Influences of processing parameters on microstructure during investment casting of nickel-base single crystal superalloy DD3

The effects of solidification variables on the as-cast microstructures of nickel-base single crystal superalloy DD3 have been investigated by using the modified Bridgman apparatus.The experiments were ...     

Evolution of micro-pores in a single crystal nickel-based superalloy during 980°C creep

The evolution of micro-pores in a single crystal nickel-based superalloy during creep at 980 °C/220 MPa was investigated by X-ray computed tomography. Time-dependent ex-situ 3D information including the number, volume fraction, distribution and morphology of micro-pores was analyzed. The results reveal that the signifi cant formation and growth of micro-pores occur at the end of secondary/beginning of tertiary creep stage. The irregular large pores as well as high density pores located at strain...     

A high thermal gradient directional solidification method for growing superalloy single crystals

The experiments here were conducted at withdrawal rates of 3 mm/min and 1 mm/min using a CMSX-6 and a CMSX-4 superalloy, respectively. The process was assessed in terms of the thermal gradient (G_L), structural refinement, microsegregation and porosity distribution, and compared to those using a Bridgman process. The G_L of the process was 200–236 K/cm, which was 10–12 times higher than that in the Bridgman process. A more refined microstructure was produced having average primary and secondary dendrite arm spacing values as low as 243 μm and 72 μm, as well as 272 μm and 76 μm in the CMSX-6 and the CMSX-4 castings, respectively. The diameter of γ′ phase in the dendrite core of CMSX-6 and CMSX-4 castings was reduced from 0.8 μm to 0.3 μm and from 1.2 μm to 0.6 μm, respectively. The average areas of (γ′ + γ) eutectic pools became smaller and more homogeneously distributed. The mean pore sizes in the castings were reduced by 57% and 43% for the CMSX-6 and CMSX-4 superalloys, respectively, and the area fractions of the pores in the CMSX-6 and CMSX-4 samples were 16% and 12% of those produced in the Bridgman samples. The segregation coefficients of the major alloying elements were closer to unity than those in the Bridgman process, which indicates that the composition distribution is more uniform.     

Effect of solidification parameters on the microstructures of a single crystal Ni-based superalloy AM3

A single crystal Ni-based superalloy AM3 was processed at withdraw rates of 3.5, 10, 50, 100, 200, and 500 μm·s-1, respectively.The as-cast microstructures and solidification segregation ratio were characterized with various withdraw rates.The shape and size of carbide microstructures were determined.As expected, the primary and secondary dendrite arm spacings (PDAS and SDAS) decrease with the increase of withdraw rate.The highest volume fraction of eutectic γ/γ' is observed at the 100 μm·s-1 withdraw rate.The volume fraction of eutectic γ/γ' does not appear to be a strong function of the withdraw rate.With increasing withdraw rate, interface morphologies change in the sequence of planar, cellular, and dendrite.There is a general refinement of the microstructure as the withdraw rate increases.EPMA analysis showed that withdraw rate does not have obvious influence on the segregation of elements.     

DD6单晶合金筏化过程中元素定向迁移行为

对[001]取向DD6单晶合金进行1000°C、300MPa条件下的拉伸蠕变实验,以研究合金的筏化现象以及元素的定向迁移行为,SEM对合金显微组织的观察表明,γ′相在温度和应力的作用下发生了筏化,且筏化方向与应力轴垂直,同时平行于应力轴的基体通道变宽,垂直方向的基体通道逐渐消失。TEM对合金的成分统计分析表明,合金中Al、Re、W元素发生了定向迁移,其中Al向与应力轴平行的基体通道扩散,而Re、W向与应力轴垂直的基体通道扩散,并主要富集于相互垂直的基体通道的交叉区域。     

Multiscale modelling and simulation of single crystal superalloy turbine blade casting during directional solidification process

