Influence of layer thickness and post-process treatments on the fatigue properties of CoCr scaffolds produced by laser powder bed fusion

Abstract

Over the last years, additive manufacturing (AM) techniques such as laser powder bed fusion (L-PBF) have been frequently adopted for efficiently producing biomedical implants. L-PBF offers the advantage of low material waste and high accuracy enabling the production of complex and highly personalized geometries. However, when manufacturing time is considered, the L-PBF production rate is relatively low compared to conventional production techniques. The aim of this paper is to present the impact of layer thickness on static and fatigue properties of CoCr scaffolds produced by means of L-PBF. An increased layer thickness (from 30µm to 60µm) leads to an improvement in terms of production rate of 40 to 50% without affecting the final geometry of the structure. A fatigue test campaign was conducted on both 30µm and 60µm layer thickness samples in "as-built" condition. The analysis of the test results with a local stress method highlighted no significant differences in terms of fatigue performances. In addition, the effect of post-process treatments, such as hot isostatic pressing (HIP) and chemical etching on static and fatigue properties were investigated. It is shown that HIP does not affect the fatigue properties of the scaffolds whilst chemical etching is capable of improving fatigue resistance when the local stress approach is considered.

Mechanical characterization and properties of laser-based powder bed–fused lattice structures: a review

“…On the other hand, Cutolo et al [100] reported an increase of the fatigue performances of chemically etched CoCr samples on a local scale (Fig. 8), while the HIP treatment seemed to be ineffective.…”

“…Heat treatment and chemical etching have been studied to reduce these issues. Many scientists [98][99][100][101] found that a heat treatment reduces the strength of the samples and increases the ductility. On the other hand, Van Hooreweder et al [102] reported that chemical etching has no influence on mechanical properties while, in another paper, Van Hooreweder et al [103] found that the different densities of the samples have a strong influence on the mechanical properties making the study of the heat treatment and chemical etching effects hard.…”

Additively manufactured porous metallic biomaterials

“…Among bioinert metals, the titanium alloy Ti-6Al-4V, 109,115,153,158,170,178,179 Co-Cr alloys, 180,181 pure titanium, 182,183 stainless steel, 174,184 NiTi alloys, [185][186][187][188] and tantalum 189 are the most important biomaterials used for fabrication of AM porous structures. Ti-6Al-4V is probably the most extensively studied material given its high mechanical properties, good biocompatibility, relatively low price, and widely available experience regarding its processing with AM.…”

Frontiers of Additively Manufactured Metallic Materials

“…Once more, the best design of these post-AM processes and their applicability to different materials are not yet well understood. As a consequence, HIP treatments are not always successful in improving the mechanical behaviour of AM metals [ 50 , 51 ]. Further research is therefore needed to clarify what the best post-treatments are for every type of material and geometrical design.…”

“…For more complex parts, it is either expensive or infeasible to perform machining or polishing in which case (electro) chemical surface treatments could be helpful in smoothening the surface of AM parts. It has been shown that a treatment improving the surface of AM metals may significantly improve their fatigue performance [ 50 ]. However, further investigation is called for to determine what the most effective types of treatments are and to integrate those post-AM processes into the production line of AM metals.…”