Properties of sand-cast magnesium alloys part vii: the effect of wall thickness on tensile properties of Mg-Al-Zn alloy castings

It has been shown in earlier investigations that, under favourable solidification conditions (e.g., in premium-quality castings), high mechanical properties may be obtained regardless of wall thickness. In commercial castings, however, it is not always possible (or economical) to avoid some reduction of mechanical properties in heavier sections of the casting. The present investigation was carried out to explore the effect of section thickness on properties of alloys AZ 80, AZ91 and AZ92 in various temper conditions. To eliminate other factors affecting solidification conditions, all castings were produced using standard foundry practice for green sand casting, without the use of any chills or insulating materials. Tensile properties of the castings were correlated with section thickness, grain size, surface area/volume ratio, cooling rate y and solidification time. It was found that tensile properties decrease gradually with decreasing surface area/volume ratio (decreasing cooling rate) until this ratio drops to about 3 (cooling rate below 30°C/min or 54°F/min); beyond this, tensile properties drop very rapidly, but specification minima can usually be met when the ratio is higher than 2 and the cooling rate exceeds 20 °C/min (36°F/min). The ratio between the diameter of round bars and the thickness of plate castings to obtain similar tensile properties is 1.4 to 1.7.