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New Trends in Forging Technologies B A Behrensa T Hagena J Kniggea I Elgalya T Hadifia A Bouguechaa aInstitute of Metal Forming and Metal Forming Machines Leibniz Universit t Hannover An der Universit t 2 30823 Garbsen Germany Abstract Limited natural resources increase the demand on highly efficient machinery and transportation means New energy saving mobility concepts call for design optimisation through downsizing of components and choice of corrosion resistant materials possessing high strength to density ratios Component downsizing can be performed either by constructive structural optimisation or by substituting heavy materials with lighter high strength ones In this context forging plays an important role in manufacturing load optimised structural components At the Institute of Metal Forming and Metal Forming Machines IFUM various innovative forging technologies have been developed With regard to structural optimisation different strategies for localised reinforcement of components were investigated Locally induced strain hardening by means of cold forging under a superimposed hydrostatic pressure could be realised In addition controlled martensitic zones could be created through forming induced phase conversion in metastable austenitic steels Other research focused on the replacement of heavy steel parts with high strength nonferrous alloys or hybrid material compounds Several forging processes of magnesium aluminium and titanium alloys for different aeronautical and automotive applications were developed The whole process chain from material characterisation via simulation based process design to the production of the parts has been considered The feasibility of forging complex shaped geometries using these alloys was confirmed In spite of the difficulties encountered due to machine noise and high temperature acoustic emission AE technique has been successfully applied for online monitoring of forging defects New AE analysis algorithm has been developed so that different signal patterns due to various events such as product die cracking or die wear could be detected and classified Further the feasibility of the mentioned forging technologies was proven by means of the finite element analysis FEA For example the integrity of forging dies with respect to crack initiation due to thermo mechanical fatigue as well as the ductile damage of forgings was investigated with the help of cumulative damage models In this paper some of the mentioned approaches are described Keywords Acoustic emission Cold forging Damage models Downsizing Heatless hardening Online monitoring Superimposed hydrostatic pressure Transformation induced plasticity PACS 02 70 Dh 81 20 Hy 83 50 Uv 62 20 M 62 20 fq 62 20 me 64 60 My INTRODUCTION In order to meet rising technical and social demands in times of scarce fossil energy and rising environmental regulations the use of light metals and load adapted structure concepts increases in most domains of mechanical engineering The automotive industry as well as the aircraft industry as one of the first users of downsized and weight reduced components have great interest in the development of innovative materials and process technologies Since the introduction of high strength and or light weight non ferrous metals in modern production engineering the group of metal forming techniques is facing great challenges The common production processes are often not suitable to utilise the full potential which these materials may offer As a result continuous attempts to improve the characteristics processing properties and combinations of materials are carried out This paper presents some of the new trends in forging processes of metastable austenitic steels and non ferrous wrought alloys on the material side Regarding new numerical computation techniques some approaches of the numerical prediction of ductile damage in hot forging dies due to thermo mechanical fatigue and the evolution of the microstructure in forming processes are introduced Furthermore considering process monitoring acoustic emission sensor systems can be a powerful tool to detect damage or crack occurrence online the forming operation The 14th International ESAFORM Conference on Material Forming AIP Conf Proc 1353 380 385 2011 doi 10 1063 1 3589545 2011 American Institute of Physics 978 0 7354 0911 8 30 00 380 ADAPTED MARTENSITIC AREAS DUE TO FORMING INDUCED PHASE TRANSFORMATION IN METASTABLE AUSTENITIC STEELS During the cold forging of metastable austenitic steels transformation induced martensite can occur This leads to an increase in flow stress and strength and offers new design possibilities for light weight and or crash components One research objective is to enable this effect of transformation induced martensite in metastable austenitic steels by the forging process 1 In detail by adjusting forging parameters such as true plastic strains strain rate and forging temperature locally strengthened areas in solid components were created The dependence of the martensite content in the experiments on the forging parameters was analysed on the basis of upsetting tests Various chrome nickel steels and one manganese hard steel were examined regarding transformation induced martensite formation under compressive stresses The quantification of the martensite evolution was carried out by means of a Feritscope and a Magnatest Then the results were approved by metallographic and numeric analyses Based