K418合金车用增压涡轮熔模铸造过程数值模拟及热裂预测

针对K418合金车用增压涡轮铸件极易出现热裂缺陷的现象,结合工厂实际生产条件,应用ProCAST铸造模拟软件对K418合金车用增压涡轮的充型过程、凝固时间以及凝固过程中的温度场、缩孔缩松形成过程和应力场进行了模拟。基于分析的结果,预测了涡轮中存在的热裂缺陷,模拟结果与实际生产情况吻合良好。分析结果表明,涡轮叶梢处容易产生热裂纹的主要原因是凝固过程中热应力不均引起的。     

K418合金显微组织及其增压器涡轮叶片热裂的研究

针对K418合金增压器涡轮叶片的热裂问题进行了研究。采用光学显微镜和扫描电镜分析了Al、Ti含量对K418合金显微组织的影响。利用Thermo-Calc热力学软件计算了K418合金中可能析出的平衡相,并分析了Al、Ti含量变化对平衡相的影响。结果表明:Al、Ti含量增加后,K418合金的显微组织发生了明显变化,γ′的尺寸增大且形状多样化,(γ+γ′)共晶增多,导致裂纹优先产生和扩展的部位增多;热裂为枝晶组织断裂,热裂纹在枝晶间产生和扩展;Al、Ti含量的增加均提高了γ′的析出温度和析出量,Al的影响尤为明显;Al、Ti含量增加后,均扩大了合金的有效结晶温度范围,导致涡轮叶片产生热裂。     

增压涡轮用镍基高温合金的凝固特性和热裂倾向性

研究了增压涡轮用镍基高温合金K418和K419的凝固特性和热裂倾向性。同时研究了合金元素的偏析行为和析出相。结果表明,凝固末期多种强枝晶间偏析元素在液相中的偏聚导致K419合金的凝固行为较K418复杂。多种元素在枝晶间剩余液相中的偏聚导致K419合金的液相线极低。K419合金凝固过程中漫长的剩余液相期的存在严重削弱了枝晶间结合力,增加了其热裂倾向性。基于一种热裂敏感区模型提出热裂倾向性系数判据,K419合金的热裂倾向性系数高于K418合金。     

TiAl合金增压涡轮断裂分析

某TiAl合金增压器涡轮在超速试验转速达8.6×104r/min时发生断裂失效。通过对失效的TiAl合金增压涡轮及涡轮叶片断口进行宏、微观观察及分析,以确定其失效原因。结果表明：涡轮和涡轮叶片断口的断裂特征主要由沿层断裂及穿层断裂组成;涡轮断裂是从增压器涡轮和涡轮轴颈相连接的圆弧过渡区域处的铸造疏松起源并发生断裂;涡轮断裂失效与层取向、铸造缺陷、圆弧过渡及离心力有关。     

K418高温合金细晶叶片铸造工艺的研究

研究了Ｋ４１８高温合金细晶叶片铸造工艺。研究结果表明，在合适的型壳材料和合理的浇注系统条件下，当型壳温度控制在１０００℃，浇注时金属液过热温度控制在２８～３８℃，精炼温度不过高，则可获得健全的等轴细晶叶片。其晶粒度等级达到ＡＳＴＭ３０～６．０。     

Al-Cu合金热裂倾向性评价

本文采用树脂砂型，开展了Al-Cu合金热裂倾向性试验，分析了试样热节处金相组织、热裂纹萌生时的温度和收缩力变化。结果表明，Cu含量小于5wt.%时，随着Cu含量升高，平均晶粒尺寸减小，靠晶界处析出呈连续网状分布的θ相增多，热裂倾向性减小。随Cu含量升高，热裂纹初始萌生温度降低，裂纹萌生时的固相体积分数先升高后降低。提出用热裂纹初次萌生时的温度减去完全凝固时的温度与合金结晶温度区间的比值判定Al-Cu合金热裂倾向性。该值越小，合金的热裂倾向性越小。     

镶铸件数值模拟及其热裂倾向性的判定

应用数值模拟技术对钢／高铬铸铁镶铸件的热裂问题进行了研究。以镶块和本体铸件间相对收缩速率的大小作为镶铸件热裂倾向性大小的依据,并给出了基于温度场数值模拟的相对收缩速率的表达式和临界热裂判据式。数值模拟结果表明：提高浇注温度和减小镶块镶入部分宽度均可减小相对收缩速率的大小。     

