Improvement in functional characteristics of aluminum-silicon cast components through the utilization of a novel electromagnetic treatment of liquid melts.

摘要

In the present dissertation the study of Electromagnetic Stirring (ES) and Electromagnetic Stirring and Vibration (ESV) techniques in several Al-Si hypo and hypereutectic alloys were conducted. Five ES and ESV workstations were designed and tested in order to develop a technology capable of conducting melt treatments on Al-Si hypereutectic alloys at temperatures above liquidus. The 390 series alloys were selected because they have shown comparable wear characteristics to cast iron (currently used for cylinder liner applications) three times higher thermal conductivity, the difference in thermal expansion coefficient thermal between the Al-Si alloys was 6%, stability and metallurgical compatibility with the 319 alloy.;The ESV technique was successfully implemented to modify the primary Si particles in Al-Si hypereutectic (390.1(I), 390.1(II), 393.2(I) and 393.2(II)) alloys at temperatures as high as ∼100°C above liquidus. The effectiveness of the ESV melt treatment is higher as the temperature gets closer to liquidus. For the first time the application of ESV treatments were able to modify the microstructure of Al-Si hypereutectic alloys from the liquid state. A novel algorithm known as the "Image Analysis Based Si Modification Level " to determine the effect of the ESV melt treatment on the microstructure.;Thermal analysis shows that the liquidus temperature increases up to 15°C as a function of the ESV melt treatment conditions and alloy composition. Therefore, thermal analysis can be used as on-line technique to predict this SiML. Additionally, this SiML, Vickers microhardness and Brinell hardness presented similar behaviour as a function of the ESV melt treatment conditions. The SiML increased from 4.3 to 7.7 and the microhardness increased up to 35% in heat treated test samples, while Brinell hardness increased 25%. Using multiple regression analysis a correlation between the ESV melt treatment conditions, the results of TA, SiML and Vickers and Brinell hardness was established. With multiple regression analysis, several algorithms were developed to predict the SiML or mechanical properties based on the ESV melt treatment conditions or the TA results. These algorithms are a powerful tool for on line determination of the SiML of the mechanical properties of ESV treated castings.;The tensile properties of the ESV treated melts were determined in test bars cast in the Stahl mould. The ESV melt treated test samples presented an improvement of ∼20% in as cast conditions and ∼30% after the heat treatment. A fractography analysis of the test bars was conducted and was found that the primary Si and the shrinkage pores are responsible for the lower mechanical properties of the test bars cast with untreated melt. Additionally, the improvement of the tensile properties of the ESV melt treated samples was due to the elimination of the porosity and the highly refined Si due to the melt treatment.;The ES technique was effectively used in a semi-solid state to refine the microstructure of the 319 alloy used to cast engine blocks. For instance the ES treatment was capable of reducing the grain of the 319 alloy size by 600%, and partially eliminates the dendrites. However, the ES treatment had negligible effect on Al-Si eutectic, Mg and Cu enriched phases and was limited to the semi-solid state and for hypoeutectic alloys.;The ESV treated melts were utilized to cast prototype pistons and cylinder liners. The cylinder liners were used to produce prototype V6 engine blocks. The prototype engine blocks were cast at NEMAK of Canada Corporation with cast in prototype Al-Si cylinder liners. The mechanical characteristics and the microstructure were evaluated following Ford's Materials Specifications for cylinder liners and regular production engine blocks in the bulkhead sections. Soundness (porosity) and mechanical properties in the V6 prototype engine blocks were assessed in comparison to the regular production engine block. The porosity measurements indicated that the prototype engine blocks presented a 40% less porosity when Al-Si cylinder liners are used. This resulted in a 12% increase of the tensile properties of the bulkheads.