Hot stamping (press hardening) is widely used to fabricate safety components such as door beams and B pillars with increased strength via quenching. However, parts that are hot-stamped from ultra-high-strength steel (UHSS) have very limited elongation, i.e., low ductility. In the present study, a novel variant of hot stamping technology called quenching-and-partitioning (Q&P) hot stamping was developed. This approach was tested on several UHSS sheet metals, and it was confirmed that this method can be used to overcome the drawbacks associated with conventional hot stamping. The applicability of Q&P hot stamping to each of these steels was also assessed. The part properties and performances of three widely used ultra-high-strength sheet metals, B1500HS, 27SiMn, and TRIP780, were evaluated through tensile testing and microstructural observations. The results demonstrated that the ductility of Q&P hot-stamped sheet metals was notably higher than that of the conventionally hot-stamped parts because Q&P hot stamping gives rise to a dual-phase structure of both martensite and austenite. Further, material tests demonstrated that the Q&P treatment had positive effects on all three selected materials, of which TRIP780 had the best ductility and the highest value of the product of strength and plasticity. Scanning electron microscopy images indicated that the silicon in the steels could limit the formation of cementite and would, therefore, improve the mechanical properties of Q&P hot-stamped products.

The superplastic deformation behaviors of as-received Ti-55 alloy and hydrogenated alloy with 0.1 wt%H were studied. The results indicate γ hydride readily decomposes and hydrogen easily escapes under vacuum or argon-protected condition near the starting temperature of α + β / β transformation. Compared with as-received alloy, the optimum superplastic temperature of hydrogenated alloy is reduced by about 125 ℃. Larger decrease of elongation in hydrogenated alloy appears if the deformation temperature and initial strain rate deviate from optimal ones. More proportion of β phase distributes uniformly around α phase grain boundaries and prominently inhabits the formation of voids in hydrogenated alloy during superplastic deformation. Hydrogen addition can lead to the improvement of discontinuous dynamic recrystallization mechanism and deformation compatibility of grain boundaries, as well as the reduction of dislocation pile-ups and tangles.

•γ hydride with bct structure shows a wedge shape, in which twinning is embedded.•Starting temperature of α/α+β phase transition decreases with increasing hydrogen concentration.•Temperature for superplasticity of Ti-55 alloy with 0.1 wt.%H addition is reduced by about 125 °C.•The reduced superplastic temperature decrease is contributed by comprehensive effects.The effects of hydrogen on microstructure evolution and superplastic behavior of a new near α high-temperature Ti-55 titanium alloy were studied. The results indicate γ hydride with fct structure exists in the specimens when the hydrogen content is even up to 0.7 wt.%. The start temperature of α+β/β phase transition decreases with increasing hydrogen concentration. The optimum superplastic temperature of Ti-55 alloy with 0.1 wt.%H is reduced by about 125 °C. Hydrogenation can significantly reduce the flow stress. The elongation of the hydrogenated alloy is more sensitive to deformation temperature and strain rate during superplastic tension. The optimum superplastic temperature decrease is resulted from the comprehensive effects of lower dislocation density, α+β/β transus temperature reduction, dynamic recovery and recrystallization, twinning induced plasticity and more deformable β phase after hydrogenation.