铸件热裂预测研究进展

简述了铸件热裂的形成机理,重点介绍了铸件热裂预测的研究方法和发展趋势,基于流变学模型的热裂数值模拟是一个很有意义的研究方向之一。     

K418合金表面激光再制造Inconel718高温合金涂层性能研究

针对K418高温合金叶轮高温、高转数工况下形变超差大,再制造成形层力学性能降低的领域研究热点,基于波形可调制脉冲激光优化工艺,再制造Inconel718高温合金成形层。采用金相显微镜、SEM、XRD、EDS、显微硬度计对该合金涂层的显微组织形貌、相结构、界面成分分布、基本力学性能进行研究。结果表明:涂层与基体间为致密的冶金结合,涂层中部为粗大的树枝晶,与激光扫描方面成30o^45o角,由涂层中部到顶部及底部,分别退化为等轴晶和胞状晶;成形层显微硬度为4100~4400 MPa,略高于基体;晶内及晶间析出少量Laves相,减少了对成形层硬脆性的影响;摩擦磨损系数较基体略有降低,但仍符合再制造要求。     

铁含量对K418合金TCP相析出规律的热力学分析

利用Thermo-Calc软件热力学计算得出铁含量对K418合金的TCP相析出规律的影响。结果表明：K418合金析出相为MC相、M23C6相、γ相、γ′相、σ相、μ相、α-Cr相、M3B2相和液相。随着铁的质量分数从2.5%降至0%,碳化物、硼化物、液相线的含量变化较小,有少量α-Cr相析出。为减少K418合金TCP相析出,铁的质量分数建议控制在1.25%以下,无σ相析出且TCP相含量较少。实际生产中制定K418合金铁含量的目标值还需考虑设备状况和冶炼烧损量。     

K418高温合金和42CrMo合金钢的激光焊接

进行了K418高温合金和42CrMo钢的激光焊接实验,分别采用OM、SEM、XRD和EDS等手段分析了焊缝的金相组织和物相组成,评价了焊缝的显微硬度和拉伸强度。结果表明：焊缝主要由树枝状非平衡凝固的FeCrNiC（力固溶体组成,此外,还有少量细小、弥散的Ni3Al（γ′）相、Laves颗粒和少量MC碳化物分布在树枝晶间区域。由于主要强化相γ′在激光辐照后的部分溶解和随后快速凝固的抑制作用,焊缝的硬度虽分布较均匀但低于母材。由于没有获得穿透的焊接接头,焊接接头的强度约只有母材的88．5％,焊缝的断裂机制是塑性和脆性断裂的混合机制。由于在焊缝中存在一些Laves颗粒,这促进了微裂纹和微孔的形成和扩展,降低了焊接接头的抗拉强度。     

Hot tearing susceptibility of MgZn4.5YxZr0.5 alloys and mechanism

The hot tearing susceptibility of MgZn_(4.5)Y_xZr_(0.5)(x = 0.5, 1, 2, 4, 6) alloys was evaluated using ClyneDavies' theoretical model and a constrained rod casting(CRC) apparatus equipped with a load cell and data acquisition system. The results obtained from these two approaches were in good agreement, illustrating that the hot tearing susceptibility of the investigated alloys is in the order of MgZn_(4.5)Y_xZr_(0.5) MgZn_(4.5)Y_xZr_(0.5) MgZn_(4.5)Y_xZr_(0.5) MgZn_(4.5)Y_xZr_(0.5) MgZn_(4.5)Y_xZr_(0.5). The microstructure and morphology of hot tearing regions were observed by means of X-ray diffraction and scanning electron microscopy. The function curves on variation of contraction force and temperature versus time were recorded by a computer during solidification of the MgZn_(4.5)Y_xZr_(0.5) alloys in a T type mold after A/D(Analog to digital) conversion. Results show that both the amount and morphology of the second phases have a great relationship with the hot tearing susceptibility. But the former plays a more important role on that of MgZn_(4.5)Y_xZr_(0.5) alloys. The role of low-melting point eutectic phases on the hot tearing susceptibility of MgZn_(4.5)Y_xZr_(0.5) alloys changes gradually with different Y contents. When Y ≤ 1wt.%, it is mainly a damaging effect on intergranular bonding force; when Y 1wt.%, it is mainly an intergranular feeding effect on formed separated dendrites.     

Hot tearing in polycrystalline Ni-based IN738LC superalloy: Influence of Zr content

Zirconium is always present in Ni base superalloys as it enhances their creep properties. In the present study, the influence of very small Zr additions, 100–400 ppm, i.e. 0.01–0.04 wt.%, on hot tearing of IN738LC superalloy is experimentally investigated using dedicated turbine blade castings. Although the Zr content remains very small, it has a strong effect on hot tearing tendency. Microstructure of hot tear in as-cast samples reveal that grain size and secondary dendrite arm spacing have no significant effect on hot tearing. On the other hand eutectic phase volume fraction and its dispersion or spreading along grain boundaries drastically affect the hot tearing propensity and strongly increase with increasing amounts of Zr. Hence grain coalescence becomes impossible at grain boundaries covered with eutectic phase films. With increasing Zr content, gain coalescence between two distinct grains with no interdendritic phase requires more undercooling. Coalescence is retarded and occurs deeper in the mush zone, i.e. at lower temperatures resulting in a higher sensitivity to hot tearing. Finally, it is shown that a reduction of Zr content to 0.02 wt.% is required to fully suppress hot tearing in polycrystalline IN738LC blades.     

