Fraunhofer Institute for Mechanics of Materials IWM
Fraunhofer Institute for Mechanics of Materials IWM

Warpage-free components after powder pressing and sintering

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Optimize density homogeneities during filling

In order to compact a powder by pressing, it must first be filled into a mold via a conveyor line. This can lead to uneven filling or segregation. It is often the case that our customers fill a flat component, notice irregularities in the component after final sintering and wish to learn the cause of this and what measures can be taken to avoid filling inhomogeneity. 

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Optimizing density homogeneities during pressing

Unwanted warping after sintering can have many causes, one important reason being density gradients that arise during pressing. Typical industry issues which arise from this and need to be addressed include achieving the highest possible density homogeneity, whether modifications to the die geometries improve the result,  the influence of different filling positions and pressing plans and if tool coating reduces the problem. The simulation sheds light on this.

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Avoid cracks during pressing

When pressing powder into a mold, inhomogeneous compaction due to wall friction or different compaction ratios can lead to inhomogeneities in the component. This can cause defects such as cracks, particularly during unloading and ejection, although these do not always occur depending on the type of powder. An oft asked question is: Why is this the case and how can cracking be avoided?

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Minimize tool load

When pressing powder, a considerable amount of force is required to compact the powder sufficiently. When our customers press components, tool wear or even tool breakage can occur after a certain cycle time. They often need to know how to check the load, what measures can be taken to prevent damage to the tools as well as the optimum pressing intensity. 

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Avoid sintering distortion

Many components produced using powder technology are sintered in the final step. During sintering, many phenomena, such as internal density differences, different shrinkage of multi-materials, gravity or friction on the sintering base, can ultimately lead to inhomogeneous shrinkage and thus component distortion.  They question whether unwanted distortion can be avoided or if they need a new pressing geometry.

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Enabling multi-material components

Some customers want to sinter a component made from two or more materials. Everything seems to be fine with the test components, but the strength of the actual component is only moderate. The reason: when cosintering components made of several materials, the shrinkage curves of the individual materials rarely match. The result: residual stresses that can lead to defects or component distortion. This can be improved by changing the geometry. We reveal how it can work.

Examples of projects

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Instrumented setup for powder pressing for comparison with the simulation.

Powder characterization to determine model parameters


Precise model parameters are required for the simulation of powder pressing. The literature usually only provides simplified descriptions for the tests and determination of the model parameters and important details are missing. However, carrying out and evaluating the relevant tests is not trivial for determining the model parameters, as powder friction and elastic deformation of the test apparatus must be taken into account in addition to pure compaction. Determining realistic model parameters is one of our core areas of expertise. Do you want to carry out your own simulations? With our model parameters, we enable you to become your own simulation expert. 

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Numerical representation of the deformation of metal powder particles using the finite element method. The color coding corresponds to the plastic deformation.

Numerical simulation of powder compaction

In many industrial applications, it is important to achieve as uniform a distribution of density as possible in the pressed green compact. However, due to wall friction, certain density gradients cannot be avoided. Especially when pressing complex components, the relationships between the powder properties and the pressing process are not trivial. In order to better understand this, Fraunhofer IWM employs state-of-the-art simulation methods. For example, we have developed a continuum mechanical material model that has been integrated into the finite element program Abaqus®. With the help of this model, we can predict how the locally varying density in the component changes during the pressing process. This enables us to understand the reasons for the special features, e.g. when filling undercuts. In addition, we carry out simulations at powder level to investigate individual mechanisms such as anisotropic yield points or incipient crack formation. This information allows us to develop even more precise material models and describe the process in detail.

  • Schmidt, I.; Trondl, A.; Kraft, T., Yielding and failure of an assembly of frictional elasto-plastic particles: A computational RVE study, Journal of the Mechanics and Physics of Solids 154 (2021) Art. 104496, 15 pages Link
  • Schmidt, I.; Trondl, A.; Kraft, T.; Modeling and simulation of damage during powder pressing; Pulvermetallarugie in Wissenschaft und Praxis "Moderne Fertigungsprozesse - Qualität und Produktivität in der Pulvermetallurgie" Vol. 29, Kolaska, H. (ed.), Heimdall Verlag Witten (2013) 113-122 Link
  • Schmidt, I.; Trondl, A.; Kraft, T.; Wonisch, A.; Simulation of the material behavior of metal powders during compaction; Proceedings of the Institution of Mechanical Engineers, Part E: Journal of Process Mechanical Engineering 224/3(2010) 187-194 Link
  • Coube, O.; Riedel, H.; Numerical simulation of metal powder die compaction with special consideration of cracking; Powder Metallurgy 43/2 (2000) 123-131 Link

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Exemplary component after pressing and sintering. The color coding corresponds to the local green density distribution.

Numerical simulation of pressing and sintering


Dry pressing is an established process for shaping components using powder technology for many materials. However, complex component shapes place high demands on the pressing technology and bring their own challenges, such as distortion due to green density gradients and cracks after ejection or firing. The simulation method developed at Fraunhofer IWM allows quantitative predictions of the green density distribution and the resulting sinter distortion. With this information, tool shapes and pressing plans can be optimized in advance of tool construction and suggestions made for improving the finished parts. Careful selection of the model parameters and suitable validation of the simulation results are our strengths.

