Foundry C was the only foundry to use 100% remelted metal and had the highest pouring temperature (Table 1), exacerbating oxidation. It appears that a melt cleaning measure using filtration 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 cast- ings they were only 1 and 1.5%. Te inclusion assessment for Foundries A and B reveals that the AZ91D alloy tends toward a lesser quantity of inclu- sions, albeit of much larger sizes, than ZE41A alloy. A similar inclusion assessment for
the fracture surfaces of tensile samples was also conducted. Tey 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 sam- ples. It is interesting to note that the inclusion areas observed in the tensile samples are much lower than those of the fracture bars. Terefore, 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. Te particle-type Mg–Al–O inclu- sions in the AZ91D alloy also resulted in a higher measured inclusion area because they tend to be equiaxed in shape and cover a larger surface area than the film-shaped Mg–O inclu- sions in the ZE41A alloy.
Mechanical Properties Whereas the ZE41A alloy is much
more susceptible to the accumulation of many film-type oxide inclusions throughout the production run, the AZ91D alloy tends to collect a few
large particle-type inclusions. Tis difference 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 influence how inclusions would agglomerate throughout the melts and alloy addition sources. Te film-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 film or particle type, because of the difference in alloy system (AZ91D or ZE41A) where each inclusion type was dominant. Te authors reason the large particle-type Mg–Al–O inclu- sions are more detrimental because of their faceted nature, larger surface area, and appearance as agglomerates on fracture surfaces. Possible future
50 | MODERN CASTING March 2017
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 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94