Feature published in the July/August 2022 issue of 3D ADEPT Mag and posted online in December 2022. Interviews reflect that context. Cycles require definition and qualification for the alloy, part, machine and application; the examples are not furnace operating instructions.
In a previous dossier dedicated to the use of furnaces in additive manufacturing, we found out that the reasons that may help catalogue the type of furnaces manufacturers will use, are often dependent on the process the part will undergo: sintering (and often debinding) on the one hand, and heat treatment on the other hand. Here is the thing, heat treatment is a bottleneck that is quite underestimated… yet a thorough understanding and handling of this process can help manufacturers to avoid a wide range of unexpected challenges.
When looking at the various post-processing steps that can be carried out in an additive manufacturing process, it’s easy to understand why powder removal is given top priority. Everything surrounding this process is relatively “new”, whereas for heat treatment, operators easily see some similarities between the steps they need to take, to heat treat their 3D printed parts and the steps to follow when the part has been manufactured via a traditional route. That’s probably where the trap is. They only see “similarities”.
This article aims to highlight the main differences between heat treatment performed for AM and heat treatment performed for conventional manufacturing processes. A specific emphasis will be laid on typical heat treatment steps & considerations to take into account for L-PBF, EB-PBF and Binder jetting.
Heat Treatment: AM vs Traditional manufacturing processes
Heat treatment changes a metal’s microstructure and properties through controlled heating, holding at temperature and cooling. “Soaking” means this holding stage. Annealing, normalizing, quenching and tempering are examples; solution treatment or aging may also be relevant to the alloy.
“The differences between heat treat cycles are based upon and driven by chemistry [understand, chemistry of the material processed]. Chemistry dictates what heat treatment step you need to take to achieve your desired properties. That’s the reason why, for instance, different alloys will require different heat treat cycles. Even within the same alloy, you might have different levels of heat treatment as well. In this case again, your chemistry will dictate what’s going to happen if you heat the part in a certain way” Anthony Mott, Global Additive Manufacturing Leader at Wabtec, told 3D ADEPT Media.
Wabtec manufactures rail equipment, among other products. It announced the start of operations at Neighborhood 91 on June 8, 2021, at the additive manufacturing campus associated with Pittsburgh’s airport. In the 2022 interview, Mott describes his team’s work with group engineers to select suitable AM applications.
Reasons to perform a heat treatment process therefore vary from one application to another and the goal to achieve. In one case, the objective might be to soften the metal while in another one, the objective might be to increase hardness or develop certain mechanical properties to the metal or alloy used.
When one tries to compare heat treatment performed for AM & heat treatment performed for traditional manufacturing processes, there “is limited to no differences between these two types of manufacturing processes. If you are using the same exact chemistry on the traditional side for instance, the material is the same and it’s what really dictates your heat treatment profile. With AM, the only change is that you may have a more narrowed temperature profile. For example, you might run a high-performance nickel alloy processed by traditional processes from 1750 to 1950 while with additive, you’ll run it from 1850 to 1875° – That’s why I talk about a much narrower window on the same profile. The reasons why you may have some deviations in temperature is because with AM, we have a melt pool that melts your material and then rapidly cools. You have much smaller microstructures, much smaller green structures vs a cast part that has very large melted cool areas. That’s why you need narrow process profiles. This will help you ensure to get consistent properties out of this thermochemical treatment”, Mott told 3D ADEPT Media.
The temperature ranges in this example retain the interview’s original wording: no unit was specified, so they must not be used as operating settings. Composition alone is insufficient for transferring a cycle; initial condition, microstructure, residual stress and part requirements also matter. “Green structures” is undefined in the transcript and should not be confused with a binder-jetted green part.
Mott’s statement highlights three essential considerations to take into account when performing heat treatment: temperature, materials and microstructure challenges.
Heat treatment by nature is about control, control of the heating temperatures, control of the cooling rates, and the quenching types that are used to land on the desired properties.
