Effect of grain refiner on hypoeutectic al-si alloy feedstock billet quality produced by gas assisted direct thermal method / Muhammad Abdin Shakirin Bin Mohd Masror

Faculty of Mechanical and Automotive Engineering Technology

Thesis (Master of Science) -- Universiti Malaysia Pahang – 2023

Includes bibliographical references

This thesis presents the research works on the effect of grain refiner on hypoeutectic Al-Si alloy feedstock billet quality produced by a gas-assisted Direct Thermal Method (GA-DTM). Even though GA-DTM has been proven in the literature to produce globular and refined microstructure feedstock for thixoforming, the effect of grain refiner addition during the operation is still unclear, with the lack of literature concerning it in this research area. Furthermore, the microstructure and mechanical properties of hypoeutectic Al-Si alloy feedstock billet that resulted from the combination of thermal and grain refinement techniques are still inexplicable. Hence further detailed experimental work is required for SSMP hypoeutectic cast alloy using a GA-DTM with the addition of grain refiner. The experiment begins with one of the Thermal Analysis (TA) methods, the Cooling Curve Analysis - Computer Aided (CCA-CA) method, to determine the thermal profile of hypoeutectic Al-Si alloy. The CCA-CA experiment was conducted under two different conditions, one without the addition of magnesium and another with the 1.0 wt.% addition of magnesium. The information from the CCA-CA was used to determine the processing parameter for the GA-DTM. The pouring temperature (590 C, 610 C, 630 C), holding time (10 s, 15 s, 20 s), and magnesium addition (0.5 wt.%, 1.0 wt.%, 1.5 wt.%) was the combination parameters for the GA-DTM experimental works. The molten hypoeutectic Al-Si alloy was poured into the cylindrical copper mould and quenched into the water to solidify at room temperature after the desired holding time. The feedstock billet samples were then prepared for microstructure analysis, density measurement, ultimate tensile strength (UTS), Vickers hardness, and fractography test. The differences in average density value for each feedstock billet were between 2.62 g/cm3 and 2.87 g/cm3. The combination of 590 °C pouring temperature, holding time of 15 s, and 1.5 wt.% of magnesium addition resulted in the highest average density value among other feedstock billet samples, which was 2.87 g/cm3. Meanwhile, in sample 2, with a pouring temperature of 630 °C, holding time of 10 s, and 1.0 wt.% produced the lowest density of 2.62 g/cm3. The higher density value indicated that the porosity content in the feedstock was low compared to the feedstock billet with a low-density value. The mechanical properties results show that the ultimate tensile test was between 14.64 MPa and 151.13 MPa, while the Vickers hardness test was between 89.56 HV and 136.52 HV. Sample 11, with the combination of 610 °C pouring temperature, holding time of 10 s, and 1.5 wt.% addition of magnesium produced the smallest grain size area at 618.64 μ𝑚2 compared to other sample microstructures. The development of the globular microstructure depends only on the heat convection of the molten alloy extracted from the copper mould. The faster the heat was expelled from the molten alloy, the slower the development of the dendritic microstructure, changing it into the globular microstructure. The research results show that magnesium was utilised to generate finer grain size since it reduced grain size from 4693.02 μm² to 3724.68 μm compared to the sample without magnesium. The grain size by 23% reduction can enhance excellent formability as less collision occurs between the particles within the microstructure