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Mechanisms and mechanics of porosity formation in ductile iron castings

100%

Perzyk M. , Witemberg-Perzyk D.

Archives of Foundry Engineering

2007

tom Vol. 7, iss. 4

147-150

Shrinkage defects in ductile iron castings can be of two basic types: shrinkage cavities associated with the liquid contraction prior to the expansion period of the iron as well as the porosity, which may appear even if the liquid shrinkage is fully compensated. In the present paper two possible mechanisms of the porosity are presented and analyzed. The first one is the Karsay's mechanism based on the secondary shrinkage concept. The second one is the mechanism acting during the expansion period of the iron, first suggested by Ohnaka and co-authors and essentially modified by the present authors. The mechanical interactions between casting and mould are determined for the both mechanisms. Their analysis leads to the conclusion, that porosity forms during expansion period of the melt. The direct cause is the negative pressure which appears in the central part of the casting due to the differences in expansion coefficients of the fast cooling surface layer and slow cooling inner region. Observations concerning feeding behavior of ductile iron castings, based on this mechanism, agree well with industrial practice. The secondary shrinkage is not only needless to induce the porosity, but the corresponding mechanism of its occurrence, proposed by Karsay, does not seem to be valid.

Gas Blowing Ultrasonic Aluminium Degassing Assessment with the Reduced Pressure Test (RPT) Method

100%

Galarraga H. , de Cortazar M. G. , Arregi E. , Artola A. , Oncala J. L. , Merchan M.

Archives of Foundry Engineering

2020

tom Vol. 20, iss. 2

111--117

Entrapped gases, solidification shrinkage and non-metallic compound formation are main sources of porosity in aluminium alloy castings. Porosity is detrimental to the mechanical properties of these castings; therefore, its reduction is pursued. Rotary degassing is the method mostly employed in industry to remove dissolved gases from aluminium melts, reducing porosity formation during solidification of the cast part. Recently, ultrasonic degassing has emerged as a promising alternative thanks to a lower dross formation and higher energy efficiency. This work aims to evaluate the efficiency of the ultrasonic degasser and compare it to a conventional rotary degassing technique applied to an AlSi10Mg alloy. Degassing efficiency was evaluated employing the reduced pressure test (RPT), where samples solidified under reduced pressure conditions are analysed. Factors affecting RPT were considered and temperature parameters for the test were established. The influence of ultrasonic degassing process parameters, such as degassing treatment duration and purging gas flow rate were studied, as well as treated aluminium volume and oxide content. Finally, ultrasonic degassing process was contrasted to a conventional rotary degassing technique, comparing their efficiency.

Struktura i porowatość odlewów ze stopu aluminium a właściwości mechaniczne

80%

Bernat Ł. , Hajkowski J. , Hajkowski M.

Archiwum Odlewnictwa

2006

tom R. 6, nr 22

41-48

W artykule przedstawiono wyniki badań krzywych stygnięcia, kryterium Niyamy, udział porowatości, parametry struktury (odległość między gałęziami w dendrytach i średni kąt nachylenia dendrytów) i właściwości mechanicznych (R_{0,2}, R_{m} i wytrzymałość zmęczeniowa) odlewu płyty ze stopu EN AC-AlSi7Mg krzepnącego w różnych warunkach cieplnych. Określono wpływ kryterium Niyamy na porowatość odlewu oraz wpływ udziału porowatości na właściwości mechaniczne przy niestałych wartościach odległości między gałęziami w dendrytach i średniego kąta nachylenia dendrytów.

In the paper the results of investigations of cooling curves, Niyama criterion, participation of porosity, microstructure parameters (the space between arms in dendrites and mean inclination angle of dendrites) and mechanical properties (R_{0,2}, R_{m} and fatigue strength) of plate casting made of EN AC-A1Si7Mg alloy solidify in different thermal conditions was presented. The influence of Niyama criterion on porosity and participation of porosity on mechanical properties with unstable parameters of spacing between arms in dendńtes and mean inclination angle of dendrites was determined.