•Constructed a new yield criterion to generally describe asymmetry & Lankford coefficients under AFR.•Obtained the convexity of proposed yield criterion using method of order principal minor determinant.•Calculated uniaxial tensile yield stresses & Lankford coefficients with acceptable accuracy.•Accurately predicted asymmetry & local deformation characteristic of yield locus for HCP material.Transforming five different stress invariants-based yield criteria in I-J2-J3 framework into η-ξ-σ¯ framework demonstrates that all of these yield criteria are the product formulation of the equivalent stress and the function of the normalized third invariant ξ and stress triaxiality η. The function of ξ is replaced by a polynomial function to construct a generalized yield function for describing both asymmetry and Lankford coefficients under associated flow rule(AFR) framework, in which the linear combination of a series of ξ with odd powers is utilized to model the effect of materials' strength differential (SD). Since there are more parameters in the polynomial function of ξ than the function of ξ in those existing yield functions, and the independent basis functions ξ, ξ2, ξ3 … are informative enough to describe the SD effect and Lankford coefficients, the proposed yield function shows potential flexibility. The conditions for the convexity of the proposed yield criterion are obtained by using the method of order principal minor determinant. To describe sheet metals' anisotropic characteristics in tension and compression, the proposed isotropic criterion is further generalized to orthotropy through introducing the fourth-order linear transformation to the deviatoric stress tensor. To improve the flexibility of the yield criterion through introducing more fourth-order linear transformations, several extended proposed isotropic criteria are added together. The effectiveness and flexibility of the constructed yield criterion have been verified by applying to AA2008-T4(a BCC material) and AA2090-T3(a FCC material). Comparisons with Yld2000-2d and Yoon 2014 criteria show that the proposed yield criterion has the same ability as Yld2000-2d to describe metals’ tensile properties and the proposed yield criterion can capture the SD effect like Yoon 2014, which validates that this generalized yield criterion can accurately describe not only the SD effect of metals but also the Lankford coefficients under uniaxial tensile loading and balanced biaxial tension based upon the AFR assumption. The flexibility of the proposed yield criterion is further validated by applying to zirconium through describing the evolution of yield surface for a clock-rolled plate with various levels of pre-strains during through-thickness compression. It is found that the proposed yield criterion can capture the asymmetry and the local deformation characteristic of the yield surfaces if the pre-strain is very large.

•A new DFC can predict ductile fracture with stress triaxiality less than −1/3.•The new DFC combines tension fracture mechanism and shear fracture mechanism.•The form of new DFC reflects void nucleation, void growth and void coalescence.•The new DFC is proved to be more accurate compared with any other selected DFCs.A new ductile fracture criterion (DFC) is developed based on two typical fracture mechanisms - tension fracture and shear fracture by analyzing the morphologies of the fractured surfaces, and is discussed through void nucleation, void growth and void coalescence. The parameters in the proposed DFC are determined by analytical and numerical methods, and the influences of these parameters on the surface of equivalent plastic strain to fracture (EPSF) and the curve of EPSF to stress triaxiality in plane stress condition are discussed. Besides, the total Lode dependence of the proposed DFC about the influences of these parameters is also illustrated. The curves of EPSFs for Al 6061-T6 and Al 2024-T351 sheet are established and compared with the curves of CrashFEM criterion, modified Mohr-Coulomb criterion (MMC), extended Lou criterion and Hosford–Coulomb criterion and the absolute relative errors of these DFCs are also compared with the experimental data of Al 6061-T6 and Al 2024-T351, demonstrating the better accuracy of the proposed DFC. Comparative studies also show that the proposed DFC can accurately predict the ductile fractures with stress triaxiality less than −1/3, in round bar and notched bar tension.

Ultrahigh-strength steel (UHSS), such as DP980, has been widely used in automotive structural components to further reduce the weight of the autobody and improve the crashworthiness performance. However, UHSS sheets demonstrate more obvious kinematic hardening, which results in severe springback. In the present work, the kinematic hardening of typical Quenching and Partitioning steel, QP980, which is the typical steel of third generation of UHSS, is tracked by using a nonsaturating kinematic (NSK) Swift model in order to simulate the loading process more accurately. For unloading process, inertia relief is adopted as a new control approach in springback calculation. The experiment results show that the NSK Swift model improves the springback prediction accuracy greatly compared with that predicted by isotropic hardening model, and the inertia relief is validated as an accurate and efficient springback calculation method. Moreover, springback compensation based on the NSK Swift model can reach acceptable tolerance as ±0.5 mm through the iterative compensation method in LS-Dyna.

In conventional stretch-flanging process by incremental sheet forming (ISF), serious thinning is a major challenge due to the ISF deformation mechanism of stretching, bending, and considerable shearing in the area far from the flange edge. To solve this problem, a new flanging tool for ISF is proposed, accompanied with the suitable tool path strategy. In order to obtain a better understanding of this new stretch-flanging approach, experiments have been conducted to study the geometric accuracy, thickness distribution, forming force, and fracture behavior of typical circle and square flange features. Besides, the effect of tool radius on the new stretch-flanging process is also investigated. Results show that the forming strategy which the tool moves horizontally in the X–Y plane is the most suitable one for the new flanging tool. Thicker but shorter flange with a more uniform thickness distribution can be obtained by the proposed stretch-flanging approach. Tool radius also has an important influence on the diameter of the final hole, the forming depth, and the fracture limit strains in the new stretch-flanging process.

