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Massachusetts Institute of Technology

Researchers at the Massachusetts Institute of Technology (MIT) have developed a new additive manufacturing technique that can print rapidly with liquid metal, producing large-scale parts such as table legs and chair frames in a matter of minutes according to the team.

The technique, known as liquid metal printing (LMP), involves depositing molten aluminium along a predefined path into a bed of tiny glass beads. The aluminium then hardens into a 3D structure.

According to the researchers, LMP is at least 10 times faster than a comparable metal additive manufacturing process, and the procedure to heat and melt the metal is more efficient than other methods.

The technique does however sacrifice resolution for speed and scale. MIT says that while it can print components that are larger than those typically made with slower 3D printing techniques, and at a lower cost, it cannot achieve high resolutions.

The researchers say that in a recent study, the team demonstrated the procedure by 3D printing aluminium frames and parts for tables and chairs which were strong enough to withstand postprint machining. They showed how components made with LMP could be combined with high-resolution processes and additional materials to create functional furniture.

“This is a completely different direction in how we think about metal manufacturing that has some huge advantages. It has downsides, too. But most of our built world – the things around us like tables, chairs, and buildings – doesn’t need extremely high resolution. Speed and scale, and also repeatability and energy consumption, are all important metrics,” said Skylar Tibbits, associate professor in the Department of Architecture and co-director of the Self Assembly Lab, who is senior author of a paper introducing LMP.

Tibbits is joined on the paper by lead author Zain Karsan SM ’23, who is now a PhD student at ETH Zurich; as well as Kimball Kaiser SM ’22 and Jared Laucks, a research scientist and lab co-director. The research was presented at the Association for Computer Aided Design in Architecture Conference and recently published in the association’s proceedings.

Drawing on the group’s previous work on rapid liquid printing with rubber, the researchers built a machine that melts aluminium, holds the molten metal, and deposits it through a nozzle at high speeds. The team says that large-scale parts can be printed in a few seconds, and then the molten aluminium cools in several minutes.

“Our process rate is really high, but it is also very difficult to control. It is more or less like opening a faucet. You have a big volume of material to melt, which takes some time, but once you get that quickly to melt, it is just like opening a tap. That enables us to print these geometries very quickly,” explained Karsan.

Bread loaf-sized pieces of aluminium are deposited into the electric furnace, which is essentially a “scaled up toaster” according to Karsan. Metal coils inside the furnace heat the metal to 700 degrees Celsius, slightly above aluminium’s 660 degree melting point.

The aluminium is held at a high temperature in a graphite crucible, and then molten material is gravity-fed through a ceramic nozzle into a print bed along a preset path. They found that the larger the amount of aluminium they could melt, the faster the printer can go.

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“Molten aluminium will destroy just about everything in its path. We started with stainless steel nozzles and then moved to titanium before we ended up with ceramic. But even ceramic nozzles can clog up because the heating is not always entirely uniform in the nozzle tip,” said Karsan.

By injecting the molten material directly into a granular substance, the researchers don’t need to print supports to hold the aluminium structure as it takes shape.

The team experimented with a number of materials to fill the print bed, including graphite powders and salt, before selecting 100-micron glass beads. The tiny glass beads, which can withstand the high temperature of molten aluminium, act as a neutral suspension so the metal can cool quickly.

“The glass beads are so fine that they feel like silk in your hand. The powder is so small that it doesn’t really change the surface characteristics of the printed object,” added Tibbits.

The amount of molten material held in the crucible, the depth of the print bed, and the size and shape of the nozzle have the biggest impacts on the geometry of the final object. Parts of the object with larger diameters are printed first, since the amount of aluminium the nozzle dispenses tapers off as the crucible empties. Changing the depth of the nozzle alters the thickness of the metal structure.

The team used LMP to rapidly produce aluminium frames with variable thicknesses, which were durable enough to withstand machining processes such as milling and boring. The researchers demonstrated a combination of LMP and the post processing techniques to make chairs and a table composed of lower-resolution, rapidly printed aluminium parts and other components, such as wood pieces.

“If we could make this machine something that people could actually use to melt down recycled aluminium and print parts, that would be a game-changer in metal manufacturing. Right now, it is not reliable enough to do that, but that’s the goal,” says Tibbits.

Jaye Buchbinder, who leads business development for the furniture company Emeco, who was not involved in the work added: “At Emeco, we come from the world of very analog manufacturing, so seeing the liquid metal printing creating nuanced geometries with the potential for fully structural parts was really compelling. The liquid metal printing really walks the line in terms of ability to produce metal parts in custom geometries while maintaining quick turnaround that you don’t normally get in other printing or forming technologies. There is definitely potential for the technology to revolutionise the way metal printing and metal forming are currently handled.”