High Velocity Forming Research Articles

Analysis of high velocity forming of metallic tubes

Metal forming is vital in many manufacturing industries for resizing and reshaping of raw metallic tubes and sheets. Conventional forming processes like forging, rolling, tube drawing and extrusion are associated with high-cost complex tools which are applicable for specific shape and size. High velocity deformation process utilizes impact energy for forming and deformation takes place at high velocity. This paper analyses the high velocity forming of 6061 aluminium tubes, formed under electromagnetic force. Numerical simulation is used to understand dynamic conditions of electromagnetic compressive forming.

Dynamic failure mechanism of copper foil in laser dynamic flexible forming

Abstract Laser dynamic flexible forming (LDFF) is a novel high velocity forming (HVF) technology, in which the foil metal is loaded by laser shock wave. Strain localization is readily to occur around the bulge edge, which will result in the ultimate dynamic failure. In this work, the microstructures before and after dynamic fracture are characterized by transmission electron microscopy (TEM) to investigate the dynamic failure mechanism. The plastic deformation regions of copper foil are composed of shock compression, strain localization and bulge. Microstructure refinement was observed in three different plastic deformation regions, particularly, dynamic recrystallization (DRX) occurs in the strain localization and bulge regions. In bulge region, extremely thin secondary twins in the twin/matrix (T/M) lamellae are formed. The microstructure features in the strain localization region show that superplastic flow of material exists until fracture, which may be due to DRX and subsequent grain boundary sliding (GBS) of the recrystallized grains. The grain coarsening in strain localization region may degrade the material flowing ability which results in the dynamic fracture.

A Comparative Study on Formability of a Different Thickness FSWed AA 5052 Blank by Conventional and EM Forming

Abstract Difficult-to-weld aluminum alloys of different thicknesses in thin sheets of 1 mm to 1.5 mm are welded by Friction Stir Welding and then mechanically analyzed. Successful welds are obtained at constant tool spindle speed of 1,500 r/min and at two tool traverse speeds of 68 mm/min and 93 mm/min. All the specimens failed at the weld region during tensile testing. Microstructure analysis suggests dynamic recrystallization with a finer grain size at the weld nugget and ductile modes of failure during tensile fracture with sharp sheared dimples at fracture surface. The welded blanks of AA 5052 H32 were then formed by an Electromagnetic Forming (EMF) process to characterize the forming behavior by high velocity forming. Furthermore, the EMF process was compared to a quasi-static forming process. It and a formability analysis were done in terms of a Limit Dome Height test and Forming Limit Diagram. It was found that the formability of welded blanks increases considerably with the EMF process, against the conventional process. However, the tensile strength of the welded blanks was lower than that of base materials, and hence, formability showed the same trend.

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.

Deformation and fracture behaviors of copper sheet in laser dynamic flexible forming

Laser dynamic flexible forming (LDFF) is a novel high velocity forming (HVF) technology, in which the high-energy laser shock waves act on the workpiece via a flexible medium. Dynamic bulge tests of copper sheet are performed to investigate the deformation and fracture behaviors in LDFF in this paper. The laser ablation states on K9 glass surfaces under different laser energies were characterized. The samples after LDFF were characterized by optical microscopy and scanning electron microscopy. For laser energy ranging from 1020 mJ to1380 mJ, as laser energy increases, the average surface roughness (Ra) in bulge region rapidly increases due to surface free forming. However, the Ra in laser shock compression region slowly decreases due to shock flattening. Strain localization occurs around the bulge edge, and the composition of oxygen element in this region is increased with laser energy. When laser energy is equal to or greater than 1690 mJ, localized deformation evolved into the dynamic failure. Apart from the elongated micro-voids, three interesting phenomena were observed: the melting marks, the serrated fracture boundaries and filaments. As the laser energy increases, the fracture mode varies from a tensile fracture mode to a shear fracture mode, then to a filament fracture mode. The filament fracture mode indicates that superplastic flow of material may exist until fracture, and oxidation behaviors make a great contribution to the superplastic flow.

Simulation of electrohydraulic free forming of DP600 sheets using a modified Rousselier damage model

Electrohydraulic forming (EHF) is a high-energy rate pulsed forming process in which the expansion of the plasma channel formed by the discharge of electrical energy between two electrodes immersed in water results in the high-velocity forming of the sheet material. This process is shown to have the capability of increasing the formability of sheet materials beyond conventional limits and being applicable to less ductile materials. In this work, DP600 sheet specimens were deformed in uniaxial tension, plane strain and biaxial tension using electrohydraulic free forming (EHFF). A modified Rousselier ductile damage model was then employed to predict the forming and damage behaviour of these specimens deformed along each strain path. This modified Rousselier model includes a modified Johnson-Cook hardening model as well as a void nucleation function and a void coalescence criterion. The limiting strains, the distribution of the scalar damage variable and the final damage morphology obtained from the numerical simulations were compared to the experimental results in order to evaluate the accuracy of the proposed micromechanical damage model. It is shown that predicted strains, damage accumulation and the final damage geometry of DP600 sheet specimens using the modified Rousselier model are in good agreement with experimental results as well as with those predicted by other phenomenological constitutive models.

