Fatigue behaviour and equivalent diameter of single Ti-6Al-4V struts fabricated by Electron Bean Melting orientated to porous lattice structures

Highlights

• Build orientation plays and important role in the mechanical properties of single struts made of Ti-6Al-4V.

• A new concept in lattice structures (equivalent diameter) has been introduced in this paper.

• Roughness values were more homogeneous in the 90° fabricated struts than in the 45° ones.

• The fatigue behaviour of oblique struts (both 1 and 0.6 mm diameter) was better than that of vertical struts.

• Fatigue strength for 10⁶ cycles was obtained between 15% and 25% of the flexural strength for each typology.

Abstract

Two types of Ti-6Al-4V single struts (1 and 0.6 mm CAD diameter) with two different build inclinations with respect to the horizontal plane: 90° (vertical orientation) and 45° (oblique orientation), were manufactured using electron beam melting (EBM). Quasi-static and fatigue three-point bending tests were performed for all typologies. In addition, each type of strut was scanned and reconstructed in 3D to assess their roughness, inner and outer diameters as well as to create finite element models. The range values of roughness obtained were more homogeneous in the 90° fabricated struts than in the 45° ones, with peak values of roughness at the 270° area in the 45° fabricated struts. The equivalent diameter values obtained from the quasi-static three-point bending tests were in line with the inner diameters measured from the microCT images. With regard to the finite element analysis, the use of cylindrical models with equivalent diameters in lattice structures could result in a good approximation for these studies. Mechanical properties obtained from static and fatigue tests of oblique struts (both 1 mm and 0.6 mm CAD diameter) were better than those of vertical struts. Therefore, the build orientation plays an important role in the mechanical properties of single struts.

Introduction

Additive manufacturing (AM) allows for direct fabrication of functional parts with complex shapes from digital models. Titanium alloy Ti-6Al-4V is the most widely used material in AM, especially, for the manufacturing of implants in orthopaedic surgery [1,2]. Besides its optimal mechanical properties, Ti-6Al-4V shows a good biocompatibility as well as suitable osseintegration properties [3]. For this alloy, the most commonly used techniques are selective laser melting (SLM) and electron beam melting (EBM). In both techniques, the metal powder is deposed and melted layer by layer.

With EBM technique it is possible to fabricate interconnected porous biomaterial with predictable and predetermined unit cells. By using different typologies and geometry parameters, it is possible to modify the variables that contribute to the success of porous implants, such as density, porosity, porous size, stiffness, strength, etc. The type of porous structure and the orientation of the struts with respect to the load direction play important roles in the mechanical properties [[4], [5], [6]]. In porous lattice structures, when the load direction is aligned with the struts, a buckling mechanism predominates, and the structures present better mechanical properties than when the load direction is oblique with respect to the struts, in which, bending mechanism predominates [5,7].

On the other hand, EBM build orientation could influence the microstructure and the mechanical properties of titanium alloy. Bruno et al. [8] studied the effect of EBM build orientation and one of their conclusions was that horizontally oriented parts exhibited similar strength to vertically oriented parts. However, the first ones showed rather lower ductility and toughness than the second ones. Formanoir et al. [9] evaluated the microstructure, texture and mechanical behaviour of Ti-6Al-4V specimens fabricated by EBM. The area of the section of the specimens was 12 mm². Results revealed that vertically built specimens exhibited lower yield strength than those built horizontally, and that the mechanical polishing induced an increase in ductility and yield strength. Zhao et al. [10] obtained different results. They found that EBM Ti-6Al-4V samples built vertically possessed higher yield strength, ultimate strength and ductility than those built horizontally. In this case, the shape of the specimens was cylindrical. In addition, they studied the fatigue mechanical properties of vertically built samples with a diameter of 7 mm, and found that the fatigue strength was lower than that of cast and annealed material. However, the samples improved after hot isostatic pressing (HIP) treatment. The same conclusion was reported by Hrabe et al. [11]. In the same way, Edwards et al. [12] concluded that fatigue behaviour is worse on titanium alloy parts obtained by EBM than that on wrought material due to defects such as porosity and surface roughness.

In most of the abovementioned works, the area of the section of the samples was over 10 mm². Nonetheless, in most of the studies on porous titanium structures obtained by EBM, and oriented to bone scaffolds and orthopaedic implants, the strut diameters are between 0.2 and 1 mm, regardless of their type [3,13]. Those small diameters are necessary to obtain lower stiffness and, accordingly, to mitigate the stress shielding effect that may appear in the location of the implant insertion [14]. With these dimensions, defects such as surface roughness and internal pores and voids [11], could have a stronger effect on the material's mechanical properties, since the ratio between the strut diameter and the defect size would be smaller. There are only a few studies reporting on the strut behaviour of porous lattice structures separately. Weibmann et al. [15] studied the mechanical properties (uniaxial compression and hardness measurements) of struts with different build orientations (0° and 45°, with respect to vertical axis). The samples for uniaxial compression tests consisted of a base area, 4 struts and a top area. They were obtained by SLM and EBM with different strut diameters (between 1.1 and 1.7 mm). They found that SLM parts had significantly lower roughness than those obtained by EBM, and highlighted the dependence of mechanical properties as a function of build orientation. Suard et al. [16] characterized, using X-ray tomography, single struts obtained by EBM, with a diameter of 1 mm and different build orientations (0°, 45° and 90°, with respect to a horizontal axis). They introduced the concept of mechanical equivalent diameter of single struts for the stiffness prediction of lattice structures.

In the present study, the main goal was to investigate the shape roughness and mechanical properties of single struts of small dimensions (as they usually appear in porous titanium structures oriented to orthopaedic implants), fabricated by EBM. Quasi-static and fatigue three-point bending tests of struts with different diameters and build orientations were carried out. Further, 3D reconstruction of morphology of single struts, using high-resolution X-ray tomography, was made. Finally, some finite element (FE) models were built from tomography images to analyse and validate the mechanical behaviour of single struts.