S. L. Semiatin

Continuous Dynamic Recrystallization During Friction Stir Welding of High Strength Aluminum Alloys

Abstract : Friction stir welding (FSW) is a solid state joining process 1,2,3 that uses a rapidly-rotating, non-consumable high strength tool-steel pin that extends from a cylindrical shoulder (Figure 1). The workpieces to be joined are firmly clamped to a worktable; the rotating pin is forced with a pre-determined load into them and moved along the desired bond line. Frictional heating is produced from the rubbing of the rotating shoulder with the workpieces, while the rotating pin deforms (i.e. stirs) the locally-heated material. To produce a high integrity defect-free weld, process variables (RPM of the shoulder-pin assembly, traverse speed, the downward forging force) and tool pin design must be chosen carefully. FSW can be considered as a hot-working process in which a large amount of deformation is imparted to the workpiece through the rotating pin and the shoulder. Such deformation gives rise to a weld nugget (whose extent is comparable to the diameter of the pin), a thermomechanically-affected region (TMAZ) and a heat-affected zone (HAZ). Frequently, the weld nugget appears to comprise equiaxed, fine, dynamically recrystallized grains whose size is substantially less than that in the parent material. The objective of the present research was to develop a basic understanding of the evolution of microstructure in the dynamically recrystallized region and to relate it to the deformation process variables of strain, strain rate, and temperature. Such a correlation has not been attempted before perhaps because of the difficulty in quantifying the process variables. To overcome such difficulties, recent work 4 to measure and model the local temperature transients during FSW was utilized, and an approximate method was employed to estimate the strain and strain rate in the weld nugget.

Hot workability of titanium and titanium aluminide alloys—an overview

Abstract The hot workability of conventional titanium alloys and titanium aluminides is reviewed. For both alloy classes, the influence of hot working variables and microstructure on failure via fracture or flow-localization controlled processes is summarized. The occurrence of wedge cracking and cavitation during bulk forming of α / β alloys with Widmanstatten microstructures or γ titanium aluminides with lamellar or equiaxed structures, is examined. In particular, the effects of grain size, grain boundary second phases and process variables on failure are presented. Observations and models of flow localization and cavitation processes which lead to failure during low strain rate, superplastic, tensile-type deformation of titanium and titanium aluminide alloys with fine equiaxed structures, are also described. In the area of flow-localization-controlled failure during bulk forming, the occurrence of shear bands and other flow nonuniformities during both conventional and isothermal hot working processes is reviewed. The influence of material properties, such as flow softening rate and strain rate sensitivity and process variables, which lead to temperature and hence flow nonuniformities, is examined. The flow localization concepts are illustrated for several hot working processes.

An automated method to determine the orientation of the high-temperature beta phase from measured EBSD data for the low-temperature alpha-phase in Ti–6Al–4V

A method was developed to determine the orientation of the high-temperature beta phase from measured electron-backscatter diffraction (EBSD) data for the low-temperature alpha phase in Ti-6Al-4V. This technique is an improvement over existing methods because it does not require a priori knowledge of the variant selection process and can accommodate variants from adjacent beta grains being incorporated in the data set submitted for analysis. It is a general method and therefore can be used to examine texture relationships in materials other than Ti-6Al-4V which undergo a burgers-type phase transformation.

Strain-Path Effects on the Evolution of Microstructure and Texture during the Severe-Plastic Deformation of Aluminum

Microstructure and texture evolution during the severe-plastic deformation (SPD) of unalloyed aluminum were investigated to establish the effect of processing route and purity level on grain refinement and subgrain formation. Two lots of aluminum with different purity levels (99.998 pct Al and 99 pct Al) were subjected to large plastic strains at room temperaturevia four different deformation processes: equal-channel angular extrusion (ECAE), sheet rolling, conventional conical-die extrusion, and uniaxial compression. Following deformation, microstructures and textures were determined using orientation-imaging microscopy. In commercial-purity aluminum, the various deformation routes yielded an ultrafine microstructure with a ∼1.5-µm grain size, deduced to have been formedvia a dynamic-recovery mechanism. For high-purity aluminum, on the other hand, the minimum grain size produced after the various routes was ∼20 µm; the high fraction of high-angle grain boundaries (HAGBs) and the absence of subgrains/deformation bands in the final microstructure suggested the occurrence of discontinuous static recrystallization following the large plastic deformation at room temperature. The microstructure differences were underscored by the mechanical properties following four ECAE passes. The yield strength of commercial-purity aluminum quadrupled, whereas the high-purity aluminum showed only a minor increase relative to the annealed condition.

Variant Selection During Cooling after Beta Annealing of Ti-6Al-4V Ingot Material

The selection of alpha variants during the cooling of Ti-6Al-4V from the beta-phase field was investigated. For this purpose, samples with a coarse, columnar beta-grain structure with a 〈100〉 fiber texture were extracted from an as-cast production-scale ingot. The alpha variants in the as-cast samples as well as those produced during several successive beta-annealing treatments were determined using electron backscatter diffraction (EBSD). The EBSD results indicated that a subset of the 12 possible variants was developed within each grain; the specific variants were a function of the cooling rate after beta heat treatment. Moreover, the generation of similar variants during successive heat treatments involving an identical cooling rate suggested a noticeable memory effect. The variant selection process was rationalized based on calculations of the strain associated with the beta-to-alpha transformation. These calculations revealed that the overall aggregate strain approached zero in both the as-cast condition as well as after beta heat treatment, suggesting the occurrence of a long-range self-accommodation mechanism.

