The selective laser melting (SLM) process is capable of producing complex 3D metallic parts that would otherwise be difficult to produce using conventional manufacturing processes like casting and machining, etc. In the SLM process, metal powder is completely melted and re-solidified, which enables it to create parts with higher density and strength as compared to other laser-based processes like Selective Laser Sintering (Kruth et al., 2005). The complete melting and solidification of metal in the SLM process can induce problems like balling, marangoni flow and solidification cracking. Before a material is qualified to be processed using SLM, suitable processing parameters are identified, so that problems are eliminated or minimized.



SLM has been used in the fields of medicine and dentistry for manufacturing of implants using different materials (Gibson, 2005; Vandenbroucke, 2008; Vandenbroucke and Kruth, 2007; Hoeges et al., 2009; Cui et al., 2012; Edson et al., 2004). The materials used for dental purposes include biocompatible metals, ceramics and composites of the two. Porcelain-fused-to-metal is currently being used for dental restorations (Cascone. 2003; Liu and Wang, 2007). The substructure in the porcelain-fused-to-metal restorations is made from biocompatible metal and coated with the hard porcelain to give it the desired strength and aesthetics. The metal substructure is made by the lost wax casting method, which involves many pre- and post-processing steps. Pre-processing steps involve taking impressions of the patient’s teeth, making a plaster model and then the wax model before the casting process. Post-processing steps include the delicate process of removing the casting from the mould (as the parts have thin walls and fine features), detaching it from the casting tree and manual finishing the part. For SLM, the part is first created as a 3D model followed by tessellation of the model, generating supports and slicing, all in a completely digital environment. The SLM process could be used to manufacture these complex metal substructures for porcelain-fused-to-metal restorations with ease and fewer processing steps. Precious metal and alloys (especially gold and gold alloys) have been used for manufacturing dental crowns and bridges for centuries. The use of high caratage gold alloy enables good shades for porcelain-fused-to-metal restorations (Cascone. 2003) but pure gold is difficult to cast in thin structures. SLM has been shown to be suitable for processing a variety of metals and alloys, therefore SLM could be used to process precious metals and alloys for producing dental crowns and bridges, etc. Electro Optical Systems (EOS) GmbH Germany, the world-leading manufacturer of laser sintering systems, in partnership with Cookson Precious Metals has successfully produced a number of products (jewellery and other items) from 18 carat gold using specifically designed and optimized Direct Laser Metal Sintering (DLMS) system (EOS Precious M080 system) (Fletcher, 2012). The specific advantage of using DLMS for processing precious metals was the reduction in raw material consumption and design freedom, where the complexity of parts was not an issue for manufacturing.dental laser

Keeping in view the specific advantages of Additive Manufacturing (AM) processes and pure gold, this research focuses on the feasibility of SLM to process 24 carat gold powder for manufacturing of gold premolar and molar dental crowns.

2. Material and equipment
A 24 carat Gold (Au) powder prepared by gas atomization was used for manufacturing dental crowns. The gold particles were spherical in shape with a mean particle size of 24 μm. Figure 1 shows scanning electron microscope (SEM) images of the gold powder. It can be seen that some of the smaller particles have attached with larger particles to form agglomerates, which can reduce flowability and hinder the smooth deposition of layers in SLM process. One of the major causes of this agglomeration is the presence of moisture in the powder. To reduce the effect of moisture on powder flowability, the gold powder was preheated to 100°C for 5 minutes before using in the SLM system for processing.

Gold powder had apparent density of 9.5 g/cm2 in its un-compacted (poured) state, but after full compaction, its tap density was found to be 10.3 g/cm2. The tap density of gold powder were measured according to ASTM B527 (1993) (ASTM, 2006) and BS EN ISO 3953 (1995) (British Standard Metallic Powders, 1995) standards.

The SLM 100 system uses a 50 watts continuous wave ytterbium doped infrared fibre laser at a wavelength of 1,070 to 1,090 nm. The SLM 100’s scanning system incorporates an f-theta lens and uses the same layer-based techniques as the majority of other AM processes. In the SLM 100 machine, a wiper spreads a single layer of powder on the surface of a 125 mm diameter steel substrate. This is followed by the fibre laser scanning a single cross-sectional layer of the part. This process is repeated multiple times to create three-dimensional parts. Having a very small laser spot size (35 μm) and smaller build area, the SLM 100 is highly suitable for processing precious metals and alloys. The SLM 100 is also suited for highly detailed and customized small parts such as jewellery items and dental crowns, cups and implants, etc. Figure 2(a) shows the schematics of the SLM 100 set-up and Figure 2(b) shows the SLM 100 chamber indicating lens, light, hopper, wiper and the build platform. The chamber was filled with argon gas with less than 1 per cent oxygen during processing of gold powder.

Conclusions
In this work, gold dental crowns (premolar and molar) were successfully manufactured using the SLM process. Optimum processing parameters were used for processing gold powder with powder bed pre-heating of up to 100°C to remove the effect of high thermal stresses in the parts. Dental premolar and molar parts were produced for 75 and 50 μm layer thickness. The reduction in layer thickness improved surface quality and also reduced layer delamination in the parts. Both premolar and molar dental parts were prepared with minimal supports for easy removal. Micro CT scan indicated internal porosity in both samples. Cross-sectional images confirmed the presence of inter-layer porosity in the samples, which is associated with the high reflectivity of gold powder and low energy reaching the interface of two consecutive layers. Average porosity of gold samples was found to be about 7 per cent. Nanoindentation was used to check mechanical properties. Gold samples showed Vickers hardness of 27.63 and modulus of 72.5 GPa. The difference in hardness and modulus of gold cubes compared to the as-cast and annealed 24 carat gold is credited to the rapid heating and cooling during laser processing. However, this needs further investigation into the microstructural variation of gold sample under different processing conditions. Although this study has shown the feasibility of SLM for processing 24 carat gold powder, further investigation is directed towards further improving quality of the final parts, i.e. investigating the effect of higher laser power (above 50 W) and finer layer thickness (below 50 μm) on the internal porosity and surface quality of gold parts.