Hariharan K

In the present work, cold rolling and cryo-rolling were performed on 99% commercially pure copper substrates. Both cold and cryo-rolling processes caused severe plastic deformation that led to an increase in dislocation density by 14× and 28× respectively, as compared to the pristine material. Increases in average tensile strengths, by 75% (488 MPa) and 150% (698 MPa), were observed in the two rolled materials as the result of the enhancement in dislocation density. In addition to strength, enhanced antibacterial property of cryo-rolled copper was observed in comparison to cold rolled and pristine copper. Initial adhesion and subsequent proliferation of bio-film forming Gram-positive bacteria Staphylococcus aureus was reduced by 66% and 100% respectively for cryo-rolled copper. Approximately 55% protein leakage, as well as ethidium bromide (EtBr) uptake, were observed confirming rupture of cell membrane of S. aureus. Inductively coupled plasma-mass spectroscopy reveals higher leaching of elemental copper in nutrient broth media from the cryo-rolled copper. Detailed investigations showed that increased dislocation led to leaching of copper ions that caused damage to the bacterial cell wall and consequently killing of bacterial cells. Cryo-rolling enhanced both strength, as well as antibacterial activity, due to the presence of dislocations.

In this work, grain refinement and deformation mechanisms of ultrafine-grained AA 6061 alloy have been investigated using uniaxial tensile tests with stress relaxation. Initially, solutionized AA 6061 alloy samples with 3 mm thickness are subjected to constrained grove pressing (CGP) with an effective plastic strain of 1.16. The CGP samples are eventually rolled at room temperature (cold rolled) to produce ultrafine-grained thin sheets of 1 mm thickness. Due to the intense plastic strain applied during CGP and cold rolling (CR), it is practically challenging to quantify the grain size refinement by optical methods. Therefore, stress relaxation, which is an alternate transient mechanical test, is used to estimate the average degree of grain refinement for a larger length scale. In this method, the transient data from uniaxial tensile tests with stress relaxation is used to calculate the activation volume, which helps to understand the deformation mechanisms of UFG structured materials. In the present work, uniaxial tensile tests with controlled single and repeated stress relaxation are performed to determine apparent and actual activation volume in different materials conditions (solutionized, CGPed, and CGP + CR). Stress–strain curves obtained from the stress relaxation tests are compared with the monotonic tensile stress–strain curves. The results showed that the activation volume determined from single and repeated relaxation tests substantially decreased after the CGP and CGP + CR processes. The results also indicated that the grain boundary sliding is the possible deformation mechanism in CGP + CR samples and dislocation-dislocation interactions in CGP and SL samples.

Stress relaxation during plastic deformation has been reported to improve ductility of metallic materials. In this study, the stress relaxation behaviour in pure magnesium is investigated during interrupted uniaxial tensile tests. During intermittent stopping of the machine for relaxation studies, the total strain is expected to remain constant. However, an anomalous non-constancy in total strain is observed in the present work. The total strain increases with relaxation time. Additional in-situ tensile tests indicate that the non-constant total strain is restricted only in the gauge area of the specimen, indicating a likely shear dominated deformation such as grain boundary sliding (GBS) responsible for the anomalous behaviour. The role of GBS during relaxation is studied using the deformation induced evolution of surface inhomogeneity. Determinations of surface profiling step heights at grain boundaries and inclination of grains were used to quantify the effect of GBS. The estimated activation volume of 4.35 b3 further confirms the role of slip induced GBS on the deformation. A new stress relaxation model accommodating GBS is proposed and is found to fit the experimental data accurately.

The flow stress reduction during plastic deformation superposed with electric current, commonly referred as 'electroplasticity' has been actively researched over the past few decades. While the existence of an electron-dislocation interaction, independent of Joule heating is established, the exact rate controlling mechanism of the observed behaviour lacks consensus. Understanding the governing mechanism is complex due to the combined effect of Joule heating and electron-dislocation interaction. The present work attempts to establish the electroplastic mechanism in AA 6063 alloy and its nanocomposites. The role of microstructure on the electron interaction is investigated by preparing four distinct microstructure from the base alloy. All the samples were subjected to constant amplitude direct current during plastic deformation. The Joule heating effect is decoupled using the experimentally measured temperature history. The potential electroplastic mechanism for the alloy is elucidated by analysing the trend of flow stress reduction with strain and strain rate. It is inferred that micro Joule heating and electron wind effect cannot completely explain the observed electroplastic behaviour in AA 6063. The SiC particles in nano-composites suppressed the electroplastic effect. The observed mechanical behaviour under electric current is in agreement with the trend predicted assuming magnetic depinning mechanism. The reduction of dislocation density quantified using X-ray diffraction is found to concur with the inferred mechanism.

