Understanding porosity and fatigue variability in Ti-6Al-4V L-PBF literature

Abstract

Being able to accurately and repeatably assess pore characteristics and resulting fatigue performance with confidence is important to material and structural engineering. Unfortunately, a review of the literature regarding porosity in laser powder bed fusion (L-PBF) reveals that different authors report different values, even when using apparently very similar inputs. The purpose of this work is to identify the reasons behind the large, reported variation in L-PBF Ti-6Al-4V porosity and fatigue characteristics. The current study has three foci: using published data to produce a meta-analysis based process mapping and linked experimental study of porosity measurement via XCT, Archimedes method and optical microscopy in L-PBF Ti-6Al-4V manufactured samples, secondly a SIF-based meta-analysis and experimental comparative study of fatigue life to examine the importance of porosity to failure and thirdly the production and testing of a suitable laser micro-drilled calibration check object for XCT porosity analysis. The meta-analysis output indicates that there is little agreement between the different measurement techniques in terms of porosity when applied to identical samples. With respect to manufacturer and meta-analysis based samples, typically, pores that initiated the fatigue failure were located at a depth of less than 175 μm from the sample outer surface and were more likely to be gas produced.

Using the calibration check object, the current study quantifies the comparability of measurements made by XCT data analysts from a range of backgrounds (undergraduate to professional) with varying levels of process automation. Results indicate that inexperienced users have a propensity to miss smaller pore sizes whilst more experienced users are able to detect the full range of pore sizes. There appears to be little agreement in XCT assessments conducted by different researchers in terms of pore count, pore size range and porosity and so the use of porosity as a guide to fatigue life is in doubt and suggests that, in particular, the assessment technique currently has too much subjectivity and variability associated with its application.