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Metal forming or shaping processes, such as sheet metal forming and bulk forming, are widely used in many industries including automotive industry. Before computational simulations were widely used in the field of metal forming industry, the trial-and-error based empirical approach was conventionally applied to the optimization of the metal forming process and product. The most commonly employed numerical approach in the metal forming process for robust process optimization is the finite element analysis. Friction, one of the parameters that significantly affect the accuracy of numerical analysis, depends on several variants such as surface quality, contact pressure, lubrication, deformation, and the forming environment. Despite the complexity and difficulty of identifying friction mechanisms, accurate friction models are essential and hence have been proposed by numerous researchers. In this paper, the friction models in the previous studies are reviewed by categorizing them into the boundary lubrication condition and mixed-boundary lubrication condition according to their evaluative influences on friction. Since friction models have been proposed based on the contact theories, an overview on the contact models is also included in this paper. In addition, the contribution of several parameters on the friction such as surface roughness and material properties of the tool, adhesion, contact pressure, sliding speed, bulk deformation is also discussed. This review paper aims to provide an understanding and insight into the friction modeling and simulation along with associated friction experiments. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive investigation on the incremental sheet metal forming assisted by ultrasonic vibration (UV). With the aid of a new acoustoplasticity constitutive model which can accurately capture the deformation response to ultra-high strain rates, extensive numerical analyses on incremental sheet metal forming under UV were conducted by using the finite element method. Corresponding experimental investigations were also carried out to examine the capacity of the constitutive and numerical models. The effects of critical parameters, such as tool size, tool revolution speed, feed rate, pitch and tool vibration amplitude, on the forming force, maximum sheet thickness reduction and springback, were investigated in detail. It was found that the models can realistically predict the incremental sheet metal forming under UV. It is expected that this research can facilitate the development of ultrasonic vibration–assisted forming techniques. [ABSTRACT FROM AUTHOR]

One of the most critical tasks within the scope of Design for Manufacturing (DfM) is to define the set of Geometrical Product Specifications (GPS) in the 3D model or in the engineering drawing that ensures the functionality and the interchangeability of parts, as well as the intended functional performance of an assembly. Several methodologies have been proposed for the optimal designation of such specifications; however, the majority of them do not effectively take into account the deformations that are inevitably induced during assembly and operation for the vast majority of mechanical components. Motivated by the widely accepted tolerancing practice for sheet metal parts in the automotive industry, where the distinction between free state and constrained state is considered, the paper investigates the influence of the deformations induced during assembly and operation on GPS. The effect of part stiffness in the resultant functional GPS of the assembly/component is explored, through CAD surfacing and non-linear numerical finite element analysis tools including the contact problem. The current stage of development of a novel, performance-based methodology for the GD&T design procedure is presented. The methodology is applied on a real-world mechanical assembly that is derived from tolerance stack up-related literature. This study illustrated is that for an unpredictably wide range of mechanical components the default, free-state GPS scheme should only be assigned after rigorous analysis of their compliance behaviour. The proposed approach will lead to deduce the correlation between production cost and performance through a further development in future study. [ABSTRACT FROM AUTHOR]

This paper focuses on numerical simulation of single point incremental forming (SPIF) process. Via a user-defined material (VUMAT) subroutine is employed to implement a non-associated mixed isotropic-kinematic hardening material model. The accuracy of the proposed non-associated flow rule model to predict the material behavior and the anisotropy in sheet metal forming simulations is examined by comparing numerical results with experimental measurements. The suggested model shows good agreement with experimental results compared to the associated Hill_R and Hill_S models. The proposed non-associated flow rule demonstrates a good efficiency to simulate the springback after sheet metal forming and to predict the thickness variation during incremental sheet metal forming process. The present non-associated model can improve the prediction of both anisotropic and hardening behaviors of material used in numerically controlled sheet metal forming technology. [ABSTRACT FROM AUTHOR]

In theoretical analyses, such as numerical simulations of the large plastic deformation of sheet metal forming, the material is usually assumed to be ideally uniform; thus, the actual material response during the process cannot be properly characterized. In this paper, a finite element method (FEM) model considering nonuniform material properties is proposed. In the model, the dispersion degree of mechanical parameters is characterized by the microhardness tested from sheet metal samples, and the discrete material properties conforming to the random distribution functions are automatically assigned to the elements in FEM by Python script. The digital image correlation (DIC) method is used to check the tensile instability deformation of sheet metal in the physical experiments of uniaxial tensile tests. The results show that the nonuniform finite element model can reveal the strain transfer and reproduce the diffuse instability in a wide range of plate samples. The deformation feature, crack and its direction are in good consistency with the experiment, and the predicted critical diffuse instability strain is of high accuracy compared with the DIC results. In addition, the influences of random distributions of mechanical properties and the mesh size effects on the simulation results of tensile instability are addressed. [ABSTRACT FROM AUTHOR]

