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Understanding the densification behaviours and formation mechanisms of defects are essential to fabricate high quality and high strength aluminium components using selective laser melting (SLM) technology. In this work, the effects of laser power and scanning speed on the densification, defects evolution and their formation mechanisms in a SLMed 2024 aluminium (Al) alloy were investigated in consideration of the corresponding laser energy input, melting mode transition and microstructural evolution. The results showed that optimizing the processing parameters effectively reduced the porosity level below 1% by avoiding the lack of fusion and keyhole melting mode, and minimizing the gas pores. However, optimization of the processing parameters could not eliminate the columnar structure associated with the SLMed 2024 Al alloy, which contributed to the hot-tearing cracks in the SLMed parts. It was found …