Showing papers in "International Journal of Material Forming in 2009"

Investigations on the thermal behavior of ultra high strength boron manganese steels within hot stamping

Abstract: The increasing demand for high strength steels regarding light weight construction design for the body in white is enforcing the automotive industry to onset more and more innovative forming technologies due to the reduced formability of such high strength steel grades. The hot stamping of boron manganese steels is hereby one of the main forming processes to be named. Besides the commonly used forming strategy where a homogeneous distribution of the material parameters is achieved, the partial press hardening is an innovative technology to be able to manufacture parts with a locally different strength and ductility profile. The thermal properties needed for this kind of process strategy had been determined using a heatable quenching tool developed at the Chair of Manufacturing Technology and are further discussed in this paper.

A viscous pressure bulge test for the determination of a plastic hardening curve and equibiaxial material data

Abstract: The importance of an accurate material modeling for the accuracy and reliability of sheet forming simulations has become increasingly evident during the last years. More advanced material models have, however, to be supported by novel methods for material characterization. The recent eight parameter yield functions Yld2000-2d and BBC2003 demand, besides data from the ordinary uniaxial tensile tests, also equibiaxial data. In the present paper a Viscous Pressure Bulge (VPB) test is described. The test yields the equibiaxial stress point and r-value, as well as a plastic hardening curve for large values of plastic strain. The test setup is based on an ARGUSS™ optical measuring system, and provides the desired result data in a very smooth and easy way. In order to verify the results from the current test, comparisons have been made with compression tests performed at Corus RD&T and hydraulic bulging tests performed at RWTH in Aachen. A discussion on how to determine the equibiaxial yield stress and how to transform the biaxial stress-strain curve to an effective stress-strain curve is included in the paper.

Effect of abrasive water jet machining parameters on aramid fibre reinforced plastics composite

Abstract: This paper presents a study on the effect of abrasive water jet machining (AWJM) process parameters on surface roughness (R a) and kerf taper ratio (T R) of aramid fibre reinforced plastics (AFRP) composite. Taguchi’s design of experiment was used as the experimental approach. Through analysis of variance (ANOVA), it was found that the traverse rate was considered to be the most significant factor in both R a and T R quality criteria. R a and T R were reduced as increasing the hydraulic pressure and reducing the standoff distance and traverse rate. However, there was no clear pattern for abrasive mass flow rate on both R a and T R. Therefore, it was confirmed that increasing the kinetic energy of water jet may produce a better quality of cuts. Mathematical models were also developed using multiple linear regression analysis to predict the performance of R a and T R in terms of AWJM process parameters. Considerably, the models are useful in predicting R a and T R in AWJM of AFRP laminate as shown in present study.

Joining of hot-dip coated steel sheets by mechanical clinching

Abstract: The joinability of hot-dip coated steel sheets using a mechanical clinching was investigated by an experiment. The distributions of the layer thickness of Zn alloy coating of the joined sheets were examined. Although the sheets were joined, the layer of coating on the side wall of the punch was scratched and the layer thickness on the bottom of the die was reduced. In order to increase the layer thickness of coating of the joined sheets, shapes of the punch and die were optimised by controlling of the deforming behaviours of the sheets using means of the finite element simulation. The layer thickness of coating of the joined sheets was improved by the modification of the punch and die shapes.

A method for the layer compression test considering the anisotropic material behavior

Abstract: The layer compression test is used in the field of material characterization to determine the onset of yielding and the hardening behavior of a sheet metal within uniaxial compression loading. For testing anisotropic materials the experimental setup has to be adjusted and the evaluation has to be adapted. This could be achieved with the installation of a second optical strain measurement system to get data of both symmetry axes. In the following paper the experimental procedure with two strain measurement systems and the evaluation methodology regarding the two sets of data will be presented and validated with experimental results. The advantage of the layer compression test is discussed in comparison with the results of a standard tensile test.

