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Powder Metallurgy

ThermoOptical measurement system TOM_metal
© Fraunhofer-Center HTL
ThermoOptical measurement system TOM_metal
Cylinder made of stainless steel (f.l.t.r.): 3d-printed green body, sintered at 1350°C in a vacuum, nitrogen and argon
© Fraunhofer-Center HTL
Cylinder made of stainless steel (f.l.t.r.): 3d-printed green body, sintered at 1350°C in a vacuum, nitrogen and argon
The animations shows the sintering of components in ThermoOptical measuring system TOM_ac
© Fraunhofer-Center HTL
Animation: Sintering of components in ThermoOptical measuring system TOM_ac - please click to play

In contrast to melting metallurgy, in powder metallurgy (PM), metal components are manufactured by forming and sintering metal powder. One of the great benefits of manufacturing using powder metallurgy is the reduction in the need for machining and processing the final product. This is particularly true for components with complex geometries manufactured in high volumes, e.g., as is the case with the powder injection molding (PIM) process. Typical metals used in the field of powder metallurgy are ferrous metals (iron / steel), aluminum, copper and bronze. In order to achieve a cost benefit when compared to melting metallurgy, it is necessary to achieve precision near-netshape sintering of the PM components. After sintering, neither a calibration nor machining should be necessary, or if required, only the machining of functional surfaces may be necessary. A further benefit of powder metallurgy is that when the manufacturing process is employed, the manufacture and mixing of the powder as well as the subsequent molding and sintering allows very fine structures to be achieved. Thus, powder metallurgy facilitates the manufacture of a wide range of components made from alloys and composites which are difficult to form by melting metallurgical processes, e.g., tungsten carbide, diamond composites, magnetic materials, nickel-base alloys and refractory metals. In order to achieve the desired structure, careful optimization of the heat treatment process is required. Micro-fissures and bubbles must be avoided, oxide layers on the metal particles must be stripped by a reduction process and over-sintering or distortion of the components must be excluded. At the same time, the manufacturing costs must be minimized. This means that the throughput of the sintering furnace should be maximized and the energy and maintenance costs of the furnace reduced.

The optimization of the heat treatment can be undertaken at the Fraunhofer-Center HTL for all kinds of powder metallurgical materials. For recording the required in-situ measurement data, ThermoOptical measurement systems (TOM) with a range of very different atmospheres are available. Sintering can be undertaken in a graphite-heated or tungsten-heated TOM furnace. Furnace atmospheres, such as inert gases or synthetically mixed reducing gases, as well as 100% hydrogen can be used. A TOM system, TOM_metal, has been specially developed for in-situ measurement of powder metallurgy. TOM_metal can be operated with a vacuum, hydrogen or other reducing gases, inert gas as well as overpressure (up to 30 bar). The in-situ measurements are undertaken on samples or small components with a volume of approx. 1 cm³ to 30 cm³. The upscaling to larger components is undertaken with a finite element (FE) procedure. The corresponding FE models are available at the HTL for the heat treatment steps of drying, debinding and sintering.

Drying is required if the PM green bodies have been manufactured by a plastic shaping process. The solvents contained in the green bodies must be gently removed, before the high-temperature processes commence. Rapid or irregular drying can cause cracks and distortion. The local drying rates depend to a great extent on the relative humidity, the temperature and the flow speed of the drying gas. The HTL possesses equipment that facilitates weighing of the samples when drying under controlled wet-solvent, temperature and gas flow conditions. This allows the drying rate to be determined during different stages of drying. The drying rate can be reduced to the desired level in the critical states. Distortion of the components due to irregular drying is simulated in FE models that take the gas flow and the local moisture gradient into consideration. For optimization purposes, the moisture and flow measurements in the industrial drying units can be taken into consideration.

