Investigation of Development of Thermo-Mechanical Analysis Method for a Wire Feeding Type Directed Energy Deposition Process

Abstract

Directed energy deposition (DED) process is one of the important additive manufacturing (AM) processes to fabricate a near net shape metallic part. The DED process can be categorized into powder feeding type and wire feeding type. The wire feeding type DED process is highly suitable for building large volume metallic part because it is capable to produce part at high deposition rate with minimal wastage. Due to rapid change of temperature during the material deposition process, undesired distortion and residual stresses are induced in parts fabricated using wire feeding type DED process. In order to control these distortion and residual stress without performing experiment, modeling of the process using finite element analyses is preferred. However, thermo-mechanical analyses for a wire feeding type DED process have yet to be developed properly. Hence, a systematic development of thermo-mechanical analysis for a wire feeding type DED process is required.

The aim of this thesis is to develop a new comprehensive method in developing a thermo-mechanical analysis model for a wire feeding type DED process. A bead deposited using a wire feeding type DED process has a curved cross section profile with a few millimeters high. The thick layer of deposited bead influences the thermo-mechanical characteristic of a DED-fabricated part. The cross section profile of bead depends on the process parameter such as power of laser and travel speed of table. In order to properly represent a deposited bead according to process parameter, a novel methodology to estimate the cross section of deposited bead for different power of laser and travel speed of table has been proposed. The cross section profiles of deposited beads for different power of laser and travel speed are derived via regression analysis of the cross section profile of bead extracted from experiments. The cross section area of the estimated profile and measured profile have been compared and discussed. Subsequently, methodologies to create bead profile for multi-layer and planar depositions are proposed. A unique polygonal-shaped bead profile and constant width model are proposed to estimate the cross section profile for a multi-layer deposition. A flat top model is introduced and discussed in order to obtain the cross section profile for a planar deposition. These proposed methodologies to estimate proper deposition bead profile for multi-layer and planar deposition of a wire feeding type DED process according to process parameter have been implemented for creation of FE models.

In order to produce an acceptable FEA results, a heat flux model must be properly selected and calibrated to represent the heat source used for a DED process. A Gaussian distribution double ellipsoidal heat flux model is typically assumed for a DED process by previous reseachers. However, it is noticed that the heat flux model is not proper for a wire feeding type DED process in this thesis, which applies a top-hat distribution beam with small radius. Hence, a top-hat distribution heat flux model with consideration of efficiency and penetration depth is introduced to properly emulate the measured intensity distribution of laser beam used in the wire feeding type DED process. The moving heat flux model is implemented in welding-based commercial FEA software SYSWELD. Various heat transfer FEAs have been carried out using different efficiency and penetration depth of heat flux model on different shape of deposited beads of FE models, created according to process parameters. From the results of thermal FEAs, the predicted geometries of heat affected zone (HAZ) have been compared to those measured depth and width of HAZ from experimental study. A novel methodology to select a proper penetration depth and efficiency of heat flux model for each combination of power of laser and travel speed of table has been introduced. From the selection of penetration depth and efficiency of heat flux model, the heat flux model is properly calibrated for each combination of power of laser and travel speed of table of a wire feeding type DED process.

The thermo-mechanical FEAs have been carried out using the calibrated heat flux model to investigate influence of process parameters such as power of laser and travel speed of table on the formation of heat affected zone (HAZ), stress influenced region (SIR) and induced thermal stress. A proper gap between deposition beads is proposed by comparing the width of HAZ and stress influenced region (SIR). The gap between adjacent deposition beads is required to avoid the effect of heat and residual stress between two adjacent parts deposited on a single substrate. In addition, the influence of angle of corner deposition on the formation of HAZ, SIR and thermal stress has been investigated and discussed. In order to minimize the thermal stress on the deposited part, the thermo-mechanical characteristics during planar and multi-layer deposition of a wire feeding type DED process have been studied according to deposition pattern and interpass time. From the results of thermo-mechanical FEAs, a proper deposition strategy has been suggested. The vertical displacements of measured points on a cantilevered specimen have been compared to the predicted vertical displacement from FEAs, in order to validate the thermo-mechanical FEA model. The influences of interpass time and deposition pattern on the formation of thermal stress of a cantilevered specimen have investigated and discussed using the validated thermo-mechanical FEA model. In order to extend the applicability of the thermo-mechanical FEA model for a wire feeding type DED process for high intensity heat source using a general FEA software, an activation algorithm of finite element has been proposed. A selection criterion of mesh size of finite element and number of front element cross section of bead have been introduced and discussed in order to properly model the heat transfer during a DED process using a high intensity heat source with small beam radius. The selection algorithm of finite elements to be activated has been implemented using MATLAB. Subsequently, heat transfer and mechanical FEAs have been performed using ABAQUS to progressively simulate the addition of material during a DED process. Finally, the proposed activation algorithm of finite element has been implemented in ABAQUS in order to obtain the thermo-mechanical characteristics of a multi-layer deposition on a cantilevered specimen. The results of the FEAs have been examined and verified by comparing them to measured displacements of experimental results.