Optimierung des Kokillengießverfahrens auf Schutzgasanwendungen

The ongoing developments in the combustion engine technology demand the reduction of piston weight and the simultaneous increase in the specific strength of the cast metal structure. The reduction of the oxygen concentration in the casting mold atmosphere has a theoretical potential to allow the realization of thin-walled castings and a parallel reduction of defect density, which are both cornerstones of the ongoing developments. In this study it was investigated to what extent the oxidation of aluminum melt can be suppressed via inert gas flush of the mold and how this practice affects the mold filling procedure. Furthermore it was investigated how the positive effects of the inert gas casting could be fortified by the introduction of novel completive measures. To demonstrate the effectiveness of the inert gas flooding of the mold cavity, firstly the casting properties of the used aluminum piston alloy were determined under air and technical shielding gas atmospheres. The results indicate that the obtained reduction of the oxidation rate does enhance the fluidity, however the improvements seem to be concentrated to the liquid flow state. After the conditions have been clarified, the practical work began with the design and validation of the for inert gas casting dedicated gating system. The preset criteria were fulfilled after six development stages, wherewith the conditions for the start of the actual test matrix have been cleared. The basis of valuation of the casting trials was represented by the results of a dye penetrant inspection, which showed to be precise in identifying the for the topic important cold shut and oxide defects. This accuracy was proved by validation tests with CT and metallographic sections. The first results obtained show that using the inert gas flooding of the mold, the formation of cold shuts and by surface flooding induced oxides can be effectively avoided. As disadvantageous result of the inert gas flooding small sized oxides were identified, which are verisimilar products of the increased surface turbulences caused by the reduced thickness of the superficial oxide layer. The castability of thin walled piston sections could be further improved by setting a structured mold surface (waffle pattern), which potently counteracted cold shuts regardless of the casting atmosphere composition. A similar effect was anticipated from the use of fluorine-containing coating additives, however according the results such coatings could be regarded as ineffective in capturing the oxide mantle of the Al-melt. The critical role of the running system on the casting quality and robustness of the casting process was demonstrated by comparing the new, for inert gas casting designed geometry with a conventional alternative. The comparison of two fiber filter types confirmed the importance of the applied pressure loss in setting the flow conditions. Particularly in case of oversized gating systems the finer mesh filter can help to calm the flow and thus to suppress turbulences. The for inert gas casting designed gating system has been able to meet the expectations by securing a reproducible control over the mold filling procedure. The resulted reduction of the velocity and thus the kinetic energy of the flow, however increased the susceptibility to cold shuts. According to the results is therefore always a compromise in the choice of flow conditions (turbulences vs. cold shuts) required. The test results are suggestive of characteristic defects. The conventional geometry favored the formation of larger oxides, whereas in the novel geometry typically that of finer oxides. To determine the characteristic peculiarities of the oxide defects and the alleged causes, the found oxides were investigated and categorized by using electron microscopy and micrographs. In addition, the effect of the categorized oxide types on the formation of second phases in the piston microstructure was studied. The won results could be used as the cornerstone of a future defect catalogue for oxides in aluminum castings. In the last part of the work, the effect of surface phenomena on the mold filling simulations has been studied. Basis of all conducted simulations were values of the “surface parameter” measured using the pendant drop method. The newly introduced attribute surface parameter represents the sum of the surface tension and the strength of the oxide layer and was measured under air and technical inert gas atmospheres at varied temperatures. By using the latest developments in the simulation programs and considering the surface parameter of the melt, a strong convergence between simulations and reality could be achieved. For further improvement of the accuracy of future casting simulations, the optimized consideration of the surface oxidation procedure seems vital. Thus the author recommends the concentration of the research work on this subject.

Grantor dc:publisher

Gießerei-Institut der RWTH Aachen

Year dc:date

2012

Author dc:creator

• Szalai, Bence Z.

Contributors dc:contributor

• Bührig-Polaczek, Andreas

• info:eu-repo/classification/ddc/620
• Gießen <Urformen>
• Kokillenguss
• Schutzgas
• Aluminium
• Oxidation
• Formfüllung
• Ingenieurwissenschaften
• Gießeigenschaften
• Oxidhaut
• Gießsystem
• Gießsimulation
• mold filling
• casting properties
• oxide films
• gating system
• casting simulations

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

ger

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-124342%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:62844