Showing papers on "Necking published in 2014"

Gradient nanostructure and residual stresses induced by Ultrasonic Nano-crystal Surface Modification in 304 austenitic stainless steel for high strength and high ductility

Abstract: In this study, the effects of Ultrasonic Nano-crystal Surface Modification (UNSM) on residual stresses, microstructure changes and mechanical properties of austenitic stainless steel 304 were investigated. The dynamic impacts induced by UNSM leads to surface nanocrystallization, martensite formation, and the generation of high magnitude of surface compressive residual stresses (−1400 MPa) and hardening. Highly dense deformation twins were generated in material subsurface to a depth of 100 µm. These deformation twins significantly improve material work-hardening capacity by acting both as dislocation blockers and dislocation emission sources. Furthermore, the gradually changing martensite volume fraction ensures strong interfacial strength between the ductile interior and the two nanocrystalline surface layers and thus prevents early necking. The microstructure with two strong surface layers and a compliant interior embedded with dense nanoscale deformation twins and dislocations leads to both high strength and high ductility. The work-hardened surface layers (3.5 times the original hardness) and high magnitude of compressive residual stresses lead to significant improvement in fatigue performance; the fatigue endurance limit was increased by 100 MPa. The results have demonstrated that UNSM is a powerful surface engineering technique that can improve component mechanical properties and performance.

Effect of the martensite distribution on the strain hardening and ductile fracture behaviors in dual-phase steel

Abstract: In order to clarify the effects of the martensite distribution on the mechanical properties of low-carbon dual-phase steel, four types of dual-phase steel with different ferrite grain sizes and martensite distributions were prepared using a thermomechanical treatment. The tensile properties of these steels were investigated; in particular, the strain hardening and the ductile fracture behaviors were discussed in terms of the strain partitioning between the ferrite and martensite and the formation and growth of micro-voids, respectively. When the martensite grains surround the ferrite grains and form a chain-like networked structure, the strain hardenability is greatly improved without a significant loss of elongation, while the necking deformability is considerably reduced. A digital-image correlation analysis revealed that the tensile strain in the martensite region in the chain-like networked dual-phase structure is markedly increased during tensile deformation, which leads to an improvement in the strain hardenability. On the other hand, the joint part of the martensite grains in the structure acts as a preferential formation site for micro-voids. The number density of the micro-voids rapidly increases with increasing tensile strain, which would cause the lower necking deformability.

Mechanism investigation of friction-related effects in single point incremental forming using a developed oblique roller-ball tool

Abstract: Single point incremental forming (SPIF) is a highly versatile and flexible process for rapid manufacturing of complex sheet metal parts. In the SPIF process, a ball nose tool moves along a predefined tool path to form the sheet to desired shapes. Due to its unique ability in local deformation of sheet metal, the friction condition between the tool and sheet plays a significant role in material deformation. The effects of friction on surface finish, forming load, material deformation and formability are studied using a newly developed oblique roller ball (ORB) tool. Four grades of aluminum sheet including AA1100, AA2024, AA5052 and AA6111 are employed in the experiments. The material deformation under both the ORB tool and conventional rigid tool are studied by drilling a small hole in the sheet. The experimental results suggest that by reducing the friction resistance using the ORB tool, better surface quality, reduced forming load, smaller through-the-thickness-shear and higher formability can be achieved. To obtain a better understanding of the frictional effect, an analytical model is developed based on the analysis of the stress state in the SPIF deformation zone. Using the developed model, an explicit relationship between the stress state and forming parameters is established. The experimental observations are in good agreement with the developed model. The model can also be used to explain two contrary effects of friction and corresponding through-the-thickness-shear: increase of friction would potentially enhance the forming stability and suppress the necking; however, increase of friction would also increase the stress triaxiality and decrease the formability. The final role of the friction effect depends on the significance of each effect in SPIF process.

