Study Of Mechanical Properties And Heat Treatment Of The 18Ni300 Maraging Steel Manufactured By Selective Laser Melting

Additive Manufacturing（AM）,the layer-by layer building-up of parts,has lately become an option for serial productions.The technology is applicable to manufacture dies with ideal design without construction of manufacturing reality,bringing a new possibility for the manufacture of a die with conformal cooling channels.Among AM techniques,selective laser melting（SLM）seems to be the most versatile process,capable to produce functional components from materials having good mechanical properties.The maraging steel 18Ni300,which combines high strength and excellent toughness,is widely used for tooling of injection moulding and die casting industries.Based on the above application background,in this study,the powder characteristics for SLM were investigated,and the microstructure and properties of SLM parts were assessed.At last,heat treatment behaviour of the 18Ni300 maraging steel manufactured by SLM is studied to improve mechanical properties.Different batches of commercially available 18Ni300 powder made by gas atomization were characterized with respect to particle size distribution,morphology,chemical composition,packing density and flowability.Domestic powders from VILORY has great advantage in price compared to SLM power from EOS,the experimental results indicate that the domestic powers have greater regular morphology with better sphericity by statistical measurements.The particle size distribution curve（PSD）of EOS powder has a large span value,which means a wider size distribution as a result of higher fractions in coarse and fine particles respectively.Therefore,powder grades tend to exhibit high packing densities.And chemical compositions of the different powders ware analysed.The element Ni in VILORY powder is lower than the EOS powder.The optimized parameters of SLM 18Ni300 have been achieved.The laser power is 180W,laser scan speed is 600 mm/s,laser scan space is 0.105mm,laser thickness is30μm,atmosphere is N₂.It is concluded that the SLM process of the 18Ni300 maraging steel is stable,and dense parts with excellent mechanical properties were obtained,but some pores and micro crack defects were still founded.Microstructural analysis reveals fine cellular and dendritic structure in the original SLM maraging steel.This work studied evolution of microstructure and properties during heat treatment,by X-ray diffraction,scanning electron（SEM）and transmission electron microscopy（TEM）.Upon aging,precipitation of nanoscale intermetallic compounds occurs,which in turn,results in remarkable improvements in yield and ultimate tensile strengths,while substantially reduces ductility and fracture toughness.When aging temperature is over 500oC,The formed reverted austenite（γ-Fe）fraction increases,while hardness decreases,with increase of aging time,more apparently at aging temperatures of higher than 540oC.Meanwhile,precipitation also involves effects of heat treatment on mechanical properties.Mechanisms of the combined effects of softening by formation of reverted austenite（γ-Fe）and age hardening induced by precipitation are discussed in this dissertation.Finally,an optimal heat treatment procedure for SLM-processed18Ni300 is proposed.