MIT flips the metal 3D printing script

Introducing yet another innovation out of Massachusetts Institute of Technology, researchers have developed an additive manufacturing technique that can rapidly print liquid metal into large-scale parts like table legs and chair frames in a matter of minutes.

To date, nearly 20 different methods of 3D printing are being utilized, most employing a technique of heating the material after it has been prepared. This is due to various factors, but generally because the material in question is so variable and preparing allows more control over the result.

Before metal powders, various plastics, polymers, and even ceramics were used creatively to improve strength, resolution, and durability. Once metal entered the picture, the possibilities grew even more as alloyed powders could provide multitudes of differing qualities that have long since been used in conventional manufacturing.

The question of how best to replicate the same qualities sought after in the centuries-old practice of melting metals was then applied to this novel technology.

The latest innovation by MIT scientists, however, goes in an entirely new direction for additive manufacturing and is more akin to the casting style of old – extruding already melted metal into the desired shape.

Liquid metal printing

Being called liquid metal printing (LMP), this technique so far extrudes aluminum along a predefined path on a bed of tiny glass beads. Aluminum, which already cools relatively quickly and has a lower melting point than other metals, was selected for its ease of use and to test the viability of this technology.

Research indicates the LMP technique is at least 10 times faster than comparable metal additive manufacturing processes, and the procedure to heat and melt the metal is more efficient than some other methods.

Right away, one can tell that resolution is sacrificed for speed, and the researchers note this is an expected outcome.

"This is a completely different direction in how we think about metal manufacturing that has some huge advantages," said Skylar Tibbits, associate professor in the Department of Architecture at MIT and senior author of the paper introducing LMP. "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."

The best example of what Tibbits details is the recent adoption of 3D printing for construction. Some companies have begun to use large-scale industrial printers to build literal houses by squeezing out concrete like one would squeeze frosting from a piping bag and building the frame of a house.

As for LMP, the MIT researchers feel that this method opens the doors to many new practices that could reduce costs by reducing time.

LMP would be highly suitable for some applications in architecture, construction, and industrial design, where components of larger structures are purely functional and don't necessarily require beauty. It could also be utilized effectively for rapid prototyping with recycled or scrap metals.

In the study, the researchers demonstrated the procedure by printing aluminum frames and parts for tables and chairs which were strong enough to withstand post-print machining. From this, they showed how components made with LMP could then be combined with high-resolution processes and additional materials to create functional furniture.

Significant speed

One method for printing with metals that is commonly used in construction and architecture is called wire arc additive manufacturing. This process is able to produce large, low-resolution structures, however they are susceptible to cracking and warping because some portions must be remelted during the printing process.

LMP on the other hand keeps the material molten throughout the process, avoiding some of the structural issues that can be caused by remelting.

Drawing on the group's previous work on rapid liquid printing with rubber, the researchers built a machine that melts aluminum, holds the molten metal, and deposits it through a nozzle at high speeds. Large-scale parts can then be printed in just a few seconds, with the molten aluminum cooling in minutes.

"Our process rate is really high, but it is also very difficult to control. It is more or less like opening a faucet," said Zain Karsan, a PhD student at ETH Zurich and another lead author of the paper. "You have a big volume of material to melt, which takes some time, but once you get that to melt, it is just like opening a tap. That enables us to print these geometries very quickly."

In addition to its fast cooling and lower melting point, the team also chose aluminum because of its accessibility as it is commonly used in construction and can be recycled cheaply and efficiently.

Bread loaf-sized pieces of aluminum are deposited into an electric furnace, "which is basically like a scaled-up toaster," Karsan added. From there metal coils inside the furnace heat the metal to 700 degrees Celsius (1,292 degrees Fahrenheit), just slightly above aluminum's melting point of 660 C.

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

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

By injecting the molten material directly into a granular substance, the researchers don't need to print supports to hold the aluminum structure as it takes shape. Much like casting has a sealed container to hold the shape, this printing technique just fills in the crevice dug into the granular bed.

Refining the technique

Before settling on glass beads, the researchers also experimented with a number of materials to fill the bed, including graphite powders and even salt. In the end, the team opted for 100-micron glass beads, which can withstand the extremely high temperature of molten aluminum and act as a natural 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," Tibbits says.

The amount of molten material held in the crucible, the depth of the print bed, and the size and shape of the nozzle, ultimately, have the biggest impacts on the geometry of the final object.

For instance, parts of the object with larger diameters are printed first, since the amount of aluminum the nozzle dispenses tapers off as the crucible empties. Thus, the researchers essentially created different tips to find that sweet spot for better control as the depth of the nozzle would alter the thickness of the metal structure.

Using LMP to rapidly produce aluminum frames with variable thicknesses, which were durable enough to withstand machining processes like milling and boring, the team demonstrated a combination of this printing method and post-processing techniques to make chairs and a table composed of lower-resolution, rapidly printed aluminum parts and other materials, like wood.

Next steps

Moving forward, the researchers want to keep iterating on the machine so they can enable consistent heating in the nozzle to prevent material from sticking, resulting in better control over the flow of molten material. But larger nozzle diameters can lead to irregular prints, so there are still technical challenges to overcome.

"If we could make this machine something that people could actually use to melt down recycled aluminum 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," Tibbits added.

Adding in his own insights from furniture manufacturing, lead for business development at Emeco, Jaye Buchbinder, sees the potential of MIT's innovation.

"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," he said. "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 revolutionize the way metal printing and metal forming are currently handled."

While not directly involved in the work, the research was funded, in part, by Aisin Group, a Japanese corporation in the automotive industry; Amada Global, another Japanese-based company in sheet metal manufacturing; and Emeco, a privately held American company that has been producing notable furniture since the 1940s.