Prediction of mechanical properties of superplastic Inconel* 718 using artificial neural networks

Abstract

This paper examines the application of artificial neural networks (ANNs) in materials science and explains the process of using an ANN to develop a constitutive relationship for the widely used nickel based superalloy Inconel 718 (IN718) in the temperature range 950-980°C with strain rates of 10^-4 and 10^-3 s^-1. These parameters are relevant to the high temperature forming of IN718 sheet. The mathematical form of the constitutive relationship derived is given and practical applications of its use are discussed.     

热处理工艺对Inconel718合金组织、力学性能及耐蚀性能的影响

对高含H2S/CO2酸性油气田封隔器材料-Inconel718镍基合金进行固溶处理和时效处理,研究不同热处理工艺条件下合金的组织、力学性能、耐蚀性能之间的关系。结果表明:随着固溶温度的升高,δ相不断溶入基体。材料经时效处理后析出第二相γ″相,硬度和强度明显高于固溶处理的样品,1000℃固溶+720℃×8h→50℃/h620℃×8h时效处理的样品硬度和强度达到最大值。高温高压H2S/CO2介质中挂片实验的结果表明,不同热处理的Inconel718合金均具有良好的耐腐蚀性能,经固溶处理的材料耐腐蚀性略优于经固溶+时效处理的材料。高温高压H2S/CO2应力腐蚀实验的结果表明,Inconel718没有发生应力腐蚀开裂迹象。综合考虑耐蚀性能和力学性能,确定Inconel718合金的最佳热处理工艺为:1000℃固溶1h+720℃×8h→50℃/h620℃×8h时效。     

Machining Characteristics of Inconel 718 by Sinking-EDM and Wire-EDM

Inconel 718 superalloy has wide applications in several industries due to its excellent mechanical properties. However, it is very difficult to machine using conventional cutting and grinding because of its high strength at elevated temperatures. Electrical discharge machining (EDM) is an alternative competitive process to machine Inconel alloys by electrical erosion. However, machinability and surface characteristics of EDMed Inconel surfaces are poorly understood. This study focuses on the machining characteristics of Inconel 718 by Wire-EDM and Sinking-EDM with a new Cu-SiC electrode, respectively. Material removal efficiency, surface roughness, surface topography, surface alloying, and electrode wear have been characterized. It is found that the high toughness of Inconel 718 would be the major contributing factor to the absence of microcracks on the EDMed surface. The new fabricated Cu-SiC electrode for Sinking-EDM has better performance in terms of material removal rate (MRR), surface roughness, and electrode wear. The higher melting temperature and fine microstructure of SiC contribute to the lower electrode wear of the new Cu-SiC electrode than the traditional Cu electrode.     

Modelling hot deformation of Inconel 718 using state variables

Abstract

A dislocation density based state variable model has been developed to describe the characteristic flow stress behaviour during hot deformation of polycrystalline superalloy Inconel 718. Model equations have been formulated to describe the role of the evolving microstructures on the macroscopic flow stress response to deformation. Following a peak in the flow stress associated with strain hardening, the model utilises mechanisms associated with dynamic recovery and recrystallisation to explain the gradual decrease in flow stress with continued deformation. Incorporation of these microstructure based state variables also enables prediction of microstructures associated with a range of hot deformation conditions. Model flow stress predictions have been validated against isothermal uniaxial compression tests conducted over a range of temperatures and strain rates relevant to industrial forging conditions.     

Thermal conductivity of Inconel 718 and 304 stainless steel

The results of thermal conductivity measurements on Inconel 718 and 304 stainless steel by the comparative and flash diffusivity techniques are reported for the temperature range 0–700°C. For 304 stainless steel, excellent agreement with published data is found for the specific heat, thermal diffusivity, and thermal conductivity. In the case of Inconel 718, the measurements show that the conductivity depends critically on the sample thermal history and the metallurgical condition of the alloy. Measurements on a solution-treated sample indicated a conductivity function close to that reported previously, while precipitated samples showed a higher conductivity, similar to the conductivityvs-temperature function used for reduction of comparative thermal conductivity data with Inconel 718 references. These results indicate that Inconel 718 is not a suitable reference for high-accuracy comparative thermal conductivity measurements unless its thermal history and associated conductivity function are known.     

Microstructure and mechanical properties of nickel based superalloy IN718 produced by rapid prototyping with electron beam melting (EBM)

Abstract

A nickel alloy of a composition similar to that of the nickel based superalloy Inconel alloy 718 (IN718) was produced with the electron beam melting (EBM) process developed by Arcam AB. The microstructures of the as processed and heat treated material are similar to that of conventionally produced IN718, except that the EBM material showed some porosity and the δ phase did not dissolve during the solution heat treatment because the temperature of 1000°C apparently was too low. Mechanical testing of the layer structured material, parallel and perpendicular to the built layers, revealed sufficient strength in both directions. However, it showed only limited elongation when tested perpendicular to the built layers due to local agglomerations of pores. Otherwise, data for the hardness, Young’s modulus, 0·2% yield tensile strength and ultimate tensile strength match those recommended for IN718.     

