Effect of high-temperature preheating on the selective laser melting of yttria-stabilized zirconia ceramic

doi:10.1016/j.jmatprotec.2015.02.036

Journal of Materials Processing Technology

Volume 222, August 2015, Pages 61-74

https://doi.org/10.1016/j.jmatprotec.2015.02.036 Get rights and content

Abstract

Selective laser melting (SLM) is one of the current rapid fabrication technology methods which has wide potential application in the aerospace, medical, consumer products and automotive industries. Currently, ceramic materials are not used as widely as metal and polymer materials due to the high melting point, high-temperature strength and low thermal conductivity, which influence the microstructure and density of ceramic samples during SLM fabrication. The most effective method of reducing cracks is the preheating at high temperature of the ceramic powder during SLM process. This paper presents the selective melting of yttria-stabilized zirconia (ZrO₂–Y₂O₃ 93–7) ceramic using a 1 μm wavelength fibre laser with high-temperature preheating at 1500–2500 °C, and an additional CHEVAL Nd-YAG laser for the preheating of the powder bed before scanning. In this paper, the influence of different laser powers and different scanning velocities on the microstructure, relative density and deformation of the ceramic sample is investigated; in particular, the effect of preheating on the morphology of the micro-cracks is discussed. Experimental results show that high-temperature preheating in 10 mm diameter range is possible with the Nd-YAG laser, and that orderly cracks are transformed into disordered little cracks by the high-temperature preheating. With preheating to 1500 °C, 2000 °C and 2500 °C, the relative density of the sample made by mixing fine powder (9–22.5 μm, 20 wt%) and coarse powder (22.5–45 μm, 80 wt%) is increased by 84% (without preheating) to 90–91%. The transformation of the monoclinic and cubic structures to a tetragonal structure is observed during the process of melting and cooling, and increasing the preheating temperature to 1500 °C, 2000 °C and 2500 °C is more suited to the formation of tetragonal crystals.

Introduction

Selective laser melting (SLM) is an additive fabrication technology, which is a powder-bed-based layer-manufacturing method that realizes the complex formed pieces directly from three-dimensional computer aided design (3D CAD) data. Before manufacturing, the 3D CAD model needs to be sliced into a 2D stack of layers, and then the scanning trace is created with fixed scanning gap. The laser beam, as the thermal source, completely melts the powder into a thin lamina, then a new layer of powder is deposited on top of the previous layer and the scanning procedure is repeated, finally the piece is fabricated layer by layer, according to Deckard (1986). With regard to the materials, polymer and metal materials have already been adequately researched and applied in the commercial domain, such as EOS (2014), but selective laser melting ceramic material, especially high-performance-ceramic yttria-stabilized zirconia, is comparatively difficult, due to its higher melting point, high-temperature strength and low thermal conductivity.

Kruth et al. (2007) reported that, although ceramic material can be sintered selectively using lasers with the binder material (metal or polymer) to achieve the process of additive manufacturing, the essence of selective laser sintering (SLS) is the sintering or melting of the metal or polymer material. Therefore, the forming of ceramic material needs further post-heating processing generally. Research on ceramic materials SLM technology has already been demonstrated in some literatures. Because of its excellent mechanical properties, Garvie et al. (1975) explored that pure zirconia is described as ceramic steel. Chevalier and Gremillard (2011) also demonstrated that, zirconia stabilized with yttrium oxide was widely used, known as a mixture of zirconia polymorphs (cubic crystal, monoclinic crystal and tetragonal metastable crystal). However, Manicone et al. (2007) reported that rapid manufacturing with yttria-stabilized zirconia (YSZ) ceramic material was more difficult to achieve than with other low melting point materials. In biomedical field, the specific geometry of the YSZ medical implants requires high geometrical accuracy. According to Piconi and Maccauro (1999), SLM technology could be adapted to the rapid manufacturing of medical implants. As mentioned above, a medical implant 3D model is divided into sections of a certain thickness by the software, and then the final product may be manufactured layer by layer. In previous studies, Regenfuß et al. (2007) indicated that the oxide and non-oxide ceramics (SiO₂, SiC) in the form of powders or suspensions were sintered or melted by an Nd:YAG laser, but the geometry of the ceramic piece was still simple with this technology. The near infrared (NIR) laser was weakly absorbed by the ceramic material. Therefore, an NIR fibre laser is normally used for metal materials currently, while NIR laser selective melting of ceramic materials is a newer research field. SLM of yttria-stabilized zirconia has been studied by Bertrand et al. (2007) using a Phenix machine and a PM 100 machine with a 50 W fibre laser. The experimental results indicate that it was possible to sinter or melt the YSZ powder without an additive, but the density and mechanical properties were insufficient. Wilkes and Wissenbach (2006) in Fraunhofer ILT developed the selective laser melting process for the direct manufacturing of the ceramic components: zirconia ceramics and tricalcium phosphate ceramics. In the SLM processing, the ceramic powder was completely melted and thus a high sample density was achieved. The problem to be solved was the limited part strength due to the micro cracks in the material. Yves-Christian et al. (2010) further indicated that Al₂O₃–ZrO₂ mixed powder was completely melted by a laser beam without any post processing. With high temperature 1600 °C preheating, Wilkes et al. (2013) demonstrated that a dense sample of the eutectic mixture of Al₂O₃ 58.5 wt% and ZrO₂ 41.5 wt% was made by complete laser melting. Then, Zocca et al. (2013) affirmed that SLM of the lithium aluminosilicate ceramic powder could get a dense layer, because of the formation of viscous liquid phase.

