Table 1. Casting Parameters Foundry Pouring temperature (°F/ °C) Cover gas
ZE41A A 1436/780 B C
1332/722 1470/798
AZ91D A 1436/780 B
1336/724 D 1400/760
CO2 + SF6 CO2 + SF6 CO2 + SF6
CO2 + SF6 CO2 + SF6
Other
Refined using Mg–Zr master alloy, Ladle poured Refined using Mg–Zr master alloy, Ladle poured
100% remelted metal, Refined using Mg–Zr master alloy, Ladle poured
Ladle poured Degassed using degassing tablet, Ladle poured
M-134 Flux 100% virgin metal, Refined and degassed using C2
Cl6 tablets, Poured directly from crucible
Samples obtained
Fracture bars, Tensile bars Fracture bars
Fracture bars, Tensile bars
Fracture bars, Tensile bars Fracture bars,Tensile bars Fracture bars, Tensile bars
same for both alloys and involved the production of permanent mold tensile castings and fracture bar castings from multiple foundries and characterizing them according to their mechanical properties, grain sizes, microstructures, and inclusion contents. T e microstruc- tures, inclusions, and grain sizes were characterized using scanning electron microscopy (SEM) and optical micros- copy. T e mechanical properties were assessed using uniaxial tensile testing. For both ZE41A and AZ91D alloy
castings, the average yield strength, ul- timate tensile strength, and elongation decreased between the start and end of the production run. T e results from examination of grain size, microstruc- ture and inclusion analysis indicate that the loss in properties was predominate- ly caused by the accumulation of oxides. For example, the AZ91D castings demonstrated a ~5-10% decrease in UTS and a ~20-30% decrease in elon- gation, with a smaller change in yield from start to end of production. For both alloys, an increase in grain
size was observed between the start and end of the production run, but the reduction in mechanical properties was mainly attributed to particle-type Mg– Al–O inclusions in the AZ91D alloy and fi lm-type Mg–O inclusions in the ZE41A alloy on the fracture surfaces of the tensile samples and fracture bars. T e AZ91D castings had very few inclusions, but they were much larger than those in the ZE41A castings. T is research recognized extensive variability of the inclusion levels in the industry and is a precursor to developing indus- try standards for melt cleanliness in magnesium alloys. T is will be a major step in enabling improved quality and enhanced use of magnesium alloys in aerospace and automotive industries.
Microstructure A representative micrograph from
Foundry A of the grain structures of the ZE41A castings produced toward the start and end of a produc- tion run is shown in Figure 1. T e samples were extracted from tensile mold castings. At the start of pouring, the average grain size of the castings from Foundry A was 22 ± 1 µm and increased to 38 ± 7 by the end of the production run. For Foundry C, at the start of pouring, the average grain size of castings was 27 ± 2 µm and increased to 32 ± 3 µm by the end of the production run. T e grain struc-
tures in Figure 1 were very spherical in shape, especially near the start of the production runs. With the minor grain coarsening toward the end of the production run, the grain struc- ture begins to deviate from its spheri- cal shape. Such a coarsening and de- viation from spherical grain structure during holding were expected due to zirconium losses, which may occur from reactions with iron crucibles or settling of zirconium particles over time. However, this coarsening is not expected to be signifi cant in reducing mechanical properties. T e grain structures of the AZ91D
Fig 1. Grain structures of ZE41A alloys (a) Foundry A start and (b) Foundry A end.
Fig 2. Grain structures of AZ91D alloys (a) Foundry B start and (b) Foundry B end. Mar/Apr 2017 | METAL CASTING DESIGN & PURCHASING | 29
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