In the same vein, it’s normal to witness some slight differences in the reaction of materials that are processed via traditional processes versus when processed with AM. “It’s difficult to 3D print pieces using materials containing high amounts of carbon, such as many steels. Carbon leads to issues in the microscopic resolidification that occurs in the AM process. It can affect expansion, contraction, shrinkage, and localized stresses, causing parts 3D printed with high carbon material to have cracking issues once completed. While carbon greatly complicates 3D printing, it is essential in many heat treatment processes”, an expert at Paulo notes. – Paulo is an organization that has dedicated its core business to address thermal processing challenges.
This observation concerns difficulties with some high-carbon alloys, particularly in fusion processes. It does not imply identical behavior for every steel or AM route.
Distortion can arise from residual stress, metallurgical transformations, thermal gradients or part restraint. Complex shapes and thin walls require particular attention, but do not guarantee tighter tolerances than machining. Several approaches can help manage these deviations:
For the team at Paulo, “the risks of distortion can be mitigated by precise adjustments to the part’s initial design to yield a geometry in the treated piece that will fit in the application.” For others, one can heat treat the parts together with the build platform to limit distortion where the cycle and build plate are compatible. Another interesting idea comes from metal additive manufacturing service provider and CNC machine shop Wagner Machine that 3D scans practically every one of its 3D printed parts after it returns from heat treating. The idea of the engineering team at Wagner is to grab every possible information that can help them understand the effect of heat treat on 3D printed parts, in order to better predict heat-treating-induced distortion.
To illustrate this point, the company shared the image of a microturbine generator housing they created using aluminum alloy F357 on an LPBF 3D printer. The part is about 23 cm in diameter by 23 cm tall (9 inches x 9 inches) and it is made with large open spaces inside its form through which fuel flows to cool the part. After careful analysis, the team admitted that these unsupported openings increased the margin for potential distortion.
However, while most of the time, the main goal of heat treatment is to stabilize the metallic microstructure and balance material properties, one tends to realize that these microstructure challenges may vary from one metal 3D printing process to another.
Considerations to take into account for L-PBF, EB-PBF and Binder jetting
Laser powder bed fusion (L-PBF), electron beam powder bed fusion (EB-PBF) and binder jetting produce different initial conditions. Metal binder jetting generally requires debinding and sintering to consolidate the part; subsequent treatments and those for fusion processes must be chosen for the required properties. Mott offers the following reference points:
“There are some changes that do happen across the three AM processes. For example, in L-PBF, the build temperature is anywhere from 50°C to 200°C. This creates very high stresses in the part, which leads to the use of a stress-relief thermos treatment followed by a full heat treatment. For binder jetting, you need to remove the binder on the part, that’s why you will need to conduct a debinding stage before a sintering process. Only after this, you could potentially jump in the same heat treatment that you would use in L-PBF. For Electron Beam Melting, the temperature you will deal with, could range from 600 to 1000°C. In this process, when you are printing, even though you are putting a lot of stress on your part, you are running a stress profile, so you won’t necessarily need a stress-relief treatment here”.
The 50–200 °C and 600–1,000 °C values are examples of preheat or bed conditions, not local melt-pool temperatures or universal ranges. EB-PBF preheating can reduce residual stresses without ruling out further treatment or qualification.
By taking the example of laser powder bed fusion (LPBF), the expert at Paulo lays emphasis on a phenomenon called microsegregation, which is the result of the solidification and resolidification of each layer during the printing process.
“In this condition, the AM process itself creates a series of microscopic melt pools (essentially weld pools) throughout the part’s interior structure. Although this can be advantageous by keeping the part’s microstructure very fine, those microscopic melt pools throughout can present segregation issues within the material, with particles separating into distinct zones and affecting the part’s overall structure. This phenomenon can actually work in your favor, since homogenization of the microstructure can happen more rapidly in some AM parts, which reduces hold times during heat treatment.”
