Optimization of mechanical properties of additively manufactured components through targeted heat treatment

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 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 in a single process step in near-net shape.

Many 3D-printed metal components require subsequent heat treatment, particularly when high demands are placed on mechanical properties such as strength and toughness.

The reasons for heat treatment of additively manufactured components are manifold. It is important to note that these components often exhibit characteristics that differ from cast, forged, or machined parts and often do not represent the optimal condition for immediate use:

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 manufacturing-related disadvantages and significantly improve mechanical properties. Therefore, heat treatment is an essential part of the process chain in additive manufacturing.

Depending on the requirements, the following treatments can be performed in the process chain of additive manufacturing:

Before the build process:

After the build process:

Drying the powder is an upstream process. Consistent part quality requires high and consistent powder quality. Moisture that the powder can absorb during storage can impair the quality of the finished part. Drying at moderate temperatures can counteract this.

In binder-based manufacturing processes, the powder is bonded with a binder, usually resins, in the first stage of component production and applied layer by layer. This results in a component with sufficient strength for in-plant transport. Before the sintering process, the binder must be removed from the component.

Debinding is carried out by heating the part to the evaporation temperature of the binder. This temperature must be precisely maintained so that the gaseous decomposition products can diffuse out of the part. During and especially after binder removal, the part is extremely unstable because there is no solid connection between the particles. Therefore, the part is stabilized after debinding through sintering to achieve the required strength.

Debinding and sintering each require specific temperatures. By using combination furnaces that cover both temperature ranges, transport between process 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:

Gas Supplies:

The high residual stresses in most additively manufactured components present a high risk of cracking as well as significant distortion potential. For this reason, stress relief annealing is almost always applied as the first process step after additive manufacturing.

The thermal relief of residual stresses occurs through the decrease in material strength with increasing temperature. Residual stresses that exceed the temperature-dependent yield strength are relieved plastically. Since the resulting plastic deformations can cause dimensional and shape changes, it is recommended to leave the components still attached to the build platform during stress relief annealing to ensure additional stability. To prevent oxidation of the metal surface, stress relief annealing must be performed in a protective atmosphere.

Gases Used:

Gas Supply Systems:

Due to the preferred grain orientation, the mechanical properties of additively manufactured components are often anisotropic. To homogenize the microstructure (uniform grain shape and size) and improve the mechanical properties (elimination of anisotropy), conventional or diffusion annealing can be performed in defined gas atmospheres.

Gases used:

Gas supplies:

Hot Isostatic Pressing (HIP) removes microporosity from additively manufactured components. By combining very high pressure (up to 3,000 bar) with heat (up to 2,000 °C), internal porosity is eliminated through plastic deformation, creep, and diffusion. By reducing internal porosity, up to 100% of theoretical density can be achieved. At the same time, ductility is increased and fatigue strength is significantly improved.

Gases used:

Gas Supply Systems:

To achieve maximum strength in additively manufactured components, thermochemical heat treatment processes such as case hardening, carbonitriding, or nitriding can be performed in the final process step, depending on the material composition.

Thermochemical heat treatment processes for additively manufactured components are subject to the following conditions:

Thin-walled and complex structures present a special challenge in heat treatment. These structures are often exposed to the risk of complete carburization or nitriding during thermochemical heat treatment, which can negatively affect mechanical properties. In addition, thin and complex structures are particularly sensitive to distortion during quenching.

Liquid quenching media such as oils and salts can severely contaminate internal structures, which complicates subsequent part cleaning. Therefore, gases are frequently used as quenching agents for complex structures. The use of high-pressure gas quenching can significantly reduce dimensional and shape changes and eliminate post-cleaning from the process chain.