Effect of laser energy density on defects behavior of direct laser depositing 24CrNiMo alloy steel

24CrNiMo alloy steel samples for preparing the brake disc of high-speed train were fabricated by direct laser deposition (DLD) with different laser energy area density (EAD). The effect of various energy area densities on the distribution, size and mechanism of pores, slag inclusions and cracks in the DLD 24CrNiMo alloy deposition layer were investigated. The results show that the increase of the EAD leads to the increase of the dilution index and area of the molten pool. The density of multilayer samples reached highest (99.25%) when the EAD was 100 J/mm(2). The morphology of pores had irregular deformation when the EAD was low while had smooth sphere morphology when the EAD was high. The cracks ratio increased with the increase of the EAD while the content of inclusion defects decreased. A behavioural process for the re-melting of pore defects at different energy densities was discussed, and the formation of inclusion defects with low EAD was studied to illuminate the EAD effects on defects. The samples deposited at the 100 J/mm(2) have no obvious defects and relatively high density, this EAD can be considered as a reference to prepare 24CrNiMo samples with low defects by DLD.