Formability of Advanced High Strength Steels

Abstract: Novel draw-bend tests of 3 dual-phase (DP) steels utilizing velocity control of both actuators revealed three patterns of failure depending on draw speed, draw speed ratio, and R/t ratio. Shear failure occurs preferentially for smaller R/t and higher deformation rates. During draw bend tests, the temperature rises are significant, up 100°C before necking, with consequent loss of strength in affected regions. A novel 1-D constitutive equation relating flow stress to strain, strain rate, and temperature was developed based on tensile tests. FE simulations using the measured constitutive response predicted the types of failures accurately, without introducing damage mechanics. The deformation-induced heating is a critical part of the failure process.

Temperature dependent friction modeling for sheet metal forming

Abstract: Stainless steel in sheet metal forming processes show a hardening behavior, which can be described only in dependency of the deformation and temperature history. Because of the temperature influence to the material properties, the temperature dependence of the friction in the process has to be taken into account. Friction tests using different temperatures showed a change of the friction regime. From the experimental observation the temperature and velocity dependence of the friction was modeled and integrated in a finite element code for metal forming. On the macroscopic scale the temperature and velocity dependent friction was integrated in a FEM code of metal forming. The FEM simulation has been applied to the biaxial stretching test and compared with the experiment. The numerical results showed a good agreement with the failure behavior of the stainless steel.

Comparison of two different simulation algorithms for the electromagnetic tube compression

Abstract: Simulation tools especially for the electromagnetic forming process (EMF process) offer the opportunity to analyse the formation process and its effects already in the development stage. The EMF process is a high-dynamic process with high transformation velocities. The deformation results from the interaction of the electrical current in the forming coil and the induced eddy current in the workpiece, represented by the Lorentz force. Most of the simulation tools and models for the EMF process are limited to small deformations. The connection between electromagnetic field and spatiotemporal evolution of the deformation of the workpiece is not negligible for example for working out a door handle recess in the automotive industry. This paper presents a comparison of a custom loose-coupled simulation model and a more accurate sequential-coupled approach. The deviations between the two simulation models increase with time, so that the loose-coupled approach leads to an overestimation of the final deformation. The correspondence of the sequential-coupled simulation with the measurement result is much better than for the loose-coupled simulation.

Design of an experimental setup to measure tool-ply and ply-ply friction in thermoplastic laminates

Abstract: Friction plays an important role in forming processes of thermoplastic laminates. A model is currently developed to predict the frictional properties of these materials and validation experiments are being conducted. The development of a friction measurement rig for thermoplastic composites is not straightforward. Controlling the transverse pressure profile on the laminate is essential for accurate measurements. Mould deflections and the setup type used (pull-out versus pull-through) have a significant effect on homogeneity of the profile. This is validated by measurements of the pressure profile and analyses of the experimental setup. In turn, the inhomogeneity of the pressure profile can make the experimentally obtained friction properties unsuitable for model validation.

MK modelling of sheet formability in the incremental sheet forming process, taking into account through-thickness shear

Abstract: This paper presents experimental measurements of the Through-Thickness Shear (TTS), (also known as out-of-plane shear) that occurs during Single Point Incremental Forming (SPIF) of low carbon steel into cone-shaped geometries. The measurements show the dependence of TTS on the cone wall angle. Also formability predictions are presented, using a Marciniak-Kuczynski (MK) type of forming limit model which can take TTS explicitly into account [1]. It is seen that the presence of TTS in the process delays the onset of localized necking and thus can be a contributing factor to the very high formability during SPIF that is observed.

Investigation on the effect of curvature on forming limit prediction for aluminium sheet alloys

Abstract: The investigation of bendability of sheet metal alloys is of great importance for the evaluation of process robustness in production of hemmed sheet metal assemblies. Furthermore, material cracks due to bending can also occur at deep drawing operations. This contribution focuses on the experimental determination of material’s formability at deep drawing, the characterisation of bendability especially for hemming operations and the determination of formability under combined deep drawing and bending loads, respectively.

