A Study on Electrochemical Polishing of Additively Manufactured Ti6Al4V Complex Parts and Its Influence on Corrosion Behavior

Abstract

Nowadays, metal additive manufacturing (AM) has become popular for creating complex geometries that are impossible to create with conventional manufacturing methods. However, the surface quality of as-built AM parts is typically not (uniformly) smooth. Among different imperfections leading to irregular surface morphology, we can mention the staircase effect due to the layer-by-layer nature of the deposition techniques, partially fused feedstock material, balling effects, spatter, and inadequate fusion, where each impact on surface finish depends on the deployed AM process method. Typically, the surface quality of an as-built part is not satisfactory for functional biomedical applications. To address this issue, many research studies are focusing on optimizing part production variables, such as feed stock material, design of the part, AM process parameters and condition of fusion deposition and binding. However, in most cases, to meet the final part requirements, post-processing after AM production is necessary. Different post-processing methods such as abrasive polishing, laser polishing, chemical etching, and electrochemical polishing are established over the years. Among others, electrochemical polishing (ECP) is a suitable post-processing method for metal additive manufactured parts, particularly those with complicated geometry and inaccessible surfaces such as lattice structures. Titanium and its alloys are commonly used in biomedical applications because of their fracture toughness, corrosion resistance, and biocompatible properties, as well as their fatigue strength. Therefore, Ti-6Al-4V is selected as AM feedstock material for lattice structure fabrication in this work.The principal objectives of this study are 1) to characterize the surface of the lattice structure’s struts to observe the influence of post-processing on the core of the lattice and ensure achieving a uniform lattice structure; and 2) to study the influence of electrochemical polishing (ECP) on corrosion behavior of AM lattice structures as well as on AM flat surfaces.In a first step (Figure 1a), the best ECP condition, based on a Taguchi method design of experiment (DOE), on a flat additively manufactured titanium surface is established. The applied potential, electrochemical polishing time, and the electrode distance (the distance between working and counter electrode) are considered as the ECP parameters in the DOE, and for each parameter two levels are considered. Therefore, four experiments are designed, and two samples are polished for each experiment. In order to observe the effect of each experiment, a Confocal characterization is performed (Figure 1b), and based on the results, the experiment with the highest improvement in roughness is selected to polish a part with lattice-like structure (Figure 1C).In a second step, it is necessary to characterize the internal struts of the lattice structures to observe the influence of post-processing on the core of the lattice and ensure achieving a uniform lattice structure after ECP. This can be accomplished by cutting the part followed by profilometry or confocal microscopy, which are destructive methods. As a non-destructive method, X-ray computed tomography (XCT) can be used to asses the internal parts of a lattice structure. However, in most literature, it is used as a qualitative tool to evaluate the uniformity of lattice structure after post-processing. In this study, the XCT output are analyzed using a novel method, which could quantitatively measure the surface roughness of internal struts and assess its improvement by ECP, in a non-destructive manner.In spite of the fact that corrosion resistance of lattice structure AM parts is important for many practical applications, few studies specifically report corrosion rates before and after post-processing of AM lattice parts. In this paper the influence of ECP on corrosion behavior of AM lattice structures and AM plane surfaces are studied. To evaluate the impact of ECP on the corrosion behavior of AM parts, linear polarization is conducted to measure both the corrosion potential and corrosion rate (Figure 1d).According to the obtained results, the highest improvement of in surface roughness is obtained in the second experiment (sample P03, P04), which contains the higher level in each parameter (20 min polishing time and 20V applied voltage). By ECP of the Ti-6Al-4V AM surface, we have proven that we can decrease the arithmetic mean surface roughness value - Sa – by 70%. Using the ECP process, we reduced the initial roughness of 12.3 μm to a polished surface of 3.5 μm.Further experiments are being conducted on the ECP of lattice structures and the evaluation of their corrosion behavior. Figure 1