  • Kraft, T.; Optimizing press tool shapes by numerical simulation of compaction and sintering - application to a hard metal cutting insert; Modelling and Simulation in Materials Science and Engineering 11/3 (2003) 381-400 Link
  • Kraft, T.; Riedel, H.; Rosenfelder, O.; Compaction and sintering of a ceramic seal: Modeling and experimental response; International Journal of Powder Metallurgy 39/6 (2003) 27-34 Link
  • Kraft, T.; Riedel, H.; Numerical simulation of die compaction and sintering; Powder Metallurgy 45/3 (2002) 227-232 Link
  • Kraft, T.; Riedel, H.; Stingl, P.; Wittig, F.; Finite element simulation of die pressing and sintering; Advanced Engineering Materials1/2 (1999) 107-109 Link

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Simulation of the powder flow (gray) from a filling shoe into a die (blue).

Evaluation of the influence of die geometry and powder characteristics on the filling behavior during die filling


In the first step of dry pressing, the filling of the die, irregularities and incomplete filling can occur due to the flowability of the powder and the die design. This impairs the success of the process. Fraunhofer IWM has developed a simulation method called SimPARTIX, which is based on the discrete element method (DEM). With this method, such processes can be analyzed in advance via computer. This makes it possible to investigate the influence of the filling speed or the filling shoe design on the filling result. In addition to determining the required model parameters, the expertise of the Fraunhofer IWM also lies in the development of specialized complex interaction laws between the powder grains, which are adapted to the respective flow behavior.

  • Bierwisch, C.; Kraft, T.; Riedel, H.; Moseler, M.; Die filling optimization via three-dimensional discrete element modeling; Powder Technology 196/2 (2009) 169-179 Link
  • Bierwisch, C.; Kraft, T.; Riedel, H.; Moseler, M.; Three-dimensional discrete element models for the granular statics and dynamics of powders in cavity filling; Journal or the Mechanics and Physics of Solids 57/1 (2009) 10-31 Link

Further details on die filling can be found on our SimPARTIX homepage

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Predicted warpage of a component after the sintering process.

Optimization of the design for sinter-based AM processes


In additive manufacturing with sintering-based processes, deviations from the desired shape can often occur during firing due to gravitational influences or shrinkage-induced friction effects on the sintering base. In contrast to conventional shaping processes, in which the compensation of such undesirable deviations is often complex, additive manufacturing processes can in principle easily take such compensation into account during the construction process. Fraunhofer IWM has developed a simulation method that automatically determines the required print geometry by simulating an "inverse" sintering process.

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Component geometry after hot isostatic pressing.

Simulation of sintering under high external load


In hot isostatic pressing or sinter molding, the ceramic or metallic green body is additionally compacted by external pressure during sintering. Special simulation models have been developed at Fraunhofer IWM to simulate this process in detail. This makes it possible, for example, to predict the deformation of the capsule during hot isostatic pressing. These simulations therefore enable unwanted distortions to be compensated for in the design of the capsule.

  • Kraft, T.; Schmidt, I.; Riedel, H.; Simulation von pulvermetallurgischen Formgebungsverfahren; Pulvermetallurgie in Wirschaft und Praxis "Formgebung: Chancen für die Metallurgie" Volume 26; Kolaska, H. (ed.); Heimdall Verlag, Witten (2010) 63-71 Link
  • Reiterer, M.; Kraft, T.; Riedel, H.; Application of a microstructure-based model for sintering and creep; Ceramic Transactions 157, Characterization and Modeling to Control Sintered Ceramic Microstructures and Properties: Proceedings of the 106th Annual Meeting of The American Ceramic Society; DiAntonio (Ed.); John Wiley & Sons, Inc., (2006) 49-58 Link
  • Reiterer, M.; Kraft, T.; Riedel, H.; Manufacturing of a gear wheel made from reaction bonded alumina - numerical simulation of the sinterforming process; Journal of the European Ceramic Society 24/2 (2004) 239-246 Link

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Prediction of the local density after surface compaction by roller burnishing using the example of a gear wheel. The color coding corresponds to the decrease in porosity.

Process simulation of the production and application behavior of porous materials


Porous materials produced using powder technology, such as sintered steels, have characteristic properties that require a special description in simulations, e.g. of the application behavior. At the Fraunhofer IWM, corresponding material models based on the Gurson model (Gologanu, Ponte-Castaneda) have been implemented in the FE program Abaqus®. These models can be used to simulate process steps for property optimization after sintering, such as the surface compaction of gears by roller burnishing, as well as the subsequent application behavior, taking into account the local service life.

  • Yazici, B.A.; Kraft, T.; Riedel, H.; Finite element modeling of PM surface densification process; Powder Metall 51/3 (2008) 211-216 Link
  • Kraft, T.; Riedel, H.; Assessment of fatigue lifetime of PM-parts; World Congress on Powder Metallurgy 2004, Vol. 3, EPMA, Shrewsbury, U.K. (2004) 111-116 Link

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Calculation of the sintering distortion of a sensor. The color coding shows the deviation from the nominal shape from low (green) to high (red).

Prediction of sintering distortions in multilayer ceramic structures

 

Ceramic multilayer structures are the basis for numerous applications such as sensors and microelectronics. Several ceramic foils printed with different precious metal pastes are stacked on top of each other, laminated and sintered together. A common problem is the component distortion that occurs during joint sintering due to different shrinkages. By using detailed sintering models and simulation of the sometimes complex printed layer composites, Fraunhofer IWM has created the basis for minimizing this undesirable distortion through targeted material and design changes.

  • T. Rasp, C. Jamin, A. Wonisch, T. Kraft, O. Guillon, Shape distortion and delamination during constrained sintering of ceramic stripes: Discrete element simulations and experiments, J. Am. Ceram. Soc. 95 (2012) 586-592. Link
  • Schmidt, I.; Kraft, T.; Simulation of the co-sintering of composite structures; International Journal of Materials Research 101/8 (2010) 933-941 Link

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