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

Process Maps for Controlling Residual Stress and Melt Pool Size in Laser-Based SFF Processes 200

Thermal control of solid freeform fabrication processes is critical for obtaining consistent build conditions and in limiting residual stress-induced tolerance losses. In this paper, thermomechanical models are presented for the building of thin-walled structures by laser-based SFF processes. The simulations are used to develop two non-dimensional plots (termed process maps) that quantify the effects of changes in wall height, laser power, deposition speed and part preheating on melt pool size (for consistent build conditions) and thermal gradients (for limiting residual stresses). Mechanical simulations are used to demonstrate the link between thermal gradients and maximum final residual stresses. Models are applied to the Laser Engineered Net Shaping (LENS) process; however, the general approach, insights and conclusions are applicable to most SFF processes involving a moving heat source. The two process maps described herein can be used together to determine optimal process variables for obtaining consistent melt pool length while limiting residual stress in the part. Results from the residual stress simulations also identify two important mechanisms for reducing residual stresses and quantify maximum stress reductions that can be achieved through manipulation of all process variables.This research has been supported by the National Science Foundation under grant DMI9700320 and by Sandia National Laboratories under grant BE-0792.Mechanical Engineerin

Effect of scanning strategies on residual stress and mechanical properties of Selective Laser Melted Ti6Al4V

During the Selective Laser Melting (SLM) process large temperature gradients can form, generating a mismatch in elastic deformation that can lead to high levels of residual stress within the additively manufactured metallic structure. Rapid melt pool solidification causes SLM processed Ti6Al4V to form a martensitic microstructure with a ductility generally lower than a hot working equivalent. Currently post-process heat treatments can be applied to SLM components to remove in-built residual stress and improve ductility. This study examined the effect of scanning strategy (scan vector lengths and scan vector rotation) and rescanning strategy on residual stress formation and mechanical properties of SLM Ti6Al4V parts. 90° alternating scanning strategy resulted in the lowest residual stress build-up for SLM Ti6Al4V parts built on both the standard and modified Renishaw platforms using a modulated Nd-YAG fiber laser. Scanning strategy did not show any direct correlation with mechanical properties. Re-scanning with 150% energy density resulted in 33.6% reduction in residual stress but the effect on mechanical properties was detrimental and samples failed prematurely. The study was based on detailed experimental analysis along with Finite Element simulation of the process using ABAQUS to understand the underlying physics of the process

Modeling of phase transformations of Ti6Al4V during laser metal deposition

[EN] The low density, excellent high temperature mechanical properties and good corrosion resistance of titanium and its alloys have led to a diversified range of successful applications. As a consequence, there is a demand of increasing the capabilities of processing such alloys. The laser cladding technique allows direct metal deposition with an excellent metallurgical bond and a pore free fine grained microstructure. A nonlinear transient thermo-metallurgical model was developed to study the technique with titanium alloys to get a better understanding of the thermal and metallurgical underlying aspects. The calculated temperatures and phase transformations are compared with experimental tests. © 2011 Published by Elsevier Ltd.This work is has been done under the research project MAT2008-06882-C04 funded by the Spanish government (MICIIN).Suárez, A.; Tobar, MJ.; Yañez, A.; Pérez, I.; Sampedro, J.; Amigó Borrás, V.; Candel Bou, JJ. (2011). Modeling of phase transformations of Ti6Al4V during laser metal deposition. Physics Procedia. 12(Part A):666-673. https://doi.org/10.1016/j.phpro.2011.03.083S66667312Part

In-situ residual stress reduction, martensitic decomposition and mechanical properties enhancement through high temperature powder bed pre-heating of Selective Laser Melted Ti6Al4V

During the Selective Laser Melting (SLM) process large temperature gradients can form, generating a mismatch in elastic deformation that can lead to high levels of residual stress within the additively manufactured metallic structure. Rapid melt pool solidification causes SLM processed Ti6Al4V to form a martensitic microstructure with a ductility generally lower than a hot working equivalent. Post-process heat treatments can be applied to SLM components to remove in-built residual stress and improve ductility. The use of high temperature pre-heating during an SLM build can assist in reducing thermal gradients, enable a more controlled cooling with the possibility to control/tailor as-built mechanical properties. In this work a high temperature SLM powder bed capable of pre-heating to 800°C is used during processing of Ti6Al4V feedstock. The effect of powder bed temperature on residual stress formation, microstructure and mechanical properties was investigated. It was found that increasing the bed temperature to 570°C significantly reduced residual stress formation within components and enhanced yield strength and ductility. This pre-heating temperature enabled the decomposition of α′α′ martensitic microstructure into an equilibrium α+β microstructure. At 570°C the yield strength and elongation of components was improved by 3.2% and 66.2% respectively

Processing Parameter Effects on Residual Stress and Mechanical Properties of Selective Laser Melted Ti6Al4V

Selective laser melting (SLM) process is characterized by large temperature gradients resulting in high levels of residual stress within the additively manufactured metallic structure. SLM-processed Ti6Al4V yields a martensitic microstructure due to the rapid solidification and results in a ductility generally lower than a hot working equivalent. Post-process heat treatments can be applied to SLM components to remove in-built residual stress and improve ductility. Residual stress buildup and the mechanical properties of SLM parts can be controlled by varying the SLM process parameters. This investigation studies the effect of layer thickness on residual stress and mechanical properties of SLM Ti6Al4V parts. This is the first-of-its kind study on the effect of varying power and exposure in conjunction with keeping the energy density constant on residual stress and mechanical properties of SLM Ti6Al4V components. It was found that decreasing power and increasing exposure for the same energy density lowered the residual stress and improved the % elongation of SLM Ti6Al4V parts. Increasing layer thickness resulted in lowering the residual stress at the detriment of mechanical properties. The study is based on detailed experimental analysis along with finite element simulation of the process using ABAQUS to understand the underlying physics of the process

Melt Pool Size Control in Thin-Walled and Bulky Parts via Process Maps

Control of melt pool size is critical for maintaining consistent build conditions in the solid freeform fabrication of complex shapes. In this research, melt pool size in laser-based solid freeform fabrication processes is studied for both thin-walled structures and bulky parts. Numerical simulations are used to construct non-dimensional plots (termed process maps) that quantify the effects of changes in part height, laser power, deposition speed and part preheating on melt pool size. Strategies for the control of melt pool size suggested by the process maps are similar for the two geometries. Insights are given as to how transitions between these two extremes in geometry can be managed to maintain a consistent melt pool size. This modeling work is being performed in tandem with process development and melt pool imaging and control research underway on the LENS process at Sandia National LaboratoriesThis research has been supported by the National Science Foundation under grant DMI9700320 and by Sandia National Laboratories under grant BE-0792.Mechanical Engineerin