Rodrigo et al.,14
examined the effects of section thickness on
the elongation-to-failure in AM60; samples were HPDC into a special mold containing flat plates as well as round (6.4 mm diameter) tensile bar samples. In testing samples cut from the as-cast plates, they found that as the section thickness increased, the failure strain also increased; the elongation-to- failure values varied from 3.7% and 8.0% (1 and 2 mm thick plates, respectively) to 11.6% and 12.7% (5 and 10 mm thick plates, respectively). Further, tests on one as-cast round sam- ple yielded an elongation-to-failure of approximately 16%.
Rodrigo et al.,15 also measured the bulk porosity in these
castings using the Archimedes method; the results varied be- tween 1.7% and 2.2% porosity present. However, Rodrigo, et al., found that these results did not correlate with the elon- gation or section thickness. A subsequent examination of the microstructural features15
revealed that there were increases
in the “skin” thickness with increasing section thickness. They observed pore bands in samples with 2mm and greater section thicknesses; the samples having a 1mm section did not exhibit pore bands. Additionally, Rodrigo et al. reported that, as section thickness increased, the eutectic phase frac- tion at the surface of the samples decreased. Their study ob- served that the 1 mm thick sample had a eutectic fraction of 21.6%; as the section thickness increased from 2 to 5 to 10 mm, the eutectic fraction decreased to 10.2%, 7.2%, and 5.5%, respectively.
Mao et al.,16 reported a significant difference in cell size be-
tween the edge and the middle with 7 µm and 17 µm respec- tively in a 2 mm as-cast sample. They also reported the pres- ence of a “skin” effect due to the differences in Knoop hard- ness measurements taken from the edge and middle of the samples. Further, they observed a general trend of increasing porosity with increasing section thickness using measure- ments from the Archimedes method; however, those authors did concede that this trend may be due to a number of factors beyond section thickness alone. In contrast to Rodrigo et al., Mao et al. found that the skin thickness was constant regard- less of section thickness.
In addition to the above studies, several other researchers have also reported the presence of defect bands in the cross- section of Mg castings. These defect bands have contained various microstructural features that have been reported to influence the mechanical properties of samples. In several studies, Dahle et al.17-19
defined defect bands as “continuous
layers of porosity or segregation that follow the contour of the casting surface.” Dahle et al. claimed that these bands arose from shearing of the mushy zone during filling and so- lidification in the HPDC process. Similar results were found by Lauki et al.20
for an AM60B alloy. Lauki et al.20 studied the presence of another group of mi-
crostructural features, externally solidified cells (ESCs). The ESCs are crystals that form due to thermal undercooling of the molten metal following pouring into a shot sleeve;
16 Bowles et al.22 also used die cast plates of AZ91 and AM60
to examine the role of microstructural features on the me- chanical properties. However, they reached some contradic- tory conclusions compared to the studies cited previously. They found that the cell size did not change dramatically from the edge of the sample to the center. They saw that large ESCs did tend to be concentrated in the center of the sample; and they proposed a definition of the “skin” as an area free from the ESCs. Bowles concluded that there is no correlation between ductility and: 1. the area fraction of porosity when <1.5%; 2. the presence of ESCs; and 3. the depth of skin thickness, even though a defect band was observed.
Further, Bowles et al. found that there was an increasing fraction of ESCs towards the center of the samples. Even though this had no effect on ductility, there did appear to be a slight influence on the yield strength. They concluded that the increased presence of the eutectic phase near the surface of the cast plates had an effect on the ductility when the bulk porosity was below 1.5%.
A number of published studies examined the influence of mold geometry and HPDC processing parameters on the re- sultant microstructure and mechanical properties. Gjestland et al.23
Reference 20 provides a more detailed description of the thermodynamics and kinetic necessary for the formation of ESCs. Lauki et al. found that the ESCs were grouped toward the middle of the cross-sectional area of the Mg castings, in contrast to being randomly distributed as is the case for Al-based HPDC castings. In a study of AM50 and AM60 plate material, Chadha et al.21
suggested that the presence of
defect bands caused the uneven distribution of porosity in samples. In contrast to the studies detailed in References 14- 16, Chadha et al. found that, as the porosity level increased, the elongation to failure in the samples decreased.
hypothesized that the mechanical property differences
in different locations in the casting were due to differences in the cooling rate of the metal in those locations. The cool- ing rates were, in turn, influenced by the processing param- eters, the geometry, and the alloy properties.
One process parameter examined by Gjestland et al.23 was
the degree of cooling occurring in the shot sleeve prior to injection; to do this, a set of experiments was conducted on uninsulated and insulated shot sleeves. Compared to a stan- dard, uninsulated shot sleeve, Gjestland et al. reported that insulating the shot sleeve produced only a slightly higher average elongation-to-failure (14-15% vs. 12% for the un- insulated sleeve.) More significantly, they observed that the uninsulated sleeves produced a greater degree of scatter in the elongation data. Examining the microstructures of both sample sets, they found that tensile specimens cut from the uninsulated shot sleeve had a higher density of ESCs present in the microstructure.
International Journal of Metalcasting/Winter 2012
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