One type of powdered-metal 3D printing is called binder-jet printing, in which thin layers of metallic particles are bound together in a computer-generated pattern using a low-temperature liquid binder jetted from a print head, similar to an ink-jet printer. After each layer, the printer bed is lowered slightly and another layer of powder is spread on top, followed by another pass of the print head.
In this way, a three-dimensional object is slowly built up, but at this stage it possesses barely any strength and needs a series of heat treatments before it is ready for prime time.
“With binder-jet printing, normally you have the problem of not getting very high density, and high density is extremely essential for mechanical properties, where a tiny pore can completely destroy your fatigue life,” said Dr. Markus Chmielus, assistant professor in the Department of Mechanical Engineering and Materials Science at the University of Pittsburgh.
Chmielus is working with Research of Additive Manufacturing in Pennsylvania, or RAMP project, co-funded by the Air Force Research Laboratory and the Commonwealth of Pennsylvania, researching heat-treatment processes for 3D-printed parts. The project also includes ExOne in Irwin, Pa., a manufacturer of binder-jet 3D printers, and Acutech Precision Machining in Meadville, Pa.
According to Chmielus, the next step after binder-jet printing is curing fragile objects in an oven to harden the binder material.
“If you just pack spherical powder particles next to each other, there are a lot of gaps in between and nothing to hold the particles together,” Chmielus said. “But curing creates no mechanical strength whatsoever; it just allows you to handle the objects and basically move them from one place to another without them falling apart.”
Densification by Sintering
That still leaves a density problem, so the next step is sintering the object.
“You want to reach very high density, as high as possible, but without distorting your part,” Chmielus said. “During sintering you go to a very high temperature so that the atoms in the powder particles start to diffuse and neighboring particles start to connect – called necking. And then over time, the porosity diffuses out, so you have very high-density parts.”
As Chmielus describes it, sintering a binder-jet-printed object is an art as much as a science.
“That’s another trick, because when you think about it, you go very close to melting, so why shouldn’t your part or piece not just become a big blob of solid metal without keeping the form?” he said. “There’s a fine line there of when your powder starts melting and when there’s actually nothing going on and you can’t get densification, so you need to be in that range of good temperature and time.”
Chmielus and his team succeeded in achieving 99.6% density of Inconel 625 binder-jet-printed objects, which he said is compatible with other traditional manufacturing processes.
Following Classic Metallurgy
Following this densification, Chmielus was able to perform a typical solution treatment followed by an aging process just as if the part had been manufactured using forging or casting.
“After sintering, in which you went very high in your temperature, you’ve probably formed a lot of different phases that you really don’t want to have in your matrix,” Chmielus said. “You now have a lot of phases in there – carbides, some gamma phases and some delta and sigma phases – but that’s not very controlled. So the next step is to get rid of all of those and basically reset your microstructure and your phases with a solution treatment.”
Resetting the Microstructure
“During the solution treatment, you take your part and heat it to a fairly high temperature, close to 1300°C for Inconel 625 – not quite melting – for a very short time,” Chmielus said. “All the alloying elements and phases that are built during the sintering process will dissolve in the matrix again, so you basically can start over, creating new phases that are beneficial.”
After resetting the microstructure with the solution treatment, Chmielus followed classic powder-metallurgy practices to achieve a specific engineered microstructure and phase development for the 3D-printed samples.
“Aging is the last step, where you heat the part to a certain temperature for a certain amount of time, based on your TTT diagram, in order to get certain phases,” he said.
Desirable phases can be developed by following a temperature profile, in which the part is held for a certain time at one temperature, dropped to a different temperature and then dropped to yet another temperature for a certain amount of time.
According to Chmielus, the TTT diagrams for Inconel 625 are always C-shaped lines.
“When you are to the right of these lines, then the phase is being formed that the line indicates,” he said. “If you are on the left or below or above that line, then this phase is not being formed.”
Phases can be good, bad or both, depending on the object being manufactured and the mechanical properties desired.
“For Inconel 625, good phases would be metal carbides (MC or M6C) or a gamma double-prime phase as the primary founding phase that creates the properties you actually want,” Chmielus said.
According to Chmielus, the gamma double-prime phase in Inconel 625 is always good because it is a precipitation-hardening phase, which means it is settling within the matrix and stretching it a bit, producing some internal stresses that are actually very beneficial for mechanical properties.
“By putting these precipitates in there, any dislocations are pinned so their motions are hindered,” he said.
Chmielus cautions that, based on his experience, testing each batch of powder that is going to be used to print metal parts is important for determining the best heat-treatment protocols to achieve the desired density and other material properties.
“For metal 3D printing, as for traditional processes that use metal powder, powder size, shape and composition will be a little different for each batch of powder,” he said. “We found that for each additive-manufacturing method and each batch of powder – even from the same manufacturer – you need to do a series of tests to identify which exact temperature you need to go to and how long you need to do your heat treatment to get the best density.”