| Mold is one of the key symbols of the modernization level of the manufacturing industry.However,with the rapid development of the manufacturing industry,how to achieve high quality mold rapid preparation and ultra-long service life has become a bottleneck in the development of the industry.Preparing molds by Direct Energy Deposition(DED)based on steel matrix nanocomposites(SMNCs)provides an effective solution to tackle this problem.However,the lack of properties associated with DED-prepared alloys and the lack of commercially available effective alloy types are the main factors limiting the development of DED-prepared molds.In addition,for SMNCs,the uniformity of the nano ceramic particles in the composite powder is the primary factor to ensure their performance.The presence of large van der Waals forces between nanoparticles results in uneven distribution of nanoceramic particles in SMNCs,which consequently causes the SMNCs material to fail to meet the expected specifications.Therefore,the development of high performance die steel and its nanocomposite powder and DED forming process has important theoretical value and practical significance for the development of rapid preparation of high performance dies.The main research and conclusions of this thesis are as follows:First,the effect of powder raw material properties on the performance of DED molded parts was systematically investigated.In718 powder prepared by Gas Atomization(GA)and Plasma Rotating Electrode Process(PREP)were deposited under the same DED process parameters,respectively.It was investigated that different atomization processes had significant effects on the morphology and purity of the powder.The PREP powder has a higher surface roughness,purity,sphericity compared to the GA powder.As a result,denser formed samples(98.88% densities)were prepared with PREP powders.It was also found that the laser absorption rate of the powder had a significant effect on the microstructural properties of the DED formed parts.The high sphericity and surface roughness reduced the laser scattering effect in the powder,resulting in a lower laser absorption rate of the PREP powder.The reduced laser absorption of the powder facilitates the further refinement of the grains in the formed sample and the generation of reinforcing phases and textures.The increased densities,grain refinement,strengthening phases and the formation of textures lead to an enhancement in hardness,tensile and compressive yield strength of24.63 %,29.76 % and 26.77 %,respectively,along the scanning direction of the formed samples prepared from PREP powders.Secondly,spherical mould steel powders suitable for DED were enriched using PREP technology.The feasibility of using DC53 powder for DED preparation and the related properties of DED formed samples were investigated and PREP was able to successfully produce DC53 spherical powders with high quality,low impurities and in compliance with DED standards.The high quality spherical powder prepared based on PREP renders crack-free processing of high carbon steel based on DED technology.It is worth noting that the excessively fast cooling rate leads to a change from diffusive to non-diffusive phase transformation in the high temperature phase austenite,thus changing the phase composition of DC53.Compared to commercially available DC53,the formation of martensite,grain and carbide particle size refinement effectively improved the hardness and wear resistance of the DC53 formed samples prepared by DED(28.2 % increase in hardness and 47 %reduction in wear rate),thus achieving the rapid preparation of high quality tool steels.Finally,a SMNCs powder with core-shell structure was prepared by ball milling time,and the effects of nano WC-Co enhanced particle content on the wettability,constitutive phase,microstructure evolution,wear and corrosion resistance of SMNCs were investigated.The balance between the homogeneous dispersion of nanoceramic particles and good sphericity of the substrate powder was found to be very important.Under the optimized ball milling time,the SMNCs powders exhibited a spherical shape with uniform nanoparticle dispersion and smooth surface.These properties are beneficial to achieve favorable DED processing performance.The homogeneous coating of nano WC-Co particles increases the laser absorption of the composite powder during the DED process,contributing to a reduction in the surface tension of the molten metal,which in turn improves the wettability between the substrate and the clad layer.The total dissolution of the nano WC-Co particles leads to more W and C atoms being dissolved in the steel matrix,which in turn leads to a reduction in the martensite phase transition onset temperature.As the nano WC-Co content increases,more austenite is retained,which also leads to a decrease in the texture strength in SMNCs.The addition of15 wt.% WC nanoparticles resulted in significant grain refinement of SMNCs along the scanning direction due to the formation of a(Fe,W)x C carbide network that limits the growth of subgrain boundaries in the parent austenite.Compared to unreinforced steel parts,the wear resistance and corrosion resistance of the composites reinforced with 15 wt.% nano WC-Co increased by 2.4 and 2.6 times,respectively.In addition,in the study of the effect of nano WC-Co on the properties of different types of steel matrix composites,it was found that the introduction of nano WC-Co had a more obvious effect on martensitic steel matrix composites.120 figures,17 tables,255 references... |
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