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  • 1.
    Ervasti, Esa
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Production Engineering, Materials Forming.
    Ståhlberg, Ulf
    KTH, School of Industrial Engineering and Management (ITM), Production Engineering, Materials Forming.
    A quasi-3D method used for increasing the material yield in closed-die forging of a front axle beam2005In: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 160, no 2, p. 119-122Article in journal (Refereed)
    Abstract [en]

    The aim of the work is to increase the material yield in closed-die forging of a front axle beam meant for heavy trucks. The amount of flash obtained in production constituted 35% of the total workpiece weight, which was equal to 115.4 kg. The strategy for improving the material yield was to modify the initial forging workpiece geometry, keeping the pre- and finishing die geometries constant. Recommending new shapes for certain cross-sections of the reducer-rolled billet did this. The goal was reached by using a quasi-3D analysis. Three critical cross-sections that showed close to plane strain conditions during forging were chosen for the analysis. The FE-code Form2D was used. The losses of material in the sections caused by axial material flow were measured from full-scale experiments and added to the optimised cross-sectional areas established from the FE-analysis. Because of advice front industry the initial cross-sections used in the 2D-forging simulations were chosen to be circular. Also the final results, after compensation for the axial material flow are presented as circular cross-sections. Utilising the results from the quasi-3D approach the theoretical material yield was increased by 2.58-7.59% for the cross-sections. Results from this work have facilitated the development of adequately shaped grooves in the reducer rolling mill so that the flash volume obtained in production has been reduced. The investigation was carried out with data from IMATRA KILSTA AB.

  • 2.
    Ervasti, Esa
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Production Engineering, Materials Forming.
    Ståhlberg, Ulf
    KTH, School of Industrial Engineering and Management (ITM), Production Engineering, Materials Forming.
    Void initiation close to a macro-inclusion during single pass reductions in the hot rolling of steel slabs: a numerical study2005In: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 170, no 1-2, p. 142-150Article in journal (Refereed)
    Abstract [en]

    The work is focussed on the start of void formation close to a hypothetical single macro-inclusion in the hot rolling of steel slabs. The inclusion is chosen to be either three times harder or three times softer than the surrounding matrix. A commercial FE-code LS-Dyna3D has been utilized for the numerical analysis. Process data from the Swedish company "SSAB Tunnpl (a) over circlet AB" are used. The initial inclusion geometry is assumed to be cylindrical with an extension through the entire width of the slab. Different distances from the slab surface, on the vertical symmetry plane in rolling direction, are chosen as locations for the enclosure. Influence of different single pass reductions and roll radii are analysed. The only boundary force used on the matrix/inclusion interface is referred to friction. By treating a very big inclusion and neglecting bonding forces in radial direction, a worst-case scenario means to be studied. Two indices, one describing the deformation of the inclusion and one the surrounding matrix are used to estimate the likelihood for start of void formation. If the matrix index is bigger than that of the inclusion, voids are assumed to be initiated. This hypothesis gives trusty results. Near the hard particle, voids are likely to be formed. It became clear that the risk is smaller when large rolls are used. Furthermore, the threat for start of void formation turned out to be slighter for high reductions and in the vicinity of the slab surface. Soft particles were just elongated and no risk for void formation could be predicted.

  • 3.
    Han, Han
    KTH, School of Industrial Engineering and Management (ITM), Production Engineering, Materials Forming.
    Determination of mean flow stress and friction coefficient by the modified two-specimen method in cold rolling2005In: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 159, no 3, p. 401-408Article in journal (Refereed)
    Abstract [en]

    The aim of the study is to investigate the possibility of determining the on-line accurate mean value of both friction coefficient and flow stress for anisotropic materials during cold rolling. For this purpose, the rolling process was carried out by FE-simulations instead of by experiments. In the FEM, elastic flattening of work-rolls and material anisotropy have been taken into account. Based on the rolling data obtained from the FE-simulations, the influence of elastic flattening on estimation of the friction coefficient has been verified by forward slip method. Also the best combination of rolling force models together with equations describing the radius of flattened work-rolls has been selected by means of the modified two-specimen method (MTSM). The work material is low carbon steel provided by SSAB. The strain rate sensitivity of the material was tested in plane strain compression tests at three different strain rates. And its anisotropy was tested in different orientations in cylinder upsetting and plane strain compression tests. Results show that the flow stress determined by off-line methods under low strain rate conditions cannot adequately describe its plastic behavior in rolling process if the rolling speed is extremely fast. Results also show that if the elastic flattening of work-rolls is not taken into account, the friction coefficient can be overestimated by up to 35% based on the forward slip method, meaning that the influence of elastic flattening on determination of the friction coefficient do exist. Comparing different combinations of the rolling force models and the equations describing the radius of flattened work-rolls, results identify that the Ekelund's model combined with the Hitchcock equation modified by Roberts gives the best result. Meanwhile, results demonstrate that the influence of anisotropy on determination of the friction coefficient can be negligible when MTSM is utilized. Finally, it is concluded that MTSM can be an efficient method to determine the on-line mean value of both friction coefficient and flow stress in the rolling process, even if materials possess anisotropy.

  • 4.
    Ssemakula, Hamah
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Production Engineering, Materials Forming.
    Ståhlberg, Ulf
    KTH, School of Industrial Engineering and Management (ITM), Production Engineering, Materials Forming.
    Öberg, Kent
    Close-die forging of large Cu-lids by a method of low force requirement2006In: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 178, no 1-3, p. 119-127Article in journal (Refereed)
    Abstract [en]

    This paper presents a FE-analysis and experimental results of a hot forging method for fabrication of large copper canister lids with a low forging load. The forging sequence is carried out in two parts, namely axisymmetric and cogging. The cogging tool is used to bite off small areas of the workpiece periphery surface at the same time as the outer material is forced into the deep die cavity of the bottom die. During this process, the bottom tool is totaled about 45 degrees after each pressing before a new bite is taken. As a result the main part of the cavity is filled at the same time as a protrusion develops at the central part of the workpiece. The cogging tool works on it later. Due to the small contact surface between the upper tool and the workpiece, the forging load needed to complete die filling is small and does not exceed the maximum load of the press. The process was earlier optimised so that no flash was needed for complete die filling. Process parameters affecting the evolution of the microstructure are analysed. Material flow and forging load in different stages of the process are studied. Microstructures obtained in the final product are presented for different locations along the radius of a vertical symmetry plane of the circular lid. Grain size evaluations are roughly co-related to the accumulated effective strain distribution obtained during the different manufacturing operations and to the temperature distribution obtained at the end of the forming process. It is concluded from the simulations that a 4600-t press is sufficient to achieve complete die filling and from experiments that an acceptably small grain size is obtained throughout the lid. From the simulation results the whole volume of the lid has experienced high strains after the manufacturing steps. No regions of low strain were found which according to experience is coincidental with a coarse-grained microstructure.

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