Superplastic behaviour of Inconel 718 sheet

Abstract

Inconel 718 is a nickel based alloy used extensively in the aerospace industry, having good service capabilities, in terms of strength and fatigue resistance, at high temperatures. Inconel 718, in the form of sheet, has the capability of being shaped using gas pressure forming techniques similar to those used for a number of aluminium and titanium based alloys. An extensive research programme has been carried out to investigate the high temperature formability of this alloy. This has involved both uniaxial tensile testing to determine such parameters as flow stress and strain rate sensitivity, and microstructural examination to investigate grain stability under both static heating and following deformation. The forming characteristics of the material have been correlated with the δ phase solvus temperature determined using SEM techniques. Optimum forming temperatures and strain rates are discussed.     

Modelling of inertia welding of IN718 superalloy

Abstract

A simple model for the inertia welding of a nickel based superalloy is proposed. The heat flow occurring in the vicinity of the joint is considered, assuming it to be one-dimensional, and this is coupled to a treatment of the stress state expected there using Hill’s general method, so that the upset can be estimated. A state variable constitutive model is included, for the IN718 alloy. It is demonstrated that many of the important characteristics of the process are predicted correctly. It is shown that the shear stress developed at the last stage of the process must be accounted if the upset is to be correctly predicted. The results are compared with those from a 2½D finite element model of the process, and the differences rationalised.     

Effects of microstructure on superplastic behaviour of nickel based superalloy NK17CDAT

Abstract

The superplastic behaviour of nickel based superalloy NK17CDAT( Astroloy) has been studied using torsion testing. The evolutions of stress and average grain size are found to follow the same classical behaviour as other nickel based alloys under similar conditions. The superplastic flow is characterised by a strain rate sensitivity exponent of 0·5 and a grain size sensitivity exponent of 3. Modelling of this alloy can be considered using a combination of microscopic mechanisms, namely, those of Gittus, Coble, and dislocation glide–climb.

MST/1135     

Tensile deformation behaviour of forged disc of IN 718 superalloy at 650 °C

The tensile properties of forged disc of IN 718 superalloy were evaluated in the strain rate regime between 10⁻⁴ and 10⁻² s⁻¹ at 650 °C. Flow oscillations were observed in stress–strain curves in the strain rate regime investigated. These flow oscillations were identified as strain increments attributed to twining mechanism at all the strain rates. However, presence of well defined serrations, temperature insensitivity of yield strength and increase in strain hardening exponent confirmed the occurrence of dynamic strain aging at 10⁻³–10⁻² s⁻¹ strain rates. Deformation behaviour was observed to be planer in nature. Fracture features remained same (transgranular) in the strain rate regime studied.     

Grain growth in IN718 superalloy fabricated by laser additive manufacturing

ABSTRACT

The grain growth mechanism of IN718 superalloy fabricated by selective laser melting (SLM) was studied. Epitaxial growth with the same crystallographic orientation or rotating by 90° across the melting pool boundary and competitive growth in the same melting pool were observed. Either of the two patterns of epitaxial growth can maintain the same grain across the melting pool boundary. Competitive growth is determined by both the heat flow direction and preferred crystallographic orientation. In SLM, the grains grow along the preferred crystallographic orientation owing to a high solidification rate. The smaller the deviation angles between the heat flow direction and the preferred crystallographic orientation, the faster the grain growth rate.     

Study of grain refinement and eutectic transformation of undercooled IN718 superalloy

Abstract

A substantial undercooling up to 250 K was produced in the IN718 superalloy melt by employing the method of molten salt denucleating, and the microstructure evolution with undercooling was investigated. Within the achieved undercooling, 0–250 K, the solidification microstructure of IN718 undergoes two grain refinements: the first grain refinement occurs in a lower range of undercooling, which results from the ripening and remelting of the primary dendrite, and at a larger range of undercooling, grain refinement attributes to solidification shrinkage stress and lattice distortion energy originating from the rapid solidification process. A ‘lamellar eutectic anomalous eutectic’ transition was observed when undercooling exceeds a critical value of ～250 K. When undercooling is small, owing to niobium enrichment in interdendrite, the remaining liquid solidifies as eutectic (γ+Laves phase); whereas, if the undercooling achieves 250 K, the interdendrite transforms from eutectic (γ+Laves phase) to Laves phase, which results from the formation of divorced eutectic arising from the huge variance of the growth velocities of γ and Laves phases.     