Microstructure development and superplasticity in Inconel 718 sheet

Abstract

Within the range of temperatures and strain rates investigated, mechanical testing identified the optimum forming condition for Inconel 718 sheet as being 965°C with a strain rate of 10^-4 s^-1. A detailed investigation of the microstructural behaviour under these deformation conditions was carried out and is reported here. Typically superplastic deformation would generate an enhanced rate of grain growth. This was not the case with the present material; instead it was found that a slight apparent grain size reduction occurred and there was evidence of substantial dislocation activity. It was concluded that the material was deforming on the borderline between 'true' superplasticity and normal slip based deformation.     

A MULTIAXIAL FATIGUE STRAIN ENERGY DENSITY PARAMETER RELATED TO THE CRITICAL FRACTURE PLANE

The cyclic strain energy density parameter W for the critical or failure plane has been successfully applied in predicting the multiaxial fatigue life of an iron-base and a nickel-base alloy. This parameter has the advantage of being independent of loading condition, allowing a universal energy-life curve to be determined for a variety of torsion, tension and bending stress and strain states. The critical strain energy density parameter has been verified using experimental data obtained from tubular and notched specimens of SAE-1045 steel and Inconel 718.     

Superplastic forming behaviour of complex shapes and post-forming mechanical properties of aluminium based SiCp reinforced metal matrix composites

Abstract

From a commercial viewpoint superplastic forming of complex shapes using only a single operation and one surface tool is appealing, especially for metal matrix composites (MMCs) that are hard to form even at elevated temperatures due to low ductility and toughness. Furthermore, secondary machining operations are difficult due to the presence of extremely hard ceramic reinforcements such as SiC. A range of aluminium alloy based MMCs have indeed been shown to exhibit superplastic properties although most of these studies have been concerned with microstructural characterisation using small uniaxial tensile specimens. This paper therefore concentrates on high strain rate biaxial superplastic forming of complex shapes (critical feature) in MMCs where a forming envelope has been defined and post-forming mechanical properties investigated. Particulate reinforced MMCs based on aluminium alloys 7475 and 7178 were superplastically formed in a die with a 45° step at a range of temperatures and pressures. Formed specimens were sectioned to investigate cavitation and cross-sectional thinning. Tensile tests were performed on parent and formed material to investigate the effect of superplastic forming on mechanical properties. The MMCs were successfully formed over the temperature range 450–550°C achieving step angles α of 22–42°. This study has shown that high strain rate superplasticity (～10^-1 s^-1)can be achieved giving a strain of 70% in only 3.5 s without SiC fracture, reinforcement–matrix decohesion or matrix cavitation making this technique economic and very attractive for commercial exploitation. Cross-sectional thinning was found to be uniform and in the order of ～25% which could be accounted for at the design stage. The high strain rate superplasticity was found to be grain size dependent (<3 µm) but greater profile definitions were achieved when forming took place just above the matrix solidus. Superplastic forming above the matrix solidus temperature resulted in the achievement of the highest step angles in the complex shapes but had a detrimental effect on mechanical properties. This is thought to be due to the liquid phase present that aids grain boundary and interfacial sliding but has a similar effect to overheating during solution treatment and brittle phases are formed at the grain boundaries.     

Multisheet structure of Inconel 718 superalloy processed by LBW/SPF technology and its microstructure

Abstract

Multisheet structure of Inconel 718 superalloy will be widely used in vehicles as heat resisting and heat shielding structure due to its lightweight, high strength and stiffness. Multisheet structure of Inconel 718 superalloy was processed by laser beam welding and superplastic forming (LBW/SPF) technology in the present paper. Multisheet structure of Inconel718 superalloy processed by LBW/SPF technology exhibits good configuration and uniform thickness distribution. Laser beam welding parameters for multisheet structure were as follows: pulse frequency was 32 Hz; pulse duration 3 ms; peak power per pulse 4500 W; welding speed 180 mm min^–1; SPF parameters were as follows: temperature T_f=965°C; forming pressure P _f=4·2 MPa; forming time t _f=130 min. Microstructure of multisheet structure was studied carefully. Microstructure in weld fusion zone was constituted of austenite dendritics and Laves phase precipitated in interdendritics. After SPF process, austenite dendritics in the weld fusion became coarser and most of Laves phases were dissolved and turned into δ precipitated phase but a few of Laves phases were still reserved. And Nb concentration in dendritics increased to 5·42% compared to 2·82% under as welded condition. Weld metal hardness increased from 331·63 under as welded condition to 391·74 under post-SPF condition which was closed to the base material hardness of post-SPF. Grain size of base material grew slightly and an amount of precipitated phase appeared in the base material undergoing SPF process. The tensile test results of base material show that tensile strength increased obviously and the ductility decreased slightly after SPF process. Therefore, LBW/SPF technology is an appropriate forming technique for multisheet structure of Inconel 718 superalloy.     

