Numerical investigations on solute transport and freckle formation during directional solidification of nickle-based superalloy ingot

In order to investigate the solute distribution and freckles formation during directional solidification of superalloy ingots, a mathematical model with coupled solution of flow field, solute and temperature distribution was developed. Meanwhile, the reliability of this model was verified by the experimental and simulation results in relevant literatures. The three-dimensional directional solidification process of Ni-5.8wt%Al-15.2wt%Ta superalloy ingot was simulated, and then the dynamic growth of solute enrichment channels was demonstrated inside the ingot. Freckles formation under different cooling rates was studied, and the local segregation degree inside the ingot was obtained innovatively after solidification. The results show that the number of freckles formed at the top gradually decreases, and so do the degree of solute enrichment at these freckles with the increase of cooling rate. Moreover, the relative and volume-averaged segregation ratio is defined to describe the segregation degree inside the ingot. The span of relative segregation ratio for positive segregation is wider than that for negative segregation, but it accounts for less of total volume. As the cooling rate increases from 0.1 K/s to 1.0 K/s, the proportion of weak segregation (-20%~20%) increases significantly from 26% to 41%, so that the segregation degree is weakened in general. By analyzing the freckles formation and segregation degree inside the ingot, the numerical simulation results can provide a theoretical basis for optimizing the actual production process to suppress the freckle defects.

Characteristics and Formation Tendency of Freckle Segregation in Electroslag Remelted Bearing Steel

Undesirable macro segregation defects, freckles, restrict the commercial production of large-sized electroslag remelting (ESR) bearing steel ingots through degradation of the mechanical properties and service lifetime. In order to clarify the freckle characteristics and formation tendency as well as the formation mechanism, freckles from an industrial large-sized GCr15SiMn ESR ingot were investigated through structural and compositional analysis, along with simulation calculation. The results show that freckles consist of (Si, Mn, Cr)-enriched equiaxed grains and occur in about the 1/2 radius region at the middle-upper part of the ESR ingot, where the secondary dendritic arm spacing (SDAS) and solidification front angle are large but cooling rate is small. The absolute value of relative Rayleigh number, Ra, also reaches its maximum in the 1/2 radius region, with a liquid fraction of 0.3–0.5, corresponding to the region where freckles form. Based on the experimental and simulation results, to evaluate the freckle formation in industrial-scale GCr15SiMn ESR ingots, the threshold value of relative Ra, a freckle criterion considering the compositional and thermal effects, was determined to be about −0.023.

Beta Fleck formation in Titanium Alloys

Beta fleck is a troublesome segregation defect in many titanium alloys. It has previously been investigated by several authors and appears to have two formation mechanisms, one similar to that of “freckle” in steels and nickel-base alloys, the other arising in the “crystal rain” effect seen in conventional steel ingots. The freckle defect has been extensively studied and several theories developed to account for its formation in both remelted ingots and directional castings. In this work we compare the findings of investigations into the nickel-base freckle formation mechanism to similar conditions in the vacuum arc remelting of titanium alloys. We find that there are strong similarities between the beta fleck formation conditions and the parameters related to the Rayleigh Number criterion for freckle formation. In particular, the dendritic solidification parameters and the density dependence on segregation coefficients both fit well with the conditions proposed to characterise freckle formation. The second formation mechanism arises in the columnar to equiax transition in solidification. The condition for the avoidance of the defect in the two cases is the shown to be the same, namely the use of a very low VAR melting rate, but that it is unlikely to be 100% successful in preventing defect formation. We propose that the techniques presently in use for alloy development in the superalloy field through optimising the composition for minimum sensitivity to freckle formation should be applied to the formulation of future titanium alloys; also that attention should be paid to developing the PAM process to provide suitable solidification conditions for defect absence in a final ingot.

Freckle formation and prevention in high strength low alloy steel ingots

Freckles considerably limit the development of larger electro-slag remelting (ESR) ingots. To simulate the freckling conditions in ESR ingots, high-strength low alloy (HSLA) steel was solidified with varying solidification front angles. Note that the freckling potential is enhanced and the orientation of freckle channel tends to be in the vertical direction with increase in solidification front angle. This is because the tilted solidification front contributes the available buoyancy and transports light (Si, Mn, Cr)-enriched liquid flows upwards toward the mush zone and then accumulates in the open segregation channel. For freckle formation in experimental HSLA steel ingots, a modified Rayleigh number (Ra) that considers the anisotropy of permeability and solidification front angle was evaluated and a threshold value of Ra that separates the freckled and freckle-free area was determined to be 0.79.