Elsevier

Additive Manufacturing

Volume 21, May 2018, Pages 544-555
Additive Manufacturing

Comparison of virgin Ti-6Al-4V powders for additive manufacturing

https://doi.org/10.1016/j.addma.2018.02.011Get rights and content

Abstract

Ti-6Al-4V powders from six different vendors were compared with respect to their microstructures, size-distributions, chemistries, surface appearances, flow behavior, and packing densities. The analysis approaches followed closely ASTM F3049, the standard guide for characterization of additive manufacturing metal powders. Chemistries, including impurity content, agreed well with the standard requirements. Powder particle microstructures revealed acicular alpha prime for all vendors. Measurable differences were observed primarily in the size-distributions and the flow behavior.

Introduction

Additive manufacturers were initially constrained in the availability of powders and powder sources, but the extraordinary popularity of additive manufacturing has led many material producers to develop powder business units. The increasing choice of powder sources raises questions about the differences between the powders from the different sources. Related but different questions are what set of powder properties need to be characterized and how variations in those powder properties ultimately affect additively manufactured part properties. The ASTM committee F42 has published a standard on characterizing properties of metal powders for additive manufacturing, F3049 [1]. According to this standard, the major categories for powder characterization include the size distribution, the morphology of the powders, the chemical composition, the flow properties, and measures of the powder bed density. For each of these categories, individual ASTM standards apply that specify measurement details. The primary aim of the present work is to compare the properties of commercially available Ti-6Al-4V powders from different sources according to the major categories in the ASTM standard F3049. Ti-6Al-4V is one of the most popular alloys for metal additive manufacturing. Since powder samples are generally available in different size ranges, for any comparison to be valid then the size ranges should be equal for powders from the different sources. Moreover, for the comparison to be useful then the size range should match that used in current additive manufacturing practices. For Ti-6Al-4V powders, common size ranges are −140/+325 mesh (45–105 μm), −120/+270 mesh (53–125 μm), or −120/+325 mesh (45–125 μm). These size ranges are typically used for electron-beam powder bed processing while laser-based powder bed additive manufacturing uses smaller powder particle size distributions, typically between about 20 μm and 60 μm. The same size distributions can also be used for additive manufacturing deposition techniques. Depending on the intended surface roughness and aptitude to adjust beam settings to size distributions, the distinction between electron beam- and laser-based powder bed additive manufacturing requirements for powder size distributions might not be very strict. Different analysis techniques are available for some of the powder characteristics, and the current work compares in particular the size distributions of powder batches measured with electron microscopy and with optical diagnosis.

The results from this work represent one part of the overall effort to understand variations in the properties of additively manufactured parts. Clearly, differences in powder characteristics from vendor to vendor might have a significant impact upon these properties. While differences and similarities between the different powders are characterized in the current work, additional work will be necessary to understand how these differences in powder characteristics affect property variations in the additively manufactured components. Thus, the current work does not seek to identify which powder sources might yield the best part qualities. Instead, the aim of this paper is to raise awareness in the additive manufacturing community of the type and extent of differences between ostensibly similar Ti-6Al-4V feedstock powders. It is also recognized that individual vendors can deliver a broad range of size-distributions and different purity levels and even custom-pedigreed powders. The current work merely compares powders with similar pedigree and suitability for powder-bed additive manufacturing.

Section snippets

Materials and methods

Ti-6Al-4V powders were acquired for this study from AP&C, Hoeganaes, Puris, TIMET, ATI, and Praxair. The powders were delivered and labeled as such in grade 5 condition (Puris, ATI, TIMET), grade 23 (ELI, AP&C, Praxair), and grade C (Hoeganaes), which refers to (outdated) MIL-T-9046F or H and is equivalent to grade 5. The TIMET powder was produced with TIMET’s plasma rotating electrode process (PREP™) while the other powders are believed to have been produced with variations of the gas

Comparison of size-distributions

The size distributions for the different powders are shown in Fig. 1. In each case the data are shown from the FEI Aspex Explorer automated SEM and from the Retsch Camsizer XT. For the Camsizer analysis, an equivalent diameter was used, xarea. This diameter is obtained from the projected particle area, A as:A=π(xarea/2)2

The volumetric size distribution, q3(xarea) denotes the volume fraction of particles with diameters equal or less to xarea. In Fig. 1 the derivative is used, q3(xarea) of the

Powder chemistry

Much emphasis is currently placed on the “powder pedigree” of additive manufacturing powders. The powder chemistries, including alloying and impurity content, are specified with different standards and as a main requirement powders have to comply with these standard chemistries. But besides the compliance with existing standards, a more fundamental question is how the chemistry and in particular the impurity content affects properties of the titanium alloy. Ti-6Al-4V or grade 5 titanium powder

Summary

Ti-6Al-4V powders from six different commercial vendors (TIMET, Puris, ATI, Hoeganaes, Praxair, and AP&C) were compared with respect to their chemistries, size distributions, surface appearance, powder flow, apparent densities and tap densities. The alloy chemistries, including impurity content, comply with the ASTM standard for additive manufacturing powders, ASTM F2924. Differences are observed in the distributions of powder particle sizes within the comparable limits; there is an additional

Acknowledgments

This work was supported by a research grant from Thermo Fisher Scientific. The microscopy studies in this paper were performed using the facilities in the UConn/FEI Center for Advanced Microscopy and Materials Analysis (CAMMA).

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