發(fā)布時間：2018-05-18 18:35

  本文選題：Al基復合材料 + 粉末觸變成形　； 參考：《蘭州理工大學》2016年碩士論文

【摘要】：本文提出了一種新型的原位自生芯-殼結構粒子Ti@Al_3Ti增強的Al基復合材料及其制備技術—粉末混合觸變成形。該新型復合材料有望克服現(xiàn)有粒子增強鋁基復合材料質(zhì)脆的問題,該技術綜合了粉末冶金技術和觸變成形技術的優(yōu)點。先以粉末冶金法的混粉和壓實步驟獲得混合粉末冷壓制塊,然后利用觸變成形的部分重熔和成形步驟,最終獲得復合材料零部件。部分重熔過程不僅可以獲得觸變成形所需半固態(tài)組織,而且通過原位反應得到Ti@Al_3Ti芯-殼結構的增強體粒子。本文以Ti-Al-2024Al混合粉末冷壓塊為研究對象,研究了Ti-Al-2024Al混合粉末冷壓塊在部分重熔過程中的組織演變,同時通過點滴實驗模擬了Ti與Al之間的反應,目的是為后期觸變成形奠定理論基礎。研究結果表明:Ti-Al-2024Al混合粉末冷壓塊在640℃加熱60min之后,可獲得細小、近球狀初生相顆粒均勻懸浮于液相中的理想的半固態(tài)組織。部分重熔過程中的微觀組織演變可以分為四個階段:2024Al粉末中共晶組織的溶解導致的粉末內(nèi)部晶粒的快速粗化(0~5min);球狀初生相顆粒的形成和顆粒間的液相薄膜的形成(5min~15min),一個球狀粉末演變成一個初生相顆粒;粉末部分熔化導致液相率增加和為了減小固液界面能而發(fā)生的初生相顆粒的輕微粗化(15min~25min);初生相顆粒的緩慢粗化(25min~)。過低或過高的重熔溫度都不能獲得理想的半固態(tài)組織,結合基體合金在不同溫度重熔60min的組織演變情況,640℃為最佳的半固態(tài)重熔溫度。組織中的孔隙數(shù)量隨著加熱時間的變化情況也分為三個階段:部分重熔初期(0~15min),組織中的液相率低,由元素擴散系數(shù)不同引起的Kirkendall效應占主導地位,導致孔隙數(shù)量隨加熱的延長而迅速增加。重熔中期(15~30min),試樣溫度上升,粉末部分熔化,組織中液相率增加,液相對孔隙的填充作用逐漸占主導地位,導致孔隙數(shù)量隨加熱時間的延長而降低。重熔后期(30min以后),相變引起孔隙數(shù)量增加占主導:Ti轉變成Al_3Ti,體積膨脹,顆粒之間的斥力增加,導致組織中的孔隙數(shù)量迅速增加。組織中孔隙數(shù)量隨溫度的變化情況表明:隨著部分重熔溫度的升高,組織中的液相增加,液相對孔隙的填充作用遠大于Kirkendall效應。從而使孔隙數(shù)量隨重熔溫度的升高而不斷減小。隨著加熱時間的延長,在液相填充和減小固-氣界面能的驅動下,組織中孔隙的形狀逐漸趨于圓整,孔隙的尺寸也逐漸減小。在640℃下加熱15~20min Ti粉顆粒與Al熔體反應形成一種由金屬間化合物Al_3Ti致密層包裹Ti芯的殼-芯的芯-殼結構粒子-Ti@Al_3Ti。隨著加熱時間的延長,Al_3Ti反應層沿Ti粉末的徑向生長,厚度逐漸增加。對于一定尺寸的Ti粉末而言,當Al_3Ti反應層達到一定厚度時,在Kirkendall效應、相變引起的體積變化和Al_3Ti脆性本質(zhì)三者的綜合作用下,孔洞和裂紋會在Al_3Ti反應層中形成,導致Al_3Ti相的破碎和剝落。接著又形成一致密層,隨后又破裂、剝離,如此反復直至Ti粉顆粒反應完。當加熱至210min以后,Ti顆粒幾乎全部反應完全,形成中間致密外層疏松的Al_3Ti顆粒聚集體。Al_3Ti反應層的厚度隨加熱時間呈拋物線規(guī)律增長,其關系式可表述為88.0(28)(35)1.0 tx。經(jīng)計算,由TTiAli3?相變引起的體積膨脹大約為261%,因體積膨脹在反應層中引起的應力大小可由(7) 計算得到。Al_3Ti反應層的厚度隨部分重熔溫度的升高呈線性增長。點滴實驗結果表明:Al_3Ti反應層是雙向生長的,但向Ti板一側的推進速度要小于往Al一側推進的速度,原因是由于Ti原子通過Al_3Ti向Al熔體中的擴散速率要大于Al原子通過Al_3Ti向Ti板中的擴散速率。通過統(tǒng)計和分析可知,粉末壓塊和點滴實驗中反應層厚度隨時間的二次擬合關系式分別為:17.021.443.02tt X(10)(10)-(28)b,06.097.112.02tt X(10)(10)-(28)a,表明混合粉末壓塊實驗中Al-Ti的反應速率快,這是由于在粉末壓塊實驗中,Ti顆粒與Al液的接觸面積要大于點滴實驗中Ti板與Al液的接觸面積。此外,點滴實驗中在反應開始階段,反應層的生長以原子擴散為主,在反應后期,由原子擴散轉為晶間擴散,反應速率逐漸減緩。點滴實驗中Al_3Ti反應層厚度隨溫度呈線性增長。
[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.
【學位授予單位】：蘭州理工大學
【學位級別】：碩士
【學位授予年份】：2016
【分類號】：TB33

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