Weight reduction in automobiles is amongst the most economical ways of reducing greenhouse gas emissions and increasing fuel efficiency. The recently invented ablation casting process is a novel technique of producing high performance light weight parts which meet this objective. In this technique a water jet demolds the water soluble sand mold and subsequently impinges upon the solidifying metal, thereby producing high cooling rates in the casting which in turn leads to microstructural refinement and higher mechanical properties.
The objective of this study was to develop a comprehensive understanding of the effect of various parameters involved in the casting of a thin walled part using the HiPerMag casting process for the wrought aluminum alloy AA 7050.
The study is divided into three major parts that deal with the composition of the sand binder system, optimization of the sand mold thickness, various aspects of the water jet parameters and the desired microstructural parameters which will result in a defect free part.
In first phase of the study, sand mold properties such as the green and dry strengths of the water soluble sand binder system used in the study were tested to ensure that they meet the molding requirements. An average green strength of 160 kPa and an average dry strength of 3825 kPa were found for the water soluble sand binder system. These values were similar to those reported in the literature for clay bonded sands and were sufficient to make molds for casting in the current study.
Secondly, a heat transfer model was developed to find a minimum mold thickness required to design a mold for the HiPerMag casting process such that the liquid metal remained sufficiently insulated before being quenched. Based on the model, for a mold with a cope thickness of 12.9 cm, the heat flux losses to the surroundings were reduced by up to 90 % versus a case where a thinner mold was used.
In addition, an analytical solution was derived for the mold thickness problem from which it was found that at a distance of 10 cm from the mold cavity there was a negligible increase in temperature of the sand at that location at large times.
Further, the minimum mold thickness was determined based on the temperature profile in the sand mold during the HiPerMag casting process. This study showed that a thin mold of about 2 cm thickness was sufficient to provide insulation to the hot metal during the HiPerMag casting process.
Thirdly, it was found that, based on cooling curve data and microstructural analysis, that a jet spacing of 15.3 cm and a time delay of 7.4 s between successive jet activations starting from the farthest jet (located near the edge of the casting), was necessary to obtain a single solidification front throughout the casting. This also ensured that grain size variation in the casting was less than 10 μm for having uniform mechanical properties.
Also, it was found for a thin walled casting, the amount of solid present in the solidifying casting at the time of water jet impingement had a negligible effect on the movement of the solidification interface.
Finally, the effect of jet momentum on surface defects was examined. It was determined that the maximum jet momentum resulting in no surface defects at temperatures close to the liquidus for Al AA 6061 alloy was approximately 2 kg.m/s. / Thesis / Master of Applied Science (MASc)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/20288 |
| Date | January 2016 |
| Creators | Sharma, Satyam |
| Contributors | McDermid, Joseph, Kish, Joseph, Shankar, Sumanth, Mechanical Engineering |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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