Sticking Efficiency

Because of the fine partially molten dendritic structure in the majority of the particles, it is believed that dendrite fragmentation will occur, breaking up the solid into small pieces, such as one to a few dendrite arm spacings (Ref 2, 3, 4, and 5). Typical solidification and flight times in spray deposition are only a few milliseconds and from the standard model of dendrite arm coarsening this leads to a dendrite arm spacing (cell size) of 1 m (Ref 31, 32). This result is characteristic of the fine scale in the dendritic microstructure observed in fully solidified powder droplets, such as the overspray particles that failed to hit or stick to the deposit (Ref 2). Fragmentation of the dendrites in the partially molten droplets as they impact the growing deposit at velocities of 100 m/s appears to provide the basis for the fine grain structure characteristic of spray formed materials (Ref 2, 3, 4).

This grain fragmentation process produced by high velocity impact is probably similar to that found in stircasting (rheocasting) (Ref 33). In stircasting, solidification of the dendritic structures under high shear rates causes break up of the dendrites to give small rosettelike particles or with faster stirring and longer times, spherical particles. The model for dendrite fragmentation found to be valid for stircasting is plastic deformation of the dendrites via dendrite arm bending, based on the low yield strength of low solute metal at temperatures close to the liquidus of the alloy. Under this condition, when dendrite arms are bent through large angles (>20°) at high temperatures, rapid dislocation recovery occurs producing high angle grain boundaries. These high angle grain boundaries are then wetted by liquid causing separation between the grains because, as shown by Gundoz and Hunt (Ref 34), the energy of a high angle grain boundary, 7gb, with a misorientation of 20° or more, is equal to that of two solid liquid interfaces, 27 sl. Clear evidence for this phenomenon was shown in stircast aluminum alloys (Ref 33), and a similar process has been proposed for spray casting (Ref 35). Thus the fragmentation of metallic dendrites appears to occur not by fracture but by deformation, recrystallization, and melting.

Irrespective of the microstructural origin of the fine spherical grains found in most spray formed deposits, the partially solid material at the surface of the deposit is expected to exhibit thixotropic mechanical properties (Ref 33). These properties include a large increase in viscosity in semisolid metallic systems when the stirring rate falls. In consequence, the behavior of the material can change rapidly from fluidlike to that of a solid as bonds build up at grain boundaries between the particles (Ref 33). For spray formed materials, this suggests that, at the deposit surface with its high frequency of high velocity impacts (^ 100 m/s), the material will be rather fluid. This fluidlike character will, however, decay rapidly at a given position as the deposit grows and the material just below the surface is no longer agitated by the spray. Videotapes of the surface of aluminum deposits at high liquid fractions (Ref 36) do show rapid surface movement similar to that seen in puddles during a heavy rainfall. This liquidlike behavior at the surface should terminate rapidly beneath the surface of the deposit where mixing by droplet impact will not penetrate deeply into the viscous semisolid material. The strength of the deposit is confirmed by the fact that, even with high rotation rates in spray formed billets and tubes, the shape of the deposit usually remains stable. If this were not the case, the process would be unworkable.

With reference to Fig. 11, the three parameters have been examined (Ref 37) in which the thermal model of the process was used to design and analyze a set of experiments which attempted to understand the process parameters that control the resulting output parameters. In performing these experiments, it was assumed that the enthalpy of the spray at the moment of impact on the deposit (usually expressed as the equivalent fraction of liquid in the spray, /(s)) was likely to be the critical control parameter. A series of experiments were run on copper-titanium alloy billets, and previous data on In625 tubes (Ref 38, 39, 40) were reanalyzed using the model. For the billets, the process parameters were selected using the model to design experiments with a wide range of /(s) in the spray. For the tubes, the experiments were analyzed to determine the operating values of/(s) in the spray at the moment of impact. In addition, using Fig. 11, the final size and shape of each spray formed billet and tube were predicted assuming 100% sticking efficiency (SE) (Ref 3, 27). The sizes predicted were then compared with the measured deposit sizes to give a first estimate of the sticking efficiency. These estimated values of SE were then used in revised runs of the three-dimensional shape model, and the comparison was repeated until, for each spray formed deposit, the predicted and measured shapes of the deposit agreed with the fitted value of SE.

Figures 21 and 22 show the resulting measured values of SE in the two studies as a function of the modeled values of/(s). The two plots show optimum values ofyl(s) to achieve maximum sticking efficiency but at very different values in the two experiments. For the copper-titanium billets, the highest SE was 0.9 (by weight) at an optimum /(s) of 0.3. In contrast, for the In625 tubes, the optimum SE occurred at a much higher value of /(s): namely 0.5. The sticking efficiency for the In625 tubes was also enhanced by reducing the centrifugal force, resulting from a slower substrate rotational speed (Fig. 22). Some material is spun off from the rotating deposit because the sticking efficiency is decreased at higher rates of rotation. The decrease in SE at fractions of liquid higher than the optimum value is also explained by this effect.

Fig. 21 Sticking efficiency obtained by matching experimental results with the shape model for spray formed copper-titanium billets compared to the modeled fraction of liquid in the spray. Source: Ref 37

Fig. 22 Sticking efficiency obtained by matching experimental results with the shape model for spray formed tubes of In625 compared to the modeled fraction of liquid in the spray for two different rotational speeds. Source: Ref 37, 38, 39, and 40

0 0

Post a comment