Effect of IRON Removal on the Structure and Mecahnical Properties of Aluminium-Silicon Alloys
The research work examined the effect of the removal of iron from aluminium-silicon alloys using dopants on the structure and mechanical properties. Manganese, cobalt and molybdenum were used as the modifying elements. The as-cast Al Si alloys were analyzed for iron content using Atomic Absorption Spectrometer (AAS) technique. The dopants or modifying elements were added as micro additives at various compositions to the percentage of iron present in the master alloys. The modified alloys were produced by melting a calculated mass of the master alloys and adding the dopants in the ratios of 1:1, 2:1, 3:1 and 4.1 to the percentage of iron in the Al Si alloy. The properties tested were ultimate tensile strength, percentage elongation, hardness and microstructure. The result of the chemical analysis of the alloys indicated that the dopants were able to reduce the percentage of iron in the alloys. On addition of 5.4%Co iron was reduced from 1.8% to 0.42%, which is far less than the critical iron level of 0.78%. The mechanical tests showed an improvement with the highest values achieved by the addition of 5.4%Co. The improvement in the microstructure is also shown in plates 1-8.
Influence of iron on castability and properties of aluminium silicon alloys: literature review
The castability and properties of Al—Si based foundry alloys are affected by the inevitable introduction of impurity elements during melting and casting. Iron has consistently emerged as the most common impurity element that generally imparts negative influences on the soundness and mechanical properties of these alloys. It leads to the formation of iron-bearing intermetallics whose type, size, morphology and amount depend on the alloy composition, melt thermal history, cooling rate and post-casting heat treatments. These compounds, particularly their size and morphology, are the main culprits responsible for the significant influence of iron on the castability and mechanical properties. The most common of these compounds are β-Al5FeSi with a needle/platelet morphology, α-Al15Fe3Si2 with a Chinese script or polyhedron morphology and π-Al8Mg3FeSi6 also with a Chinese script morphology. The iron in the α phase can be substituted by Mn or Cr whereas the π phase forms when Mg is present. The β phase (the most detrimental phase) causes an increase in the hardness accompanied by a reduction in ductility, fracture toughness, impact energy, fatigue resistance and perhaps tensile and yield strengths of the alloy. The β-Al5FeSi phase also reduces the alloy's feedability by physically blocking the inter-dendritic flow channels during the last stages of solidification. Furthermore, it is said to serve as an active pore nucleation site besides restricting pore growth and expansion. Iron also forms primary intermetallic compounds under certain conditions of melt processing and composition, particularly in the presence of Mn and Cr. These primary phases reduce the alloy's fluidity and machinability. Iron, however, reduces the tendency of the alloy to hot tear and to solder the dies during high- pressure die-casting. It can also grain refine aluminium and its alloys besides aiding the efficiency of A1–5Ti–1B grain refiners. IJCMR/482
Experimental investigation of the geotechnical properties and microstructure of lime-stabilized saline soils under freeze-thaw cycling
Abstract The Song-Nen Plain in northeastern China holds one of the three largest expanses of soda saline-alkali soil in the world. Due to its disadvantageous geotechnical properties, the saline soil in this area is unable to meet the needs of construction projects. This study evaluates the stabilizing effect of adding lime to such soils. Specifically, it investigates how the physicochemical and mechanical properties and microstructure of soda saline-alkali soil are altered by lime and how these changes are affected by both freeze-thaw cycling and the duration of curing. The results reveal that adding lime to saline soil transforms clay particles into sand and silt particles and causes the pore size distribution (PSD) to become bimodal, with small-pore and macropore populations. An increase in lime content leads to an increase in the unconfined compressive strength (UCS) but a gradual decrease in both the cation exchange capacity (CEC) and plastic index. Freeze-thaw cycles break up coarse particles and aggregate finer particles, increasing the diameter of macropores and decreasing the USC; however, the effect on specimens with >9% lime is negligible. It is further shown that the geotechnical properties of the soil change immediately after the addition of lime, indicating that the cation exchange reaction takes place very quickly. Furthermore, the pozzolanic reaction leads to an increase in the percentage of sand-sized particles and a decrease in porosity after 28 days of curing. Overall, these results indicate that it is feasible to use lime to improve the engineering properties of soda saline-alkali soil in this area.