Grain Refiner Can Reduce Hot Tearing in Permanent Mold B206
resistance and increase sensitivity to titanium-based grain refiners. However, B206 still remains prone to the defects, particularly when cast via permanent mold due to the rigidity of the metal mold. Hot tears or cracks form during casting solidification due to insufficient feeding in areas of elevated stress, such as thermal stresses generated during solidification and mechanical stresses from restrained casting contraction. Metalcasters have found that grain
M
refiners improve an alloy’s resistance to hot tearing. In a recent study testing grain refiners, hot tears appeared in a permanent mold test casting made of unrefined B206. The tears formed at the bar’s midsection, since the restraint at one end of the casting bar and the 90-degree corner at the downsprue on the other end prohibited the bar’s free contraction during solidification. The addition of titanium-boron (Ti-B) grain refiner eliminated the hot tears. The reduction of hot-tearing tendency was most significant when titanium additions exceeded 0.02% by weight. The effect of grain refinement
on casting porosity followed a trend similar to that of the grain sizes. With an increased titanium addition up to 0.05%, the casting porosity decreased (Fig. A). As the grain sizes decreased, a consistent distribution of liquid flowed through intergranular regions. Further, the intermetallic phase dispersed ho- mogenously along the grain boundar- ies, creating additional feeding paths for liquid. Consequently, liquid metal was able to compensate casting shrinkage, which decreased the alloy’s tendency to form shrinkage porosity and hot tears. Examination of cooling curves re-
vealed that titanium addition decreased the freezing range of the alloys, so the solidifying alloys remained in the semi-solid state for a shorter time and were less prone to hot tear formation. With decreased solidification ranges, the duration of mass feeding of liquid metal was extended, and porosity and hot tears were reduced.
METAL
This sidebar was adapted from the 2009 AFS Transactions Paper, “Effect of Ti-B Grain Refiner on Hot Tearing in Permanent Mold Cast B206 Alu- minum Alloy.”
March/april 2010 Metal casting Design anD purchasing 39
anipulating the titanium content of 206 aluminum alloy has been shown to improve hot tear
Fig. A. Porosity in B206 alloy decreased with up to 0.05% titanium.
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