Investigation of the effect of bismuth on the squeeze cast ZA-27 alloy

Abstract

Squeeze casting is specified as a technique where solidification is promoted under high pressure within a re-usable die hence, combining the permanent mould casting and die forging in a single operation. Squeeze casting has a greater potential to produce less defective cast components contrary to the other conventional casting techniques. In the present study, the aim was to search for the improvement of the mechanical properties of the ZA-27 alloy in three different pressure ranges in conjuction with the addition of various percentages of bismuth element. For this purpose; ZA-27, ZA-27+0.5 wt%Bi, ZA-27+1.0 wt%Bi, ZA-27+1.5 wt%Bi, i.e. in total 54 samples were cast under 0, 50 and 100 MPa squeezing pressures. Three sets of the castings (36 pieces) were machined for the tension test, microstructure examination and machinability test, and the remainder castings were prepared for hardness test, chemical composition determination and surface roughness. While doing this thesis, 36 tension tests, 12 SEM microstructure analyses were done. 85 SEM photographs, 140 optical photographs were taken to understand the microstructure-property relations, A lot of chip lengths were measured from 12 different type of parts to understand the machinability characteristics, 288 Brinell hardness measurements were taken from 12 parts to understand hardness variation, 120 roughness measurements were done in 12 parts to determine roughness variation, density calculations of 29 parts were done to determine the effect of squeezing pressure on density. It was found that, when the bismuth concentration was increased the machinability characteristics were increased, too. Also, the color of the bismuth added castings were darker than the pure ZA-27 alloys. As the squeezing pressure was increased, the porosity levels of each batch were decreased and the densities were increased. It was seen that while applying squeezing pressure to the semi-solid mixture, the lower part of the die solidified before the upper part. To eliminate this, a batch of glass wool was put under the die for insulation.