温度对718镍基合金在高酸性环境下耐腐蚀性能的影响

718镍基合金是高酸性油气井中常用的金属材料,但目前对其腐蚀机理和影响因素的研究较少。利用高温高压反应釜进行腐蚀模拟,采用失重法、扫描电镜(SEM)等手段研究了温度对718镍基合金在高含H_2S/CO_2环境下腐蚀行为的影响。结果表明:在CO_2分压3.5 MPa、H_2S分压3.5 MPa、Cl~-含量150 000 mg/L的模拟环境下,718镍基合金在150,175,205℃下均呈现全面腐蚀,未出现点蚀和局部腐蚀。但随温度升高,镍基合金718的均匀腐蚀速率逐渐增加,材料表面钝化膜出现硫化,并逐渐向腐蚀产物膜转变,质地由致密变得疏松。     

Simulation studies on deformation behaviour of AA2017 alloy in semi-solid state using FEA

Abstract

Thixo-forming is one of the near net shaped manufacturing processes in which the final product is made between the liquidus and solidus temperatures. Forming the product at this temperature range provides large benefits compared to conventional forming methods. In the present work, deformation mechanism of AA2017 alloy in the semi-solid state has been studied. As the mechanical behaviour and deformation mechanism of semi-solid metal is completely different from that of the solid state forming, it is necessary to investigate its nature at the semi solid range. To analyse the metal flow, the stress behaviour and the corresponding strain induced, extensive finite element analysis (FEA) based simulation studies have been performed. The commercial software, DEFORM 2D was used for the simulation. The flow behaviour was modelled by formulating an Arrhenius type constitutive relation.     

Joint strength and interfacial microstructure in silicon nitride/nickel-based Inconel 718 alloy bonding

Joining of Inconel 718 alloys to silicon nitrides using Ag–27Cu–3Ti alloys was performed to investigate the microstructural features of interfacial phases and their effect on joint strength. The Si3N4/Inconel 718 alloy joints had a low shear strength in the range 70.4–46.1 MPa on average, depending on joining temperature and time. When the joining time was held for 1.26 ks at 1063 K, shear, tension, and four-point bending strength were 70.4, 129.7, and 326.5 MPa on average. The microstructures of the joints typically consisted of six types of phases. They were TiN and Ti5Si4 between silicon nitride and filler metal, a copper- and silver-rich phase, island-shaped Ti–Cu phase, a Ti–Cu–Ni alloy layer between filler and base metal, and diffusion of titanium into the Inconel 718 alloys. With increasing joining temperature, the thickness increase of the Ti–Cu–Ni alloy layer was much greater than that of the reaction layer. Thus the diffusion rate of titanium into the base metal was much greater than the reaction rate with silicon nitride. This behaviour of titanium results in the formation of a Ti–Cu–Ni alloy layer in all the joints. The formation of these layers was the cause of the strength degradation of the Si3N4/Inconel 718 alloy joints. This fact was supported by the analyses of fracture path after four-point bending strength tests.     

Microstructural evolution of Inconel* 718 during ingot breakdown: process modelling and validation

Abstract

A computer model is presented that describes microstructural evolution during the ingot breakdown of nickel base superalloy Inconel 718 via the open die cogging operation. To support the development of the model, a compression testing programme has been carried out which covers the ranges of temperatures, strains, and strain rates experienced during thermomechanical processing. Analysis of the flow curves has allowed the identification of the regimes in which the various deformation mechanisms take place. Logic based rules have been incorporated into the model, and this has allowed predictions of the microstructural evolution to be made. Where possible, the results have been compared with the available experimental data and it is shown that theory and experiment are in reasonable agreement. A number of computational experiments have been carried out, to study the effects of changing the forging procedure.     

