Research On Heat Treatment Strengthening And Mechanical Properties Of 18Ni300 Laser Additive Manufacturin

Maraging steel 18Ni300 is a new type of ultrahigh-strength steel.Because of its excellent weldability,toughness,simple heat treatment process and service performance for complex working conditions,it is widely used in ship transportation,aerospace,machinery manufacturing and other fields.In order to meet the increasing requirements of enterprises for high performance,high precision and low forming cost in the steel industry,this topic was based on laser melting deposition technology,using commercial 18Ni300 metal powder as raw material,the solution treatment（840℃/1 h）and solution treatment（840℃/1 h）+aging treatment（490℃/6 h）were selected as two heat treatment methods to study the process,mechanical properties and heat treatment strengthening of 18Ni300 alloy by additive manufacturing.The specific research is as follows:（1）The process of preparing 18Ni300 by LMD was optimized with the energy area density as the variable parameter.Taking the rate of dilution,cross-sectional aspect ratio,and microstructure of a single layer as the morphological characteristics,and the hardness as the performance index,the final selection of the best process parameters is:laser power 2400 W,scanning speed 12 mm/s,powder feeding rate 1.5 r/min,the air supply volume is 4.5 L/min,and the spot diameter is 4 mm.Multi-channel single-layer and multi-channel multilayer samples were prepared with the best process parameters.（2）Study the phase composition,microstructure evolution and the effect of heat treatment on the microstructure of the 18Ni300 alloy prepared by LMD.The results show that from the perspective of grain size and growth direction,the deposited microstructure is mainly composed of equiaxed crystals,cellular crystals,and columnar crystals.The phases are basically martensite,and a small amount of austenite is distributed in granular form.Elements such as Mo,Ni,and Ti are concentrated among the crystals.After solution treatment,the impurity phase and alloying elements are fully dissolved in the austenite,and the uniform martensite structure is formed after rapid solidification;the grain boundary disappears after the solution+aging treatment,and there is a small amount of austenite reverse transformation during the aging process,Ni₃Mo and Ni₃Ti type second phase precipitates are dispersed in the whole matrix.（3）The hardness and tensile properties of 18Ni300 alloy in the as-deposited state and strengthened by heat treatment are studied.The results show that the average hardness of the as-deposited coating is 375.2 HV_0.2,the tensile strength is 662.1 MPa,and the elongation at break is 12.328%,showing good plasticity and toughness.After solution treatment,the strength decreases slightly and the plasticity becomes better;after the aging treatment,the strength is greatly increased,and the plasticity is obviously deteriorated.The sub-micron-level second precipitates are distributed in a large number of spherical or granular shapes among the dendrites.（4）The service performance of 18Ni300 alloy in the as-deposited state and after heat treatment was studied,including friction and wear performance and electrochemical corrosion performance.The deposited 18Ni300 alloy is mainly fatigued and worn seriously.;after solution treatment,adhesive wear is the main cause,which reduces the amount of wear by 68.2%compared with the deposition state;the solution+aging treatment is mainly based on oxidative wear,which reduces the amount of wear by 65.0%compared with the deposited state.After solution treatment,the phenomenon of intergranular corrosion is significantly reduced,and the corrosion resistance is improved.After solution+aging treatment,the second phase consumes the corrosion resistance elements such as Ni,Mo,and Ti in the matrix,resulting in a significant increase in the corrosion rate and corrosion resistance getting worse.