Additive manufacturing: Creating reliable parts

Homogeneous part properties and smooth surfaces by process simulations of powder spreading and melting.

Introduction

Additive manufacturing processes currently penetrate both science and industry. These methods provide specific advantages including the customization and complexity of manufactured components. Additively manufactured components are already utilized in complex systems, for example airplanes. However, the reliability of these types of components is not yet satisfactory for use in structural lightweight parts. Nonetheless, additive manufacturing methods enable the production of highly complex parts with customized mechanical properties at minimal weight.

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The task

© Photo Fraunhofer IWM

DEM simulation of spreading a new powder layer by a roller.

Fatigue of additively manufactured parts is on the one hand determined through surface roughness, typically in the range of Rz = 25 to 40 µm which is significantly larger than in traditional forming processes using gravity die casting (Rz = 3 to 8 µm). Systematic monitoring of the roughness would therefore be desirable; however this is currently not possible. On the other hand, a locally occurring residual porosity as well as layers which have not merged after melting can cause component failure. Simulation tools can help to identify relevant process parameters and to monitor and consequently make possible the production of reliable components.


At the moment, no established simulation tools exist for additive manufacturing. The availability of a continuous process simulation would be useful in view of the complexity of the processes to fulfill requirements for form and structure within the product development cycle. SimPARTIX® offers various numerical methods which make possible process simulations for additive manufacturing. These include simulating the spreading of powder yielding packing densities which influence the dynamics of the melting process as well as simulations of melting and resolidification which determine the porosity, the surface properties and finally the strength of the component.

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Results

© Photo Fraunhofer IWM

Density distribution of a thin powder layer after spreading.

The density distribution of a new powder bed layer is shown in the adjacent figure. This is one possibility to analyze a discret element simulation of the spreading process.  Alongside investigations concerning the homogeneity of the powder bed density, anisotropic attributes can also be analyzed through spatial orientation of the grains. The simulations allow extensive studies of the effects of powder characteristics and process parameters on the properties of powder beds, which are relevant for further process steps.

© Photo Fraunhofer IWM

SPH simulation of melting and solidifying of powder layers.

Smoothed Particle Hydrodynamics simulations are carried out for the actual melting and solidification process. The results of melting two consecutive powder layers are shown in the adjacent figure with color-coded density. First, a loose powder layer is spread (above left), which is then melted and resolidified (above right). Next, a second loose powder layer is applied (below left). Finally, after melting and resolidification, both layers have merged (below right). One can ascertain that a residual porosity between the original grains still exists because of insufficient energy input.

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