Investigation of modifier elements effect in the high pressure injection casting for eutectic Al-Si alloys

Aluminum-silicon alloys are especially accepted in the automotive industry because they have properties such as low density, good castability, high strength, hardness and good machinability, and are preferred in vehicles due to weight reduction, fuel economy and lower environmental emission values. In order for such alloys to be used successfully in applications, they must have better mechanical properties. By adding titanium to these alloys, the hardness, strength values and castability of the alloy can be increased by providing rapid and significant grain refinement. In this study, the effects of modification of primary aluminum dendrites and eutectic silicon crystals on the microstructure and mechanical properties of the alloy by adding alloying elements to increase strength and reduce permanent deformation in eutectic Al-Si alloys were investigated. For this purpose, it is aimed to increase the lifespan of the alloys by increasing the casting quality of the alloy and improving the mechanical properties by adding titanium and Sr. In this study, aluminum alloys with five different compositions were produced by high pressure injection molding method by adding AlTi5B1 and AlSr10 master alloy to EN AC 44300 [AlSi12 (Fe)] series aluminum alloy for grain refinement and modification processes. The effects of Ti and Sr elements added to the alloy on the microstructure and mechanical properties of the alloy were examined. Optical microscopy, scanning electron microscopy (SEM) and SEM/EDS (Energy Dispersive Spectroscopy) characterization techniques were used in microstructural investigations. The mechanical properties of the produced alloys were determined by Brinell and microhardness measurements, tensile tests and compression tests. In experimental studies, it was observed that by adding Ti to AlSi12 (Fe) alloy, the grain sizes of the alloys decreased and the hardness values of the alloy increased. In alloys containing 0.08% titanium + different amounts of strontium (0-450ppm), a linear increase in the hardness values of the alloy was achieved depending on the increase in the amount of Sr. The microhardness values obtained from the α-Al dendrites and eutectic silicon phases in the microstructure increase depending on the increase in the addition of Ti and Sr in the alloy. Likewise, a linear increase was observed in the yield, tensile and % elongation values of AlSi12 (Fe) alloys depending on the increase in the Ti and Sr ratio in the alloys. In the compression tests, a decrease was achieved in the % deformation elongation values of the alloy due to the increase in Ti and Sr addition. It was observed that the results obtained as a result of mechanical tests obtained in microstructure examinations were compatible with the microstructure and supported the results obtained. In the optical microscope images of the alloys, the addition of Ti reduces the grain size as a result of the reduction of α-Al dendrites, but no effect on eutectic silicon is observed. On the other hand, it has been observed that the addition of Sr made the Si crystals smaller and the modification process takes place in the alloy containing 450 ppm Sr, and the microstructure changes from a coarse lamellar and needle-like structure to a fibrous/spherical structure. SEM images support the optical microscope images and the α-Al dendrites and Si crystals become smaller. In addition, it is understood from point and regional SEM/EDS analyzes that there are intermetallic phases consisting of Al, Si and Fe elements. Its purpose; In the this study, it was aimed to thin the primary aluminum dendrites by adding additional alloying elements to increase the strength and reduce permanent deformation in the eutectic aluminum-silicon alloy and to investigate the effect of modification of eutectic Si crystals on the mechanical properties. It is aimed to increase the service life of the material by improving the mechanical properties of the material with the effect of Titanium used as grain refiner in aluminum alloys and Sr elements to be used for modification. In EN AC 44300[AlSi12(Fe)] series Aluminum alloys, an increase in strength is achieved by pressing (creating dislocation) at low temperatures in order to meet the desired strength under a certain load. In the study to be carried out, the mechanical properties of aluminum alloys to be produced by injection molding method, especially using Strontium and Titanium metals, will be optimized without being subjected to cold forming. By adding the alloying elements to be determined to aluminum alloys in different proportions as grain refiners and modifiers, parametric studies will be carried out and unique aluminum alloys with high strength and low permanent deformation will be developed. Results; In this study, AlTi5B1 and AlSr10 master alloys were added to AlSi10(Fe) alloy for grain refinement and modification purposes, and alloys with five different