Determination of the flow stress and contact friction of sheet metal in a multi-layered upsetting test

This paper describes the identification of hardening parameters of DC05 sheet metal and contact friction coefficients using a multi-layered upsetting test (MLUT), the modified two specimen method (MTSM) and a finite element based inverse method. The MTSM is adopted in order to identify the friction coefficient between the tools and the stacked circular specimens which are cut from the base material by spark erosion. Next, the hardening behaviour is identified inversely by combining the results of a MLUT and finite element simulations of the test setup. Finally, the results are compared with standard tensile tests and it is shown that the MLUT is a viable alternative for the identification of the local hardening behaviour of sheet metal where standard test specimens cannot be prepared due to size limitations of the specimen.     

Fundamental investigations on the material flow at combined sheet and bulk metal forming processes

The class of combined sheet and bulk metal forming (SBMF) processes denotes the forming of sheet metals with typical bulk forming operations. As a consequence, simultaneous 2D and 3D stress and strain states are present. Furthermore the local shape elements formed out of the sheet plane are in the magnitude of the sheet thickness. This paper deals with process combinations upsetting/lateral extrusion and deep drawing/upsetting. It has been shown that an insufficient mould filling is nearly inevitable using conventional tooling systems. Hence, the prospect of newly developed forming approaches will be discussed.     

Investigations of ductile damage during the process chains of toothed functional components manufactured by sheet-bulk metal forming

Sheet-bulk metal forming processes combine conventional sheet forming processes with bulk forming of sheet semi-finished parts. In these processes the sheets undergo complex forming histories. Due to in- and out-of-plane material flow and large accumulated plastic strains, the conventional failure prediction methods for sheet metal forming such as forming limit curve fall short. As a remedy, damage models can be applied to model damage evolution during those processes. In this study, damage evolution during the production of two different toothed components from DC04 steel is investigated. In both setups, a deep drawn cup is upset to form a circumferential gearing. However, the two final products have different dimensions and forming histories. Due to combined deep drawing and upsetting processes, the material flow on the cup walls is three-dimensional and non-proportional. In this study, the numerical and experimental investigations for those parts are presented and compared. Damage evolution in the process chains is simulated with a Lemaitre damage criterion. Microstructural analysis by scanning electron microscopy is performed in the regions with high mechanical loading. It is observed that the evolution of voids in terms of void volume fraction is strongly dependent on the deformation path. The comparison of simulation results with microstructural data shows that the void volume fraction decreases in the upsetting stage after an initial increase in the drawing stage. Moreover, the concurrent numerical and microstructural analysis provides evidence that the void volume fraction decreases during compression in sheet-bulk metal forming.     

控制大型饼类锻件夹杂性缺陷的镦粗成形工艺研究

镦粗变形是大型饼类锻件的主要成形方式,夹杂性缺陷是其主要质量问题。镦粗变形使锻件心部的塑性夹杂有可能成为片状,这种片状夹杂在一定的力学条件下会产生微观乃至宏观裂纹。因此控制夹杂物的变形并防止产生夹杂裂纹是饼类锻件锻造工艺所要研究的主要内容。为此,本文分别研究了旋转进砧法、梅花布砧法、排砧法和圆弧砧法等四种镦粗工艺条件下,大型饼类锻件心部夹杂物的变形、金属变形及损伤因子的分布情况。分析结果认为旋转进砧法可以有效控制锻件内部的夹杂物形貌。     

Characterizing the buckling behavior of cylindrical hollow and hybrid specimens by experimental upsetting tests

