Ehab A. El-Danaf

Evolution of grain-scale microstructure during large strain simple compression of polycrystalline aluminum with quasi-columnar grains: OIM measurements and numerical simulations

Abstract Polycrystalline deformation and its modeling by currently used crystal plasticity models has been investigated by means of an experiment involving direct measurement of deformation induced orientation changes. The experiment used a polycrystalline aluminum sample with quasi-columnar grains, whose initial lattice orientations were mapped using the Orientation Imaging Microscopy (OIM) technique. The sample was then compressed 40% (along the axis of the columnar grains), and the lattice orientations after deformation were studied by OIM. It was found that most of the grains had significant in-grain misorientations in the form of deformation bands with two morphologies — either elongated on the grain scale or nearly equiaxed. In many, but not all cases, more than one similarly oriented deformation band was found in an individual grain. The deformation was then simulated using (i) a classical Taylor-type model, and (ii) a finite element model of the polycrystalline aggregate imposing equilibrium and compatibility between and within the constituent grains (in the weak numerical sense). A comparison of the predictions with the experimental results indicated that the Taylor-type model captured well the tendency to move towards a fiber texture but failed to predict correctly which pole was rotating towards the compression axis in the individual grains, and also by its implicit assumptions could not predict any in-grain misorientation. The finite element model predicted, reasonably well, grain rotations as well as the magnitude of the in-grain misorientations in most, but not all, of the individual grains, but failed completely to predict the morphology of the deformation bands that developed within the grains.

Deformation texture transition in brass: critical role of micro-scale shear bands

Abstract The transition in deformation textures between low stacking fault energy f.c.c. metals (e.g. brass textures) and the medium to high stacking fault energy f.c.c. metals (e.g. copper textures) is addressed. A detailed microscopy investigation was conducted in parallel with texture measurements on deformed samples of copper and 70/30 brass to different strain levels in three different deformation paths, namely, plane strain compression, simple compression, and simple shear. The objective of the study was to identify the specific trends in the transition between the brass textures and the copper textures that correlated with the onset of deformation twinning and those that correlated with the onset of micro-scale shear banding. It was found that several important transitions in the evolution of the deformation textures, especially in the rolled samples, correlated not with the onset of deformation twinning but with the onset of micro-scale shear banding. These results strongly suggest that the critical feature in texture transition is not twinning directly, but the shear banding promoted by the high strain hardening rates of low stacking fault energy f.c.c. metal.

Accurate characterization of machine compliance for simple compression testing

Correction for machine compliance is an important step in analyzing the data obtained in many mechanical testing procedures. The difficulties associated with compliance correction, as they apply to the simple compression mode of testing, are explored in this paper. The commonly employed approach is to extend the procedure suggested in the ASTM standards for testing high modulus, single-filament materials, which implicitly assumes that the machine behaves as a linear spring with a constant compliance factor. It is shown in this paper that this approach results in different values for the machine compliance factor for different materials. The nonuniqueness of the machine compliance factor is attributed to the inherent nonlinearity of the machine compliance, i.e., the nonlinear dependence of the nonsample displacement on the applied load. Through a set of mechanical tests on a range of materials, it has been demonstrated that it is necessary to characterize this nonlinear compliance relationship for the machine to obtain accurate and consistent measurements.

Influence of deformation path on the strain hardening behavior and microstructure evolution in low SFE FCC metals

Abstract Following our recent studies of the influence of mechanical twinning on the strain hardening of low SFE FCC metals deformed by simple compression, the investigation was extended to two different deformation modes. These were plane strain compression and simple shear carried out on 70/30 brass, which exhibits only strain hardening, and on MP35N, a Co–Ni based alloy that also shows secondary hardening by deformation promoted precipitation. It was found that the magnitude of the primary strain hardening in both alloys, and the secondary hardening in MP35N, was dramatically reduced under simple shear compared to the other deformation paths. This reduced hardening in simple shear appears to be a consequence of the bulk of the deformation twins, and also the secondary hardening precipitates, forming on planes that were parallel to the primary {111} slip planes in this deformation path. These hypotheses are supported by deformation path change tests in which the shear samples that show low flow stress under continued shear, when subjected to simple compression showed a significant increase (jump) in the flow stress, reaching values that are similar to those of the alloy continuously compressed to the same equivalent strain. That is, the reduced strain hardening in shear deformation is due not to reduced twinning, but to the twins produced by shear providing only limited barriers to continued strain by simple shear. Shear banding was found to be more marked in plane strain compression than in simple compression after cold working, and particularly after the additional secondary hardening in MP35N.

