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Industrie und Anwendungen

Modelling of temperatures and heat flow within laser sintered part cake

Objectives


Temperature effects in the polymer laser sintering process are an important aspect regarding the process reproducibility and part quality. Depending on the job layout and position within the part cake, individual temperature histories occur during the process. Temperature history dependent effects are for example part warpage, the crystallization rate and powder ageing effects. This work focuses on temperatures and heat fl ow within laser sintered part cakes.

Procedure

Therefore, a thermal Finite Element (FE) model of a part cake is developed based on experimental temperature in
situ measurements (Figure 1). Determining of the heat flow within laser sintered part cakes requires experimental information about the three-dimensional temperature distribution and history within the powder as a reference for the model development. Since the size of the part cake increases continuously during the build phase, here only the cooling phase is selected for the model development. Experimental temperature measurements are used to specify the temperature distribution and determine the starting of the cooling phase on the one hand and to validate and check the accuracy of the model on the other hand. Thermal boundary conditions and properties of the used bulk polymer powder are analyzed and relevant parameters are identifi ed. The FE model is validated and optimized considering different job heights and ambient conditions during the cooling phase.


Achievements

A model to simulate the temperature history and heat flow within laser sintered part cakes during the cooling phase has been set up. Thermal boundary conditions of a polymer laser sintering system were analyzed. Modelled data has been compared to experimental data obtained with 48 thermocouples inside the part cake. The outer heat transfer coefficient (thermal powder contact and convection) and the thermal conductivity of the part cake were determined in a parameter study. A parameter set has been validated with an accuracy of about 6 K for all sensor positions during the whole cooling process. To improve the model, possible disturbance variables were fi gured out. A consideration of time and location dependent heat transfer coeffi cients lead to an improved model with an accuracy of 3 K. Further aspects are for example cracks within the part cake or the influence of the powder bed density on its thermal conductivity. It is finally possible to predict position-dependent temperature histories as a function of signifi cant job parameters. The model allows a transfer of the results for varied boundary conditions during cooling. In combination with an implementation of built parts, this model will be an important tool for the development of optimized process controls and cooling strategies.

Plastic freeforming of liquid-tight microfluidic components

A design adjustment of the inner structure minimizes the floating overhangs in the range of the flow channels. Due to this adjustment, the use of any kind of support material can be avoided. In this way it can be ensured that no residues of water soluble or non-biocompatible material remain in the system. A part from avoiding support material, the aim was to apply the cell culture reservoir on the polymer chip without the need for any adhesives. In the PF-process, polymer chips can be inserted into the build chamber and be printed directly. The deposition of the molten polymer droplets on the thermoplastic basic chips is similar to the welding process of polymers. The cell culture reservoirs have the purpose to absorb, store and pass the microfluidic into micro physiological systems. Therefore, the tightness of the whole system is crucial to ensure the functionality. The structure is generated by applying single polymer droplets, so that cavities are formed between the droplets. The optimization of the process parameters aimed to minimize the porosity of the cell culture reservoirs to ensure the fl uid tightness. The cell culture reservoirs were produced with a 0.2 mm nozzle and the layer thickness is about 0.15 mm.

Procedure

By adjusting the form factor (FF), the degree of fi lling and thus the pore volume can be varied. Besides the impact of the form factor, the impact of the processing temperature (material preparation and build chamber temperature) is investigated as well. These process parameters affect the mass temperature of the molten polymer droplets. A temperature increase results in a decrease of the viscosity. Expectably, a decrease of the viscosity improves the wettability of the droplets, so that less cavities are generated. The lower viscosity, therefore, is expected to result in a reduction of the pore volume. The setting behavior of the polymer droplets immediately after the deposition is mainly affected by the temperature of the build chamber.

Achievements

The figure shows the three-dimensional view of three cell culture reservoirs. The yellow colored areas mark the pores in the test samples. The integrated structures are clearly recognizable in the middle of the fi gure. It is clear to see that a low form factor and a high temperature in the build chamber result in a decrease of the pore volume.

