Pressure-carrying Medium Research Articles

Inhibiting plastic tensile instability of non-symmetric thin-walled shell component via increasing regional metal inflow based on heterogeneous pressure-carrying medium

Heterogeneous pressure-carrying medium was employed to establish a differentiated pressure field on sheet metal in flexible die forming process in this work, which aimed at matching the non-symmetric shape of target component and improving metal inflow to avoid local tensile instability. Specifically, metal inflow corresponding to the differentiated pressure field was analytically evaluated. Forming of a typical non-symmetric shell component was experimentally and numerically studied based on the proposed method. Compared with forming processes based on the uniform pressure, difference of metal inflow in two sides of the non-symmetric component increased from 2.16 mm to 3.36 mm and metal inflow in critical region increased by 11.9% when differentiated pressure field (taking heterogeneous elastomer #4–3 for example) was employed. The resultant maximum thinning ratio decreased by 4.2% and the uniformity of shell thickness increased by 16.9%. With the decrease of Shore hardness of elastomer in the formed region, stress path in the ready-to-form region transferred towards the bi-axial tension stress state, i.e., stress ratio (α) increased. And, stress triaxiality (η) in characteristic regions were regulated appropriately, which decreased the risk of tensile instability. It was attributed to the decreased normal pressure and frictional resistance at sheet/elastomer interface in the formed region.

Numerical investigation of controllable non-uniform deformation of tailor-welded blanks using heterogeneous pressure-carrying medium

Severe non-uniform deformation occurs in the forming of tailor-welded blanks (TWBs) with a large difference in properties or shell thickness of base materials, which lowers deformability of tailored shell components. Establishing a non-uniform pressure field and using it to form sheet blanks with non-uniform deformability such as TWBs may be a potential solution. In this work, the non-uniform pressure field established by heterogeneous elastomers and mechanism for controlling the deformation of TWBs using this pressure field were studied numerically. Concretely, a modified three-region model was proposed to describe the structure of TWBs based on the sectional distribution of microhardness. Finite element models of TWBs and heterogeneous elastomers were established and validated accordingly. The “higher” and “lower” pressure generated by different partitions of the heterogeneous elastomer were employed to form the “stronger” and “weaker” regions of TWBs, respectively. Forming processes using heterogeneous elastomers with different ratios of Shore hardness (ζ) as the flexible dies were simulated. The results showed that with the decrease of ζ, excessive deformation in the “weaker” side was alleviated and deformation uniformity of TWBs was greatly improved. Stress triaxiality at three characteristic regions (i.e., the “stronger” side, the “weaker” side, and the transition region) was increased simultaneously. Stress localization in the weld seam and its heat-affected zones (HAZs) was alleviated as well. In addition, results of failure analysis showed that deformation uniformity of TWBs could be greatly improved (＋19.62%) on the premise of sacrificing a few forming limits (−4.34%) by using the optimal heterogeneous elastomer.

Mechanism on the increased sheet metal formability in flexible-die forming process using elastic composites with magnetic-field-dependence as the pressure-carrying medium

Mechanism on the increased sheet metal formability in flexible-die forming process using elastic composites with magnetic-field-dependence as the pressure-carrying medium

An analytical model and its corresponding numerical algorithm for viscous pressure bulging process considering the effects of rate-dependent tangential adhesive stress

In conventional bulging process, it is usually assumed that sheet metal deforms under normal pressure regardless of the frictional force between the sheet and the pressure-carrying medium. However, there is a phenomenon that sheet metal is bulged under the combined action of normal pressure and beneficial frictional force, which is resulted from the special characteristics of the pressure-carrying medium. Effects of constant tangential stress (beneficial frictional force) caused by rubber or urethane have been studied analytically and experimentally in previous works. Yet, the tangential adhesive stress imposed on sheet metal by viscous medium is correlated with its flow velocity due to its rate-dependent property, which can be controlled and measured by loading equipment. This kind of rate-dependent tangential adhesive stress has been taken into account to establish an analytical model to analyze its effects on the viscous pressure bulging (VPB) process. Then a numerical approach is presented to solve the problem, which utilizes incremental strain to start an iteration loop by considering different magnitudes of adhesive stress. Results have shown that rate-dependent beneficial frictional force in VPB process can promote longitudinal deformation of the bulged part to help make the strain distribution more uniform. The location of maximum thickness strain point deviates from the pole the outer radius, which means that when polar strains remain at the same level, higher bulging height can be obtained for larger magnitude of rate-dependent tangential stress; this characteristic mainly accounts for the postponement of fracture instability of sheet metal in VPB. The theoretical predictions of strain components and geometric parameters are analyzed and compared with experimental results. The comparisons have shown good agreement to validate the analytical model.

