Modeling and Convergence Analysis of Directed Energy Deposition Simulations With Hybrid Implicit / Explicit and Implicit Solutions

Abstract

Conventional metal manufacturing techniques are suitable for mass production. However, cheaper and faster alternatives are preferred for small batch sizes and individualized components. Directed energy deposition (DED) processes allow depositing metallic material in almost arbitrary shapes. They are characterized by cyclic heat input, hence heating and cooling every point in the workpiece several times. This temperature history leads to distribution of mechanical properties, distortions, residual stresses or even fatigue properties in the part. To avoid experimental trial-and-error optimization, different methods are available to simulate DED processes. Currently, the wire arc additive manufacturing (WAAM) is the most competitive DED process. In this work, a simulation method for the WAAM process is established and validated, which should be capable to calculate global effects (e.g. distortions, residual stresses) of real WAAM-processes with duration of hours and thousands of weld beads. The addition of beads and layers is simulated by the element birth and death technique. The elements are activated according to the movements of the heat source (arc). In this paper, the influence of the time step, the mesh size and the material properties of the inactive elements in hybrid implicit / explicit and fully implicit solutions are evaluated with respect to the computation time and stability. This investigation concludes several recommendations for AM-modelling. For example, a low Young’s modulus (100 N/mm²) for the inactive elements show nearly no influences on the welding simulation, but introduces numerical instabilities in case of multiple welding beads. The Young’s modulus should be increased to 1.000 N/mm² for small mesh-sizes, small step-sizes and many beads, even when it introduces unwanted stresses.

Evaluation of Porosity in AISI 316L Samples Processed by Laser Powder Directed Energy Deposition

“…The height of the elements was assumed to be equal to the layer height. This approach has shown good convergence in the literature [38]. The resulting mesh is shown in Figure 2.…”

Reinforcement of Tooling Using Residual Stresses Generated by Cladding by Arc Welding

“…A fully coupled thermomechanical implicit model having hexahedral solid elements with one-point integration and viscous hourglass control was used for the structural analysis. All simulations were performed with a double-precision solver (ls-dyna_smp_d_R12.0) in LS-DYNA (Ansys, Canonsburg, PA, USA) according to the investigations of [53].…”

“…Table 1 presents the various parameters of Goldak's heat source used in this study. The parameters n = 1, v = 7.5 mm/s, and f = 1 remain constant for all the models, while the remaining heat source parameters are changed according to the bead shapes, as explained in previous work [53,[58][59][60]. The welding parameters for all simulations were kept the same.…”

Stable honeycomb structures and temperature based trajectory optimization for wire-arc additive manufacturing

“…Basic information about the AM-modeling technique with death-birth elements in the simulation environment of LS-DYNA can be found in Israr et al (2018). Details like activation of the inactive thermal and mechanical material properties, as well as convergence analysis for coupled and uncoupled, pure implicit/ explicit and hybrid solvers, estimations of computation time, time step and mesh size can be seen in Buhl et al (2019). In this paper the optimal parameters are used.…”