On the other hand, the usual method was to use a polymer as a binder material for the selective sintering of the alumina or SiC ceramic powder. Then a dense ceramic sample could be obtained after the post treatment. Friedel et al. (2005) reported that polymer derived ceramic parts (a powder mixture containing 50 vol.% polymer/50 vol.% SiC) of complex shape were fabricated by selective laser curing (SLC). The ceramic parts were built similar to the selective laser sintering/melting process. The polysiloxane loaded SiC powder layers was sequentially irradiated by a CO₂ laser beam, which locally induced curing reaction of the polymer phase at moderate temperatures around 400 °C. The laser-cured bodies were converted to Si–O–C/SiC ceramic parts in a subsequent pyrolysis treatment at 1200 °C in argon atmosphere. After pyrolysis process relative green densities 38–60% decreased to 32–50% due to the polymer shrinkage. A post-infiltration with liquid silicon was carried out in order to produce dense parts. Shahzad et al. (2013) attempted the indirect selective laser sintering of the polyamide 12 and submicrometer sized alumina. However, the relative density of parts was limited to 50%. The polymer in a deposited polymer/alumina composite microsphere layer was locally molten by a scanning laser beam, resulting in local ceramic particle bonding. Then, the binder was removed from the green parts by slowly heating and subsequently furnace sintered to increase the density. Tang et al. (2011) demonstrated a novel slurry based process to fabricate high strength ceramic parts. An average flexural strength of 363.5 MPa and a relative density of 98% were achieved.

The study on the selective laser complete melting ceramic material is relatively rare and significant. Furthermore, research on the crack generation mechanism of ceramic material is not comprehensive. Therefore, it is necessary to conduct an experimental investigation considering the effect of high-temperature preheating and the crack distribution control method. Finally, this research is also undertaken to optimize scanning velocity with different laser powers and preheating temperatures.

Section snippets

Materials

Previous experiments using a fine yttria-stabilized zirconia (ZrO₂–Y₂O₃ 93–7, AMPERIT 825.001, H.C. Starck GmbH, Germany, 22.5–45 μm) have shown that the generation of orderly vertical and horizontal micro cracks during SLM processing was the major issue according to Liu et al. (2014), seriously reducing relative density and mechanical properties. For the purpose of reducing crack generation and obtaining a dense microstructure, two types of powders (ZrO₂–7%Y₂O₃ AMPERIT 825.000, and ZrO₂–7%Y₂O₃

Temperature of the sample surface during the preheating and SLM scanning process

Fig. 7 shows the sample surface temperature versus time with different preheating temperatures (a) 1500 °C, (b) 2000 °C and (c) 2500 °C. The curve in Fig. 7(a) comprises three repeated processes. The yellow areas indicate the preheating process of the Nd-YAG CHEVAL laser. Depending on the parameters of preheating laser, the temperature may reach the predetermined value after approximately 17–27 s. The red zone is the powder melting process by SLM laser. The other areas are the new layer deposition

Analysis of SLM-processed YSZ single line scanning track

Fig. 15, Fig. 16 show the single line scanning tracks with different preheating temperatures (25 °C, 250 °C and 500 °C) and scanning speeds (0.022 m/s, 0.067 m/s and 0.400 m/s). The laser power is 60 W, and the other process parameters are identical. When the preheating temperature is 25 °C and the scanning speed is 0.400 m/s, the particles are partially melted, many ceramic particles are bonded to the single line scanning track, but it could be easily removed, and the scanning track is discontinuous.

Conclusions and perspectives

The additive manufacturing of yttria stabilized zirconia ceramic with high-temperature preheating has been investigated. The generation of long continuous cracks in the ceramic material during laser processing could be reduced by preheating the powder to a temperature of at least 2000 °C. The challenge is the residual stress caused by the different cooling speeds between the central melted zone and the boundary. The main conclusions of this paper are as follows:

(1)
Irregular pores and

Acknowledgements

The author would like to gratefully acknowledge the financial support of the China Scholarship Council.

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Effect of high-temperature preheating on the selective laser melting of yttria-stabilized zirconia ceramic

Abstract

Introduction

Section snippets

Materials

Temperature of the sample surface during the preheating and SLM scanning process

Analysis of SLM-processed YSZ single line scanning track

Conclusions and perspectives

Acknowledgements

Appl. Surf. Sci.

J. Eur. Ceram. Soc.

CIRP Ann. Manuf. Technol.

J. Dent.

Biomaterials

J. Mater. Process. Technol.

J. Eur. Ceram. Soc.

Phys. Proc.