Effect of Equal-Channel Angular Pressing Process on Properties of 1050 Al Alloy

Annealed 1050 Al samples with coarse-grained microstructure of 600 µm were equal-channel angular pressing (ECAP) processed using two routes, A and BC. The samples were processed up to four passes through a die with an internal angle of 90o using both routes. Electron back-scattered diffraction (EBSD) technique was used to study the developed microstructure after ECAP processing. The cell size distribution, misorientation, and the fraction of high angle boundaries were determined. The microstructure study was conducted on both the extrusion direction and the shear plane. The produced microstructure depends on the used route and number of passes. A study of mechanical behavior was conducted by using tensile and compression specimens from the specimens produced by ECAP in the extrusion direction. Enhanced strength was observed but with anisotropic behavior between tension and compression. The dimple size and distribution on fractured surface of tensile specimens was affected by the ECAP route and number of passes.

Effect of Solution Heat Treatment on the Hot Workability of Al–Mg–Si Alloy

The current work presents a detailed study on the high temperature processing of solution treated Al–Mg–Si alloy in the temperature range of 623 K to 773 K and at different strain rates in the range of 5 × 10−5 to 6 × 10−2 s−1. A constitutive relation that can be used in modeling the forming process of this alloy under similar hot working conditions is established. Also, the prevailing deformation mechanism was investigated through relations of the steady state stress dependence on strain rate which revealed a stress exponent of 8.5 (strain rate sensitivity; m ∼ 0.12). This stress exponent is higher than what is usually observed in Al and Al–Mg alloys under similar experimental conditions. This high stress exponent may arise from the presence of threshold stress that results from dislocation interaction with second phase particles (Mg2Si), precipitating during the deformation at high temperatures. The values of threshold stress showed an exponential increase with decreasing temperature and a dependence with an energy term Qo = 38 kJmol−1. The apparent activation energy for solution treated condition was calculated to be about 320 kJmol−1, which is higher than the activation energy for self-diffusion in Al (Qd = 143 kJmol−1) and for the diffusion of Mg in Al (115–130 kJmol−1). By incorporating the threshold stress in the analysis, the true activation energy was calculated to have a value of 111 kJmol−1, and the normalized strain rates can be represented by a power function of the effective stress with stress exponent of ∼3. Ductility was documented to reveal the best working condition for this alloy in solution treated condition. The ductility exhibited a maximum value of about 120% at 773 K at a strain rate of 0.064 s−1. The results of the current work is, also, compared to the results of another heat treatment condition (T4-naturally aged) to reveal which ever condition holds better hot forming characteristics.

Enhanced Fatigue Strength of Commercially Pure Ti Processed by Rotary Swaging

Fully reversed bending fatigue tests were performed on polished hour-glass specimens of commercially pure titanium grade 1 with three different grain sizes, that were produced by severe plastic deformation (rotary swaging) and subheat treatments, in order to examine the effect of grain size on fatigue. An improvement in fatigue strength was observed, as the polycrystal grain size was refined. The endurance limit stress was shown to depend on the inverse square root of the grain size as described empirically by a type of Hall-Petch relation. The effect of refining grain size on fatigue crack growth is to increase the number of microstructural barriers to the advancing crack and to reduce the slip length ahead of the crack tip, and thereby lower the crack growth rate. It was found that postdeformation annealing above recrystallization temperature could additionally enhance the work-hardening capability and the ductility of the swaged material, which led to a marked reduction in the fatigue notch sensitivity. At the same time, this reduction was accompanied with a pronounced loss in strength. The high cycle fatigue performance was discussed in detail based on microstructure and mechanical properties.

