Pre-strain effect on twist springback of a 3D P-channel in deep drawing

Flake powder metallurgy (FPM) combined with micro/macro-rolling (MMR) was employed to fabricate Al2O3/Al laminated composites with high dislocations storage capability. The effect of deformation mode on the laminated ultrafine grain structure and on the dislocations density was analyzed. Additionally, a model based on microstructure and processing parameters is proposed to describe the effect of size, aspect ratio and crystallographic textures of the matrix grains on dislocation density. It was shown that the dislocation density depends on Al2O3 dispersion degree and Al2O3-Al interfacial bonding level. They are further driven by mechanical micro-rolling process, as well as grain refinement, crystallography texture, and high aspect ratio resulted from macro-rolling process. It was demonstrated that the MMR process provides the laminated ultra-fined grains (UFGs) with higher dislocation storage capability with respect to equiaxed grains. The described process provides a good coordination between the Al2O3 dispersion, Al2O3-Al interfacial bonding as well as crystallographic textures, size and shape of the matrix grains.

Microstructure dependent dislocation density evolution in micro-macro rolled Al2O3/Al laminated composite

Flake powder metallurgy (FPM) combined with micro/macro-rolling (MMR) was employed to fabricate Al2O3/Al laminated composites with high dislocations storage capability. The effect of deformation mode on the laminated ultrafine grain structure and on the dislocations density was analyzed. Additionally, a model based on microstructure and processing parameters is proposed to describe the effect of size, aspect ratio and crystallographic textures of the matrix grains on dislocation density. It was shown that the dislocation density depends on Al2O3 dispersion degree and Al2O3-Al interfacial bonding level. They are further driven by mechanical micro-rolling process, as well as grain refinement, crystallography texture, and high aspect ratio resulted from macro-rolling process. It was demonstrated that the MMR process provides the laminated ultra-fined grains (UFGs) with higher dislocation storage capability with respect to equiaxed grains. The described process provides a good coordination between the Al2O3 dispersion, Al2O3-Al interfacial bonding as well as crystallographic textures, size and shape of the matrix grains. 

A 3D representative volume element (RVE) model was developed to analyze the effective thermal expansion coefficient (CTE) of carbon nanotube reinforced aluminum (CNT/Al) composites. The influences of geometric configuration, orientation and volume fraction of CNTs on CTE of the CNT/Al composites were investigated. The results show that the orientation of CNTs significantly influences the stress and strain distributions around CNTs, and when CNTs are parallel to the load direction, the constraint effect of CNTs on the matrix is the largest. Furthermore, the configuration of CNTs also plays an important role in reducing thermal expansion behavior of CNT/Al composites, and the bundled configuration exhibit the lowest CTE. In particular, with the increase of the number of CNTs in each bundle, the CTE decreases and then tends to be unchanged. When the volume fraction of CNTs is 5%, and the number of CNTs in each bundle is 25, the CTE of the composite reaches 5.12 × 10−6/°C, which is 76% lower than the CTE of the matrix. A linear decrease of the CTE of CNT/Al composites is observed as the CNT volume fraction increases. Overall, this work establishes a comprehensive understanding of factors affecting the CTE, and the obtained findings could provide a good theoretical basis for designing low CTE CNT/Al composites. 

Investigation into the influence of CNTs configuration on the thermal expansion coefficient of CNT/Al composites

Laminate carbon nanotube (CNT)/Al–Cu–Mg composites consisting of alternate ductile layers (coarse or ultrafine grain Al) free of CNTs and brittle layers (ultrafine grain) rich in CNTs, were prepared in the powder metallurgy route. It was found that the strength-ductility of the composites was improved remarkably, as compared with those of uniform composites. Mechanical incompatibility between different layers during tensile deformation produced a large number of geometrically necessary dislocations (GNDs) between the ductile layers and the brittle layers, which inhibited the strain localization, thereby enhancing the strength-ductility. Compared with the laminate composite using the coarse grain Al as the ductile layers, the strength of the laminate composite using ultrafine grain Al as ductile layers further increased by 14%, while the elongation remained unchanged. This was because the ultrafine grain rather than coarse grain of the ductile layers could lead to higher strength and had better coordination with the brittle layers. 

Enhanced strength–ductility synergy of carbon nanotube/Al–Cu–Mg composites via introducing laminate structure and grain modification

The relationship between different ball milling (BM) conditions and mechanical properties of CNT/Al composites was studied through tailoring grain interior/grain boundary affected zone (GI/GBAZ). The ultrafine-grained (UFGed) architectures of the CNT/Al composites made from flake powders with the same thickness under different BM conditions, introduce different strain rates and vary dislocation density. The results showed that the grain size of CNT/Al composite were significantly influenced by milling conditions, which caused obvious variations in mechanical properties. The enhancing in tensile properties of CNT/Al composite mostly caused by grain refinement, dislocation strengthening, and back stress hardening, which last two terms are closely linked to GBAZ thickness. The simulated results agreed well with experimental results, confirming a strain rate dependency of GBAZ thickness. This work strengthened the understanding of ball milling process on the microstructure and mechanical properties of metal matrix composites.