Studies on creep/stress rupture behaviour of superalloy 718 weldments used in gas turbine applications

Superalloy 718 in the solution-treated condition was welded autogenously by electron beam welding and gas tungsten are welding processes. The weldrnents after suitable heat treatment were subjected. to creep/stress rupture testing at 650°C and 690 MPa. The results showed that the Laves phase resulting in the weld metalis deterimental to the creep rupture life of weld metals when present With the continuous morphology found in gas tungsten are welds. The lower amounts and discontinuous morphology of the Laves phase in electron beam weldments in combination With favourable gramonentauon resulted in relatively better rupture properties for these weldments.     

Deformation behaviour in superplastic 5083 Al alloy during biaxial gas pressure forming with lubrication

Abstract

Effect of lubrication on deformation behaviour of a superplastic material has been given little attention, although it is important for industrial application. In this paper, a superplastic 5083 Al alloy under biaxial deformation was investigated by deforming the sheet into a cylindrical die cavity with and without lubrication. Several interrupted tests were performed to bulge the sheets to various depths for two different strain rates, the formed parts were then utilised to evaluate the effect of lubrication on metal flow, thickness distribution and cavitation. It was found that reducing the interfacial friction by use of a lubricant improved the metal flow after the deformed sheet had made contact with the bottom surface of die. Changes of the metal flow during forming not only developed a better thickness distribution of the formed part, but also reduced cavitation levels.     

The evolution of delta-phase in a superplastic Inconel 718 alloy

An Inconel 718 sheet alloy was tested in tension at a temperature of 965°C and an initial strain rate of 10⁻⁴ s⁻¹ corresponding to the conditions for optimum superplastic deformation. Detailed observations and quantitative measurements record the evolution of the δ-phase during tensile deformation. The experiments show that the total precipitation of the δ-phase increases with strain but there is a decrease with strain in the number density of the needle/plate δ-phase particles and a corresponding increase with strain in the number density of the blocky/globular δ-phase particles.     

Rapid forming of superplastic aluminium alloy 5083

Abstract

Decreasing the cycle time significantly for forming the commercially available superplastic aluminium alloy 5083 has been achieved. Forming results and conditions are compared with previous relevant works which are actually scarce. A circular cup having a depthdiameter ratio of 1:2 can be formed in 70 s. This ratio requires flat sheet to be stretched in area by up to three times, which should be large enough when dealing with actual industrial sheet forming. On average, the thickness is decreased by two-thirds; in fact, the thickness distribution is not uniform and the gradient is concentrated at the wall of the cup. The location of minimum thickness in rapid forming is different from that in conventional forming. Disregarding the traditional approach, the pressure-time profile employed in this work was not restricted to yield the so called optimum strain rate, which is usually low. Following the same processing profile, but proceeding in stages of partial forming, a series of progressive forming configurations was obtained in order to analyse the strain rate path leading to the successful rapid forming. For a specimen processed at 500C, the maximum volume fraction of cavities is 4 existing at the location of minimum thickness.     

Testing models for superplastic bulge forming of domes

Abstract

The predictions of a new model for the superplastic bulge forming of domes developed by the authors, and of two earlier models, are examined in a study of the variation of dome height with time, the thickness distribution along the dome profile, and the factors influencing the thickness variation. The predictions of the models are compared with experimental measurements made on two aluminium alloys, AA 7475 and Supral 220, and an aluminium bronze, bulged under constant pressure conditions. The recent model shows improved agreement with experiment compared with the two earlier models. It has been shown that the strain rate sensitivity parameter m plays a large part in determining the thickness distribution, while the height of a dome is also important.

MST /1145     

Microstructural characterisation of deformation behaviour of nickel base superalloy IN625

Abstract

Simple compression and microscopy techniques were employed to characterise the microstructural origin of the deformation behaviour of nickel base superalloy IN625 during large strain testing. The alloy exhibited a four-stage strain hardening response similar to that previously reported for low stacking fault energy face centred cubic alloys. At strains lower than about ?0·06 (stage A), a falling regime of the hardening rate was observed. This stage was followed by a second stage (stage B) of slow increasing hardening rate, which was found to be coincident with the formation of Lomer–Cottrell locks. The second falling regime of strain hardening (stage C) was seen in the strain range of ?0·25 to ?0·65. The occurrence of this stage was attributed to the increasing ease of dislocation cross-slip with increasing strain and consequently to the decreasing Lomer–Cottrell lock efficiency in hindering dislocation movement. Beyond a strain of ?0·65, a final slightly constant hardening regime (stage D) was developed. The initiation of this stage was concurrent with the onset of deformation twinning in the microstructure.     

