Effects And Mechanisms Of In-situ Nanocrystals In The Melt On The Solidification Behavior,Microstructure And Mechanical Properties Of Aluminum Alloys

Aluminum alloy is widely used in automobile and aerospace fields due to its advantages of light weight and high specific strength.It is also the most widely used lightweight material at present.The microstructure configuration of aluminum alloys has an important influence on the final mechanical properties,but the current main inoculants or modifiers can only affect part of the structure,for example,Al-Ti-B inoculants and some rare earth elements can only affect?-Al,P can only affect the primary Si particles,Sr and Sb can only affect the eutectic Si structure,and the adjustment of the morphology and size of the precipitates can often only be achieved by adjusting the heat treatment process.Therefore,it is very necessary and meaningful to explore a reliable structure control agent to comprehensively control the primary phase,eutectic phase and precipitated phase of aluminum alloy after heat treatment to obtain high-quality structure.The solidification process of the alloy directly affects the solidification structure,the distribution of alloying elements and the precipitated phases after heat treatment.The understanding and research on the effect of potential modifiers on the solidification behavior is the basis for the refinement of the alloy structure and performance enhancement.Therefore,establishing the relationship between solidification behavior and solidification structure and mechanical properties will be the focus of this article.This dissertation systematically studied the influence of nanocrystals formed by in-situ crystallization of Fe-B-Si metallic glasses on the solidification behavior and as-cast structure of hypoeutectic Al-Si alloys,and established the relationship between solidification behavior and solidification structure.It revealed the regulation mechanism of in-situ nanocrystals on Al-Si alloy structure.At the same time,the effects of in-situ nanocrystals on the solidification behavior,as-cast structure,segregation and mechanical properties of Al-(Si)-Cu alloys and Al-(Si)-Mg alloys were studied,and strengthening mechanism of Al-Mg alloy were also revealed,which provided important experimental and theoretical basis for industrial production of high-strength aluminum alloy.The innovations of this dissertation are as follows:1.The effect of in-situ micro-nanocrystals on the solidification behavior of aluminum alloys is revealed.Adding nanocrystals increases the?-Al nucleation temperature,Al-Si eutectic reaction temperature,Al₂Cu and Mg₂Si nucleation temperature;2.It reveals the influence of in-situ nanocrystals on the structure of aluminum alloy.The addition of nanocrystals significantly refines the?-Al and Al-Si eutectic structures,improves the segregation of Cu and Mg elements,and makes the Al₂Cu phase and Mg₂Si phase distribution more uniform.The?' precipitated phase and Mg₂Si precipitated phase after heat treatment are also refined.3.It reveals the regulatory mechanism of the microstructure of in-situ nanocrystalline aluminum alloy:a)?-Al dendrite regulation mechanism:the nano-crystalline Fe₂B phase has a better lattice match with?-Al,which can be used as the heterogeneous nucleation core of?-Al to promote?-Al nucleation,and more heterogeneous cores make?-Al growth space reduced,and grain growth is restricted by adjacent fine grains and hindered by nanocrystalline phases,and?-Al grains are refined;b)Eutectic silicon regulation mechanism:Nano-crystalline phases Fe₃Si and Fe₂B can be used as the heterogeneous nucleation sites of eutectic Si to promote the Al-Si eutectic reaction.The growth process of the Al-Si eutectic structure is restricted by smaller adjacent grains and hindered by nanocrystalline phases,and the eutectic structure is refined;c)Precipitation phase regulation mechanism:The nanocrystalline phases promote the formation of Al₂Cu phase and Mg₂Si phase,and the growth process is restricted by smaller adjacent grains and hindered by the nanocrystalline phases that do not participate in nucleation,Al₂Cu phase and Mg₂Si The phases are more uniformly distributed along the grain boundaries,which is conducive to the uniform distribution of elements in the?-Al crystal during solid solution,and the second phase is finer and dispersed during subsequent aging process.4.It was revealed that the in-situ micro-nanocrystals significantly strengthened Al-(Si)-Cu and Al-(Si)-Mg alloys,and the mechanisms of strengthening and toughening were revealed:a)The nanocrystals significantly strengthened Al-Cu₄ and Al-Si₇-Cu₄,Al-Mg₁ and Al-Si₇-Mg₁ alloys.The yield strength,tensile strength,fracture strain and product of strength and ductility of Al-Cu₄ alloys increased by 31.0%,24.7%,9.4%and 33.2%,respectively;the yield strength,tensile strength,fracture strain and product of strength and ductility of Al-Si₇-Cu₄ alloys increased by 27.4%,24.7%,65.7%and 130.0%respectively;the yield strength,tensile strength,fracture strain and product of strength and ductility of Al-Mg₁ alloy increased by 12.3%,5.7%,24.7%and 32.1%,respectively;The yield strength, tensile strength,fracture strain and product of strength and ductility of Al-Si₇-Mg₁ alloy increased by 16.2%,14.2%,85.0%and 131.2% respectively;b)Strengthening and toughening mechanisms of Al-(Si)-Cu and Al-(Si)-Mg alloys strengthened by In-situ nanocrystals:i.Strengthening mechanisms:fine-grain strengthening and precipitation strengthening;ii.Toughening mechanisms:Fine grain structure and grain coordinated deformation and fine eutectic Si alleviate stress concentration.