Numerical and experimental investigation of parameters affect the forming load during rubber pad sheet metal forming

The flexible die forming of sheet metal with the aid of rubber pad as a pressure-carrying medium is one of the advanced forming technologies for sheet metal forming components. In this research, rubber pad forming process was used to produce cups with different shapes. The sheet made from low carbon steel with a thickness of 0.5 mm. The rubber pad has made from polyurethane with different values of hardness and thickness which are (50, 60 and 70) shore A, and (40 and 80 mm), respectively. This pad is used to apply counter pressure to form the sheets. The former block is with three different shapes, which are flat, hemispherical, and complex. A numerical study has conducted to look into the deformation style of this process by 2D-FE simulation under the ANSYS Workbench platform. Numerical and experimental results illustrate that the forming load increases with increase rubber hardness while it decreases with increase the rubber pad thickness.

Reducing the rebound effect in micro-scale laser dynamic flexible forming through using plasticine as pressure-carrying medium

Abstract Micro-scale laser dynamic flexible forming (μLDFF) is a new kind of high velocity forming (HVF) process, which uses laser shock wave pressure to realize thin metal sheets micro-forming. Rebound effect usually occurs when forming deep complex micro-features, which has a significant effect on the forming precision. This work firstly investigates the impact and rebound behaviors of workpiece in μLDFF with rubber medium to probe the main affecting factors of rebound effect, then proposes a new μLDFF with plasticine medium to reduce the rebound effect. With the increase of laser energy, the average surface roughness of impact-affected zone (IAZ) reduces firstly by flattening behavior, and then increases afterward due to coining-like operation. The 3D deviation analyses between workpieces and micro-die are performed to characterize the fitability. When laser energy is 675 mJ, bottom region of workpiece fits well with the micro-die. When laser energy is increased to 1020 mJ, bottom region of workpiece separates from the micro-die due to rebound effect. A finite element model considering the machining marks on micro-die surface is built, and the predicted results are validated by experimental results. Coefficient of restitution (COR) and the rebound ratio (α) are used to investigate the impact and rebound process. The results show that rebound effect is dominated by laser energy, surface morphology of micro-die, and loading duration. To effectively reduce the rebound effect, the plasticine layer is first introduced as pressure-carrying medium in this work. The deformation behavior of plasticine medium is like a Reiner-Rivlin fluid. So the plasticine medium closely contacts with the sample during forming, this is equivalent to indirectly increasing the stiffness of sample and prolonging the loading duration. In addition, the plasticine layer can absorb impact-induced reflection wave. So plasticine medium can effectively improve the forming precision.

Mechanism on increased sheet formability induced by tangential adhesive stress in sheet flexible forming process employing viscoplastic pressure-carrying medium

Abstract In flexible die forming processes employing semi-solid or elastic materials as pressure-carrying medium, there exist tangential adhesive stress or friction at medium/blank interface, which may improve sheet formability and has been experimentally verified. However, the mechanism behind this phenomenon has not been clarified yet. In order to fill this gap, analytical investigations were implemented in this paper based on previous studies involving through-thickness shear stress. Concretely, the analytical model of axisymmetric shell bulging process considering tangential adhesive stress and the extended M-K model under non-planar stress state were established at first. Accordingly, the relationship between tangential adhesive stress and through-thickness shear strain was deduced together with the relationship between through-thickness shear stress (strain) and sheet forming limit. On the basis of that, the viscous pressure forming (VPF) process was taken as an example to exhibit the characteristic that forming limit changes with tangential viscous adhesive stress and through-thickness shear stress (strain). It is shown that the tangential viscous adhesive stress has positive correlation with through-thickness shear strain which results in increased sheet formability. Tube bulging tests employing viscous medium with different molecular weight as pressure-carrying medium were conducted to give an experimental evidence of the proposed analytical results.

