Ti-Al-2024Al混合粉末冷压块部分重熔过程中的组织演变

发布时间：2018-05-18 18:35

本文选题：Al基复合材料 + 粉末触变成形　；参考：《兰州理工大学》2016年硕士论文

【摘要】：本文提出了一种新型的原位自生芯-壳结构粒子Ti@Al_3Ti增强的Al基复合材料及其制备技术—粉末混合触变成形。该新型复合材料有望克服现有粒子增强铝基复合材料质脆的问题,该技术综合了粉末冶金技术和触变成形技术的优点。先以粉末冶金法的混粉和压实步骤获得混合粉末冷压制块,然后利用触变成形的部分重熔和成形步骤,最终获得复合材料零部件。部分重熔过程不仅可以获得触变成形所需半固态组织,而且通过原位反应得到Ti@Al_3Ti芯-壳结构的增强体粒子。本文以Ti-Al-2024Al混合粉末冷压块为研究对象,研究了Ti-Al-2024Al混合粉末冷压块在部分重熔过程中的组织演变,同时通过点滴实验模拟了Ti与Al之间的反应,目的是为后期触变成形奠定理论基础。研究结果表明:Ti-Al-2024Al混合粉末冷压块在640℃加热60min之后,可获得细小、近球状初生相颗粒均匀悬浮于液相中的理想的半固态组织。部分重熔过程中的微观组织演变可以分为四个阶段:2024Al粉末中共晶组织的溶解导致的粉末内部晶粒的快速粗化(0~5min);球状初生相颗粒的形成和颗粒间的液相薄膜的形成(5min~15min),一个球状粉末演变成一个初生相颗粒;粉末部分熔化导致液相率增加和为了减小固液界面能而发生的初生相颗粒的轻微粗化(15min~25min);初生相颗粒的缓慢粗化(25min~)。过低或过高的重熔温度都不能获得理想的半固态组织,结合基体合金在不同温度重熔60min的组织演变情况,640℃为最佳的半固态重熔温度。组织中的孔隙数量随着加热时间的变化情况也分为三个阶段:部分重熔初期(0~15min),组织中的液相率低,由元素扩散系数不同引起的Kirkendall效应占主导地位,导致孔隙数量随加热的延长而迅速增加。重熔中期(15~30min),试样温度上升,粉末部分熔化,组织中液相率增加,液相对孔隙的填充作用逐渐占主导地位,导致孔隙数量随加热时间的延长而降低。重熔后期(30min以后),相变引起孔隙数量增加占主导:Ti转变成Al_3Ti,体积膨胀,颗粒之间的斥力增加,导致组织中的孔隙数量迅速增加。组织中孔隙数量随温度的变化情况表明:随着部分重熔温度的升高,组织中的液相增加,液相对孔隙的填充作用远大于Kirkendall效应。从而使孔隙数量随重熔温度的升高而不断减小。随着加热时间的延长,在液相填充和减小固-气界面能的驱动下,组织中孔隙的形状逐渐趋于圆整,孔隙的尺寸也逐渐减小。在640℃下加热15~20min Ti粉颗粒与Al熔体反应形成一种由金属间化合物Al_3Ti致密层包裹Ti芯的壳-芯的芯-壳结构粒子-Ti@Al_3Ti。随着加热时间的延长,Al_3Ti反应层沿Ti粉末的径向生长,厚度逐渐增加。对于一定尺寸的Ti粉末而言,当Al_3Ti反应层达到一定厚度时,在Kirkendall效应、相变引起的体积变化和Al_3Ti脆性本质三者的综合作用下,孔洞和裂纹会在Al_3Ti反应层中形成,导致Al_3Ti相的破碎和剥落。接着又形成一致密层,随后又破裂、剥离,如此反复直至Ti粉颗粒反应完。当加热至210min以后,Ti颗粒几乎全部反应完全,形成中间致密外层疏松的Al_3Ti颗粒聚集体。Al_3Ti反应层的厚度随加热时间呈抛物线规律增长,其关系式可表述为88.0(28)(35)1.0 tx。经计算,由TTiAli3?相变引起的体积膨胀大约为261%,因体积膨胀在反应层中引起的应力大小可由(7) 计算得到。Al_3Ti反应层的厚度随部分重熔温度的升高呈线性增长。点滴实验结果表明:Al_3Ti反应层是双向生长的,但向Ti板一侧的推进速度要小于往Al一侧推进的速度,原因是由于Ti原子通过Al_3Ti向Al熔体中的扩散速率要大于Al原子通过Al_3Ti向Ti板中的扩散速率。通过统计和分析可知,粉末压块和点滴实验中反应层厚度随时间的二次拟合关系式分别为:17.021.443.02tt X(10)(10)-(28)b,06.097.112.02tt X(10)(10)-(28)a,表明混合粉末压块实验中Al-Ti的反应速率快,这是由于在粉末压块实验中,Ti颗粒与Al液的接触面积要大于点滴实验中Ti板与Al液的接触面积。此外,点滴实验中在反应开始阶段,反应层的生长以原子扩散为主,在反应后期,由原子扩散转为晶间扩散,反应速率逐渐减缓。点滴实验中Al_3Ti反应层厚度随温度呈线性增长。
[Abstract]:In this paper, a new type of in situ core shell structure particle Ti@Al_3Ti reinforced Al matrix composite and its preparation technology, powder hybrid thixoforming, are proposed. The new composite is expected to overcome the problem of the quality brittleness of the existing particle reinforced aluminum matrix composites. This technology combines the advantages of powder metallurgy and thixotropy. The mixed powder cold pressing block is obtained by powder metallurgy and compacting steps. Then the part remelting and forming steps of thixforming process are used to get the composite parts. The partial remelting process can not only obtain the semi-solid structure required for thixotropy, but also get the reinforced particles of the Ti@Al_3Ti core shell structure by the primary reaction. In this paper, the microstructure evolution of the Ti-Al-2024Al mixed powder cold press block in the process of partial remelting was studied. The reaction between Ti and Al was simulated by a drop experiment. The purpose was to lay a theoretical foundation for the later thixotropic forming. The results showed that the cold pressure of Ti-Al-2024Al mixed powder was cold. After the block is heated at 640 C for 60min, the ideal Semisolid Microstructure of the small, near spherical primary phase particles is suspended in the liquid phase. The microstructure evolution in the partial remelting process can be divided into four stages: the rapid coarsening (0~5min) of the internal grains of the powder resulting from the dissolution of the eutectic microstructure of the 2024Al powder; the spheroidal primary phase particles Formation and formation of a liquid film between particles (5min~15min), a spheroidal powder evolved into a primary phase particle; a partial melting of the powder resulting in an increase in liquid phase ratio and a slight coarsening of primary phase particles (15min~25min) in order to reduce the solid-liquid interface energy; the slow coarsening of primary phase particles (25min~). Low or excessive remelting temperature. The ideal semi solid structure can not be obtained, and the microstructure evolution of the 60min alloy at different temperatures is combined with the matrix alloy at different temperatures. The