T. Halle

Mechanical properties and microstructural changes of ultrafine-grained AA6063T6 during high-cycle fatigue

Abstract Fatigue behaviour and mechanical properties of peak-aged AA6063T6 with ultra-fine grain size, produced by equal channel angular extrusion, were evaluated with special emphasis on the microstructure before and after cyclic loading. The strength significantly increased with grain size reduction and is described by an exponential power-law constitutive relationship. A remarkable enhancement of fatigue life compared to commercial AA6063T6 with coarse grains was found in the high-cycle regime after the first two extrusions. Further extrusions eliminated this improvement. It is shown that the optimum fatigue performance correlates very well with the minimum tensile ductility. Electron backscatter diffraction revealed that the material behaviour can basically be attributed to the grain boundary characteristics. Low grain boundary misorientation angles yield the best fatigue performance in the ultrafine-grained microstructure.

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.

Manufacture of a β-titanium hollow shaft by incremental forming

Excellent mechanical properties and corrosion resistance combined with low weight qualify β-titanium materials for lightweight applications in aviation, automotive and energy engineering. Thus far, actual applications of these materials have been limited due to high material costs and limited processing knowledge. One approach for developing resource-efficient manufacturing methods is the application of incremental forming methods. This article focuses on the development of the incremental spin extrusion process, which creates hollow profiles from solid bars. This method allows hollow shape manufacturing with a much higher flexibility than other forming methods and a significantly improved material utilization in comparison to machining methods, such as deep hole drilling. Beta-titanium alloys basically have very good cold forming suitability and the resulting material properties can be controlled. The application of incremental forming methods with high hydrostatic compressive stress is a promising manufacturing approach. The β-titanium Ti-10V-2Fe-3Al material has an excellent combination of the properties strength, ductility and fatigue strength. In order to utilize these properties the forming conditions and the temperature control need to be optimized. The investigations show that the Ti-10V-2Fe-3Al material can be formed only in a narrow semi-hot forming temperature window. The paper describes the investigation and presents results on the design of partial forming process sequences, forming properties, microstructure formation and failure prevention. The process design objective is a very fine microstructure with a homogeneous secondary α-phase and very small grained β-phase.

Near-Threshold Fatigue Crack Propagation in an ECAP-Processed Ultrafine-Grained Aluminium Alloy

In the present work, the near-threshold fatigue crack propagation (FCP) at different load ratios is studied for an aluminium alloy processed by equal-channel angular pressing (ECAP). The conditions under investigation represent different stages of microstructural refinement as well as a ductility-optimized condition with superior crack growth properties, obtained by a combination of ECAP and aging. The results show a strong dependency of the threshold and its load ratio sensitivity on the grain size and grain size distribution. These observations can be rationalized on the basis of crack path tortuosity and the contribution of (roughness-induced) crack closure. Moreover, the experimental data is evaluated using the two-parametric concept of Vasudevan and Sadananda, which employs two necessary minimum conditions for crack growth, namely a critical cyclic K*th, and a critical maximum stress intensity K*max. The application of this concept shows a strong interaction of both parameters for all ECAP-processed conditions, where the ductility-optimized condition reveals superior FCP properties compared to the “as-processed” conditions.

Experimental and Numerical Determination of Cutting Forces and Temperatures in Gear Hobbing

Hobbing is one of the most productive methods for manufacturing external gears. According to the requirements of green manufacturing, the lubrication in gear hobbing has to be reduced with the final aim of dry machining. Influences due to thermal aspects during machining have to be considered, especially in hobbing of large modules or ring gears, because in this case, hobbing could be the last step in the process chain. Within the priority program (SPP) “Modeling, Simulation and Compensation of Thermal Effects for Complex Machining Processes”, founded by the DFG, special emphasis is laid on the thermally caused geometrical deviation in dry cutting. To predict the heat flux, which leads to thermal expansions and geometrical deviations of the gear, a validated model for forces and temperatures is necessary. The validation of single generation positions and chips is focused in this paper.

The Mechanical Behaviour of Ultra Fine Grained Titanium Alloys at High Strain Rates

Within this study the mechanical behaviour of ultra-fine grained Ti-6-22-22S titanium alloy was investigated and compared to coarse grained material. By severe plastic deformation using the cyclic channel die compression process, grain sizes between 300 and 500 nm were obtained. The mechanical behaviour was studied over a wide range of strain rates from 10 10 s-1 under compressive loading using different experimental techniques. A significant increase of flow stress with decreasing grain size compared to the coarse grain state was found. An evaluation of the strain hardening behaviour of the UFG material shows a significant increase of the strain hardening coefficient at high strain rates for low plastic deformation. The strain rate sensitivity of the material is found to be constant within a range of strain rates from 10 to 10 s but increases at higher plastic strains. However, compressive deformability is nearly constant up to 10 s and decreased disproportionately at higher rates of strain. With decreasing grain size a significant decrease of compressive deformability was found. The strength at failure is increased with increasing strain rate.

Microstructure and mechanical properties affecting crack growth behaviour in AA6060 produced by equal-channel angular extrusion

Crack growth in AA6060 after two and eight equal-channel angular extrusions (ECAE), showing a bimodal microstructure and a homogenous ultrafine-grained microstructure, respectively, are compared to the coarse grained counterpart. Furthermore, an optimized condition, obtained by combining one ECA-extrusion and a subsequent short aging treatment is included. Fatigue crack growth behaviour in the near-threshold regime and the region of stable crack growth is investigated and related to microstructural features such as grain size, grain size distribution, grain boundary characteristics and ductility. Micrographs of crack propagation surfaces reveal information on crack propagation features such as crack path deflection and give an insight to the underlying microstructure. Instrumented Charpy impact tests are performed to investigate crack initiation and propagation under impact conditions. Due to the recovery of ductility during the post-ECAE heat treatment, the optimized condition shows improved fatigue crack properties and higher energy consumption in Charpy impact tests, when compared to the as-processed conditions without heat treatment.

Modelling of the Mechanical Behaviour of Ultra-Fine Grained Titanium Alloys at High Strain Rates

Results of numerical simulations of the mechanical behaviour of coarse grained and UFG titanium alloys under quasi-static uniaxial compression and plane shock wave loading are presented in this paper. Constitutive equations predict the strain hardening behaviour, the strain rate sensitivity of the flow stress and the temperature softening of titanium alloys with a range of grain sizes from 20 μm to 100 nm. Characteristics of the mechanical behaviour of UFG α and α+β titanium alloys in wide range of strain rates are discussed.