Improved ductility by stress relaxation has been reported in different kinds of steels. The influence of stress relaxation and its parameters on the ductility of 304 stainless steel has not been established so far. Stress relaxation behavior during tensile tests at different strain rates is studied in 304 stainless steel. It is observed that stress relaxation can obviously increase the elongation of 304 stainless steel in all cases. The elongation improvement of interrupted tension reaches to 14.9% compared with monotonic tension at 0.05 s−1. Contradicting with the published results, stress drop during stress relaxation increases with strain at all strain rates. It is related with dislocation motion velocity variation and martensitic transformation.

As a new flexible forming method, the electricity-assisted incremental sheet forming (E-ISF) provides concentrated local heating to improve the formability and can further expand the application of the incremental sheet forming (ISF) process on conventional “hard-to-form” materials such as titanium alloy. However, E-ISF process is questioned by rough surface integrity of the formed part and severe tool wear. Driven by these challenges, several novel tools have been designed by employing inner water cooling system and rolling tool end design, and the performances were validated by forming Ti6Al4V sheet. Experimental results suggest that the tool with rolling ball end and inner water cooling reduces the surface wear on the tool tip and improves the surface finish of the formed part.

•Accuracy and efficiency of thick shell is validated compared to shell and solid.•Thickness predictive ability of thick shell is proved by dual hydroforming process.•Larger normal stress is found to increase material thinning ratio in forming zone.•Larger normal stress is found to increase ε¯p and σ¯ in forming zone.Double-sided hydroforming has been widely used in production since it can improve the formability and surface quality of sheet metals significantly. This paper investigates the application of thick shell element in finite element analysis of sheet metal forming process with pressure reacting on both sides of the blank. The computational efficiency and accuracy of thick shell element is compared with that of solid element and traditional shell element based on a numerical example. Then a double-sided blank hydroforming experiment is carried out to validate the thickness prediction ability of thick shell element, and the results show that the normal stress in the thickness direction caused by double-sided pressure reduces the material thickness and increases plastic strain and effective stress in the forming zone. Thick shell element, which can reflect the influence of normal stress on sheet metal forming process, is effective in finite element analysis of double-sided hydroforming.

•Non-saturating cyclic hardening and permanent offset are modeled simultaneously.•Tension data is used in material constants fitting for modeling cyclic loading.•Responses of the kinematic hardening model in different strain ranges are verified.•Springback and thickness predictions are performed for an automobile cross member.A recently proposed non-saturating kinematic (NSK) Swift model is verified by modeling the advanced high-strength sheet metal DP600, of which the cyclic loading is typically featured by the non-saturating hardening and permanent offset phenomenon along with other classical characteristics. Good performances are also presented to describe the further cyclic hardening behavior in different strain ranges. Noticeably only the tension data are used to determine the model parameters for this material due to the appropriate control function. Then the non-saturating kinematic (NSK) Swift model is applied to the simulation of the automobile underbody cross member in Numisheet 2005 Benchmark 2, where only the tension data is available for the material constant determination while the cyclic loading occurs in the forming process. Nice agreements are achieved between the measured data and the numerical results of the springback and the thickness prediction for two DP sheet metals (DP600 and DP965). Both the theoretical importance of the control functions in material modeling and practical significance of the obtained non-saturating kinematic (NSK) Swift model can be implied through the constitutive verification and the industrial application.

Through in situ transmission electron microscopy observation on SUS304 metastable austenitic stainless steel during stretching at room temperature, it is found that ε martensite plates were induced preferentially from the sites of dislocation pile-ups. With increasing deformation, some of ε thin martensite platelets disappear and reversibly transform to γ austenite without heating treatment, which is different from the previous result that ε martensite can entirely transform to α′ martensite. Then, some of deformation twins appear and grow along the vertical direction of ε martensite due to\( (111)_{\gamma }\, \bot (10\bar{1}0)_{\varepsilon } \). Moreover, it is directly observed that multiple transformation mechanisms via γ → ε → γ, γ → ε, γ → α′, γ → ε → α′, γ → deformation twins → α′ can co-exist.

