The influence of post-processing methods on the fatigue performance of materials prepared by selective laser melting

Researchers from Opole University of Technology in Poland have reported the latest progress in studying the effect of post-processing methods on the fatigue performance of materials prepared by selective laser melting (SLM). The related research was published in The International Journal of Advanced Manufacturing Technology under the title "Influence of post processing methods on fatigue performance of materials produced by selective laser melting (SLM)".

Although selective laser melting (SLM) can manufacture complex and customized products, the fatigue performance of parts manufactured by SLM is reduced compared to forged parts. The main factors causing this difference are the imperfect manufacturing parameters and inherent defects in the process itself. Even with the use of optimal manufacturing parameters, there is still variability in fatigue performance. However, it has been proven that using post-processing methods to reduce or eliminate defects is an effective way to improve fatigue performance.

This article provides a detailed introduction to various post-processing methods that can be used to modify surface and subsurface defects as well as internal defects. Each method demonstrates different potentials in generating beneficial residual compressive stress (CRS), altering surface and subsurface defects, and improving fatigue performance. The study found that considering the cyclic process, the impact of different post-processing methods varies. This article explores the conflicting research results that need further clarification. In addition, this article provides an in-depth review of the types and characteristics of defects that may affect fatigue performance, emphasizing the necessity of understanding these defects. The article points out the research gaps in existing post-processing methods and plans for future research directions, with a focus on technologies that have not been fully utilized but have potential for change.

Figure 1a Schematic diagram of components and process for selective laser melting (SLM); Possible internal and surface defects in SLM process.

Figure 2 Selective laser melting process parameters, key material characteristics, defects in SLM parts, and commonly used post-processing methods.

Figure 3a Schematic diagram of laser peening process and the resulting surface deformation; Laser scanning path and parameters.

Figure 4: The effect of laser peening on the performance of SLM parts; Measurement results of surface roughness and porosity before (a) and after (b) laser peening; Electron backscatter diffraction (EBSD) patterns of Ti64 treated with SLM before (c) and after (d) laser peening; Cross sectional views of AlSi7Mg in AB state (e) (a 'and b') and after laser peening (c ', d', e '); High magnification image of pores after laser peening (d '). The fracture surface of AlSi10Mg treated with SLM and post-treatment (f).

Figure 5 Shot peening process configuration and process parameters.

Figure 6: The effect of post-processing on surface and subsurface defects; (a) SLM fatigue specimen; (b) And (c) the surface roughness of unprocessed samples; (d) The sample treated with vibration light decoration failed due to crack nucleation at the surface and subsurface opening defects; (e) The crack initiation defect (√ area=98 µ m) that may cause material failure under cyclic loading.

Figure 7 SLM process parameters with defect types (a); Post processing methods and their effects on surface, subsurface, and internal defects (b); The impact of post-processing on defect resolution (c). C1, (MP) sample; Surface and subsurface pores of C2 and MP; C3，AB+SP； Uneven microstructure of C4 and SP; C5, HT specimens; The fracture morphology of C6 and HT samples; C7， Samples treated with hot isostatic pressing (HIP); C8, porosity before and after HIP treatment.

This article comprehensively reviews the relevant research on the influence of post-processing methods on the fatigue performance of selective laser melting (SLM) parts. Research has found that surface and subsurface defects, internal defects, microstructure, residual stress, and surface roughness are important factors affecting the fatigue performance of SLM parts. Laser peening has shown its potential in closing subsurface pores and generating deeper residual compressive stresses (CRS) than shot peening, although this can increase surface roughness.
However, shot peening can improve residual stress and surface roughness while forming a thicker residual compressive stress layer. The high roughness and uneven residual stress distribution generated during laser peening can lead to a decrease in fatigue performance. However, using continuous shot peening can improve fatigue performance without negatively affecting surface roughness. By creating good surface characteristics, mechanical processing has the ability to manufacture SLM parts with longer fatigue life.

However, this method has limitations in altering the transverse residual stress (TRS) and anisotropic behavior inside printed parts. Although heat treatment methods cannot improve surface properties, they can effectively change this harmful stress, but this method can lead to an increase in pores. Although hot isostatic pressing (HIP) cannot enhance surface properties, it has been proven to alter microstructure, refine anisotropy, close internal pores, and increase overall material density, thereby extending fatigue life.

The combination of hot isostatic pressing and mechanical processing, followed by polishing treatment, has been proven to produce SLM parts with superior fatigue performance even compared to traditional manufacturing parts. Not all defects affect the fatigue life of materials, but defects near the surface and large-sized internal defects (LOFs) with sharp edges are the main defects that lead to a decrease in material durability.

This article focuses on introducing post-processing methods that have not been widely studied and proposes future research directions. By evaluating the commonly used post-processing techniques in SLM, this article provides insightful guidance for improving the reliability of industries such as aerospace, energy, automotive, and biomedical implants in high stress applications.

Source: Yangtze River Delta Laser Alliance