## 1 Introduction  
Even though additive manufacturing (AM) has been investigated for over 50 years, AM of metals is still a fairly recent and fast developing field. AM has major advantages over subtractive manufacturing, most notably the design freedom and important reductions of material waste (like chips). In particular, titanium components will benefit significantly from AM, because not only can the materials' costs be reduced importantly by saving material, but also expensive (subtractive) manufacturing methods for titanium can be optimized. These drivers, together with the good welding properties of Ti-6 Al-4V, explain why Ti-6 Al-4V is among the most investigated alloys in metals AM [1; 2; 3]. One of the popular AM methods for metals is selective laser melting (SLM), which is a powder bed method using a laser to selectively melt the powder in a protective atmosphere. The component is built layer-by-layer on top of a build plate [4; 5]. The microstructure of SLM manufactured Ti-6 Al-4V is mainly \(\alpha^{\prime}\) martensite that has formed within elongated (prior) \(\beta\) grains, which extend over several consecutively deposited layers, as opposed to the different \(\alpha\)-\(\beta\) microstructures found in electron beam melted or wrought Ti-6 Al-4V [6; 7]. The martensitic transformation indicates fast cooling conditions, while the elongation of the \(\beta\) grains in the direction of the heat flux (the temperature gradient) develops by repeated epitaxial growth on previously deposited titanium [8; 9; 10]. [PERSON] et al. described that the grain growth direction is perpendicular to the local conductive heat transfer [11]. On a micro-scale, the maximum thermal gradient normal to the melt pool rather than the macro-scale heat flow is expected to determine the direction of the advancing solid/liquid interface [8]. In addition to the maximum thermal gradient, [PERSON] et al. found an influence of the scanning strategy on the texture developing upon solidification, such that the \(\beta\) phase has a strong [100] texture along the main growth direction [12]. The morphology of martensite has been shown to depend on both the processing parameters and the specimen geometry [9; 10; 11]. [PERSON] et al. described the microstructure as hierarchical, consisting of primary, secondary, tertiary, and quaternary \(\alpha^{\prime}\) martensites [13]. [PERSON] et al. observed a dependence of the microstructure on the distance to the heat sink. In a specimen built under conditions that were optimized for decomposition, the microstructure changed from martensitic in the last few deposited layers, to ultrafine lamellar \(\alpha\)/\(\beta\) with changing morphology for the investigated 10 mm specimen height [14]. The in-situ decomposition of \(\alpha^{\prime}\) martensite into ultrafine lamellar \(\alpha\)/\(\beta\) during SLM is desired for increased strength and ductility. Later, [PERSON] et al. [15] reported processing conditions resulting in a reduced thermal gradient, leading to a fully lamellar microstructure in equiaxed prior \(\beta\) grains. Accordingly, improved ductility was achieved, while the coarser lamellae lead to a decrease in tensile strength, as compared to the ultrafine lamellar microstructure presented in their earlier work presented in [14]. [PERSON] et al. [9] showed how the molten pool mode influences the microstructure. Conductive mode, which is characterized by a U-shaped (rounded) melt-pool, leads to a mainly martensitic microstructure, while keyhole mode, characterized by a V-shaped melt-pool (vertically elongated, cone like), leads to a microstructure consisting of both martensitic \(\alpha^{\prime}\) and \(\alpha\)/\(\beta\) lamellae. Keyhole-mode manufactured specimens have superior ductility, while conductive mode specimens have superior formability. Unfortunately, keyhole mode entails vaporization of material, which potentially can lead to entrapped gas and porosity during solidification, particularly when the evaporation rate is high and the melt pool is deep [9].