Thomas O. Mbuya

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 alloys 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 alloys 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

Effect of runner design on mechanical properties of permanent mould aluminium castings

Abstract A comparison of the influence of different runner designs on the mechanical properties of aluminium castings has shown that filtered rectangular runners (FRRs) yield aluminium castings with higher and more reliable mechanical properties than the conventional unfiltered rectangular runners. Unfiltered vortex flow runners have also been shown to improve the reliability of the modulus of rupture of cast aluminium alloys over unfiltered rectangular runners. In the present paper, experimental results of a comparative study on the effect of the unfiltered vortex flow and FRR designs on the tensile strength of permanent mould aluminium castings are reported. The results show that an FRR yields castings with higher and more reliable tensile strengths than the unfiltered vortex flow runner. Castings poured into a mould with an FRR had strengths between 269 and 291 MPa and a Weibull modulus of 50·2 while castings poured into the vortex flow runner had strengths between 255 and 280 MPa and a Weibull modulus of 40·3.

Wire Rope Selection for Manual Winch Application

Purpose – The purpose of this paper is to demonstrate the application of analytical decision making during the selection of engineering entities in an engineering design setting.Design/methodology/approach – The paper develops a quantitative method for wire rope selection and uses handbook data to demonstrate the use of the method in selecting a suitable type and size of wire rope for the hoisting/hauling mechanism of a hypothetical manual winch.Findings – Wire rope data can be processed into forms that are more readily useable in a quantitative selection method. Moreover, computer software such as Microsoft Excel may be used in the selection process, so long as the data are in suitable form.Originality/value – The selection of engineering entities often occurs in engineering design processes. An information processing approach to wire rope selection has been developed and demonstrated. The method demonstrated in this paper should be applicable in other situations in which a need for the selection of engi...

Microstructure and mechanical performance of a secondary cast aluminium piston alloy with minor element additions

Abstract The effect of individual and combined minor element additions (Sr, Sb, Mn, Cr and Al-5Ti-1B grain refiner) on microstructure and mechanical performance of a secondary cast aluminium piston alloy, with 1 wt-% Fe was investigated. It was observed that addition of Cr of up to 1% was better than a 0.53%Mn addition in improving tensile strength, impact energy and percent elongation of the alloy. The high mechanical performance recorded with addition of 1%Cr alloy was attributed to the significant reduction in porosity levels compared to all other minor element additions. It also resulted in a microstructure with fine compact intermetallic compounds. Other element additions also resulted in improved mechanical properties with 0.53%Mn performing better than 0.3%Mn + 0.2%Cr. Marginal improvements in mechanical performance were recorded with addition of 0.02%Sr (or 0.05%) and 0.02%Sb individually or in combination with 0.53%Mn. This was attributed to general increase in porosity and volume fraction of intermetallics.

Short Fatigue Crack Growth Micromechanisms in a Cast Aluminium Piston Alloy

The short fatigue crack growth behaviour of a model cast aluminium piston alloy has been investigated. This has been achieved using a combination of fatigue crack replication methods at various intervals during fatigue testing and post-mortem analysis of fracture surfaces and crack profiles. Crack-microstructure interactions have been clearly delineated using a combination of optical microscopy (OM), scanning electron microscopy (SEM) and three dimensional (3D) X-ray microtomography (SRCT). Results show that intermetallic particles play a significant role in determining the crack path and growth rate of short fatigue cracks. It is observed that the growth of short cracks is often retarded or even arrested at intermetallic particles and grain boundaries. Crack deflection at intermetallics and grain boundaries is also frequently observed. These results have been compared with the long crack growth behaviour of the alloy

Three‐Dimensional Analysis of Microstructure in Cast Aluminium Piston Alloys

The 3D architecture of intermetallics distribution in two model cast aluminium piston alloys is examined using synchrotron X-ray microtomography and advanced image analysis tools. The highly complex morphology and 3D interconnectivity of intermetallics is delineated using advanced 3D image analysis tools. A novel technique which circumvents quantification difficulties associated with the high interconnectivity is employed for quantifying intermetallic particles. The intermetallic particle size distribution is then analysed using extreme value statistics to predict the maximum particle size in a sample of S-N fatigue specimens and subsequently, the lower bound fatigue life in the given sample.