AN INVESTIGATION INTO FULLY MELTING A MARAGING STEEL USING DIRECT METAL LASER SINTERING (DMLS)
Mark. Stanford, Kevin. Kibble, Matthew. Lindop, Diane. Mynors and Colin. Durnall
Department of Engineering and Technology
University of Wolverhampton
Shifnal Road, Priorslee
This paper reports on the relative capability of the EOS M250 Extended platform to fully melt an 18Ni (‘300’ grade) maraging tool steel from sub-20 µm powder. The work describes the investigation of scanning routines necessary to achieve satisfactory metallurgical integrity and shape manufacturing capabilities of the process. Solidification was observed to take place by cellular and cellular-dendritic growth mechanisms in DMLS of the maraging steel. Intercellular spacing was less than 1 (m and this contributed to the excellent strength and hardness achieved for both as-sintered and aged material. Aging at 490(C for 6 h led to an increase in hardness and strength through the precipitation of Ni3(Mo,Ti) and Fe2Mo intermetallics. Ultimate tensile strength increased from 1101 to 1875 MPa, Vickers hardness increased from 387 to 603 kg/mm2 but elongation was reduced from 11.3 to 1.8% by aging the steel. These test results were within the tolerance specification for the material. The overall length of the test specimens was measured in the as-sintered and aged conditions. Uniformity was very high before and after the aging treatment, Shrinkage after aging was only 0.061%. The results confirmed that the maraging steel could be successfully manufactured on the M250 Extended platform.
Keywords: Laser Sintering, Maraging Steel, Additive Layer Manufacture
Direct Metal Laser Sintering (DMLS) was introduced in 1992 by EOS Laser Systems GmbH using its M250 platform and a CO2 laser. The current M250 Extended version of the technology allows 20 µm layer manufacture and has been used predominantly for prototype mould tooling manufacture in a cupro-nickel alloy . The technology allows for complex tooling to be made with conformal cooling capabilities. Conformal cooling has been proven to reduce lead times and enhance productivity of a number of net-shape manufacturing processes. Distinct drawbacks of the process have been related to the porous nature of the cupro-nickel alloy after sintering and the lack of wear resistance when injecting filled thermoplastic polymers. There is a need to move from prototype tooling manufacture to full production hard tooling manufacture to fully exploit the benefits of the M250 Extended additive layer manufacturing process. A fully melted durable steel is required that is hardenable and representative of traditional chromium alloyed tool steels. In this paper the capability of the M250 Extended platform to fully melt a Fe-18Ni-9Co-5Mo-Ti-Al (‘300’ grade, EOS MS-1) maraging tool steel, from sub-20 µm powder, is reported. The work describes the investigation of scanning routines necessary to achieve satisfactory metallurgical integrity, acceptable surface finish and mechanical properties. The nominal composition of the ‘300’ steel is given in Table 1.
Table 1. Nominal chemical composition of ‘300’ (EOS MS-1) maraging steel in weight % . |Fe |Ni |Co |Mo |Ti |Al | |Bal. |17-19 |8.5-9.5 |4.5-5.2 |0.6-0.8 |0.05-0.15 | |Cr |C |Mn |Si |P |S | |(0.5 |(0.03 |(0.1 |(0.1 |(0.01 |(0.01 |
Rapid tooling (RT) applications using the EOS M250 Extended technology and DirectMetal 20 (EOS DM20) powder offers significant benefits in reducing tooling production lead times. DM20 is a nickel-bronze, which has been adopted successfully for prototype and pre-serial production run tooling used in the manufacture of a range of thermoplastic products. DM20 material still offers one of the fastest build rates, produces a superior surface finish and offers...
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