Heat treatment cycles are used in the manufacturing process of functional and mission-critical parts, regardless of whether they are made by additive manufacturing or by more conventional manufacturing methods. However, while there are many similarities between the steps additive users need to take to heat treat their parts, there are also some marked differences compared to conventionally manufactured parts.
What are some of the typical heat treatment steps for metal additive parts, and why are they necessary?
The main goal of heat treatment is to stabilize the metallic microstructure and balance material properties. The types of heat-treat cycles used vary depending on both the alloy and fabrication method, but all of them play an important role.
For laser powder bed fusion (L-PBF), residual stresses are created in parts due to the rapid melting and cooling of each layer, so a stress-relief cycle is necessary to reduce distortion before removal from the build plate. Depending on the alloy used or part application, you might also want to perform a hot isostatic press (HIP) treatment to maximize density and create a more isotropic microstructure, and consider solution and age cycles for applicable alloys.
By comparison, electron beam powder bed fusion (EB-PBF) has a hot powder bed, so the stresses that are induced in L-PBF are not necessarily found in EB-PBF parts, meaning stress relief treatments are not needed. The alloy and/or application may still necessitate HIP or solution and age cycles though.
Binder jet is also different. Because this modality doesn’t melt powder in the powder bed, a new type of heat treatment is employed: sintering. This process first burns away the binder and then consolidates the particles by solid state atomic motion. A sinter step takes a 50-60% dense “green” part from the printer and creates a solid part which is greater than 99% dense.
Solution and age cycles can be used with the various additive modalities to strengthen those alloys. Solution cycles are most applicable for solution-strengthened and age-hardenable alloys. An age cycle is performed after the solution cycle for age-hardenable alloys. This lower-temperature process allows the strengthening phases to precipitate in a controlled manner.
How does heat treatment with additive manufacturing differ to conventional methods?
The difference between additive manufacturing and conventional manufacturing methods is not as different as you might think. The main difference with additive is that you have to consider how the part is made, more so than you do with conventional methods.
For example, L-PBF goes through a stress-relief process, which would be the same as a conventional post-machining stress-relief heat treat. With binder jet, you’d go through the binder burnout and sinter steps. And the high inherent process temperatures of EB-PBF mean that the parts don’t require any additive-specific steps. For all these modalities, once you’re at these points, the processes and steps are then no different from conventionally manufactured parts. You’re still dealing with the same chemistry and the same overall microstructure of the part, it’s just that the microstructure in the part is set at a different stage of the manufacturing process.
Why don’t all alloys use the same heat treat cycles?
The goal of heat treating a part is to set the microstructure and balance material properties. Each metal alloy has a different composition of elements that results in a different overall chemistry, and this means that the “stable microstructure” looks different in each alloy. The variation in chemistry means that different metallic phases form at different temperatures and rates. Some of these metallic phases require more time at temperature than others, so the heat treat recipes need to be set at different temperatures and held for different times to best suit the chemistry of each alloy. It is the balance of these metallic phases that generates desirable material properties in your alloy.
How do you verify if a heat-treat cycle was run successfully?
There are several tests that can be performed on a heat-treated material to verify that the desired material properties were achieved. Most commonly we take samples from the heat treat run, cut them up and perform a microstructural analysis to measure porosity and grain size of the part. We also perform different mechanical-property tests, with the most common being tensile and hardness testing.
Why do the part application requirements matter when selecting a heat treatment?
As we mentioned before, heat-treatment processes are a method of balancing your material properties. If you heat treat a given alloy using different thermal cycles, you will end up with different material properties that may not be the most optimal for the end application. Additionally, some may choose to eliminate certain heat-treat cycles if the part does not require it in order to save cost. So, it is important to understand the operating conditions and the goal of your finished part, as this will enable you to determine the necessary material property requirements.
One example to showcase this is 17-4PH, which is a precipitation hardened stainless steel. This alloy can be heat treated with several different precipitation cycles to optimize for strength, toughness and/or corrosion resistance. Some applications may operate in a corrosive environment and demand higher toughness but may not need high strength values. Other applications, however, could require high strength but may not be as sensitive to corrosion. Each heat-treat cycle can optimize for one property, but usually with the trade-off of another.
Does the cooling rate at the end of the furnace cycle matter?
Absolutely! Just as chemical composition of the alloy affects which microstructural phases form, and how they form, they are also significantly affected by temperature and time. At higher temperatures, the atoms and phases move around a lot more freely, and the way those phases stabilize depends on how quickly you cool the alloy. If you cool it quickly, you can freeze those high-temperature phases. However, if you cool the alloy slowly, those phases will start to change as you cool, and those elements will have time to stabilize in different ways than if they were cooled quickly. Depending on the specific alloy, the effect of cooling rate on material properties can range from minor to dramatic, which could result in properties drastically different than anticipated.
Is HIP necessary for all additive components?
No, HIP is not required for all additive components. HIP uses a combination of high temperature and high pressure to close any internal pores (macro and micro) to improve the mechanical properties of a component. We typically think of HIP as somewhat necessary for L-PBF and EB-PBF components used in critical applications where there is a need for extremely high density, such as fatigue-limited components. It is worth noting that HIP can improve fatigue life by reducing porosity, but as-printed surfaces can drive even worse fatigue behavior that may not be improved through HIP.
If the intended application is not fatigue-limited or does not require extremely high density, you can consider excluding HIP. However, you should always characterize the intended thermal state for the component first before deciding. If you can characterize the material without HIP—in terms of its density, microstructure and mechanical properties—and it meets the component requirements, then you don’t need to include HIP in your thermal post-processing. Ultimately, the decision to HIP or not is dependent on the property requirements of your components set by the application or the requirements set by regulatory bodies (if applicable).
Can all heat-treat cycles be performed in air (rather than vacuum or inert environment) if the appearance of the surface does not matter?
No, you must carefully consider the heat-treat cycle environment you used for a particular alloy, because each alloy will respond to the environment differently. Aluminum alloys, for example, can be heat treated in air with little to no effect on the resulting material properties and microstructure. Titanium alloys, on the other hand, react severely to the presence of oxygen and nitrogen in air at elevated temperatures. With titanium and other sensitive alloys, these reactions don’t just occur at the surface, but throughout the entire component. Therefore, heat treating these alloys in air will cause permanent damage and result in significantly reduced material properties. In many cases, vacuum or inert heat-treatment environments are required.
Improper selection of heat-treating atmosphere can result in microstructural phases that are detrimental to part properties. As seen in the image of Ti6Al4V below, oxygen has infiltrated the surface of a coupon and produced alpha case, the light- or white-etching surface layer. Alpha case may contain microcracks, which could cause premature part failure. Better control over furnace vacuum or including sacrificial getter material would remedy this problem.
Heat treating an additive component is not much different than heat treating conventionally manufactured components, so long as you consider how the part is made and resolve for any potential behavioral changes in the manufacturing process. Nevertheless, while the approach may not be vastly different when compared to conventionally manufactured parts, heat-treat cycles are of huge importance if you want to get the right balance of material properties in your part.
One of the critical aspects of heat treating an additive part is understanding the operating conditions and the end-use requirements of a component when designing a heat-treat cycle.
If you need any help in determining the right heat treatments for your component, or if you’d like to understand further the impacts of heat-treat cycles, please contact us to find out more.