RESIDUAL STRESS IN TI6AL4V OBJECTS PRODUCED BY DIRECT METAL LASER SINTERING

Published ArticleDirect Metal Laser Sintering produces 3D objects using a layer-by-layer method in which powder is deposited in thin layers. Laser beam scans over the powder fusing powder particles as well as the previous layer. High-concentration of laser energy input leads to high thermal gradients which induce residual stress within the as-built parts. Ti6Al4V (ELI) samples have been manufactured by EOSINT M280 system at prescribed by EOS process-parameters. Residual stresses were measured by XRD method. Microstructure, values and directions of principal stresses inTi6Al4V DMLS samples were analysed

Heat Conduction and Geometry Topology Optimization of Support Structure in Laser-based Additive Manufacturing

Laser-based metal additive manufacturing technologies such as Selective Laser Sintering (SLS) and Selective Laser Melting (SLM) allow the fabrication of complex parts by selectively sintering or melting metallic powders layer by layer. Although elaborate features can be produced by these technologies, heat accumulation in overhangs leads to heat stress and warping, affecting the dimensional and geometrical accuracy of the part. This work introduces an approach to mitigate heat stress by minimizing the temperature gradient between the heat-accumulated zone in overhangs and the layers beneath. This is achieved by generating complex support structures that maintain the mechanical stability of the overhang and increase the heat conduction between these areas. The architecture of the complex support structures is obtained by maximizing heat conduction as an objective function to optimize the topology of support structure. This work examines the effect of various geometries on the objective function in order to select a suitable one to consume less material with almost same conduction. Ongoing work is the development of an experimental testbed for verification

Efficient Multiscale Prediction of Cantilever Distortion by Selective Laser Melting

Large tensile residual stress is one major issue for metal components made by selective laser melting (SLM). Residual stress is induced by non-uniform heat input, which leads to part distortion and detrimentally affects product performance. The conventional single track simulation method is not feasible to predict the distortion of a macro part since it demands an exceedingly long computational time. The coupling multiphysics phenomenon during the SLM process further complicates this issue. In this study, a temperature-thread multiscale modeling approach has been developed to predict part distortion of a twin cantilever. An equivalent body heat flux calculated from the micro scan model was imported as the “temperature-thread” to the subsequent layer hatch model. Then the hatched layer with temperature field can be used as a basic unit to build up the macro part. The temperature history and residual stress fields during the SLM process were predicted. And the distortion of twin cantilever was calculated with a reasonable accuracy compared to the experimental data.Mechanical Engineerin

Effects of Inter-Layer Time Interval on Temperature Gradients in Direct Laser Deposited Ti-6Al-4V

Parts fabricated via additive manufacturing (AM) methods are prone to experiencing high temperature gradients during manufacture resulting in internal residual stress formation. In the current study, a numerical model for predicting the temperature distribution and residual stress in Directed Energy Deposited (DED) Ti–6Al–4V parts is utilized for determining a relationship between local part temperature gradients with generated residual stress. Effects of time-interval between successive layer deposits, as well as layer deposition itself, on the temperature gradient vector for the first and each layer is investigated. The numerical model is validated using thermographic measurements of Ti-6Al-4V specimens fabricated via Laser Engineered Net Shaping® (LENS), a blown-powder/laser-based DED method. Results demonstrate the heterogeneity in the part’s spatiotemporal temperature field, and support the fact that as the part number, or single part size or geometry, vary, the resultant residual stress due to temperature gradients will be impacted. As the time inter-layer time interval increases from 0 to 10 second, the temperature gradient magnitude in vicinity of the melt pool will increase slightly.Mechanical Engineerin

A Temperature-Thread Multiscale Modeling Approach for Efficient Prediction of Part Distortion by Selective Laser Melting

