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Product Code: ICAL05_1203

Active Control of Heat-Induced Deformation in a Laser Powder Deposition Process
Authors:
Umesh Korde, South Dakota School of Mines and Technology; Rapid City SD USA
Michael Langerman, South Dakota School of Mines and Technology; Rapid City SD USA
Matthew Hainy, South Dakota School of Mines and Technology; Rapid City SD USA
Travis Zelfer, South Dakota School of Mines and Technology; Rapid City SD USA
James Sears, South Dakota School of Mines and Technology; Rapid City SD USA
Presented at ICALEO 2005

Laser additive manufacturing has significant advantages over conventional manufacturing processes that are important in a variety of fabrication, repair, and surface treatment applications. These advantages, however, are offset to an extent by the build-up of internal thermal stresses and associated residual deformations in both the part and the substrate. Indeed, some situations may require considerable post-processing of the part following laser deposition. There is therefore a need for new deposition techniques or other strategies that could be applied in real time to alleviate this problem. This paper discusses results from ongoing research aimed at investigating the feasibility of active control to minimize residual deformations in the deposition substrate and the part. Current work focuses on the use of �smart-material� based actuators distributed over the substrate on which the part is fabricated. Our goal is to test both SMA (shape memory alloy) and PZT (piezoelectric Lead Zirconate Titanate) actuators but the present paper describes tests with PZT actuators. Results are obtained for a straight, thin-walled Titanium build on a Titanium substrate. Insulated rectangular PZT patches are firmly adhered to the upper and lower surfaces of the substrate, sufficiently separated from, and on both sides of, the build-line. DC voltage of chosen polarity can be applied independently to each actuator. While a variety of feedforward optimal switching algorithms are possible, ongoing model calculations and experiments focus on a simple strategy in which the upper and lower surfaces are each actuated uniformly, and voltages are applied to the two faces with opposite polarity (so that the substrate behaves as a bimorph) to pre-deform the substrate. The voltage magnitude is chosen such that subsequent material deposition and associated thermal deformations will �relax� the substrate back to its original geometry. Given the limitations on the actuation authority of PZT patches, this approach is expected to work only for surface cladding type applications or for slender thin-walled builds. As expected, high voltages are required, and calculations based on a thermal model are used to enable careful design and selection (placement, Curie temperature, etc.) of the PZT actuators. The paper concludes with remarks on the feasibility of extending the PZT based approach to other types of builds and a discussion of possible control strategies to enable the best use of available PZT actuation authority.

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