Effect Of Mn On Microstructure And Properties Of High Silicon Aluminum Semi - Solid Formed Parts

High-silicon aluminum alloys (Si>16%) are widely used in automotive and aerospace industry due to their attractive properties, such as low coefficient of thermal expansion, high wear resistance, high strength and hardness. Fe is the primary one of the impurity elements in the Al-Si alloy, hard and brittle needle-like Fe phases would cause serious harm which affecting the application of the alloy. There are many ways to weaken its harmful effects on the alloy, the effective method of Fe phase morphology transformation has not yet been agreed with the relevant scholars. And one of common and effective methods is add Mn to alloy. Seven different alloys which modification by P in order to refine the primary Si particles were prepared by casting method.The self-developed unitized cooling slope device and squeeze casting equipment were employed for the semi-solid Rheoforming. SHIMADZU-AG-IS electronic universal testing machine was used to test the tensile properties of samples at room temperature; wear properties of the samples were tested on MMU-5GA vacuum megathermal friction and wear testing machine; NETZSCH DIL402dilatometer was employed to measure the thermal expansion coefficient of the samples. The effects of cooling slope on the primary Si and Fe phase and Mn content on the microstructure and mechanical properties of alloys were investigated through TEM, SEM, EDS and other analytical methods.The results show that the unitized cooling slope can refine the primary Si significantly, meanwhile inhibited the formation of the need-like β Fe phase. The needle like β-AlFeSi phase can be transformed to lump α-Al-(Fe, Mn)-Si phase effectively by adding Mn to the alloy. And this transformation was almost fully conducting when the Mn content is1.20%. The tensile strength of alloy is97MPa, increased significantly compared to65MPa which the alloy without the addition of Mn. When the Mn content is1.58%, the tensile strength of the alloy reduced greatly due to the coarsening of Al-(Fe,Mn)-Si phase. It can be found that fracture mechanism of alloys is brittle fracture at room temperature by observing the tensile fracture SEM photographs. The dimple characteristics in the alloy fracture surface were increased with the increasing of Mn content. The research on the characteristics of the alloys indicating that the hard Al-(Fe,Mn)-Si phase which played a supporting role in the wear process formed after the addition of Mn elements, therefore the wear resistance of the alloy was improved. The linear thermal expansion coefficient of the alloy is19.3×10-6/℃which was determined, lower than the other hypereutectic Al-Si alloys. The main reason is the low thermal expansion of Si element and the formation of large number of Al-(Fe, Mn)-Si phase in the alloy.