Elsevier

Precision Engineering

Volume 67, January 2021, Pages 301-310
Precision Engineering

Anisotropy effect of additively manufactured Ti6Al4V titanium alloy on surface quality after milling

https://doi.org/10.1016/j.precisioneng.2020.10.003Get rights and content

Highlights

  • The effect of the anisotropy of additive manufactured Ti6Al4V on the surface quality was investigated.

  • The skewness parameter can distinguish the machined surface of differently oriented additively manufactured parts.

  • The horizontal orientation allows for better machinability when considering the burrs height and the chip morphology.

Abstract

The microstructural and mechanical anisotropy of metallic additive manufactured parts is well-established as the nature itself of the process induces the formation of columnar grains along the build-up direction. However, the effect of this anisotropy on the alloy machinability is rarely considered and a deeper understanding is needed. In the current study, Ti6Al4V prisms were fabricated by laser powder bed fusion in both vertical and horizontal orientation. After heat treatment, the prisms were slot milled at different cutting parameters to assess the influence of the build-up orientation on the surface quality, focusing on the surface topography. The results show that the skewness parameter can distinguish the machined surface of differently oriented AM parts and that the horizontal orientation allows for better machinability when considering the burrs height and the chip morphology.

Introduction

The machinability of parts fabricated by Additive Manufacturing (AM) technologies has recently aroused the interest of the scientific community working on metal machining. This is attributable to two main reasons: (i) the surface finish and geometrical tolerances of AM parts that can be quite far from those requested for the part in-service performances, and (ii) the profound difference of their microstructure compared with that of conventionally processed alloys with the same chemical composition. At present, the optimization of the AM process parameters [1] can only partially solve the high surface roughness Sa that characterizes the as-built parts [2]: in practice, almost all the AM parts need some postprocessing steps to smooth their functional surfaces [3,4]. These postprocessing operations encounter some issues when applied to AM artifacts, because of the peculiar microstructure of the latter, which, in turn, affects the local mechanical properties and thus their machinability. Due to the thermal phenomena that characterize the AM technology, long columnar grains grow across the deposited layers [[5], [6], [7]]. As a consequence, AM parts are characterized by a high anisotropy, even after heat treatment, which is necessary to stabilize the microstructure and reduce the processing defects [8,9]. The mechanical anisotropy, derived from the microstructural anisotropy, has been studied for various metal alloys, such as stainless steel [10], nickel-chromium-based superalloy [9], pure tantalum [5], aluminum alloy [11]. It has been shown that the presence of the columnar grains strongly affects the local mechanical properties of each metal alloy. However, none of the aforementioned studies gave a detailed explanation to support the correlation between microstructural and local mechanical anisotropy and machinability. Fernandez-Zelaia et al. [12] evaluated the machinability of the Laser Powder Bed Fused (LPBF) CoCrMo alloy, limiting their study to the machining force response. They showed that the cutting force was sensitive to the part mechanical anisotropy driven by both the morphology of the microstructure and crystallographic texture. Hojati et al. [13] investigated the machinability of the Electron Beam Melted (EBM) titanium alloy grade 5. Besides the cutting forces, burrs morphology and surface quality were considered, but the latter was limited to two roughness parameters (namely Sa and Sz), without any insight on the microstructure. Fortunato et al. [4] studied the tool wear, cutting forces, and surface roughness (Sa), when milling an AM maraging steel but with limited microstructural investigation. Therefore, according to the literature, it is still not possible to have a clear insight into the effect of the AM process on machinability for a given alloy. This problem would be overcome if the mechanics of chip formation would be studied in correlation with the microstructural features deriving from the AM process. Therefore, more efforts must be put to investigate how the machinability is influenced by the AM process. In recent works carried out by the Authors, it was found that the columnar grains boundaries of the LPBF Ti6Al4V alloy, known to act as preferential paths of fracture [14,15], helped the material removal during cutting when favorably oriented with respect to the tool cutting edge both in milling and turning operations [16,17]. The machinability of the LPBF Ti6Al4V alloy was assessed in terms of tool life and the effect of the tool wear on the workpiece quality and chip morphology was investigated. To further study the effect of the AM-induced anisotropy on machinability, in the present work the Authors focused on the workpiece quality and chip morphology avoiding the influence of the tool wear. To the best of the Authors’ knowledge, this is the first attempt to show the AM microstructural anisotropy effect on the machined surface quality in strict connection with the microstructural features. In this work, the effect of the microstructural anisotropy on the machinability of Ti6Al4V prisms fabricated via LPBF was investigated in terms of workpiece quality and chip morphology. The LPBF samples were produced along the Build-up Direction (BD), i.e. along the Z-axis and perpendicular to it (XY-plane), and subsequently heat treated. Later on, slot milling operations were conducted at different cutting parameters and the workpiece quality assessed in terms of surface topography and texture, defects on the machined surface, and burrs extent. A wide topographical analysis was conducted based on optical profilometries and correlated to the metallurgical analyses of the samples. The surface quality was also assessed through Scanning Electron Microscope (SEM) analysis.

Section snippets

Sample fabrication through additive manufacturing

The metal alloy under examination is the Ti6Al4V titanium alloy, whose chemical composition is reported in Table 1. This titanium alloy is broadly used in structural applications, principally in the aerospace industry thanks to its unique mechanical and corrosion resistance properties. Besides aerospace, automotive, medical and chemical fields exploit its characteristics. For this study, Ti6Al4V prisms were manufactured via LPBF using the plasma-atomized powder (15–45 μm of size range) produced

Metallurgical and mechanical characteristics

The typical LPBF Ti6Al4V microstructure after heat treatment was obtained (Fig. 4). Long columnar grains, the so-called prior β grains, developed along the BD, with α (in light) and β (in dark) phases mixture within them in lamellar form. Continuous α phase layers (αGB) can be detected at the prior β grains boundaries. The microhardness HV0.1 values of the three surfaces of each prism were comparable. In fact, the average values were 309 ± 11 g/μm2 and 298 ± 13 g/μm2 for the 0deg and 90deg

Conclusions

The surface quality of milled LPBF Ti6Al4V parts was investigated and related to the AM-induced anisotropy. The variables of this study were both cutting parameters (feed per tooth and cutting speed) and build-up orientations of the AM parts. The minimum and maximum of the variables were considered, i.e. the lowest and highest feed per tooth and cutting speed of the optimal ranges provided by the tool supplier, and horizontal (0deg) and vertical (90deg) development of the AM parts. After slot

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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