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) tech-nology.In this work,the effects of laser power and scanning speed on the densificationo defects evolution and their formation mechanisms in a SLMed 2024 aluminium (Al) alloy were investigated in considera-tion 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 that the dependence of porosity formation on SLM processing parameters was contrary to the crack den-sity.Hence,to further improve the SLM-processability of the 2024 Al alloy it is necessary to develop SLM methods in order avoid the hot-cracking within the optimized processing parameter window associated with the minimum porosity formation.