Study on microstructure and property of K418B superalloy wide gap brazed joints

Wide gap brazing(WGB) experiments using the butted specimen with a 1.5 mm gap were carried out for the repair of the failed K418B superalloy low-pressure turbine vanes.The high temperature creep rupture strengths of the brazed joints were tested,and the microstructures and fracture surfaces of the joints were observed.The results show that the microstructure of K418B alloy joint is composed of dense equiaxed grain,small and discrete compounds with a few micro-pores.During the creep rupture test,the cracks initiate preferentially at the micro-pore or the grain boundary,then propagate along the grain boundary till the fracture happens.The creep rupture strength at 700 ℃ of the brazed joints with 50% braze metal in the working part could exceed 90% that of the K418B superalloy,and the joints with 100% braze metal in the working part achieve 70% to 80% of the creep rupture strengths for the K418 B base metal.     

K418合金和42CrMo钢激光焊焊缝的微观组织

采用透射电镜及附带能谱(TEM-EDS)对K418高温合金涡轮盘和42CrMo合金钢转轴激光焊焊缝的微观组织进行了分析.结果表明:焊缝主要由树枝状非平衡凝固的FeCrNiC(γ)固溶体组成,此外,还有少量细小、弥散的Ni_3Alγ'相、Laves颗粒和少量MC碳化物分布在树枝晶之间区域;根据热动力学原理对形成这一组织特征的原因进行了简要讨论.     

TiAl合金增压器涡轮的耐久性试验研究

针对某增压器台架耐久性考核240 h的直径100 mm的Ti Al合金涡轮,通过对比分析考核前后涡轮轮毂芯部的微观组织和力学性能,并结合未考核涡轮轮毂取样的高温持久试验结果,评价了Ti Al合金增压器涡轮轮毂的耐久性。结果表明,在考核试验后,涡轮轮毂芯部的微观组织、洛氏硬度和室温抗拉强度仍保持考核前的水平;同时,参照该型涡轮服役条件下的高温持久加载后,试样的层片组织仍保持完整,仅只在层片团界析出了体积分数0.5%的等轴γ晶粒。可见,该Ti Al涡轮轮毂具有良好的耐久性,能够满足应用要求。     

Microstructure and mechanical property of MIM 418 superalloy

In this study, the influence of hot isostatic pressing(HIP) process on the 418 alloy produced by metal injection molding(MIM) technique(named as MIM 418)was investigated based on the characteristic analysis of 418 alloy powder. And comparison analysis of the microstructure and mechanical property between the MIM 418 and as-cast 418 alloys was performed by scanning electron microscopy(SEM), energy-dispersive spectroscopy(EDS),and X-ray diffraction(XRD). The results show that MIM418 alloy exhibits fine grain(~30 μm) and uniform microstructure. The defects existing in MIM 418 alloy formed during sintering process can be eliminated through HIP treatment, and the relative density increases from97.0 % to 99.5 %. The mechanical property can be improved significantly because of the elimination of defects, and the tensile strength and elongation are1,271 MPa and 16.8 %, respectively, which are increased by 34.5 % and 180 % compared with K418 alloy after solution heat treatment.     

Hot tearing behavior of NZ30K Mg alloy under progressive solidification

Progressive solidification is usually considered an effective strategy to reduce the hot tearing susceptibility of a cast component.In this study,special constrained plate castings with progressive changes in cross-section were designed,which enabled progressive solidification.The hot tearing behavior of a newly developed NZ30 K Mg alloy(Mg-3.0 Nd-0.2 Zn-Zr,wt.%)was studied under progressive solidification using various mold temperature distributions and constraint lengths.Of these,a homogeneous mold temperature distribution is found to be the best option to avoid hot tearing,followed by a local low mold temperature distribution(with a chiller),then a gradient mold temperature distribution.Unexpectedly,compared with the homogeneous mold temperature distribution,adding a chiller does not provide any further reduction in the hot tearing susceptibility of the NZ30 K Mg alloy.A high mold temperature and a short constraint length increase the hot tearing resistance of cast Mg alloys.Progressive solidification is not a sufficient and necessary condition to avoid the formation of hot tearing.The two key factors that determine the occurrence of hot tearing under progressive solidification are the maximum cooling rate and the constraint length.Decreasing these values can reduce the incidence of hot tearing.