop a rapid prototyping process and came up with what’s considered an early version of the modern stereolithography machine.

Today, most metal powder feedstock materials in AM are sourced from established powder metallurgy processes, such as pressing and sintering or hot isostatic pressing (HIP). There’s a greater chance of changing the metal’s chemical makeup because the AM process melts the powder with a laser or electron beam, altering the mechanical properties. This environment produces an opportunity to evaluate manufacturing conditions and tailor the feedstock material to obtain the best mechanical properties for optimal components. However, in the future, multi-laser systems and optimized processing parameters likely will be used to expedite the rate of current build times, with this depending on the machines and not the processed material.

Refined processing, promising results

AM can produce complex components not possible with casting and forging. The most common process with metals is using a layer of very fine powder melted by a laser or electron beam. Another layer of powder is then spread over the top and melted as well. This process continues until the part’s built, allowing for greater flexibility in design of complex parts and consolidation of sub-assemblies of many parts into one – a tremendous weight savings and advantage for aerospace components. The current range of materials used in AM are producing components for critical applications, with mechanical properties equal to and sometimes better than traditional cast or forged parts. To fully unlock the potential of cost-effective, mass-produced, specifically designed critical parts, feedstock must be developed.

Applying computer-driven digital topology optimization during the design stage, components can be produced with the same, or greater, mechanical properties of the parts they’re replacing, but with significant raw material and weight savings. According to a 2015 study, AM is particularly attractive for material-efficient production of items that would otherwise have a high buy-to-fly ratio, the ratio of the mass of the starting billet of material to the mass of the final, finished part.

Current powder supplies have been designed for mature processes, such as HIP. These processes differ in that the powder is never truly melted; it’s compressed to form a green component that’s heated to just below the melting temperature, where powder particles sinter together. Because there’s no melting, there’s very little chance of a change in the material’s chemistry. These powders are consistently used for AM because they’re well established and readily available.

Research is ongoing to understand how many times the powder can be recycled before it has a detrimental effect on part quality. If true mass production of critical components via AM is to be obtained at a cost-effective rate, powders that better withstand the effects of recycling will be key.

Mainline production

Developing materials tailored for AM, as well as improving production machines, are required to make high-volume, cost-effective parts on a wider scale. One known challenge is the depletion of certain elements in alloys when they’re melted by a laser or electron beam (aluminum in Ti64 and chromium in Inconel 718) that can reduce mechanical properties. These issues are lessened by optimizing processing parameters, such as laser power, scanning speed, or spot sizing, but developing a tailored alloy would improve mechanical properties. These developments could also reduce residual stresses of the finished component, reducing secondary processing (heat treatment) and part rejection due to being out of specification, helping drive down process cost.

Environmental, ethical, political

As the process for AM continues to evolve and leads to more production, there’s increased likelihood of environmental, ethical, and perhaps legal impacts. On May 6, 2022, U.S. President Joe Biden joined five leading U.S. manufacturers in launching Additive Manufacturing Forward, a voluntary initiative intended to help smaller suppliers increase their use of AM. The Biden Administration will reportedly support the effort with a variety of current and proposed federal initiatives that have the potential to enhance the ability to make and sell higher-performing parts, shorten wait times for needed parts, increase workers’ wages, and expand consumers’ access to better products. Another important initiative is a project including dozens of aerospace and defense corporations that’ll develop definitions, best practices, and other guidance.

Still, the prevailing theory is that the supply of critical engine forgings and components, as well as the mining of raw materials, could be limited to a few companies in countries that may not be on good terms with each other in the near future. However, as a production process, AM is maturing and is set to play an important role in the ongoing development of the aerospace industry.

About the Author: Neal Polley, PhD CEng MIMMM, has more than 20 years of experience working in materials R&D in the defense and aerospace industry, the last seven in the field of additive manufacturing. Contact him at neal_polley@yahoo.co.uk.