Infrared thermal imaging for melt pool analysis in SLM: a feasibility investigation

ABSTRACT

Melt pool dimension can help to relate process parameters and build part quality in selective laser melting (SLM) process. In this study, a near-infrared thermal imager (about 670?nm spectral range) was employed to collect powder layer thermal signal in SLM machine using nickel-based alloy as raw powder material. Radiant temperature distribution at different build heights has been acquired and melt pool sizes have been analysed. The major findings are as follows: (1) It is possible to estimate melt pool dimension based on the identified radiant liquidus temperature and appropriate thermal imager setting, but it is difficult to obtain true temperature. (2) At nominal process conditions of 600?mm/s beam speed and 180?W beam power for Inconel 718 powder, the melt pool has a length of about 0.36?mm and a width of about 0.21?mm. Build height seems to have little effect on melt pool dimensions.     

Low temperature effects on the fracture behaviour of a nickel base superalloy

The mechanical properties of a solution treated and double aged nickel - 18% iron - 18% chromium alloy (Inconel 718) were studied to assess its utility at temperatures in the ambient-to-cryogenic range. Uniaxial tensile property measurements using unnotched specimens at decreasing temperatures between 295 and 4 K show that yield and ultimate strengths increase by 20% and 29%, respectively, while ductility remains virtually constant. Fracture mechanics tests using 2.54 cm thick compact specimens revealed that the fatigue crack growth resistance of this alloy improves slightly at extreme cryogenic temperatures, and its plane strain fracture toughness, K_lc, increases from 96.3 MPa m^{$\text{1}\text{2}$} at 295 K to 112.3 MPa m^{$\text{1}\text{2}$} at 4 K. These results are compared with similar data for Inconel 750 alloys     

Hydrogen embrittlement behavior of Inconel 718 alloy at room temperature

Hydrogen embrittlement of Inconel 718 alloy was investigated. Multi-scale observation technique were employed, comprising slow strain rate tensile tests, scanning electron microscopy and transmission electron microscopy analysis. The results demonstrate that hydrogen charging deteriorates mechanical properties of the alloy. Inconel 718 alloy shows partial Portevin-Le Chatelier(PLC) effect at room temperature when hydrogen charging current density is 220 mA cm~(-2) and 590 mA cm~(-2). Moreover, plastic deformation features with dislocation cells are detected in hydrogen-induced brittle zone. Thus, it is concluded that dragging effect of hydrogen atoms on dislocations contributes to PLC effect.     

MIXED MODE SMALL CRACK GROWTH

Abstract— The fatigue crack growth behavior of small part-through cracks in 1045 steel and Inconel 718 subjected to biaxial loading has been investigated. Experiments were performed on thin-wall tubular specimens loaded in tension, torsion and combined tension torsion. Crack sizes analyzed ranged from 20 μm to 1 mm and growth rates ranged from 10^-7 to 10^-4 mm/cycle for 1045 steel and from 10^-5 to 10^-2 mm/cycle for Inconel. Nucleation and the early growth of cracks occurs on planes of maximum shear strain amplitude for both of these materials even in tensile loading. An equivalent strain based intensity factor was employed to correlate the crack growth rate under mixed mode loading conditions In loading conditions other than torsion, a transition from mode II to mode I was observed for 1045 steel. Principal strains were used to analyze mode I cracks. Cracks in Inconel 718 grow in mode II for the majority of the fatigue life. The maximum shear strain amplitude and the tensile strain normal to the maximum shear strain amplitude plane were used to calculate the strain based intensity factor for mixed mode loading.     

Creep–Fatigue Interaction of Inconel 718 Manufactured by Electron Beam Melting

Electron beam melting of Ni-base superalloy Inconel 718 allows producing a columnar-grained microstructure with a pronounced texture, which offers exceptional resistance against high-temperature loading with severe creep–fatigue interaction arising in components of aircraft jet engines. This study considers the deformation, damage, and lifetime behavior of electron-beam-melted Inconel 718 under in-phase thermomechanical fatigue loading with varying amounts of creep–fatigue interaction. Strain-controlled thermomechanical fatigue tests with equal-ramp cycles, slow–fast cycles, and dwell time cycles are conducted in the temperature range from 300 to 650 °C. Results show that both dwell time and slow–fast cycles promote intergranular cracking, gradual tensile stress relaxation, as well as precipitate dissolution and coarsening giving rise to cyclic softening. The interplay of these mechanisms leads to increased lifetimes in both dwell time and slow–fast tests compared to equal ramp tests at higher strain amplitudes. Conversely, at lower mechanical strain amplitudes, the opposite is observed. A comparison with results of conventional Inconel 718 indicates that the electron-beam-melted material exhibits superior resistance against strain-controlled loading at elevated temperatures such as thermomechanical fatigue.     

In-situ SEM study of temperature-dependent tensile behavior of Inconel 718 superalloy

Journal of Materials Science - The effect of deformation temperature on tensile behavior of Inconel 718 alloy has been studied by a self-developed in-situ high-temperature tensile stage inside a...