Incremental sheet forming (ISF) is a highly versatile and flexible process for rapid manufacturing of complex sheet metal parts. The characteristic of localized deformation is significantly different from conventional sheet metal forming process. To understand the fundamental material deformation mechanism during the ISF process is of great importance for ISF process design and optimization in achieving improved material formability, accuracy and more uniform thickness distribution. In this paper, an analytical model for single point incremental forming (SPIF) process has been developed to describe the localized deformation mechanism. With the consideration of both bending effect and strain hardening, the stress and strain states in the deformation zone are described. Analytical evaluation reveals that the deformation occurs not only in the contact zone, but also in the neighboring wall which has been already formed in the vicinity of the contact zone. In addition, the results also suggest that the fracture tends to appear at the transitional zone between the contact area and the formed wall. In order to validate the analytical results, SPIF simulation and experiments both have been conducted with good agreement obtained.

A new approach is presented in this paper to calculate the critical threshold value of fracture initiation. It is based on the experimental data for forming limit curves and fracture forming limit curves. The deformation path for finally a fractured material point is assumed as two-stage proportional loading: biaxial loading from the beginning to the onset of incipient necking, followed plane strain deformation within the incipient neck until the final fracture. The fracture threshold value is determined by analytical integration and validated by numerical simulation. Four phenomenological models for ductile fracture are selected in this study, i.e., Brozzo, McClintock, Rice-Tracey, and Oyane models. The threshold value for each model is obtained through best-fitting of experimental data. The results are compared with each other and test data. These fracture criteria are implemented in ABAQUS/EXPLICIT through user subroutine VUMAT to simulate the blanking process of advanced high-strength steels. The simulated fracture surfaces are examined to determine the initiation of ductile fracture during the process, and compared with experimental results for DP780 sheet steel blanking. The comparisons between FE simulated results coupled with different fracture models and experimental one show good agreements on punching edge quality. The study demonstrates that the proposed approach to calculate threshold values of fracture models is efficient and reliable. The results also suggest that the McClintock and Oyane fracture models are more accurate than the Rice-Tracey or Brozzo models in predicting load-stroke curves. However, the predicted blanking edge quality does not have appreciable differences.

The fracture morphologies of several advanced high-strength steels (DP590, DP780, DP980, M1180, and M1300) formed in uniaxial tension and piercing were observed by scanning electron microscope, and then quantitatively analyzed by image processing technique. The tension-induced fractographs are dominated by obvious uniform or bimodal size dimples, while shearing-induced fractographs have smooth surfaces and few dimples. The fracture zone of higher grade DP steels is smoother. As for M1180 and M1300, the fracture zones consist of very small dimples and smooth brittle surfaces. The dimple size of M1300(~1.2 μm) is smaller than that of M1180(~1.6 μm). Moreover, in the tensile fracture, the quantitative correlation between average dimple diameter (d) and tensile strength (σ) can be represented by d = 10,502.32σ−1.21. However, the relation between dimple density and tensile strength is not monotonic due to the appearance of bimodal size dimples with increase of tensile strength. For shearing-induced fracture during piercing, the fitted empirical model between the percentage of burnish zone (f) and tensile strength can be described as f = 239.9σ−0.36.

Incremental sheet forming (ISF) is a highly versatile and flexible process for rapid manufacturing of complex sheet metal parts. Although tool path plays an important role in the ISF process, there is only limited development in the tool path generation strategy and the conventional contour based strategies have been proven to cause problems in surface quality and geometric accuracy. This paper presents a new feature-based tool path generation algorithm for incremental sheet forming process. In this algorithm, tool paths are generated according to the specified critical edges. To obtain a better understanding of forming mechanism using the new tool path generation method, the thickness distribution, geometric accuracy and surface quality of the ISF formed shapes by using the feature-based tool path approach are compared with the traditional ISF tool path method based on three case studies including a truncated cone with double bottoms, a non-symmetrical cone and a car fender. The results suggest that the new tool path stretches the sheet in a different way and results in different thickness distributions. The results of these case studies also demonstrate the advantages of the feature based tool path generation especially in surface quality, geometric accuracy and forming time.