As the key parts of an aero-engine,single crystal（SX）superalloy turbine blades have been the focus of much attention.However,casting defects often occur during the manufacturing process of the SX turbine blades.Modeling and simulation technology can help to optimize the manufacturing process of SX blades.Multiscale coupled models were proposed and used to simulate the physical phenomena occurring during the directional solidification（DS）process.Coupled with heat transfer（macroscale）and grain growth（meso-scale）,3D dendritic grain growth was calculated to show the competitive grain growth at micro-scale.SX grain selection behavior was studied by the simulation and experiments.The results show that the geometrical structure and technical parameters had strong influences on the grain selection effectiveness.Based on the coupled models,heat transfer,grain growth and microstructure evolution of a complex hollow SX blade were simulated.Both the simulated and experimental results show that the stray grain occurred at the platform of the SX blade when a constant withdrawal rate was used in manufacturing process.In order to avoid the formation of the stray crystal,the multi-scale coupled models and the withdrawal rate optimized technique were applied to the same SX turbine blade.The modeling results indicated that the optimized variable withdrawal rate can achieve SX blade castings with no stray grains,which was also proved by the experiments.     

应力状态对单晶高温合金DD5显微组织的影响

利用高分辨扫描电镜观察了DD5单晶高温合金经1093℃无应力长期时效和1093℃/137 MPa拉伸蠕变试验后的组织,研究应力状态对单晶高温合金的显微组织以及TCP相析出行为的影响.结果 表明:与无应力长期时效相比,应力促进了单晶高温合金中γ'相的筏形组织的形成;剪切应力能够促进45°筏形组织的形成,筏形组织的形态与应力状态密切相关,应力可以促进TCP相的析出.     

氮对316L不锈钢焊缝凝固模式和组织的影响

采用氮含量不同的三种焊丝分别对316L奥氏体不锈钢进行了TIG焊接,通过金相显微镜和扫描电镜对其焊缝微观组织进行了观察,对比分析了焊缝的凝固模式和焊缝组织的析出行为,研究了氮对焊缝凝固模式和组织的影响.结果表明,焊缝氮含量为0.018%时,焊缝的主要凝固模式为初生相为铁素体的FA模式,δ铁素体以蠕虫状或网状分布于枝晶轴上;氮含量增加到0.088%和0.16%时,焊缝的主要凝固模式转变为初生相为奥氏体的AF模式,δ铁素体以颗粒状分布于初生奥氏体枝晶间,其数目明显减少;焊缝奥氏体组织随着氮含量的增加有明显的粗化趋势.     

Effect of carbon on microstructure and mechanical properties of DD99 single crystal superalloy

The effects of carbon on the microstructure and mechanical properties of DD99 single crystal superalloy were investigated. The results show that stress rupture life of DD99 alloy possesses peak value at carbon content of 0.03%（mass fraction）. As carbon addition is greater than 0.03%, the stress-rupture life decreases with the increase of carbon content. The tensile strength and yield strength of DD99 alloy reach peak value at 0.08% carbon and 760 ℃. On the contrary, the tensile strength and yield strength have minimal values at 0.08% carbon and 900℃. The tensile ductility of DD99 alloy basically decreases with the increase of carbon content at 760 ℃ or 900 ℃. The amount of carbides greatly increases with the addition of carbon content. Dislocation moving is retarded by carbides so that dislocation networks are apt to form, which has an important role on the mechanical properties in DD99 single crystal superalloy.     

