Part dimensional errors in free upsetting due to the elastic springback

Pojava elastičnog vraćanja materijala radnog predmeta prisutna je u svim procesima plastičnog deformiranja metala. Ovaj faktor prepoznat je kao jedan od glavnih čimbenika dimenzijske (ne)točnosti. Stoga, da bi se proizveli dijelovi sukladno zahtjevima koji se odnose na njihovu geometriju ovaj fenomen se mora dobro razumjeti i uzeti u obzir prilikom projektiranja alata i procesa deformiranja. Nažalost, znanje vezano za ovaj fenomen je ograničeno te stoga predviđanje i proračun iznosa elastičnog vraćanja ponekad predstavlja vrlo težak zadatak. U ovom članku predstavljeno je opće analitičko rješenje za izračun elastičnog vraćanja. Izvedene analitičke jednadžbe mogu se primijeniti za različite procese oblikovanja pod uvjetom da su vrijednosti glavnih naprezanja na samom kraju procesa deformiranja poznate. Koristeći ovaj pristup izračunate su elastične deformacije i amplituda elastičnog vraćanja radnog komada kod procesa slobodnog prešanja cilindrične gredice. Dobiveni rezultati provjereni su pomoću MKE analize.Elastic springback of workpiece material which occurs in any forming process has been recognized as one of most relevant factors regarding part dimensional accuracy. Therefore, in order to manufacture component in accordance with the geometrical specifications engineers must have a good understanding of this phenomenon and take it into account during the design tool and forming process. Unfortunately, this knowledge is often insufficient and therefore the prediction of elastic springback is sometimes a very tough task. The paper presents a general approach for the calculation of elastic recovery. Given analytical equations can be applied for different forming processes under the condition that values of the principal stresses at the very end of forming process are known. By using this approach elastic strains and amplitude of elastic springback of workpiece in case of free upsetting of cylindrical billet were calculated. Obtained results were verified by FEM analysis

EXPERIMENTAL AND NUMERICAL INVESTIGATION OF THE T-STUB ELEMENTS WITH FOUR BOLTS IN A ROW UNTIL BOLT FRACTURE

For the past several decades, codified design of steel connections in civil-engineering is based on the component approach. For a very common end-plate connection, tension component, named T-stub, usually dictates the connections’ behaviour. This T-stub element is greatly investigated in the configuration with two bolts in a row, but configuration with four bolts in a row is usually neglected, both in the studies and codes. This paper presents an experimental investigation of T-stub elements and important aspects of their numerical modelling. Special attention is dedicated to the material testing and modelling since all of the tests were performed until bolt fracture. Uniaxial tests of steel specimens were performed using extensometers, strain gauges, and Aramis system, while bolt material is additionally tested by microscopic examination and hardness testing. In order to obtain satisfactory calibration of numerical models developed in Abaqus, knowing material parameters including damage initiation and propagation is crucial. Several iterative numerical-experimental procedures for obtaining the true stress-strain curves are outlined and compared, along with well-known Bridgman method. The advantages of using Aramis system in calibrating numerical model, for both material and assembly are demonstrated. In the end, comparisons of numerical and experimental behaviour curves are presented and satisfactory results are obtained

EXPERIMENTAL AND NUMERICAL INVESTIGATION OF THE T-STUB ELEMENTS WITH FOUR BOLTS IN A ROW UNTIL BOLT FRACTURE

For the past several decades, codified design of steel connections in civil-engineering is based on the component approach. For a very common end-plate connection, tension component, named T-stub, usually dictates the connections’ behaviour. This T-stub element is greatly investigated in the configuration with two bolts in a row, but configuration with four bolts in a row is usually neglected, both in the studies and codes. This paper presents an experimental investigation of T-stub elements and important aspects of their numerical modelling. Special attention is dedicated to the material testing and modelling since all of the tests were performed until bolt fracture. Uniaxial tests of steel specimens were performed using extensometers, strain gauges, and Aramis system, while bolt material is additionally tested by microscopic examination and hardness testing. In order to obtain satisfactory calibration of numerical models developed in Abaqus, knowing material parameters including damage initiation and propagation is crucial. Several iterative numerical-experimental procedures for obtaining the true stress-strain curves are outlined and compared, along with well-known Bridgman method. The advantages of using Aramis system in calibrating numerical model, for both material and assembly are demonstrated. In the end, comparisons of numerical and experimental behaviour curves are presented and satisfactory results are obtained

Upsetting of bimetallic components in closed die

Metal forming technology has been applied to effective production of metallic components in various fields. One of the newer metal forming techniques is so called „joining by forming“. With this method two or more separate parts are joined together using plastic deformation. Self-piercing riveting, clinching and coin and medal production are some of the joining by forming operations, which have been successfully applied in industry. The present study is bound to a process of joining of two bimetallic axisymmetric components from various materials by upsetting bimetallic components in a closed die. The external component is a ring and the central part is a cylinder – like element. By upsetting those two elements in closed die, inseparable workpiece has been obtained. Therefore, two different cases (geometries) have been investigated. In the first case the simple ring and profiled inner cylinder are combined while in the second case vice versa combination is applied. Also, forming load, material flow and filling of joint section were analyzed. The process has been investigated experimentally. In short, current investigation has yielded considerable insight into the process of joining two metallic components