This paper presents a variant of the traditional 'mortise-and-tenon' joint, which has been used for thousands of years by carpenters and blacksmiths to connect wood or metal parts. The new proposed joint is utilized to fix longitudinally in position two metal sheets (or plates) perpendicular to one other by sheet-bulk metal forming, at room temperature. The development is performed by means of a combined finite element and experimental investigation focused on the identification of the major process parameters and on the understanding of their influence on the overall joining feasibility. Destructive testing is carried out to characterize the performance of the new proposed joint, and an analytical expression is provided to determine the maximum tensile force that the joint can safely withstand. [ABSTRACT FROM AUTHOR]

Numerical simulation is becoming one of the main methods to investigate various engineering problems with sophisticated conditions. A considerable amount of research is conducted on numerical analysis of sheet metal forming process to address different aspects of the problem. Among the numerical simulation methods for sheet metal forming process, finite difference method (FDM) and finite element method (FEM) have been the main methods. In this paper, the progresses in simulation techniques, advantage and disadvantage of numerical methods for simulating sheet metal forming process are discussed based on these numerical methods. Currently, FEM being the main simulation method for sheet metal forming, development in solution strategies and formulation, element selection are further brought into attention. Historical development of anisotropy and yield criteria, which are theoretical foundation for numerical simulation, and their application in simulation software are briefly classified. Formability of sheet metal is presented from the numerical simulation point of view. Numerical investigations on springback are reviewed in terms of simulation techniques and factors influencing springback. Simulation techniques for novel sheet metal forming techniques such as laser forming and incremental sheet forming (ISF) are presented. [ABSTRACT FROM AUTHOR]

In this paper, an iterative learning control (ILC) model is constructed with the state function, forming quality function, and learning updating law to optimize the sheet forming process. The state function, which is hardly calculated explicitly, is replaced by finite element simulation to predict the workpiece state with a specific drawbead geometry scheme. Then, the forming quality function is defined by accumulating the elements' thinning strain deviation with different weights. Afterwards, the learning updating law is designed to provide a new scheme of drawbead geometry based on the forming quality function multiplied by a learning gain. The ILC model guarantees a quick convergence and greatly improves the optimization efficiency for industrial application. To apply the ILC model to optimizing the drawbead geometry automatically, schemes of seamless integration of computer-aided engineering system into the CATIA platform and the drawbead geometry parameterization are adopted. Lastly, the rapidity and practicability of the optimization method for drawbead geometry are verified by the numerical experiment of the engine hood inner panel. [ABSTRACT FROM AUTHOR]

Plastic deformation of the sheet metal occurs synchronously with elastic deformation of the elastomer in flexible die forming process using elastic die. Thus, adjusting elastic deformation state of the elastomer to adapt to stress state of the sheet metal may be a potential way to improve sheet-forming capability. In this work, various deformation modes of the flexible dies were established using elastomers with different structures, which aims at developing a new way to improve sheet-forming capability through controlling in-plane stress state of the sheet metal. Digital image correlation (DIC) technique was used to capture the displacement and strain field of the sheet metal in bulge tests. Finite element analysis (FEA) was conducted to obtain pressure field of the flexible die and in-plane stresses of the sheet metal. The results showed that forming load, bulged height, and bulge strain could be adjusted by using flexible dies with different structures. The sharp increase of normal pressure in local region at the flexible die/sheet interface leads to non-uniform distribution of radial and circumferential stresses of the sheet metal. A design strategy of flexible die with a hollow structure (i.e., type 6) was proposed to reduce localization of interfacial normal pressure. Due to evenly-distributed normal pressure and beneficial friction at the flexible die/sheet interface, radial and circumferential stresses subjected by the sheet metal were relieved effectively. And the consequent increase of bulge height and bulge strain indicates the improvement of sheet-forming capability. [ABSTRACT FROM AUTHOR]

