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The volume porosity was assessed using Archimedes’ princi- ple. The cleanest melt possessed the lowest porosity whereas contaminated melts possessed higher values of percentage porosity. An increase in porosity in the thicker section was found for the melt with low oxide additions. In contrast, the oxide rich melts tend to show lower porosity in the thicker sections. This is due to pores connected to the cut surface becoming filled with water during the weight measurement in liquid. Thereby, the scale displays a misleading weight compared to the weight when the pores were within the bulk of the metal. Thus, the volume porosity may sometimes be underestimated to a large degree.

Pores size distributions, shape factor, number density and equivalent diameter were obtained by image analysis. The latter two measurements correlated well with each other. An increase or decrease in pore number density was always ac- companied by a decrease or increase in the equivalent diam- eter. In other words fewer but larger pores were found and vice versa.

The castings produced with contaminated melts possessed more porosity with rising section thickness and the aver- age equivalent pore diameter decreased. The general in- crease in equivalent diameter was due to the higher solidi- fication time in the thicker sections which allow hydrogen to diffuse longer, consequently leading to extensive pore growth. Also the pore size distributions for both of the heavier contaminated melts revealed an increase of smaller pores in the thicker sections of the step castings. The com- bination of hydrogen concentration, high amount of poten- tial nucleation sites and higher solidification time led to 1.) smaller pores, and 2.) no increase in the fraction of large pores. Laslaz and Laty22

reported the same behaviour. In

their studies they found a significant increase in porosity for similar hydrogen content when the melt was oxidized by turning chip addition. Moreover, rounder pores were found for melts with higher oxide additions and elongated pores in the clean melt. This was not found in the present investigation due to the domination of shrinkage porosity caused by inadequate feeding.

mechanical Properties Acknowledgments

The impact of oxide additions on mechanical properties was fairly severe when compared to a non-contaminated melt. The mechanical properties (UTS and elongation) also deteriorated with increasing section thickness. This is in accordance with findings reported earlier by Akhtar et al.4-6 In their work the hydrogen content was changed and, at the same time, a constant oxide level maintained. When hydro- gen content was low, in the presence of excess nucleation sites, oxides were the main cause of fracture.

Scanning Electron Microscopy (SEM) micrographs from the fractured surface showed the presence of iron rich in-

International Journal of Metalcasting/Spring 2012

The European Project NADIA (New Automotive compo- nents Designed for and manufactured by Intelligent process- ing of light Alloys) is gratefully acknowledged for financial support. Special mention is given to Dr. Shahid Akthar from the Department of Materials Science and Engineering, Nor- wegian University of Science and Technology (NTNU) for his invaluable assistance. The help and encouragement of Dr. Derya Dispinar, Arne Nordmark and Kurt Sandaunet from SINTEF Materials and Chemistry during the experi- mental trials and the interpretation of analysis data are very much appreciated.


termetallic phases and interdendritic porosity, and both lead to early fracture, hence cause loss in strength and duc- tility. However, both heavily contaminated melts exhibited better mechanical properties than the melt with moderate contamination (10% addition of turnings). It is believed that an increase in the fraction of smaller pores at expense of larger pores is less detrimental for the mechanical prop- erties of the castings. The presence of inclusions different to oxide films contributes to the variations in the mechani- cal properties. Moreover, it should have been stated that the gating system in the present die was not optimized to give adequate feeding so the mechanical properties in the oxide-free reference casting are not optimal.


The influence of oxide additions on the porosity and me- chanical properties of a gravity die cast A356 aluminium al- loy has been studied. Castings were made under controlled, reproducible conditions of oxide additions, hydrogen level, melting, pouring and casting temperatures. The following conclusions can be drawn:

• PoDFA tests revealed that although large inclu- sions were removed from the melt, fine oxide films were introduced by contamination of the A356 al- loy with turning chips.

• Oxide films might have caused blockage of the feeding path leading to surface linked porosity in the contaminated melts.

• The addition of oxides led to the generation of larg- er fractions of small pores.

• Mechanical properties, namely Ultimate Tensile Strength (UTS) and elongation, decreased from thin to thick sections in step casting unconstrained from the contamination level of the melt.

• High pore number densities in combination with small pore diameters are less detrimental for me- chanical properties.

• The use of bifilm index for melt quality assessment as a stand-alone method is questionable and needs further revision.

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