【作者】 张连勇；

【导师】 周彼德；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2007， 博士

【摘要】 相变致冷技术是一种新的凝固技术,它是采用铜模铸造的方法,用相变材料作为冷却介质,利用相变材料在相变过程中吸收的潜热对熔体进行冷却的一种技术。本文采用此种凝固技术,制备了铸造A356铝合金和铜基大块非晶合金。本文利用Ansys软件对相变致冷技术中的铸件和铸型温度场进行模拟。模拟结果表明,熔体的冷却速度随着相变介质的种类和体积的变化而变化。对于Na、Sn和Zn三种相变介质来说,采用介质Na使熔体获得的冷却速度最高,Sn次之,Zn最弱,这些是由介质不同的热物性所造成的。铸件冷却曲线和介质升温曲线的模拟结果与实验结果相比基本吻合。对A356合金的研究结果表明,凝固条件对合金的组织是有影响的。介质对合金的细化效果依Na、Sn、Zn的次序依次减弱,而铜模对铸件的细化能力在Na和Sn之间。铸件与介质体积比不同,细化效果也不同。随着铸件与介质体积比的增加,合金的组织先逐渐细化,而后又逐渐粗化,介质与铸件的体积比对合金的细化效果存在一个最佳值。当其体积比为5.25时,合金的组织最细小。变质剂与细化剂对合金的组织也有影响。对于采用相变致冷方法制备的A356合金,单独加入Al-10%Sr变质剂(加入量为0.04%)及单独加入Al-5%Ti-B细化剂(加入量为0.03%)时,合金的组织显著细化,显微硬度也明显提高。与单独加入变质剂和细化剂相比,同时加入变质剂和细化剂时,加入量的最佳值发生变化,这是由于变质剂和细化剂形成了化合物,合金组织的细化效果减弱。与铜模冷却方法相比,相变致冷方法可以细化合金的铸态组织。DSC和XRD的研究结果表明,在用铜模冷却方法制备的样品中,Si在α-Al中的固溶度为2.4 at.%,在用相变致冷方法制备的样品中,Si在α-Al中的固溶度为2.7 at.%,合金的强度提高了22.9%,伸长率提高了33.3%。对合金的热处理研究结果表明,传统的T6和T5工艺可以不经过固溶处理,而是直接对合金进行时效处理,最佳的T6处理工艺为:175℃×8h,最佳的T5处理工艺为:140℃×4h,其断裂强度分别320MPa和305MPa。对铜基大块非晶的研究结果表明,用相变致冷方法可以制备出Φ4 mm的Cu50Zr40Ti10和Φ5 mm的Cu47.5Zr47.5Al5大块非晶合金。差示扫描量热分析研究结果表明,这两种大块非晶合金的晶化过程都是一种依赖于升温速率的动力学过程。采用原位高温X射线衍射分析对Cu50Zr40Ti10大块非晶合金在连续加热条件下的晶化过程进行了研究,结果表明:Cu50Zr40Ti10大块非晶合金在连续加热的过程中首先析出了一种单斜CuZr固熔体相,该相为亚稳相,晶化结束后的稳定相为正交Cu10Zr7相、正交Cu8Zr3相和四方Cu3Ti2相。对两种大块非晶合金的室温压缩力学性能进行了研究,结果表明:Cu50Zr40Ti10大块非晶合金在压缩过程中具有较高的强度和良好的塑性。更多还原

【Abstract】 A new rapid-cooling technology using a copper mould cooled by a phase-transition medium was used to prepare cast aluminium-A356 alloy and Cu-based bulk metallic glass by solidification of the melt in this paper.The temperature fields of the casting and the mould with phase-transition cooling were simulated using the Ansys software package. The simulated results showed that the cooling rates of the melt could be controlled by using different phase-transition materials and their volumes. Sodium, tin and zinc were selected as the phase-transition materials in this paper. The cooling rate of melt obtained by using Sodium as phase-transition medium is the highest one of these three media, followed by tin, zinc. This was caused by their different thermal physical properties. The simulated curves of melt and media were in good agreement with the experiment curves.The microstructures and mechanical properties of aluminium-A356 alloy cast with this technology were studied in this paper. The results showed that the solidification conditions affected the microstructures of this alloy. The effect of refining on the microstructures was weakened in the order: sodium, tin, and then zinc. The refining ability of copper was between sodium and tin. The different ratios of volumes between the casting and the media also affected the microstructures of this alloy. The microstructure was refined as the ratios increasing. The microstructure was the finest when the ratio was 5.25, and when higher than this ratio the microstructure became coarser. The microstructure was modified by adding Al-10%Sr and refined by adding Al-5%Ti-B to the melt. The microstructure was refined obviously and the microhardness was improved when the amount of Al-Sr10% was 0.04% or the amount of Al-5%Ti-B was 0.03%. However, the microstructure became coarser when both Al-10% Sr and Al-5%Ti-B were added in the melt, and the microhardness decreased. Compounds formed by the combination of modifier and refiner weakened the effect of refining on the microstructure.The microstructures of the casting solidifying with a phase-transition material and copper were investigated by scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and X-rays diffraction (XRD). The results showed that the microstructure obtained by the phase-transition cooling method was finer than that obtained by copper mould. The solubility of silicon inα-Al was 2.7 at. % in specimens solidifying in phase transition cooling medium and 2.4 at. % in specimens solidifying in copper mould. The ultimate tensile strengths (UTS) and elongation of A356 alloy cast by phase-transition cooling method were improved 22.9 % and 33.3%, respectively. The optimal heat treatment of T6 is 175℃×8h and the UTS is 320MPa. The optimal heat treatment of T5 is 140℃×4h and the UTS is 305MPa.Bulk metallic glasses of Cu50Zr40Ti10 with a diameter of 4 mm and Cu47.5Zr47.5Al5 with a diameter of 5 mm were prepared by using phase-transition cooling methods and their crystallization kinetics and mechanical properties have also been investigated in this paper. The crystallization kinetics of these two kinds of bulk metallic glasses was investigated by means of DSC under nonisothermal conditions. The results showed that the crystallization process of these two kinds of bulk metallic glasses could be regarded as a kinetically modified thermodynamic phase transformation process.Real-time detection of the crystallization behavior of Cu50Zr40Ti10 bulk metallic glass from the glassy state was studied using X-ray diffraction during in-situ heat treatment. The results showed that monoclinic CuZr metastable phase is firstly precipitated in the glassy matrix during the continuous heating process, and the equilibrium phases were orthorhombic Cu10Zr7, orthorhombic Cu8Zr3 and triclinic Cu3Ti2 at the final stage of crystallization.The compressive deformation behaviors of these two kinds of bulk metallic glasses at room temperature were investigated at strain rate of 10-4 s-1. The results showed that both the strength and the strain of Cu50Zr40Ti10 bulk metallic glass are much high than those of Cu47.5Zr47.5Al5 bulk metallic glasses.更多还原