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1. Introduction
Rapid prototyping (RP), also referred to as layer manufacturing (LM), is a family of modern technologies, which generates three-dimensional (3D) solid objects using a layer-by-layer approach. With this technology, there is no need to remove unwanted material, as the part is built exactly to the desired shape.
Direct metal laser sintering/selective laser melting (DMLS/SLM) is a powder-based RP technique which builds parts by laser sintering, producing a bulk of "near-net" shapes. The laser beam scans over the powder bed and elevates the temperature up to the material melting point and melts the surface. The use of SLM parts has numerous applications in the automobile, aerospace and biomedical industries which are increasingly being developed. This is mainly due to DMLS/SLM's extensive capacity in melting materials in comparison with the capacity of other RP technologies.
However, DMLS/SLM have some limitations such as surface roughness, part accuracy, and the formation of layered residual stresses, which are reinforced by the high thermal gradients due to full melting and solidification in a very short time. These limitations make it very difficult for DMLS-/SLM-fabricated parts to achieve 100 per cent densities; however, densities of around 98-99 per cent are achievable and the mechanical properties of SLM parts are comparable to those conventionally produced (Yasa et al. , 2011).
To control and vary the part properties, DMLS/SLM processing parameters, including laser power, laser scan speed, layer thickness and preheating, need to be varied and controlled (Simchi, 2004; Caulfield et al . 2007; Dingal et al ., 2008; Gu and Shen, 2009; Wei et al. , 2011). It has been identified that there are certain parameters that have a crucial effect on part properties, like laser power, laser scan speed, laser scan spacing, layer thickness and/or preheating. Leong and Phua (2001) indicated the effect of laser power and laser scan spacing on the internal structure of bioactive implants and found that an increase in laser power results in more dense parts. Savalani et al. (2012) has also highlighted layer thickness as the most important parameter for the densification of parts. Gu and Shen (2009) examined the effects of different processing conditions of SLM on the microstructural characteristics and densification and behaviour of metallic components. This investigation showed that both the...