Raghavan Srinivasan

Microstructural Modeling of Metadynamic Recrystallization in Hot Working of IN 718 Superalloy

The hot deformation behavior of IN 718 superalloy has been characterized in the temperature range 900–1100°C and strain rate range 0.001–1.0 s−1 using compression tests on process annealed material, with a view to obtain a correlation between grain size and the process parameters. At a strain rate of 0.001 s−1, the material exhibits dynamic recrystallization (DRx) at 975°C and superplasticity at 1100°C, while metadynamic recrystallization (MDRx) occurs in the temperature range 950–1100°C and strain rate range 0.01–1.0 s−1. Unlike in the DRx domain, the grain size (d) variation in the MDRx regime could not be correlated with the standard Zener–Hollomon (Z) parameter due to strong thermal effects during cooling after hot deformation. However, it follows an equation of the type d=cexp(−Q/RT), where c, p and R are constants, Q the activation energy for MDRx and T the temperature. The value of p is very low (0.028) and the apparent activation energy is about 275 kJ mole−1, which is very close to that for self-diffusion in pure nickel. The data obtained from several investigators are in agreement with this equation. Such an equation combines the mild dynamic effect in MDRx with a stronger post-deformation cooling effect and may be used for predicting the grain size of IN 718 hot forged in the MDRx regime.

Optimum Design of Forging Die Shapes Using Nonlinear Finite Element Analysis

An optimization method is developed for the design of intermediate die shapes needed in the plane strain and axisymmetric forging operations. The approach is based on backward deformation simulation using nonlinear rigid viscoplastic finite element method and shape optimization techniques. The advantage of this optimization approach is that it has the ability to determine the intermediate die shapes from the final product shape by applying constraints on the plastic deformation of the material. This paper presents axisymmetric disk and plane strain case studies to demonstrate the new design procedures for minimizing variations in deformation rates during a multistage forging operation

Scaling Up of Equal Channel Angular Pressing (ECAP) for the Production of Forging Stock

Over the past two decades equal channel angular processing (ECAP) and other severe plastic deformation (SPD) processes have been shown, in the laboratory scale, to produce material with promising properties for industrial applications. In particular, ultrafine grain (UFG) metals produced by ECAP process, for example, have been shown to exhibit higher strain rate sensitivity at lower temperatures and higher strain rates. These factors translate to improved hot formability. However, scale up of these processes to manufacture industrial size components has not been widely undertaken. In this study, billets of annealed AA6061 with 12.5 mm (0.5-in), 50 mm (2-in) and 100 mm (4-in) square cross section were ECAP processed. For the first time, these larger SPD billets were used as starting stock for subsequent hot forging. Several parts were forged on an industrial scale press with the UFG material, as well as conventional stock materials. These parts varied in complexity, as well as size in order to cover the variability in industrial components. This paper will present the effect of scaling up on the mechanical properties, microstructure, and the hot workability of the alloy from the laboratory scale (12.5 mm) to industrial scale (100 mm). Results show that both the forging temperature of the billets and the starting billet size can be substantially decreased compared to conventional forging practice. Therefore, the use of SPD materials, as forging stock, results in decreased energy usage and increased material yield. Results presented will include examples of forged parts, estimated energy savings associated with the use of SPDUFG stock, and properties after forging and subsequent heat treatment.

MREF-ECM process for hard passive materials surface finishing

Abstract Hard passive alloys, such as nickel-based superalloys, titanium alloys, and molybdenum alloys, are widely used as engine components, isothermal hot dies and forging tools, die castings, etc. in various industries. The finishing of these materials by traditional manual methods, especially in the manufacturing of dies and moulds, is both tedious and time-consuming. A new MREF-ECM (modulated reverse electric field electrochemical machining) polishing process is being developed for hard passive alloys surface finishing. The results obtained from the experimental study are reported in this paper. An important parameter, MREF-ECM electric field waveform is investigated to optimize the ECM polishing process for IN718.

