| Hot cracking is the main defect of casting Al-Cu alloy.The study of hot cracking mechanism shows that the liquid film formed during the final solidification of the alloy has an important effect on the occurrence of hot cracking.Because the liquid film produced in the late solidification stage is difficult to be detected directly,the relationship between liquid film properties and hot cracking cannot be proved directly.In this paper,the semi-solid microstructure was obtained after the secondary heating remelting of the semi-solid billet,and the liquid phase surrounded the spherical solid phase in the form of a liquid film.The liquid film during the solidification of casting alloy was simulated,and the shrinkage stress was measured during the solidification process,so as to realize the direct detection of the liquid film properties.It provides a detecting method and research method for the study of hot crack mechanismA device for measuring the shrinkage force of liquid film in the later solidification period was designed.By heating the prefabricated semi-solid sample to semi-solid,the liquid film is obtained,and then the temperature and stress generated in the solidification process of the liquid film are detected.The relationship between liquid film stress and solidification temperature was obtained quantitatively by establishing the relationship between grain size,liquid phase ratio and shape factor and temperature,and then the tendency of hot cracking and the occurrence condition of hot cracking were determined.In order to obtain the semi-solid billet ingot with ideal spherical structure,the semi-solid billet ingot was prepared by mechanical vibration method.When the casting temperature was determined to be 760?,the ideal spherical structure could be obtained,and the microstructure uniformity was good.In order to realize the detection of liquid film properties by the designed detection device,the relationship between the secondary heating remelting process and the semi-solid microstructure of the sample should be established.The experimental results show that the main process parameters affecting the semi-solid remelting microstructure are heating temperature and holding time.At the same secondary heating temperature,the intergranular liquid film increased gradually with the extension of holding time.The higher the secondary heating temperature or the longer the holding time,the closer the shape factor of the solid grain in the semi-solid microstructure is to 1,that is,the roundness gradually increases.The relationship between the secondary heating temperature,holding time and the liquid volume fraction was established by analysis.At the same time,induction and resistance heating methods were used to compare the secondary remelting structures obtained.Results The size and shape factors of the semi-solid particles under the two heating modes were basically the same.The semi-solid liquid film solidification shrinkage detection device designed in this paper was used to detect Al-4Cu alloy,and it was found that the shrinkage stress accumulation coefficient of the liquid film decreased first and then increased with the extension of holding time.When the heating temperature is 600 ? and 630 ?,respectively,the stress accumulation coefficient is the minimum when the holding time is 60 min.When the sample solidifies,the cooling rate affects the stress accumulation coefficient.When the heating temperature is 600?and the cooling rate is about 5 ? /s,2 ? /s and 0.6 ? /s for 25 min,the stress accumulation coefficient is 0.00737,0.0028 and 0.00308,respectively. |
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