AZ91HP, when poured at 690C (1274F), into a ZP131 mould coated using MAGCOAT achieved the maximum tensile strength. Aluminium alloy castings attained rela- tively lesser UTS values compared to normal expected levels. Metallographic examination of the aluminium samples suggests a modified eutectic morphology, as shown in Fig. 3. The initial suspicion was that aluminium was reacting with the mould materials, and perhaps, was not suitable for casting in these moulds.
The morphology of the plate-like constituent of Si par- ticles appears to be a result of no strontium modification, perhaps due to strontium burn-off, which is a known phe- nomenon. The resulting large brittle, plate-like silicon structure is detrimental to the mechanical properties.24 The silicon needles are believed to promote stress con- centrations, leading to a reduction in material toughness and typical brittle fractures.25
Fractography analyses indi- cate the presence of silicon particles on the fractured sur-
faces of current tensile test specimens, acting as centres of crack initiation and growth. Apart from the size and shape of the eutectic silicon particles, Fe intermetallics, mani- fested as large plate-like iron particles also affect the ten- sile and fracture properties of A356 alloys.26
The size of
the Fe intermetallics in A356 aluminium alloys depends on the cooling rate and the amount of iron present in the alloy. While higher cooling rates decrease the plate size, increasing iron content increases the same and an inverse logarithmic relationship exists between the Fe intermetal- lic particle length and the percent elongation and UTS of unmodified A356 permanent mould cast samples. From the photomicrograph in Fig. 4, obtained from one of the samples of the current work, an Fe intermetallic particle of around 200µm can be seen, which could lead to reduced ductility and strength.27
Further, macro examination of
the specimens revealed rounded and evenly distributed pores as shown in Fig. 5, indicative of hydrogen porosity, which could have led to the poor tensile strength, as there
Figure 3. A356 casting produced in ZCast moulds poured at 770°C (1418°F) using Isomol 200 mould coating shown at 200X (σUTS
= 134.30MPa δ = 0.74% HB = 66.70 Grain size: 2.45).
Figure 4. A photomicrograph showing plate-like Fe intermetallics in A356 casting produced in 3D printed moulds.
Figure 5. These improperly degassed A356 specimens show signs of hydrogen porosity in micro and macro structures. (Left) Microporosity
(Right) Macroporosity International Journal of Metalcasting/Summer 2011 31
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