In the research of the compound forging of steel-aluminum parts mostly the upsetting test is used. In this paper the factors influencing the buckling behavior during the upsetting of mono hollow cylinders and hybrid steel-aluminum specimens are examined and characterized by two parameters. The buckle ratio is used as a measurement for the uniformity of the part geometry in radial direction and the buckle distance provides information about the part geometry in axial direction. The results gained by the upsetting of hollow cylinders show that the reproducibility depends on the wall thickness of the hollow steel cylinder. Above a forming temperature of T = 750 °C a transformation from mode II (double barrel profile) to mode I (single barrel profile) occurs. Furthermore, it could be shown, that the strain rate has no influence on the buckling behavior in both the upsetting of hollow cylinders as well as the upsetting of hybrid specimens. The results gained by the upsetting of hybrid steel aluminum specimens show that with increasing forming temperature the hybrid specimens with a wall thickness of the steel casing of s = 1 mm form more homogeneously in radial direction. In addition the scatter of the results decreases with increasing forming temperature.     

Welded preforms for forging

While tailor-welded preforms are commonly used in sheet forming, to date, similar technology has not been developed for bulk forming. In the present paper, the results of a study that was conducted to investigate the feasibility of solid-state welded preforms in forging is presented. Both same-metal and bi-metal combinations were considered and results indicate that in general friction welded preforms have adequate workability based on upsetting and side pressing tests, although flow tends to be concentrated in the lower strength alloy for bi-metal preforms. Mechanical properties obtained from tensile testing of specimens machined from side pressed preforms indicate that mechanical properties of welded preforms are comparable to that of the as-received base metal pieces. While further work is needed to develop the process, results to date confirm that friction-welded preforms have potential for use in bulk forming applications.     

Contact Conditions in Skin-pass Rolling

The special contact conditions in skin-pass rolling of steel strip is analysed by studying plane strain upsetting of thin sheet with low reduction applying long narrow tools and dry friction conditions. An extended sticking region is estimated by an elasto-plastic FEM analysis of the plane strain upsetting. This sticking region causes a highly inhomogeneous elasto-plastic deformation with large influence of work-hardening and friction. A numerical analysis of skin-pass rolling shows the same contact conditions, i.e. an extended sticking region around the center of the contact zone. The calculated size of the sticking region with varying contact length and pressure/reduction is experimentally verified by plane strain upsetting tests measuring the local surface deformation of the work pieces after unloading.     

Investigations on workability of commercial purity aluminum processed by equal channel angular pressing

Equal channel angular pressing (ECAP) is an important process for producing ultra fine grains in bulk metallic materials by means of severe plastic deformation. Workability of metals and alloys is an important parameter as it influences the fracture resistance of the material and the ease of subsequent forming by conventional techniques. In this study, the effect of various passes and processing routes of ECAP on the workability of commercially pure aluminum has been investigated. Aluminum specimens were subjected to ECAP using 90° angle ECAP die. ECAP was carried out using two processing routes for up to three passes. Microstructure characterization and mechanical property measurements were carried out. Workability was determined by means of upsetting tests on hexagonal collar specimens machined from specimens processed by ECAP. A Cockcroft fracture criterion was used to evaluate experimental results. It is observed that processing to two passes through Route C results in enhanced mechanical properties with only a slight decrease in workability.     

Experimental Investigation on Friction Welding of UNS S32205 Duplex Stainless Steel

UNS S 32205 duplex stainless steel specimens were joined by continuous drive friction welding process. The experiments were conducted as per the Taguchi（L16 orthogonal array） method. The friction welding process parameters such as heating pressure, heating time, upsetting pressure, upsetting time, and speed of rotation were fixed with low,medium, and high levels of range based on the machine capacity, and the required knowledge was acquired from the preliminary experiments. The joint characterization studies included micro structural examination and evaluation of mechanical properties of the joints. Microhardness variation, impact toughness, and tensile strength of the joints were evaluated. Neither a crack nor an incomplete bonding zone was observed. The tensile strength of the joints was higher than the strength of the base material, and the friction and upsetting pressures were found to influence the joint strength. The tensile strength of all the welds was observed to be increasing with an increase in the rotational speed. The toughness of the friction welds was evaluated at room temperature and also at subzero（cryo） temperature conditions. The toughness for friction welds was found to be superior to the fusion welds of duplex stainless steel at room temperature and cryo conditions. Weldments exhibited better corrosion resistance than the parent material.     

