Study On Effect Of Co On Microstructure And Mechanical Properties Of Hypereutectic Al-Si Alloy

Hypereutectic Al-Si alloy, which has low thermal expansion coefficient, small density, high wear resistance, corrosion resistance, high temperature strength and good castability, is especially good for production of friction and wear resistant parts that used in transportation and machinofacture. However, the microstructure of traditional hypereutectic Al-Si alloy is composed of coarse polygonal primary Si particles and coarse acicular shape eutectic Si, which makes the mechanical and cutting performance worse and cannot meet the expanding demand. The development of new modifier, alloying elements, heat treatment, and forming technology are requiring nowadays. This research is focused on the alloying of hypereutectic Al-Si alloys and the improvement of mechanical properties, especially tensile strength at elevated temperature. The effects of Co on microstructure and mechanical properties of hypereutectic Al-Si alloy were first systematically studied in this thesis. This paper includes the combined effects of cobalt (Co) and ultrasonic vibration, the combined effects of Co and rare earth (RE), the combined effects of Co and iron (Fe), and the effect of heat treatment.The combined effects of Co and RE in AlSi2oCu2Ni1were explored.0.3%～1.5%Co were added respectively into AlSi2oCu2Ni1REo.6alloy and0%RE alloy with P modification. The results show that, after the addition of Co, long acicular intermetallic compounds Al-RE-Co-Ni-Si were formed. The RE-containing acicular compounds were increased with the increasing of Co content. The addition of Co has an adverse effect on tensile strength of the AlSi2oCu2Ni1REo.6Cox alloys. For the alloy without RE, the Co-bearing compounds presented as small block shape when Co was equal or less than0.7%and turned into dendritic or fish-bone shape when Co was more than0.7%. The addition of Co has a positive effect on tensile strength of the AlSi2oCu2Ni1alloys that added equal or less than0.7%Co. There was little effect of Co on microstructure of primary Si and eutectic Si.The combined effects of Co and Fe, especially on tensile strength at elevated temperature in AlSi2oCu2Ni1were discussed. The results indicate that, the addition of Co could effectively modify the morphology of Fe-rich intermetallics and improve the ultimate tensile strength of AlSi2oCu2Ni1alloys. For example, after the addition of0.3%～1.5%Co, the acicular iron-rich phases turned into Chinese-script Al15(Fe, Mn, Co)3Si2, and dendritic or fish-bone quaternary phases Al9(Fe, Co, Ni)2were formed in the AlSi2oCu2Ni1alloys. The volume fraction of Fe-bearing intermetallic compounds was increased with the increasing of Co content, when various levels of Co (from0%to1.5%) added respectively into both AlSi2oCu2Ni1Feo.7and AlSi2oCu2Ni1Fe1.0alloys. The Fe-bearing compounds presented as long acicular β-A15(Fe, Ni)Si phases when Co was0%while turned into Chinese script, granular, rod-like or dendritic a-Al15(Fe, Co, Ni)3Si2phases when Co added. When0.91%Co added in alloys with0.7%Fe, or1.3%Co added in alloys with1.0%Fe, there was a reduction oOf the length, width, and aspect ratio of the intermetallics, respectively. Co is a very effective element in promotion of tensile strengths at room temperature and elevated temperature (at350℃) of both AlSi2oCu2Ni1Feo.7and AlSi2oCu2Ni1Fe1.0alloys. The optimum Co:Fe ratio in these two alloys studied is1.3. The optimum Co:Fe ratio in these two alloys studied is1.3. The maximum tensile strengths at room temperature and elevated temperature (at350℃) of alloys with0.7%Fe were275and101MPa respectively. Compared with the minimum tensile strengths that obtained when Co was0%, they respectively increased by13%and28%. While the corresponding data were296and120MPa respectively for alloys with1.0%Fe. Compared with the minimum tensile strengths that obtained when Co was0%, they respectively increased by24%and35%.The hypereutectic Al-Si alloys treated by ultrasonic vibration (USV) were also studied. The combined effects of USV and Co addition (XCo,0%≤X≤1.5%) on AlSi2oCu2Ni1alloys and AlSi2oCu2Ni1Feo.7alloys were discussed. The USV treatment applied around liquidus temperature (started at710℃and maintained120s) of the melt could not only lead to refinement and volume fraction reduction of primary Si, but also results in a great decrease of grain size and aspect ratio of both Fe-containing and Co-bearing compounds by promoting nucleus formation, and hindering preferential growth on certain direction. The addition of equal or less than0.91%Co in AlSi2oCu2Ni1Feo.7alloys, the addition of equal or less than0.7%Co in AlSi2oCu2Ni1alloys, as well as application of USV has a positive effect on tensile strength not only at room temperature but also at elevated temperature of both the alloys. Take AlSi2oCu2Ni1Feo.7alloy for example, the maximum tensile strengths at room temperature and elevated temperature (300℃or350℃) of alloys in traditional casting were275MPa and101MPa, respectively, with0.91%Co, while the corresponding data were284MPa and132MPa, respectively, for alloys treated with USV.Effect of heat treatment on microstructure and mechanical properties of hypereutectic Al-Si Alloy with Co-bearing intermetallic compounds were firstly explored. The morphology transformation of (Fe+Co)-rich intermetallics in AlSi20Cu1.85Ni1.05Fe1.26C01.35alloy during non-equilibrium heat treatment were found and studied. When solution heat treated at510℃for various times (from0to11hours), the coarse plate-like8-Al4(Fe, Co, Ni)Si2particles vary slowly through concurrent dissolution along widths and at the plate tips as solution treatment time increased. The volume of the original δ-Al4(Fe, Co, Ni)Si2phase decreased as atoms of Si, Fe and Ni diffused. What is more, the change rate of atoms in the matrix was larger at points such as the crevice locations or the tips than at the flat face of the e, Co, Ni)Si2phase. The transformation process is explained by diffusion-induced grain boundary migration. The8-Al4(Fe, Co, Ni)Si2phase, which appears as a coarse plate-like particle in two dimensions, is actually a cuboid in three dimensions. The length of this cuboid is close to the width, while the height is much smaller. Solution heat treatment has an important influence on UTS, yield strength, and hardness of this alloy. The optimum solution heat treatment time is7hours when temperature is510℃. The maximum UTS and yield strength of the AlSi20Cu2Ni1Fe1.26C01.35alloy after T6treatment were258and132MPa, respectively, while the maximum hardness was131HB (without USV). Compared with those of the samples in the as-cast state, they increased by53%,42%, and28%, respectively.