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Influence of oxIde AddItIons on the PorosIty develoPment And mechAnIcAl ProPertIes of A356 AlumInIum Alloy cAstIngs


T. Ludwig, M. Di Sabatino and L. Arnberg


Department of Materials Science and Engineering, Norwegian University of Science and Technology, Trondheim, Norway D. Dispinar


SINTEF Materials and Chemistry, Trondheim, Norway Copyright © 2012 American Foundry Society Abstract


Three A356 aluminium alloy melts with different oxide con- tents were produced by mixing alloy turnings with ingot mate- rial. The melts had a constant hydrogen level and were cast in a step mould steel die. The resulting casting porosity was characterized using the Archimedes’ method and image anal- ysis. Melt quality was accessed using a Porous Disc Filtration Apparatus (PoDFA) and the Reduced Pressure Test (RPT). It was shown that oxide additions lead to higher amounts of fine structured oxide films resulting in an increase of the fraction of small pores. Shrinkage porosity with its inherent


Introduction


Whenever liquid aluminium is exposed to air, the surface quickly oxidizes. Accordingly, when the liquid metal is poured into the mould, the oxide layer on the melt surface as well as new oxides, which are formed along the falling stream of liquid metal, can be entrained into the metal flowing through the gating system and into the casting cavity. These oxides will then fold over each other to form bifilms.1


During


solidification some of these bifilms that were entrained into the metal stream will be found within the casting and consti- tute crack-like defects which may lead to premature failure.


The second important interaction of liquid aluminium and its surroundings is the absorption of hydrogen. Hydrogen is the only gas capable of dissolving in large quantities in liq- uid aluminium and its alloys. However, it is rejected from the liquid metal on solidification due to a lower solubility of hydrogen in aluminium in solid state. Hydrogen is then free to diffuse to potential nucleation sites, namely fine dispersed inclusions and/or entrained oxide bifilms.1-2


Hydrogen there-


fore contributes to pore growth, depending on the solidifica- tion rate of the cast part and the initial hydrogen concentra- tion of the melt. Hence, controlling the amount of entrained oxides, which are excellent sites for pore nucleation, and hydrogen concentration in the melt are of major concern.


It is well known that increased hydrogen content in the melt leads to higher porosity in the cast part. Akthar3-6


International Journal of Metalcasting/Spring 2012 studied the


partially elongated and interdendritic pore morphology was predominant. Ultimate tensile strength (UTS) and elongation from thin and thick sections in the step castings deteriorated with increased oxide content; UTS decreased from 3-18 % and elongation from 18-38 % with oxide additions as the sec- tion thickness increased. The melt cleanliness analyses from PoDFA were consistent with the oxide additions.


Keywords: porosity, oxides, bifilm, mechanical properties, A356 alloy


influence of hydrogen in aluminium, achieved by upgassing and degassing the melt in highly reproducible experiments. During those casting experiments the amount of oxide inclu- sions was kept constant at a low value.


Timelli et al7 recently presented results on the fluidity of


aluminium casting alloys. They deliberately contaminat- ed a melt with oxide inclusions by adding 50 wt% scrap and compared the findings with a clean reference melt. The fluidity of the alloy was reduced by 20% at a pouring temperature of 750C (1382F). They concluded that the addition of scrap to a melt generates more oxides. This reduces the fluidity and diminishes the critical solid frac- tion at the flow tip. Also, Di Sabatino et al8


reported a


notable decrease in fluidity by adding turning chips to a clean melt. A different approach of gaining control over and predicting the effect of oxide entrainments was re- ported by Yang et al.9


It was previously found that en-


trapped oxide films are accompanied by casting defects in the solidified casting such as shrinkage porosity, crack and dross formation.10-11


Yang et al9 developed a model


termed Oxide Film Entrainment Tracking (OFET, 2-D) to simulate oxide film defect distribution in the liquid alu- minium alloy. Models for the prediction of microporosity exist as well.12


However, the effect of oxide inclusions in


melts with varying amounts of purposely generated ox- ide films on the porosity development and distribution in aluminium cast alloys, have not been studied under con- trolled conditions.


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