APPLICATION OF SELECTIVE LASER MELTING IN MANUFACTURING OF FIXED DENTAL PROSTHESES

The additive technologies characterize with the building of one layer at a time from a powder or liquid that is bonded by means of melting, fusing or polymerization. They offer a number of advantages over traditional methods: production of complex personalized objects without the need of complex machinery; manufacturing of parts with dense as well as the porous structure and predetermined surface roughness; controllable, easy and relatively quick process. The methods, mostly used in prosthetic dentistry, include stereolithography, selective laser sintering, and selective laser melting. The aim of the present paper is to review the features of the Selective Laser Melting (SLM) process and the possibilities of its application for production of fixed dental prostheses. The features of the SLM process, the microstructure and mechanical characteristics of dental alloys as well as the properties of fixed dental prostheses, fabricated via SLM, were discussed. It was revealed that the SLM Co-Cr dental alloys possess higher mechanical and tribo-corrosion properties, comparatively good fitting ability and higher adhesion strength of the porcelain comparing to the cast alloys. All this is a good precondition for successful application of the SLM process in the production of fixed dental prostheses, mainly of frameworks for metal-ceramic and constructions covered with polymer/composite, intended for areas with high loading.


INTRODUCTION
In the late 1980's a radically new approach in the production technologies has been developed -manufacturing of objects by addition of material layer by layer, or so called "additive technologies".These technologies are alternative of the technologies, operating on the principle of material removal.Using additive technologies, the objects are manufactured by polymerization, melting or sintering of materials in consecutive layers with prelimi-nary set thickness with no need of additional tools.Thus, complex geometrical shapes and volumes can be produced without a large amount of waste material, which is impossible to be done by other technological processes.The additive technologies, mostly used in the dental medicine, include stereolithography, fused deposition modeling, selective electron beam melting, selective laser sintering, selective laser melting and ink-jet printing [1][2][3][4].
The aim of the present paper is to reveal the features of the selective laser melting process and the possibilities of its application for production of fixed dental prostheses.

Selective laser sintering / selective laser melting
The technology for manufacturing of objects from powder using a laser consists of two processes: selective laser sintering and selective laser melting.In this technology layers of powder material are melted and layered over each other using a laser until the real part is fabricated [2,5].The term "Selective Laser Sintering" (SLS) is used in the processing of polymers and ceramics, while in the manufacturing of metals and alloys the terms "Selective Laser Melting" (SLM) or "Direct Metal Laser Sintering" are used [1,6].
SLS/SLM technology is very suitable for application in dental medicine, especially in prosthetic dentistry, because the whole range of dental materials can be used for manufacturing of dental constructions -thermoplastic polymers, waxes, metals and alloys (Ti and Ti alloys, Co-Cr alloys, stainless steel), ceramics and thermoplastic composites.Using SLS the maxilla-facial prostheses, functional skeletons and individual scaffolds for tissue engineering can be fabricated of polymers and composites.When the metals and alloys are processed by SLM, bulk as well as porous orthopedic and dental implants [1, 7, 8], dental crowns, bridges and frameworks for partial prostheses can be produced (Fig. 1) [5,6,[9][10][11][12][13][14][15].During the manufacturing process, a large amount of various dental constructions can be fabricated on the machine table, which considerably increases the productivity of this kind of technology.

Features of the SLM process
The first steps of the SLM process are dividing the virtual 3D model of layers and adding of supports, which are typical for each additive manufacturing process.The SLM production of objects is performed in an atmosphere of inert gas -argon [6].During the SLM process, the layers of metallic powder (stainless steel 316L, Co-Cr and Ti alloys, commercially pure titanium) with a predetermined thickness between 20 µm -75 µm are placed on the machine table.The shape of each object's layer, defined by the CAD data, is melted by a laser, equipped with fiber optics.Worktable descends down the Z-axis at a distance equal to the layer's thickness.The process of laser melting is repeated until the object completion (Fig. 2-a).Then the details are removed from the worktable (Fig. 2-b) and supports are cleaned (Fig. 2-c).It is very important the supports be properly designed because they are of the same density as the detail and sometimes it is difficult to be removed.In the development of each SLM production process, it is important to estimate the object's density, accuracy, surface roughness, hardness, strength and residual stresses.
According to Rehme & Emmelmann [16], the main goal of the SLM process is to produce details with the highest possible density.It depends on the stable melted pool, which can be controlled by the temperature gradient during heating/cooling [17,18] and the amount of energy needed for complete melting of the metal powder [19].The main process parameters which are crucial for the production of high-quality constructions are the scanning rate, laser power, layer thickness and treated area.
Due to the high-temperature gradients during the laser treatment processes the high residual stresses are generated in the object [6,20].The residual stresses in the SLM are the result of the mechanism caused by the temperature gradient in each melted pool during melting of the metal powder [6].During SLM process each melted volume is heated fast, followed by rapid cooling, leading to expansion and contraction of the material.Since only one scanned trace melts and the all other melt volumes cool and contract separately, tensile stresses generate between the melt volume and the already scanned traces and rows.Because the object builds along Z-axis, its thickness increases.This protects it from destruction but generates stresses that can affect the geometry and mechanical properties.They can occur as immediately after removing the object from the worktable and at a later stage.

