Density variation in binder jetting 3D-printed and sintered Ti-6Al-4V

doi:10.1016/j.addma.2018.06.017

Additive Manufacturing

Volume 22, August 2018, Pages 746-752

https://doi.org/10.1016/j.addma.2018.06.017 Get rights and content

Abstract

Binder jet printing is one additive manufacturing technique utilized in today’s industry that uses an adhesive to bind powders together selectively in a bed. Post-printing processes are necessary for binder jet printed parts to increase key properties in materials such as density, but the full effects of this post-processing are not yet well understood. This study aims to enhance the understanding of how the process of sintering can affect the density evolution of a Ti-6Al-4 V binder jet printed part. Results show that the density is lower at the edges of the part and higher in regions of significant topological curvature, likely due to variations originating from the printing process that are propagated. These printing process effects can be due to binder- or powder-related occurrences, which are described in relation to the obtained results. Binder effects include high-velocity impact, particle disruption, and excessive spreading. Powder effects include printhead and recoater speed, satellite particles, and changing pressure throughout the powder bed. These factors affected the coordination number of particles in the green part, and caused sintering to progress more slowly in certain areas.

Graphical abstract

Introduction

Additive manufacturing is the three-dimensional printing (3DP) of parts into complex shapes. This process is utilized for its reduction of material waste, energy usage and high internal and external geometrical complexity [[1], [2], [3], [4]]. Powder bed binder jet printing is one example of 3DP that involves the adding of layers one at a time and binder to translate sliced digital model data into the 3D shape, in contrast with an energy beam based 3DP method [5]. After printing, the part must be post-processed to achieve functional densities.

Density evolution in post-processing is necessarily a product of the previous processing steps. For instance, the density of the as-printed green part will influence the density of the sintered part. One of the factors that can influence the density of the green part is packing density of the powder, which changes with the shape and size distribution of the powder [6,7]. Other contributing factors include binder saturation/penetration and powder spreading [8,9].

In this research study, a titanium alloy was binder jet printed into a complex shape. Titanium has been used in industrial settings for over fifty years because of its availability and favorable mechanical and corrosion-resistant properties. The most commonly used alloy of titanium is Ti-6Al-4 V (Ti64), which was the material used in this experiment. Ti64 is well-understood in industry and is often used in researching energy-beam-based additive manufacturing techniques [10].

Heated processing of Ti alloys can be a challenge due to their reactivity, and must be done in an inert atmosphere. Sintering Ti alloys with slow diffusers (such as Al and V) is difficult because they tend to prohibit densification; therefore, the sintering densification of Ti-6Al-4 V is dictated by Ti self-diffusion [11]. Sintering aids can be used to aid in increasing density, or another process – such as HIP – can be performed after sintering [12,13].

The driving force for sintering is a reduction in total interfacial energy, which can either be achieved through densification or by grain coarsening [14]. During sintering, the coordination number of the powder particles increases as density increases, and the rate of sintering is a function of the initial coordination number [15]. Parts with lower coordination number have decreased sintering forces and higher repulsive viscous forces [16].

With the growth of additive manufacturing and the need for post-processing in binder jet printed parts, an understanding of the evolution of densification within large parts during sintering is necessary. Effects that may carry over from the printing process must be enumerated so that they can be accounted for or studied further. This study quantifies the variations in density across a large, uniformly-sintered binder jet printed part, and identifies potential causes for these variations.

Section snippets

Material and methods

One sample was printed in an ExOne M-Flex binder jetting 3D printer with Ti-6Al-4 V -100/+325 (−147 μm / + 44 μm) powder that was gas atomized by Carpenter. Layer thickness for printing was 150 μm, and binder saturation was set at 70%. In the studied part, default print settings were used except for an approximately 2/3 decrease in the heating lamp and recoater speeds. This adjustment was made because the binder was being quickly absorbed by the powder, possibly due to the large size

Density

Fig. 3 shows the density distribution throughout the sintered part cross-section. Densities ranged from 19.5% to 100.0%. The corner location with the 19.5% section was a localized minimum, contained to approximately one square millimeter of sample and is excluded from the legend in Fig. 3 so that other density variations can be seen more clearly. Over the entire sample, the mean density was 81.9 ± 11.1% and the median density was 84.2%. Most visible is the difference in density between the flat

Discussion

Results of this study on a 3D printed and sintered large Ti64 part showed a large variation in density throughout the cross section. The very low density corner of ∼20% density (seen in Fig. 3) was an aberration. It could have been caused by a printing flaw such as improper powder spreading or binder saturation level, or more likely by particles dislodging during sample preparation. Also, handling of the green part can sometimes be a factor in losing powder at the sample surface. It is supposed

Conclusions

Density and microstructure are important features of binder jetting additive manufactured parts, and their evolution must be better understood. Based on this study, the density throughout a part after sintering is non-uniform, though the microstructure is consistent for the current material and sintering parameters. Density was lower at the edges of the part and on the flat portions. The differences in density at the edges are attributed primarily to the printing process, due to the interaction

Acknowledgements

The authors would like to acknowledge Fred Yolton and Adam Byrd from Carpenter. E. Stevens was partially supported by the Department of Defense (DoD) through the National Defense Science & Engineering Graduate Fellowship (NDSEG) Program. S. Schloder would like to thank the PPG Foundation for funding and the Swanson School of Engineering for supporting undergraduate research.

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Full length articleDensity variation in binder jetting 3D-printed and sintered Ti-6Al-4V

Abstract

Graphical abstract

Introduction

Section snippets

Material and methods

Density

Discussion

Conclusions

Acknowledgements

Acta Mater.

Powder Technol.

Scr. Mater.

Acta Mater.

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J. Manuf. Sci. Eng.

Prog. Mater. Sci.

Powder Technol.

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J. Mater. Eng. Perform.

Full length article
Density variation in binder jetting 3D-printed and sintered Ti-6Al-4V