Numerical and Experimental Investigation of Interface Bonding Via Substrate Remelting of an Impinging Molten Metal Droplet

Abstract

A molten metal droplet landing and bonding to a solid substrate is investigated with combined analytical, numerical, and experimental techniques. This research supports a novel, thermal spray shape deposition process, referred to as microcasting, capable of rapidly manufacturing near netshape, steel objects. Metallurgical bonding between the impacting droplet and the previous deposition layer improves the strength and material property continuity between the layers, producing high-quality metal objects. A thorough understanding of the interface heat transfer process is needed to optimize the microcast object properties by minimizing the impacting droplet temperature necessary for superficial substrate remelting, while controlling substrate and deposit material cooling rates, remelt depths, and residual thermal stresses. A mixed Lagrangian–Eulerian numerical model is developed to calculate substrate remelting and temperature histories for investigating the required deposition temperatures and the effect of operating conditions on remelting. Experimental and analytical approaches are used to determine initial conditions for the numerical simulations, to verify the numerical accuracy, and to identify the resultant microstructures. Numerical results indicate that droplet to substrate conduction is the dominant heat transfer mode during remelting and solidification. Furthermore, a highly time-dependent heat transfer coefficient at the droplet/substrate interface necessitates a combined numerical model of the droplet and substrate for accurate predictions of the substrate remelting. The remelting depth and cooling rate numerical results are also verified by optical metallography, and compare well with both the analytical solution for the initial deposition period and the temperature measurements during droplet solidification.

Numerical and experimental analysis of residual stresses induced in metal coatings thermally deposited (HVOF) on Al2O3 substrates

“…The duration of these two phases is much shorter than that necessary for the droplet to solidify and, thus, the initial spreading phase can be neglected (Amon et al, 1996). This assumption decouples the problem from the fluid dynamics and permits it to be simplified to a thermo-mechanical problem.…”

On the mechanism of mushy layer formation during droplet-based processing

“…They found that h sharply increases to at the upper surface exceeds the solidus temperature may a maximum, then rapidly decreases, and finally reaches a be fully liquid instead of mushy, since a pure metal or a relatively steady state. Amon et al [55] also calculated h dureutectic alloy solidifies at a single temperature.…”

The fusion process of successive droplets impinging onto a substrate surface

“…Near net shape parts are fabricated by sequentially depositing molten droplets layer by layer [2]. Up to now, the generation of molten metal droplets has been developed into two main modes: continuous jetting [3][4][5][6][7][8] and dropon-demand (DOD) jetting [9,10]. The former mode can produce high-frequency droplets with a piezoelectric ceramic actuator.…”