Substructuring in the implicit simulation of single point incremental sheet forming

Abstract: This paper presents a direct substructuring method to reduce the computing time of implicit simulations of single point incremental forming (SPIF). Substructuring is used to divide the finite element (FE) mesh into several non-overlapping parts. Based on the hypothesis that plastic deformation is localized, the substructures are categorized into two groups: the plastic—nonlinear—substructures and the elastic—pseudo-linear—substructures. The plastic substructures assemble a part of the FE mesh that is in contact with the forming tool; they are iteratively updated respecting all nonlinearities. The elastic substructures model the elastic deformation of the rest of the FE mesh. For these substructures, the geometrical and the material behaviour are assumed linear within the increment. The stiffness matrices and the internal force vectors are calculated at the beginning of each increment then they are statically condensed to eliminate the internal degrees of freedom (DOF). In the iteration process the condensed stiffness matrices for the elastic substructures are kept constant. The condensed internal force vectors are updated by the multiplication of the condensed stiffness matrices and the displacement increments. After convergence, any geometrical and material nonlinearity for the elastic substructures are nonlinearly updated. The categorization of substructures in plastic and elastic domains is adapted during the simulation to capture the tool motion. The resulting, plastic and condensed elastic, set of equations is solved on a single processor. In an example with 1600 shell elements, the presented substructuring of the SPIF implicit simulation is 2.4 times faster than the classical implicit simulation.

Manufacturing of lightweight frame structures by innovative joining by forming processes

Abstract: The majority of the materials used in modern lightweight frame structures are aluminium alloys. But some aluminium alloys, e.g. EN-AW6060, are not or hardly joinable using conventional joining techniques. In this case innovative joining methods like friction stir welding; joining by dieless hydroforming or by electromagnetic compression lead to good joint’s quality. Friction stir welding has been widely investigated for joining sheet metal, but joining tubular workpieces creates new challenges in handling and processing, which will be described here. Joining by Dieless Hydroforming has been introduced in both plant and automotive engineering in the 1970ies. This joining method was used to join heat exchangers or camshafts made of steel. Recently, joining of aluminium has been targeted and is addressed below. Joining by Electromagnetic Compression offers the opportunity to manufacture high strength joints by e.g. form-fit. Groove’s design has been adopted targeting failure of the basic tube material, which is described here.

Regularization and perturbation technique to solve plasticity problems

Abstract: This paper investigates new procedures to solve plasticity problems by using the asymptotic numerical method (ANM). As the elastic-plastic behavior involves two unilateral conditions, we replace these two conditions by regular functions depending upon the stress field and its time derivative which permits one to take into account elastic-plastic transition and elastic unloading. Several applications in structural plasticity problems are presented to assess the ability of the proposed algorithm.

Thermal stability of laser treated die material for semi-solid metal forming

Abstract: This paper presents laser surface modification work performed to improve the lifetime of die materials. Die material AISI H13, with typical hardness in the range of 42 to 48 HRC, offers high wear and corrosion resistance. However the cyclic high temperature conditions along with exposure to high viscosity molten metal in semi-solid forming cause the die to wear and crack with resultant shortened die lifetime. In this study, the thermal stability of die material at elevated temperature was investigated through micro-hardness testing and a metallographic study. AISI H13 samples were laser glazed using CO2 continuous wave mode laser with 10.6 µm wavelength. Samples were attached to a specially designed rotating chuck to enable it to be rotated at speeds up to 1500 rpm and allow flat surface glazing to take place. The micro- hardness was measured for as-glazed samples and annealed samples which were held at temperatures ranging from 550oC to 800oC with 50oC intervals. The metallographic study conducted examined the formation of three zones at different depths which were the glazed zone, the heat affected zone and the substrate. As a result of rapid heating and cooling from the laser glazing process, a metallic glass layer was developed which exhibited an average micro-hardness of 900 HV when exposed to 3.34E+10 W/m2 laser irradiance within a range of 0.0011 to 0.0018 s exposure time. Crystallization in glazed zone increased as the annealing temperature increased. As the annealing temperature reached above approximately 600oC, the micro-hardness decreased to approximately 600 HV (equivalent to approx. 54 HRC) due to local crystallization. These findings show potential direct application of glazed dies for non-ferrous semi-solid forming and the requirement for thermal barrier protection for application at higher temperatures.

