156. Transient thermo-mechanical modelling of stress evolution and re-melt volume fraction in electron beam additive manufacturing process
Singapore University of Technology and Design, Singapore
Electron beam melting and selective laser melting processes are in demand because of their capability to produce highly dense and homogeneous structures. The quality of parts produced using powder bed fusion additive manufacturing processes mainly depends on the energy source, raster scan pattern, and stress evolution due to concentrated heat addition. Also, the study on temperature profile over the scanning period is important as it accounts for the energy build up that pre-heats the material in-front of the scan head. The amount of raw powder material that has been converted to bulk solid upon melting - solidification cycle is governed by the penetration depth of the energy source and the absorption profile. In order to analyze the relationship between these various process parameters such as input power, penetration depth, raster scan hatch spacing, powder porosity and their effects on the temperature profile, stress evolution and re-melt volume, a three-dimensional thermo-mechanical simulation of the EBAM process on AISI 316L stainless steel is implemented using the COMSOL® multiphysics modelling platform. The phase changes are modeled using the apparent heat capacity method. With an effort to improve the accuracy of the modeling results, temperature dependent thermo-mechanical properties are incorporated. Also, linear, and exponential thermal absorption profiles are considered and their results are compared in-terms of amount of re-melt volume and stresses. Our results show that the hatch spacing plays a critical role in governing the temperature distribution and the residual stress patterns in the solidified regions of the substrate. The volume fraction of re-melt materials shows a non-linear and non-monotonic dependence on the electron beam size. The plot of stress evolution at a point located at the end of raster shows a double peak and valley as the beam approaches it and moves away during the second raster. The first peak is observed when the powder is about to melt as it experiences sudden rise in temperature. The valley represents the drop-in stress levels due to elevation of temperature above the melting point. When the beam head moves away, the second peak is observed because of re-solidification and temperature contours that are close to the melting temperature. Affected by the cumulative thermal budget buildup, the region around the raster turn experiences maximum residual stress. The peak temperature in the substrate is attained at the start of the scanning process and drops subsequently as the scan head moves further, which is supported by the rise in thermal conductivity of the re-solidified bulk material. The results of this study pave way for further robust design of the EBAM process from a simulation point of view.
127. Filament Temperature Dynamics in Fused Deposition Modelling and Outlook for Control
University of Bristol, United Kingdom
Additive Manufacturing (AM) is a collection of processes capable of building solid objects directly from a computer-based geometry file. Through construction of the objects on a layer-by-layer approach, complex and functional geometries are easily achievable. Such components are often impossible to manufacture through conventional subtractive methods, due to internal geometries and material gradations. As virtually no different tooling is required, AM processes are ideally suited for customised components in small production runs.
Fused Deposition Modelling (FDM) is an AM process involving the extrusion of molten plastic through a nozzle moving relative to the print bed; usually affixed to a gantry system. This process is commonly used on low-cost 3D printers, but an increasing availability of higher strength and reinforced filaments has motivated research into its applications within the manufacturing sector.
Extensive work has been conducted on topological and process parameter optimisation. The valuable work in these fields has created tools for designing and manufacturing components with potentially superior properties to conventionally manufacturable parts.
For adoption into the supply chain for aerospace components, quality control systems must be implemented to ensure manufacturing consistency. Previous studies have researched feedback for improved control over the XY gantry, pre-emptive correction of thermal distortion, and iterative learning processes for road deposition widths at features. There have been efforts to model the extrusion process, but very little work has been conducted on the filament temperature change during flow rate variations. This parameter is key to ensuring good bonding throughout the component. A majority of modern extruder temperatures are controlled through a thermistor mounted within the heater block, assuming the filament will be extruded at the same temperature.
This paper investigates the effect of filament temperature on bond formation, and errors incurred through fluctuations during a typical printing process. During the build process, the filament flow rate rapidly changes, especially during the initial section of the deposited road. The extruded temperature was monitored by both a thermal camera and a nozzle-mounted thermistor, providing information on the fluctuations in extruded filament temperature. The printer nozzle test setup was driven through a fast prototyping DSpace system to allow efficient real-time measurement and control. Initial results have shown a step increase in flow rate from 6.4mm2s-1 to 31.9mm2s-1 causes a 15°C drop in extrusion temperature; with a predicted 30% reduction in sintering bond formation. Tests will be conducted to identify if this effect is mitigated through control of the nozzle temperature, as opposed to the standard block temperature controller. Further investigations will study the effect of retraction and priming motions on the extruded filament temperature.
