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

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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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

Process Mapping of Transient Melt Pool Response in Wire Feed E-Beam Additive Manufacturing of Ti-6Al-4V

Wire feed electron beam additive manufacturing processes are candidates for manufacturing and repair in the aerospace industry. In order to implement feedback or feedforward control approaches, the time needed for a change in process variables to translate into changes in melt pool dimensions is a critical concern. In this research, results from 3D finite element simulations of deposition of Ti-6Al-4V are presented quantifying the transient response of melt pool dimensions to rapid changes in beam power and travel velocity. Results are plotted in beam power vs. beam velocity space, following work by the authors developing P-V Process Maps for steady-state melt pool geometry. Transient responses are determined over a wide range of process variables. Simulation results are compared to initial results from experiments performed at NASA Langley Research Center.Mechanical Engineerin

Separation of crack extension modes in composite delamination problems

This work concerns fracture mechanics modeling of composite delamination problems. In order to predict delamination resistance, an applied stress intensity factor, K, or energy release rate, G, must be compared to a mode-dependent critical value of K or G from experiment. In the interfacial fracture analysis of most applications and some tests, the mode of crack extension is not uniquely defined. It is instead a function of distance from the crack tip due to the oscillating singularity existing at the tip. In this work, a consistent method is presented of extracting crack extension modes in such cases. In particular, use of the virtual crack closure technique (VCCT) to extract modes of crack extension is studied for cases of a crack along the interface between two in-plane orthotropic materials. Modes of crack extension extracted from oscillatory analyses using VCCT are a function of the virtual crack extension length, delta. Most existing efforts to obtain delta-independent modes of crack extension involve changing the analysis in order to eliminate its oscillatory nature. One such method involves changing one or more properties of the layers to make the oscillatory exponent parameter, epsilon, equal zero. Standardized application of this method would require consistent criteria for identifying which properties can be altered without changing the physical aspects of the problem. Another method involves inserting a thin homogeneous layer (typically referred to as a resin interlayer) along the interface and placing the crack within it. The drawbacks of this method are that it requires increased modeling effort and introduces the thickness of the interlayer as an additional length parameter. The approach presented here does not attempt to alter the interfacial fracture analysis to eliminate its oscillatory behavior. Instead, the argument is made that the oscillatory behavior is non-physical and that if its effects were separated from VCCT quantities, then consistent, delta-independent modes of crack extension could be defined. Knowledge of the near-tip fields in a planar orthotropic material interfacial fracture analysis is used to determine the explicit delta dependence of VCCT parameters. Once this delta dependence is determined, energy release rates are defined with this delta dependence factored out. This modified VCCT method is applied to results from two finite element test cases. It is shown that, as predicted, delta-independent modes of crack extension result. The modified VCCT approach shows potential as a consistent method of extracting crack extension modes. It uses the same information from a finite element analysis (i.e., nodal forces and displacements) as the traditional VCCT method does. The A-independent modes extracted using the modified VCCT approach can also be used as guides to test the convergence of finite element solutions

Separation of crack extension modes in orthotropic delamination models

In the analysis of an interface crack between dissimilar elastic materials, the mode of crack extension is typically not unique, due to oscillatory behavior of near-tip stresses and displacements. This behavior currently limits the applicability of interfacial fracture mechanics as a means to predict composite delamination. The Virtual Crack Closure Technique (VCCT) is a method used to extract mode 1 and mode 2 energy release rates from numerical fracture solutions. The mode of crack extension extracted from an oscillatory solution using the VCCT is not unique due to the dependence of mode on the virtual crack extension length, Delta. In this work, a method is presented for using the VCCT to extract Delta-independent crack extension modes for the case of an interface crack between two in-plane orthotropic materials. The method does not involve altering the analysis to eliminate its oscillatory behavior. Instead, it is argued that physically reasonable, Delta-independent modes of crack extension can be extracted from oscillatory solutions. Knowledge of near-tip fields is used to determine the explicit Delta dependence of energy release rate parameters. Energy release rates are then defined that are separated from the oscillatory dependence on Delta. A modified VCCT using these energy release rate definitions is applied to results from finite element analyses, showing that Delta-independent modes of crack extension result. The modified technique has potential as a consistent method for extracting crack extension modes from numerical solutions. The Delta-independent modes extracted using this technique can also serve as guides for testing the convergence of finite element models. Direct applications of this work include the analysis of planar composite delamination problems, where plies or debonded laminates are modeled as in-plane orthotropic materials

Scaling Effects in Laser-Based Additive Manufacturing Processes

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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

Accelerating Process Development for 3D Printing of New Metal Alloys

Addressing the uncertainty and variability in the quality of 3D printed metals can further the wide spread use of this technology. Process mapping for new alloys is crucial for determining optimal process parameters that consistently produce acceptable printing quality. Process mapping is typically performed by conventional methods and is used for the design of experiments and ex situ characterization of printed parts. On the other hand, in situ approaches are limited because their observable features are limited and they require complex high-cost setups to obtain temperature measurements to boost accuracy. Our method relaxes these limitations by incorporating the temporal features of molten metal dynamics during laser-metal interactions using video vision transformers and high-speed imaging. Our approach can be used in existing commercial machines and can provide in situ process maps for efficient defect and variability quantification. The generalizability of the approach is demonstrated by performing cross-dataset evaluations on alloys with different compositions and intrinsic thermofluid properties