A novel technique for outlining
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diameter rod. The experimental procedure involves only two steps: (1) the ingot is cast as usual without addition of grain refiner, and (2) a significant amount of chopped AI-5 wt pct Ti-0.2 wt pct B alloy rods (about five times the standard amount) is rapidly added to the casting basin while the ingot is still being cast in a steady-state condition. The first step allows the alloy to establish a nongrain refined (or coarse-grained) microstructure in the solidified 'region. The second step produces a fine-grained microstructure as a result of the addition of the grain refiner. The crater profile is expected to be outlined by the large difference in grain structures in the regions before and after addition of grain refiner. Figure 1 shows a crater profile, outlined using the
Fig. 1 - - T h e solidification crater profile of 0.15 m x 0.41 m (6 in. • 16 in.) 5182 alloy DC ingot cast by the level pour method. VOLUME 23A, AUGUST 1992--2323
Fig. 2 - - S c h e m a t i c drawing of the DC casting process.
method described above, in a 0.15-m (6-in.) thick by 0.41-m (16-in.) wide 5182 alloy ingot cast by the level pour method. The melt composition of 5182 alloy was A1-4.5 wt pct Mg-0.35 wt pct Mn-0.1 wt pct Si-0.2 wt pct Fe. The initial melt charge for this ingot was virgin metal, and therefore, no innoculant particles were present in the melt prior to the addition of grain refiner. The location for the addition of grain refiner is shown schematically in Figure 2. Figure 3 shows details at the crater interface in the areas labeled A and B in Figure 1. Area A is near the ingot surface, and area B is near the ingot center. The distinct interfaces are clearly delineated by the drastic difference in grain morphology and size between the two regions. The absence of a transition zone along the crater interface between the two regions suggests that the solidification front (i.e., liquidus isotherm) is well represented by the outlined crater profile. This technique for outlining the profile of the solidification crater has also been successfully applied to a large commercial-size ingot cast by the horizontal DC method. Figure 4 is a schematic drawing of the horizontal DC casting process. In this experiment, the initial
Fig. 3 - - D e t a i l s of grain structure at the crater interface in the regions labeled A and B in Fig. 1.
2324--VOLUME 23A, AUGUST 1992
METALLURGICAL TRANSACTIONS A
ingot at the W/2 location, where W represents the width of the ingot. Note that the crater profile has been highlighted by the dotted line marked on the figure in order to make the boundary more visible. This result indicates that the sharpness of the solidification interface is not seriously affected by the presence of residual Ti and B in the initial charge prior to the addition of the grain refiner. The method described here has proven to be a simple and effective technique for outlining the crater profile of commercial-size DC ingot cast vertically or horizontally. However, this method is not limited to the DC casting process; it can be us
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