Schematic of directed energy deposition and its involved multi-physics. phenomena.

Contexts in source publication

Context 1

... distinct properties boost the development of various kinds of 'smart' materials (e.g. shape memory polymer, shape memory alloy, etc.). Near equiatomic nickel-titanium shape memory alloys (SMAs), as Fig. 1 shows the schematics of directed energy deposition. A laser beam with high energy density scans across the substrate surface and creates a melting pool on its optical focus. Simultaneously, the powder stocks are injected into the molten pool by a carrier inert gas through a coaxial powder ...

Context 2

... the depth of the melt pool has no distinct influence on the final part geometry. The deposition rate is a more interesting parameter that can estimate cladding time and cost-efficiency. Fig. 10 is the schematic diagram of the cross-section of the cladding layer. The deposition rate is calculated by the following ...

Context 3

... verify the proposed numerical model, a series of single tracks with different cladding parameters were deposited. The simulated and experimental results of the deposition rate are shown in Fig. 11. From Fig. 11(a) to 11(c), it is observed that the deposition rate sharply increases along with the increasing laser power, regardless of the scan speed variation. This deposition rate improvement can be explained by the wider molten pool and increased powder absorption coefficient caused by the rising laser power. The scan speed has a ...

Context 4

... verify the proposed numerical model, a series of single tracks with different cladding parameters were deposited. The simulated and experimental results of the deposition rate are shown in Fig. 11. From Fig. 11(a) to 11(c), it is observed that the deposition rate sharply increases along with the increasing laser power, regardless of the scan speed variation. This deposition rate improvement can be explained by the wider molten pool and increased powder absorption coefficient caused by the rising laser power. The scan speed has a tiny influence ...

Context 5

... Furthermore, within the same powder feed rate, the deviation of simulated and experimental results gradually increases when the laser power increase. This can be attributed to the fact that the intrinsic fluid flow is decided by thermocapillary and temperature gradients which are very sensitive to the laser power variation. Besides, as shown in Fig. 11(c), when the laser power is higher than 67.8 W, the deposition rate measured by the experiment shows a downward inflection point while the value calculated by simulation is still going up. This downward inflection point is caused by the burning of flying powders by high laser power which is not considered in our numerical ...

Context 6

... result in chemical segregation. To verify this assumption, four different parameter combinations including scan speed, power, and powder variations were chosen to compare in the simulation and experimental results. The cross-sections of the simulated molten pool with velocity fields and corresponding cladding layer after corrosion were shown in Fig. 12. With the presence of a negative thermocapillary coefficient, an outward fluid flow pattern was observed in all four molten pools but the velocity fields present different intensities. From Fig. 12(a) to Fig. 12(b), it is observed that the fluid stronger velocity field caused by higher laser power input, promoting the fluid convection ...

Context 7

... results. The cross-sections of the simulated molten pool with velocity fields and corresponding cladding layer after corrosion were shown in Fig. 12. With the presence of a negative thermocapillary coefficient, an outward fluid flow pattern was observed in all four molten pools but the velocity fields present different intensities. From Fig. 12(a) to Fig. 12(b), it is observed that the fluid stronger velocity field caused by higher laser power input, promoting the fluid convection through the whole area. As seen from Fig. 12(c) to Fig. 12(d), a higher well-developed fluid velocity field is observed after increasing the powder feeding rate but a smooth cross-section without ...

Context 8

... The cross-sections of the simulated molten pool with velocity fields and corresponding cladding layer after corrosion were shown in Fig. 12. With the presence of a negative thermocapillary coefficient, an outward fluid flow pattern was observed in all four molten pools but the velocity fields present different intensities. From Fig. 12(a) to Fig. 12(b), it is observed that the fluid stronger velocity field caused by higher laser power input, promoting the fluid convection through the whole area. As seen from Fig. 12(c) to Fig. 12(d), a higher well-developed fluid velocity field is observed after increasing the powder feeding rate but a smooth cross-section without traces of corrosion ...

Context 9

... thermocapillary coefficient, an outward fluid flow pattern was observed in all four molten pools but the velocity fields present different intensities. From Fig. 12(a) to Fig. 12(b), it is observed that the fluid stronger velocity field caused by higher laser power input, promoting the fluid convection through the whole area. As seen from Fig. 12(c) to Fig. 12(d), a higher well-developed fluid velocity field is observed after increasing the powder feeding rate but a smooth cross-section without traces of corrosion is ...

