In another study,24
Gjestland et al. saw that the flow pattern
of the metal into the casting had a much larger influence on elongation. Using three different gating configurations, a series of tensile bar casting samples were created. The gating design with the longest fill time yielded the lowest elongation-to-failure values (7.4 +/- 2% vs. 13.3 +/- 3% for the other two designs.) The authors also concluded that fill rate was significant. They found that consistent mechanical properties were obtained in samples that had a constant, fast- filling rate; in samples cast at a non-constant rate (such as an early partial filling, a period of “stagnation” and then further flow to fill the part), the overall solidification was slower and the defects in the casting increased.
Wood et al.9 examined excised samples from a thin-walled
AM60 casting. They performed tomography on the samples prior to testing and found a correspondence between the level of microporosity and the elongation-to-failure. Their results showed the lack of a porosity defect band present in the microstructure. Additionally, their results indicated that different levels of porosity were present in different regions of the casting; the regions of porosity appeared to be ran- domly distributed throughout the section.
Wang et al.25 also report deterioration of the mechanical
properties in the AM60B component as a function of loca- tion. They suggested that this was due to defects present in the melt. They found that for oxide contents below 1200 ppm, however, the location where the cast coupon was ex- cised plays a larger role in the behavior observed than the oxide content itself.
An article by Cao and Wessen26 discussed the influence of seg-
regation bands on the mechanical properties of Mg alloys. They noted that there were two types of segregation bands. The Type 1 segregation bands were determined by the level of ESCs pres- ent in the sample. ESCs tend to form more readily in Mg than Al because much less heat needs to be extracted for solidifica- tion to occur. Mg and Al have similar specific heat, latent heat of fusion and melting points but because Mg has a much lower density than Al, the amount of energy for solidification is less for a given volume of material. Cao et al., found that in areas characterized by unidirectional flow of the molten metal, the segregation bands pushed the ESCs to the center of the sam- ples; this was seen in the large flat areas found in specific com- ponents as well as in test plates. In areas where there was more turbulence, such as wide or corner areas in actual components, the ESCs tended to be distributed randomly.
The Type 2 segregation bands were not characterized by ESC placement and instead were related to a change in the appearance of the microstructure close to the edge of the sample. The micrographs presented showed a change in contrast possibly related to chemical composition or poros- ity—the authors made no explanation and used this category as a “catch-all.” They did report that both types of segrega- tion bands could be observed in the samples they examined.
International Journal of Metalcasting/Winter 2012
Cao and Wessen found that the defect level decreased with increasing casting temperature and fast shot velocity. Inten- sification pressure increases tended to produce castings with considerably fewer defects as defined by their scale. They also performed 3-point bend experiments and found a slight decrease in maximum load and deflection at break with in- creased defect level.
In a later study, Cao and Wessen27 described the mod-
eling of some samples of Mg alloys with between 3-15 wt % Al. These samples were produced under carefully controlled solidification conditions where unidirectional solidification was performed. The authors found that in- creasing Al content led to increasing hardness and yield strength, but decreasing fracture elongation. The authors also noted that they could increase the hardness of the eutectic phase itself by increasing the cooling rate. They suggested that in samples that are relatively defect-free, eutectic volume fraction and hardness could have an in- fluence on the overall tensile behavior.
In summary, as the preceding review of the literature has detailed, there is a substantial amount of disagreement with regard to level of influence that the microstructure has on the mechanical behavior in Mg castings. There is, however, agreement that the processing parameters and geometry play a considerable role.
experimental information
To create the samples needed for this study, a complex- shaped test casting, designated “Ladder” casting was used (Figure 1). This casting was designed to create a wide range in flow lengths and cross-sectional areas for research pur- poses. These castings had a minor amount of external, di- mensionally-small cross-ribbing to help the flow of metal into the casting. This did, however, result in the gage length section of some of the samples to have these ribs.
A set of 20 castings were obtained as part of a larger Design of Experiments (DOE) study. In this study, the effects of chang- ing two processing parameters (the fast shot speed and the oil
17
They also indicated that the segregation bands did not form in thinner (~2 mm) sections of the AM50 component cast- ings examined and that thicker sections (>3 mm) were more sensitive to this phenomenon.
In the same study,26 Cao and Wessen also attempted to quan-
tify the effect of processing parameters on the defect popu- lation in the samples. They used a qualitative procedure of examining x-rays of various samples and rating the defect population on a scale from 0 to 2. A rating of 0 corresponded to a case where no obvious tearing or porosity could be de- tected and higher ratings were given in those samples where those defects were evident. The rating system was then aver- aged across 12 samples for each condition investigated.
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