Influence of porosity on fatigue of additive manufactured titanium alloy Ti-6Al-4V

Abstract

Process-induced defects have been identified as one of the principal failure sources in metal additive manufacturing (AM) under cyclic loading; yet, understanding how they impact fatigue behaviour, such as S-N curves, fatigue crack growth rates remains open. In this work, the high-cycle fatigue behaviour under constant amplitude, axial loading was studied for an AM titanium alloy Ti-6Al-4V that fails from sub-mm size, porosity type defects. More specifically, the dispersion of fatigue life due to porosity and the propagation behaviour of cracks initiated from pores were investigated.
Two different AM processes, namely wire+arc additive manufacturing and laser powder-bed fusion, were used to manufacture titanium alloy Ti-6Al-4V used in this work. A trade-off was made by working with process-induced defects, e.g. defect morphologies were representative of real case scenarios, but the dimensions of defects were uncontrolled. A targeted experimental programme was developed to concentrate on the influence of defects by circumventing other parameters that might impact the fatigue life such as the surface roughness, build direction and more. Size and spatial distribution of process-induced defects were measured using a laboratory scale X-ray Computed Tomography prior to fatigue testing. Load-controlled fatigue testing was conducted by repeating tests at the selected stress levels in order to obtain statistically significant data. Furthermore, fatigue crack growth rates were measured for cracks initiating directly from surface pores using the replica technique. After each fatigue test, the crack initiating pore size was quantified by analysing the fracture surface using a scanning electron microscope. The experimental results were analysed both from a linear elastic fracture mechanics perspective and a total life perspective using the local elastic stresses in the vicinity of a pore. Key findings are summarised below.
A popular approach to evaluate defect criticality is assuming a volumetric defect as a planar crack by projecting its area to the plane perpendicular to the applied load direction. This planar crack is then assessed using a Kitagawa-Takahashi diagram or similar since cracks at the sub-mm scale could grow below the threshold value of stress intensity factor range (∆K_th). It was found that the analogy of assuming pores as cracks was lost after a certain transition size and fatigue life reduction saturated despite the increase of crack initiating pore diameter roughly by a factor of four.
Total fatigue life of AM Ti-6Al-4V could vary up to three orders of magnitude for nominally similar test specimens, as seen in the exploratory literature studies. Defects are considered as one of the causes of this scatter and in this work, defect location, e.g. embeddedor surface, was found to be the dominant factor rather than the defect size. In fact, within a same defect category, such as the surface pores, the scatter of fatigue life due to the variation in crack initiating defect size was less than a conventional manufactured Ti-6Al-4V, where the scatter assumed to be related to the distribution of unfavourably oriented surface grains.
Individual fatigue life stages were also investigated by following surface cracks initiated from pores using the replica technique. First detected cracks were less than 50 µm length and occupied roughly 50% of the total fatigue life. This suggests that the crack initiation stage was significant and it should be accounted in fatigue life prediction approaches, noting that the measurements were limited to surface observations. During the crack propagation stage, small cracks could grow faster than the long crack growth measurements at the same nominal ∆K value, i.e. the so-called small crack behaviour. Such a behaviour was observed near the threshold region, however, it was less significant compared to conventional manufactured counterparts. Finally, a similitude to long crack growth rates was achieved,when the measured crack size added to the crack initiating pore size.

Date of Award	Mar 2023
Original language	English
Awarding Institution	Coventry University
Sponsors	The Welding Institute (TWI)
Supervisor	Xiang Zhang (Supervisor), Matthew Dore (Supervisor) & Yanhui Zhang (Supervisor)