| dc.description.abstract | The objective of this study was to investigate the effects of iron content on the microstructure and mechanical properties of scrap locsmotive engine aluminium pistons. Cast aluminum alloys mainly A356 and 319 find widespread applications in general engineering for automotive, aerospace and in domestic applications due to their high quality and lightness combined with excellent castability.
During this study, iron in the form of iron filings was added into the molten aluminum in a graphite crucible to achieve four levels of iron content namely;O.84wt.% Fe, 1.2wt.% Fe, 1.5wt.%Fe and 1.8wt.% Fe. Strontium modification and manganese addition was done for some alloys so as to study their neutralizing effects on 13 - phases. The melt treatment, such as degassing, skimming, fluxing and refining was carried out prior to casting the specimens into a permanent cast-iron mold.
Prior to complete solidification, cooling rates were recorded for thermal analysis using K-type thermocouples. Some cast specimens were heat-treated before carrying out mechanical tests and metallographic analysis. Microstructural analysis was carried out using a microscope and camera connected to hardness testing machine and the microstructure characterizer software. Mechanical analyses were conducted using tensile, fatigue and hardness testing machines at the University of Nairobi in the department of mechanical and manufacturing engineering workshop.
Cooling rate analysis indicated that altering the chemical composition strongly influences the solidification behavior and types of phases formed. Change in iron and manganese was found to cause a variation in the reaction temperature while strontium addition tends to lower the nucleation temperature.
From the microstructural analysis, it was observed that intermetallic compounds are formed at high iron contents or at high concentrations of manganese. Both grain size and volume fraction of intermetallic compounds were found to increase from 5.3 urn to 7.3 urn and from 4.2% to 8.6%
respectively as the iron content increases from 0.84wt.% Fe to 1.8wt.% Fe. With addition of 0.6wt.% Mn to an alloy of 1.2wt.% Fe, grain size was found to increase further from 6.3 urn to 7.1 um, while volume fraction increased from 4.9% to 5.8%. The formation of intermetallic phases m~nly [3 - phase with iron addition were found to adversely reduce tensile strength from 207 MNm-2 with 0.84wt.% of iron to 168 MNm-2 when iron content increased to 1.8wt.%. Increase on iron content was also found to reduce ductility but increases hardness from 85 HVN with 0.84wt.% Fe to 97 HVN with 1.8wt.% of iron. Both percentage elongation and area reduction were found to drop from 2.5% and 3.6% with 0.84wt.% Fe to 1.5% and 2.2% respectively when iron content was increased to 1.8wt_% Fe. It was also found that increasing iron content from 0.84wt.% to 1.8wt.% decreases fatigue life from about 1.745xl06 cycles to about 1.404x106 cycles at an applied constant load of 2894 newtons.
The above detrimental effects on microstructure and mechanical properties were as a result of increased [3-phases with increase in iron content. Addition of manganese, strontium modification and heat-treatment was found to reduce such harmful effects of [3-phases by converting and defragmenting the brittle and needle like beta phases to less harmful Chinese script a-phases. Such improvement of mechanical properties with strontium modification and heattreatment is also as a result of coarsening and spheroidization of silicon particles. | en_US |