compositions containing 0.08% Ti + different amounts of Sr were produced by high pressure injection molding method. The effect of titanium and strontium addition on the microstructure and mechanical properties of AlSi10(Fe) alloy was examined. Hardness and microhardness measurements, tensile and compression tests were carried out on the produced alloys, and optical and SEM microscope examinations were carried out. The following conclusions were drawn from the experimental studies carried out: In the hardness examinations of the produced alloys, it was observed that 0.08% titanium added to the AlSi10(Fe) alloy using AlTi5B1 master alloy clearly increased the hardness values by reducing the grain size of the alloy. By adding AlSr10 master alloy to AlSi10(Fe) alloy containing 0.08% Ti, alloys containing 150ppm, 300ppm and 450 ppm strontium by weight were produced, and a linear increase in the hardness values of these alloys was observed depending on the increase in the Sr amount of the alloy. This increase increased the hardness of alloys containing 150ppm, 300ppm and 450ppm Sr by weight by an average of 2.5 Brinell hardness values for each amount of increase. The difference between the Brinell hardness values of the AlSi12(Fe) alloy without addition and the alloy with 0.08% Ti + 450ppm Sr added is 19%, respectively. α-Al dendrites were modified by adding titanium to the AlSi10(Fe) alloy, and the eutectic silicon phase was modified by the addition of strontium. Due to the increase in the strontium ratio, the microhardness values of the phases increased. The difference between the microhardness values of α-Al dendrites and eutectic silicon phase in the AlSi12(Fe) alloy without addition and the alloy with 0.08% Ti + 450ppm Sr added is 181% and 127%, respectively. Due to the addition of 0.08% titanium to the alloy and the increase in the strontium ratio, the yield and tensile strength values increased linearly after the tensile tests, and the highest yield and tensile strength and % elongation values were obtained in the alloys containing 450ppm Sr. The difference between the yield, tensile strength and % elongation values of the AlSi12(Fe) alloy without addition and the alloy with 0.08% Ti + 450ppm Sr added is 21%, 31% and 69%, respectively. In the compression test, the percentage (%) deformation elongation is due to the fact that the added Ti and Sr elements reduce the grain structure of the alloy and the transformation of eutectic silicon into a fibrosis/spherical structure absorbs the applied Fm force. This situation increases the strength of the alloy. Thus, the addition of Ti reduces the permanent deformation elongation of the alloy. In addition, the permanent deformation elongation decreases proportionally due to the increase in the amount of Sr in the alloy. The difference between the % permanent deformation elongation values of the AlSi1(Fe) alloy without addition and the alloy with 0.08% Ti + 450ppm Sr added is 26%, respectively. In microstructure examinations, it was observed in optical microscope images that the addition of Ti to the alloy reduced the grain size of the alloy by shrinking the α-Al dendrites. However, it has been observed that this situation does not affect eutectic silicon. On the other hand, it was observed that the addition of Sr to the alloy made the silicon crystals smaller. When the amount of Sr in the alloy reached 450 ppm by weight, it was observed that the microstructure changed from a coarse lamellar and needle-like structure to a fibrous/spherical structure, and thus it was concluded that the modification took place. The results obtained in SEM/EDS analyzes support the results obtained in optical microscope images. In the images, it was determined that α-Al dendrites and Si crystals became smaller. Additionally, from SEM/EDS point and regional analyzes of the alloy, it was observed that there were intermetallic phases consisting of Al, Si and Fe elements. Suggestions; Based on the results obtained as a result of experimental studies, the following suggestions can be offered to researchers who will work on similar subjects: The effects of titanium + different of strontium addition rates on the AlSi10(Fe) alloy used in this study on the corrosion properties of the alloy can be examined. The mechanisms can be explained more clearly by examining in detail the microstructure and mechanical properties obtained by grain refinement using different microstructure characterization techniques such as transmission electron microscopy. The effects of alloys containing different amounts of strontium and phosphorus added to the AlSi10 (Fe) alloy on the microstructure and mechanical properties studied in this study can be investigated. In this study, high pressure casting method was used in the production of the alloy. Apart from this method, the effect of pressure on microstructure and mechanical properties can be examined in alloys produced by low pressure casting methods such as sand and gravity mold casting with the same composition.