For debinding, the organic molding additives contained in the PM green bodies are, generally, thermally removed. The organic additive is pyrolysed in the process. If the debinding process proceeds too rapidly, the resulting gases cannot be dissipated quickly enough from the pores to the component surface. The resulting overpressure in the pores can cause component damage. In addition, there are further undesirable phenomena which can occur during debinding, such as the absorption of pyrolysis gases by colder green bodies in continuous furnaces, or the segregation of the liquid binder in the pores. Furthermore, it is necessary to ensure that the carbon that may be created during pyrolysis does not have undesired effects on the progress of sintering or the production properties. Similarly to drying, it is necessary to identify the fastest safe temperature profile at which the debinding can be carried out without faults. Analog to drying, the debinding experiments at the HTL are undertaken at a controlled atmosphere and temperature. The degree of debinding is detected in-situ by measurement of the sample mass. The optimization of the debinding parameters are undertaken analog to drying using FE simulation and the verification of then optimized conditions through additional debinding experiments.

With sintering, a stabilization of the defined residual porosity can be achieved. The structure should be free of fault and homogeneous, and the component may not distort. The high cost for calibration or machining / finishing of PM components requires a process delivering near-netshape component manufacture, where the shrinking due to sintering has been incorporated into the dimensioning of the green bodies. Divergences from the target geometry result during sintering due to irregular distribution of the porosity in the green body (see below) as well as due to the influence of friction, gravitation and temperature differences with heat treatment. Temperature difference within the components leads to distortion, whereas temperature differences in the furnace lead to scatter of the component dimensions. Unwanted processes may also be at play in the microstructure during sintering. During compaction, thermodynamic driving forces ensure that an original homogeneous green body is locally more highly sintered or that the different phases segregate. At the end of compaction, there is increased grain growth, just as is the case with the thermodynamic effects that impair the material properties. Further phenomena that need to be mastered during sintering are the gas phases' processes, which result due to the reactions of the material to be sintered with the furnace atmosphere as well as by the release of gases. The sintering speed is measured precisely at HTL with the TOM system. Furthermore, other important material properties during sintering governing heat transfer and creep deformation are recorded. Using coupled FE models, the thermomechanical effects in the component are simulated, and the process parameters are optimized.
Also special heat treatment procedures for re-compaction of the PM components, such as hot isostatic pressing (HIPing) or the melt infiltration, can be optimized with the procedure at HTL. Using TOM_metal, sintering can be undertaken even at low pressures until essentially closed pores result. The residual compaction can then be undertaken at high gas pressure. Initially, the compaction process can be observed and optimized in-situ. Using TOM_ac, the direct melt infiltration of porous components can be observed. Possible temperature gradients in the furnace are considered for the re-transfer of the optimized process parameters to large industrial furnaces. The FE procedure for simulation of the temperature distribution in the furnace system can be used.
The homogeneity of the green bodies is a prerequisite for successful heat treatment. HTL has high-performance computer-tomography (CT) and self-developed software to evaluate the pore distribution in green bodies. Irregular distribution of porosity is thus measured and eliminated by optimization of molding. The measurement procedures are undertaken at varying scale sizes: From the microstructure right up to the component level.
HTL has a 3D printer available for the manufacture of powder metallurgical components. The printer operates according to a powder-bed binder jetting process using binder-jetting technology. This facilitates the manufacture of porous PM green bodies with complicated geometries. The green bodies can be subsequently compressed by sintering or melt infiltration. Advantages of melt infiltration are that it avoids the occurrence of dimensional changes enabling the manufacture of innovative and advanced composites. Using the 3D printer, prototypes for PM components can be manufactured. Even prototypes for compression molding or PIM molds can be manufactured using the 3D print process.
For powder metallurgical materials consisting of several phases, at HTL, special simulation methods can be applied to determine the relationship between the structure and the material properties. For example, the stiffness, the creep properties and the thermal stresses of hard metals can be calculated by reproducing the microstructure on a computer and calculating the macroscopic behavior with an FE simulation. This procedure is used for the microstructure design of materials and can be undertaken supplementarily to experimental structure optimization.

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Waldemar Walschewski

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Waldemar Walschewski

Fraunhofer-Center for High Temperature Materials and Design
Gottlieb-Keim-Str. 62
95448 Bayreuth, Germany

Phone +49 921 78510-512