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新型粉末高溫合金多火次等溫鍛造過程中晶粒細化機制

Mechanism of grain refinement of an advanced PM superalloy during multiple isothermal forging

  • 摘要: 為探索多火次等溫鍛造對新型粉末高溫合金晶粒細化的影響, 本文對實驗合金進行了每火次變形量40%左右的三火次等溫鍛造, 采用商用有限元軟件DEFORM 2D模擬鍛造過程中的等效應變分布圖, 采用電子背散射衍射技術對各火次后的鍛坯進行顯微組織觀察和分析.研究表明: 等溫鍛造過程中, 鍛坯軸向剖面大致分為三個區域, 位于上、下兩端面附近的Ⅰ區變形量最小, 位于兩側附近的Ⅱ區次之, 位于剖面中心的Ⅲ區變形程度最大.經過三火次等溫鍛造后, 鍛坯Ⅱ、Ⅲ區再結晶充分, 獲得等軸細晶組織, 平均晶粒尺寸2~3 μm.然而Ⅰ區形成再結晶不完全的"項鏈"組織, 在變形晶粒周圍分布大量細小的再結晶晶粒, 變形晶粒內小角度晶界含量較多, 位錯密度較高.通過對三火次后的鍛坯進行合適的熱處理, Ⅰ區"項鏈"組織得到細化, Ⅱ、Ⅲ區組織發生晶粒長大, 整個盤坯為較均勻的細晶組織, 平均晶粒尺寸為6~8 μm.

     

    Abstract: Nickel-base powder metallurgy (PM) superalloys are widely used as high temperature components in gas turbine engines owing to their outstanding mechanical properties and workability under intense heat. In order to meet the performance requirements of a new generation aircraft engine with a higher thrust-weight ratio, the fourth generation PM superalloy has been studied at home and abroad. Its operating temperature has been raised to 815-850℃. The alloy in this study was a newly-designed fourth generation PM superalloy, which exhibited excellent high temperature stress rupture and creep properties compared with the previous three generations' PM superalloys, FGH4095, FGH4096, and FGH4098. Based on the performance characteristics of PM superalloys of different grain sizes, dual microstructure heat treatment (DMHT) has been used to produce a turbine disk which has a fine-grained bore and a coarse-grained rim. Therefore, it was first necessary to obtain a uniform fine-grained disk. It has been demonstrated that the fine-grained disk can be gained through hot isostatic pressing (HIP) and multi-steps of high temperature working. In order to study the influence of multiple isothermal forging (ITF) on the grain refinement of the advanced PM superalloy, three steps of ITF were employed; each deformation was about 40%. The effective strain distribution of the alloy during ITF was simulated by using the commercial finite element software DEFORM 2D. Microstructures of those forgings were investigated by means of the electron back scattered diffraction (EBSD) technique. The experimental results show that during ITF, the axial section of the forging is divided into three regions. Region Ⅰ, located in the upper and lower end faces, has the smallest deformation. Region Ⅱ is located at both sides of the section, and its deformation is larger than that of region Ⅰ. And region Ⅲ, located in the center of the section, obtains the maximal deformation. After three steps of ITF, Regions Ⅱ and Ⅲ of the forging are fully recrystallized, and equiaxed fine-grained microstructures with an average grain size of 2-3 μm are generated. Nevertheless, necklace structures form near Region Ⅰ of the forging. A great amount of fine recrystallized grains distribute around the non-equiaxed deformed grains. The deformed grains contain plenty of low-angle grain boundaries (LAGBs), which mean that the dislocation density is very high. Through proper heat treatment, the necklace structure in Region Ⅰ is refined. Meanwhile, grain growth occurs in Region Ⅱ and Ⅲ. These findings suggest that fine-grained disks with uniform microstructures can be achieved, and the average grain size is 6-8 μm.

     

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