Effect Of Cooling Rate And Modification On The Microstructure And Tensile Properties Of Al-Si-Mg Alloy

ABSTRACT 

The effect of cooling rate and modification on the microstructure and tensile properties of Al-Si-Mg alloys was carried out. Castings were produced using a stepped pattern that has different section thicknesses.  Sand casting technique was used.  A thermocouple inserted into the mould cavity recorded the cooling rates for the different sections. The Al5.43%Si3.83%Mg which was melted in the crucible furnace was modified with sodium (Na) and strontium (Sr) in separate cases. The molten metal was poured at 750oC and allowed to cool and to solidify in the mould. Tensile tests, hardness test and microstructural analyses were carried out using standard specifications and methods. The tensile test results showed that the component with minimum section thickness (5mm) had the highest cooling rate and gave the highest tensile   strength of 110N/mm2. It was also revealed from the hardness test results that the section with minimum thickness (5mm) had the highest hardness of 46.5 HRB. From the photomicrographs produced, the 5mm thick section with the fastest cooling rate showed finer grains than the other sections, with the thickest section of 40mm showing the most coarse microstructure at slowest cooling rate. Also structures modified with sodium and strontium displayed better microstructural and tensile properties when compared with the unmodified structures. Modification effect is better with sodium than strontium.