Pack Rolling Research Articles

Microstructure, creep, and tensile behavior of a Ti–12Al–38Nb (at.%) beta+orthorhombic alloy

The microstructural evolution, creep, and tensile deformation behavior of a Ti–12Al–38Nb (at.%) alloy were studied. Monolithic sheet materials were produced through conventional thermomechanical processing techniques comprising nonisothermal forging and pack rolling. TEM studies showed that depending on the heat-treatment schedule, this alloy contains two constituent phases including: β (disordered body-centered cubic) and O (ordered orthorhombic based on Ti2AlNb). Heat treatments at all temperatures above 800°C, followed by water quenching, resulted in fully-β microstructures. Below 800°C, fine Widmanstatten O-phase needles precipitated within the β grains. Fully-β microstructures exhibited room-temperature (RT) elongations of more than 27%. The second-phase O precipitates provided strengthening at the expense of elongation. However, RT elongations of more than 12% were recorded for aged microstructures containing 30 vol.% O-phase. Metallographic observations revealed that slip was compatible between the two phases. Tensile creep tests were conducted in the temperature range 650–705°C and stress range 50–172 MPa. The deformation observations and measured creep exponents and activation energies suggested that the creep mechanisms were dependent on stress. For applied stresses less than 123 MPa, the creep exponents were between 1.6 and 2.0 and low dislocation densities were observed. For higher stresses, the stress exponents were between 3.5 and 7.2 and higher dislocation densities were observed. The calculated activation energies for the low-stress regime were approximately half those calculated for the high stress regime. These data suggest that Coble creep operates at low stress levels and dislocation climb is active at high stresses. Microstructural effects on the tensile properties and creep behavior are discussed in the light of existing models.

The phase evolution and microstructural stability of an orthorhombic Ti-23Al-27Nb alloy

The phase evolution and microstructural stability were studied for an orthorhombic Ti-23Al-27Nb alloy. Monolithic sheet materials were produced through conventional thermomechanical processing techniques comprising nonisothermal forging and pack rolling. Phase evolution studies showed that, depending on the heat treatment schedule, this alloy may contain several constituent phases including: α2 (ordered close-packed hexagonal D019 structure), B2 (ordered body-centered cubic, bcc), β (disordered bcc), and O (ordered orthorhombic based on Ti2AlNb). Differential thermal analysis studies indicated that the B2 transus temperature was 1070 °C. Heat treatment and transmission electron microscopy studies showed that the α2+B2 phase field extended between 1010 and 1070 °C. From 875 to 975 °C, a two-phase O+B2 field existed. Sandwiched between these two-phase regimes was a narrow three-phase α2+B2+O field. Below 875°C, an O+β field existed. All heat treatments at or above 875°C, followed by quenching, resulted in equiaxed microstructures. However, below 875 °C, the B2 phase transformed into a mixture of O and bcc phases with lenticular morphologies. Cellular precipitation of O+β platelets at O/B2 and α2/B2 grain boundaries occurred depending on solutionizing and aging temperatures, which is explained by the compositional gradient between the bcc phases.

Microstructural evolution during rolling of Ti22Al23Nb sheet

The evolution of microstructure during hot pack rolling of the orthorhombic titanium aluminide alloy Ti22Al23Nb was investigated. The starting material was a prior hot-rolled sheet with an elongated ordered β (B2) grain structure whose grain boundaries were decorated with orthorhombic (O) phase. Two different types of rolling schedule were utilized, one consisting of rolling below the β (B2) transus alone and the other comprising supertransus preheating (in order to dissolve the second phase) followed by rolling through the transus under decreasing temperature conditions. For both schedules, the rolled structure consisted of elongated β (B2) grains and a fine dispersion of second-phase (O) particles. The absence of recrystallization of the B2 grains, particularly for sequences involving supertransus preheating, was explained by measurement of the precipitation kinetics for the second phase. These measurements indicated very rapid grain boundary precipitation, even close to the transus, an effect that leads to the pinning of the β (B2) grain boundaries.

Deformation and microstructure development during hot-pack rolling of a near-gamma titanium aluminide alloy

Deformation behavior and microstructure development during hot pack rolling of the near-gamma titanium aluminide alloy Ti-45.5Al-2Cr-2Nb (atomic percent) were established. Deformation behavior was investigated through rolling at various nominal furnace temperatures and parallel modeling studies using a finite difference approach to predict temperature transients during workpiece transfer from the furnace and during the rolling operation itself. Agreement between measured rolling pressures and predictions based on a rule-of-mixtures (ROM) average of the flow stresses of the pack components (at the predicted temperatures and strain rates within the roll gap) was excellent. As-rolled microstructures were interpreted in terms of the Ti-xAl-2Cr-2Nb pseudobinary phase diagram, predicted temperature transients during rolling, and the static (no deformation) phase-transformation behavior of the program material. These results demonstrated the strong influence of furnace preheat temperature on microstructure development, as well as the tendency for temperature transients due to radiation heat losses and roll chilling to suppress phase transformations.