Selective laser melting (SLM) is a powder bed based additive manufacturing process to manufacture functional parts. The high-temperature process will produce large tensile residual stress which leads to part distortion and negatively affect product performance. Due to the complex process mechanism and coupling multi-physics phenomena, the micro-scale single laser scan modeling approach is not practical to predict macro part distortion since it demands an exceedingly long computational time. In this study, a temperature-based multiscale modeling approach has been developed to simulate material phase transition of powder-liquid-solid for fast prediction of part distortion. An equivalent body heat flux obtained from the micro-scale laser scan can be imported as “temperature-thread” to the subsequent layer hatching process. Then the hatched layer with temperature filed can be used as a basic unit to build up the macro-scale part with different scanning strategies. The temperature history and residual stress fields during the SLM process were obtained. In addition, the part distortion can be predicted with a reasonable accuracy by comparing with the experimental data.Mechanical Engineerin

Residual Stress in Metal Specimens Produced by Direct Metal Laser Sintering

Direct Metal Laser Sintering (DMLS) has great potential in additive manufacturing because it allows the production of full-density complex parts with the desired inner structure and surface morphology. High temperature gradients, as a result of the locally concentrated energy input, lead to residual stresses, crack formation and part deformations during processing or after separation from the supports and the substrate. In this study, an X-ray diffraction technique and numerical simulation were used for investigation of the residual stress in DMLS samples fabricated from stainless steel 316L and Ti6Al4V alloy. Conclusions regarding directions and values of stresses in DMLS objects are given.Mechanical Engineerin

Thermal Stresses Associated with Part Overhang Geometry in Electron Beam Additive Manufacturing: Process Parameter Effects

For powder-bed electron beam additive manufacturing (EBAM), support structures are required when fabricating an overhang to prevent defects such as curling, which is due to the complex thermomechanical process in EBAM. In this study, finite element modeling is developed to simulate the thermomechanical process in EBAM in building overhang part. Thermomechanical characteristics such as thermal gradients and thermal stresses around the overhang build are evaluated and analyzed. The model is applied to evaluate process parameter effects on the severity of thermal stresses. The major results are summarized as follows. For a uniform set of process parameters, the overhang areas have a higher maximum temperature, a higher tensile stress, and a larger distortion than the areas above a solid substrate. A higher energy density input, e.g., a lower beam speed or a higher beam current may cause more severe curling at the overhang area.Mechanical Engineerin

Predictive Model for Thermal and Stress Field in Selective Laser Melting Process -- Part II

Finite Element Analysis (FEA) is used to predict the transient thermal cycle and optimize process parameters to analyze these effects on deformation and residual stresses. However, the process of predicting the thermal history in this process with the FEA method is usually time-consuming, especially for large-scale parts. In this paper, an effective predictive model of part deformation and residual stress was developed for accurately predicting deformation and residual stresses in large-scale parts. An equivalent body heat flux proposed from the single layer laser scan model was imported as the thermal load to the layer by layer model. The hatched layer is then heated up by the equivalent body heat flux and used as a basic unit element to build up the macroscale part. The thermal history and residual stress fields of two solid parts with different support structures during the SLM process were simulated. Layer heat source method has the capability for fast temperature prediction in the SLM process, while sacrificing modeling details for the computational time-saving purpose. Thus numerical modeling in this work can be a very useful tool for the parametric study of process parameters, residual stresses and deformations

General Rules for Pre-Process Planning in Powder Bed Fusion System -- A Review

Powder bed fusion (PBF) is one of the current additive manufacturing techniques that can fabricate almost fully dense functional metal components. Through a layer by layer fabrication methodology, complex geometries to meet the requirements of aerospace, automotive, biomedicine industries, etc. can be produced. The success of a build largely depends on having a flawless pre-process planning, including build orientation selection, support structure optimization, process parameter chosen, etc., which closely relates to the quality of the final products. Geometric inaccuracy and poor surface quality can occur due to a bad build plan. This review presents the crucial general planning rules for the build process. Build orientation selection, support structure optimization, and process parameter chosen in terms of residual stress reduction are the mainly concerns, which have been surveyed and discussed. The overall objective of this work is to help setup build plans that can ensure precise dimensions and high surface quality among the built components