ABSTRACT
Aluminum on its own is soft, brittle and has low tensile strength, so it is not readily used engineering construction work. To improve on this, draw back this study aimed at improving the tensile strength, hardness and microstructure of aluminum by alloying it with magnesium. The casting was carried out using sand casting.
In cause of this study 20kg of aluminum scrap were purchased at Uwelu Motor Spare Parts, Benin City and were degreased with premium motor spirit (PMS), while the magnesium alloy were purchased at Owode-oniran market Lagos State, and were grinded to powder form. The sand mold was prepared using green sand with the gate located at the bottom of the mold to allow easy flowability of the molten metal. The patterns for the different test were made, tensile strength pattern 200mm by 14mm and hardness pattern 250mm by 25mm. These patterns were placed in the mold and covered with the molding sand and rammed. Aluminum scrap of 2kg was melted in a crucible furnace and heated to a temperature of 7500C and impurities were removed with the aid of a skimmer. The powdered magnesium alloy of 4% composition was added to the molten aluminum and mixed together with the stir casting machine and poured into the mold cavity at a temperature of 7300C and allowed to solidify. This process was repeated for 6, 8, 10, 12 and 14% composition of magnesium alloy. The Al-Mg cast parts were machined to the various test specification to enable the tensile strength and hardness test to be carried out. The tensile strength test specimen of dimension 100mm by 13mm was carried out using Avery-Dimensional Universal Testing Machine, while the hardness test specimen dimension of 20mm by 20mm was carried out using Brinell Hardness Testing Machine. The microstructural test specimen of 10mm by 10mm was carried out using the Light Optical Microstructural Testing Machine and a micrograph of 100x magnification was taken. The analysis of variance (ANOVA) was carried out on tensile strength and hardness test results for magnesium alloy, and compared with results from silicon alloy by Ezewanka and Ekpo 2020.
It is inferred from the results of this study that as the percentage by weight composition of magnesium increases, the tensile strength and hardness also increases. Comparing this with other literature shows same trend that increase in percentage composition of magnesium in Al-Mg alloy also increases tensile strength and hardness of the cast parts. The microstructure of Al-Mg alloy with 4% composition of magnesium shows a well-defined grain sizes, for Al-Mg alloy with 8% composition of magnesium shows the presence of eutectic magnesium in the inter-dendritic region, which is an arrangement of primary and secondary dendrites, this improves the tensile strength and hardness properties of the cast parts. Al-Mg alloy with 14% weight composition of magnesium shows a uniform distribution of Mg in Al-matrix, which causes complete fusion with the primary aluminum grains changing its original coarse grain form, thereby increasing the strengthen effect of the cast parts. The analysis of variance for both tensile strength and hardness shows that there is sufficient evidence to accept the null hypothesis which indicates that both alloying element (silicon and magnesium) have the same effect on tensile strength and hardness of the cast parts.
