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Here we propose to employ electron beam surface melting (EBSM) to refine microstructural length scale and obtain suited microstructure morphology of Nb-Si based alloys. As characterized by scanning transmission electron microscopy (STEM) and atom probe tomography (APT), nano-lamellar eutectics are created in Nb-20Si (at.%) alloys upon EBSM. Theoretical analysis based on the competitive growth models demonstrates the preferred formation of Nb solid solution (Nbss)/Nb 3 Si eutectics under selected processing parameters. The large cooling rate during EBSM, higher than 8 × 104 K/s, promotes the formation of the nano-scale eutectics. Compared to the initial coarse eutectics by arc melting, simultaneous enhancement of hardness and fracture toughness is enabled owing to the controlled microstructural length scale and morphology. Display Omitted • Nano-lamellar eutectics are created by electron beam surface melting. • Theoretical analysis based on the competitive growth models is performed. • Simultaneous enhancement of hardness and fracture toughness is achieved. [ABSTRACT FROM AUTHOR]

The control of solidification microstructure is critical to successful laser processing of single-crystal (SX) nickel-base superalloys and a practical tool for the microstructural control is processing–microstructure maps. However, the maps presented in literature do not consider the effects of powder feeding during laser additive manufacturing (LAM) of SX superalloys. This paper therefore presents a simple and feasible strategy to deal with the effects of powder feeding and to extend the combined numerical model used to calculate processing–microstructure maps. A characteristic ratio of epitaxial SX growth was defined to quantitatively compare the final solidification microstructure. Resulting processing–microstructure maps can estimate the influence of most processing variables, especially powder feeding rate, on the extent of epitaxial SX growth and the position of columnar-to-equiaxed transition. Using the processing parameters selected according to these processing–microstructure maps, a multi-layer SX deposit with fine dendrites was successfully fabricated by LAM. This successful SX LAM indicates that these new processing–microstructure maps involving powder feeding are reliable and useful because they can determine proper processing windows for LAM of SX superalloys and further advance the understanding of the processing–microstructure relationship in powder-feeding LAM process. [ABSTRACT FROM AUTHOR]

Experimental process optimization is essential to obtain reliable processing conditions prior to performing actual laser additive manufacturing (LAM) of single-crystal (SX) nickel-base superalloys. The influence of processing parameters on deposited productivity and epitaxial SX growth in powder-feeding LAM process was systematically investigated by the orthogonal experiment (OE) method, a statistical experiment design method. This method can rapidly and economically estimate the effect of each processing parameter by a small number of experiments. Resulting relationship between the processing variables and each of deposited productivity and microstructure formation contributes to the selection of detailed processing conditions to balance the factors crucial to successful SX LAM, which means that appropriate adjustment of the processing parameters during actual SX LAM is easy to be performed. On the basis of the analyses of the OE results, a combination of relatively high power, low scanning velocity and moderate powder feeding rate is beneficial to both deposited productivity and epitaxial SX growth during powder-feeding LAM, and allows the preparation of good multilayer SX deposits with fine dendrites. [ABSTRACT FROM AUTHOR]

Primary dendritic arm spacing (PDAS) is an important microstructure feature in the nickel-base single-crystal (SX) superalloys by laser rapid directional solidification (LRDS). A combined numerical model was developed in this paper to investigate the influence of laser processing parameters on the PDAS. This model consists of (1) the theoretical PDAS models which relate PDAS to the solidification conditions and (2) the heat-flux calculations of laser processing which provide the solidification conditions as a function of the processing parameters. It is therefore able to immediately relate the PDAS to the processing parameters and obtain corresponding processing–microstructure maps. To verify the predicted accuracy, the PDAS values calculated by different theoretical models were compared with those produced under different processing conditions. Results show that the PDAS firstly decreases and then increases at a lower laser power whereas it decreases with increasing scanning velocity at a higher laser power, leading to nose-shape contour lines. The predicted accuracy depends on appropriate selections of material thermo-physical properties. These processing–microstructure maps can accurately capture the trends of the PDAS variation with the processing parameters and contribute to the control and optimization of dendritic microstructure while determining relevant processing windows for controlled SX laser processing. [ABSTRACT FROM AUTHOR]

Abstract: The compositional variation and the microstructural evolution in laser additive manufactured Ti/Ti–6Al–2Zr–1Mo–1V graded structural material were investigated. An anomalous compositional variation and its good correlation with hardness were found. Although, during the forming process, the ratio of Ti–6Al–2Zr–1Mo–1V in feeded powders abruptly changes from 0% to 100%, all the elements exhibit a gradual variation, which is preferred to a graded material because such gradient change can decrease the discontinuity of its microstructures and properties and can remarkably reduce the costs due to the simplified manufacturing process and the rational distribution of materials. [Copyright &y& Elsevier]