Origami-based sheet metal (OSM) bending is a promising new die-free folding technique for sheet metal. OSM bending principle is based on deforming the material along a pre-defined fold line, which is determined using material discontinuity (MD) produced by laser or waterjet cutting. The objective of this work is to study and evaluate the fracture in OSM bending under the influence of various MD types, kerf-to-thickness (k/t) ratios, and sheet thicknesses. The research goal is to provide information on selecting an optimized k/t ratio and type of MD that allows for fracture-free bending. Four different ductile fracture criteria (DFC) are used and calibrated from experimental data to forecast fracture. The DFC calibration is used to produce a set of critical damage values (CDV) for assessing the possibility of fracture in the OSM bending. In addition, the study provides fracture evaluation using finite element analysis (FEA) integrated with experimental cases for a broader range of OSM bending parameters and MDs. The results demonstrated that an MD with a higher k/t ratio is less likely to fracture during the OSM bending, whereas a higher sheet thickness increases the possibility of fracture. Furthermore, the study identifies the k/t ratio limit that ensures successful bending without fracture and categorizes MD types into two groups based on fracture likelihood. The fracture in the first group is dependent on the limiting k/t ratio, whereas the possibility of fracture in the second group is independent of the k/t ratio due to its topology. [ABSTRACT FROM AUTHOR]

The clinched process of heterogeneous materials is increasingly used in automobile, aerospace, and household appliances manufacturing. To investigate the forming quality of clinched joint for dissimilar sheet metal, the clinching experiments are carried out, and the cross-section morphology of clinched joints is observed. The finite element model for forming steel-aluminum clinched joint is established. The influence of process parameters (forming process parameters, geometry parameters, and concave die structural parameters) on the forming quality of steel-aluminum clinched joint is analyzed. The evaluation of the joint after forming includes the critical dimension, deformation, and neck-lock ratio. Then, the strength of the joint is measured by the tensile shear test. The changing law of strength and the neck-lock ratio is analyzed. The design strategy of different process parameters is proposed. The results show that the forming process of the joint is well predicted by numerical simulation, and the joint quality is good. The neck lock ratio of the joint with the highest tensile and shear strength is close to 1 when the forming force is 40 kN. The tensile strength and shear strength of the joint are increased by 125% and 62.35%. The design strategy is verified by experiments. The shear strength of the joint is increased by 16.49%. [ABSTRACT FROM AUTHOR]

For sheet metal parts with the vertical-wall partially and irregularly distributed on the sides (VWPIS) that contains both the difficult-form-surface (i.e., vertical-wall) with large forming angle and easy-to-form surface (i.e., gently tilted surface) with small forming angle, a selective multi-stage two point incremental forming (SMTPIF) on the basis of the forming angle was proposed. This SMTPIF method performs multi-stage forming only for the difficult-form-surface and gives different forming strategies and forming parameters to the different surface regions according to the forming angle. The algorithms for surface zoning, generation of the small forming angle surface for vertical-wall, generation of one-stage forming model by combining the easy-form-surface and small forming angle surface were presented. Meanwhile, the algorithm for the generation of the toolpath that is suitable to the SMTPIF was also given. The effectiveness of the proposed algorithm and the SMTPIF was verified by using the case studies, the finite element analysis, and the forming experiments. The results indicate that the SMTPIF can fabricate the VWPIS with short forming time and no sinking phenomenon. [ABSTRACT FROM AUTHOR]

This study aims to develop a universal, robust, and linear method to obtain an initial guess configuration for the multi-step finite element method (FEM) solver of sheet metal forming. Using the decoupling theory, the deformation at each step in the multi-step FEM solver of the sheet metal forming is decoupled into two independent deformation modes: bending-dominated deformation and stretching-dominated deformation. The configuration of the bending-dominated deformation constrained by the sliding constraint surface is considered as the initial guess configuration for the current step in multi-step FEM solver. To get an accurate initial configuration at each step, the method of Laplace-Beltrami operator (LBO) on a simplicial surface is employed to obtain the initial guess configuration effectively. Several numerical examples are provided for validation and verification of the proposed method through its applications for complicated sheet metal workpieces of finite element simulations. The results show that the proposed method on the simplicial surface for the initial guess configuration within a few iterations to be significantly effective. [ABSTRACT FROM AUTHOR]

There are some problems in the CNC incremental forming with the sheet metal part containing vertical wall, such as excessive thinning and easy fracture. A new CNC incremental forming method was proposed based on the feature recognition for the multidirectional adjustment of the sheet metal posture. According to the feature recognition, the multi-feature of the sheet metal part was decomposed into a number of the forming feature units according to the size and distribution of the forming angles, so that the forming angles of the surface are less than the forming limit angle. The feature recognition and forming feature unit generation algorithm, sheet posture control method, and sheet posture surface generation algorithm were studied, and the finite element analysis and the forming experiments based on the proposed algorithms were also presented. The research results show that the proposed method can change the sheet metal posture in real time during the CNC incremental forming process and realize the non-rupture forming with the large forming angle parts such as vertical wall parts. [ABSTRACT FROM AUTHOR]