Papers by Author: Terry R. McNelley

Abstract: ntense plastic deformation is generally effective in producing grain refinement. IPD methods include equal channel angular pressing/extrusion (ECAP/ECAE), high-pressure torsion (HPT), accumulative roll bonding (ARB), and friction stir processing (FSP), among others. In this work, we summarize the main results on grain refinement by these processing methods and present our own data on microstructure and texture evolution in metals and alloys during ECAP, HPT and FSP. Whereas ECAP and HPT are usually performed with the work piece material initially at room temperature or even at liquid nitrogen temperature to enhance refinement, FSP involves a brief but complex thermomechanical cycle with peak temperatures up to 0.7 0.9 T_Melt. Apparently, materials undergo dynamic recrystallization (DRX) during FSP. DRX also occurs also in metals and alloys of low T_Melt due to adiabatic heating during HPT performed at room temperature. The paper is devoted to revisiting of previous as well as new results and a comparative analysis of microstructure and texture evolution in commercially pure aluminum and selected pure metals and alloys during ECAP, HPT and FSP in order to illustrate the limits of grain refinement.

Abstract: FSP is an allied technology of friction stir welding (FSW). These processes are reviewed and the additional considerations such as processing pattern and step over distance in FSP are introduced. The application of FSP to cast metals including AA5083, Al-7Si and NiAl bronze is summarized. As-cast microstructures may be converted to a wrought condition in the absence of external shape change and the extent of grain refinement and homogenization of microstructure is documented. The FSP-induced superplastic response of AA5083, enhanced ductility of Al-7Si and surface hardening of NiAl bronze are summarized as examples of properties developed by FSP.

Abstract: FCC materials were subjected to large strain deformation by three techniques: equal channel angular pressing (ECAP), plane-strain machining and friction stir processing (FSP). Based on the orientation imaging mapping (OIM) analysis of the deformed regions, the most likely microstructure refinement mechanisms have been identified for each of the techniques and compared among one another.

Abstract: Multi-pass FSP was conducted on continuously-cast (CC) AA5083 materials in the as-cast condition. Stir zone grains were refined to ~1.0 – 3.5μm in size and highly superplastic response was obtained during tension testing of the as-processed materials at 450°C (>1200 pct. elongation at 10-1 s-1). Current models of recrystallization do not predict adequately the highly refined grains and predominantly random textures that are observed in stir zones. Grain refinement during FSP is accompanied by refinement and redistribution of non-deforming constituents in the absence of particle fracture. The mechanics of the homogenization process remain to be established and requirements for redistribution mechanisms will be summarized. Comparison reveals that results from FSP of the as-cast material are superior to those attained in conventional processing.

Abstract: Parameters for multi-pass FSP include the pattern of tool traverse and step-over distance between successive passes. Multi-pass FSP was conducted on as-cast NiAl bronze and as-cast AA5083 in order to modify stir zone (SZ) microstructures and mechanical properties. Highly refined and homogeneous SZ microstructures may be produced by FSP. Refined and equiaxed grain structures reflect recrystallization during FSP; mechanisms leading to homogenization by redistribution of microstructure constituents remain to be determined. Refined microstructures exhibit enhanced ambient-temperature properties and superplasticity at elevated temperatures.

Abstract: Among the phenomena leading to formation of high-angle boundaries during deformation processing at low homologous temperatures is the subdivision of prior grains and formation of deformation bands. Evidence for this phenomenon during processing of AA2004, a superplastic aluminum alloy, is reviewed; fragmentation of deformation bands leads to equiaxed grains and high-angle boundaries that support superplasticity. In addition to subdivision, groups of grains undergo lattice rotation toward one or the other variant of the β orientation fibers during plane-strain deformation of pure aluminum by ambient temperature rolling. The formation of equiaxed grains from banded structures during simple shear by equal channel angular pressing (ECAP) is also considered.

Abstract: Friction stir processing (FSP) has been employed for localized modification and control of microstructures in NiAl bronze materials, which are widely utilized for marine components. The thermomechanical cycle of FSP results in homogenization and refinement and the conversion of microstructures from a cast to a wrought condition within stir zones in the material. However, the direct measurement of stir zone temperatures, strains, strain rates and cooling rates is difficult due to steep gradients and transients in these quantities, and this is an impediment in the assessment of FSP-induced microstructures and properties. Quantitative microstructure analyses following FSP of cast NiAl bronze materials have been used to develop estimates of stir zone thermomechanical cycles. The estimation procedures will be reviewed and the microstructure-based estimates will be compared to results from computational models and embedded thermocouples measurements. Stir zone microstructures comprise a mixture of primary α grains and transformation products of the β that formed during processing. Recrystallization in the primary α occurred due to particle-stimulated nucleation in this low stacking fault energy material. Factors that influence the distribution of strength and ductility in the stir zone appear to include the mixture of microstructure constituents and gradients in microstructure due to gradients in processing conditions.

Abstract: Friction stir processing (FSP) is a severe plastic deformation (SPD) method that has been applied to as-cast NiAl bronze (NAB) materials, which are widely used for marine components. The thermomechanical cycle of FSP results in homogenization and refinement, and the selective conversion of microstructures from a cast to a wrought condition. The physical metallurgy of NAB is complex and interpretation of the effects of FSP on microstructure has required detailed analysis by optical and electron microscopy methods. Annealing and isothermal hot rolling have been employed to confirm microstructure-based estimates of stir-zone peak temperatures. The variation of mechanical properties was assessed by use of miniature tensile samples and correlated with microstructure for samples from stir zones of single and multi-pass FSP. Exceptional improvement in strength – ductility combinations may be achieved by FSP of NAB materials.

Abstract: The influence of strain path during equal-channel angular pressing (ECAP) has been evaluated in pure aluminum by orientation imaging microscopy (OIM) and transmission electron microscopy (TEM). The material was examined after four pressing operations by route BC in a 90° die, or eight pressing operations by route BC in a 135° die. The von Mises equivalent strains were essentially the same for these two ECAP procedures. The microtexture data indicate that the distortion during ECAP corresponds to a simple shear in a direction approximately parallel to diechannel exit and on a plane perpendicular to the flow plane. For both procedures the OIM data reveal prominent meso-scale band-like features. Lattice orientations in each band correspond to a texture orientation but the particular combinations of orientations depend upon ECAP die angle. High-angle boundaries in the structure correspond to interfaces between the bands.