The Influence of Porosity on Fatigue Crack Initiation in Additively Manufactured Titanium Components

Abstract

Without post-manufacture HIPing the fatigue life of electron beam melting (EBM) additively manufactured parts is currently dominated by the presence of porosity, exhibiting large amounts of scatter. Here we have shown that the size and location of these defects is crucial in determining the fatigue life of EBM Ti-6Al-4V samples. X-ray computed tomography has been used to characterise all the pores in fatigue samples prior to testing and to follow the initiation and growth of fatigue cracks. This shows that the initiation stage comprises a large fraction of life (>70%). In these samples the initiating defect was often some way from being the largest (merely within the top 35% of large defects). Using various ranking strategies including a range of parameters, we found that when the proximity to the surface and the pore aspect ratio were included the actual initiating defect was within the top 3% of defects ranked most harmful. This lays the basis for considering how the deposition parameters can be optimised to ensure that the distribution of pores is tailored to the distribution of applied stresses in additively manufactured parts to maximise the fatigue life for a given loading cycle.

Fatigue behaviors of laser hybrid welded AA7020 due to defects via synchrotron X‐ray microtomography

“…Both elastic and elasto‐plastic FEM simulations show that near surface pores (0.6 < s / r < 1.2) are more harmful to the fatigue cracking resistance of hybrid laser welded AA7020‐T651 joints in terms of notable stress concentration. Similar conclusions have also been derived for different materials with various defects …”

“…Compared with surface pores, the subsurface pores appear to be less harmful to the fatigue regardless of the size. The similar conclusion was confirmed on AM Ti alloys even though relatively less data are available . More experiments and theoretical aspects are required to show the point.…”

“…For in situ fatigue SR‐μCT at the ID19 with the stress ratio R = 0.1, a total of 11 smoothed hybrid laser welded joints were completed at the distance of 15 cm to 2048 2 resolution detector and 64 m from X‐ray source, approximately producing a pixel size of 0.7 μm and overall 1995 radiographs under the photon energy of 19 keV and exposure time of 0.1 seconds. Previous and our studies have indicated that those pores lower than 10 μm in equivalent diameter usually have a very limited effect on the tensile and fatigue properties . It is reasonably derived that more flexible and broad imaging parameters can be selected at the BL13W1 of SSRF.…”

“…While the critical defect size of rheocast A357 alloys is considered as 155 μm, which is larger than the critical value 25 μm of Sr‐modified A356 alloys . It is thus suggested that the critical defect size should be determined when assessing the crack initiation and short crack behaviors of welded joints …”

Investigation of the anisotropic cyclic damage behavior of selective laser melted AISI 316L stainless steel

“…The results obtained by nondestructive CT analysis can be used to identify critical pores with a high volume and characterize their size, shape, and position in the specimen before fatigue testing. For a titanium alloy processed by electron beam melting technique, it could be shown that not only the size but also the proximity to the surface and the pore aspect ratio, which was detected by CT measurements, have a high influence on the fatigue behavior …”