Extrusion through controlled strain rate dies

The workability of a material during deformation processing is determined by (a) the die geometry which, in turn, determines the flow field during deformation, and, (b) the inherent workability of the material under the imposed processing conditions of strain rate and temperature. Most common alloys have good inherent workability and can be successfully formed over wide ranges of temperature and strain rate. Products can be successfully formed from these alloys even with dies which impose large variations in strain rate during deformation. However, many of the new alloys and composites can be deformed only in very narrow processing regimes, and control of the strain rate during deformation of such materials becomes important. For example, extrusion of a whisker-reinforced aluminum alloy composite is possible only when the strain rate is controlled to within one order of magnitude. This paper describes the development of a method for obtaining preliminary shapes of controlled strain rate extrusion dies, a special case being the constant strain rate die. The theoretical basis for such die design processes is presented, followed by some examples of die geometries. Since this design procedure ignores the material flow properties, the designed die shapes must be verified using the finite element method or physical modeling. Results of simulations with the program ALPID are also presented.

Computer Simulation of the Forging of Fine Grain IN-718 Alloy

In recent years, there has been great emphasis on the use of computer-aided tools in process design. The key to the success of any computer modeling is the accurate knowledge of the mechanical and thermal properties of the various components of a manufacturing system. In order to develop a data base of forging properties of the nickel-base alloy IN-718, isothermal constant strain-rate compression tests were conducted on the annealed fine-grain material over the temperature range 871 °C to 1149 °C (1600 °F to 2100 °F) and strain-rate range 0. 001 to 10 s−1. Empirical relationships among flow stress, strain rate, and temperature developed based on these tests, along with experimentally measured heat-transfer and friction coefficients, were used in the program ALPID to simulate nonisothermal forging of “double-cone” specimens. The simulation results were compared with actual forging in an industrial forge press. The good agreement between simulation and forging results indicates that when a complete data base of materials properties is available, computer modeling can be used effectively to study the forging process.

Properties of AA6061 Processed by Multi-Axial Compressions/Forging (MAC/F)

Multi-axial compressions/forgings (MAC/F) were conducted at room temperature to obtain severe plastic deformation (SPD) of AA6061 alloy. Microhardness measurements taken across the cross-section of each of the MAC/F processed samples indicated that the hardness distribution is nonuniform during initial compressions/forgings and becomes uniform with subsequent compressions. Microhardness and tensile testing showed that the hardness, yield strength, and elongation of the MAC/F processed material followed similar trends to that for AA6061 processed by equal channel angular pressing (ECAP). The MAC/F material exhibited high strain rate sensitivity and high percentage of elongation to failure in the temperature range of 300–350°C. This observation is in agreement with the high formability of SPD-processed AA6061 over the same temperature range.

Phase relationships in Nb-18.7 a/o Si in-situ composite

This study will identify the crystallographic orientation relationships between the niobium and silicide phases in the cast, the cast and extruded, and the directionally solidified (DS) materials. Habit plane relationships will be experimentally identified and compared with atom site models of the crystallographic planes.

Copper Metallic Substrates for High Temperature Superconducting Coated Conductors

Biaxially cube textured polycrystalline Cu(200) substrate tapes were produced for high temperature superconducting (HTS) coated conductor applications. A comparison is made between Cu substrates fabricated by reverse cold rolling followed by recrystallization, from stock materials that were obtained in the form of extruded rod and rolled plate. Detailed x-ray diffraction (XRD) studies and orientation imaging microscopy (OIM) were performed to measure the in-plane alignment, out-of-plane alignment, and microtexture at various deformation levels and annealing temperatures. The rod starting geometry proved to have superior biaxial alignment with a predominant (220) deformation texture after rolling. Phi (Φ) scan and psi (Ψ) scan XRD reveals that the best in-plane and out-of-plane alignment, measured in terms of full width half maximum (FWHM) values of 5.4° and 5.8°, were obtained at 99.5% reduction in thickness and 750 °C annealing temperature. OIM microtexture results indicate that more than 97.5% of grains had less than 10° misorientation with no observable twinning.

Optimization of microstructure during deformation processing using control theory principles

Abstract A two stage approach based on modem control theory has been proposed to control the microstructure development during hot working. This method was utilized for optimal design of hot extrusion process. In the first stage, equations for dynamic recrystallization of plain carbon steel were utilized to obtain an optimal deformation path such that the grain size of the product would be 26 μm. In the second stage, the geometric mapping was utilized to develop an extrusion die profile such that the strain rate profile during extrusion matches with the optimal trajectory computed in the first stage. An extrusion experiment was performed to validate the proposed methodology, by utilizing the extrusion die geometry obtained in the second stage. The as-extruded grain size was observed to be in close agreement with the optimal design performed in the first stage. The results of the present investigation revealed that the principles of control theory can be reliably applied for the optimization and control of microstructure during deformation processing.