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Hydrogen impurity and porosity inaluminum castings: An examination of hydrogen solubility in aluminum melts, thermodynamics of different alloys and its impact on die castings' quality
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
2014 (English)Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

This paper examines and discusses the role of hydrogen porosity caused by melt impurityon the quality record, microstructure and mechanical properties in aluminum highpressure die castings. An introduction into casting processes, relevant thermodynamics,melt refining processes as well as post casting processes such as heat treatment isgiven. Diffusion is also discussed as a process that controls the most importantphase transitions in solid castings. Two different types of castings are investigated: amechatronics housing and a structural body part of a car. For each of these two, testbatches are analyzed. One each was produced with uncleaned melt while the other wasproduced according to serial production standard, i.e. with melt previously cleaned byrotary impeller degassing. The mechatronics housing featured the expected result asthe worsened melt purity caused an increase in porosity and a drop in the casting’squality record. For the structural body part, the shock absorber carrier, no significantdifference could be found.Neither during tensile testing, nor during welding tests, microstructural investigationsand its quality record. The thermodynamics of important alloys are investigated anddiscussed regarding their impact on hydrogen solubility. Some differences betweenthe alloy systems were found but none that would explain the differences in porosityforming that were mentioned before. The impact of the high pressures during hpdc isalso studied and discussed as a possible cause for the observations. It was found thathpdc’s high pressures cause the alloys’ hydrogen solubility alter so much compared toambient pressure that according to thermodynamics solid aluminum at this elevatedpressure can accommodate all the hydrogen impurity present. A model is eventuallydeveloped, which introduces the concept of local pressure in die castings. It explains theconnection between the homogeneity of the casting’s cross-section and its solidificationbehavior. The model thus argues why porosity formation was different in the twodifferent types of castings. The paper concludes with studying diffusion of hydrogen inaluminum in a furnace atmosphere to explain why even during welding no differencein porosity between pure and impure melt was found.

Place, publisher, year, edition, pages
2014. , 149 p.
Keyword [en]
aluminum alloy, hydrogen impurity, porosity forming, thermodynamic solubility, high-pressure die casting
National Category
Metallurgy and Metallic Materials
URN: urn:nbn:se:kth:diva-159691OAI: diva2:786913
Educational program
Master of Science - Materials Science and Engineering
2014-11-28, Red Lab, Brinellvägen 23, Stockholm, 14:00 (English)
Available from: 2015-02-16 Created: 2015-02-06 Last updated: 2015-02-16Bibliographically approved

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