Microsegregation is a locally nonuniform distribution of chemical elements during solidification. Its evolution during treatment depends on alloy and thermal history; reduced holding time cannot be generalized. NIST’s metal AM research illustrates the importance of linking process, microstructure and properties.
The table below summarizes what characterizes heat treatment for each of these metal AM processes:
| Comparison | Laser powder bed fusion | Electron beam powder bed fusion | Metal binder jetting |
|---|---|---|---|
| As-produced condition | Selective laser melting of powder, layer by layer. | Selective electron-beam melting of powder in a preheated bed. | Selective binder deposition in powder: a bound green part requiring consolidation. |
| Steps to assess | Stress relief according to a qualified cycle; microstructure treatments and possibly HIP. | Preheat can reduce residual stresses; microstructure treatments and possibly HIP as required. | Debinding and sintering matched to binder and metal, then further treatments as required. |
| Limitations | Plan build-plate removal and distortion; no universal cycle for every alloy. | Omitting separate stress relief needs justification; a hot bed does not remove inspection needs. | Control shrinkage, density and sintering atmosphere; printed geometry is not yet the final part. |
Hot isostatic pressing (HIP) combines heat and pressure to reduce certain internal pores. It does not repair every defect or remove the need to verify microstructure, dimensions and properties afterwards.
How can the result of a cycle be checked?
Anthony Mott shares three tips that can help operators verify if they have successfully performed their heat treat cycles:
- Thermocouples can be attached to the thickest areas in order to ensure that the process is conducted under the desired temperature.
- Operators can also use a “sacrificial coupon” that they will print with the part. You print a cylinder with a hole, and you put a thermocouple inside that hole. It will act as a representation of what your part will go through when temperature variation will occur.
- And the last one that’s most typically done is to print test coupons for tensile or density, break them or analyze them to ensure that you are achieving the desired properties in the right area of your part.
These checks belong within a qualification plan: sensor calibration and placement, coupon representativeness, cycle traceability and acceptance criteria. A tensile or density coupon alone does not establish conformity throughout the part.
What now?
Despite the advancements and precautions one can take during the manufacturing process, it is sometimes challenging to address the mechanical properties variability that occurs when the manufactured part is subjected to different heat treatments and process parameters. The truth is, even if the manufacturing technologies used are performant, the result of each fabrication is greatly influenced by the application and the product reliability. We may not be at the end of the “what ifs” that govern the heat treat cycles and processes when they are combined with AM, but by sharing their holistic experiences, users can learn from the road of success or failures of each other.
Terminology and sources
This dossier was first published in the July/August 2022 edition of 3D ADEPT Mag.
Here are a few explanations on the main terms used in this article.
The main stages are heating, holding at temperature and cooling.
During the heating stage, the foremost aim is to make sure that the metal heats uniformly. The purpose of the soaking stage is to keep the metal at the appropriate temperature until the desired internal structure takes shape. In the cooling stage, you’ll want to cool metal back to room temperature, but there are different ways to do this depending on the type of metal.
Reference: the three stages of heat treatment, Kloeckner Metals.
The four examples below are particularly relevant to steels; they are not an exhaustive list of metal AM treatments.
Annealing: a family of cycles used, among other purposes, to lower hardness, improve ductility or relieve stress, depending on initial condition.
Normalizing: a treatment for some steels aimed at a more homogeneous, refined-grain structure. It should not be confused with quench hardening.
Quenching: rapid cooling from a defined thermal condition, using a medium and rate suited to the alloy. Desired effects and distortion risks depend on the material.
Tempering: a treatment used particularly after steel quenching to adjust the balance between hardness and toughness. Temperature and duration must suit the grade.
Definitions rephrased by the editors using technical references: Bodycote on annealing and normalizing and Kloeckner Metals. The statements attributed to Paulo come from its technical article of August 27, 2021.
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