Plastic Flow and Microstructure Evolution during Thermomechanical Processing of a PM Nickel-Base Superalloy

Plastic flow and microstructure evolution during sub- and supersolvus forging and subsequent supersolvus heat treatment of the powder-metallurgy superalloy LSHR (low-solvus, high-refractory) were investigated to develop an understanding of methods that can be used to obtain a moderately coarse gamma grain size under well-controlled conditions. To this end, isothermal, hot compression tests were conducted over broad ranges of temperature [(1144xa0K to 1450xa0K) 871xa0°C to 1177xa0°C] and constant true strain rate (0.0005 to 10xa0s−1). At low temperatures, deformation was generally characterized by flow softening and dynamic recrystallization that led to a decrease in grain size. At high subsolvus temperatures and low strain rates, steady-state flow or flow hardening was observed. These latter behaviors were ascribed to superplastic deformation and microstructure evolution characterized by a constant grain size or concomitant dynamic grain growth, respectively. During supersolvus heat treatment following subsolvus deformation, increases in grain size whose magnitude was a function of the prior deformation conditions were noted. A transition in flow behavior from superplastic to nonsuperplastic and the development during forging at a high subsolvus temperature of a wide (possibly bi- or multimodal) gamma-grain-size distribution having some large grains led to a substantially coarser grain size during supersolvus annealing in comparison to that produced under all other forging conditions.

Mechanisms and Kinetics of Static Spheroidization of Hot-Worked Ti-6Al-2Sn-4Zr-2Mo-0.1Si with a Lamellar Microstructure

The effect of imposed strain ε, annealing temperature TA, and annealing time τ on the static spheroidization behavior of Ti-6Al-2Sn-4Zr-2Mo-0.1Si having an initial lamellar microstructure was investigated. For this purpose, the samples were compressed isothermally at 1173xa0K (900xa0°C) to εxa0=xa01.0 and subsequently annealed at 1228xa0K (955xa0°C)xa0≤xa0TAxa0≤xa01253xa0K (980xa0°C) for 10xa0minutesxa0≤xa0τxa0≤xa024xa0hours. For each test condition, metallography was performed to evaluate the change in aspect ratio (AR) and thus quantify the structural evolution from a lamellar to an equiaxed morphology. The average AR decreased rapidly during short annealing times as a result of sub-boundary–induced boundary splitting, but it decreased at a considerably slower rate during subsequent long-time, diffusion-controlled termination migration. The overall time to complete the static globularization was thus governed largely by termination migration. To model the observations, a kinetic equation describing the static spheroidization of two-phase titanium alloys was developed. A comparison of experimental results and predictions showed that the model can provide a reasonable prediction of the time required to complete diffusion-controlled migration of the edges of thin lamellar fragments that are circular or elliptical in shape.

Texture evolution in vacuum arc remelted ingots of Ti-6Al-4V

Abstract The textures of the alpha and beta phases of a production-scale Ti-6Al-4V VAR ingot were determined using orientation imaging microscopy (OIM). Alpha-phase textures were determined directly from specimens that were cut from various regions of the ingot. To determine the texture of the beta phase, the measured orientations of alpha-phase variants from a number of prior-beta grains and specialized analysis software, which was based on the burgers relation between the alpha and beta phases, were utilized. The results of the analysis demonstrated that the columnar grains in the ingot had formed as a result of solidification of the beta phase along 〈100〉 preferred-growth directions. By contrast, the equiaxed grains at the center of the ingot had random alpha- and beta-phase textures.

Effect of Process Variables on Transformation-Texture Development in Ti-6Al-4V Sheet Following Beta Heat Treatment

The effect of preheat time, prestrain, cooling rate, and concurrent deformation during cooling on the preferential selection of hcp alpha variants during the decomposition of the high-temperature, bcc beta phase in two-phase titanium alloys was established using Ti-6Al-4V sheet material. For this purpose, sheet tension samples were pre-soaked in the beta phase field for 0 or 10xa0minutes (to vary the beta grain size), subjected to a prestrain of 0 or 0.1, and cooled at a rate of 11 or 155xa0K/min (11xa0or 155xa0°C/min) under conditions comprising free ends, fixed ends, or concurrent deformation at a strain rate between ~10−5xa0and 3xa0×xa010−4xa0s−1. Electron-backscatter diffraction was used to determine the orientations of the alpha variants so formed, from which the underlying high-temperature, beta-grain microstructure and orientations were reconstructed. These measurements revealed that the parent beta texture changed due to grain growth during preheating. A comparison of the alpha- and beta-phase textures indicated that preferential variant selection was most noticeable under conditions involving a slow cooling rate especially when prestrain or concurrent straining was imposed.

Characterization of Microstructure, Texture, and Microtexture in Near-Alpha Titanium Mill Products

Microstructure, texture, and microtexture in Ti-6Al-2Sn-4Zr-2Mo-0.1Si billet/bar of three different diameters (57, 152, and 209xa0mm) were quantified using backscattered electron imaging and electron backscatter diffraction. All three billets exhibited a microstructure comprising a large fraction (≥70 pct) of primary alpha particles, the average size of which decreased and aspect ratio increased with increasing reduction/decreasing billet diameter, or trends suggestive of low final hot working temperatures and/or slow cooling rates after deformation. Appreciable radial variations in the volume fraction and aspect ratio of alpha particles were noticeable only for the smallest-diameter billet. Alpha-phase textures were typical of axisymmetric deformation, but were relatively weak (~3× random) for all billet diameters. By contrast, bands of microtexture, which were multiple millimeters in length along the axial direction, were relatively strong for all of the materials. The intensity and radial thickness of the bands tended to decrease with decreasing billet diameter, thus indicating the important influence of imposed strain on the elimination of microtexture and the possible influence of surface preform microstructure following the beta quench on the evolution of microstructure and microtexture.