Thermal finite element model to compute melt pool dimensions for directed energy deposition additive manufacturing process with experimental validation

Directed energy deposition (DED), a metal additive manufacturing process, is the manufacturing of parts in layers by injecting metal powder into the melt pool created by the thermal energy provided by a heat source. Compared to traditional subtractive manufacturing methods, DED attracts attention as an emerging manufacturing technology in different industries with the reduction in using molds and tools, the need for assembly, the ability to design and manufacture parts with complex features, and the ability to manufacture on parts that need repair. Because of the nature of the process, the high temperature and temperature gradients that occur with high heat input are critical and still very important factors for directed energy deposition, just as with other additive manufacturing methods. The correct estimation of the structure and geometry of the melt pool and the effect it creates for the manufactured part during the process significantly affect the quality of the part. For this purpose, in this thesis, a thermal finite element process model has been developed to better understand and predict the melt pool geometry and properties. The model includes temperature and state-dependent physical properties for materials used in the additive manufacturing process with novel features, treatment of surface losses as a volumetric heat flux to eliminate the need for the re-definition of the surface after the addition of each layer with an additional user-defined subroutine and evaporative heat losses added to the surface losses to avoid the high temperatures. To observe the effect of laser power and scanning speed on the structure of the melt pool, single-track and multi-layer samples were additively manufactured with Inconel 718 material for comparison and validation with the developed model. In experimental methods, the in-situ collected data using an infrared thermal camera were prepared and analyzed via developed image process method and used in comparison with the model for melt pool area prediction. In addition, ex-situ melt pool characterization with an optical microscope was also examined, and the obtained findings were used for comparison with the developed model. The proposed thermal model accurately estimates the melt pool sizes in terms of area, depth, and width of both single-track and multi-layer depositions and inter-layer boundaries for multi-layer depositions with revealing the effect of laser power and scanning speed process parameters