Abstract Scope |
Introduction:
Grade 91 steel components are used in the nuclear industry for their excellent strength, creep resistance, and radiation tolerance. Directed energy deposition (DED) is a potentially attractive method to manufacture Grade 91 steel components for the nuclear power industry. Structurally sound, defect-free, and reliable parts of Grade 91 steel made by DED require the selection of appropriate process conditions to achieve superior microstructure and properties. Temperature fields and pool dimensions significantly affect the microstructure and properties of the component. For example, multi-layer laser DED (DED-L) exposes components to repeated thermal cycles that can significantly alter the microstructure of the previously deposited layers. Since Grade 91 is an austenitic martensitic steel, the amount of martensite formation depends largely on the cooling rates during the deposition process. Careful selection of processing conditions such as laser power, scanning speed, powder mass flow rate, and preheat temperature is important for tuning the thermal cycles, cooling rates, and pool shape and size that are responsible for the evolution of microstructure. Here we develop and use a heat transfer and fluid flow model to investigate the effects of important processing conditions on thermal cycles, molten pool shape and size, and cooling rates during multi-layer DED-L of Grade 91 steel.
Technical approach:
The heat transfer and fluid flow model solves the equations of conservations of mass, momentum, and energy in a 3D, discretized solution domain that includes deposit, substrate, and shielding gas. Thermophysical properties of Grade 91 steel required for the heat transfer and fluid flow model are computed by thermodynamic calculations using a commercial software JMatPro. The model considers the process parameters such as laser power, scanning speed, laser radius, layer thickness, substrate thickness, preheat temperature as well as the thermophysical properties of Grade 91 steel as inputs. The model calculates the 3D, transient temperature and velocity fields, thermal cycles at different locations of the component, molten pool shape and size, and cooling rates. Calculations are done for a large window of the process conditions to evaluate the effects of important process variables on thermal cycles, molten pool shape and size, and cooling rates.
Results and discussions:
Temperature variations with time for different processing conditions were compared with the corresponding independent experimentally determined values. Besides, the fusion zone shape and size obtained from the modeling were compared with several independent experimental data. The agreement provides confidence that the heat transfer and fluid flow model can provide a reliable basis for estimating the necessary thermal cycle for understanding the evolution of microstructure. The peak temperatures, cooling rates, and pool geometry for various processing conditions were evaluated. It is found that for a set of typical deposition parameters commonly used in DED-L, the cooling rates are sufficiently rapid to form martensite during the cooling of austenite. However, the extent of martensite formation depends both on the local cooling rate and temperature. This finding is important because it provides hope that by tuning process parameters, microstructures of Grade 91 steel could be optimized.
Summary and conclusions:
In summary, a 3D, transient heat transfer and fluid flow model of the DED-L process is developed, tested, and used to predict thermal cycles, molten pool shape and size, and cooling rates for Grade 91 steel. Effects of important process variables on these attributes are examined. This research is shown to be useful in selecting appropriate laser power, scanning speed, preheat temperature, and powder mass flow rate to control the extent of martensite formation in the additively manufactured Grade 91 steel components. |