Enhanced mechanical properties of CNT/Al composite through tailoring grain interior/grain boundary affected zones

The strain rate dislocation behavior in carbon nanotubes (CNTs) reinforcing aluminum is described through an explicit, analytic grain interior/grain boundary affected zone (GI/GBAZ) composite model. The model describes how the mechanical behavior of metal matrix composites, under different strain rates, depends on the different response of dislocation accommodation in GBAZ and GI which in turn origins from geometrically necessary dislocation (GNDs) and statistically stored dislocation (SSDs), respectively. An in-situ tracking of the strain rate dependent strain distribution and computational simulations were jointly employed to provide insight into the dislocation behavior. Given the different distributions of dislocations in the ultrafine grains of CNT/Al composites, mechanical response depends on how dislocations arrange in GI and GBAZ. Results from experiments and simulations, based on Crystal plasticity (CP) finite element modelling (FEM), clarified that the dominant deformation mechanism of the CNT/Al composite is strain rate dependent. The results show that the GI/GBAZ model can successfully reveal the dislocation dependency of strain rate. The results might shed some light on understanding of strain rate dependent dislocation behavior in metal matrix composites reinforced with nanocarbon structures such as CNTs.

Modelling of strain rate dependent dislocation behavior of CNT/Al composites based on grain interior/grain boundary affected zone (GI/GBAZ)

Springback prediction of sheet metal forming is always an important issue in the industry, because it greatly affects the final shape of the product. The accuracy of simulation prediction depends on not only the forming condition but also the chosen material model, which determines the stress and strain redistributions in the formed parts. In this paper, a newly proposed elastoplastic constitutive model is used, in which the initial and induced anisotropies, combined nonlinear isotropic and kinematic hardenings, as well as isotropic ductile damage, are taken into account. The aluminum alloy sheet metal AA7055 was chosen as the studied material. In order to investigate springback under non-proportional strain paths, three-point bending tests were conducted with pre-strained specimens, and five different pre-straining states were considered. The comparisons between numerical and experimental results highlighted the hard effect of both kinematic hardening and ductile damage on the springback prediction, especially for a changed loading path case.

Springback Prediction of Aluminum Alloy Sheet under Changing Loading Paths with Consideration of the Influence of Kinematic Hardening and Ductile Damage

The invention discloses a three-dimensional free bending forming tube bender and belongs to the technical field of metal complex component advanced manufacturing. The three-dimensional free bending forming tube bender comprises a fixing frame, a pushing mechanism, a fixing mold, a bend moving mechanism and a bend deflection mechanism, wherein the pushing mechanism comprises a first ballscrew; thefirst ballscrew is provided with a clamping device; the clamping device is provided with a first tube channel; the fixing mold is provided with a second tube channel; the bend moving mechanism comprises a movable platform; a circular through hole is formed in the movable platform; a motion transmission branch chain is arranged between the movable platform and a fixing frame; the bend deflection mechanism is arranged on the outer side of the movable platform and comprises a deflection platform; a third tube channel is formed in the deflection platform; and a driving mechanism is arranged between the deflection platform and the movable platform. Bend free bending forming in complex spaces can be achieved without replacing the bending mold, and the production efficiency is high.

Three-dimensional free bending forming tube bender

Ductile damage model has been widely regarded as a valuable method to predict the failure of sheet metal. Based on the thermodynamic theory and continuum damage mechanics, the fully coupled ductile damage model can be developed, which also can better predict the initiation and growth of the fracture. But the identifications of model parameters with theoretical methodology are difficult due to the complex coupling relationships existing among all state variables. The inverse methodology is regarded as a good method to resolve the problem. In this paper, the recently proposed fully coupled ductile damage model is chosen to investigate the deformation behavior of DP600 steel, in which the mixed saturation isotropic and kinematic hardenings are taken into account and fully coupled with the ductile damage. During the identification process, the least square formula of the error between experimental and numerical results is selected as the target function. The trust region algorithm and genetic algorithm are used with the help of MATLAB for the identification of three damage parameters. Finally, by comparing the experimental and numerical results, the validations of two algorithms are proved. The efficiency of the optimization process with trust region algorithm is higher, but with lower accuracy. Meanwhile, the optimization process is greatly affected by the chosen initial values of the ductile damage parameters.

Hybrid Inverse Parameter Identification of Fully Coupled Ductile Damage Model for Steel Sheet DP600 with Two Different Algorithms: Trust Region and Genetic Algorithms

Jiashuo Qi

Papers

Enhanced mechanical properties of CNT/Al composite through tailoring grain interior/grain boundary affected zones

Modelling of strain rate dependent dislocation behavior of CNT/Al composites based on grain interior/grain boundary affected zone (GI/GBAZ)

Springback Prediction of Aluminum Alloy Sheet under Changing Loading Paths with Consideration of the Influence of Kinematic Hardening and Ductile Damage

Three-dimensional free bending forming tube bender

Hybrid Inverse Parameter Identification of Fully Coupled Ductile Damage Model for Steel Sheet DP600 with Two Different Algorithms: Trust Region and Genetic Algorithms