Electromagnetic forming analysis of AA5182 at elevated temperatures

Electromagnetic forming is a high-velocity, unconventional forming process, in which a high magnetic field is used to form a metal workpiece. Light weight metals such as aluminium and magnesium and their alloys are generally formed through this type of forming process. Increasing the temperature would lead to a decrease in the flow stress of a material. Thus, a metal can be deformed easily at elevated operating temperatures for the case of conventional forming. In this paper, high-velocity electromagnetic forming analysis of aluminium alloy AA5182 has been carried out in a commercially available FEM package, LS Dyna®. The deformation behaviour, in the case of electromagnetic forming, has been studied for various elevated temperatures in order to determine the feasibility of the process at high temperatures.

Thermal effects in magnetic pulse forming of magnesium alloy sheet

Thermal effects in magnetic pulse forming of magnesium alloy sheet were investigated by finite element software ANSYS. The adiabatic temperature produced in magnetic pulse forming was due to the high forming velocity. Distribution of adiabatic temperature is affected by discharge parameters, such as voltage and capacitance. Distribution of adiabatic temperature is predicted by C-P model and J-C model which are always used to describe the forming behaviors of material in high velocity forming. Predicted distribution of adiabatic temperature by C-P model and J-C model increases with increasing of discharge voltage, but decreases clearly with capacitance increasing, particularly at the center position of the deformed part. The highest temperature is 77.7 and 94 °C at C = 1152 μF/U = 2.75 kV (limit forming parameters) predicted by C-P model and by J-C model, respectively. The relationship between velocity and adiabatic temperature is also analyzed. Preliminary study for the effect of adiabatic temperature on the micro-structure of AZ31 sheet in magnetic pulse forming is explored by the analysis for fracture morphology of the deformed sheet.

Response of shock wave deformation in AA5086 aluminum alloy

The AA5086 aluminum alloy sheets with different starting textures were subjected to shock wave deformation with an input impulse of ~0.2 Ns. Microstructural examination indicate no significant change in grain size; however, the evolution of substructure manifesting intra-granular misorientation was evident. The improvement in hardness indicates the absence of recovery and strain hardening during shock deformation. Shock deformed samples show characteristic texture evolution with high Brass {110}〈112〉 component. The study demonstrates the viability of high velocity forming of AA5086 aluminum alloy sheet using shock wave.

A model for deformation in high energy rate micro forming process

This paper presents an investigation on high energy rate micro forming (HERMF) of a thin aluminum foil using a high velocity forming (HVF) technique. A model is proposed for the work of deformation and efficiencies involved in every stage of energy transfer. The forming action has been achieved through incident shockwaves, which cause plastic deformation in the blank to take the shape of the die cavity. The shockwaves are generated by a rapid capacitive discharge of energy across a fuse wire inside the water. The paper presents an analysis to predict the amount of energy required for a desired depth of hemispherical deformation, based on the principles of plastic deformation and volume constancy. The energy transferred from the capacitor bank to the fuse wire is modeled using Kirchhoff’s voltage law and the Joule heating principle. An indigenous micro-electro-hydro-forming (EHF) setup developed in-house is used to validate the analytical model developed through a set of experiments. Readings are optimized against the number of parameters present in the entire process. Validation for the suggested analytical model is carried out by comparing theoretical and experimentally measured values of the depth of deformation of a 20-μm thick aluminum foil. The results obtained are encouraging, and the highest error in the predictive ability of our model was found to be 30 %. To the best of our knowledge, this is the first attempt at analytically modeling the depth of deformation for a given value of supplied electrical energy in the micro-domain.

Electromagnetic Formability of an Aluminium Ice Tray

Electromagnetic forming (EMF) is a typical high speed forming process using the energy density of a pulsed magnetic field to form work sheets made of metals with high electrical conductivity like aluminium alloys, which have low formability at low strain rate. Under high velocity forming, metallic materials exhibit an increase of flow stress and ductility with increasing deformation rate. Therefore, materials of lower ductility can be deformed to higher strains using high strain rate deformation processes such as electromagnetic or explosive forming techniques. In this paper analysis of an electromagnetic sheet metal forming process is carried out by using commercial finite element software LS-DYNA®, which incorporates a sequential coupling method involving electromagnetic field, structural and thermal solutions. Study of process parameters for forming aluminium ice tray (used in refrigerator) by Electromagnetic forming process has been carried out, simulation was done involving a die, Al worksheet and a spiral coil.

State of the art of electromagnetic energy for welding and powder compaction

The innovation of advanced materials and their application to engineering structures can be related to their ease of fabricating defect-free parts. The identification of various energy sources has resulted in various new processing techniques in manufacturing and electromagnetic energy is one which offers promising qualities for high-velocity forming, welding, and compaction of metals. Bimetallic joint designs were proposed in automobiles to reduce the weight and to take advantage of properties of different metals in the same construction. Joining by fusion is impossible in certain cases due to huge differences in melting point, thermal conductivity, volumetric specific heat, and coefficient of thermal expansion. In the present study, magnetic-pulse welding of a Cu/Al joint and an ODS alloy is investigated. The formation of intermetallic phases and interface morphology is presented along with a quantitative analysis for temperature in the Al/Cu joint. Also, initial investigations on the application of electromagnetic energy for compaction of aluminum powders are reported.

