Enhanced laser weldability of an aerospace superalloy by thermal treatment

ABSTRACT

Weldability improvement of the Inconel 738 alloy, subjected to different pre-weld heat treatments and, thermomechanical fatigue of the unwelded and welded alloy is investigated. A new pre-weld heat treatment is found to significantly reduce the susceptibility of the extremely difficult-to-weld alloy to weld cracking. The heat treatment also produces welded material with enhanced resistance to thermomechanical fatigue failure at high mechanical strain ranges, in comparison to the unwelded alloy. Furthermore, by comparing polycrystalline and single crystal materials, without altering the chemistry/microstructure of the superalloy, the presence of grain boundaries in the alloy is found to aid damage mechanisms during in-phase thermomechanical fatigue.     

Melting during heat treatment of a sheet of a niobium alloy – Failure analysis

The niobium alloy Nb‐5.4 At% Hf‐2 At% Ti is an important structural material used for high temperature applications. It is commonly produced in the form of sheet; however, it is also available in other mill forms. The sheets are vacuum‐annealed to improve the formability prior to fabricating different components out of them through metal forming. Annealing in vacuum is adopted in view of the high proneness to oxidation of niobium‐ base materials. In the present instance vacuum annealing of a sheet of the niobium alloy was carried out at 1175 ^oC; a plate of the nickel‐base superalloy 600 was used as charge support. It was noticed that the vacuum annealing resulted in melting of the charge as well as the charge support material. The plate of superalloy 600 was then replaced by a sheet of commercially pure titanium and next trial of annealing heat treatment carried out; there was no problem of melting. Failure analysis was carried out. The analysis brings out how phase diagrams can be used to choose the right material for supporting the charge in order to prevent melting during heat treatment. The failure is very expensive, if the material to be heat treated is costly, as it happens to be in the present case.     

Alumina composite coatings with enhanced high-temperature electrical insulating properties on Ni-based superalloy substrates

Electrical insulation of nickel-based superalloy substrate, especially at high temperature range, is one of the major challenges for the reliability and stability of the integrated thin-film sensors. Here, we report a solution-processed approach to fabricating high-temperature, electrically insulating coatings on Ni-based superalloy substrates. NiCrAlY coatings were fabricated by DC magnetron sputtering and heat-treated, and then Al₂O₃ films were deposited by sol-gel method. X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy were used to characterize the composition, phase, microstructure and morphology of these composite coatings. The electrical resistance of the composite coating was measured as a function of temperature up to 800 °C. Electrical resistance greater than 1 MΩ were consistently achieved from 600 °C to 800 °C. Moreover, this insulating coating survived thermal shock and thermal fatigue tests without cracking or delaminating. A type S thin-film thermocouple was prepared on the composite coating to verify its high-temperature electrical insulation performance.     

Crack growth mechanisms in an aged superalloy at high temperature

Turbine blades made of Ni-base superalloy IN738 in stationary gas turbine had been removed during routine maintenance procedures after 10,000–20,000 h of operation. Crack growth in this material was studied by performing lab tests with specimens directly cut from the hot area of these blades with testing temperatures of 20 °C and 700 °C. The crack paths in these tests were analyzed and compared to those of service cracks. It was found that even though intergranular crack growth was dominant at elevated temperatures there was still a certain amount of transgranular crack propagation in all cases. Transgranular crack growth under operating conditions led to local recrystallization in the plastic zone ahead of the crack tip. The implications of the recrystallization process on the crack growth rate are discussed in the paper.     

凝固速率对单晶高温合金元素含量和显微结构的影响

本文研究了凝固速率对单晶高温合金凝固界面形态、合金元素含量、γ’相及γ／γ’相界面结构的影响。结果表明，随凝固速率的减小，凝固界面从细枝、粗枝向胞状、平面转变，γ’相形成元素挥发严重，弥散分布的γ’相含量减少，γ’相严重合并粗化，γ／γ’相界面失去共格，亚晶界量减少。     

镍基铸造高温合金热等静压处理的研究

就热等静压处理对镍基铸造高温合金的缺陷消除、组织和力学性能的改善作了研究。研究结果表明，通过热等静压处理后的合金，不仅可有效地消除合金中的缺陷（疏松和气孔），获得致密的合金，而且还可改善合金的显微组织，提高合金的持久、拉伸和疲劳性能，显著地减小性能分散度。     

Modelling high-temperature creep of anisotropic material

Cast single-crystal superalloys are being used as turbine blades in modern aeroengines with 〈001〉 axis oriented along the direction of principal stress. This allows use of higher operating temperature with associated improvement in efficiency. Although 〈001〉 is the natural direction of crystal growth in these alloys it is often not possible to attain perfect alignment. Consequently, besides time-dependent deformation this would induce a change in cross-sectional shape and crystal orientation of the blade as a results of service exposure. Therefore in order to exploit their full potential it is necessary to develop a predictive system which could model effect of crystallographic anisotropy on hightemperature creep deformation of such materials. The paper reviews the recent developments in this direction and suggests that modelling not only helps in identifying the dominant mechanisms of deformation but also suggests simple experiments to verify them.     

Performance of alumina-based ceramic inserts in high-speed machining of nimonic 80A

The study of machining forces and cutting tool wear during the machining is important for designing and selection of machining system and improving the productivity. This study reports the machinability of Nimonic 80A superalloy with alumina-based ceramic inserts. The objective is to analyze the reason for higher cutting forces generated during machining and tool wear mechanism on machining parameters. The cutting forces and tool wear are found to be mainly influenced by the cutting speed. The main causes of tool failure while machining Nimonic 80A are adhesion and abrasion. The role of tool wear is more dominant on the surface finish at lower cutting speed. Also, with an increase in cutting speed, thermally activated wear quietly increases at tool surfaces. The mechanistic approach is used to model the main cutting force. Developed cutting force model agrees well with experimental cutting force values.     

Forming behavior and workability of Hastelloy X superalloy during hot deformation

The hot deformation behavior of Hastelloy X superalloy has been characterized using hot compression tests in the temperature range of 900–1150 °C and strain rates varying between 0.001 and 0.5 s⁻¹. The results showed that both kinds of softening mechanisms, dynamic recovery and dynamic recrystallization, occurred during hot working. Hot workability of this alloy has been analyzed by calculating flow localization parameter and construction of workability map for occurrence of shear band. In addition, on the basis of flow stress data obtained as a function of temperature and strain rate in compression, power dissipation map and instability map for hot working have been developed.     

The characteristics of serrated flow in superalloy IN738LC

Serrated flow was investigated in superalloy IN738LC, a nickel-base γ′ age-hardened alloy. In this material serrated flow appeared between 350 and 450 °C and strain rate of (8.77 × 10⁻⁵ to 8.77 × 10⁻³) s⁻¹. Activation energy for this process was calculated to be 0.69–0.86 eV which is in good agreement with the values reported for similar alloys. Results show that the diffusion rate of substitutional solute atoms at this temperature range is too low to cause this effect. This suggests that the interaction of solute atoms and moving dislocation is responsible for the observed serrated flow in this alloy.