The solidified grain structure (macrostructure) of castings is predicted by process simulation using a newly extended front-tracking technique which models the growth of solid dendritic fronts through undercooled liquid during metallic alloy solidification. Such fronts are either constrained, as is the case with directed columnar growth from mould walls, or unconstrained, as is the case for multiple equiaxed growth from individual nucleating particles distributed throughout the liquid. Non-linear latent heat evolution is treated by incorporating the Scheil equation. Thermal conductivity changes with the solid fraction. A log-normal distribution of activation undercooling to initiate free growth from equiaxed nuclei is used, and the routines to deal with such growth followed by impingement of dendritic grains upon one another are verified by comparison with the results of analytical studies of simplified systems. The extensions to the model enable the predictions of equiaxed grain structure and, importantly, the columnar to equiaxed transition in inoculated alloy castings. The model is validated via comparison with experimental results. The front-tracking method is proposed as a suitable formulation for modelling alloy castings that solidify with a dendritic structure, either columnar, equiaxed, or both.
In this article, the columnar-to-equiaxed transition (CET) in directionally solidified castings is investigated. Three CET prediction methods from the literature that use a simulation of the growing columnar front are compared to the experimental results, for a range of Al-Si alloys: Al-3 wt pct Si, Al-7 wt pct Si, and Al-11 wt pct Si. The three CET prediction methods are the constrained-to-unconstrained criterion, the critical cooling rate criterion, and the equiaxed index criterion. These methods are termed indirect methods, because no information is required for modeling the equiaxed nucleation and growth; only the columnar solidification is modeled. A two-dimensional (2-D) front-tracking model of columnar growth is used to compare each criterion applied to each alloy. The constrained-to-unconstrained criterion and a peak equiaxed index criterion agree well with each other and some agreement is found with the experimental findings. For the critical cooling rate criterion, a minimum value for the cooling rate (between 0.07 and 0.11 K/s) is found to occur close to the CET position. However, this range of values differs from those cited in the literature (0.15 to 0.16 K/s), leading to a considerable difference in the prediction of the CET positions. A reason for this discrepancy is suggested, based on the fundamental differences in the modeling approaches.
Titanium alloy powder used for laser-based powder bed fusion (L-PBF) process is costly. One of the solutions is the inclusion of a powder recycling strategy, allowing unused or exposed powder particles to be recuperated post manufacture, replenished and used for future builds. However, during a L-PBF process, powder particles are exposed to high levels of concentrated energy from the laser. Particularly those in close proximity to the melt pool, leading to the formation of spatter and agglomerated particles. These particles can settle onto the powder bed, which can then influence the particle size distribution and layer uniformity. This study analysed extra-low interstitial (ELI) Ti6Al4V (Grade 23) powder when subjected to nine recycle iterations, tracking powder property variation across the successive recycling stages. Characterisation included chemical composition focusing upon O, N, and H content, particle size distribution, morphology and tapped and bulk densities. On review of the compositional analysis, the oxygen content exceeded the 0.13% limit for the ELI grade after 8 recycles, resulting in the degradation from Grade 23 level.
The Bridgman furnace is widely used in industry and research. This paper outlines a working 1-dimensional model for tracking the columnar solidification front in a Bridgman furnace where the pulling velocity, and hence front position, change as a function of time. The front tracking model is applied to a fixed grid of control volumes using an explicit numerical finite difference scheme to solve the heat equation over a finite domain. The model is demonstrated by way of a notional scenario, namely, Bridgman furnace solidification of a 16-mm diameter rod of Al7wt.%Si. The results show how the evolution of temperature distribution, thermal history, and front position are affected by a step change in pulling velocity.
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