Densification during the supersolidus liquid-phase sintering of nickel-based prealloyed powder mixtures
- PDF / 675,302 Bytes
- 8 Pages / 612 x 792 pts (letter) Page_size
- 40 Downloads / 232 Views
I. INTRODUCTION
SUPERSOLIDUS liquid-phase sintering (SLPS) involves heating a prealloyed powder to temperatures between the solidus and liquidus to cause rapid densification.[1–7] The SLPS technique is possible using mixtures of two prealloyed powders.[8–11] Densification of prealloyed powders is often enhanced by the addition of simple additives which form a low-temperature liquid.[12–16] However, SLPS of mixtures containing two or more prealloyed powders is more complex, as interaction between the powders also needs to be considered. Tandon and German[8] investigated the processing parameters in the densification of mixed nickel-based superalloy powders. One powder was doped with boron to lower the melting-temperature range by formation of the Ni-B eutectic. The other powder had a higher melting range. In such systems, the low-melting powder forms a supersolidus liquid that densifies the mixed-powder structure and assists in rapid homogenization. The target final composition is achieved by interdiffusion of the constituent powders. Adjusting the powder mixture can control the densification process. An increase in the low-melting powder fraction accelerates densification. Alternatively, an increase in the high-melting powder fraction broadens the temperature range over which densification occurs. Thus, by making the sintering process sluggish, the addition of the high-melting powder provides a means to control the sintering rate, reduce the temperature sensitivity, and delay compact slumping. Fabrication of structural components requires conditions sufficient to cause complete densification and homogenization without distortion.[1] Accordingly, the fraction of the low-melting powder should be optimized. ANAND LAL, Research Assistant, RONALD G. IACOCCA, Director, Materials Characterization, and RANDALL M. GERMAN, Brush Chair Professor in Materials, are with the Department of Engineering Science and Mechanics, P/M Lab, The Pennsylvania State University, University Park, PA 16802-6809. Manuscript submitted October 15, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
A recent study by Lal et. al.[9] examined a system similar to the one explored by Tandon and German.[8] The study isolated critical events during the microstructural development, including events that lead to homogenization in the sintering of mixtures of two nickel-based alloy powders. The extent of liquid formation and the chemical interactions between the two powders were influenced by various process variables such as sintering temperature and time, heating rate, and ratio of the two powders. The diffusion of the melting depressant (boron) from the low-melting to the highmelting powder was critical to microstructural development. Boron diffusion from the low-melting powder continuously increased its solidus temperature and resulted in incomplete melting, even with peak temperatures above its liquidus. On the other hand, depending on the temperature, boron diffusion resulted in either boride precipitation or liquid formation in the high-melting powde
Data Loading...
