Microstructural control in hot working of IN-718 superalloy using processing map

The hot-working characteristics of IN-718 are studied in the temperature range 900 °C to 1200 °C and strain rate range 0.001 to 100 s−1 using hot compression tests. Processing maps for hot working are developed on the basis of the strain-rate sensitivity variations with temperature and strain rate and interpreted using a dynamic materials model. The map exhibits two domains of dynamic recrystallization (DRX): one occurring at 950 °C and 0.001 s−1 with an efficiency of power dissipation of 37 pct and the other at 1200 °C and 0.1 s−1 with an efficiency of 40 pct. Dynamic recrystallization in the former domain is nucleated by the δ(Ni3Nb) precipitates and results in fine-grained microstructure. In the high-temperature DRX domain, carbides dissolve in the matrix and make interstitial carbon atoms available for increasing the rate of dislocation generation for DRX nucleation. It is recommended that IN-718 may be hot-forged initially at 1200 °C and 0.1 s−1 and finish-forged at 950 °C and 0.001 s−1 so that fine-grained structure may be achieved. The available forging practice validates these results from processing maps. At temperatures lower than 1000 °C and strain rates higher than 1 s−1 the material exhibits adiabatic shear bands. Also, at temperatures higher than 1150°C and strain rates more than 1s−1, IN-718 exhibits intercrystalline cracking. Both these regimes may be avoided in hotworking IN-718.

Microstructural control in hot working of IN-718 superalloy using processing map

The hot-working characteristics of IN-718 are studied in the temperature range 900 degrees C to 1200 degrees C and strain rate range 0.001 to 100 s(-1) using hot compression tests. Processing maps for hot working are developed on the basis of the strain-rate sensitivity variations with temperature and strain rate and interpreted using a dynamic materials model. The map exhibits two domains of dynamic recrystallization (DRX): one occurring at 950 degrees C and 0.001 s(-1) with an efficiency of power dissipation of 37 pct and the other at 1200 degrees C and 0.1 s(-1) with an efficiency of 40 pct. Dynamic recrystallization in the former domain is nucleated by the delta(Ni3Nb) precipitates and results in fine-grained microstructure. In the high-temperature DRX domain, carbides dissolve in the matrix and make interstitial carbon atoms available for increasing the rate of dislocation generation for DRX nucleation. It is recommended that IN-718 may be hot-forged initially at 1200 degrees C and 0.1 s(-1) and finish-forged at 950 degrees C and 0.001 s(-1) so that fine-grained structure may be achieved. The available forging practice validates these results from processing maps. At temperatures lower than 1000 degrees C and strain rates higher than 1 s(-1), the material exhibits adiabatic shear bands. Also, at temperatures higher than 1150 degrees C and strain rates more than 1 s(-1), IN-718 exhibits intercrystalline cracking. Both these regimes may be avoided in hot-working IN-718.

Optimization of hot workability of an Al-Mg-Si alloy using processing maps

The hot workability of an Al-Mg-Si alloy has been studied by conducting constant strain-rate compression tests. The temperature range and strain-rate regime selected for the present study were 300-550 degrees C and 0.001-1 s(-1), respectively. On the basis of true stress data, the strain-rate sensitivity values were calculated and used for establishing processing maps following the dynamic materials model. These maps delineate characteristic domains of different dissipative mechanisms. Two domains of dynamic recrystallization (DRX) have been identified which are associated with the peak efficiency of power dissipation (34%) and complete reconstitution of as-cast microstructure. As a result, optimum hot ductility is achieved in the DRX domains. The strain rates at which DRX domains occur are determined by the second-phase particles such as Mg2Si precipitates and intermetallic compounds. The alloy also exhibits microstructural instability in the form of localized plastic deformation in the temperature range 300-350 degrees C and at strain rate 1 s(-1).

Influence of carbide particles on the dynamic recrystallization of as-cast 304 stainless steel: a study using secondary ion mass spectrometric (SIMS) analysis

The domain of dynamic recrystallization (DRX) in as-cast 304 stainless steel material occurs at higher temperatures (1250 degrees C) and lower strain rates (0.001 s(-1)) than in wrought 304 stainless steel (1100 degrees C and 0.01 s(-1)). The above result has been explained earlier on the basis of a simple theoretical DRX model involving the rate of nucleation versus rate of grain boundary migration. The present investigation is aimed at examining experimentally the influence of carbide particles on the DRX of ascast 304 using secondary ion mass spectrometric (SIMS) analysis. Isothermal compression tests at a constant true strain rate have been performed on wrought 304 and as-cast 304 materials in the temperature and strain rate ranges of 1000 to 1250 degrees C and 0.001 to 1 s(-1) respectively. The SIMS analysis carried out on the deformed samples revealed that the large carbides present in the as-cast 304 material strongly influence the DRX process. In as-cast 304 material, the presence of large carbide particles in the microstructure shifts the DRX domain to higher temperature and lower strain rate in comparison with wrought 304 material.

