Fracture in metal forming can occur in three different modes: (i) tensile; (ii) in-plane shear; and (iii) out-of-plane shear (respectively the same as modes I, II and III of fracture mechanics). The ...circumstances under which each mode will occur are identified in terms of plastic flow and microstructural ductile damage by means of an analytical framework to characterize fracture loci under plane stress conditions that also takes anisotropy into consideration.
Experimental results retrieved from the literature give support to the presentation and show that plastic flow and failure in sheet forming results from competition between modes I and II whereas in bulk forming fracture results from competition between modes I and III.
•New analytical framework to characterize fracture loci in metal forming under plane stress conditions.•Characterization of fracture loci in metal forming in the light of crack separation modes I, II and III.•Independent treatment of surface cracks for sheet and bulk metal forming processes.•Representation of fracture loci in the principal strain space and in the space of the effective strain at fracture vs. stress triaxiality.
In this paper, a multi-scale model for AA3104-H19 and AA2024-T3 aluminium alloy with strong cold rolled crystallographic texture is proposed. This model consists of two components: (i) a full-field ...crystal plasticity spectral method using fast Fourier transformation (CPFFT) that can be used to predict anisotropic mechanical properties in both the in and out-of-plane uniaxial and multi-axial stress states and (ii) a 3D new phenomenological anisotropic yield function with enough flexibility to describe the in and out-of-plane mechanical anisotropies of strongly textured aluminium alloy sheet. This multi-scale model was implemented into a commercial finite element (FE) software Abaqus via a user-defined material subroutine UMAT to predict the anisotropic plastic deformation in a multi-step forming process (deep drawing). Results showed that this new modelling approach is able to accurately predict earing profiles with multiple ears in deep drawn cups based on the crystallographic texture information data obtained from orientation density functions (ODFs).
Over the past 15 years, numerous tribological systems for warm and hot aluminium sheet metal forming have been investigated by various researchers. Two previously disregarded aspects will be examined ...in this paper with an enhanced strip drawing test: the temperature influence on friction and wear performance in an extensive comparison of different lubricant classes in tool lubrication and the bonding performance of different lubricants to the die surface. In depth analyses of the coefficient of friction (COF) and wear performance (wear-free draw-in) for nine different lubricants and additionally several different formulations / mixtures were carried out for sheet temperatures from 20 °C to 425 °C. Furthermore, approaches for future lubricant developments are given.
•The flat die strip drawing test resolves detailed lubrication effects of temperature supported sheet metal forming.•Oil-, wax-, polymer- and solid lubricants show distinct behaviours in temperature supported forming of aluminium sheets.•Mixing different lubricant classes can greatly improve overall lubrication performance.•Polymer based bonding agents play a key role in solving tribological challenges of tool lubrication.
A hybrid experimental-numerical procedure was implemented to determine the failure surface of a dual-phase 780 steel sheet as a function of the effective plastic strain, triaxiality, and Lode ...parameter using butterfly specimens with in situ digital image correlation strain measurement and supporting finite element calculations. A butterfly-type test specimen was employed to experimentally obtain stress states ranging from simple shear to plane strain tension including mixed tensile and shear loading. The numerically-derived failure surface was implemented into the phenomenological GISSMO damage model in the commercial finite element code LS-DYNA and the accuracy of the failure surface was determined using finite element predictions of the characterization experiments. A series of independent validation experiments related to sheet metal forming were performed including a hole tension test, a conical and flat punch hole expansion test, and a hemispherical punch test. The finite element models utilizing the damage model were able to accurately reproduce the load–displacement and surface strains of the sheet material for both the characterization and validation experiments. Prediction of the failure orientation and location compared favorably to each of the validation tests.
This paper presents a constitutive equation for anisotropic inelastic deformation rate applied to sheet metal forming based on recently developed invariants for nonlinear orthotropic ...elastic–inelastic material response. These invariants are based on Eulerian evolution equations for microstructural vectors that characterize directions of anisotropy and elastic deformations which cause stress. These microstructural vectors are used here to model directions of orthotropy for sheet metal forming. This Part I of a two-part paper modifies an anisotropic yield function for sheet metal analysis based on stress components determined by elastic deformations of microstructural vectors. Specific expressions are proposed for the direction of inelastic distortional deformation rate, which automatically satisfies the rate of material dissipation inequality, and for the direction of inelastic spin. The material parameters are calibrated for the aluminum alloy AA6022-T43, steel MP980 sheets and cold rolled low carbon steel sheets. The results of stress anisotropy and R-value distributions are shown to be in good agreement with the experimental data. Part II describes the implementation of this model into a finite element program and presents a comparison of Eulerian and Lagrangian formulations.
