Work hardening and shot peening in AM
The fatigue killer: How residual compressive stresses prevent cracking in titanium components.
1. Introduction: When residual stresses become dangerous
Powder bed 3D printing (PBF-LB) is a process of thermal extremes. The melting and immediate solidification every second leaves the component pumped full of tensile residual stresses. These tensions literally want to pull the component apart.
Under cyclic mechanical loading (e.g. a rotating drive shaft or vibrating turbine blades), these tensile residual stresses add up to the external forces. The result: cracks appear very quickly on the rough surface and the component breaks (material fatigue). The solution to this critical aerospace problem is called shot peening.
What is shot peening?
Shot peening is not to be confused with sandblasting (which primarily cleans or roughens). During shot peening, tiny, perfectly round steel, glass or ceramic balls are shot onto the surface of the metal component with enormous kinetic energy (compressed air).
2. The physical principle: compressive stress as a shield
Each ball hits the surface like a tiny blacksmith's hammer. This creates small, round dents.
The underlying material is thereby plastically deformed (strain hardening). Because the underlying core material wants to compensate for this deformation, a huge "protective armor" of compressive residual stresses is created on the surface layer (the uppermost micrometers of the component).
This protective armor compresses the material. If an external tensile force acts on the component during operation (which wants to tear it apart), this force must first overcome the embedded compressive stresses. Cracks that try to penetrate the component from the outside are literally "squeezed shut" again by the compressive stress.
3. Smoothing and closing pores
For 3D printed parts, shot peening offers a massive double effect:
- Surface smoothing: The extremely hard, uneven "sandpaper" surface (partially melted powder) is physically leveled (plasticization) by the massive ball impact. The Ra value drops drastically.
- Pore closure: Subcutaneous pores (located just below the surface) that were created in the laser melting process are closed by the brutal impact of the bullets. These notches (predetermined breaking points) disappear.
4. Challenges with complex AM geometries
As brilliant as shot peening is, it is difficult to use in 3D printing.
- Almen intensity: The pressure of the balls must be precisely calibrated (measured using the Almen test). If the impact is too strong, delicate walls distort (thin wall distortion).
- Internal channels: Bullets shot from a nozzle ricochet off walls (line of sight). Deep, curved cooling channels in an injection molding tool cannot be shot peened from the inside. Expensive special processes such as abrasive flow machining have to be used here.
5. Conclusion: The fatigue killer
For additively manufactured titanium or aluminum components in aviation, motor sports or for medical knee implants, shot peening is not an optional gimmick, but mandatory. It often increases the fatigue life of a component tenfold and transforms the weakest point of 3D printing (the rough edge layer under tensile stress) into its strongest protective armor.