Expression of Additive Manufacturing Surface Irregularities through a Flaw-Based Assessment

Abstract

Metal additive manufacturing (AM) has progressed from a pure research topic to an indispensable manufacturing process for many industries within the past 15 years. For the space industry, the performance of a variety of products, including structures, antennas, propulsion parts, and optical equipment is being improved through this manufacturing technique. Process-inherent defects are still encountered and may lead to significant safety factors, particularly for highly loaded, mission-critical applications. An analysis of the fatigue properties of AlSi10Mg produced by selective laser melting (SLM) has shown that the fatigue strength and fatigue life of machined specimens could be accurately predicted by adopting a defect-tolerant design concept. In details, the fatigue strength appeared to be controlled by the defect with the maximum stress intensity factor (SIF) present in each specimen. The fatigue properties could be predicted by treating the lack-of-fusion defects as short cracks. For near-net-shaped (i.e., not machined) surfaces, a previous investigation studied the influence of building direction, platform temperature, powder layer thickness, surface finish, and heat treatment on fatigue properties of AM AlSi10Mg and X-ray computed tomography (XCT) was performed to research the defect population. The work showed that processing parameters and surface treatment influenced fatigue properties, as expected for conventionally produced material. This study conducted an accurate analysis of the subsurface features at the fracture origin of AlSi10Mg specimens, manufactured by SLM and treated with different surface finish. On the basis of these results, a simple fracture mechanics assessment inside the Kitagawa diagram is proposed, which relies on the size of critical defects and the residual stress obtained on the surface. The applicability of this approach to define the quality of AM material in the near-net-shaped condition is discussed.

Quality control of AlSi10Mg produced by SLM: Metallography versus CT scans for critical defect size assessment

“…In these cases, PS analysis might result a simpler and more cost-effective process but with potentially useful 3D information lost. Similar considerations can be drawn regarding surface quality assessment for additively manufactured materials in the net-shape condition [67,68] , which is why standardized procedures are being presented, see for example [69].…”

High cycle fatigue behavior and life prediction for additively manufactured 17-4 PH stainless steel: Effect of sub-surface porosity and surface roughness

“…In addition to such pores, the NS surface condition also contains deep valleys due to the surface roughness, which can introduce stress concentrations and negatively affect the fatigue performance of the material [10,17,26,43]. For the present material, the maximum valley depth measured on the polished sections was approximately 65 μm (see Figure 6a).…”

“…Regarding the surface condition, fatigue assessment is more complex due to strong influence of various process parameters (e.g., laser power, scanning speed, layer thickness, contouring strategy, build orientation) on the part's surface roughness [10,[17][18][19][20][21][22]. Even if roughness measurements are often adopted to evaluate the surface quality of AM parts, various studies have demonstrated that common average or peak-to-valley measurements are not enough to obtain a robust correlation with fatigue life [10,21,[23][24][25].…”

Analysis of prospective SIF and shielding effect for cylindrical rough surfaces obtained by L-PBF

“…A batch of net-shaped L-PBF AlSi10Mg fatigue specimens was provided by ESA/ESTEC to Politecnico di Milano. These specimens are part of a test campaign, whose results were already discussed in [43,44].…”