• PDF / 929,461 Bytes
• 10 Pages / 608.4 x 786 pts Page_size
• 101 Downloads / 260 Views

DOWNLOAD

REPORT

---

7/1/04

7:52 PM

Page 2893

Effects of the Entrained Surface Film on the Reliability of Castings J. MI, R.A. HARDING, and J. CAMPBELL The tilt pouring and gravity top pouring of an Al-4.5 pct Cu alloy have been studied. A computercontrolled rollover casting wheel was used to perform the tilt pouring. Filling sequences with tranquil or turbulent flow patterns have been visualized using real-time X-ray video radiography and modeled using a computational fluid dynamics (CFD) code. The area of the free surface film entrained into the bulk of liquid metal in different filling conditions has been calculated using a filling sequence free from surface turbulence as a baseline. The tensile properties of the castings have been quantitatively assessed for reliability using a two-parameter Weibull distribution function. The study reveals that the liquid metal flow in the mold filling process can be accurately simulated using a CFD code. In addition, the computed total surface area of the entrained surface film can be used as a criterion to judge the deterioration of reliability. The high Weibull modulus achieved by filling a mold without surface turbulence was reduced by a factor of 2.5 of its original value by entrained surface films. Entrainment of bubbles required surface turbulence, but folded films could be entrained simply by contraction of the free surface, creating excess surface film that necessarily folds inward.

I. INTRODUCTION

where

THE surface turbulence generated when a mold is filled with liquid metal to produce a casting has been recognized as the cause of the entrainment of the surface film into the bulk of the liquid metal. The entrainment mechanism is a folding process that results in surface films being folded dry side to dry side during filling to produce bi-films, which persist in the solidified castings. The lack of bonding across the folded interface constitutes a crack. The random size and distribution of cracks act to reduce the mechanical properties and the reliability of the castings.[1] The liquid metal flow in a mold can be observed and recorded by such techniques as a heat-resistant glass window embedded onto the mold[2,3] or by X-ray video radiography.[4,5] However, it is not possible for these techniques to determine the complete flow field and to provide quantitative data about the free surface generated during the filling, and it is therefore beneficial to supplement these studies with computer simulation. It is common practice to use the Gaussian normal distribution to assess the scatter in properties (such as tensile strength) of a material, and this can be characterized by the standard deviation. However, this ignores the cutoff point at the theoretical maximum tensile strength. The Weibull statistical function[6,7] is a useful means of describing data that are skewed about their mean value. When it is used to characterize fracture strength distributions, the Weibull distribution is frequently used in the two-parameter form as x l FW  1exp c a b d s

[1]

J. MI, Research Fellow, form