Grain Structure in Spray Formed Deposits

The defining feature of the microstructure of spray deposited materials is the fine equiaxed grain structure with minimum solute segregation (Ref 2, 3, 4). It is generally agreed that the fine grain size arises from dendrite fragmentation on impact of the spray followed by grain coarsening during solidification. The times for solidification increase with the fraction of liquid in the spray at a given geometry (Fig. 24). However the coarsening is slower than that predicted using standard coarsening theory, which results in a small grain size (Fig. 25). A detailed investigation of coarsening under conditions of spray forming was carried out by Annavarapu and Doherty (Ref 32) using aluminum AA2014 (Fig. 26) and copper-

titanium (Fig. 27) reheated into the solid-liquid phase field. It was found that the measured coarsening rates were much slower than those expected for dendritic structures, and the coarsening rate fell as the fraction of solid in the mixture increased. This is in qualitative but not quantitative agreement with the experimental results for copper-titanium (Fig. 14). Other studies on grain coarsening in spray formed structures (Ref 44, 45) yield similar results. Slower coarsening in solidliquid mixtures with increasing solid fraction is, however, in direct conflict with experimental studies (Fig. 28) and modeling of coarsening in high purity two phase solid-liquid systems (Ref 32). In these studies, coarsening is accelerated as the volume fraction of solid increases due primarily to shorter diffusion distances. At the highest volume fraction of solid where the liquid is present as small droplets trapped at the boundaries, coarsening is controlled apparently by the drag of these droplets (Ref 32). This conflict appears to arise from the presence of solid second phase impurity particles in the spray formed material which inhibit grain boundary movement (Ref 32, 45). Currently this is established qualitatively, and a detailed quantitative investigation of this phenomenon is required to allow the thermal modeling of deposit solidification to yield reliable predictions of the resulting grain size in different parts of the deposit. Figure 29 shows the result of an experimental study of the grain size variation in a spray formed deposit (Ref 37).

Fig. 24 Thermally modeled total local solidification times, tf(s), in copper-titanium billets at an axial location 25 mm from the base of the deposit for different fractions of liquid in the spray. Source: Ref 3

Fig. 25 Measured and thermally modeled grain sizes in copper-titanium billets as a function of the fraction of liquid in the spray. das-f dendrite arm spacing-solidification time correlation. Source: Ref 3

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Fig. 26 Experimentally measured coarsening rates for 2014 (Al-Cu-Mg-Fe-Si) both as conventional dendrite arm coarsening (KD = 782 m3/s), and for reheated spray cast Al 2014 at fs = 0.65 (625 °C, K = 393 m3/s), fs = 0.85 (600 °C, K = 186 m3/s), fs = 0.89 (575 °C, K = 140 m3/s), and fs = 0.93 (550 °C, K = 80 m3/s). Source: Ref 32

Fig. 27 Experimentally measured coarsening rates for copper-titanium both as conventional dendrite arm coarsening (KD = 750 m3/s) and for reheated spray cast copper-titanium billet at fs = 0.62 (1015 °C, K = 91

m3/s), fs = 0.73 (1000 °C, K = 40 m3/s, and fs = 0.86 (975 °C, K = 5 m3/s). Source: Ref 32

Fig. 28 Measured normalized coarsening rates; Kp normalized by the value of Kp at a fraction solid, fs of 0.6, as a function of fs. This is the situation in high purity binary alloys containing one solid phase. Source: Ref 2, 32

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Fig. 29 Measured grain sizes as a function of location in a spray deposited billet of In690. Source: Ref 37

The physical processes that give rise to the fine grain structures in spray forming also give rise to another desirable characteristic, low microsegregation and macrosegregation. Insights into these phenomena were reviewed by Biloni and Boettinger (Ref 46). Rapid solidification to a high fraction of solid (giving fine scale solid particles that are then allowed to cool more slowly with significant coarsening) is expected to result in decreased microsegregation. In addition, the fact that the deposit forms with a high fraction of solid reduces significantly the solidification shrinkage within the deposit, which combined with the restricted permeability of the fine structure, should limit severely the fluid flow processes that cause macrosegregation. These ideas appear to fit qualitatively with the experimental observations of homogeneous spray formed deposits (Ref 4). However few attempts have been made to test the ideas of the expected reduced microsegregation and macrosegregation quantitatively. This is an area that needs additional study.

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