The manufacture of precise metal components by cold forging poses serious challenges to the process reliability under unstable process conditions. The detection of geometrical imperfections, such as ...concentricity deviations, is necessary for the operator to adjust the forging tool rack properly. In this paper, a novel piezoelectric thin-film sensor disc is introduced to detect such concentricity deviations based on the measurement of eccentric load, that is arising from elastic punch deformation. Experimental results showed, that the concentricity deviation of the produced parts efficiently can be predicted by processing measured force data using a support vector regression algorithm.
Drawing is a manufacturing process that consists of indirect deformation by pulling the material through a tool with a conical geometry. The process is usually performed at room temperature (cold ...forming), so the selection of effective lubricants is critical. If lubrication is inadequate, there is a high risk that both, tool and the manufactured wire, will fail. In this study, annealed AISI 1020 steel rods were drawn and the effectiveness of three different industrial lubricants was tested. During the process, the drawing force values were recorded and used to determine the friction coefficient that developed under each lubrication condition. Numerical simulations were performed to further understand the process. Based on the experimental and numerical results, qualitative and quantitative analysis were performed for each condition. Among the different lubricants used in this study, zinc stearate showed the lowest value for drawing force, 18.8 kN, followed by Lub A and B with values of 20 kN and 20.6 kN, respectively. The numerical models showed excellent approximation to the force values determined in the tests. The values for the coefficient of friction obtained by both the numerical analysis and the empirical model indicate that zinc stearate has the highest lubricating effect among the lubricants focused on this study.
The use of lubricants without conversion layers is a global target of the cold forging industry. Compared to conventional lubricant systems based on zinc phosphate coatings, these fluctuate more in ...their tribological properties. In a worldwide round robin test, thirteen different lubricants were compared in tests with six different tribometers. The results show significant differences in friction coefficients both between different lubricants and between tribometers. A multiple regression model describing the dependence of the coefficient of friction on tribological loads allows for a consistent merging of the data. The model is successfully validated by a full-forward extrusion process.
Cold forging is an ecological production technology for high-strength metal parts. Tool fatigue is the most significant constraint preventing the enhancement of process limits towards complex part ...geometries. To better analyse tool fatigue, a test method for dies under high inner pressure is researched. The load usually transmitted by the workpiece is applied by compression of elastomers. Results show that the test is able to accurately model the fatigue behaviour of forging tools including crack propagation and the influence of prestressing systems. Compared to conventional fatigue tests, the analysis of different reinforcements and tool geometries is enabled.
The critical pad load to suppress wrinkling in free bending is derived by linking the pad lifting behaviour and the wrinkle disappearing condition that depends on the clearance between the punch and ...the blank generated by pad lifting at the early stage of bending. The clearance is estimated based on the simulated results that the contacting zone of the blank with the pad at the early stage of bending is confined to the vicinity of the punch shoulder. The usefulness of the critical pad load is confirmed by forming experiments of a T-shaped product from high strength steel sheets.
Injection lap riveting Ferreira, Francisco R.; Pragana, João P.M.; Bragança, Ivo M.F. ...
CIRP annals,
2021, 2021-00-00, Letnik:
70, Številka:
1
Journal Article
Recenzirano
This paper presents an injection lap riveting process to connect two sheets placed one on top of the other. The process is carried out at ambient temperature and differs from self-pierce riveting ...because its joining principle is based on plasticity and friction without fracture and formation of new surfaces. The working principle is based on two consecutive operations - first, a dovetail ring hole is machined in the lower sheet and then a semi tubular rivet is injected through the upper sheet into the dovetail ring hole of the lower sheet, by compression with a punch. The presentation is based on a combined experimental and numerical investigation and special emphasis is put on the influence of the dovetail ring hole geometry in material flow, riveting force, and pull-out and shear destructive forces. The last past of the paper includes details of a prototype cutting tool that was developed by the authors for producing the dovetail ring holes in-site and fostering the portability and applicability of the process.
Cold forming has required considerably superior hard coating on a die to manufacture net-shaped products. Especially, under poor lubrication conditions or a dry condition, the hard coating is ...indispensable for the success of manufacturing. Accordingly, the die has required the superior hard coating to prevent adhesion, reduce friction, and control wear. Recently, it has been also necessary to improve die life and product qualities under severe tribological conditions. However, the comprehension of damage such as cracking, flaking, and galling is insufficient for metal forming. This study focuses on initiation and growth of cracking in the hard coating. Assuming the hard coating is subjected to tensile stress, nominal stress, or frictional shear stress, the performances of typical four hard coatings are investigated by two tests. One is a uniaxial tensile test, and another is a rotating cross-cylinders wear and friction test. These tests are applied to estimate the mechanical performance of hard coatings: CrN, TiN, VC, and CrAlN. After the two test and analyses with detail observation, the CrAlN coating collectively exhibits the highest anti-cracking property of the tested four hard coatings.
