Selective laser melting (SLM), as an additive manufacturing technology method, focuses on direct transformation and fabrication from a three-dimensional computer-aided design (3D–CAD) model to fully functional ready-to-use metal components [
1]. During the SLM process, the metal powder is uniformly spread on the building platform and selectively melted by a high-energy laser beam. After scanning the cross-section of a layer, the platform is lowered by a layer’s thickness, and a new layer is prepared and scanned. This printing process is repeated until the components are completed. With this layer-wise manufacturing approach, the complicated geometries in a component are split into simplified two-dimensional slices. Therefore, SLM provides a promising approach to effectively and efficiently manufacturing innovative products.
Titanium alloys such as Ti–6Al–4V have high specific strength, superior thermal stability, and strong corrosion resistance, and therefore have been widely applied in both aerospace and biomedical industries [
2,
3]. However, the machinability of titanium alloys is a difficult problem to address based on traditional processing technologies. For instance, when cutting Ti–6Al–4V materials, the traditional manufacturing technologies may cause a few critical problems, such as sticking phenomenon, blade wear-out, and material wastage. Since SLM is the process of joining metal powders, it shows great advantages in fabricating titanium alloy parts while being able to avoid those problems. However, compared to traditional manufacturing technologies, the distinct manufacturing process makes SLM form different microstructures on Ti–6Al–4V parts, and hence different part performance. Therefore, the performance of SLM Ti–6Al–4V components under various monotonous or cyclic loading conditions becomes a critical issue to be investigated. In the literature, this issue has been studied intensively by academic scholars. Some authors in their work [
4,
5,
6] identified that SLM Ti–6Al–4V parts usually consisted of acicular α’ martensite instead of equilibrium α and β phases exhibited in the wrought material and, meanwhile, had higher yield stress, higher ultimate tensile strength, and lower ductility than the wrought material, whereas the residual stresses introduced to the parts could still be effectively eliminated by using traditional post heat treatment (2 h at 800 °C) [
7,
8]. Some studies investigated the high cycle fatigue (HCF) properties of SLM Ti–6Al–4V material under the elastic deformation scenario incurred by low amplitude cyclic stress. Edwards & Ramulu [
9] conducted the HCF tests for SLM Ti–6Al–4V with a load ratio of R = 0.2 at different constant maximum stress levels, from 100 to 600 Mpa. Based on the test results, the authors generated approximate S–N curves for specimens with the different surface condition and build orientation and concluded that the curves for SLM Ti–6Al–4V were significantly lower than that for the wrought material. Xu et al. [
10] selected proper SLM processing parameters so that the fatigue life of SLM Ti–6Al–4V samples was able to approach that of the wrought material. Wycisk et al. [
11,
12] tested the HCF properties for heat-treated (3 h at 600 °C) SLM Ti–6Al–4V samples with different surface conditions (as-built, machined, and shot-peened) and found that the application of machining surface treatment was able to enhance HCF performance to the level of wrought Ti–6Al–4V. Rafi et al. [
13] evaluated the SLM Ti–6Al–4V HCF data at a load ratio of R = 0.1, in line with the Metallic Materials Properties Development and Standardization (MMPDS). The results showed that the Ti–6Al–4V specimens fabricated by SLM had a better fatigue performance than the cast. Leuders et al. [
14] presented the S–N curves for SLM Ti–6Al–4V in the as-built condition and in different heat treatment conditions. They found that the HCF performance could be optimized through the hot–isostatic–pressing (HIP) treatment with a pressure of 1000 bar and temperature of 920 °C. Kasperovich and Hausmann [
15] developed a two-step approach to increasing the HCF resistance of SLM Ti–6Al–4V: firstly, minimizing the inherent defects of SLM Ti–6Al–4V through the optimization of SLM processing parameter, and secondly, adjusting the microstructure of SLM Ti–6Al–4V by appropriate thermal treatment. As a result, the fatigue resistance of SLM Ti–6Al–4V could be comparable with the wrought material. The studies reviewed above investigated the HCF properties for SLM Ti–6Al–4V at low-amplitude cyclic stress level. A few authors [
16,
17,
18] investigated the cyclic elastoplasticity of SLM Ti–6Al–4V during high strain amplitude cycling. From their work, SLM Ti–6Al–4V with α′ martensite presented a much narrower hysteresis loop than that of the wrought bimodal Ti–6Al–4V. However, there are few papers so far which explicitly investigate and evaluate the low cycle fatigue (LCF) performance parameters for SLM Ti–6Al–4V, whereas such research is very important and beneficial to engineering design and operation in practice.
This paper investigated the LCF performance of SLM Ti–6Al–4V materials, in which a set of monotonic tensile tests and strain-controlled LCF tests were performed on both as-built and annealed SLM Ti–6Al–4V samples. The cyclic softening/hardening properties of the SLM Ti–6Al–4V were characterized by stress–strain hysteresis loops and the progression curves of the stress amplitude with cycles. The cyclic stress–strain and strain–life curves of SLM Ti–6Al–4V were fitted according to the Ramberg–Osgood and Basquin–Coffin–Manson models, respectively. In addition, the LCF failure mechanism was analyzed using scanning electron microscopy (SEM). The results were compared with those of the wrought Ti–6Al–4V specimens.