Criticality of porosity defects on the fatigue performance of wire + arc additive manufactured titanium alloy

Abstract

This study was aimed at investigating the effect of internal porosity on the fatigue strength of wire + arc additive manufactured titanium alloy (WAAM Ti-6Al-4V). Unlike similar titanium alloys built by the powder bed fusion processes, WAAM Ti-6Al-4V seldom contains gas pores. However, feedstock may get contaminated that may cause pores of considerable size in the built materials. Two types of specimens were tested: (1) control group without porosity referred to as reference specimens; (2) designed porosity group using contaminated wires to build the specimen gauge section, referred to as porosity specimens. Test results have shown that static strength of the two groups was comparable, but the elongation in porosity group was reduced by 60% and its fatigue strength was 33% lower than the control group. The stress intensity factor range of the crack initiating pore calculated by Murakami's approach has provided good correlation with the fatigue life. The kink point on the data fitting curve corresponds well with the threshold value of the stress intensity factor range found in the literature. For predicting the fatigue limit, a modified Kitagawa-Takahashi diagram was proposed consisting of three regions depending on porosity size. Critical pore diameter was found to be about 100 micrometres.

CMT-Based Wire Arc Additive Manufacturing Using 316L Stainless Steel: Effect of Heat Accumulation on the Multi-Layer Deposits

“…Wire arc additive manufacturing (WAAM), which is a DED method, is a manufacturing process that involves the melting of the supplied wire using an arc as the heat source, and is a droplet-based AM technique with a fast deposition rate. Compared to powder-based methods, it causes fewer defects, such as pores, enabling the realisation of high-quality products [3,4]. Moreover, most of the heat-source equipment used in WAAM are cheaper compared to laser or electron beam equipment because it uses a conventional commercial welding machine; in addition, the cost of the metal wire used as feedstock is approximately 10% that of the powder [5].…”

Relationship between internal defect size and fatigue limit in selective laser melted Inconel 718

“…The effects of the internal defect size on the mechanical properties of aluminum and titanium alloys have been investigated (Romano et al, 2018) (Wycisk et al, 2014) (Masuo et al, 2018) (Biswal et al, 2019) (Beretta and Romano, 2016). However, for Inconel 718, the relationship between the internal defect size and fatigue limit is still unclear.…”

Fatigue behaviour of additively manufactured Ti-6Al-4V alloy: The role of defects on scatter and statistical size effect

“…The same observations were made in the work of Chastand et al [3] in which the authors observed several defect types such as surface defects, un-melted zones and small defects. In order to model the effect of these defects on the fatigue strength, linear elastic fracture mechanics based approaches have been widely used [5][6][7] and show globally satisfactory predictions. However, very little work clearly characterises the fatigue behaviour related to each defect type at the origin of the crack initiation sites.…”