Computational Fluid Dynamic Analysis for Modified Steam Duct in Air Cooled Condenser

Cooling system is a subsystem of a power plan that has an indispensable effect on efficiency of power plant. Air Cooled Condenser (ACC) system as a cooling system, is usually used in many power plants. Steam duct as a part of ACC conveys the steam flow with high velocity form steam turbine exhaust to head of tube bundle which is located at height of 25 to 30 meters. In this paper, pressure drop in steam duct is calculated using computational fluid dynamic method. Then, in order decrease pressure drop and consequently increase ACC efficiency, steam duct configuration is modified. The steam flow is modeled as an incompressible, turbulent and single phase flow. The solution strategy is based on SIMPLE and Segregated Implicit algorithm and the k-ε RNG model for turbulence are employed. The second order upwind differencing scheme are applied to discrete governing equations. Numerical results confirm reducing of total pressure drop in modified diffuser comparing with primary geometry.

Finite Element Analysis of Formability of Metallic Sheets in High-Velocity Forming Processes

Finite Element Analysis of Formability of Metallic Sheets in High-Velocity Forming Processes

Effect of velocity on ductility under high velocity forming

Effect of velocity on ductility under high velocity forming

Formability of steel sheet in high velocity impact

High velocity deformation can be quite effective in increasing the forming limits of metal sheets as well as effectively treating some other common metal forming problems, such as wrinkling and distortion. This study examines the high velocity formability of cold rolled sheet steel as developed in impact with a curved punch. To study forming over large areas, a steel sheet sample was electromagnetically launched, at a shaped punch at velocities of 50–220 m/s, using a flat spiral electromagnetic coil and an aluminum driver sheet. Estimates of impact velocity were obtained with the help of a high-speed camera. It was observed that the impact velocity of the samples increased nearly linearly with the energy of launch. The failure strains of all the steels were dramatically increased beyond those obtained in tensile tests as expected from the forming limit diagram. The forming limits of steels with both very low and high quasi-static ductility were similar in high velocity forming. Formability seemed to depend largely on local boundary conditions as dictated by the punch/tool geometry used. The strain distribution obtained depended on the shape of the punch used for impact. Thus it appears that in high velocity impact, the quasi-static ductility of the material is not of primary importance to the material's formability. At high velocities, high strains to failure can be obtained with steels having very low quasi-static ductilities. Based on these observations, one may be able to develop forming operations that take advantage of this dramatic improvement in formability. This could enable the use of high-strength low-cost materials in a number of complex components.

The failure of spherical particles under impact

A wide ranging study has been carried out of the forms and mechanisms of failure in solid spheres impacting hard, plane solid surfaces over a range of diameters, impact speed and impact angle. Results illustrated here include examples of soda-lime glass and PMMA. In general the forms of failure observed vary with impact velocity. However, although there is a range of low velocity forms, most materials tend towards a common high velocity form. At the highest velocities, propagating fracture fronts appear as the mechanism of comminution of glass spheres into fine fragments. The dynamic processes behind these velocity variations are discussed in this paper, as well as the influence of material properties.

Hyperplastic Forming: Process Potential and Factors Affecting Formability

AbstractThis paper has two distinct goals. First, we argue in an extended introduction that high velocity forming, as can be implemented through electromagnetic forming, is a technology that should be developed. As a process used in conjunction with traditional stamping, it may offer dramatically improved formability, reduced wrinkling and active control of springback among other advantages. In the body of the paper we describe the important factors that lead to improved formability at high velocity. In particular, high sample velocity can inhibit neck growth. There is a sample size dependence where larger samples have better ductility than those of smaller dimensions. These aspects are at least partially described by the recent model of Freund and Shenoy. In addition to this, boundary conditions imposed by sample launch and die impact can have important effects on formability.

Energy-loss rates of heavy and light charged particles in a two-dimensional electron gas

Energy-loss rates for charged external particles moving in a two-dimensional electron gas of zero temperature are calculated. The transition probability per unit time for excitations in the host system is determined by using weak coupling, i.e., linear-response theory. Contributions arising from electron-hole and collective excitations to the total loss rate are separated at the levels of the random-phase approximation and a generalized mean-field approach for the linear-response function. Substantial phase-space reduction in the excitations of the fermion system is demonstrated for light external particles and interpreted as the effect of recoil. The results obtained for heavy external particles are compared with recent strong-scattering predictions based on the kinetic theory of the energy-loss process. A kinematical effect that influences the high-velocity form of the loss rate for different masses of the projectiles is pointed out.

Electron velocity distribution function in a fully ionized plasma slab with a large temperature gradient: solar flare transition region

The high velocity tail of the electron distribution function has been calculated by numerically solving the high velocity form of the Landau equation for a thermal structure representative of a solar flare. The calculations show an anisotropy and an enhancement of the tail populations above the Maxwellian value throughout the flaring loop.