An analysis of non-axisymmetric backward extrusion

U usporedbi s konvencionalnim metodama obrade, proizvodnja složenih dijelova hladnim istiskivanjem često nudi tehničke i ekonomske prednosti. U ranoj fazi industrijske primjene hladnim istiskivanjem su se uglavnom izgrađivali aksijalno simetrični dijelovi. U toku daljnjeg razvoja, tehnologija hladnog istiskivanja primijenjuje se i za izradu složenijih ne-aksijalno simetričnih oblika. Samim time povećan je značaj i proširena područja primjene protusmjernog deformiranja i u drugim oblastima industrije, posebice u automobilskoj industriji. U radu su prikazana istraživanja tehnologije protusmjernog deformiranja ne-aksijalno simetričnih dijelova, s akcentom na izradu dijelova pomoću žigova kvadratnog poprečnog presjeka. Ispitivane su vrijednosti deformacione sile i pritisaka na čelo žiga, kao i naponsko-deformacijsko stanje u dva karakteristična presjeka radnog komada. Proces je analiziran metodom konačnih elemenata (MKE) i eksperimentalno. Za eksperimentalna istraživanja konstruirani su i izrađeni specijalni alati.Manufacturing of complex parts by cold extrusion often offers technical and economical advantages when compared with the conventional production methods. In the initial stage of industrial application of cold extrusion mainly axisymmetric components were produced. In the course of further development, application of cold extrusion has been extended to more complex and non-axisymmetric shapes. These have increased the significance and broaden the application fields of cold extrusion within other production technologies, especially in the automobile industry. Current paper elaborates non axisymmetric cold extrusion. Focus of the work is placed on the backward extrusion by the punch with square cross section. In this process load – stroke has been determined as well as punch pressure, and strain and stress distribution in two characteristic different workpiece cross sections. The process has been analyzed by FE method and experimentally. For the experimental investigation special tooling has been designed and made

Possibilities to measure contact friction in bulk metal forming

Trenje je neizbježna pojava u svim postupciima plastičnog deformiranja metala i ima negativan utjecaj na parametre procesa, radni vijek alata, kao i kvalitetu radnog komada. U hladnom prostornom deformiranju veliki pritisci djeluju između alata i radnog komada. Ovi pritisci mogu biti limitirajući faktor prilikom primjene tehnologija deformiranja. Smanjenjem kontaktnog trenja, smanjuju se i kontaktni pritisci. Poznavanje parametara trenja (faktora trenja µ i/ili omjera trenja m) je od velikog značaja prilikom proračuna parametara procesa (npr. sila ili energija) i odabira odgovarajućeg sredstva za podmazivanje, kao i numeričku simulaciju obrade. Postoje mnogobrojne metode za određivanje parametara trenja u obradama deformiranjem. Neke od ovih metoda opisane su u ovom radu. Također je predstavljen i novi model za određivanje parametara trenja u obradama deformiranjem. U radu je izvršeno i mjerenje sile trenja u procesu protusmjernog istiskivanja za različite vrste maziva.Friction occurs in all metal forming operations and, in general, it has a negative impact on process parameters, die life, as well as workpiece quality. In cold metal forming processes high interface pressures between die and material take place. These pressures can be the limiting factor of application of cold forming. By reducing interfacial friction, contact pressures can be reduced too. Knowledge of friction amount (factor of friction µ and/or friction ratio m) is essential for calculation of the main process parameters (load, energy), for choosing a proper lubricant but also for numerical modelling of forming operations. There exist a number of experimental methods to determine friction in metal forming processes. Current paper deals with the possibilities to evaluate friction. It analyses and assesses a number of existing friction models in cold metal operations. Proposal of a new friction model for cold metal forming operations is presented and discussed. Measurement of friction force in backward extrusion for different lubrication conditions was also preformed

Generation of a Layer of Severe Plastic Deformation near Friction Surfaces in Upsetting of Steel Specimens

Narrow layers of severe plastic deformation are often generated near frictional interfaces in deformation processes as a result of shear deformation caused by friction. This results in material behavior that is very different from that encountered in conventional tests. To develop models capable of predicting the behavior of material near frictional surfaces, it is necessary to design and carry out tests that account for typical features of deformation processes in a narrow sub-surface layer. In the present paper, upsetting of steel specimens between conical and flat dies is used as such a test. The objective of the paper is to correlate the thickness of the layer of severe plastic deformation generated near the friction surface and the die angle using a new criterion for determining the boundary between the layer of severe plastic deformation and the bulk