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

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 mm(3)/s. The as-printed 300M steel specimens exhibit the heterogeneous microstructure with the spatial alternating stacks of low-temperature tempered martensite and hightemperature tempered martensite. Too large hatch spacing (>= 220 mu m) can make the tensile properties of HPSLMed 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 um to 180 mu m. The tensile strength (similar to 1217 MPa) and elongation (similar to 12.65%) of the vertical specimens are higher than those (similar to 1149 MPa, 11.80%) of the horizontal specimens while the yield strength (similar to 1058 MPa) of the vertical specimens is lower than that (similar to 1120 MPa) of the horizontal specimens. Additionally, the impact toughness (96.5 J/cm(2)) of HP-SLMed 300M steel parts is much superior to that of weldments and forgings.

Automated summary

The use of high laser power in selective laser melting technology can improve productivity and eliminate surface defects, resulting in high-density 3D printed parts. The combined use of high laser power and low scanning speed with a soft scraper can achieve a relative density of over 99.9% for 300M steel parts. Additionally, the as-printed parts exhibit a heterogeneous microstructure with alternating stacks of low-temperature tempered martensite and high-temperature tempered martensite.