3D printing of fine alumina powders by binder jetting
Graphical abstract
Introduction
In the last decades, Additive Manufacturing (AM) has made huge progresses and improvements, finding multiple applications in industrial sectors, as those of aerospace, energy, biomedical and automotive [[1], [2], [3], [4], [5], [6], [7]]. However, these upgrades have mainly concerned polymeric and metallic materials, leaving ceramics to an early development stage [8,9]. In the recent past, much interest has arisen from the possibility of producing functional ceramic materials through AM due to the need of small scale and customized manufacturing, which would be excessively expensive if performed with traditional techniques such as molding [[10], [11], [12], [13], [14]].
Among various processes, 3D printing by binder jetting (BJ) has gained attention thanks to its simplicity, compared to other direct printing processes that involve high power sources as laser or electron beams [[15], [16], [17], [18]].
BJ consists of a series of independent steps:
- a)
The building up of a powder bed layer by layer and the selective deposition of a binder, thanks to a Drop-on-Demand piezoelectric printhead, which joins the particles according to the 3-dimensional CAD model provided. The amount of binder added, calculated as the ratio between the volume of binder solution deposited and that of voids in the powder bed (estimated from the powder tapped density) is called binder saturation.
- b)
The curing of the bed at mild temperature (roughly at about 200 °C) to promote the polymerization of the binder and to extract the green bodies from the unbound powder (“de-powdering” process).
- c)
The debinding process for the removal of the residual polymer (formation of the brown) by heat treatment in air or inert atmospheres.
- d)
The sintering process to achieve high or full density, performed at high temperature.
Therefore, this technique makes it possible to heat treat the ceramic material in high-performance furnaces while keeping the printing process itself simple, low energy demanding and cheap.
α-Al2O3 is one of the most employed ceramics by the industry due to its functional properties (e.g. high wear resistance, biocompatibility, electrical insulation, etc.) and relative low cost, thus it is of great interest to produce it by BJ when complex shapes are required. Previous studies (Table 1) have highlighted the difficulty to achieve full-density alumina components and several strategies have been adopted to enhance the densification, such as the use of multimodal powder distributions [[19], [20], [21], [22], [23]], post-process slurry infiltration [[23], [24], [25]], particles coating [26] and nanosuspensions as a binder [27]. Although these multiple approaches have led to some improvements, they were not significant in most cases, as can be seen from the literature review in Table 1. Meaningful final density increases were not recorded without the infiltration of sintering aids, which, however, lead to chemical composition variations [[28], [29], [30], [31]] that influence the final properties.
In our study, first we employed computer simulation to verify the possibility of predicting the packing efficiency of a specific powder, given its size distribution. Different methods were applied to describe the system during printing. Then, the process of production of the samples was kept as simple as possible, thus using a unimodal size distribution of the powder, and avoiding post-sintering processing, to identify those factors, strictly related to BJ, which most influence both green and sintered densities.
Section snippets
Modeling and simulation of powder packing
A numerical analysis has been performed first, to estimate the dependence of the powder packing from its Particle Size Distribution (PSD) and the geometrical and mechanical characteristics of the Al2O3 spherical granules.
The simulation of the powder packing behaviour was first developed through the so-called Lubachevsky-Stillinger (LS) Algorithm, a geometrical model that creates jammed configurations of rigid spherical particles starting from a random generation of points that grow in size
LS algorithm and DEM modelisation
To obtain consistent results, the LS simulations were carried out by processing not less than 2000 spherical particles having a pseudo-random diameter defined in the range 0.1−50 μm. The final packing was contained inside a cubic periodic box having a volume equal to W and dimensions that were automatically computed by the algorithm to circumscribe the N spheres, each of which was characterized by a volume equal to Vi. The relative density of the jammed configuration was computed as in Eq. (3):
Conclusions
Binder jetting is a relatively recent additive process; however, it has the potential to expand the manufacturing of functional ceramics, which have numerous fields of application.
In this study, our goal has been to improve the properties of alumina components obtained by binder jetting, while keeping the printing and post-printing processes as simple as possible, thus demonstrating the feasibility of this technique for industrial productions. Optimal powder size distribution has allowed to
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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.
Acknowledgments
Authors would like to acknowledge the “Functional Sintered Materials (Funtasma)” Interdepartmental Laboratory of Politecnico di Milano, where this research activity was developed. The fruitful discussion on DEM simulations with Prof. C. Di Prisco and Dr. I. Redaelli is gratefully acknowledged. Support by the Italian Ministry for Education, University and Research through the project Department of Excellence LIS4.0 (Integrated Laboratory for Lightweight and Smart Structures) is also
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