The importance of additive manufacturing, also known as 3D printing, has grown exponentially in the recent years thanks to its high flexibility to manufacture intricate parts. This process can be divided into many different sub-categories, depending on the feedstock form and the source used. However, the principle stays the same: building the part one layer after another. Selective Laser Melting (SLM) is one them, combining between a fine metallic powder bed and a laser beam scanning the surface to selectively fuse the particles together. SLM is already used in the medical field where custom shapes implants are needed, or in the automotive and aerospace industries where it eases the production time of certain parts. SLM exhibits very high heating and cooling rates, up to 105-106 K/s as demonstrated in this work. Therefore, it poses several challenges such as the formation of pores, the accumulation of residual stresses and the creation of metastable phases in the microstructure. Post-processing techniques such as long heat treatments are used to counter the effect of these problems. A better way to address these different issues would be to adapt the processing parameters to reduce or remove the generation of such unwanted features. To do so, it is necessary to understand the underlying causes creating these defects. Ex situ characterisation of SLM-printed parts can be done using microscopy techniques. It consists in exploring the microstructure changes for a certain range of parameter values (laser power, scanning speed, etc). However, this is very time consuming. Simulations could be used as a prediction tool, but the degree of complexity needed here is quite high, which makes them computationally expensive. This PhD thesis is part of the CCMX-Challenge AM3 (Additive Manufacturing and Metallic Microstructures) and is dedicated to the development of a new device reproducing the SLM manufacturing process to characterise the phase evolution of metallic microstructures in real time, or operando, by X-ray diffraction (XRD) techniques. A Ti-6Al-4V (wt%) alloy is used to demonstrate the capabilities of this new device, thanks to its compatibility with the X-ray energies available at the Swiss Light Source (SLS) synchrotron beamlines of the Paul Scherrer Institut (PSI). This dissertation introduces the design of the machine and its software, as well as a characterisation of the printed Ti-6Al-4V samples to determine the optimum printing parameters. To demonstrate the measurement capabilities of the machine, different operando XRD experiments will then be presented, giving new insights into the phase evolution during SLM processing. These observations will be linked to the sample microstructure after printing. Additionally, the unique properties of this setup allow to capture the rapid temperature evolution within the probed materials during the XRD measurement. The obtained temperature profile will be compared to some results derived by Finite Elements Method (FEM) analysis on already existing models established for Ti-6Al-4V.