Three series of directional solidification experiments on refined Al-20wt.%Cu alloys have been carried out with different temperature gradients, and for each of them a wide range of cooling rates were applied. The experiments were performed in horizontal configuration to minimize the impact of gravity-driven phenomena and characterized in situ and in real-time by using the X-radiography technique. The influence of the temperature gradient on the microstructure formation (impact of Temperature Gradient Zone Melting: TGZM), the nucleation distance, the average grain size and morphology (elongation factor and grain orientation) have been analysed quantitatively. The experimental results are discussed with current theoretical models and similar experimental works.