Context 10

... coefficient, an outward fluid flow pattern was observed in all four molten pools but the velocity fields present different intensities. From Fig. 12(a) to Fig. 12(b), it is observed that the fluid stronger velocity field caused by higher laser power input, promoting the fluid convection through the whole area. As seen from Fig. 12(c) to Fig. 12(d), a higher well-developed fluid velocity field is observed after increasing the powder feeding rate but a smooth cross-section without traces of corrosion is ...

Context 11

... enlarged view of point C in Fig. 12 (d) is shown in Fig. 13(c). It is found that a fine NiTi phase and Ti2Ni phase are evenly and alternatively distributed throughout the whole area. This phenomenon is ascribed to the limited Ti melting and redundant NiTi powder adding, inducing the saturation reaction of NiTi + Ti → Ti2Ni. The newly formed distribution shows excellent ...

Context 12

... enlarged view of point C in Fig. 12 (d) is shown in Fig. 13(c). It is found that a fine NiTi phase and Ti2Ni phase are evenly and alternatively distributed throughout the whole area. This phenomenon is ascribed to the limited Ti melting and redundant NiTi powder adding, inducing the saturation reaction of NiTi + Ti → Ti2Ni. The newly formed distribution shows excellent resistance to the etching ...

Context 13

... that a fine NiTi phase and Ti2Ni phase are evenly and alternatively distributed throughout the whole area. This phenomenon is ascribed to the limited Ti melting and redundant NiTi powder adding, inducing the saturation reaction of NiTi + Ti → Ti2Ni. The newly formed distribution shows excellent resistance to the etching solution as indicated by Fig. ...

Context 14

... temperature gradient normal to the liquid-solid interface G and the solidification rate of liquidsolid interface R are two important parameters, which determine the solidification microstructures as shown in Fig. 14 [12,33,48]. The product of G x R determines the grain size, while the quotient of G / R determines grain morphology. The morphology of the microstructure changes from the planar to cellular to columnar to equiaxed dendritic as the G / R decreases. The grain size varies from coarser structure to finer structure with the increase of G x R. The ...

Context 15

... predict the effect of temperature gradient G and solidification rate R on the microstructure evolution, the values of G, R at the instantaneous deepest point of solidification interface in the reference crosssection plane (as shown in Fig. 15) for three time instances, t=0.38 s, 0.40 s and 0.42 s was ...

Context 16

... is equal to the scan speed) for the all three points, due to the molten pool quasi-steady state. Temperature gradient, solidification rate, and the shape factor at points A, B, C in Fig. 7 For well-mixed samples, the typical microstructure characteristics of a single track at these three different locations in a cross-section are shown in Fig. 16. The grains of the track from the bottom to the top experience planar, cellular, columnar, and equiaxed grains are shown. In the initial stage of the molten pool solidification process, the temperature gradient G at the bottom is very high but the solidification rate R at the same point is relatively low, which results in the shape ...

Context 17

... high. Moreover, large amounts of Ti elements exist in the liquid-solid boundary. The combined effect of both induces the Ti3Ni planar grain growth on the solidification interface as shown in Fig. 16(c) and Fig. 16(d). Along with the solidification progresses, the newly formed planar grains will impede the heat exchange between the substrate and the molten pool, decreasing the temperature gradient G on the area near the solidification interface. Meanwhile, the direction of heat flow between the molten pool and the substrate is ...

Context 18

... high. Moreover, large amounts of Ti elements exist in the liquid-solid boundary. The combined effect of both induces the Ti3Ni planar grain growth on the solidification interface as shown in Fig. 16(c) and Fig. 16(d). Along with the solidification progresses, the newly formed planar grains will impede the heat exchange between the substrate and the molten pool, decreasing the temperature gradient G on the area near the solidification interface. Meanwhile, the direction of heat flow between the molten pool and the substrate is perpendicular to the ...

Context 19

... solidification front towards the surface, the temperature gradient G is further decreased, leading to the increment of undercooling. This undercooling will also be further aggravated by the formation of new cellular grains, accelerating the secondary nucleation in the liquid metal. Therefore, the Ti2Ni columnar dendrites are formed as shown in Fig. 16(b). At the top area, the temperature gradient G is lowest due to the poor heat dissipation condition. This lower G is beneficial to the nucleation rate. The solidification rate R at this area is extremely high, which further reduces the time of grain growth. The increased nucleation rate, as well as decreased growth time, induces the ...

Context 20

... G is lowest due to the poor heat dissipation condition. This lower G is beneficial to the nucleation rate. The solidification rate R at this area is extremely high, which further reduces the time of grain growth. The increased nucleation rate, as well as decreased growth time, induces the formation of Ti2Ni equiaxed grains on the top surface in Fig. ...