Micro-Macro Relationship between Microstructure, Porosity, Mechanical Properties, and Build Mode Parameters of a Selective-Electron-Beam-Melted Ti-6Al-4V Alloy

Abstract

The performance of two selective electron beam melting operation modes, namely the manual mode and the automatic 'build theme mode', have been investigated for the case of a Ti-6Al-4V alloy (45-105 µm average particle size of the powder) in terms of porosity, microstructure, and mechanical properties. The two operation modes produced notable differences in terms of build quality (porosity), microstructure, and properties over the sample thickness. The number and the average size of the pores were measured using a light microscope over the entire build height. A density measurement provided a quantitative index of the global porosity throughout the builds. The selective-electron-beam-melted microstructure was mainly composed of a columnar prior β-grain structure, delineated by α-phase boundaries, oriented along the build direction. A nearly equilibrium α + β mixture structure, formed from the original β-phase, arranged inside the prior β-grains as an α-colony or α-basket weave pattern, whereas the β-phase enveloped α-lamellae. The microstructure was finer with increasing distance from the build plate regardless of the selected build mode. Optical measurements of the α-plate width showed that it varied as the distance from the build plate varied. This microstructure parameter was correlated at the sample core with the mechanical properties measured by means of a macro-instrumented indentation test, thereby confirming Hall-Petch law behavior for strength at a local scale for the various process conditions. The tensile properties, while attesting to the mechanical performance of the builds over a macro scale, also validated the indentation property measurement at the core of the samples. Thus, a direct correlation between the process parameters, microstructure, porosity, and mechanical properties was established at the micro and macro scales. The macro-instrumented indentation test has emerged as a reliable, easy, quick, and yet non-destructive alternate means to the tensile test to measure tensile-like properties of selective-electron-beam-melted specimens. Furthermore, the macro-instrumented indentation test can be used effectively in additive manufacturing for a rapid setting up of the process, that is, by controlling the microscopic scale properties of the samples, or to quantitatively determine a product quality index of the final builds, by taking advantage of its intrinsic relationship with the tensile properties.The selective electron beam melting process (SEBM) is a tool-free additive manufacturing technology that enables the fabrication of complex, nearly net-shaped 3D products of dense, micro-architectured structures, cellular structures, and open cell foam structures in which an electron beam is computer-controlled to selectively melt the powder feedstock and rise the build layer-by-layer. When SEBM is applied to a Ti-6Al-4V alloy, a wide range of outstanding properties (e.g., excellent biocompatibility, specific strength, stress corrosion cracking resistance, good ballistic resistance, and thermal stabil...

Additive manufacturing of WC-Co hardmetals: a review

“…These internal, contouradapted cooling channels allow higher cutting speeds and, consequently, a remarkable increase in the productivity of the machining process. The main additive manufacturing technologies suitable for metals are selective laser melting (SLM), selective electron beam melting (SEBM), laser powder deposition, binder jet additive manufacturing (BJAM), and wire arc additive manufacturing (WAAM) [39][40][41][42][43][44][45][46][47][48][49]. So far, AM technologies have been successfully applied to stainless steels [50][51][52][53][54], Ni alloys [55,56], Ti alloys [57,58], refractory metals [59,60], Al alloys [61,62], etc.…”

Tensile and Creep Properties Improvement of Ti-6Al-4V Alloy Specimens Produced by Electron Beam Powder Bed Fusion Additive Manufacturing

“…Depending on the process conditions and the product size, EB-PBF gives rise to such a thermal distribution in melting phase which, combined with fast directional solidification, at high cooling rate from temperatures higher than the β-transus, results in fine lamellar microstructures with "basket weave" morphology or less usually in a martensitic acicular morphology. Furthermore, EB-PBF process parameters are determining not only for bulk microstructural features, but for surface features also, in both cases with consequent effects on mechanical properties [20,21]. Surface conditions are affected by powder grain size also: in general, it gives rise to considerable surface roughness that is appreciated in surgery implants supporting osseo-integration but reduces strength and ductility.…”

Processing–Structure–Property Relationships in Metals

“…The book gathers manuscripts from academic and industrial researchers with stimulating new ideas and original results. It consists of one review paper regarding state of art and perspectives of alloys for aeronautic applications [1] and fifteen research papers [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] focused on different materials and processes.…”

“…Finally, the papers by Tocci et al [15] and Maizza et al [16] deal with additive manufacturing (AM), an innovative technology for the production of parts and prototypes based on layer-by-layer build-up that allows an unrivaled design freedom, not reachable via conventional manufacturing routes combined with high quality and outstanding mechanical properties. The examined samples were produced by means of different techniques: direct metal laser sintering (DMLS) of Scalmalloy powder [15] and selective electron beam melting (SEBM) of Ti-6Al-4V alloy [16]. Both papers highlight the specific microstrucural features of the materials related to the parameters of the production process and the consequences on mechanical performances.…”

“…Both papers highlight the specific microstrucural features of the materials related to the parameters of the production process and the consequences on mechanical performances. Moreover, Maizza et al [16] give quite an original contribution to the benchmark of AM products by applying the macroinstrumented indentation test for the nondestructive determination of local tensile-like properties.…”