•Anisotropic inelastic sheet metal behavior is modeled using microstructural vectors.•Microstructural vectors determine elastic deformations and axes for orthotropic inelastic response.•Hyperelastic constitutive equation for Cauchy stress.•Validation with two materials shows that this model can capture anisotropies of R-value and yield stress.•Inelastic spin can shift R-value curves.
In this study, the epitaxial laser metal forming(E-LMF) process was used to deposit a nickel-based single crystal superalloy under different cooling conditions, and its microstructure and formation ...mechanism were investigated. The results reveal that an epitaxial microstructure with an almost crack-free is formed under air-cooling, while water-cooling increases the hot crack sensitivity of the deposited due to the thermal stress accumulation. In the case of water-cooling, almost all cracks arise in the overlap zone, with a few cracks appearing in other regions. The element distribution analysis and grain boundary angle analysis show that micro-segregation, as well as high angle grain boundaries, result in typical solidification cracks. In the case of air-cooling, the crystal orientation of the deposited zone is consistent with that of the substrate, although there are some stray grains in the top zone. As the bottom layers are annealed for a longer time than the upper layers, the γ' phase is precipitated more at the bottom than at the top.
•The multi-track nickel-based single crystal superalloy block with an almost crack-free was obtained.•Although water-cooling can maintain the temperature gradient and aids epitaxial growth, it significantly increases the hot crack sensitivity of the nickel-based singlecrystal superalloy block fabricated by the E-LMF process due to excessive thermalstress accumulation.•The microstructure evolution and formation mechanism of the multi-track nickel-based single crystal superalloy block were clarified.
Dislocation glide is a general deformation mode, governing the strength of metals. Via discrete dislocation dynamics and molecular dynamics simulations, we investigate the strain rate and dislocation ...density dependence of the strength of bulk copper and aluminum single crystals. An analytical relationship between material strength, dislocation density, strain rate and dislocation mobility is proposed, which agrees well with current simulations and published experiments. Results show that material strength displays a decreasing regime (strain rate hardening) and then increasing regime (classical forest hardening) as the dislocation density increases. Accordingly, the strength displays universally, as the strain rate increases, a strain rate-independent regime followed by a strain rate hardening regime. All results are captured by a single scaling function, which relates the scaled strength to a coupling parameter between dislocation density and strain rate. Such coupling parameter also controls the localization of plasticity, fluctuations of dislocation flow and distribution of dislocation velocity.
Mass production of customized parts using conventional manufacturing technologies is challenging. The increasing use of hybrid approaches is a promising way to address mass customization. Due to the ...widespread use of the term hybrid in manufacturing technologies, the amount of literature is increasing dramatically. Therefore, this paper aims to provide an overview of the current state of the art on the combination of additive manufacturing (AM) and metal forming. First, the term hybrid is classified, followed by examples of hybrid AM. The focus of this paper is on the combination of either sheet metal forming or bulk metal forming with either powder bed fusion or directed energy deposition. Examples of both sequences - AM before or after forming - are presented and analyzed for their potential use in hybrid parts. From the selected literature, the possibilities of this approach are derived as well as the ongoing need for future research. AM offers great potential for adjusting the microstructure and resulting material properties to improve formability. Furthermore, forming operations need to be adapted to the different material properties found in hybrid parts.
Producing load-adapted and functionally integrated components by flexible and resource efficient processes has gained importance within industries like the automotive sector in recent years. A ...promising new class of processes that enables a local adaption of the sheet thickness is Sheet-Bulk Metal Forming (SBMF). While the incremental procedure (iSBMF) only requires a moderate forming force, forming of high strength steels leads to a tool load resulting in a significantly reduced tool life. One approach to reduce tool loads is the utilization of the so called electroplastic effect (EPE). This study for the first time identifies the potential of the EPE on a temporary reduction of the forming force during the iSBMF of gears targeting an improvement of the tool life. The steel grades DC04 and HSM700 HD are characterized considering the EPE under uniaxial tension. Based on the characterization, the current density and temperature increase are modelled numerically and analytically for the incremental gear forming process. Moreover, the impact of EPE on strain hardening, grain texture and forming force is determined. By a local insulation of the forming tool based on a PVD coating and the application of an electrical current, a temporary force reduction of up to 55 % is observed whereas the strain hardening effect remains almost unaffected.
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