The models herein developed provide an insight into the effect of flow rate variation on temperature, and subsequently bond quality, to enable potential weak spots in a structure to be identified. This represents a potential step change in quality assurance of FDM components, where the results can be applied to improved build path design and ensuring the inter-layer bond quality throughout the component.
372. A design strategy based on topology optimization techniques for an additive manufactured high performance engine piston
University of Modena e Reggio Emilia, Italy
In this paper, a methodology for a motorcycle piston design involving optimization techniques is presented. In particular, a design strategy is preliminary investigated aiming at replacing the standard aluminum piston, usually manufactured by forging or casting, with an alternative one made of steel and manufactured via an Additive Manufacturing process. In this methodology, the minimum mass of the component is considered as the objective function and the stiffness of important parts of the piston is employed as design constraint. The results demonstrate the general applicability of the methodology presented for obtaining the trusses layout
and thickness distribution of the structure.
10. Optimisation of Additive Manufactured Sand Printed Mould Material for Aluminium Castings
Northumbria University, United Kingdom
The foundry industry provides near net shape metal casting for a wide range of industries, producing components in ferrous and non-ferrous metals castings in a range of sizes from miniature to large castings such as zips to ships propellers. It has played a fundamental role in the development of man-kind and our current life styles from the Bronze and Iron Age are known for their ability to cast products and tools for example weapons and armour.
The sand casting process has little changed over the years, except for automation and mechanisation of the process, providing productivity advances, the fundamental process of sand compacted around a mould pattern, which is then removed to cast the metal has remained. For mass production this is economical and efficient however for development and prototyping the requirement for tooling and the production method design constraints means this stage often takes a long time and costly.
Additive manufacturing has been used to manufacture sand moulds for metal sand casting using laser sintering and sand bonding. This research focuses on optimisation of the build parameters for the Additive Manufacturing sand print bonded process developed by ExOne GmbhH Germany for Automotive Aluminium components.
The approach taken in this research is to evaluate characteristics of casting produced and relates to the permeability, dimensional accuracy, tensile and compressive crush strength, density, impact strength and high temperature resistance of the mould tool produced. These properties are required to compare the 3D Sand Printing (3DSP) process to Selective Laser Sand Sintering (DLSS) and traditional Furan based casting sand mixtures. The automotive turbo charger casing was used to validate the build parameters optimisation process.
This research would be of interest to designers and manufacturing engineers wishing to take advantage of the implications of having new design freedom, tool less manufacturing with short lead times in a wide range of materials using fundamentally tried and tested foundry industry casting technique. This research has demonstrated 3DSP process has the capability to manufacture sand patterns to permeability, accuracy, tensile and compressive strength comparable to traditional sand casting process.
385. Cost modelling and sensitivity analysis of wire and arc additive manufacturing
University of Bath, United Kingdom
With the proliferation and diversification of metal additive manufacturing (AM) processes in recent years, effective decision tools for process selection are of increasing importance. Wire and Arc Additive Manufacturing (WAAM) is an emergent metal AM technology with limited studies on the cost effectiveness compared to alternative manufacturing methods. This paper addresses this gap through the development of a novel time activity based cost model which, for the first time, includes post-processing activities. Modelling of the WAAM processing chain enables a sensitivity analysis to be carried out. A tool path based deposition cost is also introduced which accounts for geometric complexity and improves the accuracy of deposition time estimations. The results provide a comparison for two case study components that indicate WAAM has significant potential to be a cost effective manufacturing approach compared to electron beam, direct metal laser sintering and conventional CNC machining methods. Key cost drivers in WAAM have been shown to be indirect costs for both large and small components. However, smaller components are more influenced by direct costs and benefit from increases in parts per build plate. In contrast, shielding cost was highlighted as an area of particular impact for large components.