Introduction and Objectives

The Plastic Freeforming (PF) enables the successful construction of application-specifi c reservoirs and cell culture segments directly on a universal micromachining platform (polymer chip). The cell culture reservoirs were manufactured from the copolymer ABS. The focus was on the optimization of the process parameter concerning the fluid tightness and the bonding on the polymer chip made of PC.

Surface roughness optimization by simulation and part orientation

Objectives

The layered structure of Additive Manufacturing processes results in a stair-stepping effect of the surface topographies. In general, the impact of this effect strongly depends on the build angle of a surface whereas the overall surface roughness is caused by the resolution of the specifi c AM process. The aim of this work is the prediction of surface quality in dependence of the part building orientation. Furthermore, these results can be used to optimize the orientation of the part to get a desired surface quality for functional areas or an overall optimum. In AM the build height is most often a cost factor, therefore the part orientation tool takes not only the predicted surface quality into account. The job height is an optimization objective for this tool as well.

Procedure

Based on experiments a surface roughness database was generated. To support this database an additional surface roughness Rz simulation tool was developed (Figure 1a / 1b). Usually not every area of a part can be optimized, as the surface quality is highly dependent on the build angle. Therefore, a pre-assignment of functional or important areas takes place for the orientation simulation. The selected surfaces get an increased weighting factor for the preferred build alignment. The model uses the digital STL format of a part as this is essential for all AM machines. Each triangle is assigned with a roughness value and by testing different orientations an optimized position can be found. Even if this tool is validated and build on the LS process, this method can be applied to all AM technologies.

Achievements

With the alignment optimization tool for AM processes, which uses a surface roughness database and build height as optimization objectives, it is possible to validate the part orientation for AM parts.

Innovative AM optimization

Objectives

In order to save resources and adapt parts better to their requirements, companies are focusing on part optimization for lightweight design. Unfortunately, the existing software for topology optimization is characterized by several shortcomings: Modeling is a lengthy, labor-intensive process, the computing time is long and extensive expertise and manual reworking is required. Usually several software tools are needed to achieve a satisfying result. However, these software tools are not aligned to an AM-specific design and the transition between the various tools results in errors and reduces the quality of results. The aim was therefore to develop a dedicated software solution for automated topology optimization with integrated retransition into proper, additive manufacturable geometries. The result is a software called AMendate, which will soon be available on the market and offers topology optimization, automated in one single solution, from CAD to CAD.

Procedure and Achievements

Instead of a polygon-based approach, the software is based on an innovative voxel grid, which enables a multitude of unique selling points: The model is created automatically, a high resolution can be achieved and the resolution can be varied within the optimization calculation. This results in highly complex, optimal structures. An intelligent smoothing algorithm automatically transfers the voxel result to smooth surfaces. The result requires neither further interpretation nor further engineering. The optimization algorithm automatically takes into account all relevant design rules for additive manufacturing for a directly printable result. This gives the user a better result much faster and more cost-effectively. Time savings of up to 80% can be achieved by eliminating and automating several time-consuming process steps. The automated and integrated topology optimization enables an optimization from CAD to CAD within hours instead of days. The newly developed software and its innovative approach enable considerable speed growth. This is driven by a software architecture that fully utilizes the computing power of current high-tech graphics cards and the seamless, automatic workflow. Another significant advantage is the direct stress oriented optimization, which provides better optimization results and a balanced stress distribution over the entire component. This makes AMendate a significant step towards the automated design of optimized parts, which will promote the introduction of additive manufacturing in other industries.

KOBFS - Lufo

Objectives

The reduction of process times in additive manufacturing is a major focus of research. The aim of the investigation was to reduce the time, required for a process route, of the additive manufacturing process for the Ti6Al4V titanium alloy with subsequent HIP process.