Effects of initial δ phase on hot tensile deformation behaviors and fracture characteristics of a typical Ni-based superalloy

Abstract: Uniaxial tensile tests of a typical Ni-based superalloy are conducted under the deformation temperature range of 920–1010 °C and strain rate range of 0.01–0.001 s −1 . The effects of initial δ phase (Ni 3 Nb) on the hot tensile deformation behaviors and fracture characteristics are discussed in detail. The results show that: (1) For the studied Ni-based superalloy with a large amount of δ phase, the flow stress curves are composed of three distinct stages, i.e., work hardening stage, flow softening stage and the final fracture stage. (2) The initial δ phase has significant effects on the deformation behaviors of the studied superalloy. δ phase can cause the obvious work hardening at the beginning of hot deformation, and then accelerates the flow softening by promoting the dynamic recrystallization with further straining. With the increase of initial δ phase, the strain rate sensitivity coefficient decreases firstly and then increases. (3) The combined effects of localized necking and microvoid coalescence cause the final fracture of specimens. The increase of initial δ phase increases the density of nucleus for the formation of microvoids, and promotes the nucleation and coalescence of microvoids.

Mechanical properties of bulk single crystalline nanoporous gold investigated by millimetre-scale tension and compression testing

Abstract: In this work, the mechanical behaviour of millimetre-scale, bulk single crystalline, nanoporous gold at room temperature is reported for the first time. Tension and compression tests were performed with a custom-designed test system that accommodates small-scale samples. The absence of grain boundaries in the specimens allowed measurement of the inherent strength of millimetre-scale nanoporous gold in tension. The elastic modulus and strength values in tension and compression were found to be significantly lower than values measured with nanoindentation-based techniques and previously reported in the literature, but close to those reported for millimetre-scale polycrystalline samples tested using traditional compression techniques. Fracture toughness was found to be very low, in agreement with the macroscopic brittleness of nanoporous gold, but this is due to the localization of deformation to a narrow zone of ligaments, which individually exhibit significant plasticity and necking.

Identification of Post-Necking Hardening Phenomena in Ductile Sheet Metal

Abstract: A combined theoretical/experimental approach accurately quantifying post-necking hardening phenomena in ductile sheet materials that initially exhibit diffuse necking in tension is presented. The method is based on the minimization of the discrepancy between the internal and the external work in the necking zone during a quasi-static tensile test. The main focus of this paper is on the experimental validation of the method using an independent material test. For this purpose, the uniaxial tube expansion test is used to obtain uniaxial strain hardening behavior beyond the point of maximum uniform strain in a tensile test. The proposed method is used to identify the post-necking hardening behavior of a cold rolled interstitial-free steel sheet. It is demonstrated that commonly adopted phenomenological hardening laws cannot accurately describe all hardening stages. An alternative phenomenological hardening model is presented which enables to disentangle pre- and post-necking hardening behavior. Additionally, the influence of the yield surface on the identified post-necking hardening behavior is scrutinized. The results of the proposed method are compared with the hydraulic bulge test. Unlike the hydraulic bulge test, the proposed method predicts a decreased hardening rate in the post-necking regime which might be associated with probing stage IV hardening. While inconclusive, the discrepancy with the hydraulic bulge test suggests differential work hardening at large plastic strains.

Determination of the Effect of Stress State on the Onset of Ductile Fracture Through Tension-Torsion Experiments

Abstract: A tubular tension-torsion specimen is proposed to characterize the onset of ductile fracture in bulk materials at low stress triaxialities. The specimen features a stocky gage section of reduced thickness. The specimen geometry is optimized such that the stress and strain fields within the gage section are approximately uniform prior to necking. The stress state is plane stress while the circumferential strain is approximately zero. By applying different combinations of tension and torsion, the material response can be determined for stress triaxialities ranging from zero (pure shear) to about 0.58 (transverse plane strain tension), and Lode angle parameters ranging from 0 to 1. The relative displacement and rotation of the specimen shoulders as well as the surface strain fields within the gage section are determined through stereo digital image correlation. Multi-axial fracture experiments are performed on a 36NiCrMo16 high strength steel. A finite element model is built to determine the evolution of the local stress and strain fields all the way to fracture. Furthermore, the newly-proposed Hosford-Coulomb fracture initiation model is used to describe the effect of stress state on the onset of fracture.

