Optimization of the mechanical properties of additively manufactured components through targeted heat treatment
Heat treatment of 3D printed components
Subtractive manufacturing processes, such as milling and turning, remove material until the desired geometry of the component is achieved. In contrast, additive processes, commonly known as additive manufacturing or 3D printing, build up components layer by layer from materials such as metal, plastic, ceramic, glass, sand or other substances until the finished component is created. With these processes, complex components can often be manufactured to almost their final contour in a single process step.
Many 3D printed metal components require subsequent heat treatment, especially when high demands are placed on mechanical properties such as strength and toughness.
There are many reasons for the heat treatment of additively manufactured components. It is important to note that these components often have characteristics that differ from cast, forged or machined components and are often not in the optimum condition for immediate use:
- Components from additive manufacturing can have pores or cracks that are caused by incomplete melting of the powder or inhomogeneous cooling of the metal.
- Due to the thermal history, additively manufactured components often exhibit locally different grain size distributions. The grains often show a pronounced preferred direction in the vertical direction due to solidification.
- This directional microstructure often leads to direction-dependent mechanical properties (anisotropy). This means that both the static and dynamic strength as well as the toughness of an additively manufactured component can show significant differences between the transverse and longitudinal direction.
- Additive manufacturing is characterized by a local thermal energy input, rapid cooling and the repetition of the process when applying each new layer. This leads to local compressive and tensile stresses, which can result in a complex residual stress state of the component.
These characteristics often impair the load-bearing capacity of additively manufactured components and limit their use in highly stressed applications. However, optimized heat treatment can eliminate or reduce many of the production-related disadvantages and significantly improve the mechanical properties. Heat treatment is therefore an essential part of the process chain in additive manufacturing.
Depending on the requirements, the following treatments can be carried out in the additive manufacturing process chain:
Before the construction process:
- Drying the powder
After the construction process:
- Debinding
- Sintering
- Stress-relief annealing
- Normal annealing
- Diffusion annealing
- Hot isostatic pressing (HIP)
- Thermochemical heat treatment processes
Drying the powder is an upstream process. Consistent component quality requires a high and consistent quality of the powder. Moisture that the powder can absorb during storage can impair the quality of the finished component. Drying at moderate temperatures can counteract this.
Debinding and sintering in metal 3D printing
In binder-based manufacturing processes, the powder is bound with a binder, usually resins, in the first stage of component manufacture and applied in layers. This results in a component with sufficient strength for internal transportation. The binder must be removed from the component before the sintering process.
Debinding is carried out by heating the component to the evaporation temperature of the binder. This temperature must be precisely maintained so that the gaseous decomposition products can diffuse out of the component. During and especially after removal of the binder, the component is extremely unstable as there is no solid bond between the particles. For this reason, the component is stabilized by sintering after debinding in order to achieve the necessary strength.
Debinding and sintering each require specific temperatures. By using combination ovens that cover both temperature ranges, transportation between the work steps can be avoided.
The debinding and sintering of metal components is usually carried out under protective gases that protect the components from oxidation. The use of hydrogen-containing protective gases is also possible. The use of hydrogen and the vapors produced during debinding require an adapted safety system for the furnace system.
Gases used:
- Nitrogen
- Hydrogen
- Argon
- 3D Heat Mix (2.65 % H2 in Ar)
Gas supplies:
- Nitrogen supply
- Argon supply
Stress relief annealing for 3D-printed metal components
The high residual stresses in most additively manufactured components pose a high risk of cracking and a considerable potential for distortion. For this reason, stress relief annealing is almost always used as the first process step after additive manufacturing.
The thermal reduction of residual stresses occurs due to the decrease in the strength of the material with increasing temperature. Residual stresses that exceed the temperature-dependent yield point are reduced plastically. As the resulting plastic deformations can cause dimensional and shape changes, it is advisable to leave the components unseparated on the component platform during stress relief annealing to ensure additional stability. To prevent oxidation of the metal surface, stress relief annealing must be carried out in a protective atmosphere.
Gases used:
- Nitrogen
- Argon
Gas supplies:
- Nitrogen supply
- Argon supply
3D components Normalizing and diffusion annealing
Due to the preferred direction of the microstructure, the mechanical properties of additively manufactured components are often anisotropic. In order to homogenize the microstructure (uniform grain shape and size) and improve the mechanical properties (elimination of anisotropy), normalizing or diffusion annealing can be carried out in defined gas atmospheres.
Gases used:
- Nitrogen
- Argon
Gas supplies:
- Nitrogen supply
- Argon supply
Hot isostatic pressing (HIP) for printed metal components
Hot isostatic pressing (HIP) removes the microporosity of additively manufactured components. By combining very high pressure (up to 3,000 bar) with heat (up to 2,000 °C), the internal porosity is eliminated through plastic deformation, creep and diffusion. By reducing the internal porosity, up to 100 % of the theoretical density can be achieved. At the same time, ductility is increased and fatigue strength is significantly improved.
Gases used:
- Argon
Gas supplies:
- Argon supply
Thermochemical heat treatment processes
In order to achieve maximum strength in additively manufactured components, thermochemical heat treatment processes such as case hardening, carbonitriding or nitriding can be carried out in the final process step, depending on the material composition.
Thermochemical heat treatment processes for additively manufactured components are subject to the following conditions:
- The components should not be too complex, filigree, thin-walled, asymmetrical and rough.
- The surface must be active (free of oxides or other passivation layers).
Thin-walled and complex structures pose a particular challenge during heat treatment. During thermochemical heat treatment, these structures are often exposed to the risk of complete carburization or sticking, which can have a negative effect on the mechanical properties. In addition, thin and complex structures are particularly susceptible to distortion during quenching.
Liquid quenching media such as oils and salts can heavily contaminate internal structures, which makes subsequent component cleaning more difficult. For this reason, gases are often used as quenching agents for complex structures. The use of high-pressure gas quenching can significantly reduce dimensional and shape changes and remove subsequent cleaning from the process chain.