Controlling of material flow in the quasi-bulk forming of thin-walled corrugated rings through optimization of contact pressure

Corrugated rings made of difficult-to-deform superalloy sheet with extremely thin wall are hardly manufactured since the forming processes are rather sensitive to parameters, such as axial feeding, cavity pressure, contact pressure, and so forth. The quasi-bulk forming method proposed in the author’s previous work, which gets rid of axial compression, was demonstrated to be an effective solution. In this work, forming strategy based on the controlling of material flow in this process is further developed. Since the pressure-carrying medium (i.e., viscous medium) is of high viscosity, the sealing pressure of this process is lowered down in comparison with that of forming processes utilizing nonviscous fluid as forming medium. The contact pressure at blank ends becomes adjustable as a result. Effects of contact pressure at sealing/blank interface (Pm) on the blank draw-in were investigated numerically. An optimal load path of Pm was obtained. Regulatory mechanism of contact pressure was presented, and contact pressure was considered to be the resultant pressure caused by the static pressure (Ps) and cavity pressure (Pv). Thereafter, process optimization was conducted through adjusting the path of Pv to approximate to the optimal path of Pm. On the basis of that, geometrical parameters of die cavity were improved and forming experiments were conducted accordingly to form two sorts of nickel-based superalloy corrugated rings. Sufficient blank draw-in was observed and thickness thinning is acceptable. The forming strategy, which is based on the adjustment of contact pressure, was verified to be feasible to improve the manufacturability of thin-walled corrugated rings.

Determination of the flow stress of a magnetorheological fluid under three-dimensional stress states by using a combination of extrusion test and FEM simulation

Magnetorheological fluid (MR fluid), a kind of smart material, has been used as a new pressure-carrying medium in magnetorheological pressure forming (MRPF). The mechanical property of MR fluid under the pressure significantly affects the sheet formability. However, there is little knowledge on the deformation behavior of MR fluid under three-dimensional stress states. In this paper, a new procedure via a combination of extrusion test and FEM simulation has been proposed to determine the flow stress of MR fluids. The experimental device for extrusion test of MR fluids was designed. The flow stresses of a MR fluid (MRF-J01T) under four different magnetic fields were determined through the proposed procedure. In addition, the obtained flow stresses were used in the following FEM simulations to verify the accuracy by comparing with the experimental results. The simulation results were in good agreement with the experimental data, which supports the correctness and practicability of the proposed method.

Experimental Studies on Flexible Forming of Sheet Metals Assisted by Magnetic Force Transfer Medium

To improve the thickness uniformity and increase the forming limit of sheets to enhance their overall quality, a magnetorheological fluid (MRF) was injected into the punch cavity to act as the force transfer medium and fulfill the function of flexible pressing during the sheet bulging process. The rheological properties of the MRF were changed under the influence of a magnetic field produced by loading different currents, which allowed variation of stress states and deformation modes in the 0.75-mm-thick 304 stainless steel sheets. With increasing current (up to 3.5 A), the sheet-forming limit increased by 16.13% at most, and the fracture morphology experienced a certain change. Additionally, both the bulge height and the wall thickness distribution had obvious changes with a punch stroke of 10 mm. According to the experimental analysis, the MRF can be used successfully as a pressure-carrying medium in the sheet forming process.

Forming thin-walled circular rings with corrugated meridians via quasi-bulk deformation of metal blank and viscous medium

Excessive thinning of wall thickness caused by the unstable deformation of metals is the main failure mode in the manufacturing of thin-walled circular rings with complex section. A novel quasi-bulk forming method is proposed in this work to deal with this problem through achieving a stable state of metal flow. Metallic materials and viscous medium with different strain rate sensitive exponents (m) were respectively employed as core materials (i.e. pressure-carrying medium) in finite element analysis (FEA) to investigate the effects of core materials on the deformation behaviors of ring-shaped metal blank. Numerical results indicate that viscous medium and metal blank deform synchronously like a composite billet when appropriate viscous medium acts as core material. It is attributed to the adhesive attraction between viscous medium and metal blank caused by the high viscosity of viscous medium. In this condition, the composite billet easily fills the die cavity, which is similar to the way that round billets fill the die cavity in bulk forming processes. In order to verify the validity of quasi-bulk forming method, two types of thin-walled circular rings with corrugated meridians were formed through proposed method in the forming experiments. The maximum wall thickness reduction of formed parts is less than 8%. The present study demonstrates the quasi-bulk forming method can not only inhibit wall thickness thinning of thin-walled parts but simplify the process control. Thus, it may become a key process to fabricate thin-walled circular rings with large diameter variation per unit axial length (ψ).

