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.     

Effect of surface nanostructure on tensile behavior of superalloy IN718

Peak aged nickel based superalloy Inconel 718 was subjected to ultrasonic shot peening for different durations of 45, 60 and 90 min. The transverse and longitudinal sections of the ultrasonic shot peened (USSP) specimens were examined by scanning electron microscope (SEM) and transmission electron microscope (TEM) and the shot peened microstructures were characterized. A nanostructure of nearly 12 nm grain size was observed to develop in surface region of the USSP specimen. X-ray diffraction (XRD) of the USSP samples did not reveal any phase change. Microhardness of the shot peened region was increased by 20% in the as shot peened condition and it was reduced to 8% in the stress relieved condition. Yield and tensile strengths were found to increase only marginally by 3.5% and 2.15%, respectively and the ductility was reduced by 3.4% following ultrasonic shot peening. The results are discussed in terms of grain refinement in the shot peened region.     

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.     

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.     

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.     

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.     

Yield strength anomaly and dynamic strain ageing behaviour of recently developed advanced ultra-supercritical boiler grade wrought Ni-based superalloy IN 740H

The yield strength anomaly (YSA) and dynamic strain ageing (DSA) behaviour of advanced ultra-supercritical boiler grade wrought nickel-based superalloy IN 740H is studied by conducting tensile tests in temperature range 28–930°C and by employing strain rates 1 × 10^?2, 1 × 10^?3, 1 × 10^?4 and 1 × 10^?5 s^?1 followed by extensive electron microscopic examination. Increase in yield strength accompanied by impairment of ductility indicates that YSA exists in alloy IN 740H in temperature range of 650–760°C. The electron microscopic observation confirms that YSA is due to pinning of dislocations by γ′ precipitates and shearing of γ′ precipitates in IN 740H. DSA is observed in the temperature range of 200–500°C and is predominant at 300°C. The nature of serrated plastic flow due to DSA is dependent on the temperature and strain rate.     

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.     

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     

Superplastic behavior of a powder metallurgy superalloy during isothermal compression

In this work,the flow behaviors and microstructure evolution of a powder metallurgy nickel-based superalloy during superplastic compression is investigated.Based on the strain rate sensitivity m determined by flow data,superplastic region is estimated at relatively low temperature and strain rate domains,specifically around 1000 ℃/10~(-3)s~(-1).Thereafter,the cylinder specimens are isothermally compressed at 1000 ℃/10~(-3)s~(-1) and 1025 ℃/10~(-3)s~(-1) with different strains,to exam the superplasticity and related mechanisms.The experimental results indicate that the accumulated dislocations are mainly annihilated by dynamic recovery and dynamic recrystallization(DRX),and the grain boundary sliding(GBS)contributes to the total strain during superplastic compression as well.In addition,the cavities and cracks at triple junctions or interfaces between matrix and second phase particle have not been detected,which is different from superplastic tensile deformation.     

超塑成形技术在轨道交通领域的应用

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