Fatigue properties of titanium alloy custom short stems fabricated by electron beam melting

doi:10.1016/j.jmst.2020.02.047

Abstract

Abstract:

The fatigue properties of titanium alloy short-stems with four different lengths, manufactured by electron beam melting (EBM) technology, were investigated by in vitro test and finite element (FE) analysis. FE simulation results indicate that the maximum tensile stress concentrates at the lateral side of the stem body. The magnitude of the concentrated tensile stress increases and the corresponding area of the axial section decreases with increasing of stem length. Results from fatigue tests demonstrate that fatigue cracks mainly initiate from the rough surface of the stem where the maximum tensile stress concentrates. The fatigue strength decreases with the increase of stem length, which is attributed to the higher stress concentration on the longer stem surface. In addition, it is found that post EBM treatment via hot isostatic processing (HIP) is able to enhance the fatigue properties of the stems, since the pores generated during EBM are mostly closed during HIP. Our work also demonstrates that the stress concentration on the stem surface can be effectively mitigated and the corresponding fatigue properties of the EBM-fabricated titanium alloy short stem can be considerably improved by optimizing the design in the stem length.

Key words: Custom short stem, Titanium alloy, Electron beam melting, Fatigue property

Liao Wang, Shujun Li, Mengning Yan, Yubo Cheng, Wentao Hou, Yiping Wang, Songtao Ai, Rui Yang, Kerong Dai. Fatigue properties of titanium alloy custom short stems fabricated by electron beam melting[J]. J. Mater. Sci. Technol., 2020, 52: 180-188.

Figures/Tables 17

Fig. 1. (a) Maximize the fit and fill of proximal femoral canal in three specific axial planes for the design of the custom short stem. P1: axial section at 20 mm above the lesser tranchanter. P2: axial section at the middle of the lesser tranchanter. P3: axial section at 10 mm below the lesser tranchanter. (b) 3D models of the custom short stems with four different lengths (20-80 mm). Custom short stems manufacturing via electron beam melting technology, including the stem body (B) and surface porous coating (P).

Fig. 2. SEM image of Ti-6Al-4 V powder used in this study.

Table 1 Chemical composition of Ti-6Al-4 V alloy powder (wt%).

Ti	Al	V	C	Fe	O	N	H
Bal.	6.42	3.94	0.01	0.18	0.13	0.01	0.0016

Fig. 3. Experimental setup of the short stems for fatigue test according to ISO 7206-4 standard.

Fig. 4. The stem position, mesh and boundary conditions of the three dimensional FE model. For simulating the ISO7206-4 standard, the models of the stem were oriented in adduction of 10° and flexion of 9°. A maximum vertical force of 2300 N was applied on the femoral head, and all the nodes of the external surface of the cement were fixed.

Fig. 5. Optical images showing the microstructures of the EBM Ti-6Al-4V short stems: (a) as-fabricated; (b) after HIP.

Fig. 6. SEM images showing the microstructures of the EBM Ti-6Al-4V short stems: (a) as-fabricated; (b) after HIP.

Fig. 7. XRD patterns of the EBM Ti-6Al-4V short stem.

Fig. 8. SEM images of (a) the trabecular structure surface and (b) the cross section of the porous coating on the studied short stem.

Fig. 9. (a) Test regions for the studied short stems (circled by red rectangular dash line). Micro-CT results of (b) HIPed and (c) as-fabricated (AF) samples fabricated by EBM. (d) Distribution of pore diameter in the as-fabricated sample.

Table 2 Tensile properties of the EBM Ti-6Al-4V samples before and after the HIP.

Material	UTS (MPa)	YS (MPa)	EL (%)	RA (%)
As-fabricated	1037 ± 15	951 ± 15	14.5 ± 1	43 ± 4
HIP	981 ± 10	892 ± 10	15.5 ± 1	52 ± 3

Table 3 Fatigue properties of the studied custom short stems (cycles).

Model	20	40	60	80
As-fabricated	5,000,000	5,000,000	1,585,153	393,983
HIP	5,000,000	5,000,000	2,113,568	2,347,178

Fig. 10. Images showing the fracture position of the studied short stem.

Table 4 The height of fatigue fracture position and its corresponding sectional area of the studied custom short stems.

Model	20		40		60		80
Model	Height (mm)	Area (mm²)	Height (mm)	Area (mm²)	Height (mm)	Area (mm²)	Height (mm)	Area (mm²)
As-fabricated	Unfailure		Unfailure		48		70
HIP	Unfailure		Unfailure		46		65
Maximum tensile stress	63.7	177.9	33.1	141.8	48.6	134.9	68.7	123.7

Fig. 11. SEM images of fracture surface of the studied model 60/60HIP, 80/80HIP stem after high-cycle fatigue, with different notations representing fracture characteristics. (I) Fatigue crack initiation region. (II) Fatigue propagation region. (III) Fast fracture region. (a) Crack initiation site. (b) Secondary crack. (c) Dimple.

Fig. 12. Maximum tensile stress distributions obtained by FE simulation in the studied short stems with varying stem lengths under the same loading condition.

Fig. 13. Morphologies and areas of the fracture cross section at the site with the maximum tensile stress of the studied short stems with varying stem lengths simulated by FE method.

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