Die Gefahr der Versprödung von Stählen beim Schweißen in kaltumgeformten Bereichen führt zu vielen Fertigungsrestriktionen bzw. zu hohen Fertigungskosten. Der Grund der Versprödung liegt in der ...Grobkornbildung der Rekristallisationszone. In diesem Beitrag wurden drei hochfeste Stähle hinsichtlich ihrer Versprödungsneigung bei unterschiedlichen Umformgraden über ein Temperaturspektrum untersucht. Ziel war es, die Temperaturen während des Schweißens in der Rekristallisationszone zu simulieren und damit das Kornwachstum abzuschätzen. Dazu wurden die Werkstoffe auf maximale Dehnungen bis kurz vor der Gleichmaßdehnung gelängt. Die anschließende Wärmebehandlung erfolgte zwischen 500 °C und 800 °C mit verschiedenen Haltezeiten. Über die gesamte Versuchsreihe konnte kein nennenswerter Zusammenhang zwischen Kornwachstum, Umformgrad, Glühdauer und Glühtemperatur festgestellt werden. Die Ergebnisse deuten darauf hin, dass beim Längen hochfester Stähle der kritische Umformgrad nicht erreicht werden kann und somit keine Grobkornbildung in der Rekristallisationszone stattfindet.
Translation
Grain size development of high‐strength steels after cold forming and heat treatment
The risk of embrittlement of steels during welding in cold‐formed areas leads to many production restrictions or high production costs. The reason for embrittlement is the coarse grain formation in the recrystallization zone. In this paper, three high‐strength steels were investigated with respect to their tendency to embrittlement at different degrees of deformation over a temperature spectrum. The aim was to simulate the temperatures during welding in the recrystallization zone and thus to estimate the grain growth. For this purpose, the materials were elongated to maximum elongations up to just before the uniform elongation. The subsequent heat treatment was carried out between 500 °C and 800 °C with different holding times. Over the entire series of tests, no significant correlation was found between grain growth, the degree of deformation, the annealing time and annealing temperature. The results indicate that the critical degree of deformation cannot be reached during the elongation of high‐strength steels, and thus no coarse grain formation takes place in the recrystallization zone.
The residual stresses and strains in cold-formed steel members are a result of the manufacturing process. It has been shown that the variation of residual stresses through the thickness of ...cold-formed steel members is not linear. In this study a numerical algorithm is developed to calculate the through-thickness variation of residual stresses and strains. The algorithm calculates the stresses and strains by viewing the manufacturing process as a combination of elasto-plastic bending and springback in a wide plate under plane strain conditions. In order to calculate the plastic deformations, the Prandtl-Reuss flow rule associated with von Mises yield criterion is used. With regard to satisfying the boundary conditions on the surface, the bisection method is used to find the location of the neutral axis. The results obtained via the proposed algorithm are verified with the available closed formed solutions, finite element analysis results and experimental measurements. A parametric study is performed to evaluate the effect of the coil radius and cross-sectional and material properties on the residual stresses and strains. It is shown that, while in the corner regions the most important parameter is the corner radius, it is the coil radius and yield stress that play a significant role in the variation of residual stresses and strains in the flat regions.
•A method is proposed to obtain the residual stresses and strains from cold-forming.•Cold-forming is modeled as a set of elastoplastic and springback deformations.•Results are verified with finite element modeling and experimental measurements.•Plasticity is modeled through Prandtl-Reuss flow rule and von Mises yield criterion.•A parametric study is performed on the variability in residual stresses and strains.
This paper presents the experimental investigation on the material properties and residual stress distributions of cold-formed high strength steel hollow sections. The nominal yield stresses (0.2% ...proof stresses) of the hollow sections were 700, 900 and 1100MPa. The sections were composed of 9 square hollow sections, 2 rectangular hollow sections and 6 circular hollow sections. 66 tensile coupons were tested to determine the modulus of elasticity, 0.2% tensile proof stress, ultimate tensile strength and elongation at fracture of the material. The results were also compared with the American, Australian and European standards. Based on the Ramberg–Osgood expression, a new constitutive model for high strength steel material is proposed. On three of the hollow sections, strength variation and cold-forming effect were studied by testing coupons extracted from different locations of the cross-sections. Residual stresses were also investigated both in longitudinal and transverse directions on three hollow sections using wire-cutting method. A total number of 82 longitudinal strips and 3 transverse rings were cut from the original sections.
•Cold-formed high strength steel hollow sections were examined.•Material characterizations were conducted through 66 tensile coupon tests.•Cold-forming effects were investigated on halves of the three hollow sections.•A new material model for high strength steel was proposed.•Residual stresses on three sections were measured and reported.
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