Post-Process Treatments for Additive-Manufactured Metallic Structures: A Comprehensive Review

This review summarizes the recent research progress on the post-processing techniques to improve the as-built metallic structure fabricated by additive manufacturing. The comprehensive comparison reveals that hot isostatic pressing (HIP) is the only one of few that can eliminate the internal porosity (< 0.01%) with high efficiency and availability. Traditional heat treatments (HTs) are still the best approach to improve or alter the microstructure for the overall bulk properties. Various surface post-processes are presented that are more flexible and cost-effective than common machining and polishing. Laser-based techniques are the most popular treatment for surface modification to induce compressive stress, remove the surface pores, reduce the surface roughness, or apply coatings for functionality. Other mechanical methods like shot peening (SP), laser shock peening (LSP), ultrasonic nanocrystal surface modification (UNSM), and abrasive-based treatments facilitate plastic deformation to impact surface beneficially for hardness, fatigue, or corrosion resistance. In addition, impact-type surface treatments (SP, LSP, and UNSM) are more effective on enhancing the mechanical properties due to greater energy imparted to the surface than abrasive-type treatments. Electrochemical polishing is capable to achieve the best surface smoothness ( $${R}_{a}$$ < 0.1 $$\mathrm{\mu m}$$ ), while friction stir processing shows the best effectiveness on homogenize the structure without significant grain growth at nanoscale. Advanced but costly surface techniques are only suitable to satisfy specific requirement and application of high value metallic component. By optimizing the process parameters and adopting in situ tools, the as-built structure can be enhanced with minimized necessity for post-processing. Overall, a complete solution of post-treatment includes HIP, HTs, surface modification, and/or special process.