本文關(guān)鍵詞： 同步輻射 鋁合金 凝固組織 枝晶生長(zhǎng) 結(jié)構(gòu)效應(yīng) 細(xì)化劑　出處：《上海交通大學(xué)》2015年博士論文　論文類(lèi)型：學(xué)位論文

【摘要】：微觀組織控制是獲得高質(zhì)量、低缺陷鑄件的關(guān)鍵所在,特別是對(duì)于具有復(fù)雜結(jié)構(gòu)(比如變截面、T型結(jié)構(gòu)、薄壁結(jié)構(gòu))的大型復(fù)雜薄壁件尤為重要。眾所周知,熱場(chǎng)、溶質(zhì)場(chǎng)以及流場(chǎng)的分布是凝固微觀組織形成的關(guān)鍵因素。本文基于熱與質(zhì)對(duì)合金結(jié)晶的影響,采用同步輻射方法研究凝固微觀組織和多場(chǎng)分布的關(guān)系。結(jié)果顯示了Al-Cu合金中的一種新的雙胞晶生長(zhǎng)模式,TiB2顆粒細(xì)化機(jī)理以及凝固過(guò)程中微觀組織形成的特征。采用同步輻射成像技術(shù)和傳統(tǒng)的電子背散射衍射(EBSD)技術(shù)研究了亞共晶Al-Cu合金中的枝晶形態(tài)及樹(shù)枝雙胞晶的生長(zhǎng)位相。實(shí)驗(yàn)結(jié)果表明,新型雙胞晶空間結(jié)構(gòu)為二維結(jié)構(gòu),二次枝晶臂與主干成垂直生長(zhǎng)。雙胞晶形態(tài)可以等軸枝晶、柱狀枝晶或與正常枝晶臂共存等形式存在。EBSD結(jié)果表明,樹(shù)枝雙胞晶尖端和二次枝晶臂的生長(zhǎng)方向分別為110和001。樹(shù)枝雙胞晶的形成幾率強(qiáng)烈地依賴于Cu濃度。隨著合金濃度的逐步提高,雙胞晶出現(xiàn)幾率逐步提高,并在Al-20 wt%Cu時(shí)達(dá)到最高值。根據(jù)實(shí)驗(yàn)數(shù)據(jù)和理論計(jì)算的結(jié)果,修正了亞共晶al-cu合金動(dòng)力學(xué)晶體生長(zhǎng)形貌圖,確定樹(shù)枝雙胞晶為分形枝晶和密集枝晶之間的過(guò)渡形態(tài)。利用同步輻射成像技術(shù)研究了上述所提到的特征結(jié)構(gòu)中亞共晶al-cu合金微觀組織的演化過(guò)程。根據(jù)成像照片呈現(xiàn)的吸收襯度差異與成像的時(shí)間相關(guān)性,分析了溫度場(chǎng)、溶質(zhì)場(chǎng)和流場(chǎng)對(duì)凝固組織演變的影響規(guī)律。結(jié)果表明,對(duì)于凝固空間存在空間結(jié)構(gòu)變化的變截面結(jié)構(gòu)和t型結(jié)構(gòu),生長(zhǎng)界面前沿溫度分布呈明顯的結(jié)構(gòu)相關(guān)性。不同的結(jié)構(gòu)導(dǎo)致不同的溶質(zhì)富集區(qū)域和熔體流動(dòng)狀態(tài),從而造成不同的凝固順序和組織特點(diǎn)。對(duì)于薄壁結(jié)構(gòu)而言,壁厚是凝固組織演變的關(guān)鍵影響因素,提升澆注溫度和鑄模溫度對(duì)改善薄壁結(jié)構(gòu)的充填與補(bǔ)縮作用不大。冷卻速度隨壁厚減小而增加,等軸晶粒均勻度隨壁厚減小而減小。為了進(jìn)一步分析細(xì)化凝固組織對(duì)均衡組織的影響因素,本文利用x射線小角散射(saxs)研究了添加tib2顆粒對(duì)al-15wt%cu熔體結(jié)構(gòu)的影響。研究結(jié)果表明,al-cu熔體中的團(tuán)聚體表現(xiàn)為質(zhì)量分形特征,且呈現(xiàn)松散的三維團(tuán)絮狀空間結(jié)構(gòu)。在熔點(diǎn)到800℃范圍內(nèi),tib2顆粒的加入能夠破壞這種空間網(wǎng)鏈狀結(jié)構(gòu),大幅減小熔體團(tuán)聚體的尺寸。同時(shí),同步輻射成像結(jié)果表明,tib2細(xì)化劑的加入,可以很好細(xì)化凝固組織,消除溶質(zhì)富集造成的大面積共晶,有效改變凝固區(qū)域的溶質(zhì)和流場(chǎng)分布,提高凝固組織的均衡性。綜上所述,本文對(duì)亞共晶Al-Cu合金枝晶形態(tài)轉(zhuǎn)變機(jī)制,特征結(jié)構(gòu)處凝固組織演化規(guī)律以及細(xì)化劑抑制結(jié)構(gòu)效應(yīng)作用機(jī)理的研究,為調(diào)控鑄件凝固組織提供了科學(xué)依據(jù)。研究結(jié)果為枝晶形態(tài)研究提供了有益補(bǔ)充。
[Abstract]:Microstructure control is the key to obtaining high quality, low defect castings, especially for large and complex thin wall parts with complex structures (such as variable section T structure, thin wall structure). The distribution of solute field and flow field are the key factors in the formation of solidification microstructure. The relationship between solidification microstructure and multi-field distribution was studied by synchrotron radiation method. The results show that a new two-cell crystal growth model, TIB _ 2 particle refinement mechanism, and the characteristics of microstructure formation during solidification of Al-Cu alloy are shown. The dendritic morphology and the growth phase of dendrite in hypoeutectic Al-Cu alloys were studied by synchrotron radiation imaging and traditional electron backscatter diffraction (EBSD) technique. The spatial structure of the new double cell crystal is two dimensional structure, the secondary dendritic arm grows perpendicular to the main stem. The morphology of the double cell dendrite can be equiaxed dendrite, columnar dendrite or coexisting with the normal dendritic arm. The results show that the secondary dendritic arm grows perpendicularly with the main dendritic arm. The growth direction of dendritic tip and secondary dendritic arm are 110 and 001 respectively. The formation probability of dendritic double crystal is strongly dependent on Cu concentration. According to the experimental data and theoretical calculation results, the morphologies of hypoeutectic al-cu alloys were modified. The transition form between dendritic dicellular crystal and dense dendrite is determined. The microstructure evolution of the meso-eutectic al-cu alloy mentioned above has been studied by synchrotron radiation imaging. According to the imaging radiation, the microstructure of the meso-eutectic al-cu alloy has been studied. The difference in absorption contrast presented by the film is correlated with the time of the imaging. The effects of temperature field, solute field and flow field on the evolution of solidification structure are analyzed. The temperature distribution at the front of the growth interface has obvious structural correlation. Different structures lead to different solute enrichment regions and melt flow states, resulting in different solidification sequence and microstructure. The wall thickness is the key factor influencing the evolution of solidification structure. Raising the pouring temperature and casting mold temperature has little effect on improving the filling and shrinkage of thin-walled structure. The cooling rate increases with the decrease of wall thickness. The uniformity of equiaxed grains decreases with the decrease of wall thickness. In this paper, the effect of adding tib2 particles on the structure of al-15wt%cu melt has been studied by means of X-ray small angle scattering. The results show that the aggregates in the melt of al-cu show the fractal character of mass. In addition, in the range of melting point to 800 鈩，

本文編號(hào)：1541642