Orbital forming of tailored blanks from sheet metal

Orbital forming as an incremental bulk forming process can be applied on sheet metal to create a defined circular sheet thickness characteristic. This paper presents an investigation of the process parameters with their effects on the part geometry and properties. With FE-simulation and experiments the influencing parameters have been analyzed. The forming force, tumbling angle and lubricant were identified as the main influencing factors on the form filling. But the number of rotations governs the inhomogeneity of the thickness distribution. The potential of the process is shown by a comparison to an upsetting process.     

Influence of tool roughness and lubrication on contact conditions in skin-pass rolling

The special contact conditions in skin-pass rolling of steel strip are examined by experimental as well as numerical analysis studying plane strain upsetting of thin sheet with low reduction applying long narrow tools with smooth and roughened surfaces under dry friction and lubricated conditions. The influence of friction on the extent of a central sticking region is determined by an elasto-plastic FEM analysis of the plane strain upsetting. The experimental results obtained by measuring the local surface extension using markers made by Micro Vickers indentation verify the FE analysis and show significant influence of tool roughness and lubrication on the contact conditions for varying pressure. The central sticking region was larger for larger friction or tool roughness. At increasing pressure a sudden change in deformation pattern appeared with drastic elongation and sliding in case of lubrication. This deformation pattern is also affected by the tool roughness.     

Hot-upsetting tests on steel cylinders after solidification

A frequently occuring set of forming sequences involves deformation of the workpieces at a strain rate of less than 0.02 s⁻¹ immediately after solidification. The determination of the stress-strain relationship in these circumstances requires new specific devices and related procedures for the achievement of upsetting. Cylinders to be upset are obtained by fusion of an adequate volume of steel in a silica container which is placed on the lower platen of a testing machine. The specimens are upset immediately after in-situ solidification. The influence of strain-rate hardening and of the occurence of a central shrinkage cavity are analysed using elasto-plastic and elasto-visco-plastic finite-element models for reference manganese-alloyed steels. Besides basic upsetting experiments on specimens after solidification, new experiments are performed on room-temperature specimens in which different cylindrical holes have been drilled in the centre zone to represent a well-defined cavity. It is shown that the occurrence of a central cavity, which appears more or less after complete solidification, does not change significantly the state-variable values in the specimens, nor the strain-rate hardening, so that at the present time elasto-plastic pore-free modelling remains relevant in most cases.     

Single point incremental forming of PVC

Polymers represent an important percentage of the raw materials currently utilized in manufacturing applications but their conventional processing techniques are mainly suitable for mass production. Growing requirements of low volume, high quality, customized products are triggering the need for developing innovative flexible polymer processing techniques that are capable of dealing with the new agile manufacturing trends involving very short life cycles and very short development and production lead times.This paper is concerned with these issues and is focused on the possibility of employing the single point incremental forming technology currently being developed for flexible sheet metal forming applications, for producing low cost, small-batch, high-quality polymeric sheet components. Experimentation is based on the utilization of an ordinary CNC milling machine and a single point forming tool to shape commercial PVC sheets and the overall investigation is centred on the characterization and evaluation of the formability limits of the process as a function of the major operating parameters.Results show that single point incremental forming of commercial PVC sheets at room temperature seems promising for the manufacture of complex polymer sheet components with very high depths.     

自动锻压机上一种新型校直机机构

自动锻压机在冷镦零件前,需要将线材笔直地送进机器内部进行镦锻。然而目前大部分线材的直线度并不能满足零件镦锻精度的要求,需要在线材送进机器前进行校直。本文介绍一种可满足多种不同规格线材送进功能的校直机构。     

Vibrational working of metal

Conclusions Vibrational working (upsetting) produces a more uniform structure and hardness distribution within the reduced specimen than static loading. This is apparently the main reason for the better forming properties of metals in vibrational working. The strain hardening of the end surfaces of the vibrated specimens is less than that of those subjected to a static load [for the same reduction].     

