ity decreased with increasing eutectic fraction. Rare-earths and Si containing Mg alloys show a high tendency to hot tear; however, other factors such as fluidity and the presence of oxides possibly influenced hot tearing tendency.
Effects of Grain, its Size, and Morphology on Hot Tearing
Various investigators6,19,19-21,27-36 have studied the effects of
grain refinement on hot tearing. Predominance of results indi- cates that grain refinement improves resistance to hot tearing. In contrast, the work of Rosenberg et al.19
indicates that grain
refinement did not have any effect on hot tearing in non-fer- rous binary alloys. A decade later, Metz and Flemings studied the formation of hot tears by exploring the response to im- posed strain in the Al-4%Si and Al-4%Cu system.6
Metz and Flemings found that in coarse-grain castings, the development
of strength and hot tearing susceptibility coincides with the formation of a dendritic network at about 0.25 fraction solid. In grain refined castings shear strength did not develop until approximately 0.40 fraction solid. Grain refinement improved resis- tance to hot tearing by accommodating local strains. In studying the rheological behavior of Sn-15%Pb alloy, Spencer et al.34
confirmed that fine-grained materials are more resistant to hot tearing.
Warrington and McCartney studied the effect of grain structure on hot tearing susceptibility in aluminum alloys 7010 and 7050 using a test in which the solidification conditions were similar to the ingot shell zone of semi-continuous direct chill casting.37
They found that without grain re-
finement, columnar structures formed in castings having high cracking susceptibility. With moder- ate additions of grain refiner, the alloys formed equiaxed-dendritic grains and gained resistance to cracking. However, with increasing refinement and equiaxed grains, cracking susceptibility in- creased, thus a reversal was noted.
Easton et al. studied how grain refinement affects hot tearing by both experimental and modeling ap- proaches.29,30
Wrought aluminum alloy 6061 was
cast in a hot tearing rig with no grain refiner ad- dition (0.001% Ti), and 0.005, 0.01, and 0.05% Ti additions using Al5Ti1B grain refiner. The load de- velopment with temperature in the solidifying test bar was measured. Figure 5 shows the measured load versus temperature curves. Cracks and casting microstructures are shown in Figure 6. It was found that the onset of load development was delayed, and the load developed during cooling decreased with increasing grain refinement. The severity of cracking also decreased with increasing grain re- finer additions. A modified RDG (Rappaz, Drezet and Gremaud) hot tearing model38
developed by 28
Figure 4. Variation of cracking fraction of Al-Mg alloys for different levels of superheat.20
Grandfield et al.39
was used to investigate the effect of grain
refinement on hot tearing. The original RDG model was de- veloped for columnar grain structures. The modified model incorporated grain refinement effects from three perspectives:
1. Grain refinement changes grain morphology from columnar to equiaxed, and as a result the perme- ability length scale is changed from secondary den- drite arm spacing (for columnar grain) to grain size (for equiaxed grains),
2. changing the upper and lower limits of the region over which feeding occurs,
3. changing the capillary pressure by changing the liquid film thickness between grains.
From the predictions and experimental results, Grand- field et al.39
concluded that grain refinement decreased
Figure 5. Load development of alloy 6061 during solidification with different grain refinement levels. Fraction solid value and phase formation as temperature decreases are also indicated.30
International Journal of Metalcasting/Winter 11
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