The ring mold, backbone mold, cold finger test belong to the first category. Other similar tests (modified from these three tests) also belong to this category; these include: flanged bar test,7,48


cylindrical bar test,19,49 casting test,50,51 ball-bar casting test,42 “U” shape casting test,52 mark of N-Tec Ltd.) hot tearing mold test.


The N-Tec hot tearing mold is a variation of the backbone mold. The casting consists of five ‘Dog Bone’ sections of different lengths, constrained at each end, and with a hot spot along the gauge length. The hot tearing mold and the set up are shown in Figure 9. Metal is poured into the mold by means of a pouring sleeve that is located in the mold by a clamp ring. The top of the sleeve is used as the pivot point for the ladle when pouring. The latter sets the pouring height to a repeatable level and ensures accurate and consistent test results. Moreover, a heater plate is placed under the mold, so that the temperature can be precisely controlled. In the N-Tec mold, hot tearing usually appears in the longer finger sections due to increasing constrained shrinkage in the longer sections. This test provides both a qualitative and a semi-quantitative measure of hot tearing susceptibility. A hot tearing index is obtained by recording the number of cracked ‘Dog Bones’. Hot tearing (characteristic) can be obtained by plotting all visible cracks on a control sheet.


A common point in the many tests that fall within category one (see above) is that each section has a heavier end to provide a restriction during contraction. When the metal is solidified, the contraction of the section will take place, and hot tears will occur at the hot spot if the section is greater than a critical length. Hot tearing severity is evaluated by total crack length and/or the crack width, or critical length (diameter) that is tear free. These data are qualitative or semi-quantitative assessment. Moreover, internal cracks in the casting are not accounted for via such tests.


In category two tests, the approach is to investigate hot tear- ing by simultaneously measuring strength development of the solidifying metal, i.e., to measure contraction force and/ or linear displacement during solidification. Eskin et al. re- viewed numerous testing techniques for measuring the me- chanical properties of semi-solid model and commercial alu- minum alloys.5


include tensile test at higher temperatures,5,53,54


The methods direct chill


casting tensile test,55 stress


and strain measurement in half-ring mold test,5,56 tensile strain test,57 tear test rig.58,59


variable and hot


More recently,


several testing systems, which measure contraction force and/ or linear displacement devel- oped during solidification of the casting have been devel- oped and some typical ones are described below.


32 I-beam and N-Tec (trade-


Instone et al. developed a constrained solidification test rig to characterize tensile strength development and hot tearing be- havior during solidification.58,59


The apparatus simulates the


DC casting process where two solidification fronts meet at the center of the test bars and measures the load imposed in the semi-solid region during solidification. A schematic view of the mold is shown in Figure 10. The mold has a pouring reservoir at the center, which ensures that hot tears form at the central location of the bars. One test bar is used for temperature mea- surements and for microstructural examination. The other test bar is connected to a displacement transducer and a load cell to measure load and displacement developed during solidification. The schematic diagram of the test rig is shown in Figure 11. A modified Instron tensile testing machine with a 5kN load cell is used as the basis of the test rig.


Figure 10. Plan view of the test mold developed by Instone et al.58


Figure 11. Schematic diagram of Instone’s hot tear rig.59 International Journal of Metalcasting/Winter 11


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