The microstructure characteristics and plastic deformation behavior of SUS304 metastable austenitic stainless steel sheets have been investigated during tensile process at different strain rates at room temperature. The yield stress continuously increases with strain rates due to low fraction of martensite transformed from austenite at 0.2% plastic stain. While the ultimate tensile stress (UTS) and elongation gradually decreases and then slightly increases with increase in strain rate from 0.0005 s−1 to 0.1 s−1, which is attributed to the variation of the martensite fraction that is affected seriously by adiabatic heating. A higher temperature increase in the tensile specimens restricts the martensitic transformation at high strain rate. The strain rate of 0.1 s−1 is considered as a transition deformation rate from quasi-static state to plastic forming, where the transformed martensitic content is very small in a higher strain rate range. Anomalous stress peaks in the later half stage of deformation occur at a very low strain rate (i.e., 0.0005 s−1) result from X-shaped strain localization repeatedly sweeping over the specimen. With increasing strain rates, the variation of dimple number density follows similar trend as that of UTS and ductility because martensite fraction mostly influences void nucleation and growth.

Laminated steel sheet consists of two steel sheets and a polymer layer which bonds them. During forming process, mechanical properties of polymer layer significantly influence the shape of final product. In this study, a continuum model in which nonlinear visco-elasticity is taken into account has been developed for polymer layer of laminated steel sheet and implemented in a commercial finite element program by material subroutines. Lap-shear test and T-peel test have been conducted to obtain parameters of this continuum model. Two different methods are compared to establish a better method for modeling the polymer layer deformation in lap-shear test simulation. One is cohesive zone element and the other is contact method. In order to assess calculation efficiency, both explicit and implicit procedures are used to simulate lap-shear test, and T-peel test is simulated by implicit procedure to evaluate accuracy. The result indicates that cohesive element is easier to solve convergence problem and implicit procedure may save much simulation time. T-peel test data can be used to describe the normal mechanical behavior of polymer layer in an acceptable range. Finally, V-bending forming process has been studied to investigate the effect of polymer layer on the springback and final deformation shape through experiment and numerical simulation. The result indicates that the comparison between numerical simulation and experiment is in good agreement. The finite element model can accurately predict the final shape after bending and springback.

The hydroforming process is used widely across many industrial fields. High applied pressure during hydroforming makes it necessary to consider the influence of the through-thickness normal stress, while traditional approaches based upon a plane-stress assumption are not appropriate in such cases. Reliable constitutive models that consider the through-thickness normal stress are summarized in this paper, which focuses on the state of the art in the following several aspects: determine the flow stress curve with proper experimental methods and choose the measurement and computational methods to minimize the error as much as possible; select the proper three-dimensional anisotropic yield criterion for the specific material; Define the forming limit model and construct corresponding experimental verification method. The review of existing work has revealed several gaps in current knowledge of the hydroforming process accounting for the through-thickness normal stress. Conclusions are drawn concerning some critical issues and potential future developments in hydroforming modeling.

The static softening behavior of aluminum alloy A6082 was investigated by interrupted hot tests conducted on Gleeble-1500 simulator at deformation temperatures from 573 to 773 K and strain rates from 0.1 to 10 s−1, with a pre-strain from 0.3 to 0.7 and variable inter-pass delay times. The offset method was applied to convert the changes in flow stress between two passes to static softening fraction. The microstructural changes were characterized by the quantitative metallography of quenched specimens. The results showed both static softening and static recrystallization curves exhibited a simple sigmoidal shape; the static softening is related to the static recrystallization in a nonlinear manner with 50% static recrystallized volume fraction corresponding to 80% static softening fraction; an increase in temperature, strain rate or pre-strain yields a decrease in the time for 50% static recrysallized volume fraction, on which the temperature has the most remarkable influence; Si and Mn additions accelerate the process of static recrystallization. Finally, the equations of static recrystallization kinetics of this alloy were developed with a good agreement between the predicted and experimental results.

The dynamic recrystallization (DRX) behaviors in SPHC steel were investigated with hot compression tests at deformation temperatures of 950–1 150 °C, strain rates of 0.1–15 s−1, and initial austenite grain sizes of 86–232 μm. The effects of deformation temperature, strain, strain rate and the initial austenite grain size on the microstructural evolution during DRX were studied in detail. The results show that DRX is observed under the condition of the Zener-Hollomon parameter being less than 1.07×1013 s−1. The deformation activation energy for SPHC steel is calculated to be 299.4 kJ/mol by regression analysis. Austenite grain size of DRX is refined with decreasing temperature and increasing strain rate under steady state conditions, but it is not influenced by the initial grain size. The mathematical equation of DRX grain size of SPHC steel is obtained.