Room-temperature forming limit diagram and tensile behaviour up to 200° C of an AI-Ca-Zn superplastic alloy

The forming limit diagram and strain distribution under punch stretching at room temperature of an AI-Ca-Zn (superplastic) alloy have been evaluated. Tensile behaviour up to 200° C is reported. The fracture surfaces have been examined by scanning electron microscopy and the results are analysed to support the failure criterion proposed earlier by Marciniaket al.     

On the pressure forming of two superplastic alloys

Superplastic forming of the Ti-6Al-4V and Sn-Pb eutectic alloys was attempted using the pressure forming (sheet thermoforming) process. It has been demonstrated that true hemispheres could be formed out of sheets of both the alloys. The thickness strains in both the alloys were less than those predicted theoretically and this could be traced to material flow from the flange and gripped regions. This flow, however, was greater in case of the titanium alloy than the Sn-Pb alloy, on account of the greater strain-rate sensitivity of the former material. Due to the same effect, the thinning factor actually increased with deformation in the titanium alloy, but it decreased on increasing deformation in the Sn-Pb alloy. Within the experimental range, the hold-down pressure (titanium alloy) and initial sheet thickness (Sn-Pb alloy) had very small effects, although the deformation became slightly more uniform on decreasing the hold-down pressure or increasing the initial sheet thickness. The thickness and circumferential strains increased with deformation and in particular when the bulge height (h ₀) to base diameter (D ₀) ratio was greater than 0.35, non-uniformity in deformation along the bulge profile became noticeable. These strains were largest at the pole and its vicinity. On account of its lower strain-rate sensitivity, these effects were more pronounced in the Sn-Pb alloy than in the titanium alloy. Although initially the bulging rate was rapid, later the (h ₀/D ₀) ratio increased linearly with the forming time and at any instant the bulge profile corresponded to an arc of a circle.     

Single pass laser joining of Inconel 718 superalloy with filler

Abstract

Microstructure and mechanical property of CO₂ laser beam welded IN 718 superalloy were studied by electron microscopy and hardness testing. The use of a welding filler wire produced a sound fusion zone with no cracking but grain boundary microfissuring occurred in the heat affected zone (HAZ) and was observed to be significantly influenced by pre-weld heat treatment and laser welding speed. Crack-free weld was produced by a pre-weld heat treatment that minimised non-equilibirum grain boundary boron segregation and inhibited grain growth. While post-weld heat treatment (PWHT) reduced the difference between the hardness values of the base alloy, HAZ and the fusion zone, it resulted in increased HAZ cracking, which was likely aided by pre-existing cracks. The PWHT cracking was, however, avoided by subjecting pre-weld material to the heat treatment condition that produces crack-free weld during welding process.     

Microstructure evolution during dynamic recrystallization of hot deformed superalloy 718

Microstructure evolution during dynamic recrystallization (DRX) of superalloy 718 was studied by optical microscope and electron backscatter diffraction (EBSD) technique. Compression tests were performed at different strains at temperatures from 950 °C to 1120 °C with a strain rate of 10⁻¹ s⁻¹. Microstructure observations show that the recrystallized grain size as well as the fraction of new grains increases with the increasing temperature. A power exponent relationship is obtained between the dynamically recrystallized grain size and the peak stress. It is found that different nucleation mechanisms for DRX are operated in hot deformed superalloy 718, which is closely related to deformation temperatures. DRX nucleation and development are discussed in consideration of subgrain rotation or twinning taking place near the original grain boundaries. Particular attention is also paid to the role of continuous dynamic recrystallization (CDRX) at both higher and lower temperatures.     

A finite element study of thermal relaxation of residual stress in laser shock peened IN718 superalloy

The residual stresses in laser shock peened (LSP) Inconel 718 Ni-base superalloy and their thermal relaxation behavior were investigated based on three-dimensional nonlinear finite element analysis. To account for the nonlinear constitutive behavior, the Johnson-Cook model has been employed and the model parameters for high strain rate response of IN718 are calibrated by comparison with recent experimental results. Based on the LSP simulation, the thermal relaxation behavior was studied through coupled thermal-structure analysis in LS-DYNA. More specifically, the effects of test temperature, exposure time and degree of initial plastic deformation are analyzed and discussed. It is observed that stress relaxation mainly occurs during the initial period of exposure, and the relaxation amplitude increases with the increase of applied temperature and as-peened plastic deformation. Based on the simulation results, an analytical model based on Zener-Wert-Avrami function is proposed to model the thermal residual stress relaxation.     

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...