Study On Modification Of Intermetallic Componds In Mg₂Si/Fe-rich Recycled Aluminum Matrix Composites

Aluminum alloys have been widely used in transportation, manufacturing and aerospace industries and other fields because of their excellent characteristics, such as low density, high specific strength and specific stiffness. The total amount of scrapping aluminum due to discard as useless of aluminum products per year are about 50% of the global aluminum market demand. Therefore, the development of recycled aluminum industry for promoting efficient recycling of aluminum is important. The main challenge of recycled aluminum is high content of impurity Fe, and it’s difficult to remove. Based on this problem, Fe-rich recycled aluminum alloy was used as metal matrix to prepare Mg₂Si/Fe-rich aluminum matrix composites by in-situ reaction between Mg and Si in the present study. The Fe-rich and primary Mg₂ Si phases in Mg₂Si/Fe-rich aluminum matrix composites were modified by alloying element additions to improve their refinforcing abilities. Some conclusions were obtained as follows:（1） A356-1.3%Fe recycled aluminum alloy was used as raw material, Mg₂Si/Fe-rich recycled aluminum matrix composites（denoted as Mg₂Si/A356-1.3Fe） were prepared. The Mg₂Si/A356-1.3Fe composites mainly composed of α-Al, β-Fe（Al5FeSi）, Si and Mg₂Si（primary and eutectic） phases. β-Fe（Al5FeSi） phase mainly assumed as a long needle. With increasing in the Mg₂ Si content, morphology of primary Mg₂ Si phase gradually changed from granular to coarse dendrite. When Mg₂ Si content was 20%, dendrite primary Mg₂ Si phase was uniformly distributed in the matrix with average size of about 70 μm. The hardness was about HB100.（2） The elements of La, Ce, Sr, Ca, Mn, Cr and B were chosen as modifying agents to separately modify 20%Mg₂Si/A356-1.3Fe recycled aluminum matrix composites. B element was proved to modify effectively primary Mg₂ Si phase. Neutralizing elements of Mn and Cr could effectively modify β-Fe phase. Also, low amount ofs levels of Sr and RE play a certain role in modification of β-Fe phase. The optimal amount ofs of these effective modifying agents were 0.2%B, 1.0%Mn, 0.8%Cr, 0.1%Sr and 0.2%RE. Over modification phenomenon occurred when the amount ofs exceeded the optimal contents. Cr element could effectively modify β-Fe phase and refine primary Mg₂ Si phase to some extent. Sr element could effectively refine eutectic Si phase. The rest elements could not effectively modify β-Fe and primary Mg₂ Si phases simultaneously.（3） Compared to B-Mn-Sr, B-Cr compound modifying agents had higher efficiency to modify primary Mg₂ Si and β-Fe phases simultaneously in Mg₂Si/A356-1.3Fe recycled aluminum matrix composites. All of the needle-like β-Fe phase were changed into α-Fe phase like Chinese-script characters or skeleton. The aspect ratio of Fe-rich phase was reduced by 90%. Morphologies of primary Mg₂ Si phase were changed from coarse dendrite to small granular. Its average size was reduced by about 50% from 70 μm to 37 μm, about 50% lower. Furthermore, eutectic Si phase could be also refined by trace addition of Sr element. The tensile strength and elongation to failure of 20%Mg₂Si/A356-1.3Fe composites were increased by 30% and 90% after being modified by B-Cr-Sr compound modifying agents, respectively.（4） Compared to A356-1.3%Fe recycled aluminum, the tensile strength and elongation to failure of the composites modified by B and Cr were increased by 27% and 73%. Also, wear resistance of the composites was improved by 3 times due to 20%Mg₂Si addition. The wear properties of the composites were not obviously improved by modification of intermetallic compouds.Mg₂Si/Fe-rich recycled aluminum matrix composites were successfully prepared through in-situ reaction in melt. Simultaneous modifications of primary Mg₂ Si and β-Fe phases were achieved for the first time to improve mechanical properties of the composites. The present study could provide a useful route to extend the application fields of recycled aluminum alloy.