Defects, densification mechanism and mechanical properties of 300M steel deposited by high power selective laser melting

Abstract

The usage of high laser power is one of the popular strategies to improve the productivity of selective laser melting technology. Herein, the single-track, single-layer and bulk 300M steel specimens were produced by high power selective laser melting (HP-SLM) to investigate the defects, densification mechanism, microstructure, tensile properties and impact toughnesses of HP-SLMed 300M steel parts. The results show that three main types of defects including protrusions, depressions and spatters exist on the surfaces of the prior solidified layers during the HP-SLM depositing process. The flexible powder laying strategy such as a soft scraper can be used to eliminate the instability of the powder-spreading process caused by the protrusions and spatters. Besides, the 300M steel parts with a relative density of more than 99.9% can be printed via the combined usages of high laser power (2000 W) and low scanning speed (400 mm/s) with a soft scraper. Meanwhile, the theoretical depositing rate for the HP-SLM process can reach 17.28 mm3/s. The as-printed 300M steel specimens exhibit the heterogeneous microstructure with the spatial alternating stacks of low-temperature tempered martensite and high-temperature tempered martensite. Too large hatch spacing (≥ 220 µm) can make the tensile properties of HP-SLMed parts degraded dramatically due to the unfused powders and unfused spatters. However, the tensile properties of the as-printed parts vary marginally as the layer thickness varies from 120 µm to 180 µm. The tensile strength (~1217 MPa) and elongation (~12.65%) of the vertical specimens are higher than those (~1149 MPa, 11.80%) of the horizontal specimens while the yield strength (~1058 MPa) of the vertical specimens is lower than that (~1120 MPa) of the horizontal specimens. Additionally, the impact toughness (96.5 J/cm2) of HP-SLMed 300M steel parts is much superior to that of weldments and forgings.