Nucleation Kinetics and Microstructure Evolution of Traveling ASTM F75 Droplets

Abstract

ASTM F75 alloy, a cobalt-chromium-molybdenum alloy with high wear resistance, good strength, and excellent biocompatibility, is popularly used to manufacture artificial dental, hip, and knee replacements. [1] A novel method for using the alloy as a bioimplant involves fabricating a porous mating surface for the bone tissues to grow into for enhanced osseointegration (attachment of the bone to the implant). [2][3][4][5] This anchors the implant firmly to the bone, while making the implant's density and strength similar to those of the human bone. The osseointegration surface is fabricated by sintering balls of diameter 300-700 mm to obtain a porous surface. The present work explores the feasibility of producing F75 balls suitable for the bio-implant applications by the uniformdroplet spray (UDS) process, [6][7][8][9] with particular interest in developing a method to predict the solidification microstructure in relation to the process parameters.The UDS process is a droplet-based manufacturing process where mono-sized droplets are generated by the controlled breakup of a laminar jet of molten metal ejected through an orifice while applying a regulated perturbation. Figure 1(a) schematically illustrates the UDS droplet generator. Details of this process can be found elsewhere. [6][7][8][9] Droplets produced by the UDS process have high sphericity and nearly identical sizes (less than AE3% of target size) [7] and thermal history, which allows for precise control of droplet solidification microstructure. The thermal state of the traveling UDS droplets can be simulated using an in-flight solidification model [10,11] under Newtonian cooling conditions justified on the small Biot numbers that normally apply to UDS droplets. The process parameters of the UDS process, being decoupled, [6] allow for a systematic investigation of the solidification behavior of droplets as a function of experimental variables. The experimental variables are primarily represented by the droplet-cooling rate and volume of the droplet, which determine the amount of undercooling and the path of subsequent solidification, and hence the resultant solidification microstructure, of the droplet. In the UDS process, the droplet-cooling rate is controlled through the droplet size and the choice of cooling gas. The droplet volume, which affects not only the cooling rate but also the number of heterogenenous nucleation sites in a droplet, is controlled through the molten jet diameter, the mass flow rate and the frequency of the imposed perturbation, all of which can be varied independently from each other. This ability, unique to the UDS process, permits stringent control of solidification microstructure for virtually all the droplets in a UDS.In rapid solidification processing, high cooling rates are generally used to produce large prior supercoolings, which promote rapid solidification under a larger driving force, particularly during recalescence. In an alloy, the solid-liquid interface under such conditions normally takes a dendritic morphology, with...

Solidification behavior of highly supercooled polycrystalline silicon droplets

“…The observation of multi-grained microstructure in the droplets suggests the possibility of fragmentation of the silicon dendrites, which is commonly observed in alloys and reported earlier [16]. The fragmentation behavior of dendrites of pure materials is not expected to occur in the same manner as that for alloys.…”

“…However, fragmentation itself is considered to have occurred in all droplets r390 mm regardless of the sectioning and orientation, as predicted by the model and metallographically verified. Although fragmentation is ascertained to occur in pure silicon droplets when sufficiently supercooled Z172 K, the fragmentation is not as extensive as is commonly observed in alloy droplets [16]. This may be due to the rapid growth of the planar dendritic front in the postrecalescence stage which does not allow for complete fragmentation with formation of fine equiaxed grains.…”

“…The nucleation distances of the droplets were determined by the splat quenching technique where the droplets were impinged on rotating blades positioned below the orifice. This method for determining the nucleation distance was first used by Bialiauskaya and Ando [15] and later adapted by Roy and Ando [16]. Details of the experimental set-up are provided in Table 1.…”

Production, characterisation and application of monosize alloy droplets

“…Metals and alloys that have been sprayed by the UDS process include Sn and Sn base solder alloys, Al alloys, Mg alloys, Cu, Co alloys, Si and Fe alloys. [9][10][11][12][13][14][15][16][17][18][19][20] Figure 2 shows monosize balls of an Sn base solder alloy, 10 a Mg alloy (AZ91D) 11 and an Fe-4 mass-%B alloy 12 produced by allowing the droplets to solidify in flight. Work conducted to date suggests that droplets produced under a set of conditions have a diameter within ¡3% of the target diameter and that production of monosize droplets as small as 30 mm in diameter is possible.…”

“…Figure 7 shows 240 μm amorphous balls of an Fe–Ni–Co–Mo–B–Si alloy produced under a condition for amorphous solidification determined on a CCT curve calculated for crystalline solidification 47 Figure 8. shows cross-sections of a 300 μm droplet of a Co alloy (ASTM F75)13,17 and a 300 μm droplet of silicon produced by the UDS process 16. The fine microcrystalline structure of the F75 ball was obtained under a process condition that caused dendrite fragmentation 4.…”