Philipp Frint

Microstructural Features and Mechanical Properties after Industrial Scale ECAP of an Al 6060 Alloy

Future applications of ultrafine-grained, high performance materials produced by equal-channel angular pressing (ECAP) will most likely require processing on an industrial scale. There is a need for detailed microstructural and mechanical characterisation of large-scale, ECAP-processed billets. In the present study, we examine the microstructure and mechanical properties as a function of location and orientation within large (50 x 50 x 300 mm³) billets of an Al 6060 alloy produced by ECAP (90° channel angle) with different magnitudes of backpressure. The internal deformation is analysed using a grid-line method on split billets. Hardness is recorded in longitudinal and cross-sectional planes. In order to further characterise the local, post-ECAP mechanical properties, tensile tests in different layers are performed. Moreover, low voltage scanning transmission electron microscopy observations highlight relevant microstructural features. We find that the homogeneity and anisotropy of mechanical properties within the billets depend significantly on the geometry of the shear zone. We demonstrate that deformation gradients can be reduced considerably by increasing the backpressure: The opening-angle of the fan-shaped shear zone is reduced from ψ ≈ 20 ° to ψ ≈ 7 ° when the backpressure is increased from 0 to 150 MPa. Backpressures of 150 MPa result in excellent homogeneity, with a relative variation of tensile mechanical properties of less than 7 %. Our investigation demonstrates that ECAP is suitable for processing homogenous, high performance materials on a large scale, paving the way for advanced industrial applications.

Effects of particle reinforcement and ECAP on the precipitation kinetics of an Al-Cu alloy

The precipitation kinetics of Al-Cu alloys have recently been revisited in various studies, considering either the effect of severe plastic deformation (e.g., by equal-channel angular pressing – ECAP), or the effect of particle reinforcements. However, it is not clear how these effects interact when ECAP is performed on particle-reinforced alloys. In this study, we analyze how a combination of particle reinforcement and ECAP affects precipitation kinetics. After solution annealing, an AA2017 alloy (initial state: base material without particle reinforcement); AA2017 + 10 vol.-% Al2O3; and AA2017 + 10 vol.-% SiC were deformed in one pass in a 120° ECAP tool at a temperature of 140°C. Systematic differential scanning calorimetry (DSC) measurements of each condition were carried out. TEM specimens were prepared out of samples from additional DSC measurements, where the samples were immediately quenched in liquid nitrogen after reaching carefully selected temperatures. TEM analysis was performed to characterize the morphology of the different types of precipitates, and to directly relate microstructural information to the endo- and exothermic peaks in our DSC data. Our results show that both ECAP and particle reinforcement are associated with a shift of exothermic precipitation peaks towards lower temperatures. This effect is even more pronounced when ECAP and particle reinforcement are combined. The DSC data agrees well with our TEM observations of nucleation and morphology of different precipitates, indicating that DSC measurements are an appropriate tool for the analysis of how severe plastic deformation and particle reinforcement affect precipitation kinetics in Al-Cu alloys.

Influence of ECAP temperature on the formability of a particle reinforced 2017 aluminum alloy

Severe plastic deformation methods are commonly used to increase the strength of materials by generating ultrafine-grained microstructures. The application of these methods to Al-Cu alloys is, however, difficult because of their poor formability at room temperature. An additional reduction of formability of such alloys occurs when ceramic particles are added as reinforcement: this often triggers shear localization and crack initiation during ECAP. This is the main reason why equal-channel angular pressing (ECAP) of aluminum matrix composites (AMCs) can generally only be performed at elevated temperatures and using ECAP dies with a channel angle larger than 90° (e.g. 120°). In this study we present a brief first report on an alternative approach for the improvement of the formability of an AMC (AA2017, 10 % SiC): ECAP at low temperatures. We show that, using a temperature of -60 °C and a channel angle of 90° (corresponding to an equivalent strain of 1.1), ECAP of the AMC can be successfully performed without material failure. The mechanical properties of the strongly deformed AMC are analyzed by tensile testing. Our results indicate that the increased formability of the AMC at low temperatures can be attributed to the suppression of unstable plastic flow that affects formability at room temperature.

Electrochemical Properties of AL-6060 Alloy After Industrial-Scale ECAP

The equal-channel angular pressing (ECAP), as the most famous method of severe plastic deformation, has the potential for upscaling from the laboratory to industrial scale. Thus, it is important to examine the practice-relevant properties of large billets deformed by this process. Mechanical properties and corrosion resistance significantly affect the service life of the structural components. The mechanical properties of Al-6060 alloy after industrial-scale ECAP (the cross-sectional sizes of the billets were 50 × 50 mm2) were analyzed by P. Frint. In the present work, we investigate the effect of one pass of the in-dustrial-scale ECAP on the electrochemical properties of Al-6060 alloy by means of potentiodynamic polarization tests in a 0.1 M NaCl solution. The corroded surfaces were analyzed by means of optical microscopy and scanning-electron microscopy. In order to characterize the homogeneity of the corrosion behavior of the ECAP-processed material, all analyses were taken in different zones perpendicular to the extrusion axis. The results indicate that one pass of ECAP does not deteriorate the electrochemical behavior of Al-6060 alloy.

Analysis of the complex stress state during early loading in cylindrical compression-shear specimens

In most engineering applications, materials are subjected to complex load cases rather than the simple uniaxial ones typically used for material characterization. To experimentally study the material behavior under a combination of compression and shear, an inclined compression specimen can be used. This specimen has been applied in various earlier experimental studies, typically to investigate shear localization under quasi-static or impact loading. In this contribution, we analyze the stress state in a compression-shear specimen in detail using an elastic-ideal plastic finite element simulation. The effects of specimen aspect ratio (height/diameter), inclination angle, and friction conditions between specimen and tool plates are investigated using the material parameters of different conventional steels as input. Shear stress distributions in characteristic shear directions on specific planes in the specimen that control the subsequent plastic deformation behavior are evaluated. Our results show that, even in the absence of friction, shear stresses are distributed heterogeneously in the inclined specimen, which differs from the stress distribution in a conventional compression specimen. Moreover, the highest shear and equivalent stresses always occur at the edges of the short diagonal plane of the specimen, independent of the investigated parameters. This study contributes to a more detailed understanding of the elasto-plastic mechanics in compression-shear specimens, and it specifically provides information for the analysis of the onset of early plastic deformation.

Strain hardening and microstrains during cyclic incremental forming of carbon steel and pure iron

The evolution of subsequent yield loci after large plastic strains has been studied for pure iron and steel with ferrite-pearlite structure. Large strains have been applied by torsion of thin walled tubes with different strain paths: monotonic and cyclic incremental torsion. Points on the yield loci have been probed by biaxial torsion-tension or torsion-compression load using a single-probe method. The results show strong influence of the microstructure towards anisotropic hardening. Microstrain values from XRD profile analysis are added and confirm the findings for hardening behaviour.