Microstructure and properties of dental alloys fabricated via SLM
The features of the SLM technological process define the specific microstructure and higher mechanical properties of as treated dental alloys.
Meacock et al. [21] established that details of Co-Cr-Mo alloy, produced by laser sintering, characterize with homogenous microstructure and higher hardness (460 HV0,2) comparing to that of objects, manufactured by other technologies.The investigations of Jevremovic D. et al. [22] and Lin Wu et al. [23] showed the considerably higher tensile strength of Co-Cr-Mo samples, fabricated via SLM (about 1300 MPa), while this property of cast samples is about 760 MPa.Dolgov et al. [24] confirmed that the samples of Co-Cr alloys, produced by SLM, has higher yield strength (R0.2= 720 MPa) and module of elasticity comparing to the cast samples (R0.2= 410 MPa).Yanjin Lu et al. [25] investigated the microstructure, mechanical properties and electro-chemical behavior of Co-Cr-W alloys, manufactured by SLM in two different scanning strategies -linear and zonal.Their results showed that the density, tensile strength, hardness and electro-chemical behavior do not depend on the scanning strategy and the samples of the both types meet the requirements of the standard for dental constructions ISO 22764:2006.

Properties of fixed dental prostheses manufactured by SLM
The team of Dikova Ts. et al. [19] confirmed the higher hardness of Co-Cr dental bridges, produced by SLM (356HV-407HV) in comparison with that of the cast bridges (327HV-343HV).The hardness distribution along the depth of each element of the SLM bridges is more even, characterizing with lower values' deviations comparing the hardness distribution in the cast bridges.The higher hardness and more homogenious microstructure of SLM Co-Cr dental alloys determine their higher wear and corrosion resistance in tribocorrosion tests in artificial saliva [26].
Concerning to the surface quality, it was established 6. Thomas D. The Development of that the surface roughness of Co-Cr dental bridges, manufactured by SLM, is nearly 4 times higher than that of the cast constructions [27].This promotes 23% higher adhesion strength of the ceramic coating to Co212-f alloy, fabricated by SLM, comparing that of the ceramic to the cast alloy Biosil F (83,1 MPa and 67,5 MPa accordingly) [24].Considerably higher roughness and partially melted powder on the surface of the SLM samples lead to increase the mechanical as well as the chemical components of the adhesion of the porcelain to the alloys.The high roughness and inability for good finishing and shaping of the occlusal surfaces (Fig. 2-c) are obstacles for application of this technology for manufacturing of full metal constructions.But they could be an advantage in metalceramic and dental constructions, covered with composites or polymers, intended especially for areas with high loading [23,24].
In the investigation the adjusting of 4-part dental bridges, produced of Ni-Cr alloy by standard lost-wax casting technology, milled of zirconia and manufactured of Co-Cr alloy by SLM, Pompa G. et al. [28] established that the SLM bridges possess the best marginal fitting.The research of Dzhendov D. et al. [27] showed that in adjusting tests of 4-part SLM Co-Cr bridges there is 0.05-0.20 mm gap between the gypsum model and the crown-retainers, which is in the range needed for cementation of the construction.
The present study shows that Co-Cr dental alloys, manufactured by SLM, characterize with higher mechanical and tribo-corrosion properties as well as higher adhesion strength of the ceramic coating comparing to the cast alloys.Thus they meet the requirements of the standards for dental constructions.
The higher properties of the SLM dental alloys are due to the fine and more homogeneous microstructure and the rougher surface, determined by the specific features of the SLM process.This demonstrates that the SLM process can be successfully applied for manufacturing of metal frameworks for fixed dental prostheses mainly of metal-ceramic or covered with polymer/composite.

CONCLUSION
The SLM Co-Cr dental alloys possess higher mechanical and tribo-corrosion properties, comparatively good fitting ability and higher adhesion strength of the porcelain comparing to the cast alloys.All this is a good precondition for successful application of the SLM process in the production of fixed dental prostheses, mainly of frameworks for metal-ceramic and constructions covered with polymer/composite, intended for areas with high loading.