Quantification of the influence of increased pre-stretching on microstructure-strength relationships in the Al–Cu–Li alloy AA2195

AbstractThe effect of increasing pre-stretching to higher levels, than are currently used in industrial practice, has been investigated on the strength, microstructure, and precipitation kinetics seen during artificial ageing an Al–Cu–Li alloy AA2195 - focussing on the behaviour of the main strengthening phase, T1. Increasing the pre-strain level, to the maximum obtainable before plastic instability (15%), resulted in an increase in the T8 yield strength to ∼ 670 MPa with a corresponding reduction in ductility from ∼11 to 7.5%. Microstructure data have been used to deconvolute and model the effects of increasing pre-strain on the main strengthening components that contribute to this large strength increase. The precipitation strengthening model proposed by Dorin et al. [1] has been successfully employed to calculate the strengthening contribution of the T1 phase and the increase in strength due to strain hardening has been modelled using X-ray line broadening measurements of dislocation density, using the modified Williamson–Hall approach. Refinement of the T1 phase was observed to continue to higher pre-strains than previously thought, but it is predicted that this leads to a reduction in the strengthening contribution from precipitation. In contrast a low level of recovery was observed during stretching, and artificial ageing, resulting in an increasing contribution form strain hardening with pre-strain. Thus, it is shown that increasing the pre-strain prior to ageing results in a reduction in the strengthening provided by the T1 phase, in favour of an increase in the strain hardening contribution

The Influence of Porosity on Fatigue Crack Initiation in Additively Manufactured Titanium Components

Without post-manufacture HIPing the fatigue life of electron beam melting (EBM) additively manufactured parts is currently dominated by the presence of porosity, exhibiting large amounts of scatter. Here we have shown that the size and location of these defects is crucial in determining the fatigue life of EBM Ti-6Al-4V samples. X-ray computed tomography has been used to characterise all the pores in fatigue samples prior to testing and to follow the initiation and growth of fatigue cracks. This shows that the initiation stage comprises a large fraction of life (>70 %). In these samples the initiating defect was often some way from being the largest (merely within the top 35 % of large defects). Using various ranking strategies including a range of parameters, we found that when the proximity to the surface and the pore aspect ratio were included the actual initiating defect was within the top 3 % of defects ranked most harmful. This lays the basis for considering how the deposition parameters can be optimised to ensure that the distribution of pores is tailored to the distribution of applied stresses in additively manufactured parts to maximise the fatigue life for a given loading cycle

Ultrafine grained plates of Al-Mg-Si alloy obtained by Incremental Equal Channel Angular Pressing : microstructure and mechanical properties

In this study, an Al-Mg-Si alloy was processed using via Incremental Equal Channel Angular Pressing (I-ECAP) in order to obtain homogenous, ultrafine grained plates with low anisotropy of the mechanical properties. This was the first attempt to process an Al-Mg-Si alloy using this technique. Samples in the form of 3 mm-thick square plates were subjected to I-ECAP with the 90˚ rotation around the axis normal to the surface of the plate between passes. Samples were investigated first in their initial state, then after a single pass of I-ECAP and finally after four such passes. Analyses of the microstructure and mechanical properties demonstrated that the I-ECAP method can be successfully applied in Al-Mg-Si alloys. The average grain size decreased from 15 - 19 µm in the initial state to below 1 µm after four I-ECAP passes. The fraction of high angle grain boundaries in the sample subjected to four I-ECAP passes lay within 53-57 % depending on the examined plane. The mechanism of grain refinement in Al-Mg-Si alloy was found to be distinctly different from that in pure aluminium with the grain rotation being more prominent than the grain subdivision, which was attributed to lower stacking fault energy and the reduced mobility of dislocations in the alloy. The ultimate tensile strength increased more than twice, whereas the yield strength - more than threefold. Additionally, the plates processed by I-ECAP exhibited low anisotropy of mechanical properties (in plane and across the thickness) in comparison to other SPD processing methods, which makes them attractive for further processing and applications

Evolution of microstructure and crystallographic texture during dissimilar friction stir welding of duplex stainless steel to low carbon-manganese structural steel

Electron backscattered diffraction (EBSD) was used to analyze the evolution of microstructure and crystallographic texture during friction stir welding of dissimilar type 2205 duplex stainless steel (DSS) to type S275 low carbon-manganese structural steel. The results of microstructural analyses show that the temperature in the center of stirred zone reached temperatures between Ac 1 and Ac 3 during welding, resulting in a minor ferrite-to-austenite phase transformation in the S275 steel, and no changes in the fractions of ferrite and austenite in the DSS. Temperatures in the thermomechanically affected and shoulder-affected zones of both materials, in particular toward the root of the weld, did not exceed the Ac 1 of S275 steel. The shear generated by the friction between the material and the rotating probe occurred in austenitic/ferritic phase field of the S275 and DSS. In the former, the transformed austenite regions of the microstructure were transformed to acicular ferrite, on cooling, while the dual-phase austenitic/ferritic structure of the latter was retained. Studying the development of crystallographic textures with regard to shear flow lines generated by the probe tool showed the dominance of simple shear components across the whole weld in both materials. The ferrite texture in S275 steel was dominated by D 1, D 2, E, E¯ , and F, where the fraction of acicular ferrite formed on cooling showed a negligible deviation from the texture for the ideal shear texture components of bcc metals. The ferrite texture in DSS was dominated by D 1, D 2, I, I¯ , and F, and that of austenite was dominated by the A, A¯ , B, and B¯ of the ideal shear texture components for bcc and fcc metals, respectively. While D 1, D 2, and F components of the ideal shear texture are common between the ferrite in S275 steel and that of dual-phase DSS, the preferential partitioning of strain into the ferrite phase of DSS led to the development of I and I¯ components in DSS, as opposed to E and E¯ in the S275 steel. The formations of fine and ultrafine equiaxed grains were observed in different regions of both materials that are believed to be due to strain-induced continuous dynamic recrystallization (CDRX) in ferrite of both DSS and S275 steel, and discontinuous dynamic recrystallization (DDRX) in austenite phase of DSS

The development of matrix and interface microstructures and their effect on the mechanical behaviour of SiC particulate reinforced Al matrix composites

SIGLEAvailable from British Library Document Supply Centre- DSC:D062168 / BLDSC - British Library Document Supply CentreGBUnited Kingdo