Fig. 1. The top half of a permanent mold has been removed to show the solidifi ed test casting in the cavity, just before ejection.
subsequent casting. This results in a smaller as-cast grain size while reduc- ing the tendency for hot cracking. The new alloy, designated B206 by the Aluminum Association, could become a practical material choice for automo-
Online Resource
Visit
www.metalcastingdesign. com to read the full paper, “Heat Treatment and Corrosion Resis- tance of B206 Alloy,” D. Jean, J.F.
Major, J.H. Sokolowski, B. Warnock and W. Kasprzak, 2009 AFS Transactions (09-040).
Fig. 2. Cracks in the permanent mold test casting appeared near where the arms joined the heavier section.
tive and aerospace applications where weight reduction is critical. Casting trials were undertaken at
Nemak’s Research and Development Laboratories in Nuevo Leon, Mexico, to determine the castability and mechani- cal properties of B206 and its various alloy compositions. According to the results of the trials, hot tear sensitiv- ity was found not to depend on alloy composition over the ranges studied. Mechanical properties were controlled primarily by the solidifi cation rate,
soundness and, to a lesser extent, alloy composition. The best combination of mechanical properties was obtained when B206 alloy castings were heat treated to the T4 temper.
Casting Trials Twenty alloys were prepared for the
casting trials. A single element was added at a time to a base alloy composition to demonstrate the effect of each element on the alloy properties. Two hot crack tests were employed.
The fi rst was performed on permanent mold castings (Fig. 1). The test casting had a sharp radius at the points where the arms joined the heavier sections at each end, so the stress generated by shrinkage of the six arm sections dur- ing freezing was concentrated into a small area (Fig. 2). Each casting was examined and
given a numerical hot cracking number according to the following scheme: 1 point for a fully broken arm, 0.75 points for a severe crack, 0.5 points for a modest crack, 0.25 points for a crack detectable only under a magnify- ing glass and 0 points when no cracks were present. The number for each arm was added to produce a total for each of the fi ve castings poured. Permanent mold hot tear castings
also were poured from 319 alloy to establish a relative calibration for the test results. Aluminum 319’s hot tear- ing number was determined to be 0.75. Table 1 shows the crack rating for the B206 alloys’ compositions. The second test was performed on
nobake sand molded castings. The design of these castings was similar to the permanent mold casting, except each of the arms was 1 in. longer. No cracks were found in any of the sand
38 Metal casting Design anD purchasing March/april 2010
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