Application of Magnetorheological Fluid in Bulge Tests of Sheet Metal

Magnetorheological (MR) fluid is a kind of smart material. In this paper, magnetorheological (MR) fluid is applied as a flexible-die, the rheological properties of which can be changed rapidly under a magnetic field, in sheet metal forming process. Bulge tests using a kind of MR fluid, MRF-J01T, are carried out on self-designed experimental device. Stainless steel 1Cr18Ni9Ti combined with different process parameters are adopted to investigate the effect of property of MR fluid on the formability of material tested. The results showed that MR fluid can be utilized successfully as a pressure-carrying medium in sheet forming process. The rheological properties of MR fluid can greatly influence the dome height and thickness strain distribution of specimens.

Sectional finite element analysis of coupled deformation between elastoplastic sheet metal and visco-elastoplastic body

The present paper is devoted to developing a new numerical simulation method for the analysis of viscous pressure forming (VPF), which is a sheet flexible-die forming (FDF) process. The pressure-carrying medium used in VPF is one kind of semisolid, flowable and viscous material and its deformation behavior can be described by the visco-elastoplastic constitutive model. A sectional finite element model for the coupled deformation analysis between the viscoelastoplastic pressure-carrying medium and the elastoplastic sheet metal is proposed. The resolution of the Updated Lagrangian (UL) formulation is based on a static explicit approach. The frictional contact between sheet metal and visco-elastoplastic pressure-carrying medium is treated by the penalty function method. Coupled deformation between sheet metal and visco-elastoplastic pressure-carrying medium with large slip is analyzed to validate the developed algorithm. Finally, the viscous pressure bulging (VPB) process of DC06 sheet metal is simulated. Good agreement between numerical simulation results and experimental measurements shows the validity of the developed algorithm.

Effect of Loading Location of Viscous Medium on Sheet Deformation in Viscous Pressure Bulge (VPB) Test

Viscous pressure forming (VPF) uses a highly viscous but flowable material as pressure-carrying medium (PCM). Due to the relative low flowability of viscous medium compared with fluid, nonuniform pressure distribution in viscous medium can be used to control and regulate the deformation sequence of the workpiece through controlling the loading mode of viscous medium. In the present study, viscous pressure bulge (VPB) tests with three kinds of loading location of viscous medium (central zone, corner zone and the whole deformation zone) are conducted and the influences of loading location of viscous medium on sheet deformation behavior are investigated via numerical simulations and experiments. It is found that changing the loading location of viscous medium can greatly affect the deformation behavior of sheet metal. When the viscous medium is injected from the die corner zone, a local high pressure formed at the corner zone of sheet metal and a higher limiting dome height and strains are obtained.

Prediction of wrinkling and fracturing in viscous pressure forming (VPF) by using the coupled deformation sectional finite element method

Viscous pressure forming (VPF), which uses a kind of semisolid, flowable and viscous material as pressure-carrying medium (PCM), is a new flexible-die forming (FDF) process of sheet metals. In VPF, wrinkling and fracturing are two major failure modes. It is more difficult to predict the occurrence of wrinkling and fracturing in VPF than the conventional sheet metal forming processes due to the coupled deformation of sheet metal and viscous medium. In this paper, a sectional finite element method (SFEM) for analysis of the coupled deformation of sheet metal and viscous medium is developed. The C-B wrinkling criterion based on the energy method and a ductile fracture criterion on the basis of Lemaitre damage theory are proposed to analyze the VPF process of aluminum ladder part via combining with SFEM. The wrinkling and fracturing defects occurred in VPF are thus predicted. The effects of blank holder pressure and the interface condition between sheet metal and viscous medium on the wrinkling and fracturing formation in VPF are investigated. Based on the simulation results of SFEM, the comparison with the experimental data is conducted and the efficiency of the developed approach is thus validated.

High strain-rate bulge forming of sheet metals using a solid bulging medium

The biaxial bulge-forming of sheet metals is a standard test to evaluate the formability and mechanical behaviour of materials. A new dynamic bulge-testing method is simulated and analysed in this study that can be performed on a conventional split Hopkinson pressure bar (SHPB) system. The aim of this work is to develop a numerical and theoretical technique for the analysis of materials and structures, which are dynamically loaded and subjected to the different levels of strain rates. The commercial finite element code ABAQUS/Explicit has been selected as the numerical test bed for the simulation of the dynamic bulging test. In the new bulge-forming technique, flexible rubber is used as the pressure-carrying medium instead of hydraulic fluid. The theoretical analysis is based on conventional hydraulic bulge test principles and SHPB relations. To verify the accuracy of the developed technique, analytical and finite element methods are compared and show good correlation with each other.

