Selective Laser Melting Of Maraging Steel And Its Composite,Gradient Materials

Additive manufacture(3D printing)is recognized as a world-altering technology,which will lead to the third industrial revolution.Selective laser melting(SLM)as a typical metallic additive manufacture technology has many attractive advantages over traditional manufacturing methods.In this work,SLM of grade 300(18Ni300)maraging steels(MS)was investigated in terms of parameters optimisation,component tailoring,fabrication path,and structure design,etc.The SLM-processed MS,ceramics reinforced MS metallic matrix composites(MMCs),MS-based gradient multi-materials,and the novel design of MS conformal cooling moulds were systematically investigated.These researches aim at overcoming the drawbacks in SLM of MS,such as low productivity,unicity of material,high manufacturing costs,etc.The research outcomes could improve performance,productivity,and gradient functionality of MS,expanding the scopes of engineering applications of MS.The main contents include:(1)High-performance MS were fabricated by SLM recuring to laser parameters and heat treatment(HT)optimizations.The laser parameter optimization experiments reveal that the optimum laser energy density is 67 J/mm3,achieving a relative density of 99.986%based on the computed tomography(CT)results.Two kinds of HTs,i.e.,age treatment(AT)and solution-age treatment(SAT)were elaborately applied based on the detected phase transformation temperatures.The observed submicron grain size(0.31?m on average)suggests an extremely high cooling rate up to 107 K/s.Massive needle-shaped nanoprecipitates Ni3X(X=Ti,Al,Mo)are clearly present in the martensitic matrix,which accounts for the age hardening.Strengthening mechanism is elaborately analyzed and explained by Orowan bowing mechanism and coherency strain hardening.The probable phase transformations during HTs are:?+??490? aging?+?+?'+Ni3(Ti,Al,Mo)precipitates?+??840? solution ??490? aging ?+?'+Ni3(Ti,Al,Mo)precipitates.The mechanical properties of the AF and SAT MS meet the standard wrought conditions in overall.Building orientation based mechanical anisotropy is faint,which could be further eliminated with HTs.Besides,the HTs significantly relieve the residual stress from 464 MPa in AF samples to 103 MPa in AT samples.The tribological resistance is almost doubled,and the corrosion resistance is also improved after HTs.(2)SiC ceramics reinforced MS metallic composites(MMCs)were in-situ processed by SLM.The laser parameters were optimized based on the results of porosity,roughness,and hardness.The cracking phenomenon in high SiC content MMCs can be suppressed by laser re-melting,substrate pre-heating,applying support structures,and altering the build directions The microstructural evolutions experience cellular structures?columnar structures?dendritic structures?acicular and necklace structures,with increasing of SiC fractions.Massive nanosized nanoprecipitates and clusters are present in as-fabricated MMCs samples,which may in-situ precipitate by heterogeneous nucleation as SiC particles could function as preferential nucleation sites.Increasing of SiC fraction causes larger residual stress,more austenite phase fraction,as well as higher nucleation rate of precipitates in MMCs samples.Besides,the hardness and tensile yield strength of MMCs are also enhanced with the increase of SiC fraction Strengthening mechanisms analysis of MMCs reveals that Orowan strengthening and dislocations strengthening together with load-bearing strengthening account for the increment of strength in MMCs.Additionally,the tribological performance of MMCs is significantly improved compared with MS,especially,when the SiC addition is higher than 6vol.%,the wear rate reduces more than 20%.Elated to find that hot isostatic pressing post-treatment of MMCs increases density remarkably,the porosity of samples can be reduced dramatically from 0.109%to 0.001%.(3)Several kinds of MS-based gradient multi-materials were processed by SLM,and the interfacial microstructures and properties were intensively characterized and analyzed.The influence of interfacial parameters on the interfacial performance follows the order:surface condition<laser melting strategy<laser parameters.For SLM processing gradient multi-materials,the adequate diffusion of alloy elements,and an in-situ metallurgical reaction of two materials at the interface,will improve interface bonding.The interfacial mechanical properties of 304 steel-MS(304-MS)and 4Cr13 steel-MS(4Cr13-MS)multi-materials are comparable to parent materials,and distinctly better than 45 steel-MS(45-MS)multi-materials,when integrating the results of tensile,flexural and fatigue tests into consideration.Unlike the alloy element free 45 steel,high alloy elements contents(such as Cr and Ni)exist in 304 steel and 4Cr13 steel.During SLM processes,intensive Marangoni convection and mass transfer in the interfacial melt pools of 304-MS and 4Cr13-MS samples enhance the interfacial elements diffusion and metallurgical bonding.Moreover,Cu-MS gradient multi-materials were also produced by SLM.A defect-free and metallurgical bonded interface with a 30-40?m inter-diffusion region is formed with suitable laser parameters.Gradient submicron grains with strong<111>orientation exhibit at the interface,which owe to high cooling rate and large temperature gradient caused by high thermal conductive Cu.The bonding mechanism is revealed by transmission electron microscope(TEM)and illustrated in detail,interfacial intense Marangoni flows contribute to interfacial bonding.Fractures in mechanical test present on the copper side rather than the interface.Interfacial tensile,flexural strength and fatigue property are superior to that of the Cu(4)The novel design of MS in the applications of conformal cooling injection tooling is also studied.Aiming at increasing the cooling rate,large conformal cooling channel injection tooling with self-support structures is designed and fabricated by SLM MS.Meanwhile,some assembly part of injection tooling is designed as lattice structures to save the materials and manufacture time.Laser parameters of SLM lattice structures were optimized,the laser energy density needed for processing lattice structures is lower than solid parts.The 20 mm diameter channel with self-support structures is successfully processed by SLM,and mould flow analysis reveals that self-supported larger channel increases cooling efficiency more than 20%.Besides,applying lattice structures in injection tooling is also investigated by experiments.The effect of wall-thickness on the strength of solid-lattice composite structures are studied,which providing theoretical calculation method as a guideline to design.At last,an injection tooling product,which integrated lattice portion and self-support channels,is processed by SLM,demonstrating the feasibility of producing the above innovatively designed toolings by SLM technique.