Study On Microstructure Evolution Mechanism And Kinetics Of Direct Laser Deposition 12CrNi2 Alloy Steel

The nuclear power emergency diesel generator camshaft is a key component of the nuclear power emergency system.It has the characteristics of complex shape,large size and high service conditions.The traditional manufacturing technology of forging and heat treatment has short production cycle and low material utilization,and thus the development of new technologies has broad application prospects.Direct laser deposition(DLD)technology is one of advanced manufacturing technologies for complex structural metal parts in recent years.Since DLD alloy steel is a non-equilibrium metallurgic process,the problem of deformation and crack of parts caused by complex phase transformation needs to be solved.Therefore,the research on the microstructure evolution mechanism and kinetics of laser direct deposition alloy steel not only has important scientific research significance,but also has important practical application value.In this paper,FL-DLS21 direct laser deposition forming system was used to prepare 12CrNi2 alloy steel samples by changing the laser power and scanning speed.OM,XRD,SEM,EBSD,TEM,X-Ray CT,thermal simulator,paperless recorder,laser high temperature confocal and other analytical test methods were to research systematically the non-equilibrium microstructural evolution and dynamic mechanism of alloy steel as-deposited samples,the results show that:The microstructure of DLD 12CrNi2 alloy steel sample consisted of a large amount of bainite,a small amount of martensite and ferrite.It is found that with the increase of laser power or the decrease of scanning speed,the microstructure of as-deposited samples mainly transformed from lath bainite(LB),granular bainite(GB)to polygonal massive ferrite.The equations of the influence of process parameters on the phase content and grain size of as-deposited samples were established.In addition,based on the classical bainite phase transformation theory,combined with the experimental results,this paper clarified the bainite nucleation and growth mechanism,the bainite transformation temperature was 561℃,and the growth rate of LB was 1.5 μm/s.The process parameters of DLD 12CrNi2 alloy steel samples with good structure and performance was optimized.When the laser power was 2000 W and the scanning speed was 5 mm/s,the porosity of as-deposited sample reached 0.018%;when the laser power was 2000 W and the scanning speed was 6 mm/s,the prepared sample had the best tensile strength(824 MPa)and wear resistance(the friction coefficient is 0.282);when the laser power was 2200 W and the scanning speed was 5 mm/s,the impact toughness of the as-deposited sample was optimal,reaching 100 J/cm2.This study provides a good theoretical and technical reference for DLD alloy steel parts with excellent comprehensive properties.