Key Engineering Materials Vols. 611-612

Abstract: This study investigates the influence of heat treatment on the properties of welded steels 1.4404 and 1.4571 of high thickness concerning the forming properties by rolling and the final properties. The weld seam is a region in the work piece where the material can have a higher resistance to deformation. The strength of the weld seam is often higher than in the parent metal. Therefore a suitable pre-heat treatment (1050°C for 30 min) was applied to the weld seam and heat affected zone within the cold rolling process to achieve a more homogeneous distribution of strength and ductility over the entire work piece. A second heat treatment (1050°C for 60 min) after the rolling was done to obtain the solution annealed structure of the steels. To compare the effect of the heat treatments and of the deformation during rolling (work hardening) tensile tests were performed after each process step. Here both materials behave slightly differently. Metallographic investigations show how the microstructure is influenced and give a clear picture of the fracture occurring. Force – time behaviour during rolling was monitored and provides information about the improvements by the preliminary heat treatment. Cold rolling of welded plates is characterized by a force peak just when the weld seam is within the gap of the rolls. The application of heat treatment has been found to lower that force peak and ensures less distortion in shape during the cold rolling passes.

Abstract: Titanium alloys, due to their high mechanical properties coupled with light weight and high corrosion resistance, are finding a widespread use in the aeronautic industry. The use of titanium in replacing the conventional alloys, such as aluminum alloys and steel, is reduced by both the high cost of the raw material (it costs anywhere from 3 to 10 times as much as steel or aluminium) and the machining costs (at least 10 times that to machine aluminium). For such a reason new technologies have been studied and developed. In particular many researchers are searching for technologies, such as the precision hot forming, that allows to obtain components with a low buy to fly ratio. Many of the airframe component structures are designed to fit against the inside radius of the fuselage curvature. By combining traditional stretch forming technology with hot titanium forming techniques, the HSF guarantees a saving in material and machining time, which are two serious cost issues for todays aircraft manufacturers. In addition, the process allows for consistent quality in a productively efficient manner, assuring the sustainable attainment of delivery and build schedules. In order to develop and improve the HSF process a modeling of the process itself was executed in order to study the stresses and strains undergone by the material among the deformation. The FEM model was validated through the residual stresses, and in particular the residual stresses provided by the model were compared with the ones experimentally measured using the hole drilling technique. Good agreement, in terms of stress range, was recorded both for the maximum and the minimum stress.

Abstract: Experimental works were carried out to investigate microstructure and crack formation during compression tests of 1.9wt%Cultrahigh carbon steel according to temperature and strain rate. As-received ultrahigh carbon steel is composed of precipitated cementites and pearlite matrix. In addition, numerous voids were observed in the matrix of as-received material. The compression tests at 800 ^oC showed that the voids within the matrix are closed with increase of reduction ratio. On the other hand, when the reduction ratio increased numerous micro-cracks were newly formed in the bulky cementites and at the interfaces between hard cementite and soft matrix. It was also observed that because the volume fraction of cementite is reduced when temperature increased, volume fraction of newly formed micro-crack significantly decreased. Cast microstructures were observed after compression test at 1130 ^oC due to local melting. From experimental results and microstructure anlayses, it was concluded that the forging temperature should be controlled at the temperature of more than 900 ^oC and less than 1130 ^oC.

Abstract: Paper presents deformation behaviour and microstructural response of selected medium-carbon high-strength steels commonly used for high-duty components deformed under high-strain-rate and warm work temperature range. The investigation of material behaviour is oriented at analysis of hot and warm workability of material and microstructure evolution resultant from deformation mechanisms, strain induced recrystallization and hardening at temperatures of lower forging regime and high strain rate deformation. The effect of these factors on microstructure after forging and subsequent direct-cooling was studied. Metallographic work aided with numerical methods of simulation of the metal flow and microstructure evolution during forging were used to correlate thermo-mechanical parameters observed with microstructure and mechanical properties after forging and cooling.

Abstract: The hammer forging is a well-known technology to incrementally produce geometrically complex forgings by compressing the material against the dies using several forming blows. When forging aeronautical components with this technology, it is crucial to control the final grain size of the part since this variable highly influences the high temperature low cycle fatigue properties. Nowadays, it is common practice to use the finite element models coupled with recrystallization models to optimize the process parameters and strategy. However, a very important variable to conduct these simulations is the real available hammer energy, which must be calibrated, not being an easy task since very high forces are generated in the impact of the anvils. In the present paper, the copper-column upsetting method is compared with a novel method where a high speed camera has been used to compute the anvils’ velocity and corresponding energies. The compressive behavior of the copper samples has been characterized using Rastegaev compression tests. The experimental and calculated results using the high speed camera are compared to the ones obtained using high purity copper samples. These measurements have enable to quantify the influence the friction and the elastic rebound have during the energy transfer from the anvils to the billet. This makes possible a precise future characterization of hammers using the conventional copper-column upsetting method if high speed cameras are not available in workshop.

