Microstructure Evolution And Performance Control Of Laser Melting Deposited High Strength Steel

High strength alloy steel has excellent mechanical properties and is widely used in parts with high strength and high load,such as transmission shaft,crankshaft and brake disc.Forging has the problems of long delivery time,low material utilization,complex process and low yield.Laser melting deposition can realize the rapid,full dense and near net shape of large-size and complex structural parts,shorten the delivery time and improve the service performance of components.At present,the control of mechanical properties of as-deposited high strength steel mainly depends on changing process parameters and post heat treatment process,and does not make full use of the temperature cycle characteristics of laser melting deposition.In this paper,the scanning strategy is used to change the temperature cycle characteristics of laser melting deposition.We clarify the relationship between temperature cycle characteristics,microstructure and mechanical properties,and put forward a method to adjust the microstructure and mechanical properties of high strength steel by scanning strategy.This method can simplify the heat treatment process and has the advantages of improving the production efficiency and reducing energy consumption.The carbon equivalent and undercooled austenite stability of 12Cr Ni2,24Cr Ni Mo and 4Cr5Mo Si V1 high strength steel increase successively and thus these high strength steel have different microstructure evolution under the same temperature cycle characteristics.Taking the above three kinds of high-strength steel as the research object,the control methods of microstructure and mechanical properties are obtained.The relevant research results of high strength steel with different compositions provide a basis for the application of the control idea.The temperature cycle characteristics including interlayer temperature,maximum melting pool temperature,cooling rate and tempering time in the laser melting deposition of high strength steel are systematically studied by finite element simulation method.Three scanning strategies with different interlayer cooling time and interlayer laser incident energy are designed,and the thermal cycle characteristics of the deposition layer under different scanning strategies are obtained.The results show that the cooling time between the deposition layers and the incident laser energy between the deposition layers will affect the cooling rate of the deposition layer and its residence time at 250℃-750℃.Taking 12Cr Ni2 high strength steel as an example,when the strategy of continuous deposition is adopted,the average cooling rate of the deposited layer decreases from 517℃/s of the first layer to 144℃/s of the sixth layer.The residence times of the bottom（layers 1-2）,middle（layers 3-4）and top（layers 5-6）of the deposited layers at 250℃-750℃are 364 s,327.9 s and 181.7 s,respectively.When the interval between the deposition layers is prolonged and the laser incident energy is reduced layer by layer,the cooling rate of the deposited layers in the sixth layer can reach 449℃/s,and the residence time of the bottom,middle and top of the deposited layers at 250℃-750℃are 84.3 s,76.5 s and 59.7 s respectively.The influence of temperature cycles on the microstructure evolution of high-strength steel was clarified,and the microstructure evolution of high-strength steels with different undercooled austenite stability under different temperature cycles were obtained.During the continuous cooling process of the deposited layers,ferrite,bainite and martensite transformation mainly occur in the deposited layers of 12Cr Ni2and 24Cr Ni Mo high strength steel.When the strategy without time interval is adopted,the cooling rate of the deposited layers decrease significantly with the increase of the number of deposition layers,and the proportion of M-A islands,proeutectoid ferrite and bainite in the deposited layers increase.When the scanning strategy of interlayer cooling for 2 min and reducing the laser incident energy layer by layer is adopted,the deposited layers maintain a high cooling rate,resulting in a significant decrease in the number of ferrite,bainite and M-A islands in the deposited layers of the two high-strength steels,and an increase in the number of lath martensite.The mechanical properties of three kinds of high strength steel under different temperature cycles are obtained,the relationship between temperature cycles,microstructure and mechanical properties is clarified,and the method of regulating the mechanical properties of high strength steel through temperature cycles is put forward.When the strategy of continuous deposition is adopted,the proportion of granular bainite in 12Cr Ni2 high strength steel is significantly increased,and its ultimate tensile strength and elongation are 790 MPa and 22.8%,respectively.When the interlayer temperature and laser incident energy are adjusted layer by layer,12Cr Ni2 high strength steel is mainly composed of lath martensite,and its ultimate tensile strength and elongation are 1022 MPa and 16.2%,respectively.The solid solution strength of24Cr Ni Mo high strength steel is higher than that of 12Cr Ni2 high strength steel.When the deposited layers is mainly composed of granular bainite,its ultimate tensile strength and elongation are 814 MPa and 19.1%,respectively.When the deposited layer is mainly composed of lath martensite,its ultimate tensile strength and elongation are1207 MPa and 14.2%,respectively.By adjusting the interlayer temperature and laser incident energy layer by layer,the size of lamellar martensite and Cr₂₃C₆precipitates in the deposited layers of 4Cr5Mo Si V1 high strength steel can be refined.The fine grain strengthening effect of high strength steel is improved,and thus the ultimate tensile strength of high strength steel is increased from 1875 MPa to 1981 MPa,and the elongation is increased from 6%to 8%.