Additively manufactured metal components promise design freedom and high customization. In safety-critical applications such as aerospace, automotive, and energy technology their use has so far often been hindered by insufficient fatigue strength: surface roughness, tensile residual stresses, and microstructural defects such as gas pores and/or lack of fusion act as crack initiators and can lead to premature component failure.
Mechanical surface post-treatment methods such as shot peening, deep rolling, and burnishing can specifically eliminate or reduce these weaknesses: they increase the hardness of the surface layer, reduce defect density, and introduce compressive residual stresses. Until now, however, a practical calculation model for their targeted application to AM components was lacking. Researchers at Fraunhofer IWM have now closed this gap.
From stress analysis to fatigue life prediction – in a continuous calculation chain
Within a project funded by the Federal Ministry for Economic Affairs and Energy (BMWE), a model-based calculation chain was developed and validated on the AM materials AlSi10Mg, 316L, and Ti6Al4V produced with LPBF (laser powder bed fusion) and CMF (cold metal fusion). It links three steps:
Component assessment and process selection: Depending on the component geometry and the location of the highest stress, the appropriate post-treatment method is determined.
Process simulation: Surface layer properties such as residual stresses, roughness, and hardening are predicted and optimized as a function of the process parameters (e.g. peening pressure, contact force, path overlap).
FKM-based fatigue life assessment: The fatigue life prediction is carried out in accordance with the FKM guidelines established among design engineers and SMEs and can be directly integrated into existing development processes. The FKM fatigue life assessment was adapted for the printed and post-treated materials AlSi10Mg, 316L, and Ti6Al4V.
The result via model tests is an increase in fatigue strength of up to 40 percent and an extension of service life by several decades. The calculation results were experimentally validated through fatigue experiments at both specimen and component level.
»One significant reason why post-treatment methods are rarely used for AM components is the lack of understanding of the effect chain from process parameters to service life improvement, and the absence of a fatigue life verification method. With our approach, we are now opening new possibilities for novel applications,
Added value for design and product development
The method can already be applied during pre-development and enables a computational comparison of various post-treatment strategies before the first component test takes place. A further advantage: since components predominantly fail at their surface, a tailored post-treatment can partially compensate for process-induced scatter in the AM printing process. Based on fundamental materials properties such as yield strength and tensile strength, the effects can also be transferred to new materials.
For the first time a computer-aided design tool was created for the mechanical surface post-treatment of additively manufactured metal components.
Project funding
The project results were produced within the project Mechanical Surface Post-Treatment of Additively Manufactured Metal Components for the Targeted Improvement of Fatigue Strength (funding reference IGF 22833 N), funded by the Federal Ministry for Economic Affairs and Energy (BMWE), and were subsequently extended and expanded.
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Fraunhofer Institute for Mechanics of Materials IWM