DD6单晶高温合金的等温氧化行为(英文)

研究第二代单晶高温合金DD6在1050℃和1100℃下的等温氧化行为,分别采用SEM、XRD和EDS对氧化产物进行分析。结果表明,在1050℃和1100℃温度下,DD6合金的100h氧化动力学曲线遵循亚抛物线规律。在1050℃时,氧化产物分为两层,外层为NiO和少量的Al2O3,内层为Al2O3。而在1100℃时,氧化产物分为三层,外层为Al2O3和少量的NiO,中间层为Cr2O3和TaO2,内层由Al2O3组成。在这2种温度下的氧化物下面都形成了无γ′相的区域。     

碳对镍基单晶高温合金碳化物形貌的影响(英文)

在抽拉速率为50μm/s的条件下制备5种不同含碳量的单晶高温合金,研究碳对单晶高温合金中碳化物形貌的影响。研究发现,铸态组织中存在4种形貌的MC型碳化物,呈针状、球状、块状以及中文汉字状。随着碳含量的增加,碳化物的体积分数增大而共晶组织的体积显著减少。同时,碳化物的尺寸随着碳含量的增加亦呈增大趋势。     

真空热处理对DD6单晶合金表面氧化的影响

采用扫描电子显微镜（SEM）自带的INCA250能谱仪对DD6单晶合金真空热处理过程中真空度、真空充气压力、加热温度、保温时间对其表面氧化作用的影响规律进行了研究。结果表明,在不同真空热处理工艺制度下,该合金表面Al、Cr元素含量与O元素含量的变化呈现出一定的相关性,形成的氧化物主要为Al2O3、Cr2O3。表面氧化行为主要受到加热温度、真空充气压力的影响,保温时间对其影响相对有限。该合金的最佳加热温度为1315 ℃,最佳真空充气压力为80 Pa。     

电子束焊热作用对DD407单晶熔凝区裂纹形成的影响

使用电子束焊对DD407单晶高温合金进行焊接,采用光学显微镜和扫描电镜观察分析了熔凝区裂纹形态及分布特征,采用EDS测定了试样枝晶干与枝晶间区域的化学成分,计算了元素的偏析比并分析了裂纹形成原因。结果表明：在电子束热作用下,DD407单晶熔凝区中主要产生纵向裂纹、横向裂纹、弧坑裂纹、晶间裂纹,当其它工艺参数不变时,熔凝区凝固裂纹含量随着电子束流的增大而先增大后减小;DD407单晶在焊接凝固过程中,杂晶区合金元素偏析严重,导致低熔点共晶组织聚集并形成低熔点液态薄膜,在凝固收缩拉应力作用下,使得杂晶区常形成凝固裂纹。     

Effect of competitive crystal growth on microstructural characteristics of directionally solidified nickel-based single crystal superalloy

Directionally solidified single crystal superalloy test bars were prepared by the spiral grain selection method.The microstructural evolution and orientation characteristics of the starter block and spiral part were studied,and the influence of the competitive growth of crystals on the microstructural characteristics was analyzed.The results show that the divergent grain groups,with small size and randomly oriented grains,appear at the bottom of the start block due to the chilling effect,which is an important area for competitive growth.As the height of the starter block increases,the primary dendrite spacing increases,and the grain density decreases;furthermore,the proportion of grains with an orientation deflection angle less than 10°gradually increases.The<001>texture gradually becomes stronger as the height of the starter block increases,which indicates that the competitive growth of crystals gradually weakens.At the initial stage of the crystal selection in the spiral part,the obstacle of adjacent grains and spiral passage is the main working mechanism.The grains located at the inner side of the front edge of the spiral passage have the growth advantage.The single crystal screening process is achieved at about two-thirds of the spiral height,and the single crystal with the orientation deviation angle of 6.7°from the casting axis is prepared.     

原始组织对高温合金热处理组织和力学性能的影响

对具有不同枝晶间距的CMSX 2单晶高温合金的凝固组织特征及其对热处理组织和持久性能的影响进行了研究。考察了不同温度下不同枝晶间距组织的固溶情况及热处理后的组织均匀性 ,测试了持久性能。结果表明 :随着凝固组织的细化 ,合金的组织均匀性得到提高 ,从而有效提高了合金的固溶温度 ,增大了热处理均匀化效果 ;CMSX 2合金铸态超细组织单晶的持久寿命是粗枝晶单晶的 1.4 5倍 ,热处理态超细枝晶单晶的持久寿命是粗枝晶单晶的 1.85倍。单晶高温合金组织超细化是显著提高合金高温力学性能的有效手段     

