Heat Treating 3D-Printed Metals
Metal 3D printing, or additive manufacturing (AM), is an advanced manufacturing method that opens up new possibilities for designing objects with optimized geometries and minimized weight using far less material and energy – important drivers for a future sustainable, energy-efficient industrial base.
Printing a solid object with metal powders using micrometer-thick digital “slices” generated from a computer-aided design is not the end of the story. Just as with casting or machining metal components, a series of post heat treatments is required to reduce internal stresses, increase density and develop the final shape, finish and (most importantly) microstructural phases, resulting in the desired physical properties.
3D-printed components destined for use in aerospace, nuclear, gas turbine, marine or medical applications will require an additional hot isostatic pressing (HIP) treatment to fully densify the metal, eliminating pores that can lead to catastrophic failures (more about this later). New HIP technologies are making it possible to accomplish heat treatments in the same vessel where the HIP takes place, making for a faster, cheaper and more energy-efficient manufacturing process.
Metal 3D-printing machines are varied but come in two basic types: high-temperature laser or electron-beam printers and low-temperature binder-jet printers (Fig. 1).
Binder-Jet Metal 3D Printing
Binder-jet metal 3D printing involves binding thin layers of a powdered metal (e.g., Inconel) with a liquid binder, which is dropped from a print head onto a powder bed, similar to printing ink on paper with a laser-jet printer. The print head follows a computer-generated pattern of micro-thin slices of the object to be printed.
After a layer of binder is printed, the powder bed is heated in a curing process to bind the binder and powder together. Next, the bed is lowered and another thin layer of powder spread across it. Once again, the liquid binder is deposited and bound to the layers below, eventually building up the object layer by layer.
As the object “grows” it is supported by the loose powder filling in the powder bed. This avoids the need for a build plate or printed support structures, which would have to be removed later. After printing is complete, the excess powder is carefully vacuumed away (to be reused) and the object is then heated in a furnace to burn off the binder material and sinter the powder particles together.
With other metal 3D-printing technologies such as direct-metal laser sintering (DMLS), a high-temperature directed heat source (e.g., a laser or electron beam) heats up a thin layer of powder in a computer-controlled pattern, which then cools and bonds to the previous layers. The rapid and uneven heating and cooling of layers cause residual stresses to develop between the build plate and the object and within the object itself, which then must be relieved through stress-relieving heat treatment. This typically takes place in a vacuum furnace, in which the part is heated just below the material’s transition temperature and held for long enough to allow the stresses to relax.
HIP for AM
After stress relief, components may be required to go through the HIP to eliminate pores and heal defects, achieving 100% of the maximum theoretical density. Both cold and hot isostatic pressing have been used for decades to treat castings made from powdered aluminum, steel and superalloys, but HIP can also be used to treat 3D-printed metal objects (Fig. 2).
“Any structural or critical components such as those for aerospace and medical applications (Fig. 3) will tend to be HIPed to ensure the material achieves optimum fatigue and creep properties,” said Susan Davies, Ph.D., advanced technology leader HIP for heat-treating service provider Bodycote.
The HIP process involves placing the printed object inside a pressure vessel and then pumping it full of inert gas, typically argon, in order to build up the pressure on all sides of the part, including inner surfaces such as inside a tube. For HIP, high temperatures are applied at the same time as the pressure so that the yield strength of the alloy is exceeded.
“This allows any porosity from the AM build to close by the pore matrix structure plastically deforming and the pore surfaces coming in intimate contact,” Davies said. “Plastic deformation is then followed by creep and diffusion mechanisms, which allow the mating surfaces to be bonded and achieve bulk material properties.”
According to Davies, different AM processes will result in different forms of defects or weaknesses in the build structure, so the benefits from HIP will depend on the actual process used. Even if an AM component is HIPed after stress relieving, however, it still requires heat treatment post HIP to achieve optimum mechanical properties comparable with wrought and cast alloys.
HIP Combined with Heat Treatments
Because both heat treatments and HIP take place in a furnace, some HIP manufacturers now offer equipment able to perform both HIP and heat treating, which can reduce cycle time, increase productivity and provide significant cost savings.
“People are becoming more aware of the fact that the cooling rates you can achieve inside a hot isostatic press are similar to what you can do in a vacuum furnace – or better,” said Peter Henning, business unit director-AMD for HIP manufacturer Quintus Technologies AB, Västerås, Sweden.
Natural cooling in an HIP system can take from 8-12 hours, well over half the typical cycle duration, according to Henning. In comparison, units equipped with rapid-cooling technology can easily cool a full workload in a medium-sized HIP from 1260°C to 300°C (2300°F to 570°F) in less than 30 minutes.
An even more advanced version of HIP uses variable cooling and heating rates and pressure levels to more precisely control the quality and mechanical properties of treated parts through rapid quenching. According to Henning, controlled cooling rates up to 3000°C/minute can be achieved by combining pressure and temperature control.
“In the past, HIP was strictly hot, but the latest technology allows for both hot and cold areas inside the pressure vessel,” Henning said. “Hot areas in a graphite furnace can reach up to 2000°C, and a molybdenum furnace gets up to 1400°C. For quenching, you quickly replace the hot-zone compressed gas with cold compressed gas from outside the hot zone but still inside the pressure chamber.”
Because the cold argon gas is highly pressurized, it has a higher density than water, so it acts as a quenching agent, similar to oil or water at normal pressures. Cold areas inside the HIP are kept at a controlled temperature by cooling water outside the pressure vessel.
Opening New Possibilities
This HIP-quenching technology has benefits beyond increased productivity and cost savings. It opens even new possibilities for design based on the more uniform quenching and cooling made possible.
“Normally, you have a cooling medium with a fixed temperature, so at first you have a large ΔT. As the hot component cools and it slowly approaches the temperature of the cooling medium, the ΔT changes over time,” Henning said. “If you put a piece with thick and thin sections in a quenching bath, the sections will experience very different quenching because the thin part will very quickly adopt the temperature of the cooling media, while the thick part will have some time to adjust to the temperature. So, you may have a crack form or distortion because of the stresses.”
By contrast, for quenching in a HIP, the components and the cooling medium can start at the same temperature and then the ΔT between the component and medium can be controlled over time, resulting in a more-uniform cooling.
“In this case, what we do is we restrict cooling of the thinner part to the temperature of the cooling media,” Henning said. “We can hold back the cooling of the thinner section while waiting for the thicker section to cool, and then we gradually move the thinner section down to the final cooling temperature so that there will be much less thermal stresses formed due to different temperatures in the material.”
As an added benefit, Henning said that using compressed gas for quenching, rather than water or oil, means that no agitation can occur on the component surface since the cooling medium already is a gas.
Optimized for 3D Printing
As the industrial metal 3D-printing marketplace grows, advances in heat-treatment technology and practice will continue to improve.
“AM components can be HIPed and heat treated using conventional specifications, but there is an opportunity to optimize the HIP and heat treatment for AM components to minimize distortion during processing,” said Bodycote’s Davies.
Improved heat-treatment technologies should help cut costs and enhance the performance of 3D-printed parts.
“With the HIP densification and simultaneous heat treatment, the cost of operations goes down, and HIP becomes accessible to other high-performing components,” Henning said.
For more information: Holly B. Martin is a science writer and technical copywriter based in Winchester, Va. Holly specializes in creating white papers, blog posts and articles that convert “engineer-speak” into readable content that communicates important business information. Visit www.hollybmartin.com