optimum remelting temperature is 640 C. The number of pores in the tissue is also divided into three stages with the change of heating time: the initial stage of partial remelting (0~15min), the low liquid phase ratio in the tissue, and the element diffusion system. The number of different Kirkendall effects led to the leading position, resulting in a rapid increase in the number of pores with the extension of heating. In the middle of remelting (15~30min), the temperature of the sample rises, the powder is partially melted, the liquid phase ratio in the tissue increases, and the filling effect of the liquid relative pore gradually dominates, resulting in the decrease of the number of pores with the heating time. Remelting In the later period (after 30min), the increase of pore volume leads to the increase of the number of pores: the transformation of Ti into Al_3Ti, the expansion of the volume, the increase of the repulsive force between the particles, and the rapid increase in the number of pores in the tissues. The number of pores in the tissue changes with the temperature, and the liquid phase increases with the increase of the remelting temperature and the filling of the liquid relative pore. The effect is far greater than the Kirkendall effect. Thus, the pore number decreases with the increase of the remelting temperature. With the heating time prolonging, the pore shape of the tissue tends to be round and the size of the pores gradually decreases with the increase of the heating time. The 15~20min Ti powder particles and the Al melt are heated at 640. The reaction formed a core shell structure particle -Ti@Al_3Ti. containing Ti core in the dense layer of Al_3Ti intermetallic compound. With the prolongation of the heating time, the Al_3Ti reaction layer grew along the radial direction of the Ti powder, and the thickness gradually increased. For a certain size Ti powder, when the Al_3Ti reaction layer reached a certain thickness, the Kirkendall effect and phase transition were found. Under the combined effect of volume change and the three elements of Al_3Ti brittleness, the holes and cracks are formed in the Al_3Ti reaction layer, resulting in the fragmentation and exfoliation of the Al_3Ti phase. Then, the uniform dense layer is formed, and then it is broken and stripped until the Ti particle reacts. When the heat is added to 210min, almost all Ti particles react completely, shape and shape. The thickness of the Al_3Ti particle aggregate.Al_3Ti reaction layer, which is loose in the middle dense outer layer, increases with the heating time, and its relation can be expressed as 88 (28) (35) 1 TX. and the volume expansion caused by the TTiAli3? Phase transition is about 261%. The stress caused by the volume expansion in the reaction layer can be calculated by (7).Al_3 The thickness of the Ti reaction layer increases linearly with the increase of the partial remelting temperature. The results of a drop experiment show that the Al_3Ti reaction layer is bi-directional, but the speed of propelling to the side of the Ti plate is less than that of the Al side. The reason is that the diffusion rate of Ti atoms in the Al melt through Al_3Ti is greater than that of Al atoms through Al_3Ti to Ti plates. According to the statistics and analysis, the two fitting formulas of the thickness of the reaction layer with time are: 17.021.443.02tt X (10) (10) - (28) - B, 06.097.112.02tt X (10) (10) (10) - (28) a, indicating that the reaction rate of Al-Ti is fast in the testing of mixed powder press, which is due to Ti particles in the powder pressing experiment. The contact area of the Al liquid is greater than the contact area between the Ti plate and the Al liquid in the drop experiment. In addition, the growth of the reaction layer is dominated by atomic diffusion in the initial stage of the reaction, and the reaction rate gradually slows down from the atom diffusion to intergranular diffusion in the later stage of the reaction. The thickness of the Al_3Ti reaction layer increases linearly with the temperature.
【学位授予单位】：兰州理工大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：TB33

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