螺旋选晶器对单晶高温合金晶粒取向的影响(英文)

对于航天推进和发电燃气轮机,为了实现更高的效率和性能并同时降低成本和排放,需要发展先进的镍基单晶高温合金。随着合金的发展,合金的铸造技术已经从定向凝固(DS)发展到单晶(SX)。单晶取向与择优取向的偏差已经成为单晶高温合金最重要的铸造缺陷之一。在自行设计的高温度梯度定向凝固装置中,采用改进的Bridgeman法,用螺旋选晶器拉制出DZ417G合金的单晶。使用XRD和EBSD对其取向进行分析研究。结果表明:较小角度的螺旋选晶器可以有效地减少单晶取向与择优取向的偏差。     

Effect of long-term thermal exposure on microstructure and creep properties of DD5 single crystal superalloy

The effect of thermal exposure on the microstructure and creep properties of the Ni-based single crystal superalloy in different test conditions was studied. Long-term exposure was performed at 1,000 °C and 1,100 °C for 500 h prior to the creep tests. The creep lifetime is found to be improved after the long-term exposure at 1,000 °C for 500 h as a result of the formation of secondary M₂₃C₆ in the interdendritic region. The coarsening of γ′ precipitates accompanied by the formation of TCP phase lead to the degradation of alloy, which is responsible for the reduction of the creep lifetime of Ni-base single crystal superalloy after long-term exposure at 1,100 °C for 500 h. The creep lifetime of 1,000 °C thermally exposed sample under the conditions of 1,093 °C/137 MPa is lower than that of heat-treated state. Thermal exposure at 1,100 °C for 500 h causes the creep lifetime to drop drastically.

     

热处理过程中DD6单晶高温合金的组织演化规律与力学性能

研究了DD6单晶高温合金在热处理过程中的显微组织演化规律以及初熔组织的生成机理。通过研究不同固溶时效处理对γ′相形貌、尺寸分布和体积分数的影响且分析了完全热处理后合金的显微硬度和拉伸性能,从而确定了合金最佳的热处理工艺。结果表明,通过差热分析法和金相观察法确定合金的初熔温度在1300~1310 ℃。在1315 ℃固溶处理4 h,枝晶间/枝晶干γ′相尺寸趋于一致,呈立方状均匀排列。在固溶处理过程中,γ/γ′共晶组织熔化生成了不规则初熔组织。在不同的一次时效工艺下,1120 ℃时效4 h空冷后,γ′相立方度更好,尺寸分布更均匀。合金最佳的热处理工艺为1290 ℃×1 h+1300 ℃×2 h+1315 ℃×4 h, AC+1120 ℃×4 h, AC+870 ℃×32 h, AC。合金在完全热处理后,随拉伸温度从室温升高至850 ℃时,强度达到峰值,温度继续升高,强度下降;在760 ℃拉伸时塑性最差,随着拉伸温度从760 ℃升高到950 ℃,塑性提高。     

合金状态对单晶高温合金DD6再结晶的影响

对单晶高温合金DD6进行表面吹砂处理,然后在1100～1250℃加热4h,研究其铸态与标准热处理状态的表面再结晶行为。结果表明:当加热温度为1200℃时,铸态合金出现等轴再结晶晶粒;而加热温度为1150℃时,热处理态合金出现等轴再结晶晶粒,铸态与热处理态合金的再结晶是由胞状再结晶晶粒和等轴再结晶晶粒组成的;随着加热温度的升高,等轴再结晶晶粒逐渐长大,胞状再结晶晶粒逐渐减少,胞状再结晶内部粗大的γ′相由长条状向颗粒状转变。由于铸态合金含有一定数量的粗大γ′相以及γ+γ′共晶组织,在相同的条件下,铸态合金的再结晶倾向小于热处理态合金的。