For a more accurate forming calculation and numerical simulation of hydraulic turbine blade, experimental studies on the flow stress of stainless steel 0Cr13Ni5Mo were carried out upon Gleeble-1500 thermal simulator under different deformation conditions. The results then were analyzed and the effects of all influencing factors were summarized consequently. New mathematic models were conceived. Utilizing the software Matlab, regression coefficients were calculated by the least square method. The model has an eminent capability of curve-fitting performance with impact structure whose correlation coefficient is up to 0.908 0 and the cosine coefficient is 0.995 8. All mathematic models and process parameters can be used in engineering calculations or computer simulations.

•The increased friction has two contrary effects in SPIF process: enhance the deformation stability but decrease the formability.•Friction affects the SPIF deformation behavior: while the dominant deformation is sheet stretching, high friction causes considerable through-the-thickness-shear.•An ORB tool will reduce friction in SPIF process, which results better surface finish, lower force and higher formability.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.

•In general, the material formability enhances in frictional heat assisted SPIF, but this process is not monotonous.•The mechanisms behind formability change in frictional heat assisted SPIF have been explored in detail.•It physically confirms that the through-the-thickness shear is a positive factor for formability improvement when friction is in a certain range.•Laser surface textured forming tool is able to reduce the friction at tool–workpiece interface and change the magnitude of heat generation.Single point incremental forming (SPIF) is a new sheet metal forming process which achieves higher formability, greater process flexibility and reduced forming force compared to conventional sheet forming operations due to its characteristic of localized deformation. In recent years, a novel SPIF process assisted by localized friction heat is developed to further improve the material formability. Physically, the frictional heat is generated by the high relative motion at tool–workpiece interface resulted from tool rotation. However, the mechanisms behind formability difference induced by tool rotation at both low and high speed ranges are required to investigate in detail. In this paper, a series of experiments with an increase of tool rotation speeds ranging from 0 to 7000 rpm are conducted to form AA5052-H32 aluminum alloy sheets into a truncated funnel. Additionally, the obtained results are analyzed in terms of formability, forming forces and temperature trends to find out the different roles of friction and heat during the forming process. As a result, the formability behaviors at varying tool rotation speeds can be categorized into four stages according to different reasons. It indicates that friction is the dominant factor in low tool rotation speed range (0–1000 rpm) but will be substituted by thermal effect and potential dynamic recrystallization in high tool rotation speed range (2000–7000 rpm). Furthermore, due to the proved lubrication enhancement and hydrodynamic enhancement generated by surface textures, a laser surface textured forming tool is also utilized to show its influence on forming forces, measured temperatures and the corresponding formability. Finally, it demonstrates that the fabricated laser surface texturing (LST) is capable to reduce the friction at tool–workpiece interface and change the magnitude of heat generation.

Single point incremental forming (SPIF) is a new sheet metal forming process characterized by higher formability, product independent tooling and greater process flexibility. The inability of conventional single pass SPIF to form vertical walls without failure is overcome by forming multiple intermediate shapes before forming the final component, i.e., multi-pass single point incremental forming (MSPIF). A major issue with MSPIF is significant geometric inaccuracy of the formed component, due to the generation of stepped features on the base. This work proposes analytical formulations that are shown to accurately and quantitatively predict the stepped feature formation in MSPIF. Additionally, a relationship is derived among the material constants used in these analytical equations, the yield stress and thickness of the blank material, such that the computational effort required for the calibration of these constants can be minimized. Finally, the physical effects of yield stress and sheet thickness on the rigid body translation are further discussed.

•Developed a new hole flanging process by incremental sheet forming using a new tool.•Established an analytical model to investigate the deformation mechanism.•Improved the hole-expansion ratio by 130% with more uniform thickness distribution.One of the major challenges in conventional incremental sheet forming (ISF) is the extreme sheet thinning resulted in an uneven thickness distribution of formed part. This is also the case for incrementally formed parts with hole-flanging features. To overcome this problem, a new ISF based hole-flanging processing method is proposed by developing a new ISF flanging tool. Comparative studies are conducted by performing hole-flanging tests using both ISF conventional ball-nose tool and the new flanging tool to evaluate the sheet deformation behavior and the quality of the final part. Stress distribution and strain variation are investigated by analytical approach and numerical simulation. Experiments have been conducted to validate the analytical model and simulation results, and to further study the fracture behavior. Results show that the new flanging tool generates greater meridional bending than stretching deformation in conventional ISF. The combination of bending-dominated deformation mode with localized deformation of ISF ensures more uniform thickness distribution on hole-flanging part with better resistance to fracture.