Microstructure evolution and mechanical properties of Cu/Zn multilayer processed by accumulative roll bonding (ARB)

Abstract: Cu/Zn multilayer was processed by accumulative roll bonding (ARB) up to eight cycles. The evolution of microstructure and its correlation with mechanical properties was investigated. Necking and rupture of constituents were observed at intermediate cycles. The intermetallic CuZn5, microcracks and Kirkendall porosities were formed during ARB. After eight ARB cycles, a laminated composite with wavy interfaces and lenticular fragments of Cu was produced. The maximum tensile strength was achieved after four cycles, which was about 1.4 times higher than that of pure Cu. The hardness of individual layers increased continually with increasing the ARB cycle. Tensile fracture surfaces reveal dimples in both Cu and Zn at early ARB cycles.

Effect of heat treatment on tensile deformation characteristics and properties of Al3003/STS439 clad composite

Abstract: The tensile deformation and fracture behaviors of roll-bonded Al3003/STS439 clad composite were studied. The mutual constraint and interaction of joined Al and STS with different deformation characteristics tend to shape the deformed specimen geometry to appear similar and induce the simultaneous fracture of Al and STS. The examination of the fractured clad revealed more uniform deformation of less ductile metal layer was induced by co-deforming more ductile metal layer in the clad composite, enhancing the overall tensile ductility. The suppression of neck formation in STS layer of Al3003/STS439 clad composite was observed to be induced by the resistance to neck formation of joined Al layer, rendering the enhanced ductility of joined STS layer over the separated STS layer. Excellent interface bonding and absence of brittle intermetallics at the interface are prerequisites for the enhanced ductility of clad composites because constraint by the adjacent joined metal layer can be made by the stress and strain transfer through the interface. The strength of the roll-bonded Al3003/STS439 clad composite is in close agreement with that calculated from the rule of mixture.

Effects of strain rate on dynamic mechanical behavior and microstructure evolution of 5A02-O aluminum alloy

Abstract: This paper studies the effects of strain rate on dynamic mechanical behavior and microstructure evolution of 5A02-O aluminum alloy at room temperature. Based on the results of the dynamic tensile tests and compressive tests at strain rates of 1000–5000 s −1 by split Hopkinson bar as well as the results of quasi-static tests at strain rate of 0.001 s −1 , it is shown that with increasing strain rate, the flow stress and tensile strength significantly increase and notable strain hardening and thermal softening behaviors are observed for 5A02-O with elongation of 63.00% and softening ratio of 73.23% at the strain rate of 4000 s −1 . The strain rate sensitivity for 5A02-O is enhanced in the range of 1000–3000 s −1 . Scanning electron microscopy (SEM) observations illustrate that the fracture surfaces are characterized by larger and deeper dimple-like structure with more precipitates at higher strain rates, which indicates the ductile failure mode. The enhancement of ductility is interpreted via the inertia effect which may contribute to diffuse necking, slow down the necking development and delay the onset of fracture. Furthermore, transmission electron microscopy (TEM) observations show that higher strain rate leads to higher dislocation density, smaller cell size with thinner cell wall and the appearance of dislocation wall with parallel dislocation lines. Dislocation cells are incomplete under dynamic deformation. In addition, the micro-hardness of 5A02-O increases with increasing strain rate.

Investigation and comparative analysis of plastic instability criteria: Application to forming limit diagrams

Abstract: The prediction of forming limit diagrams (FLDs) is of significant interest to the sheet metal forming industry. Although a large variety of plastic instability criteria have been developed during the previous decades, there is still a lack of comparison of their respective theoretical bases. The aim of this paper is to present the theoretical formulations of a representative selection of diffuse necking and strain localization criteria based on the maximum force principle, the Marciniak–Kuczynski method and the bifurcation approach. The theoretical foundations and underlying assumptions for each criterion are specified prior to their application to several materials to determine the associated FLDs. The capability of the criteria to predict the formability of thin metal sheets is discussed and a classification of some of the criteria is attempted according to their order of occurrence in terms of the localization prediction.

