| Inconel 718 alloy formed by laser directed energy deposition(LDED)has been successfully applied in aerospace,weapons and equipment,etc,and its extensive employment prospect has attracted great attention from the academic and industrial fields in the future.Nevertheless,the LDED-Inconel 718 alloy carries the inferior mechanical properties owing to Nb,Mo element segregation,non-equilibrium phase,holes and cracks and so forth,which seriously limit the strategic utilization of LDED technology in the forming of high performance,complex and high-end parts.Accordingly,the effect of Nb content on the microstructure and mechanical properties of LDED-Inconel 718 alloy in as-deposited state,fully heat-treated state and direct aging state was studied by focusing on Nb modification mechanism and heat treatment process.The forming of Inconel 718 with Nb reinforcing by means of LDED was implemented on the surface of typical multi-feature and variable structure shaft.The major contents and conclusions are as follows:The effects of Nb content on the microstructure of Inconel 718 in deposited and heat-treated states were studied.The effects of Nb content on elemental segregation,crack,dendrite structure and phase transition were investigated.The results demonstrated that,for the as-deposited state samples with Nb addition,the advanced dendritic structure and fine grains occurred.the Nb content in matrix y augmented with the increment of Nb content in the alloy,and the interdendritic element segregation rate decreased with augment of Nb content.The amount of Laves phase and y" phase both increased with Nb addition,and Laves phase in morphology turned into network-like structure from granular structure.Moreover,numerous cracks emerged in the 11.0 wt.%Nb deposition layer.For the fully heat-treated state,different degree of recrystallization occurred in samples with various Nb contents.The size,amount and distribution of Laves phase,?" phase and ? phase in intergranular and intragranular can be conspicuously altered via increasing Nb content.There were many stacking faults containing edge dislocations in the 9.0 wt.%Nb deposition layer.For the directly aged state,the dendrite structure,Laves phase in amount and morphology were similar to the as-deposited state ones.The strengthening phase ?" increased with the increment of Nb content,and it was distributed both in interdendritic and grain interior region.Simultaneously,high density dislocations appeared in the 9.0 wt.%Nb deposition layer.The effect of Nb content on mechanical properties of Inconel 718 in as-deposited and heat-treated states was researched.Microhardness test,ambient and element temperature tensile test were carried out to evaluate the effect of Nb content on the microhardness and tensile properties of Inconel 718 before and after heat treatment.The results mirrored that microhardness,tensile strength at ambient and elevated temperature of the as-deposited,fully heat-treated and directly aged state enhanced in accordance with the addition of Nb content,while the elongation reduced with the augment of Nb content.Fine grain strengthening and solution strengthening were the main factors to improve the hardness and strength of the as-deposited state deposition layer.The interaction of Laves phase,?" phase,? phase and carbide with dislocation led to the enhancement of hardness and strength of heat-treated state deposition layer.The improvement effect of heat treatment method on hardness and strength was sorted as follows:direct aging>full heat treatment>LDED.The LDED forming strategy on the surface of multi-feature and variable structure shaft was conducted.As a function of the geometric characteristics of the sample shaft,the method of realizing the uniformity of deposition layer and relevant geometric motion parameters were investigated.By virtue of the analysis of the movement relationship of the transmission shaft and the selection of the mechanical structure,the special flexible fixture design was carried out.Combined with the geometric motion parameters to achieve the uniformity of deposition layer and the movement relationship of each shaft of the fixture,the mathematical models of the positioning position,compound motion velocity,relative trajectory between varying shaft segments and laser beam were established,and the numerical control program used to deposit various shaft segments was obtained.Finally,the LDED forming and laser polishing of the Nb-reinforced Inconel 718 were conducted on the surface of the sample shaft,and the surface quality,thickness uniformity and surface roughness of the deposited and polished layer were characterized. |
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