Prediction of Elastic Behavior of Sintered Metal Powder from the Ultrasonic Velocities of Green Compacts
- PDF / 528,674 Bytes
- 9 Pages / 593.972 x 792 pts Page_size
- 39 Downloads / 186 Views
I.
INTRODUCTION
POWDER metallurgy constitutes a major route for fabricating a large variety of metallic components with wide usage in various engineering and structural applications. The process involves making a green compact of metal powders in the shape of the component and the subsequent sintering of the same. The compaction process may involve cold uniaxial pressing, cold isostatic pressing (CIP), hot isostatic pressing (HIP), or some other techniques, as necessary. Usually, irregularly shaped powders are used to ensure that the resulting green compact is sufficiently strong, due to the interlocking and plastic deformation of the particles with their neighbors. The powder-processing steps invariably leave some amount of porosity in the material, which controls its physical properties. Also, the microstructure of the final product is highly dependent on the powder morphology as well as on the compaction method used for making the green compact.[1–3] In order to produce quality products, various parameters, such as the relative density or porosity and the elastic moduli, need to be monitored using nondestructive evaluation (NDE) techniques during the sintering process. Among the NDE techniques, ultrasonic methods have been used widely for characterizing these materials, with the help of longitudinal and shear wave velocities. These ultrasonic velocities, along with the relative density or porosity, provides an estimate of the elastic moduli of the porous powder compacts from the theory of physical acoustics, at various stages of sintering. In order that ultrasonics can be used as an effective quantitative nondestructive evaluation (QNDE) tool, the variation of these elastic properties with porosity K.K. PHANI and DIPAYAN SANYAL, Scientists, are with the Central Glass and Ceramic Research Institute, Kolkata, India. Contact e-mail: [email protected] Manuscript submitted May 18, 2007. Article published onlined February 16, 2008 790—VOLUME 39A, APRIL 2008
must be known. To achieve this objective, various empirical and theoretical relationships have been proposed in the literature,[4–6] for ceramics and metal compacts. Among these empirical models, the early work of Panakkal et al.[7] and the recent work of Yeheskel[8] relating to the elastic behavior of sintered iron powder need to be specifically mentioned. Panakkal et al.[7] had derived a linear relation between the effective modulus (M) and the ultrasonic longitudinal velocity (VL), based on the theory of elasticity. This theory is based on spherical pores of very low concentration (p £ 0.05). Therefore, it is not expected to hold for a wide range of porosity variations, as well as for different methods of compaction, resulting in different nonspherical pore morphology in each case. In a recent article, Yeheskel[8] suggested a third-degree polynomial relation between the normalized moduli and the normalized longitudinal ultrasonic velocity, assuming that the Poisson’s ratio is given by a power-law function of the normalized velocity. The empirical constants involved
Data Loading...
