Grain Refinement And Laser Remelting Of Cast Al-Si Alloys

Due to its excellent castability,corrosion resistance and high specific strength,the cast Al-Si alloy is suitable for the preparation of structural components with complex shapes,and has been widely used as a lightweight material in the automotive manufacturing field.At present,cast Al-Si alloys account for about 70%of automotive Al alloys,and further improvement of its performance is directly related to lightweight design and has been the focus of attention.The improvement of alloy properties through grain refinement and efficient densification is one of the most promising methods,and the related mechanisms need to be further explored.In this paper,we studied the refinement of Al-Si alloy by Al-Nb-B-Ti and Al-Nb-B-V intermediate alloys,investigated the effect of intermediate alloys on the microstructure of Al-Si alloy,and explored its refinement mechanism and anti-Si toxicity mechanism;used laser remelting method to laser treat Al-Si alloy with Ce addition,and studied the modification of eutectic Si by rare-earth Ce under rapid solidification conditions.The mechanism of the modification of eutectic Si by rare earth Ce under rapid solidification conditions was investigated,and the strengthening and toughening mechanisms were discussed.The main research contents and results of this thesis are as follows:（1）The Al-3.5Nb-1B-1Ti intermediate alloy prepared in this work has excellent grain refining effect on Al-10Si alloy.In order to reveal the role of Ti in enhancing the resistance of Al-Nb-B to Si poisoning,the Nb B₂/Al interface reconstructed by Ti adsorption was studied using high-resolution electron microscopy and first-principles calculations.The orientation relationship of（Nb,Ti）B₂/Al interface is determined by HRTEM/HRSTEM experiments,namely（0001）（Nb,Ti）B₂/（112）Al₃（Nb,Ti）/（111）α-Al.Al-3.5Nb-1B-1Ti were sandwich-structured（Nb,Ti）B₂ particles,with Nb_richB₂ as the outermost layer,Ti_richB₂ in the middle and Nb B₂ substrate as the initial substrate.Calculations based on first principles show that the（Nb,Ti）B₂/Al interfacial structure at the Ti B₂ end is unstable and tends to be reconstructed by interdiffusion between the Ti atoms in the Ti end and the Nb atoms in Nb B₂,culminating in a sandwich-shaped（Nb,Ti）B₂ structure,a process that is driven by a reduction in interfacial energy.Such a reconstruction weakens the interfacial adsorption tendency of Si,thus ensuring better resistance to silica poisoning.（2）The Al-3.5Nb-1B-1V intermediate alloy prepared in this work exhibits excellent grain refinement of the Al-10Si alloy.In order to reveal the role of V in enhancing the resistance of Al-Nb-B to Si poisoning,the V adsorption reconfigured Nb B₂/Al interface was investigated using high resolution electron microscopy and first principles calculations.The orientation relationship of the（V,Nb）B₂/Al interface was determined by HRTEM/HRSTEM experiments as（0001）（V,Nb）B₂/（112）Al₃V/（111）α-Al.The main nucleation particle in Al-3.5Nb-1B-1V is the（V,Nb）B₂ particle.The adsorption energy at the（0001）Nb B₂/（111）Al interface was investigated using first nature principle calculations and it was found that the V atoms adsorbed onto L=5,but the difference in interfacial energy was not sufficient to drive the interdiffusion of V and Nb atoms.However,the adsorption of V solute atoms onto the（0001）Nb B₂ substrate at the B-terminus leads to the appearance of the VB₂ region,resulting in the formation of a Nb_richB₂/V_richB₂/Al core-shell structure.the formation of the VB₂ interface is the main reason for its good resistance to Si poisoning.（3）The surface strengthening layer of the Al-Si alloy with 0.05 wt.%Ce after laser remelting consists of remelting zone,heat affected zone and substrate zone.Due to the fast solidification rate during LSR,Ce atoms can adsorb on{111}Si planes and lead to the appearance of twins and layer dislocations by activating the IIT mechanism,which promotes the morphological transformation of eutectic Si from fibrous to granular.Before T6treatment,the increase in RMZ hardness is mainly attributed to precipitation strengthening（Si-rich clusters and eutectic Si particles）and dislocation strengthening.After T6 treatment,the RMZ The strengthening mechanism in the RMZ is mainly the precipitation of Mg₂Si and the precipitation of smaller dispersed Si particles.Small Si particles with high density of twins,stacking layer dislocations and Al-rich clusters reduce the probability of crack sprouting,and their diffuse distribution combined with fine crystals increases the barriers to crack extension and facilitates the plastic toughness.