Optimal design for additive manufacturing of heterogeneous objects using ultrasonic consolidation

Abstract

Rapid prototyping (RP) technologies, such as Laser Engineering Net Shaping (LENS®) and Ultrasonic Consolidation (UC), can be used to fabricate heterogeneous objects composed of more than one material, wherein spatially varied microscopic structural details produce continuously or discretely changing mechanical or thermal properties on a macroscopic scale. These objects are engineered to achieve a potentially enhanced functional performance. Past research on the design of such objects has focused on representation, modeling, and desired functional performance. However, the inherent constraints in RP processes, such as system capability, size and shape of raw materials, and processing time, lead to fabricated objects that may not meet the designer's original intent. To overcome this situation, the research presented in this paper focuses on developing an approach— Design for Additive Manufacturing (DfAM)—to implement identified manufacturing constraints into the design process. Previous work has applied DfAM to the design of heterogeneous objects fabricated using the LENS® process. Two manufacturing constraints for this process, namely the achievable volume fractions and the processing time, were identified and incorporated into the DfAM. In this paper, the DfAM approach is extended to the design and manufacture of heterogeneous objects for the UC process. Constraints on the possible volume fraction values and on the gradient material direction are two identified manufacturing limitations, which are incorporated into the design process. An element-based finite element (FE) representation is extended to model layered heterogeneous objects. Each element is composed of metal foils of different materials according to specific design parameters. An evolutionary-based optimizer is used for its ability to handle the type of multi-modal problems encountered in the design of heterogeneous objects. The multi-criteria design problem, consisting of finding the optimal material composition along the build direction, that satisfies the functions of minimum weight and structural deformation, is implemented and solved. A three-dimensional I-beam made of two materials—aluminum for lightweight and steel for better strength characteristics—is used to illustrate the DfAM approach and its implementation for the design of heterogeneous objects using the UC process.