Microstructure And Mechanical Properties Of Laser Solid Forming And Repairing 34CrNiMo6 Steel

34CrNiMo6 steel is widely used in important components for aircraft engine such as camshaft,connecting rod et al.due to its excellent comprehensive mechanical properties.However,these components are easy to be damaged due to the harsh service environment,which increases the cost and affects the normal operation of the equipment.Therefore,it has important significance for ensuring the normal operation of equipment and reducing production costs to restore the performance of damaged parts using appropriate repair technology,Laser solid forming(LSF)is a kind of additive manufacturing technology characterized by the powder feeding simultaneously,which can digitally manufacture the high performance metallic parts with without die in a short time.In addition,due to the advantages of high laser energy density,small heat input,low material dilution rate,and small deformation of the substrate,LSF can be deposited directly on the damaged parts according to the damaged shape,so as to achieve high-performance repair and remanufacturing of large volume defects.That provides an important way for rapid forming repairing of heavy-load damaged 34CrNiMo6 steel.It should be pointed out that the repair efficiency rate is relatively low due to the way of point of point cladding deposition when the large size of defect is repaired using LSF technology.However,block substrate has been directly melted in the damaged location using high energy beam or friction stir processing,and then combined with LSF for final forming repairing,it is expected to improve the repairing efficiency and reduce the repairing cost.Therefore,considering that LSF is the basis of the laser forming repairing,microstructure and mechanical properties of LSFed 34CrNiMo6 steel have been systematically studied,the influence of cooling conditions and heat input on its microstructure and mechanical properties have been investigated.In the above,the microstructure and mechanical properties of34CrNiMo6 steel using laser forming repairing,laser fused block repairing,electron beam fused block repairing and friction stir processing repair have been analyzed.The main conclusions are shown as follows:(1)The microstructure of LSFed 34CrNiMo6 steel is obvious effected by the size of the components.For single-pass multi-layer samples,the microstructure of the deposited zone is mainly mixed structure of tempered martensite and bainite.The microstructure at the top of the heat affected zone of the substrate is quenched martensite structure.The microstructure of the substrate zone is tempered sorbite.Thus the hardness of the heat affected zone is the highest,the hardness of the deposited zone is second,and the hardness of the substrate is the lowest.The softened zone appeared in single-pass multi-layer sample when the layer increases to 20 due to the heat accumulation.The microstructure of deposited zone of the single-pass 20-layer sample is mainly mixture of bainite and martensite.The amount of bainite gradually increases from the top to the bottom,and the amount of martensite gradually decreases.For five-pass 20-layer samples,the microstructure at the top of the sample is bainite and island-like martensite,the size of the bainite and martensite at the middle gradually increases,and the microstructure at the bottom is mainly ferrite,pearlite and bainite.Its microhardness gradually decreases from the top to the bottom of the deposition.The hardness of the top is the highest(400 HV),and the hardness of the substrate is the lowest(340 HV).(2)Effect of cooling condition on the microstructure and mechanical properties of LSFed 34CrNiMo6 steel thin-wall component have been obtained.The microstructure along the deposited direction is substantially uniform under water-cooling condition,which consists of bainite and retained austenite.With the decrease of the cooling rate(metal cooling),microstructure of the deposited zone is significantly uneven.The transformation from top to the bottom is bainite→ferrite+pearlite→ferrite.The cooling rate is further reduced(air cooling),the evolution of microstructure is similar to that under the metal cooling conditions.But the sizes of massive ferrite and carbides at the bottom obviously grows.The tensile strength of the LSFed34CrNiMo6 steel is 951 MPa under water-cooled conditions.With the decreases of the cooling rate,the tensile strength firstly increases and then decreases,while the elongation rate increases.(3)Heat input directly affects the forming efficiency and the mechanical properties of the component.The microstructure presents the ferrite at the bottom to the bainite at the top of the deposited layer under low heat input(Q*=1,Q*=(laser power/scanning speed)/(reference laser power/reference scanning speed),the base value of dimensionless heat input is represented by 1)and medium heat input(Q*=1.9).But morphology of the carbides in bainite presents short rods or granular(approximately 65 nm in diameter)with low heat input,and flaky(approximately 170nm in thickness)under middle heat input conditions.When the heat input is high(Q*=2.9),the microstructure at the top of the deposited zone is composed of massive ferrite and pearlite,and the microstructure at the bottom is fine ferrite and carbides.With the increase of heat input,the microhardness,tensile strength and yield strength gradually decrease,while the elongation and percentage reduction of area gradually increase.After the quenching and tempering treatment,the strength of the samples significantly increases,but the plasticity slightly decreases.With increasing the heat input,the deposition efficiency gradually increases,but the mechanical properties(strength and microhardness)of the LSFed parts gradually decrease.The forming process under the condition of medium heat input(Q*=1.9)is the best considering the deposition efficiency and mechanical properties,and the tensile strength of the LSFed parts reaches 858 MPa.(4)During quenching and tempering heat treatment,with increasing the quenching temperature,the austenite grain size of the as-deposited LSFed 34CrNiMo6steel(Q*=1.9)decreases firstly and then increases,and the hardness of the samples increases firstly and decreases.When the quenching temperature is 830 ~oC,the austenite grain size is the minimum with 8.6μm.At this time,the hardness of the sample can reaches 644 HV.With increasing the tempering temperature,the amount of carbides precipitated on the substrate gradually increases,and its morphology changes from rod-like to spherical.The hardness of the deposited zone gradually decreases.(5)The optimal heat treatment system of LSFed 34CrNiMo6 steel has been established:quenching treatment(830 ~oC,1h/oil quenching)+tempering treatment(540 ~oC,2h/air cooling).The tensile strength and yield strength of LSF-B(Q*=1.9)sample are greatly improve after the optimized heat treatment process.The tensile strength increases from 980 MPa under the standard quenching and tempering conditions to 1230 MPa(increased by 25.5%)and the yield strength increases from916 MPa under the standard quenching and tempering conditions to 1123 MPa(increased by 22.6%).But the elongation and percentage reduction of area obtained by the two quenching and tempering conditions are basically the same.The mechanical properties of LSFed 34CrNiMo6 steel after the optimal heat treatment process have reached the forging standard.The fracture mode of heat-treated LSFed samples is ductile fracture.(6)The LFRed 34CrNiMo6 steel can be divided into four areas:repaired zone,remelting zone,heat-affected zone and substrate zone.The hardness of different layers gradually increases from heat-affected zone to repaired zone.LFRed samples presents better wear resistance compared with the substrate zone.The microstructure of repaired zone of the three block repaired samples is a mixed microstructure of martensite and a small amount of bainite,which makes the highest hardness.The heat-affected zone of the friction stir processing repaired specimen has been softened,and the hardness is only 240 HV.After the quenching and tempering treatment,the microstructure and hardness of different zones of the repaired specimen become uniform,and the mechanical properties reaches the forging standards.Compared with the electron beam frit repaired and laser frit repaired specimens,the friction stir processing repaired specimens has higher tensile strength.