Processing Characteristics, Macro/microstructures And Mechanical Properties Of Laser Solid Forming 2Cr13 Stainless Steel

Laser solid forming?LSF?is a kind of additive manufacturing technologies characterized by the powder feeding simultaneously.It can be used to fabricate metallurgical components of high performance.Compared to the traditional processing technologies,laser solid forming possesses the different forming principle and complicated physical metallurgical processes.To date,many forming and physical metallurgical phenomena have not been clarified.Steel is the standard alloy used for investigating the phase transformations of metallurgical materials,also the broadest used engineering material.However,the investigation and application of laser solid forming of steel are quite insufficient.2Cr13 stainless steel is a kind of martensitic stainless steel used widely,with good erosion resistance and mechanical properties,which possesses significant engineering value.In the present work,the forming behavior,oxidation behavior and micro/macrostructures evolution and mechanical properties of laser solid forming 2Cr13 stainless steel were investigated.Main conclusions are as follows:?1?With the same processing parameters,the width and height of the single-track clad deposited under air is the largest.When the power density is high,the width and height of the single-track clad deposited under argon is larger than that under nitrogen.When deposited under air and nitrogen,the bulk deposit possesses better shape and dimension precision than that deposited under argon.The influence of the atmosphere is attributed to the exothermic oxidation reaction under air,the more extensive plasma plume effect under argon and the larger deformation of the molten pool under air and nitrogen.?2?During the depositing layer by layer,the boundary single-track clad shape is determined by the“superposition effect”between the substrate and powder accumulation height and the deformation of the molten pool.The inward contraction of the boundary single-track clad layer by layer results in that the outside walls title inwardly.The deformation of the molten pool leads the outside rim of the boundary single-track clad to protruding outward,which promotes the transition of the boundary single-track clad shape from the spherical-cap-like to the spherical-segment-like and counteract its inward contraction to some extent.It can promote the deformation of the molten pool by increasing the height of the single-track clad,which is to the benefit of the outside wall growing vertically.?3?The oxidation reaction increases the O content of the laser solid formed 2Cr13stainless steel,however decreases the C and Cr content.When the environmental oxygen partial pressure is lower than the critical value(2.1×10^-3 atm in present work),the deposited2Cr13 stainless steel is purified.The O content of the deposit can be lower than that of the fed powder by about an order of magnitude when the environmental oxygen partial pressure is low enough.The oxidation/deoxidation mainly takes place in the molten pool stage.By considering the transfer of gaseous oxides in gas phases,a novel mechanism was developed to explain the oxidation/deoxidation transition.It is indicated that volatile oxides play a significant role in the purification.Both the decrease of the environmental oxygen partial pressure and increase of the contents of elements forming volatile oxides promote the purification.?4?For deposition under air,the oxide inclusion in the deposit consists of the angular spinel and the allotriomorphic or globular compound oxide of spinel and silicate,with the relatively large size?the largest one is about 100?m?.With decreasing the oxygen partial pressure,the oxide inclusion is mainly the globular silicate,with more uniform size of about0.8?m.The oxide inclusion characteristics in LSFed 2Cr13 stainless steel are determined mainly by the precipitation and solidification of the slag before and after the liquid metal solidification respectively.Ordinarily,the number and size of the oxide inclusion particles are dominated by the nucleation of the slag.When the O content is high enough,the collision-coalescence between the slag particles results in the decrease of the number of the oxide particles but the increase of the size.?5?The microstructure evolution of the LSFed 2Cr13 stainless steel is affected by the heat history.For single-track depositing,the solidification path is:?L?????L+??????L???,which results in the microstructure of martensite in the dentrite stem,?ferrite in the center of the dentrite and the interdendritic skeletal austenite.For multitrack and multilayer deposition,the substrate temperature increases gradually due to the thermal accumulation.When the substrate temperature is higher than the Ms temperature,the austenite will transform following the reaction of??Carbides and???+Carbides.When the continuous deposition time is long enough,the precipitation of M₂₃C₆ carbides along the prior austenite grain boundary is sufficient,resulting in the formation of broad carbides precipitation zone.Meanwhile,the austenite transforms into?ferrite.When the continuous deposition time is not long enough,both the precipitation of M₂₃C₆ carbides and austenite transforming into?ferrite are not sufficient.The austenite will transform into martensite after the deposition ends.When the deposition is conduction again,the martensite will be tempered.?6?Grains in the deposit are generally columnar nearly along the deposition direction.For single wall deposit and small bulk deposit,the grains come from solidification,which appear straight.For large bulk deposit,except for the upper part,the columnar grains appear curved and aline as“C”type.In the upper part,along the deposition direction,the“equiaxial grains”,“neck-lack grains”and“straight columnar grains”appear sequentially.The recrystallization of the austenite leads to the formation of the“equiaxial grains”.Under the nearly directional temperature field,the directional recrystallization leads to the formation of the curved columnar grains and“C”type arrangement.?7?The tensile properties of LSFed 2Cr13 stainless steel are anisotropic.Compared to the conserve tensile?the tensile direction perpendicular to the deposition direction?,the strength is lower while the plastic is better for longitudinal tensile?the tensile direction parallel to the deposition direction?.For conserve tensile,from the top to the bottom of the deposit,the ultimate tensile strength decreases obviously,while the yield strength decreases slightly.The plastic keep invariant almost.The ultimate tensile strength is dominated by the extent of the martensite tempering.With more sufficient tempering,the ultimate tensile strength is lower.The yield strength is dominated by the dimension of the grains along the tensile direction.For converse tensile,the columnar grains turn to the tensile direction,which deteriorates the plastic.