Numerical simulation of single and dual pass conventional spinning processes

Abstract: Due to the complex nature of sheet metal spinning processes, recent trends in analysis of the process are moving toward numerical techniques. These numerical methods, for instance finite element modelling, enable the study of parameters that can not easily be measured directly such as transient strains and stresses. Additionally, it allows a prediction of dynamic instabilities that may be used to control and achieve better product quality. In this investigation, a finite element dynamic explicit model has been used to simulate single and dual pass conventional spinning processes. The initial models are validated against published experimental data and show very good correlation. A variety of roller feed rates, roller passes and roller configurations are then simulated. Effects of roller feed rate on the axial force, radial force and thickness strain are established. The effect of roller pass and roller configuration on the axial force and thickness strain are also assessed.

Investigations on hard machining of Impax Hi Hard tool steel

Abstract: In this paper, experimental investigations are carried out by end milling process on hardened tool steel, Impax Hi Hard (Hardness 55 HRC) a newly developed tool steel material used by tool and die making industries. Experiments are performed with an aim to study performance investigations of machining parameters such as cutting speed, feed, depth of cut and width of cut with consideration of multiple responses viz. volume of material removed, tool wear, tool life and surface finish to evaluate the performance of PVD coated carbide inserts and ball end mill cutters. It has been observed through scanning electron microscope, X-ray diffraction technique (EDX) that chipping and adhesion are active tool wear mechanisms and saw-toothed chips are formed while machining of Impax Hi Hard steel. It is also noticed out that tool life is not enhanced while machining with minimum quantity lubricant than dry machining. From the investigations, it is observed that hard machining can be considered as an alternative to grinding and EDM, traditional methods of machining difficult-to-machine materials i.e. hardened steel with hardness greater than 50 HRC with a scope of improved productivity, increased flexibility, decreased capital expenses and reduced environmental waste.

Process and tool design for precision forging of geared components

Abstract: Precision forging is an innovative manufacturing process for the flash less, near-net-shape production of high performance components. Outstanding material characteristics as well as a reduced process chain and a high material efficiency are the essential advantages of precision forging. Only defined functional surfaces need a finishing after the forging and integrated annealing process. The increased mechanical and dynamical component strength forward the trend of light weight construction. The requirements regarding accuracy in dimension demand high standards of process and tool design. An important part of the current research work is beyond the basic process design and tool construction the correction of geometry and material influences on the shrinking of the hot forged parts.

Semi-discrete shell finite elements for textile composite forming simulation

Abstract: The composite textile reinforcement draping simulations allows the conditions for a successful process to be determined and, most importantly, the positions of the fibres after forming to be known. This last point is essential for the structural computations of the composite part and for resin injection analyses in the case of LCM processes. Because the textile composite reinforcements are multiscale materials, continuous (macro) approaches and discrete (meso) approaches that model the yarns have been developed. The finite element that is proposed in this paper for textile fabric forming is composed of woven unit cells. The warp and weft directions of the woven fabric can be in arbitrary direction with respect to the direction of the element side. This is very important in the case of multi-ply deep drawing and when using remeshing. The element is efficient because it is close to the physic of the woven cell while avoiding the very large number of unknowns in the discrete approach.

Experimental study of heat transfer in hot stamping process

Abstract: Hot stamping and quenching of boron steel is a novel technology to produce structural automotive parts with an excellent crash performance. It offers the opportunity to produce thinner parts with complex shapes and higher mechanical properties. In order to increase the simulation expertise of the hot stamping process,, an experimental device has been designed and developed to accurately measure the thermal contact resistance (TCR) under representative process conditions. This parameter characterizes the heat transfer intensity between the tools made in Z160 steel and Usibor 1500P blank (a precoated carbon/manganese steel) at the contact interface. In this paper, we present an approach to determine experimentally the evolution of the TCR under different contact pressure (2 to 30 MPa).

