Effect Of Erbium Element On Microstructure And Mechanical Properties Of Hypoeutectic A356 Alloys

A356 alloys,due to its advantages of light weight,high strength,good corrosion resistance,excellent castability,are widely used in many industrial fields,such as automotive and national defence industries.But coarse needle/plate like eutectic silicon(Si),which forms during its solidification can induce stress concentration and seriously damages the mechanical properties,leading premature crack initiation and fracture,limiting its application fields.In order to enhance its tensile properties,eutectic silicon should be modified.Due to the simple and efficient of chemical modification,it has become the main method to optimize the eutectic silicon.In this paper the effects of Erbium(Er)on modification of eutectic Si in A356 alloys were investigated,through metallographic analysis,scanning electron microscopy and transmission electron microscopy analysis method.The interactions of modifying agents with different cooling rate,modification temperature,holding time on the morphology of eutectic silicon at the time of solidification were studied.Interaction experiment between Er and P elements in high purity Al-7Si alloys was carried out to discuss the modification mechanism.Based on comprehensive experiments and theoretical analysis,this paper mainly achieved the following valuable research conclusions as follows:1.Microstructure observations show that Er can change the large plate-like eutectic silicon into short coral shape,refining its average grain area and aspect ratio as well as the secondary dendrite arms value of primary ?-Al.The optimum addition amount is 0.3% and the fracture mode changes from brittle to ductile.After T6 heat treatment,the tensile strength and elongation of the alloy has been further enhanced.0.3% Er increases the quality index of the as-cast alloy by 37.2%,and the quality index after T6 heat treatment increases by 23.3%.2.The relationship between modifying agent and experimental parameters such as cooling rate,holding temperature,holding time are established.Results show the modification effects are highly affected by the cooling rate.At low cooling rate the structure of eutectic silicon modified by Er is short rod like.But at high cooling rate the morphology of eutectic silicon is coral-like structure,indicating that the modification effect of Er is highly depending on the cooling rate.Through the experiments of overheating and holding time,the result shows that heating the alloy at 740°C for 30 minutes,eutectic silicon will achieve a fully modified morphology.Overheating at higher temperature or holding time longer will oxidize the rare element and reduce the modification effect,coarsening the eutectic silicon.After remelting the specimen repeatedly,the modification can be stable in 3 times.3.The experiment of interaction between Er and P in high-purity Al-7Si alloy is designed to elucidate the mechanism of modification.The experiment shows that Er could not change the nucleation in high-purity alloy.With the P concentration increases,eutectic silicon grain becomes coarsening.EDX analysis shows that Er can form Al-Si-Er phase,restricting the nucleation of eutectic silicon in two ways: packaging or dissolving in AlP particles.The more P content in the alloys,the more Er content is required to modify the eutectic silicon.Thermal analysis shows that Er element depresses the eutectic reaction temperature,increasing the recalescence,enlarge the supercooling.Increasing the supercooling promotes the morphological transformation of eutectic silicon,indicating that the transformation of Er morphology may mainly through poisoning the nucleation mechanism.The TEM photos show that the twin crystals in the Er modified eutectic silicon matrix are parallel to each other,indicating that modification mechanism of Er may be different from Sr and Na.The modifying mechanism mainly through poisoning eutectic silicon nucleation.