In March, ExOne announced that it had qualified binder jet 3D printing of aluminium 6061 with Ford Motor Co., delivering parts with 99% density and properties comparable to traditional manufacturing. The achievement is expected to unlock new benefits in manufacturing, such as new lightweight part designs that can now be delivered at super-fast binder jetting speeds that support high-volume serial production.
Delivering a commercially viable achievement in this area has evaded researchers for more than a decade, and the story of how it was achieved demonstrates the value of global research partnerships, innovative thinking, and experience.
While collaborative and individual patents are expected to be filed by ExOne and Ford as a result of this ongoing project, partners such as Abbott Furnace Company also played a key supporting role in advocating new patent-pending approaches to help make binder jetting of Al 6061 more production friendly.
While most binder jet 3D printers sold today are paired with a batch sintering furnace, ExOne found that Al 6061 preferred a continuous furnace, the very type that would be advantageous in high-volume production.
“The use of a continuous furnace turned out to be a key element of our patent-pending approach,” said Rick Lucas, ExOne Chief Technology Officer and VP, New Markets. “In our roadmap, 6061 aluminium was always the peak. It’s the most commonly used aluminium in the world; it’s what manufacturers want. But it’s also the most challenging to print and sinter. A lot of great minds came together to make this happen, building on decades of foundational work.”
Abbott’s new Vulcan Delube System is a key part of ExOne’s successful sintering of Al 6061.
THE QUEST TO SUCCESSFULLY SINTER HIGHER STRENGTH ALUMINIUM ALLOYS
Generally, the higher the strength of aluminium, the more difficult and expensive it is to form with traditional manufacturing.
Common manufacturing methods, such as metalcasting and machining, each present limitations on the types of geometries that can be produced as well as the specific aluminium alloys that can easily be shaped into a part. Each alloy also contains its own properties for items such as strength-to-weight ratio, weldability, energy absorption, and resistance to corrosion, cracking and heat that cause a lot of trade-offs.
At the same time, the world needs strong aluminium alloys to deliver a more fuel-efficient and sustainable future. The material is lightweight, relatively strong, highly recyclable and can therefore help lower energy costs and carbon emissions in automobiles, airplanes, military equipment and more.
The strong demand and challenges surrounding aluminium have long caused higher strength aluminium alloys to be a target of 3D printing companies. While laser powder bed fusion (LPBF), can print certain aluminium alloys, it’s largely been held back by its slower processing speeds and relative expense.
Binder jet (BJT) 3D printing, which selectively deposits binder into a bed of powder particles and is up to 100 times faster than LPBF, has been seen as a potential pathway to success. However, sintering bound-powder aluminium parts to high densities has been a well-known challenge. The metal injection moulding (MIM) market has been trying to develop a method to successfully do so for decades. So, while both MIM and BJT could bind aluminium particles into a precision “green” form, neither had found a reliable way to sinter these forms to high densities while retaining shape and properties.
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With strong incentives to find a solution, however, research continued worldwide down two main branches: methods to successfully sinter standard aluminium powders bound together in a form, and laser printing higher-strength aluminium alloys with modified powders or other approaches.
A controlled atmosphere version of the X1 160Pro will be offered in late 2022 and will be capable of production 3D printing Al 6061.
EXONE PULLS ALL THE RIGHT LEVERS
At ExOne, aluminium development had been underway with varying intensity for many years. In 2019, that work again intensified after the launch of ExOne’s patented Triple Advanced Compaction Technology (ACT), which delivers the consistent green parts needed for success.
The project was led by Patrick Dougherty, whose Ph.D. research had focused on metal powder mechanics, particle flow, and tribology at Carnegie Mellon University. Dougherty began by studying the wide body of work and efforts to date on the subject, while also conducting in-depth interviews with researchers around the world on their experiences and insights. His goal was to find the gaps between failures and near successes, and pinpoint possible avenues that weren’t fully explored.
Binder jetting of aluminium is challenging for several key reasons.
One, the powder is highly reactive at the moderate-to-fine particle sizes that are used to deliver high-density results in binder jetting. This is because aluminium particles instantaneously form oxide layers when exposed to air and generate heat in the process. While it’s possible to engineer some of this risk out of the powder, and manufacturers deal with these risks every day with gasoline and other materials, it’s best to 3D print in a controlled atmosphere chamber free of oxygen and with safety controls. (ExOne will begin selling an X1 160Pro with these features in 2022.)
ExOne can now binder jet 3D print Al 6061 parts with high density, tight geometric control and properties comparable to traditional manufacturing.
Second, this oxide layer is also what makes aluminium difficult to sinter, as the oxide essentially forms a hard, chemically stable shell around the aluminium particle with a much higher melting point. A particle of aluminium may begin to liquefy during sintering, but it can remain inside the shell – never fusing with adjacent particles.
Getting adjacent particles to fuse together in a controlled way, a process that begins with necking, is a vital part of successful sintering. In fact, when sintering any bound metal parts, it’s not desirable to ever reach a fully liquid state in the furnace, which would erode the geometry of the part and negate the point of 3D printing it in the first place.
Dougherty and a team of collaborators agreed to pursue several potential strategies to overcome these challenges. “There were a lot of alleyways we went down with a lot of great partners that just didn’t work,” Dougherty said. “But I’ll never forget the first one that actually densified into a nice, tidy little square.”
In late 2020, ExOne began reporting consistent and repeatable densities of 99% for Al 6061 – as well as favourable material properties on par with traditional die-cast aluminium alloys and tight geometric control during sintering.
“The ExOne team is proud to offer Al 6061 as we aggressively expand the use of binder jetting for high-volume production,” said John Hartner, ExOne’s CEO. “We’re confident this achievement will enable new designs that will help lower energy consumption, decarbonise products, and deliver all-new performance benefits in high volumes that will improve the world.”