Procedure

Therefore, this study pursues comprehensive investigations on the mechanical properties of the titanium alloy TiAl6V4, which was processed in an optimized process chain. By optimizing the process parameters, the production speed is drastically increased, the process-induced defects are adjusted in a controlled manner and eliminated by thermal post-treatment (HIP). The result is a faster processability of the titanium alloy and thus better economic efficiency. A HighSpeed parameter window was determined during singletrack tests. Subsequently, mechanical characteristic values of the specimens are determined in tensile tests and fatigue tests. Both static and dynamic measurement results are very sufficient in comparison to the conventional route.

Achievements

The exposure time during the additive manufacturing process was reduced to 50%. The subsequent HIP treatment also reduces pores up to approx. 6%. Compared to samples that were not generated with high-speed parameters, the mechanical characteristics are identical. In addition, the HighSpeed parameter does not generate any increased residual stresses in the component. This leads to a process time reduction of around 25%. These were validated with a gimbal specimen whose process time could be reduced from 49 hours to 38 hours.

Optimization of Material Properties of selective Laser-Melted Aluminum Alloy 7075

Objectives

Objectives Experience from conventional manufacturing shows a good performance of the high-strength aluminum alloy EN AW 7075 which leads to frequent use in automotive and aerospace sector. Scientific investigations on the processability of this alloy in the SLM process shows that prepared samples have anisotropic behavior due to process-induced hot cracks (Figure 1). Furthermore, it was not possible to determine solid results regarding the fracture mechanical characterization. Due to its chemical composition, aluminum alloy EN AW 7075 has a high solidification interval. As a result, the melting and solidification of the material results in a high affinity for the formation of hot cracks. For industrial use, these hot cracks must be avoided.

Procedure

An investigation documented in literature shows that addition of 4 Wt.-% Silicon avoids hot cracking. This is due to a reduction of the thermal expansion coefficient. Furthermore another investigation shows that an increase of the pre-heating temperature of the building platform reduces the number of hot cracks in selective laser melted EN AW 7075. In practice, pure silicon is not always available for the modification of the powder, which is why a practice-oriented approach to the production of a mixed alloy made of high-strength EN AW-7075 and silica-rich AlSi10Mg is pursued in this context.

Achievements

The production of a mixed aluminum alloy based on the high-strength alloy EN AW-7075 led to process-reliable processing using SLM. When using an adequate process parameter set, the samples produced have an almost pore- free appearance without detectable hot cracks (Figure 2). A subsequent heat treatment leads to an increase in the mechanical and fracture mechanical characteristics of the mixed aluminum alloy. The approach of mixing two available powders makes it possible to compensate for negative mechanical and fracture mechanical properties and to generate additively processable material.

Surface Finishing

Partner

This post processing study was realized in cooperation with Walther Trowal GmbH & Co. KG, a company specializing in grinding technology based in Haan.

Objectives

The surfaces of additive components are not as smooth as for conventional machined parts due to the manufacturing process. Therefore, AM manufactured components require a surface post treatment. Due to the complex geometries of AM components with undercut and difficult to reach areas, conventional machining like drilling or milling are not suitable as overall surface treatments. Even sand blasting often does not meet the requirements. In cooperation with Walther Trowal the vibratory finishing technique was investigated as a surface treatment for additive manufactured components. This offers attractive possibilities for improving the surface.

Procedure

Walther Trowal has developed various processes and special machines for subsequent surface treatment. The Direct Manufacturing Research Center at Paderborn University has provided various components with hard-to-reach areas and narrow cross-sections. These are treated by vibratory grinding for a period of around 24 hours. The polishing and treatment additives developed at Walther Trowal are specially selected for the materials to be prepared.

Archievements

After the treatment, the surface quality of the components is outstanding. Even very narrow and hard to reach areas have a smooth surface. The process is also used by many well-known companies for difficult-to-machine materials such as titanium and cobald chromium alloys. The process is characterized by high quality and reproducibility, which makes it attractive for other applications.

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