Localized Preheating Approaches for Reducing Residual Stress in Additive Manufacturing

Uniform preheating can be used to limit residual stress in the solid freeform fabrication of relatively small parts. However, in additive manufacturing processes, where a feature is deposited onto a much larger part, uniform preheating of the entire assembly is typically not practical. This paper considers localized preheating to reduce residual stresses, building on previous work using a defined thermal gradient through the part depth as a metric for predicting maximum final residual stress. The building of thinwalled structures is considered. Two types of localized preheating approaches are compared, appropriate for use in laser- or electron beam-based additive manufacturing processes. In evaluating the effectiveness of each approach, a simplified thermomechanical model is used that can be related directly to analytical thermomechanical models for thermal stresses in unconstrained thin plates. Results are presented showing that one of the methods yields temperature profiles likely to yield reduced residual stresses at room temperature. Mechanical model results confirm this, showing a significant reduction in maximum stress values. A more complete thermomechanical simulation of thin wall fabrication is used to verify the trends seen in the simplified model results.Mechanical Engineerin

An analytical and experimental study of crack extension in center-notched composites

The normal stress ratio theory for crack extension in anisotropic materials is studied analytically and experimentally. The theory is applied within a microscopic-level analysis of a single center notch of arbitrary orientation in a unidirectional composite material. The bulk of the analytical work of this study applies an elasticity solution for an infinite plate with a center line to obtain critical stress and crack growth direction predictions. An elasticity solution for an infinite plate with a center elliptical flaw is also used to obtain qualitative predictions of the location of crack initiation on the border of a rounded notch tip. The analytical portion of the study includes the formulation of a new crack growth theory that includes local shear stress. Normal stress ratio theory predictions are obtained for notched unidirectional tensile coupons and unidirectional Iosipescu shear specimens. These predictions are subsequently compared to experimental results

Scaling Effects in Laser-Based Additive Manufacturing Processes

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Analysis of piezocone penetration under different drainage conditions with the two-phase Material Point Method

The piezocone penetration test (CPTU) is commonly used to identify the soil profile and to estimate material properties. Depending on the soil type, ranging from clay to sand, undrained, partially drained or drained conditions may occur during cone penetration. In silt and sand-clay mixtures the CPTU penetration is characterized by partially drained conditions, which are often neglected in data interpretation. The effect of drainage on CPTU measurements has been mainly studied experimentally. Numerical analyses are rare because taking into account large soil deformations, soil-water and soil-structure interactions, as well as non-linear soil behaviour is still a challenging task. This paper presents and discusses numerical simulations of CPTU in saturated soils with the two-phase Material Point Method. Soil behaviour is described with the Modified Cam Clay model. This study investigates the effects of pore pressure dissipation during penetration, cone roughness and horizontal stress state, comparing the results with experimental data. The paper discusses the effect of neglecting partial drainage in deriving the shear strength parameters for silty soils and suggests a procedure to estimate the consolidation coefficient performing CPTU at different penetration rates

Process Maps for Laser Deposition of Thin-Walled Structures

In solid freeform fabrication (SFF) processes involving thermal deposition, thermal control of the process is critical for obtaining consistent deposition conditions and in limiting residual stress-induced warping of parts. In this research, nondimensionalized plots (termed process maps) are developedJrom numerical models of laser-based material deposition of thin-walled structures that.map out the effects of changes in laser power, deposition speed and part preheating on process parameters. The principal application of this work is to the Laser Engineered Net Shaping (LENS) process under development at Sandia Laboratories; however, the approach taken is applicable to any solid freeform fabrication process involving. a moving heat source. Similarly, although thinwalled structures treated in the current work, the same approach could be applied to other commonly fabricated geometries. A process map for predicting and controlling melt pool size is presented .and numerically determined results are compared against experimentally measured melt poollengthsfor stainless steel deposition in the LENS process.Mechanical Engineerin

Transient Changes in Melt Pool Size in Laser Additive Manufacturing Processes

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Material Issues in Layered Forming

A brief overview of key issues in layered thermal processing is given. Incremental sintering and layered fusion ofpowder and molten droplets are discussed. The criteria for remelting the solid substrate are derivedfrom a one dimensional heat transfer model. Temperature gradients which occur during solidification and subsequent cooling. are responsible for the build up of internal stresses which can be estimated through establishing an elastic beam model. The difficulties as well as opportunities regarding the generation of multi-layer multi-material structures are also described in this article.Mechanical Engineerin

Process Scaling and Transient Melt Pool Size Control in Laser-Based Additive Manufacturing Processes

This modeling research considers two issues related to the control of melt pool size in laser-based additive manufacturing processes. First, the problem of process size scale is considered, with the goal of applying knowledge developed at one processing size scale (e.g. the LENSTM process, using a 500 watt laser) to similar processes operating at larger scales (e.g. a 3 kilowatt system under development at South Dakota School of Mines and Technology). The second problem considered is the transient behavior of melt pool size due to a step change in laser power or velocity. Its primary application is to dynamic feedback control of melt pool size by thermal imaging techniques, where model results specify power or velocity changes needed to rapidly achieve a desired melt pool size. Both of these issues are addressed via a process map approach developed by the authors and co-workers. This approach collapses results from a large number of simulations over the full range of practical process variables into plots process engineers can easily use.This research was supported by the National Science Foundation Division of Design, Manufacture and Industrial Innovation, through the Materials Processing and Manufacturing Program, award number DMI-0200270.Mechanical Engineerin