Effect of Equal Channel Angular Pressing on the Surface Roughness of Solid State Recycled Aluminum Alloy 6061 Chips

Solid state recycling through hot extrusion is a promising technique to recycle machining chips without remelting. Furthermore, equal channel angular pressing (ECAP) technique coupled with the extruded recycled billet is introduced to enhance the mechanical properties of recycled samples. In this paper, the surface roughness of solid state recycled aluminum alloy 6061 turning chips was investigated. Aluminum chips were cold compacted and hot extruded under an extrusion ratio (ER) of 5.2 at an extrusion temperature (ET) of 425°C. In order to improve the properties of the extruded samples, they were subjected to ECAP up to three passes at room temperature using an ECAP die with a channel die angle of 90°. Surface roughness ( and ) of the processed recycled billets machined by turning was investigated. Box-Behnken experimental design was used to investigate the effect of three machining parameters (cutting speed, feed rate, and depth of cut) on the surface roughness of the machined specimens for four materials conditions, namely, extruded billet and postextrusion ECAP processed billets to one, two, and three passes. Quadratic models were developed to relate the machining parameters to surface roughness, and a multiobjective optimization scheme was conducted to maximize material removal rate while maintaining the roughness below a preset practical value.

Effect of equal-channel angular pressing on the surface roughness of commercial purity aluminum during turning operation

Aluminum has been increasingly used in automotive and aerospace applications due to its beneficial specific strength and chemical properties. Due to its extensive use, machining of aluminum parts has become specifically significant in recent years. One important aspect of machining is the surface quality represented by the surface roughness values. In this article, the effect of equal-channel angular pressing on the surface roughness (Ra, Rq, Rt and Rz) of commercial purity aluminum machined by turning was studied. Five starting material conditions, defined as the annealed and equal-channel angular pressing processed up to four passes, were investigated. The independent variables were the cutting speed, depth of cut and feed rate. The fourth parameter (number of equal-channel angular pressing passes) was considered as categorical factor and, hence, was not included in the mathematical model. A full central composite circumscribed design matrix was built to allow the optimization of surface roughness using response surface methodology. The significance of process parameters and their interactions in estimating surface roughness was investigated using analysis of variance. The two parameters, with significant effect on surface roughness, were found to be the feed rate and number of equal-channel angular pressing passes. Minimum depth of cut (0.15 mm) and minimum feed rate (0.05 mm/rev) are needed to achieve minimum surface roughness parameters: Ra (0.06 µm), Rq (0.057 µm) and Rz (0.71 µm) and Rt (1.2 µm). The cutting speed, for these optimum roughness values, ranged from 207.5 m/min for Ra to 193 m/min for Rz. The optimum roughness values were generally achieved with the higher strength materials. Optimum values for Ra, Rq and Rz happened at the four equal-channel angular pressing passes–processed material, while the optimum value of Rt happened at the three equal-channel angular pressing passes–processed material.

Mechanical Characterization of Cryomilled Al Powder Consolidated by High-Frequency Induction Heat Sintering

In the present investigation, an aluminum powder of 99.7% purity with particle size of ~45 µm was cryomilled for 7 hours. The produced powder as characterized by scanning, transmission electron microscopy, and X-ray diffraction gave a particle size of ~1 µm and grain (crystallite) size of  nm. This powder, after degassing process, was consolidated using high-frequency induction heat sintering (HFIHS) at various temperatures for short periods of time of 1 to 3 minutes. The present sintering conditions resulted in solid compact with nanoscale grain size (<100 nm) and high compact density. The mechanical properties of a sample sintered at 773 K for 3 minutes gave a compressive yield and ultimate strength of 270 and 390 MPa, respectively. The thermal stability of grain size nanostructured compacts is in agreement with the kinetics models based on the thermodynamics effects.