Metal Forming Prevention of Internal Cracks in Forward Extrusion by Means of Counter Pressure: A Numerical Treatise C. Soyarslan 1),2) and A. E. Tekkaya 3) 1) Atılım University, Department of Manufacturing Engineering, 06836, Ankara, Turkey 2) Middle East Technical University, Department of Civil Engineering, 06531, Ankara, Turkey 3) University of Dortmund, Institute of Forming Technology and Lightweight Construction, 44227, Dortmund, Germany In the context of forward bulk extrusion, where product defects are frequently observed, the effect of counter pressure on damage accumulation materializing a Continuum Damage Mechanics (CDM) approach is presented. A Lemaitre variant damage model accounting for unilateral damage evolution coupled with a multiplicative finite plasticity is utilized for this purpose. After a presentation of the crack governing mechanism, it is demonstrated that application of counter pressure introduces a marked decrease in the central damage accumulation, which in turn increases the formability of the material through keeping the tensile triaxiality in tolerable limits. It is also shown that, for a crack involving process, through systematic increase of the counter pressure, the crack sizes diminish; and at a certain level of counter pressure chevron cracks can be completely avoided. Keywords: forward extrusion; counter pressure; chevron cracks; ductile damage; finite elements. DOI: 10.2374/SRI08SP170; submitted on 12 November 2008, accepted on 5 March 2009 Introduction Cold forward extrusion, as designated in Figure 1, is a bulk metal forming process where a workpiece is forced through a die with the aid of a punch in order to obtain an area reduction. Besides its considerable cost efficiency, extrusion is advantageous due to gained hardness, fatigue resistance, accuracy, net shape and surface qualities of the product. With reference to Figure 1, the governing process parameters consist of reduction in area, [1 - (d 1 /d 0 ) 2 ], die semi-cone angle, α, friction at the die-workpiece interface, working temperature, material ductility and micro- structure. The workability of the metal, which stands for the extent of deformation that can be achieved without cracking, strongly depends on the choice of these parameters where improper combinations may result in defected extrusion products. These defects emerge in the form of surface cracks (also called snake-skin, or fir-tree) or internal cracks (also called chevron cracks or central bursts). Central bursts are internal arrow shaped cracks normal to the central axis of the workpiece. Their formation results in a drastic decrease in the load carrying capacity of the product. Standard surface investigation methods fall short in detection of these defects due to their insidious nature. Hence, elimination of possibly defected products necessitates a proper investigation by nondestructive ultrasonic testing. This burden has attracted many researchers to examine the governing mechanism of crack formation in forward extrusion for more than seven decades through experimental, analytical and numerical methods. Jennison’s [1] experimental works go back to the early 1930s. For conical dies, using the upper bound theorem, percent reduction versus die semi-cone angle diagrams have been introduced in the analytical studies of Avitzur [2] for perfect plasticity and by Zimerman and Avitzur [3] for hardening plasticity. Although such curves serve a very practical purpose of selecting process geometries, their generalization to different material ductilities and internal structures in addition to different frictional and thermal conditions is questionable. Numerical analysis which materializes finite elements emerges at this level as a suitable tool where problems involving complicated geometries and boundary conditions can be solved for many thermo-mechanical material models, based on micro-mechanical or phenomenological assumptions, frictional conditions, which are readily accessible through certain mathematical models. With this motivation, there have been numerous numerical studies on damage accumulation in forward extrusion which materialize Figure 1. Forward extrusion process geometry. steel research int. 80 (2009) No. 9 4