Viano et al. used this test rig to investigate hot tearing in Al- Cu alloys.59
The temperature and load recorded as a function
of time are shown in Figure 12. The first derivative of the load is also plotted to determine the point where load be- gins to develop. Figure 13 shows load recorded at the solidus temperature as a function of solute content.
More recently, Davidson et al. modified the hot tear test rig developed by Instone to directly observe the hot spot region during solidification.17
A mold lid with a glass window is
located directly above the hot spot region. A video camera was used to record the images of the hot spot region during solidification. Using this technique, it is possible to observe the initiation and propagation of the tear.
Based on the idea suggested by Novikov,3 Eskine et al. devel-
oped an apparatus to measure the linear contraction of alloys during solidification.60 shown in Figure 14.61
The schematic diagram of the setup is It consists of a T-shaped mold made
of graphite and a moving block which can slide in the hori-
zontal direction along the mold cavity. A connection screw is embedded into the moving block and is used to attach the so- lidifying metal. A linear displacement sensor (linear variable differential transformer [LVDT]) is attached to the moving wall to measure the linear displacement of the solidified shell. Stangeland et al. measured the linear solidification contrac- tion of aluminum alloys using such a setup and related the experimental results to thermal strain that causes hot tearing.62
Cao et al. developed an instrumented constrained rod casting method for quantitatively analyzing hot tearing and used it to study Mg-Al and Mg-Al-Ca alloys.63
The experimental
set-up is shown in Figure 15. It consists of a steel mold, a load cell, and a thermocouple. The bottom rod is connected to the load cell at one end and a thermocouple is inserted at the junction area between the rod and the sprue, near the crack initiation location. The mold is equipped with a graphite-pouring cup with a graphite sleeve to maximize test reproducibility and graphite felt to minimize the interference of the rising tension near the sprue end of the rods. Bottom
(a) 0.25 wt% Cu Figure 12. Temperature and load development as a function of time.59
(b) 4 wt% Cu
Figure 13. Load recorded at solidus T as a function of solute content.59
International Journal of Metalcasting/Winter 11
Figure 14. Diagram of the casting mold with a moving wall.61 33
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