It is a well known practical obser-
vation that hot tearing can be reduced or eliminated in controlled casting conditions that prevent formation of large temperature and stress gradients. In line with that, preheating perma- nent molds to a high enough tempera- ture is done to alleviate or eliminate hot tearing. If feeding serves only to close the
space between separated dendrites, inad- equate feeding cannot be the reason for hot tearing; there are open and filled hot tears. Feeding is not considered in this work, although it is implied by the RDG model, which will be used here. Experiments in which several magnesium alloys were poured into preheated molds determined the criti- cal preheat temperature above which hot tearing does not occur. Magne- sium alloy AZ 91 did not experience hot tearing if the mold was preheated
Te underlying hypothesis is that hot tearing may be avoided if thermal
expansion and shrinkage of the mold and casting occur simultaneously.
to 635F (335C) or more. Among mag- nesium alloys tested, AZ 91 had the lowest susceptibility to hot tearing. Te researchers tried to simulate
similar conditions and determine what hot tearing criterion matches prior experimental observations. To test the sensitivity of the prediction, AZ 91 alloy was simulated because it has relatively low susceptibility to hot tearing and a somewhat lower pouring temperature,
which sometimes help decrease the danger of hot tears. Other hypotheses also tested were the relation between thermal stresses and mechanical proper- ties of the mushy zone, and mismatch between expansion of the mold and casting shrinkage. As the results of this simple simulation showed, strain rate could be used as a reliable predictor of hot tearing in simulations of casting magnesium alloys.
2
Procedure Commercial casting sim-
ulation software was used, with properties for AZ 91 adopted from its database.
Liquidus and solidus temperatures of AZ 91 are 1,114F (601C) and 797F (425C), respectively. Pour- ing temperature was set to 1,292F (700C), although previous research- ers used 257F to 320F (125C to 160C) superheat. Te simulation model, Fig. 1,
resembles roughly the one of two molds used in the prior studies. Te dimensions are reproduced while those not given are made to resemble the photographic record. Tis type of specimen, sometimes referred to as “harp” casting, frequently is used for determining hot tearing susceptibility. Venting holes, which continue
from the tip of spherical ends, are added in the simulation software to avoid an unnecessarily large number of control volumes. However, it was found their existence does not affect the results presented here. Control points are placed between the rod and the radius at both ends of all the rods,
Fig. 1. Geometry of the mold (one half) used in the simulation is illustrated. Only main dimensions are shown, in millimeters. Venting holes are added in the simulation software.
around 0.008 in. (0.2 mm) from the surface, in the casting and mold. Te data presented in the Results section are obtained from these points. Te casting alone is modeled with 150,610 metal cells, while the complete model has 5,528,736 control volumes. Instead of finding the pressure drop under applied strain rate, the critical strain rate that causes hot tearing was estimated. It is not easy to determine the critical strain rate precisely. It can be hard to determine exact values of the temperature gradient and the solidification rate. Te biggest problems seem to be viscosity and especially the cavitation threshold. It is well known that the magnitude of cavitation threshold is highly variable and unpredictable, partly because a valid model for the existence of cavita- tion nuclei still does not exist. The critical strain rate increases with the increase in cavitation threshold and secondary dendrite arm spacing. It is evident that this model predicts that material becomes very intolerable to imposed strain rate and very susceptible to hot tearing as the temperature
October 2014 MODERN CASTING | 39
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70