The capability of accurately modeling nonlinear behaviors is essential to simulation-based engineering. Giving better descriptions of actual constitutive behaviors, nonlinear kinematic hardening models are frequently considered as an ad hoc approach by directly prescribing the hardening laws. The necessity has been recognized for accommodating this effective yet empirical methodology into extreme principles which theoretically underlie the derivation of evolutionary equations in irreversible dissipative processes. In contrast to the published efforts, this paper presents a systematic approach for characterizing both nonlinear kinematic and isotropic hardening behaviors of rate-independent polycrystalline metals. With the modified principle of maximum mechanical dissipation and the method of Lagrangian multipliers, the typical rate-independent constitutive laws are derived. Enlightening decompositions of the mechanical dissipation and its implications are discussed. Control functions are introduced to provide useful specifications about formulating hardening models. In contrast to the ad hoc origins, the relationship of many existing hardening models (both nonlinear kinematic and isotropic types) has been clarified through the unified framework. Moreover both saturating and non-saturating behaviors of the two hardening types can be properly modeled and numerical implementations are presented. Particularly permanent softening can be automatically given by non-saturating kinematic hardening modeling along with other features of cyclic loading. With this approach this phenomenon is explained from the viewpoint of energy and reproduced with only one back-stress and single yield surface. Finally comparisons between the methodology in this work and other classical theories are given to clarify the relationships and analogies. Pressure-dependent yield is also discussed to show the generality of the approach.Highlights► The approach provides heuristic specifications to formulate new hardening models. ► The relationship of existing models is clarified against their ad hoc origins. ► Non-saturating kinematic hardening is proposed to describe permanent softening. ► The classical plastic potential theories can be generalized by the approach. ► The approach is applicable to pressure-independent and pressure-dependent yield.

Preform tool shape optimization using response surface method (RSM) was developed in this work. Neural network approximation model was employed for response surface construction in order to overcome the limitation of quadratic polynomial model in solving non-linear problems. A two-step axisymmetric forging problem was studied as an example using proposed method. Optimum was achieved by using pattern search optimization method to search response surface describing relationship between preform shape and die cavity fill ratio. In addition to that, with respect to the complexity of the optimum solution, the knowledge-based redesign concept was proposed. Simplified preform shape description model was built based on the knowledge extracted from previous optimization and additional shape optimization in terms of a new optimization objective was conducted to obtain a better redesign preform shape. Finally, comparison was made between the original optimal shape and redesigned one; better result was achieved by using the concept proposed.

•Tool position/supporting force result varied deformation behavior and formability.•Slave tool introduces “stress triaxiality drop”, which increases the DSIF formality.•Tool squeezing increases formability more obviously under plane-strain condition.Double side incremental forming (DSIF) is an emerging technology in incremental sheet forming (ISF) in recent years. By employing two forming tools at each side of the sheet, the DSIF process can provide additional process flexibility, comparing to the conventional single point incremental forming (SPIF) process, therefore to produce complex geometries without the need of using a backing plate or supporting die. Although this process has been proposed for years, there is only limited research on this process and there are still many unanswered open questions about this process. Using a newly developed ISF machine, the DSIF process is investigated in this work. Focusing on the fundamental aspects of material deformation and fracture mechanism, this paper aims to improve the understanding of the DSIF process. Two key process parameters considered in this study include the supporting force and relative position between master and slave tools. The material deformation, the final thickness distribution as well as the formability under varying conditions of these two process variables are investigated. To obtain a better understanding from the experimental results, an analytical model has been developed to evaluate the stress state in the deformation zone. Using the developed model, an explicit relationship between the stress state and key process parameters can be established and a drop of stress triaxiality can be observed in the double contact zone, which explains the enhanced formability in the DSIF process. Based on the analytical and experimental investigation, the advancements and challenges of the DSIF process are discussed with a few conclusions drawn for future research.