Load-Signature analysis for pack rolling of near-gamma titanium aluminide alloys

The objective of the present work was to demonstrate the sensitivity of rolling-load signature analysis as a means to monitor hot-pack-rolling processes for the fabrication of sheet of near-gamma titanium-aluminide alloy sand other difficult-to-work materials. In previous work, a simple method was developed for the prediction of temperature transients during two steps in the pack-rolling process: the transfer of the pack from the furnace to the rolling mill and the actual rolling operation itself. The accuracy of the temperature-transient calculations was established through load-signature data obtained during trials for Ti-48Al (atomic percent) rolled at a single nominal (furnace) temperature. In the present work, additional results are presented and discussed for hot pack rolling at various nominal temperatures and for a range of near-gamma titanium-aluminide alloys.

Effects of thermomechanical processing on titanium aluminide strip cast by the melt overflow process

The objective of this research project was to investigate the feasibility of producing titanium aluminide foils from direct cast strip using ribbon technology's plasma melt overflow process. Niobium-modified Ti 3Al alloys were melted in a cold copper crucible using a transferred plasma arc and then direct cast into strip on a rotating chill roll. Samples cut from the as-cast Ti 3AlNb (α 2) titanium aluminide strip were encapsulated into a pack. The packs were heated to the rolling temperature and then hot rolled at low strain rates. Foils 70 μm (0.003 in) thick, having a uniform α 2-B2 microstructure with oxygen contents as low as 900 wt.ppm were obtained after pack rolling. The strips and foils were characterized in terms of microstructure and chemical composition in the as-received, heat-treated and pack-rolled conditions. The results indicated that it was technically feasible to produce foils from direct cast titanium aluminide strip using pack-rolling technology. The advantage of this technology lies in its cost-effectiveness, since the relatively low cost direct-cast titanium aluminide strip was thermomechanically processed into foil with the desired microstructure without any intermediate processing steps.

Design, Analysis and Experimental Verification of the Pack-Rolling Process

The paper presents a design and analysis approach for the pack rolling process to determine the stability in terms of the correct dimensions of the material and the desired microstructure without defects. Pack rolling involves the rolling of “difficult-to-form” materials within a pack or cover material. Simulation of the process requires modeling the physics of the pack rolling process; the temperature distribution, the velocity discontinuities between the cover plate and the workpiece, and the deformation process. An analytical model was developed for simulating the pack rolling process which utilizes a kinematically admissible velocity field to determine the relative slip between the cover plate and the workpiece, a heat transfer module determines the temperature distribution, and the deformation analysis is modeled by a modified slab method. A 2D finite element analysis with coupled heat transfer for pack rolling is presented. The pack rolling model was correlated with experiments and the more accurate finite element model.

Temperature transients during hot pack rolling of high temperature alloys

Temperature transients during hot pack rolling of high temperature alloys

Processing and properties of titanium foils

Advanced aerospace structures call for the use of structural honeycomb materials for improved load carrying capacity combined with reduced weight. Foils produced from titanium-based alloys are candidate materials for honeycomb applications because of their low density and good intermediate temperature strength properties. The production of foils from titanium-based alloys is difficult, however, principally because of atmospheric contamination during processing. The current state of the art of Ti6Al4V foil production is reviewed and a newly developed foil production method based upon elevated temperature pack rolling is presented. This novel processing method yields a fully dense foil with microstructures, oxygen content and mechanical properties comparable with more massive bar and plate products.

Three diverse target preparations: 14C (12 mg/cm 2), 71Ga 24Mg (12 mg/cm 271Ga, 3 mg/cm 224Mg), and 66,67Zn (1.8–14.9 mg/cm 2)

A natural-carbon analog of fluffy, intractable 14C powder was produced. With it, a method was developed to produce a pressed disk of 14C of 12.7-mg/cm 2 thickness and 1.27-cm diameter, bound with 2.1 wt.% of adhesive. Aluminized Mylar cover foils and a fritted-disc filter were used to contain the target for ( p, p′) experiments. Reduction of 71Ga 2O 3 to the metal was accomplished with an efficiency of 94.3% in a small electroplating cell. Magnesium was chosen as the companion element because 50 at.% could be tolerated in the (p, n) experiment, and GaMg has a melting point of 646 K. A 1.27-cm diameter target, supported at the edge by a Mg foil, was produced in several simple steps. Directly rollable 66,67Zn foils were obtained from an electroplating cell with a Pt screen anode and a highly polished tungsten-carbide cathode. Plating times of 3 h provided metal-recovery efficiencies ranging from 94.2 to 96.5%. The as-deposited foils had many holes but were hole-free and shiny after reduction of 25% by pack rolling.

The part of targets in heavy-ion positron spectroscopy

Quasi-atomic K-hole and positron-production experiments together with the positron-line phenomena are presented which emphasize the importance of target proporties in heavy-ion collisions near the Coulomb barrier with beams of 208Pb, 232Th, and 238U. Various self-supported, supported, or sandwiched targets — either metallic or as chemical compounds — have been used; preparation techniques (high-vacuum sublimation- and evaporation condensations, pack rolling, sputter deposition, and molecular plating) are mentioned casually for targets ranging from C, Ti, Ni, Ta, and Pb up to radioactive samples of 232Th, 238U, and 248Cm. The influence of target deterioration is discussed in view of the energy loss and the charge states of the ions. In-beam heavy-ion scattering as well as proton backscattering are exemplified as target control procedures. A target wobbler and a target wheel are given as examples for the reduction of deteriorations due to heavy-ion bombardments.

