PSI - Issue 7

Z.H. Jiao et al. / Procedia Structural Integrity 7 (2017) 124–132

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Z.H. Jiao et Al./ Structural Integrity Procedia 00 (2017) 000–000

1. Introduction As a well-known light alloy, Ti6Al4V is characterized by having excellent mechanical properties and corrosion resistance combined with low weight and biocompatibility. This material is ideal for many high-performance engineering applications, for example in aerospace, motor racing and biomedical implants [1]. In aerospace engineering, Ti6Al4V components are traditionally manufactured through intense milling of bulk parts, the hot forming of sheets and assembly welding [2]. These processes are low efficiency, high cost and wasting materials. Additive manufacturing (AM) is the process of joining materials to make objects from three-dimensional (3D) model data in a layer-wise manner. It can achieve rapid manufacturing of novel components and structures with complex geometries. Compared to traditional ‘‘subtractive’’ manufacturing, AM shows greater application value on Ti6Al4V components manufacturing because of its compressing supply chain and reducing material waste [3, 4]. Selective laser melting (SLM) is an AM process in which successive layers of powder are selectively melted by the interaction of a high energy density laser beam. Molten and re-solidified material forms parts, while non-melted powder remains in place to support the structure [5]. This layer-wise production technique offers some advantages over conventional manufacturing techniques such as high geometrical freedom, short design, manufacturing cycle time and made-to-order components. SLM produced Ti6Al4V alloy obtains widespread attention for its potential applications on aero-engine and aircraft parts. In recent years, much research has focused on the mechanical performances of this alloy in order to assess whether its properties are safe and reliable for aerospace applications. Galina Kasperovich et al [6] concentrated on the improvements of tensile properties and high cycle fatigue resistance of SLM produced Ti6Al4V after annealed or hot-isostatically pressed. P. Edwards et al [7] investigated the influence factors of the fatigue properties of SLM produced Ti6Al4V. It was found that the fatigue performance was influenced with a variety of issues, such as microstructure, porosity, surface finish, residual stress and anisotropy. Van Hooreweder et al [8] and Liu et al [9] also studied the fatigue performance based on other aspects such as notch effect and defect effect. V. Cain et al [10] studied the effects of post-build processing as well as specimen orientation on the fracture toughness and fatigue crack growth behavior of SLM produced Ti6Al4V. It showed that post-build processing and specimen orientation had a strong influence on dynamic behavior of this alloy. Leuders et al [11] focused on the fatigue crack growth rates and fatigue resistance of SLM produced Ti6Al4V, and discussed relationship with micro-structural features and residual stresses. Although many results have been published regarding relationship among processes, microstructure and properties, The mechanical properties studies are limited for evaluation of SLM produced Ti6Al4V. Detailed researches on durability and damage tolerance properties are lacked, especially at elevated temperatures that aero-engine concerned. This study is performed to contribute to the understanding of the mechanical behavior of Ti6Al4V alloy produced using SLM process. Tensile and fatigue crack growth (FCG) performances at room and elevated temperatures are studied. The ultimate strength, yield strength, elongation, reduction of area and fatigue crack growth rate (FCGR) are compared between SLM produced Ti6Al4V and conventionally manufactured Ti6Al4V such as casting, forging and bar. Different orientations of SLM produced Ti6Al4V are concerned during tensile and FCGR tests in order to comprehensively evaluate the influence of anisotropy on mechanical properties. Finally, Fracture tomography is investigated to better clarify the crack propagation mechanism at room and elevated temperatures. 2. Materials and methods 2.1. Materials Cylindrical rods ( Φ 13×73 mm) and rectangular blocks (52×50×12 mm) of Ti6Al4V alloy were manufactured by SLM and post heat treatment processes. The position and orientation of the rods and blocks, as well as the reference axis system on the build platform of the BLT-S300 machine are shown in Fig. 1. The powders used in this study, shown in Fig. 2, have a particle diameter between 15μm and 60μm. The chemical co mposition norm and actual values are shown in Table 1. Metallographic images of SLM produced Ti6Al4V alloy are given in Fig. 3. They show a characteristic inherent texture according to X, Y and Z-axis. Texture is similar for the vertical planes, shown in Fig. 3(a) and (b), and differs from the horizontal plane, shown in Fig. 3(c). Morphology of vertical planes is characterized by elongated columnar grains while morphology of horizontal plane is characterized by equiaxed grains. The horizontal plane magnification morphology shown in Fig. 3(d) is characterized by equiaxed prior β -