Accuracy improvement of miniaturized machine tool: Geometric error modeling and compensation

A novel capacitance–sensor based multi-degree-of-freedom (DOF) measurement system has been developed for measuring geometric errors of a miniaturized machine tool (mMT) overcoming the size limitations. In the present work five geometric error components of a three-axis mMT are measured simultaneously along each axis and the squareness errors are determined by the slopes of straightness error profiles. Least-squares fitting method is used to represent the analytical models of geometric errors. A kinematic chain consisting of various structural members of mMT is introduced to establish the positional relationships among its coordinate frames. Based on this kinematic chain a general volumetric error model has been developed to synthesize all geometric error components of a miniaturized machine tool. Then, a recursive compensation method is proposed to achieve error compensation efficiently. Test results show that the positioning accuracy of miniaturized machine tool has been improved with compensation.     

Approach for bidirectional laser bending of sheet metal with one-sided accessibility

Laser bending by means of the temperature gradient mechanism (TGM) has so far been limited to bending the free end of a sheet metal strip toward the irradiating laser beam. This investigation aims to present a new approach which, despite one-sided accessibility, allows targeted incremental laser bending in both directions by the TGM and the upsetting deformation mechanism (UM). The method is based on blind holes along the bending line, whereby the bend direction can be determined by irradiating either the blind hole bottoms or the bridges between the holes. The possibility of bidirectional and alternating laser bending is demonstrated.     

Formation mechanism of basin-like depression defect in electric upsetting process of Ni80A superalloy

A kind of surface instability, basin-like depression defect companied by mixed grain structure at the bottom of large-scale valve during electric upsetting process, would significantly influence the microstructures and mechanical properties of components. In order to analyze the forming process of the basin-like depression defect, a finite element model for the electric upsetting process of Ni80A superalloy was developed using multi-field and multi-scale coupling analysis method. Subsequently, a series of parameters loading path schemes for force and current were designed by varying the initial value, peak value and value level, and their effects on basin-like depression and mixed grain structure were simulated and uncovered. It is concluded that the changes of heating speed and pressurization speed result in the different flow velocities between the inner and outer layers of billet, thus exerting the basin-like depression. Simulation results also indicate that these defects can be optimized through the parameter coordination between force and current. Finally, the validity and reliability of the finite element model were verified by physical experiments in electric upsetting process.     

Evaluation of the plastic yield locus for embossed sheet using biaxial tensile tests

3D-structured (embossed) aluminium sheets have been used as heat insulation materials in automotive exhaust parts because the embossments on the sheets increase the surface area and reinforce the stiffness of exhaust components. Unlike the press-forming process for flat (non-embossed) sheets, however, that for embossed aluminium sheets is constrained by many restrictions given the distinct mechanical properties and geometric 3D shape of the latter. In designing sheet-stamping tools, manufacturers have recently used CAE technologies based on finite element analysis. Guaranteeing the effectiveness of CAE technologies necessitates information about the plastic yield criterion, which is determined primarily by performing a biaxial tensile test on cruciform-shaped specimens. We measured the yield locus of an embossed aluminium 3004-P sheet by using the camera vision method instead of strain gauge measurement because of the difficulty in attaching a strain gauge to the central region of the aluminium body. The measured yield locus of the studied sheet shows that its yield stress in equi-biaxial stress is smaller than the flat sheet yield locus measured by the strain gauge method. The shape of the yield locus of the embossed aluminium sheet also adequately corresponds with Logan-Hosford anisotropic yield function.     

一种新型实用的四柱式管端加厚液压机

介绍了一种新型实用的四柱式管端加厚液压机组。本机组主要适用于油管和钻杆管端加厚(内加厚、外加厚及内外加厚)工艺及套管管端的定径工艺。