Additive manufacturing (AM), commonly known as 3D printing, has revolutionized the way products are designed, prototyped, and produced. Unlike traditional subtractive manufacturing methods, which remove material to create a part, AM builds objects layer by layer, providing unmatched design flexibility and material efficiency. The motto for AM, "Freedom for nothing," highlights the great design freedom available to engineers, enabling them to develop solutions that would not be feasible with traditional manufacturing processes.

However, despite these advantages, additive manufacturing does not produce a finished part in its as-built condition. Post-processing is a crucial step in the production cycle, necessary to enhance the functionality, aesthetics, and, most importantly, the mechanical properties of printed parts. Furthermore, post-processing accounts for a significant fraction of the total cost of AM. A recent study conducted at the University of Pennsylvania estimated that pre- and post-processing contribute to approximately 40% of the total cost of an additively manufactured part.

The necessity of post-processing in AM arises from several factors, including surface finish, mechanical strength, dimensional accuracy, and material properties. Most 3D-printed parts exhibit rough surfaces, support structures, residual stresses, and even microstructural defects, all of which require refinement to meet industry standards. The specific post-processing steps depend on the printing technology, material, and application.

Post-Processing for Mechanical Components

When focusing on mechanical components produced through AM processes, fatigue strength is one of the main challenges. Without post-processing, the final fatigue strength of an AM part is often insufficient for demanding applications. Shot peening and related treatments are highly attractive solutions for addressing this issue.

In recent experimental tests conducted by my research group and me, we found that shot peening significantly enhances the fatigue strength and endurance of the AlSi10Mg alloy. Specifically, at 5 million cycles, the fatigue strength of shot-peened AlSi10Mg was more than three times higher than that of the as-built condition. Additionally, under a fixed stress level, the endurance was extended by up to 600%.

However, since the initial surface condition of AM parts differs significantly from conventional shot-peening applications, careful selection of process parameters is essential. A preliminary set of experiments is strongly recommended to determine the optimal parameters.

Combining Shot Peening with Other Post-Processing Treatments

Shot peening can also be combined with other treatments to further improve post-processing outcomes by enhancing surface finishing and reducing surface roughness. This is necessary because shot peening alone cannot produce a mirror-like surface due to its inherent material impact characteristics.

To achieve a smoother finish, the following post-processing techniques can be used:

• Chemical polishing or electrochemical polishing, which smooths surface irregularities.
• Tumbling, which improves surface quality through mechanical abrasion.

Our experiments indicate that the synergistic combination of shot peening and one of these final treatments can significantly enhance the performance of AM parts compared to shot peening alone. This suggests that a multi-step post-processing approach may be the optimal solution.

Challenges and Future Outlook

The main challenges remain cost and the integration of the complete post-processing cycle into production without creating bottlenecks. However, I remain optimistic. With continuous advancements in automation and material-specific solutions, shot peening-based treatments will likely see wider adoption across various industries.

Contributing Editor MFN and Full Professor of Technical University of Milan

20156 Milan, Italy

mail: mario@mfn.li