Influence of temperature and strain rate on the flow stress of an FeAl alloy

The deformation behavior of an FeAl alloy processed by hot extrusion of water atomized powder has been investigated. Compression tests are performed in the temperature range 1073–1423 K and in the strain rate range 0.001–100 s−1 up to a true plastic strain of 0.5. The flow stress has been found to be strongly dependent on temperature as well as strain rate. The stress exponent in the power law rate equation is estimated to be in the range 7.0–4.0, decreasing with temperature. The activation energy for plastic flow in the range 1073–1373 K varies from 430 kJ mol−1 at low stresses to 340 kJ mol−1 at high stresses. However, it is fairly independent of strain rate and strain. The activation area has similarly shown a stress dependence and lies in the range 160–45b2. At 1423 K and at strain rates lower than 0.1 s−1 a strain rate sensitivity of 0.3 is observed with an associated activation energy of 375 kJ mol−1. The plastic flow in the entire range of temperature and strain rate investigated appears to be controlled by a diffusion mechanism. The results have revealed that it is possible to process the alloy by superplastic forming in the range 1373–1423 K at strain rates lower than 0.1 s−1.

Stress-Strain Prediction of Jointed Rocks using Artificial Neural Networks

The applicability of Artificial Neural Networks for predicting the stress-strain response of jointed rocks at varied confining pressures, strength properties and joint properties (frequency, orientation and strength of joints) has been studied in the present paper. The database is formed from the triaxial compression tests on different jointed rocks with different confining pressures and different joint properties reported by various researchers. This input data covers a wide range of rock strengths, varying from very soft to very hard. The network was trained using a 3 layered network with feed forward back propagation algorithm. About 85% of the data was used for training and remaining15% for testing the predicting capabilities of the network. Results from the analyses were very encouraging and demonstrated that the neural network approach is efficient in capturing the complex stress-strain behaviour of jointed rocks. A single neural network is demonstrated to be capable of predicting the stress-strain response of different rocks, whose intact strength vary from 11.32 MPa to 123 MPa and spacing of joints vary from 10 cm to 100 cm for confining pressures ranging from 0 to 13.8 MPa.

Elasto-plastic analysis of jointed rocks using discrete continuum and equivalent continuum approaches

Results from elasto-plastic numerical simulations of jointed rocks using both the equivalent continuum and discrete continuum approaches are presented, and are compared with experimental measurements. Initially triaxial compression tests on different types of rocks with wide variation in the uniaxial compressive strength are simulated using both the approaches and the results are compared. The applicability and relative merits and limitations of both the approaches for the simulation of jointed rocks are discussed. It is observed that both the approaches are reasonably good in predicting the real response. However, the equivalent continuum approach has predicted somewhat higher stiffness values at low strains. Considering the modelling effort involved in case of discrete continuum approach, for problems with complex geometry, it is suggested that a proper equivalent continuum model can be used, without compromising much on the accuracy of the results. Then the numerical analysis of a tunnel in Japan is taken up using the continuum approach. The deformations predicted are compared well against the field measurements and the predictions from discontinuum analysis. (C) 2012 Elsevier Ltd. All rights reserved.

Analysis of Strain Rates and Microstructural Evaluation during Metal Forming: Role of Surface Texture and Friction

The surface texture of a die plays an important role in friction during metal forming. In the present study, unidirectional and random surface finishes were produced on hardened steel plate surfaces. To understand the influence of surface texture on friction, experiments were conducted using Al-Mg alloy pins that slid against steel plates of different surface textures. In the sliding experiments, a high coefficient of friction was observed when the pins slid perpendicular to the unidirectional grinding marks and low friction occurred when the pins slid on the random surfaces. Finite element simulations were performed using the measured friction values to understand the stress and strain evolutions in the deforming material using dies with various friction. The numerical results showed that the states of stress and strain rates are strongly influenced by the friction at the interface and hence would influence the final material microstructure. To substantiate the numerical results, laboratory compression tests were conducted. Different surface textures were obtained in order to experience different friction values at different locations. A large variation in the microstructure at these locations was observed during experiments, verifying that surface texture and die friction significantly influence fundamental material formation behavior.

Soil stabilization with waste materials based binder

This paper describes a laboratory trial to study the effectiveness of a waste-based binder to stabilize expansive soils. The proposed binders viz., Fly ash and/or Ground granulated Blast furnace slag (GGBS) were mixed with the expansive soil along with a small amount of lime to increase soil pH and enable pozzolanic reactions. The geotechnical characteristics of the various combinations of samples were investigated through the compaction tests, unconfined compression tests etc. It was found that the addition of GGBS with and without fly ash and lime has significant influence on the geotechnical characteristics of the soil.

Friction Welding of Thixocast A356 Aluminium Alloy

In this paper, a numerical model for friction welding of thixo-cast materials is developed, which includes a coupling of thermal effect and plastic deformation using a finite element method (FEM). As the constitutive equations for flow behavior of materials for a thixo-cast material are expected to be different from those of conventionally cast material of the same alloy, the necessary material data are experimentally determined from isothermal hot compression tests of the A356 thixocast alloy. The Johnson-Cook model has been employed to represent the flow behavior of the thixocast A356 alloy. The purpose of this FEM analysis is to provide better understanding of the friction welding process of thixo-cast material, and to obtain optimized process parameters before an actual welding is carried out.

