Matsuda et al. studied the effect of added elements on solidi- fication crack susceptibility in the Al-2%Zn-2-3%Mg system (welding alloy).33


The elements added were Ti + B, Ti, Zr, Fe,


Mn, B, Si, Be, Ni, Cr, V, Mischmetal, and Cu, and the added amount was up to 0.5% (up to 0.06% for Ti-B). Experiments were conducted using the ring casting crack test, and crack length was used as an index to measure susceptibility. It was found that among the thirteen added elements the most favor- able ones were Ti + B, Ti, and Zr, and the detrimental element was Cu. The effects of Ti + B, Ti, and Zr, are shown in Figure 7. The optimal amounts were found to be greater than 0.05% for Ti-B, 0.14% for Ti, and 0.24% for Zr. It was observed that when several elements were added together, the elements reacted with each other with unexpected results. For example, small addition of Ti was favorable to the Ti-B addition, but small addition of Zr canceled the beneficial effect of Ti-B. The effects of these elements are related to their grain refining contribution on the alloy system. When large columnar grains are dominant, the crack length is likely to reach the saturated value. When the grains are equiaxed, and thus smaller in size, the crack length (hot tearing susceptibility) is reduced; the re- lation between the measured crack length and mean grain size is shown in Figure 8.


Clyne and Davies20 studied the effect of grain refinement for


two Al-Mg alloys; one being Al-2%Mg, which has a low cracking susceptibility, and Al-1%Mg, which has a high cracking susceptibility. Both alloys had a starting columnar grain structure with no grain refiner (Ti) addition. It was found that the low susceptibility alloy showed an increased crack- ing tendency over a narrow range of Ti contents, although its grain size was reduced and had become equiaxed. However, the high susceptibility alloy was unaffected by Ti additions except at high levels (>0.2%Ti) even though the grain was altered from columnar to equiaxed. In sum, they pointed out that there was a complex interaction between impurity con- tent, grain structure, and cracking susceptibility.


Pumphrey et al.’s experiments demonstrated the effect of grain morphology.21


In their study the grain structure


changed from columnar to transitional to equiaxed in Al-Si, Al-Cu, Al-Mg, and Al-Zn binary systems with increasing al- loy element content. This morphology change corresponded to the crack length reductions. Their results are in agreement with practical foundry shop observations.


Processing Parameters Melt Superheat (Pouring Temperature) As pointed out by Pellini,15


Several studies show that the effects of superheat is a func- tion of the test method utilized, and other factors such as cooling rate, presence of grain refiners, and metal fluidity, etc. need to be considered.43


In preparation of this review


has clearly pointed out, the hot tearing is the result of many factors, and by measuring crack length, or the development of a crack(s) is an indirect and convoluted approach. By re- lating a cause to a symptom is not helpful. What is needed is a discriminating and quantitative test that relates the cause to the development of stresses in the casting, which gives rise to the development of hot tears.


Mold Temperature


Mold temperature directly affects casting cooling rate and thus the resultant cast microstructure and its performance. In fact, most studies of hot tearing control the cooling rate and the local solidification time by controlling mold temperature, i.e., Clyne and Davies20


and Spittle and Cushway’s.27 Lim-


ited work on this topic was found in the published literature. Bichler et al.41


“at any meeting where the subject


of hot tearing is discussed, the steel foundry community would divide into groups with distinctly opposite opinions regarding carbon and pouring temperature effects depending on the gen- eral types of casting produced and the foundry practices used by the individuals expressing the opinions”. For example, in an earlier study of hot tearing of steel, Singer et al. believed


30 studied the effects of mold temperature on Mg


alloy, AZ91D. The tests were conducted at pouring temperature of 700C (1292F) and mold temperatures of 140, 180, 220, 260, 300, 340, and 380C (284, 356, 428, 500, 572, 644, and 716F). It was found that mold temperature had a significant effect on hot tearing. Hot tearing severity decreased progressively with increasing mold temperature. Bichler’s work shows that a mold temperature of 220C (428F) was critical, which corresponded to a cooling rate of 18-20C/s. At mold temperatures below 220C (428F) cracks initiated from all surfaces, propagated towards the center, and were connected across the entire cross section.


International Journal of Metalcasting/Winter 11


that high pouring temperature would minimize hot tearing.40 While, Middleton and Protheroe showed that hot tearing was likely to occur and was more severe at high casting tempera- tures than at low temperatures.9


These conflicting experimental


results and contradictory opinions were also seen in non-ferrous alloys. Pumphrey et al. studied six aluminum binary alloy sys- tems and their experimental results showed that at any given al- loying element level the cracking susceptibility decreased with decreasing superheat.21


alloy, AZ91D, Bichler et al. found that the variation of pouring temperature did not have any significant effect on hot tearing.41 It is most unfortunate, however the literature is full of such con- tradictions with respect to alloy factors, as we have seen in the previous section, as well as processing parameters.


Couture and Edwards42 thought that two factors have a play in


this controversy. A high superheat might spread the hot spot, which would reduce hot-tearing tendency; and a high superheat might also increase liquid film life, which would increase the tendency to hot tear. However, Briggs thought high superheat levels can increase the temperature gradients during solidifica- tion and result in the promotion of columnar dendritic growth.8 Generally, alloys with columnar structures have higher hot tear- ing tendency than alloys with equiaxed structures.


However, in the study of a magnesium


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