X-ray Micro Tomography Study of Internal Defects of Electron Beam Melted Ti6Al4V and Their Effect on Fatigue Behavior

Abstract

In this work, the fatigue behaviour of Ti6Al4V manufactured using electron beam melting, its dependency on porosity, distance from the base plate and build layer height were investigated. XCT scans of the fatigue sample gauge lengths were correlated to SEM investigations of the fracture surfaces. A comparison between the top and bottom halves of the builds in terms of defect population and fatigue behaviour was also made. Larger pores were detected in samples with a larger build layer height and lower position in the build chamber. Results also indicate that part geometry and pore location, specifically closeness to the surface, are important factors regarding the initiation location of fatigue fractures at 1 % strain. Furthermore, a fatigue critical lack of fusion defect was undetectable in the XCT scan.

Defects in Electron Beam Melted Ti-6Al-4V: Fatigue Life Prediction Using Experimental Data and Extreme Value Statistics

“…This is likely because of the flat geometry of LOF defects in combination with the limitations of the resolution. This was previously discussed in [ 20 ].…”

“…As demonstrated in the 1980s in the works of Murakami [ 19 ], defects close to the surface are more likely to initiate crack growth than defects further from the surface. This has also more recently been shown to be the case for defects in AM materials [ 20 , 21 , 22 ]. Due to the generally large number of defects present in AM materials, there is almost always a near-surface defect determining the fatigue life [ 23 ].…”

“…The parameters for the scans are presented in Table 2 . Segmentation and analysis of the tomography data were carried out using Dragonfly PRO 4.1 [ 39 ] employing a segmentation algorithm for internal porosity specified in [ 20 ]. Any segmented volume smaller than 81 voxels was considered unreliable data and therefore excluded from the analysis.…”

“…For the porosity distribution simulations two assumptions were made: (i) Defect projections in the plane perpendicular to the loading direction were assumed to be circular. This is a good, but not perfect assumption based on the observed XCT data [ 20 ]. (ii) The defect location distribution was assumed to be uniform in the plane perpendicular to the building direction.…”

“…The defect size distribution was approximated by a generalized Pareto distribution (GPD) according to the peak-over-threshold (POT) extreme value statistics method in which only the tail of the distribution above a certain cut-off diameter u is analyzed [ 37 ]. The tails of the defect size distribution as evaluated by XCT have previously been found to be exponentially distributed [ 20 ]; consequently, the shape parameter of the GPD approaches 0, and the cumulative density function of the generalized Pareto distribution becomes a two-parameter exponential distribution: where is a scale parameter for the distribution of defect sizes larger than the cut-off parameter u . The p th percentile of the distribution can be derived from Equation ( 2 ) as: where p is the cumulative probability of recurrence.…”