Analysis of forces, accuracy and formability in positive die sheet incremental forming

Abstract: Sheet Incremental Forming (IF) is a forming technology which consists of a sheet clamped along its edges by a suitable blank-holder while an hemispherical head punch is moved along a defined path and locally deforms the sheet. To improve the part geometrical accuracy, a die (which can be positive or negative) is placed behind the sheet with respect to the punch position. In this case, the process is called Two Point Incremental Forming (TPIF). In the present paper TPIF with positive die was studied through an experimental tests campaign using deep drawing steel sheets. The die geometry used in this research was chosen so to be representative of the process and it was tested using different tool paths. Forming forces were measured using self designed instrumented punch and table while the final workpiece geometry was detected using a coordinate measuring machine (CMM). The collected data allowed to study the effects of the different tool paths on the maximum forming forces, the geometry errors, the maximum reachable wall angle and the drawing depth during forming and after springback. The results were also compared with a previously performed experimental campaign where similar tests with negative die were conducted.

Formulation of springback and machine setting parameters for multi-pass three-roller cone frustum bending with change of flexural modulus

Abstract: Roller bending process can be used to plastically deform a sheet or plate to hollow shapes of constant (i.e. cylindrical, elliptical) or varying (i.e. cone frustum) cross sections. Cylindrical and conical shells are the basic components of vessels and structures used for the various engineering applications. Reported work discuss the formulation of springback and machine setting parameters for continuous multi-pass bending of cone frustum on three-roller bending machines with non-compatible (cylindrical) rollers. Effect of change of flexural modulus during the deformation on springback prediction is analyzed. Effect of plate initial radius, number of bending passes and step interval for two successive bend radii on the springback is presented. Analytical results of multi-pass cone frustum bending were verified with the cone frustum bending experiments and found to be in good agreement. Work reported is expected to improve the accuracy and productivity by facilitating the fabrication engineers.

Accurate 3D friction stir welding simulation tool based on friction model calibration

Abstract: Friction Stir Welding (FSW) is one of the most effective solid states joining process and has numerous potential applications in many industries. A FSW numerical tool, based on Forge® F.E software, has been developed. Its main features are an Arbitrary Lagrangian Eulerian (ALE) formulation and an adaptive remeshing procedure based on error estimation. A 3D FSW simulation based on friction models calibration has been presented using Eulerian and ALE formulation. Two friction models have been studied to model friction in the tool-plate interface in aluminium alloy 6061-T6: Norton’s and Coulomb’s. Comparisons with experimental results considering various travel speed has been performed.

An experimental investigation of cylindrical wire electrical discharge turning process using Taguchi approach

Abstract: This paper is concerned with Cylindrical Wire Electrical Discharge Turning processes being especially developed for difficult to machine materials. First, the effects of the machining parameters power, voltage, pulse off time, and spindle rotation rate on the machining performance, which comprises the material removal rate, the surface roughness, and the roundness, was studied. The machining parameters were evaluated by Taguchi's method. In particular, an L9(34) standard orthogonal array was employed as experimental design while determining the influence on the machining performance using the analysis of variance technique. In a second step, an optimisation of the machining parameters was sought using signal to noise ratio analysis. Therefore, the response surface methodology was determined in a regression analysis, which was used to model the influence of the parameters on the performance. Final experiments with a sophisticated home-made rotary spindle allowed the quantification of the effectiveness of the proposed method.

Innovation by forming technology: motivation for research

Abstract: This paper focuses on new forming techniques for the improvement of product quality, the reduction of process chains and the saving of our available resources. The presented and discussed technologies cover extrusion of reinforced profiles, bending of profiles, single point incremental forming of polymers, and tool systems made of polymers coated by hard metal as well as re-manufacturing strategies. Beside the process principles and advantages several results from numerical and experimental investigations will be given in order to show the motivation for fundamental and application oriented research in the field of forming technology.

Improvement of mechanical properties of AL (A356) cast alloy processed by ECAP with different heat treatments

Abstract: Severe plastic deformation (SPD) by equal channel angular pressing (ECAP) has been used as a reliable method to achieve an ultra-fine and homogenous microstructure in the bulk materials. In this research, study was conducted on the structures and mechanical properties of A356 (7%Si) Al alloy produced by severe plastic deformation through ECAP. Special attention was paid to the effect of heat-treatments such as annealing, solution, quenching and pre/post-aging on the properties of the materials. The heat treated as cast structure was successfully deformed in ECAP process. The yield strength, ultimate strength, and hardness of the ECAP samples were improved after pre/post-artificial aging. Mechanical properties improvement was achieved after ECAP process in the samples with the pre-aging treatment. The results show near 114% improvement of the ultimate tensile strength (UTS) and 166% elongation from the as-cast structure. From experimental work, it was also found that the acceleration of precipitation kinetics after ECAP process is not important factor to achieve improvement of mechanical properties, but distribution of Si particles with precipitates formed during T6 is important in high UTS achievement along with high elongation.

