highest pouring temperature (Table 1), exacerbating oxidation. It appears that a melt cleaning measure using fi ltration or argon bubbling is necessary to enable use of 100% recycled metal. All of the foundries produced some castings that appeared completely inclusion free. For Foundries A and B, the median
inclusion areas were 2–4 times higher for AZ91D than their ZE41A coun- terparts. Also, for Foundries A and B, the maximum areas of the inclusions in the AZ91D castings were 10 and 25%, whereas in the ZE41A castings they were only 1 and 1.5%. T e inclusion as- sessment for Foundries A and B reveals that the AZ91D alloy tends toward a lesser quantity of inclusions, albeit of much larger sizes, than ZE41A alloy. A similar inclusion assessment for
the fracture surfaces of tensile samples was also conducted. T ey show similar trends, with Foundry C having the largest inclusions for ZE41A and Foundry B for AZ91D. Foundry B did not provide any ZE41A tensile samples. It is interesting to note that the inclusion areas observed in the tensile samples are much lower than those of the fracture bars. T erefore, the tensile sample fracture surfaces also can be used as a representative means to determine the relative amounts of inclusions in samples but likely underestimates their maximum size. T e particle-type Mg–Al–O inclusions in the AZ91D alloy also re- sulted in a higher measured inclusion area because they tend to be equiaxed in shape and cover a larger surface area than the fi lm-shaped Mg–O inclu- sions in the ZE41A alloy.
Mechanical Properties Whereas the ZE41A alloy is much
more susceptible to the accumulation of many fi lm-type oxide inclusions throughout the production run, the AZ91D alloy tends to collect a few large particle-type inclusions. T is diff erence in the accumulation of inclusions between the two alloys is likely a contribution of many factors, including oxidation tendencies of each alloy, melt density and viscosity which would infl uence how inclusions would agglomerate throughout the
Mar/Apr 2017 | METAL CASTING DESIGN & PURCHASING | 31
melts and alloy addition sources. T e fi lm-type inclusions in the
ZE41A were more distributed in the samples, while the particle-type inclusions in the AZ91D appeared as agglomerates with a large surface area. It is not possible to relate the decrease in the mechanical properties accord- ing to inclusion type, whether it be fi lm or particle type, because of the diff erence in alloy system (AZ91D or ZE41A) where each inclusion type was dominant. T e authors reason the large particle-type Mg–Al–O inclusions are more detrimental because of their faceted nature, larger surface area, and appearance as agglomerates on fracture surfaces. Possible future avenues for research would be to induce inclu- sions of various sizes and shapes into magnesium alloy melts and measure changes in microstructure and me- chanical properties.
Conclusions
T e types of inclusions observed in ZE41A and AZ91D magnesium alloy castings from multiple foundries were investigated. T e following are the major results: Mechanical properties decreased in
both alloys from the start to the end of the production run for all foundries. T is prin cipally depended on the increase in number and size of entrapped inclusions. Grain size increased in both alloys
from the start to the end of the produc- tion run for all foundries, especially AZ91D. For ZE41A, a loss of grain- refi ning zirconium with holding time was the attributing cause, whereas for AZ91D a transformation of grain-refi ning man- ganese–aluminum particles to less potent compositions during holding was the reason for the increase in grain size. T e fracture surfaces of tensile
samples can be used as a representa-
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