on test series with a constant strain rate of 0 1 s 1 and varying forging temperatures of TF 5 20 and 60 C a significant influence of the forging temperature on the martensite content was detected Due to cooling the specimens to TF 5 C an immense increase in the martensite evolution is activated compared to those experiments taken at room temperature On the other side increasing the forging temperature reduces the effect of the martensitic phase conversion At a temperature of TF 60 C a phase conversion is close to be completely suppressed Besides a distinct strain rate dependency of the martensite content was determined This is the result of an increased adiabatic heating during the upsetting tests at high strain rates which also suppresses phase transformation due to increased inner temperature FORGING UNDER SUPERIMPOSED HYDROSTATIC PRESSURE Forming under superimposed media based hydrostatic pressure is a promising approach to handling low ductile steels and non ferrous metals in a forming process The influence of superimposed hydrostatic pressure on the flow stress as well as on the formability for various hydrostatic pressures forming temperatures and aluminium and magnesium alloys was analysed 2 3 By increasing the formability of the workpieces larger local plastic strains could be achieved Considering aluminium wrought alloys in upsetting tests under superimposed pressure of 85 MPa the formability of workpieces from thermosetting alloy AlSi1MgMn AW 6082 was more increased compared to those from self hardening alloy AlMg4 5Mn0 7 AW 5083 As a general tendency the self hardening alloys show a lower increase in formability when forged under superimposed pressure To determine the process limits considering highest increase in formability and complexity of workpiece geometry a new tool concept see Figure 1 left was designed for applicably superimposing pressures up to max 200 MPa Ejector Closing plate Die Ram Die inserts Sealings Load cell Guide frame Upsetting Lateral Extrusion Ducts Top die insert Bottom die insert Work piece FIGURE 1 left Tool concept for superimposed pressures of max 200 MPa right Die inserts for upsetting and lateral extrusion processes The tool design uses a closing plate an ejector and easily changeable die inserts see Figure 1 right Hereby various forging methods such as upsetting and extrusion processes can be carried out The hydraulic pressure is 381 provided by an active hydraulic power unit and controlled by cascading chokes of specified diameters during the forming operation By using the formability increase the raw part geometries can be selectively varied to forms which can not be forged in a conventional forging process By forming these selected preforms under superimposed hydrostatic pressure locally adapted strain hardening can be induced ACOUSTIC EMISSION ANALYSIS A NEW FACILITY FOR ONLINE MONITORING OF FORGING PROCESSES Through this work three main subjects have been studied the feasibility of applying acoustic emission technique to forging processes determination of suitable parameter sets for signal acquisition under harsh industrial working conditions and finally comparison of possible AE signal analysis methods that can be applied in order to reach a conclusion about the quality of the manufactured component or the condition of the forging die 4 The main part of investigation was performed using a high end professional AE system AMSY 5 Vallen Systeme Germany The 8 channels system is capable of 10 MS s sampling rate per channel and is equipped with different types of resonant and broadband sensors various filters and transient recorder for real time acquisition The use of broadband sensors up to 2 4 MHz and the application of both hardware filters 400 1600 kHz as well as energy and duration based criteria e g E 50 x109 V2s K Voges Schwieger A Bouguecha J Knigge K Weilandt S H bner Local Strain Hardening of Metal Components by Means of Martensite Generation Advanced Materials Research Vol 137 2010 pp 1 33 Trans Tech Publications Switzerland 2 B A Behrens T Hagen A Klassen J Knigge J Mielke I Pfeiffer Forging of Aluminium Components under a Superimposed Hydrostatic Pressure to Induce Local Strain Hardening Advanced Materials Research Vol 137 2010 pp 191 217 2010 Trans Tech Publications Switzerland 3 I El Galy I Pfeiffer B A Behrens Application of Acoustic Emission to Monitor Forging Processes of Magnesium Alloys under Superimposed Hydrostatic Pressure Material Science and Technology 2009 MS Pittsburgh USA 23 29 Oct 2009 4 I El Galy B A Behrens Online monitoring of hot die forging processes using acoustic emission Part I Journal of Acoustic Emission Vol 26 2008 5 I El Galy B A Behrens Online monitoring of hot die forging processes using acoustic emission Part II Proceedings of 29th European Conference on Acoustic Emission Testing Vienna 2010 6 K Ono New goals for acoustic emission in material research Keynote lecture in acoustic emission beyond the millennium Tokyo Japan 2000 7 E Doege B A Behrens Handbuch Umformtechnik Springer Verlag Berlin 2007 8 E Haibach Betriebsfestigkeit Verfahren und Daten zur Bauteilberechnung 3rd Edition Springer Verlag Berlin 2006 9 P F Bariani S Bruschi T Dal Negro A New Constitutive Model for Hot Forging of Steels Taking Into Account the Thermal and Mechanical History CIRP Annals Manufacturing Technology Volume 49 pp 195 198 2000 10 C C Liu J H Wu C C Kuo Low Cycle Fatigue of DIN 1 2367 Steels in Various Treatments Fracture of Nano and Engineering Material