Numerical study on the deformation behaviors of the flexible die forming by using viscoplastic pressure-carrying medium

The flexible die forming (FDF) of sheet metal with the aid of viscoplastic pressure-carrying medium (VPCM) is one of the advanced forming technologies for forming complex sheet metal components with large plastic deformation. The technology has been used in industries by employing different VPCMs, the epistemological understanding of the deformation and process behaviors of this process, however, has not yet been fully addressed. In this paper, numerical study is conducted to look into the deformation behaviors of this process by explicit 3D-FE simulation under the ABAQUS platform, in which the counter pressure variations of VPCM is applied via user subroutine VDLOAD and the ductile fracture criterion is implemented by using VUMAT. Three case study parts, viz., barrel, conic and parabolic parts with large Limit Drawing Ratio (LDR) are studied. The comparison between the conventional deep drawing (CDD) and VPCM-based FDF is conducted in terms of wall-thickness reduction, hydrostatic pressure, principal stress distribution and damage factor. The uniqueness and the deformation behaviors of the VPCM-based FDF are then highlighted. The simulation results show that the higher VPCM pressure could result in the higher hydrostatic pressure throughout the process and further resist wall thinning and prevent fracture of the sheet metal. The formability is thus increased significantly.

Inner and outer pressure forming of nickel based super-alloy thin-walled part with variable diameter sections

A novel forming method of nickel based super-alloy thin-walled part with variable diameter sections was proposed by using inner and outer pressure with the visco-elasto-plastic pressure-carrying medium at room temperature, and the principle of the method was provided. Experiments and FE simulations were carried out to analyze the deformation characteristics for the part with larger variable diameter ratio (35%). The results show that visco-elasto-plastic pressure-carrying medium can meet the requirements of the room-temperature deformation condition for nickel based super-alloy sheet. The inner and outer pressure forming with the visco-elasto-plastic pressure-carrying medium can meet the requirements of dimensional accuracy for the thin-walled part with variable diameter sections. The thinning of wall-thickness is less than 4%. This method provides a new approach for near-net shape forming of nickel based super-alloy thin-walled parts with variable diameter sections.

EXPERIMENTAL STUDY ON EFFECT FACTORS OF VISCOUS ADHESIVE STRESS IN VISCOUS PRESSURE FORMING

Because of the coupled deformation characteristic between viscous medium and sheet metal, however, it is difficult to analyze the deformation behavior of sheet metal under the interaction of viscous adhesive stress. Based on the above situation, viscous medium tensile test is carried out by using a kind of polymethylvinylpolymer as viscous pressure-carrying medium. Using the viscous adhesive stress as tensile stress, sheet metal deformation can be measured directly. The influence of viscous adhesive stress on the deformation of sheet specimen is investigated and some effect factors of viscous adhesive stress are analyzed preliminarily. Experimental results show that the viscous adhesive stress increases with an increasing of pressure difference on the surface of sheet metal, flowing velocity and hydrostatic pressure of viscous medium. Research results provide a good foundation for utilizing viscous adhesive stress to improve the formability of sheet metal in viscous pressure forming.

Sheet bulging experiment with a viscous pressure-carrying medium

Usually, flexible die forming technology and conventional stamping technology have their own separate and limited application range, as well as their own particular shortcomings. So a more advanced forming method is needed to meet the requirements of increasing component complexity. Viscous pressure forming (VPF), which uses a semi-solid viscous material as the pressure-carrying medium, is suitable for forming parts requiring large sheet deformation using a sequential forming method. In this paper, a sheet metal bulging experiment with three kinds of pressure-carrying media, viscous medium, polyurethane and steel, was studied. The results show that the specimens formed with VPF, compared with those formed with polyurethane and steel, have less wall-thinning and consequently a more uniform wall thickness distribution, and the shape of part is closer to the desired hemisphere. When bulged with back-pressure, the formability of sheet metal can be improved significantly. Thus VPF is a promising sheet metal forming method.