Microstructural deformation pattern and mechanical behavior analyses of DP600 dual phase steel

Abstract: Deformation pattern is the most important issue which can be effective on the prediction of mechanical behavior of dual phase steel in microscale. In this paper, simulation with large and small deformation theories using the real microstructure is employed to investigate the deformation pattern in microstructure of DP600 dual phase steel. Deformation pattern is mostly effective on the transition of rotation and displacement between ferrite and martensite phases and stress–strain behavior. Different deformation theories affect the predicted stress–strain behavior of dual phase steel using the finite element method. The obtained results from both numerical and experimental methods are used to investigate the effect of deformation in the microstructure. The performed experiments were interrupted in three stages to obtain the deformation pattern of components microstructure: necking initiation, localization and failure. Metallographic and SEM images were prepared to study the ferrite matrix and martensite grains deformation, and failure pattern in the microscale. Localization which causes severe deformation, rotation and displacement of the microstructure is defined as the failure criteria and the main source of void formation and rupture phenomenon. The predicted results such as stress–strain behavior and deformation pattern using small and large deformation theories are compared with the experimental findings and observable voids in the SEM images.

Varying tensile fracture mechanisms of Cu and Cu-Zn alloys with reduced grain size: From necking to shearing instability

Abstract: In ultrafine-grained (UFG) or nanocrystalline (NC) materials, achieving high strength often induces loss of ductility due to the formation of shear fracture without obvious necking feature. To investigate mechanical properties and fracture mechanism with reduced grain size, the macroscopic tensile fracture behaviors of UFG or NC Cu and Cu–Zn alloys were systematically investigated. It is found that the tensile strength and uniform elongation of UFG or NC Cu and Cu–Zn alloys display simultaneously increasing trend. The limitation of ductility can be attributed to the occurrence of shear bands in these materials; their main characteristic, the shear fracture angle, decreases with decreasing grain size as well as the degree of necking. The macroscopic fracture mechanisms are explained using an Ellipse criterion in terms of modification of the stress state in the fracture zone. It is suggested that the reduction of necking degree results from the geometrical hardening effect due to the change of the stress state. As the degree of necking decreases with decreasing grain size, the tensile shear fracture angle decreases too in the UFG/NC materials. Thus improving the geometrical hardening ability may effectively inhibit the formation of shear fracture, as the nucleation of shear bands becomes difficult.

Modification of the grain structure, γ phase morphology and texture in AZ81 Mg alloy through accumulative back extrusion

Abstract: The capability of accumulative back extrusion (ABE), as a recently developed severe plastic deformation technique, has been considered to modify the microstructural characteristics of a Mg–Al–Zn alloy composing higher Al content. The results indicate that applying the ABE process up to five passes led to simultaneous modification of the γ phase morphology, grain structure and deformation texture of the experimental alloy. The morphology of the eutectic γ phase has been sequentially altered from initial coarse network to elongated and finally spherical morphologies with an average globularity and diameter of 0.85 and 7.5 μm, respectively. This has been justified considering the mechanical fragmentation and thermal disintegration of the eutectic particles through necking phenomena. In addition the initial coarse grain structure of the cast alloy (~330 μm) has also been significantly refined ( d ~1 μm). This substantial grain refinement is attributed to the dynamic recrystallization enhancement in connection with pinning effect of crushed γ particles during successive passes. The typical basal texture has been modified to a weak random texture having grains with their normal distribution in a desirably wide range of deviation angles from normal direction. The occurrence of particle stimulated nucleation, particle pinning, and shear banding phenomena are suggested as the main reasons causing the weak scattered deformation texture.