Abstract: Residual stresses and lack of straightness appear during the cooling of sheet piles where the initial temperature field is not homogeneous. To meet the standards, the long hot rolled pieces are straightened using a series of rollers placed alternately above and below the pieces with shifts which create a succession of bendings. The process is modeled to study the impact of the industrial parameters (the duration of the cooling and the rollers positions), to improve the final geometry and to reduce the residual stresses. Tests are carried out on this structural steel to observe the material behavior, then material laws are chosen and the parameters of these laws are defined using an inverse method. Two sets of material data are obtained: for the first one, the hardening is supposed to be isotropic, and for the second one, additional tests are performed to describe isotropic and kinematic hardenings. The cooling followed by the straightening is then simulated by the finite element method with these two sets of data. The comparison of the rollers forces, the deformation and the residual stresses show the impact of the kinematic hardening on such a process where the material undergoes a succession of tensions and compressions. The real forces applied by the rollers, the real curvature of the interlocks at the end of the straightening process and the distribution of the residual longitudinal stresses measured on the web using the ring core method are used to validate the numerical model.

Abstract: Rolling of thin sheets generally induces flatness defects due to thermo-elastic deformation of rolls. This leads to heterogeneous plastic deformations throughout the strip width and then to out of plane displacements to relax residual stresses. In this work we present a new numerical technique to model the buckling phenomena under residual stresses induced by rolling process. This technique consists in coupling two finite element models: the first one consists in a three dimensional model based on 8-node tri-linear hexahedron which is used to model the three dimensional behaviour of the sheet in the roll bite; we introduce in this model, residual stresses from a full simulation of rolling (a plane-strain elastoplastic finite element model) or from an analytical profile. The second model is based on a shell formulation well adapted to large displacements and rotations; it will be used to compute buckling of the strip out of the roll bite. We propose to couple these two models by using Arlequin method. The originality of the proposed algorithm is that in the context of Arlequin method, the coupling area varies during the rolling process. Furthermore we use the asymptotic numerical method (ANM) to perform the buckling computations taking into account geometrical nonlinearities in the shell model. This technique allows one to solve nonlinear problems using high order algorithms well adapted to problems in the presence of instabilities. The proposed algorithm is applied to some rolling cases where “edges-waves” and “center-waves” defects of the sheet are observed.

Abstract: Ring Rolling is a versatile metal-forming process to manufacture seamless rings of various cross-sectional geometries. Rings with a “dish shape” are used in different areas such as offshore, aeronautics or the energy sector. Current ways to produce dish shaped rings have the disadvantages of limited or inflexible geometries and either high material waste, additional costs for special tools or long process time. Instead, when manufacturing dish shaped rings on conventional radial-axial ring rolling mills, ring producers will be able to expand the range of their products easily. In a prior investigation, the general feasibility of an alternative to the current manufacturing processes was shown in experiments and in finite element method (FEM) simulations, avoiding major additional machining and material costs. Resulting from an analysis of the geometrical requirements and material flow mechanisms for dishing and ring climbing, a rolling strategy was derived, applying a large height reduction of the ring. A major problem of this rolling strategy is that whenever the contact between the ring and the main roll is lost in the process, the ring starts to oscillate around the mandrel and neither dishing nor ring climbing can be observed. In order to ensure a permanent contact between ring and main roll and in order to stabilize the ring in its inclined position in the rolling mill, additional stabilizing measures of the process will be developed and investigated. With the developed FE-model, a stabilizing measure by the use of pressure rolls and automatic guide roll movement for ring climbing was tested and appears promising for the application in a real experimental environment.

Abstract: The machining of titanium alloys is challenging in every aspect. In order to avoid waste material by cutting processes and to improve mechanical properties, forming processes offer many advantages but harbor also challenges. To face these challenges, especially techniques like isothermal forging are promising methods. Isothermal forging is an appropriate process for achieving a microstructure with excellent properties for high performance applications in aviation technology and turbine construction. One of the main challenges in this special process is the determination of a tool material with a high temperature resistance as well as a high resistance against the work load of forging processes. Given their high hardness, temperature resistance and wear resistance, technical ceramics feature properties classifying them as generally suitable for this application. This article deals with the complete design of an isothermal forging process with ceramic tool material for titanium forming. The material characterization of the forming material by flow curve determination is performed to receive data for FE analyses. Afterwards, a ceramic tool system for isothermal forging is designed and manufactured. The tests show that especially the brittleness of technical ceramics restricts their application as tool material for isothermal titanium forming. Additional investigations on isothermal forging using carbide metal as tool material show the benefit of isothermal titanium forging. The results of metallographic analyses are given.

Abstract: On component-side the process limits of bulk metal forming processes are limited by defects in parts, like folds or cracks. Considering the time and cost aspect, the complexity of forged parts and the demands on quality are increasing. Therefore in production processes it is inalienable to detect failures in parts in earlier process stages, to avoid additional costs and loss of time. One of the comparatively less investigated failures in bulk metal forming processes are forging folds, which predominantly occur at the equatorial section in upsetting processes of hollow parts. Some previous studies showed the main influences on the formation of forging folds [1, 2, 3]. In these studies i.a. at the Institute of Forming Technology and Machines an algorithm was developed and implemented in a commercial FE-Software-System, which is able to detect forging folds in bulk metal forming processes. However, the algorithm is only valid for two-dimensional models.