Iron-related porosity in Al-Si-(Cu) foundry alloys

An experimental program has been undertaken to explore the effect of iron concentration on porosity levels in Al-Si alloy sand castings. The effect of iron concentrations above, below and equal to the critical iron content for alloys with either 5 or 9% Si and either 0, 1 or 3% Cu has been determined. Increasing iron concentrations were found to increase porosity in all alloys except the copper-containing Al-5% Si alloys which displayed a porosity minimum at the critical iron content. Porosity was observed to be higher in the Al-9% Si castings than the Al-5% Si castings. Differences in the primary phase volume fraction and morphology may explain this observation. The results of this experimental work do not support the existing published theories that have been proposed to explain the effect the iron on porosity. An alternative theory is therefore developed. (c) 2006 Elsevier B.V. All rights reserved

Interactions between iron, manganese, and the Al-Si eutectic in hypoeutectic Al-Si alloys

Sand-cast plates were used to determine the effect of iron and manganese concentrations on porosity levels in Al-9 pet Si-0.5 pet Mg alloys. Iron increased porosity levels. Manganese additions increased porosity levels in alloys with 0.1 pet Fe, but reduced porosity in alloys with 0.6 and I pet Fe. Thermal analysis and quenching were undertaken to determine the effect of iron and manganese on the solidification of the Al-Si eutectic. At high iron levels, the presence of large beta-Al5FeSi was found to reduce the number of eutectic nucleation events and increase the eutectic grain size. The preferential formation of alpha-Al15Mn3Si2 upon addition of manganese reversed these effects. It is proposed that this interaction is due to beta-Al5FeSi and the Al-Si eutectic having common nuclei. Porosity levels are proposed to be controlled by the eutectic grain size and the size of the iron-bearing intermetallic particles rather than the specific intermetallic phase that forms

Influence of process parameters on the microstructure and casting defects of a LPDC engine block

The growing demand in the automotive industry for lighter vehicles has led to increasing use of Al-Si based alloys in the production of engine blocks. Low-pressure die casting (LPDC) is an enhanced process generally used for parts with premium requirements, therefore it is one of the most promising technologies for the production of engine blocks. This work is aimed to study the effects of Sr modification and holding pressure on the microstructure and casting defects of a low-pressure die cast A356 engine block. The microstructural scale, evaluated by secondary dendrite arm spacing, the amount of porosity and inclusions, and the morphology of eutectic Si particles were investigated by metallographic and image analysis. The results were correlated with the variation of input process variables such as holding pressure and Sr level. The measured amount of porosity is low, therefore confirming LPDC as a useful foundry process for the production of Al blocks for high performance engines

Optimization of the Strontium Modification Process in Gravity Permanent Mould Tilt Cast AlSi6Cu4 Cylinder Heads

Sr addition rates between 80 and 400 ppm were tested in industrial-scale casting campaigns. An adequate level of modification is achieved across the entire section of the cylinder head at a Sr addition rate of 160-200 ppm, and further increase in Sr levels fails to offer additional benefits. Porosity increases at Sr addition levels exceeding 200 ppm. The fraction of porosity is as high as 2.5% at a Sr addition rate of 400 ppm. Mechanical properties do not change over these Sr addition rates. It can be argued that the adverse and favourable effects of Sr addition more or less balance each other. However, the underlying issue of the industrial-scale experimental work is that the mechanical properties targeted for this particular AlSi6Cu4 cylinder head casting have been confidently and consistently achieved with the gravity permanent mould tilt casting process, without a separate heat treatment

Ultrasonic melt treatment of light alloys

The application of ultrasonic vibrations when casting aluminium alloys can improve the final quality of castings. Moreover, ultrasound has no direct environmental impact as compared to traditional melt treatment routes. Ultrasound has been used with different purposes in aluminium casting: (1) degassing of aluminium alloys leading to high density and virtually gas porosity free castings; (2) promoting nucleation, thus leading to highly refined microstructures, including refinement and dispersion of intermetallic compounds; (3) improving castings mechanical properties either by promoting heterogeneous nucleation and development of equiaxed globular structures or dendrite fragmentation. This article presents the experimental work carried out so far at the University of Minho, Portugal, in this field and focuses on the results and benefits of ultrasonic melt processing when compared with traditional alternatives like the chemical routes.POCI-01-0145-FEDER-006941. FEDER/COMPETE funds and by national funds through FCT and was developed on the aim of the research Postdoctoral grant SFRH/BPD/76680/2011. Also, this work has been supported by FCT with the reference project UID/ EEA/04436/2013, by FEDER funds through the COMPETE 2020—Programa Operacional Competitividade e Internacionalização (POCI) with the reference project POCI-01-0145-FEDER-00694