Fatigue performance improvement of porous titanium: topological optimization design and selected area laser melting forming

Contributors: Gao Yunpeng, Lu Zhongliang
Contributed by: State Key Laboratory of Mechanical Manufacturing System Engineering　

The research team of Shenyang Institute of Metals, Chinese Academy of Sciences has made porous CP-Ti (pure titanium) with topologically optimized structure through selective laser melting (SLM) forming technology, which has extremely excellent fatigue performance at 106 cycles It has a normalized fatigue life of about 0.65.

They believe that there are three main reasons for the excellent fatigue performance of porous CP-Ti formed by SLM:

First, the porous CP-Ti structure designed by topology optimization can distribute stress evenly and reduce stress concentration. They compared the porous CP-Ti with topologically optimized structure to the rhombic dodecahedron structure, as shown in Figure 1. At a strain of 1%, the maximum tensile stress of the topology-optimized structure is about 1/2 of the diamond dodecahedral structure, and is distributed along the horizontal brace, and the stress concentration is relatively reduced;

Secondly, after fatigue testing, twins and dislocations appear in the porous CP-Ti with topologically optimized structure (Figure 1(b) and (c)). The twins make the porous CP-Ti show higher strain hardening and plasticity. Strain hardening reduces the increment of plastic strain in each cycle, and higher plasticity can make porous CP-Ti accumulate more plastic strain and delay the initiation of fatigue cracks;

(A) TEM bright field image; (b) bright field image of twin slices; (c) electron diffraction pattern of the corresponding twin region; (d) atomic scale image of twin interface
Finally, the fatigue crack propagation path in the porous CP-Ti sample with topological optimization design is in a zigzag shape instead of a straight line (Figure 3). The deflection and bifurcation of fatigue cracks reduce the propagation speed of the cracks, resulting in a low fatigue crack growth rate, thereby extending the fatigue life.

references:
Liu Y J, Ren D C, Li S J, et al. Enhanced fatigue characteristics of a topology-optimized porous titanium structure produced by selective laser melting[J]. Additive Manufacturing, 2020: 101060.