Experimental Studies on the Mechanics of Cohesive Frictional Granular Media

The mechanical behaviour of cohesive-frictional granular materials is a combination of the strength pervading as intergranular friction (represented as an angle of internal friction - Phi), and the cohesion (C) between these particles. Most behavioral or constitutive models of this class of granular materials comprise of a cohesion and frictional component with no regard to the length scale i.e. from the micro structural models through the continuum models. An experimental study has been made on a model granular material, viz. angular sand with different weights of binding agents (varying degrees of cohesion) at multiple length scales to physically map this phenomenon. Cylindrical specimen of various diameters - 10, 20, 38, 100, 150 mm (and with an aspect ratio of 2) are reconstituted with 2, 4 and 8% by weight of a binding agent. The magnitude of this cohesion is analyzed using uniaxial compression tests and it is assumed to correspond to the peak in the normalized stress-strain plot. Increase in the cohesive strength of the material is seen with increasing size of the specimen. A possibility of ``entanglement'' occurring in larger specimens is proposed as a possible reason for deviation from a continuum framework.

Evolution of microstructure during hot deformation of pearlitic steel

Hot deformation of pearlitic steel was carried out to examine the overall deformation response to microstructural evolution. To understand the mechanisms operative during hot deformation, compression tests were carried out at various temperatures in the range 400(-)600 degrees C and strain rates in the range 0.001-10 s(-1). The flow curves were analyzed to examine the occurrence of dynamic recrystallization. The evolution of microstructure in hot deformed samples is analysed using EBSD.

Hot deformation behavior of Ni-Fe-Ga-based ferromagnetic shape memory alloy - A study using processing map

Ni-Fe-Ga-based alloys form a new class of ferromagnetic shape memory alloys (FSMAs) that show considerable formability because of the presence of a disordered fcc gamma-phase. The current study explores the deformation processing of this alloy using an off-stoichiometric Ni55Fe59Ga26 alloy that contains the ductile gamma-phase. The hot deformation behavior of this alloy has been characterized on the basis of its flow stress variation obtained by isothermal constant true strain rate compression tests in the 1123-1323 K temperature range and strain rate range of 10(-3)-10 s(-1) and using a combination of constitutive modeling and processing map. The dynamic recrystallization (DRX) regime for thermomechanical processing has been identified for this Heusler alloy on the basis of the processing maps and the deformed microstructures. This alloy also shows evidence of dynamic strain-aging (DSA) effect which has not been reported so far for any Heusler FSMAs. Similar effect is also noticed in a Ni-Mn-Ga-based Heusler alloy which is devoid of any gamma-phase. (C) 2014 Elsevier Ltd. All rights reserved.

High-Temperature Workability of Thixocast A356 Aluminum Alloy

The present work highlights the role of globular microstructure on the workability of A356 aluminum alloy at elevated temperature. The hot deformation behavior was studied by isothermal hot compression tests in the temperature range 573 K to 773 K (300 A degrees C to 500 A degrees C) and strain rate range of 0.001 to 10 s(-1). The flow stress data obtained from the tests were used to estimate the strain rate sensitivity and strain rate hardening. Flow stress analysis of the alloy shows that the effect of temperature on strain hardening is more significant at lower strain levels and strain rate sensitivity is independent of strain. The results also reveal that the flowability of conventionally cast alloy increases after changing the dendritic microstructure into a globular structure through semisolid processing route. Thixocast alloy exhibits lower yield strength and higher elongation at elevated temperature in comparisons to conventionally cast values. This property has an important implication toward thixo-forming at an elevated temperature. (C) The Minerals, Metals & Materials Society and ASM International 2015

Mechanism of improvement in the strength and volume change behavior of lime stabilized soil

An attempt has been made to bring out the influence on strength and volume change behavior of fabric changes and new cementitious compound formation in a soil upon addition of various lime contents and with curing periods. The effects of changes in fabric of treatment with various lime contents (0, 2,4 and 6%) and with curing periods (0, 7, 14 and 28 days) have been evaluated by one-dimensional consolidation tests, in terms of void ratio changes and compressibility. The strength of soil treated with different lime contents with curing periods up to 28 days, and with the optimum lime content of 6% up to one year has been determined by unconfined compression tests. Comparison of effects of lime on the strength and volume change behavior of the soil brings out that the formation of flocculated fabric and cation exchange significantly reduces the compressibility of soil but marginally increases the strength. Cementation of soil particles and filling with cementitious compounds of the voids of flocculated fabric in the soil marginally reduces the compressibility but significantly increases the strength. Thus, the mechanism of volume change behavior of soil treated with lower lime content at short curing periods is distinctly different from that of the soil treated with optimum lime content at longer curing periods. This is consistent with the increase in the permeability caused by the addition from 2 to 4% lime and the decrease following the addition of 6% lime. Changes consistent with mechanical behavior have been determined by scanning electron microscope, X-ray diffraction and thermal analyses, energy dispersive X-ray spectrometer and pH value in microstructure, mineralogy, chemical composition and alkalinity, respectively. (C) 2015 Published by Elsevier B.V.