No abstract

In this study, in order to compare effect of unidirectional compression and rolling on final microstructure of strain induced melt activated (SIMA) A356 aluminum alloy, rectangular samples with dimensions of 3cm×5cm in area and 1cm in thickness and cylindrical specimens with 2.5cm in diameter and 1cm in length, have been prepared for rolling and compressing processes, respectively. Then, these samples were plastically deformed at a same strain in ambient temperature. Afterward, the strained samples were cut into equal quarters. In the next stage, to produce globular microstructure, these specimens were partially remelted in 580°C for different times. Results obtained from light microscopy showed that specimen's thickness and so, its strain affected zones influence on the globulization of dendrites. In addition, it was seen that at a given strain and constant diameter, increase of H/D ratio led to increase of needed time for reaching a certain sphericity in cylindrical samples. Also, it was showed that microstructural evolutions during SIMA processing of both rolled and unidirectional compressed samples were relatively identical. However, at a same condition, ultimate size of globulized dendrites in the rolled samples was smaller than those of compressed ones.

In this work, effective parameters of SIMA process to obtain non dendritic microstructure in A356 alloy were investigated. In addition, the effect of SIMA process on the evolution of morphology of silicon and intermetallic phases in this alloy was studied. Microstructure images obtained from optical microscopy and SEM observation showed that increase in plastic work up to 40% and then holding of samples in the semi solid state at temperature of 580oC, causes that primary dendritic structure changes to non dendritic, fine and globular structure, but optimum reheating time completely depended on initial thickness of samples. If all parameters of SIMA process are the same, the grain boundaries of thinner samples begin to wet and following globalization will be completed in shorter reheating time rather than thicker ones. Moreover, it was found that the intermetallic phases lost their angular or needle morphology and gradually changed to rounded morphology and even to globular form. Also the optimum reheating time thoroughly depends on primary casting microstructure as the finer casting microstructure begin to globalize faster than thicker one under more little stains.

In this study, the effect of small additions of lithium on the microstructure and morphology of Fe-containing intermetallic compounds in A360 alloy has been investigated. Different Fe contents (1, 1.5 and 2%) were added into A360 alloy, and Li concentrations also varied from 0 to 0.5 wt. %. The image analysis results showed that, the maximum length of β-Al5FeSi plate in Li-modified specimens is changed significantly in comparison with non-modified ones. It is evident that the maximum reduction in the average length of Fe containing platelets occurs in specimens with 2% Fe. Also, the lowest changes in the length of intermetallic phases -as Fe content decreases- occur in the 0.5 wt% Li. At higher Li concentrations, the presence of Al-Li-Si intermetallics in the microstructure can provide undesirable condition. This intermetallic can be so damaging in mechanical properties especially in coarse shapes.