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1 Introduction
Ti-6Al-4V is an α-ß titanium alloy widely used in the aeronautical and medical fields because of its chemical and mechanical features along with well-documented biocompatibility ([16] Ping Li et al. , 2006). It is mostly processed by hot working, but in the last few years rapid prototyping techniques, based on sintering/melting of powders, have been developed for the production of complex geometries, directly from 3D CAD-data. One of the main advantages of these techniques is the possibility to create complex networks with open and interconnected porosity both in the core material and onto the surface ([7] Hollander et al. , 2006; [17] Ponader et al. , 2007).
The energy for consolidation of the powders is provided by either a laser beam (selective laser melting (SLM)) ([7] Hollander et al. , 2006; [1] Bourell et al. , 2002; [15] Over et al. , 2003) or an electron beam (electron beam melting (EBM)) ([3] Cormier et al. , 2002; [13] Mitchell, 1999). The common feature of these technologies is the melting and rapid solidification of a powder layer; under specific process conditions, solidification results in a fully dense solid body.
The optimization of the process has the following main tasks:
- The production of a fully dense material (except for the porous surface layers, when needed), which is obtained by the optimum combination of the four main process parameters: the power of the energy source (P), the scan speed (v), the hatch distance (d) and the thickness of the single layer (l) using a fixed beam diameter (D) ([19] Simchi and Pohl, 2003).
- The control of the residual stresses, caused by the localization of the thermal input, which may lead to distortion of the pieces, as well as to the formation of cracks.
- The control of the as-built microstructure, which is strongly influenced by the large solidification undercooling of the melting pool.
Residual stresses are almost unavoidable. They are influenced by the building strategy ([8] Kruth et al. , 2004) and are significantly smaller in EBM than in SLM since the former process is carried out with preheating of the powder bed to an average temperature of about 600°C ([5] Facchini et al. , 2009), which reduces thermal gradients.
Titanium alloys typically undergo...