Processing titanium aluminide foils

In an effort to extend the available manufacturing technologies for Ti3Albase aluminides, the potential success of a new hot-rolling process for the production of high-quality foils has been investigated. Processing of Super-α2 aluminide foils approximately 0.15 mm thick takes full advantage of the combined benefits of controlled rolling and a unique pack rolling process. Based on the results of a study of phase transformation kinetics, microstructure and hardness of continuously-cooled and isothermally transformed specimens, the processing parameters are optimized to provide enhanced surface quality and superior ductilities in the as-rolled condition.

Bonding of metals during pack rolling

Bonding of metals during pack rolling

Effect of oxygen in iron on the inhibition of diffusion in Al?Fe bimetals

Reactive diffusion during heat treatment of aluminum-iron bimetals can be suppressed by oxygen contained in the steel, due to the formation of an aluminum oxide film at the interface that blocks mass transfer of both components. This can be utilized in cladding steel with aluminum and aluminum alloys by cold pack rolling, followed by recrystallization annealing.

The deformation structure of molybdenum 34% rhenium

Polycrystalline molybdenum 34%(at.) rhenium has been deformed by cold rolling. The variation in twin and dislocation microstructures produced by longitudinal, transverse, cross and pack rolling is reported. Deformation is initially accompanied by much twinning; twin formation being more difficult when the material is rolled in a direction transverse to the original rolling direction. A dislocation cell structure begins to form after about 10 per cent deformation by single direction rolling, but forms at a lower deformation by rolling in two directions. Within limits the dislocation cell size and wall thickness can be varied by the use of different rolling schedules. The structure of the pack rolled material is shown to be due to constraints imposed by the jacket. Failure of this material results from the growth of voids under an internal tension.

Diffusion redistribution of elements in transition layers of bimetals

1. The main element diffusing into the transition zone of bimetals consisting of carbon steel clad with stainless steel is carbon. Carbon diffuses into the cladding layer of chromium or chromium — nickel steel, creating an inverse concentration gradient. The diffusion of carbon occurs in the bimetal even at 425°C. The character of the carbon distribution depends on the method of producing the bimetal. 2. The diffusion of carbon into stainless steels Kh18N10T and 0Kh13 leads to the formation of carbides M23C6 and M7C3 in the form of films 500–2000 A thick in the transition zones. 3. To limit the diffusion of carbon and iron into the cladding layer it is expedient to use a nickel. interlayer, which is easily accomplished in pack rolling. 4. In a chromium — nickel clad bimetal the nickel interlayer goes into solution during heat treatment and in its place is formed an Fe−Cr−Ni−C alloy whose composition depends on the heat treatment; the bonding strength increases in this case. 5. In the St.3 + Monel, bimetal the properties of the transition zone are affected mainly by the diffusion of iron into Monel, which leads to the formation of brittle intermediate layers, particularly after heat treatment. The embrittlement of this bimetal can be prevented by use of a nickel interlayer. 6. By plotting the results of electron probe analysis of the composition of transition layers on phase and equilibrium diagrams it is possible to predict the characteristics of these layers and the reason for changes in them.

Textured 6.5% Silicon-Iron Alloy

Sheet of an iron base alloy containing 6.5% silicon and second phase inclusions was prepared by hot rolling and pack rolling. Two structures were obtained by varying the rate of heating to the final annealing temperature. A slow rate of heating yielded {110} grains in a secondary recrystallization structure, and a fast rate, the primary recrystallization and normal grain growth structure. Only during the slow rate of heating were inclusions effective in suppressing normal grain growth and in promoting growth of {110} secondaries. The secondary recrystallization texture was of the {110} 〈001〉 type. The normal grain growth texture had a similar degree of alignment of 〈100〉 directions with the rolling direction and a nearly random distribution of planes of the 〈100〉 zones parallel to the rolling plane. Similar dc and ac magnetic characteristics were observed for these two structures since they had the same degree of alignment of 〈100〉 directions (easy directions of magnetization) with the rolling direction.

Metallurgical Problems in Fabrication of Zirconium-Clad Fuel Elements for Pressurized Water Reactors

To achieve adequate corrosion resistance in high-temperature water, the chemical composition and fabrication of zirconium cladding material must be carefully controlled. Present technology permits the satisfactory fabrication of a variety of fuel materials which are compatible with zirconium cladding. Homogeneity and melting crucible problems still exist with some of the fuel alloys. Zirconium-clad fuel elements of the bonded type are produced by pack rolling or coextrusion of the fuel and cladding. The optimum working temperature represents a balance between the need for minimizing differences in plasticity between fuel and cladding and for adequate diffusion at the clad-fuel interface. Unbonded fuel elements require close dimensional control of fuel and cladding and high-quality closure welds which are made possible by the excellent welding characteristics of Zircaloy.