Hot tearing criteria evaluation for direct-chill casting of an Al-4.5 pct Cu alloy

  • PDF / 346,226 Bytes
  • 10 Pages / 612 x 792 pts (letter) Page_size
  • 37 Downloads / 206 Views

DOWNLOAD

REPORT


I. INTRODUCTION

A hot tear is a fracture formed during solidification due to shrinkage and hindered contraction and lack of feeding, and it initiates in the mushy zone. Because of the complex mechanisms acting during the solidification of metals, the prediction of the hot tearing phenomenon is not an easy task. The complex nature of mushy properties adds additional difficulties when incorporating these in a hot tearing model. Several mechanisms of hot tearing have recently been reviewed.[1] Various criteria that might enable the prediction of hot tears have been proposed.[2–10] These criteria can be classified into those based on nonmechanical aspects such as feeding behavior,[2–4] those based only on mechanical aspects,[5–7] and those that combine these features.[8–10] Three criteria[2–4] based only on nonmechanical aspects are proposed by Feurer, Clyne and Davies, and Katgerman. Feurer’s criterion calculates the maximum feeding rate in relation to the shrinkage rate in the vulnerable temperature range, whereas Clyne and Davies’ criterion is based on the simpler approach of time spent in the vulnerable temperature range. Katgerman’s criterion combines both approaches. Also, three criteria[5–7] based only on mechanical aspects are proposed by Novikov, Prokhorov, and Magnin et al.. All of these mechanical criteria introduce an experimentally determined fracture strain that is compared with the thermal contraction and plastic strain (Prokhorov), the thermal contraction strain only (Novikov), and the plastic strain only (Magnin et al.). These six criteria[2–7] are the oldest criteria proposed in the literature and are rather simple in formulation so that they are easily implemented in finite-element method (FEM) simulations. Other criteria combining both mechanical and nonmechanical aspects[8–10] were proposed recently and include several parameters, which are complex in formulation and SUYITNO, Postgraduate Student, W.H. KOOL, Senior Scientist, and L. KATGERMAN, Professor, are with the Department of Materials Science and Engineering, Delft University of Technology, 2628 AL Delft, The Netherlands. Contact e-mail: [email protected] Manuscript submitted October 18, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A

implementation in FEM simulations. In the Rappaz– Drezet–Gremaud (RDG) model,[8] the pressure drop in the mush is introduced as the main factor for hot tear development. The pressure drop is equal to the difference of the metallostatic pressure and the local pressure in the mush and depends on the location in the mush. A hot tear will develop if the local pressure in the mush becomes lower than a critical pressure for cavity formation (i.e., if the pressure drop becomes sufficiently high). However, in the model, a constant, externally applied strain rate is used and not a local strain rate. An extension of this model is proposed,[9] in which the strain rate is determined from the rheological properties of the mush. Further, the concept is introduced that the formation of a cavity will not automatically