Research On Solidification Behaviour Of Aluminum Alloys By Sub-rapid Solidification

Twin roll thin strip continuous casting technology which will change the traditional thin strip production process,is a revolutionary technology in the field of materials and metallurgy.Its basic principle is that the molten steel be poured into a pair of counter rotating with cooled water internally.The steel liquid is solidified and slightly deformed into a thin strip.Thin strip continuous casting technology connects casting and rolling together,simplifing the production process of hot rolled steel from and realizing the integration of casting and rolling.The speed of strip cooling rate is between 102?103?/s,which is sub-rapid solidification range.The solidification structure has the characteristics of sub rapid solidification.Sub-rapid solidification has a high cooling rate.It can significantly refine the grain and the second phase,improve the uniformity of composition and microstructure,extent the solid solubility of alloy elements and the formation of a new metastable phase.The mechanical properties,processing properties and corrosion resistance of the alloy can be greatly improved.Therefore,it is an effective way to improve the properties of alloys,the application prospects can be extended,the microstructure can be obtained by using the sub-rapid solidification technique which is different from the conventional solidification method.In this paper,the rapid solidification of aluminum alloy is realized by using the sub-rapid solidification experiment device designed by ourselves,the characteristics and formation mechanism of the solidification structure of aluminum alloy at different cooling rates were studied.The mathematical relationship between the secondary dendrite arm spacing and solidification rate is derived.The main research contents and results are as follows:(1)According to the theory of thermodynamics,processed water cooling device for preparing aluminum alloy casting.The one-dimensional cooling sub-rapid solidification of aluminum alloy was realized.(2)Established a study on the relationship between the secondary dendrite arm spacing and the cooling rate,by measuring and recording the solidification temperature curves of at least two points during the solidification process,obtaining the cooling rate of the points.Then the cooling rate of different positions in the molten body is derived by using these two points.(3)By measuring the secondary dendrite arm spacing by microscope,the mathematical relationship between the two dendrite arm spacing and solidification rate is derived:SDAS=838.73v(?).(4)The characteristics of solidification structure of aluminum alloy under sub rapid solidification were studied,and the microstructure of the alloy was compared with that of conventional solidification.Results show:Under conventional freezing conditions,the solidification microstructure of aluminum alloy is coarse and uneven;Mainly composed of the primary ?-Al and a large number of distribution of intermetallic compound.Solute element Si,Cu is enriched in the solidification process,and the formation of intermetallic compound phase at the grain boundaries.But only a small amount of alloying elements are dissolved in the grain(primary ?-Al).Micro segregation of elements is serious.Grain crystal of of Al alloy castings by sub rapid solidification has been refined.The relatively high cooling rate inhibits the diffusion of solute elements,formed a solidification structure dominated by the over saturated ?-Al phase.The ? phase at grain boundaries is very rare or disappeared.The uniformity of the composition of casting and the microstructure were increased.A large number of solute elements are dissolved in the matrix Al phase.Al atoms were replaced by the Cu atoms in the crystal structure in the form of substitution.The lattice constant of crystal Al is reduced.Its XRD diffraction peak shifted.