Hybrid additive manufacturing combining bulk components from traditional manufacturing route can overcome the size limitation in typical metal additive process. Investigating the joining process of bulk and additive components is crucial in hybrid processes. In the present work, a comprehensive study, including both experiments and simulations, has been carried out to describe the linear friction welding of electron beam melted Ti6Al4V. Three different welding configuration have been considered: conventional-conventional, EBMelted to EBMelted and the dissimilar conventional-EBMelted. Successive weld joints were achieved in all the three configurations. The mechanical behavior and the phase transformation in the weld region is highly influenced by the base material microstructure. The thermo mechanical behaviour during the friction welding is simulated using finite element method. The temperature history is predicted with very good accuracy in all the three configurations. The experimental data along with the simulation results are used to further explain and understand the phenomena occurring during the welding.

Experimental determination of strain-fatigue life curve is expensive considering the time and effort involved. Several empirical relations based on monotonic tensile properties and/or hardness is available for estimating strain-fatigue life properties. Existing criteria to evaluate the accuracy of these empirical relations are limited by the data points in the strain-fatigue life curve. A new criterion based on the prediction errors of fatigue constants is proposed to overcome the limitation. Around eight prediction methods are evaluated for two hundred twenty-six steel grades from literature using the existing and the proposed criteria. It is observed that the conclusions from the new criterion are more reliable than the existing criteria. © 2010 Elsevier Ltd. All rights reserved.

The dislocation density-based model after Kocks–Mecking–Estrin (KME) is widely used to characterize the thermally activated plastic deformation and dislocation kinetics. According to the model, the slope of the stress–strain curve decreases linearly with stress, which contradicts the experimental observation. In the current study, the evolution of dislocation density in the model is generalized to account for the nonlinearity of the slope.

The formability limit in a typical incremental sheet forming (ISF) far exceeds the conventional estimate assuming necking failure. The material subjected to incremental forming fails by fracture. Although damage models have been used to correlate the failure in incremental forming, a detailed understanding of the failure mechanism spanning strain paths is not clear yet. In this work, three parts with varied shapes (hybrid five lobe, pyramid and variable wall angle conical frustum (VWACF)) were developed using ISF to cover a range of possible strain paths. The failure is predicted using established uncoupled phenomenological damage models. Damage models were calibrated for AA1050 aluminium sheet by the method of inverse approach using carefully designed tensile tests and FE simulations. Three different damage models were implemented as user subroutine in commercial software code, ABAQUS/Explicit and the results predicted were compared. The linear damage accumulation used to develop fracture locus under monotonic loading could not predict the failure limit in a benchmark single groove test. Therefore a non linear damage accumulation rule (NLDA) is implemented to simulate ISF. The parameters of the NLDA were calibrated from the single groove test. The fracture forming limits during ISF was established using circular grid analysis near the failure zone of the formed part. A good agreement can be found between the experimental observations and numerical predictions for fracture location and part height. It was observed that the overall predictive capability of Hosford Coulomb (HC) damage model is better among three damage models investigated for the given range of loading conditions. However all the three models under-predicted the experimental fracture strain measured in ISF. The possible explanation for the discrepancy is explored.

The fatigue behaviour of cold rolled and annealed sheet metals are influenced by the anisotropy of mechanical properties due to crystallographic texture. However, the existing fatigue strain-life models are primarily meant for isotropic material behaviour. In the present work, the Coffin-Manson equation for strain-life is modified to include the effect of anisotropy using phenomenological plasticity models. It is observed that the variation of strain hardening exponent is critical to model the strain-life behaviour. Variation of strain hardening exponent with orientation is modelled using existing anisotropic yield criteria. The prediction of fatigue life using the proposed model correlates well with the experimental results of Al6061-T6 along different orientations. The proposed model can be used to predict the fatigue properties along any orientation from the fatigue data along one orientation and monotonic mechanical properties along longitudinal, transverse and diagonal directions. © 2012 Blackwell Publishing Ltd.

Transient stress relaxation test can be used to estimate activation volume for plastic deformation. The analysis is complicated due to the non-constancy of mobile dislocation density during relaxation. The reloading yielding post stress relaxation has a direct correlation with the change in mobile dislocation density during relaxation. However, this feature is ignored in the past studies. In the present work, the reloading yielding phenomenon in dual phase (α+β) titanium alloy (Ti–6Al–4V) is investigated using phenomenological stress relaxation model. Based on indirect experimental evidences, it is shown that the reloading yielding is due to regeneration of new dislocations and not due to unpinning of sessile dislocations, as assumed previously. The improvement of ductility due to stress relaxation follows a similar trend reported in literature. The ductility improvement is analyzed using both microstructure changes and thermal gradient. Fractographs of failed samples were investigated to understand the fracture characteristics and quantitative analyses were performed using image processing to correlate the microstructure mechanisms responsible for enhanced ductility. Thermal history at select locations were recorded using thermocouple and were used to analyze the contribution of thermal gradient in ductility. The necessity of a comprehensive approach to model ductility improvement in non-monotonic stress relaxation test is explored.