Finite element study and sensitive analysis of the deep-drawing formability of commercially pure titanium

Abstract: This paper describes a 3D finite element model used for optimizing the deep-drawing formability of a commercially pure titanium cap designed for the cosmetic industry. The results obtained from a specific tooling and various tensile tests highlight a strong anisotropic behavior of the material (earing profile on the parts and Lankford coefficients very sensitive to the loading direction). The mechanical behavior taking this anisotropy into account is described according to an elastic-plastic model based on the quadratic Hill’s criterion. A special attention is paid to studying the sensitivity of the FEM predictions with respect to the numerical parameters. The type and number of elements in the thickness of the blank and the friction coefficient have a significant influence on the numerical results. The comparison with the experiments taking the springback into account shows that the FE model is suitable for describing the behavior of titanium during a forming process such as deep-drawing. A Forming Limit Diagram is finally given to predict the feasibility and optimize the forming operation.

Improved predictability of forming limit curves through microstructural inputs

Abstract: Forming Limit Curves (FLCs) were determined experimentally through limiting dome height (LDH) tests in AA1050, AISI 316L and AISI 304L. FLCs were also simulated through FE (finite element) analysis. Simulations involved both constant and varying (with strain and strain path) material properties — namely, strain hardening exponent (n) and normal anisotropy (\(\bar r\)).Varying n values were estimated from limited experimental data (from tensile tests) and the implicit assumption that n scales with in-grain misorientation developments and formation of strain induced martensite. \(\bar r\), on the other hand, could be estimated from crystallographic texture only for AA1050. Simulations with varying \(\bar r\) in AA1050 had shown a clear, though numerically marginal, improvement. On the other hand, varying n could remarkably improve the FLC predictability in 316L and 304L, especially in the biaxial region.

The pulse electrochemical micromachining (PECMM)

Abstract: The pulsed electrochemical micromachining (PECMM) closes the gap between the normal ECM with gap widths in the middle micrometer range and the EC-nanomachining. As a matter of fact, there is a distinction between the extension of the pulsed electrochemical machining down to smaller gap widths and the enlargement of the machining surface of the EC-nanomachining. The goal is the economical production of large plane microstructures. Due to the necessary application of energy, new requirements have arisen concerning the process energy sources (PES) and their pulse units (PU). This paper will show the influence of the feeder of the gap on the shape of the voltage impulses appearing at the gap, and which repercussions the gap may have on the PES. Furthermore, it is of interest to capture the electrical process values (e.g. gap current) very precisely, to react fast on abrupt process changes. Two circuit concepts of pulse units have been developed. Both allow a process control in the relevant parameter range, providing a gap current signal remaining true to shape.

Strength, ductility and impact toughness of the magnesium alloy az31b after equal-channel angular pressing

Abstract: The room temperature strength, ductility and impact toughness of the commercial Mg wrought alloy AZ31B was investigated after equal-channel angular pressing (ECAP). It is shown, that the introduction of a beneficial texture allows a tremendous increase of ductility. An equivalent strain of ∼4.4 at 260°C processing temperature increases the elongation to failure for example from ∼12% to ∼35% (strain rate 10–3 s-1). Simultaneously, the yield stress drops from 225 MPa to 80 MPa. Since the strain hardening capability is increased, an ultimate tensile stress close to that of the commercial material is reached. Instrumented Charpy impact toughness tests reveal also an increase of toughness, whereas crack initiation contributes in particular. Additionally it is shown, that the decrease of the processing temperature necessitates the reduction of the feeding rates to ensure homogenous plastic deformation without shear localisations on the one hand, but gains a higher strength, ductility and toughness on the other hand.