| The current study comprises an analysis of the effects of the handling of liquid metal on the microstructure and mechanical properties of industrially-produced Al-Si-Mg and Al-Cu alloy castings using the lost-foam casting method. Moreover, this work allows for the transfer of technology by means of a comparison between castings prepared in a laboratory under ideal conditions and those actually produced in industry, in the hopes of ameliorating the quality of the end result products.;A study of the effects of the casting parameters on the microstructures of the 356, 357 and 220 obtained from lost-foam casting was carried out using various analysis techniques including optical microscopy, image analysis and scanning electron microscopy. An extensive analysis of the mechanical properties was conducted using a large number of tensile samples. In light of the results obtained, it was observed that the chemical composition of the strontium master alloys used had no effect on the microstructure or the mechanical properties of the alloys studied.;The increase in the size of eutectic silicon particles in the presence of boron and strontium demonstrate that these two elements react together to form the SrB6 compound which reduces the respective effects of the individual elements. That is to say, the grain refinement and eutectic modification provided by boron and strontium are reduced. In addition, the microstructures of samples obtained from all mold types revealed that this interaction is independent of the solidification rate.;Rapid cooling of the metal brings about a reduction in the size of the eutectic silicon particles in the as-cast condition. However, the T6 heat treatment can cancel this effect if the silicon particles enter the growth phase. The growth of these particles in the bolt boss section is not as rapid as in the combustion chamber section, indicating that the Si particle size depends greatly on the geometry of the section of the casting in question.;First of all, the phases and other microstructural elements of the alloys used (220, 356 and 357) were characterized using thermal analysis. Next, the effects of the handling of the liquid metal (modification, grain refinement and degassing) of the pieces produced via the LFC method on their microstructure and mechanical properties (tensile and hardness) were evaluated and compared to samples stemming from other types of molds (standard metallic ASTM B108, an L-shaped large section metallic mold, and a refractory mold which provided directional solidification).;Hydrogen plays a predominant role in the formation of porosity. On the other hand, the foam pattern leaves an impression on the casting surface which, having the appearance of pores, may be confused with actual gas or shrinkage pores. These impressions are mainly present for the samples coming from the combustion chamber section due to its narrow width (∼10 mm). Moreover, the solidification rate affects the porosity by reducing the time required for it to form. Thus, a high rate of solidification generates fewer pores. Therefore, the occurrence of porosity is dictated by a combination of the hydrogen level present and the solidification rate employed, as was shown by the results obtained from the refractory mold which exhibited directional solidification.;The hardness values as well as the elastic limit vary according to the chemical composition of the alloy studied. Gains of 17% and 24% were observed for the hardness and elastic limit for the 357 alloy compared to the 356 alloy. This difference is attributed to the different concentrations of magnesium which, during the T6 heat treatment, precipitate as Mg2Si. The hardness and elastic limit of the 220 alloy increased by 18% and 15%, respectively, compared to that measured for the 356 alloy. In this case, the hardening phase, Al2Cu, is responsible for this increase. All increases in hardness are independent of the type of mold used. The addition of hydrogen reduces the hardness by ∼25%, regardless of the alloy or casting technique used. Moreover, due to the increased formation of porosity in the presence of H2, the alloy ductility is severely affected as fracture occurs much more easily. The fracture of the aluminum alloys will depend on the microstructure, strain rate used and the temperature of testing. (Abstract shortened by UMI.)... |
