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Binder Jetting in Series Manufacturing of Powder Metallurgy Components

GKN Powder Metallurgy is a leading international technology company with more than 5000 employees at 27 production locations in nine countries. GKN PM is divided into the divisions GKN Hoeganaes (yearly production of around 250 000 t metal powder), GKN Sinter Metals (world market leader with around 10 million sintered parts/day) along with the most recent division GKN Additive. GKN Additive is already using Metal Binder Jetting (MBJ) – a powder-bed-based Additive Manufacturing (AM) process – for industrial-scale series manufacturing (yearly >10 000 parts) of complex components. Key feature of MBJ technology is the application of a liquid binder for selective bonding of metal powder layers. The green body formed is then densified in a sintering process to relative densities of over 99 %. Dr Patrick Köhnen (PK), Technology Manager Binder AM, gave us some information about the current situation with MBJ at GKN PM.

cfi: What criteria have led to the favouring of binder jetting among the array AM processes? 

PK: Direct-melting, metal-based AM technologies, like Laser Powder Bed Fusion (LB-PBF), enable the production of geometrically complex components with high geometric and dimensional accuracy. However, currently feasible build rates are reducing wide industrial-scale series production in cost-driven industry segments such as the automotive sector and machine and plant engineering. We get the necessary increase in productivity and reduction in component costs with the implementation and further development of sinter-based AM technologies, like Metal Binder Jetting (MBJ), which enable considerably higher build rates of components weighing up to 500 g. The key feature of MBJ technology is the application of a liquid binder by means of a multi-nozzle print head for selective binding of metal powder layers. The binder in the green body formed in this way is then removed in a debinding and sintering process and densified to relative densities up to 99,5 %. Besides the boost to productivity, MBJ offers the potential to process special materials, such as, for example, high-carbon tool steels, which are difficult to realise with other AM processes. Moreover, in comparison with other metal-based AM technologies, more isotropic properties can be obtained and residual stresses reduced. 

cfi: What had to be adjusted compared with established shaping process (presses and MIM) in respect of the conditioning of the powders or the adaption of the pressure and sintering parameters to achieve the required properties? 

PK: At GKN PM, we have the advantage that, with GKN Hoeganaes, we have powder development in house. GKN Additive has emerged from an internal start-up from 2014. The colleagues from GKN Hoeganaes in the USA support us with tailored powder development. In addition, at the MIM plant in Bad Langensalza we can sinter the AM components in the continuous MIM furnaces in production conditions. The difference to classical powder metallurgy processes like, for example, MIM is largely that the green part is not produced in a mould in an injection moulding system, but with a printer. As other binders are used, the debinding process has to be adapted correspondingly. During debinding, the binder polymer is broken down by means of pyrolysis with the exclusion of oxygen. To improve the component properties and robustness of the process, besides targeted MBJ material development, adaption of pressure and sintering parameters and digitalization and automation of the MBJ process chain are necessary for series production. At the R&D center at the Bonn location, process and product development are undertaken for subsequent industrial series production at the production site. 

cfi: What development times are needed based on the current level of knowledge on component development? 

PK: In the run-up to the process, the Business Development Team defines with the customer which AM processes are advantageous for the application. Arguments can be the integration of functions, aspects concerning lightweight engineering or, as indicated earlier, very complex geometries. In addition, aspects like the required unit numbers and development times can play a part. An added value must be identified to pass the come out well in the cost comparison, too. At the R&D center, we then start with product development and hand over the sintered components to the customer for performance testing. Only after approval has been granted is further development conducted under production conditions. 

cfi: How intensive is the cooperation with the printer manufacturers? 

PK: In the cooperation with HP for the development of the printer, the foundation was laid for an industrially useful system with the high quality standards of an established machine manufacturer. Essential for this was the close cooperation with the freely adaptable process parameters to enable the process development in line with the needs of the application. To this end, work was done early with pre-series machines at GKN PM and the development of the S100 series machine accelerated. 

cfi: How do you rate the potential of MBJ for Powder Metallurgy? 

PK: In 2021, AMPOWER published a study which, based on the market situation from 2020, expects a fivefold increase in industrial MBJ capacities by 2025. With future material and process development and the elaboration of adapted design guidelines and simulation models to predict the sintering process, increases in respect of the annual number of units, component complexity, process robustness and a significant widening of the applications for MBJ can be anticipated. In our company, MBJ is an important technology to open up new application segments. 

cfi: What new applications can you open up with AM components? 

PK: We already have successful projects, e.g. with John Deere (fuel by-passes in tractors that have to work at low temperatures) or filters for Schneider Electric. Lightweight components for the automotive industry have also been realised. Topics also include filter elements for media distribution (gas and oil), heat exchangers, wearables and design products. With our in-house toolmaking, we are also working with AM technology to become faster and more cost-efficient. 

cfi: From today’s perspective, where do the challenges lie for long-term establishment of MBJ technology for series manufacturing? 

PK: MBJ is a relatively recent AM process, which, however, profits considerably from experience in classical Powder Metallurgy. Besides building up confidence in the new technology, which can be achieved principally with case studies, the available range of materials and the manufacturing-friendly design are limiting factors. In concrete terms, we are working with internal and external teams to widen the range of materials for e.g. nickel-free steels, copper materials or high-strength special steels. Moreover, we are engaging intensively with sintering simulation. With simulation, suggestions for design modifications for improving the production result can be elaborated and the robustness of the process improved. Controlling sintering shrinkage, which is particularly noticeable in the case of complex components and has an adverse impact on dimensional accuracy, is important. Physically based FEM sintering simulation enables calculation of the shrinkage and warpage of MBJ components for a defined sintering profile. The shrinkage and deformation vectors from the sintering simulation are taken into consideration to generate a pre-deformed component model with which dimensional accuracy requirements can be met based on compensation for any anticipated deformation. 

cfi: How important is post-processing with HIP? 

PK: For cost reasons, we try to avoid it with adapted material developments and optimised sintering curves. However, that is not possible for all materials and applications. With the combination of MBJ and HIP, very good mechanical properties can be obtained, comparable with that of forged components. 

cfi: For the costs of series production, the level of automation is a key factor. How far has your development come in this regard? 

PK: Great potential for automation can be found specifically in powder handling and processing, quality control and finishing. Precondition of automation is the digitalization and control of the MBJ process chain with the incorporation, evaluation and cross-linking of process data in Manufacturing Execution Systems (MES) based on the use of digital interfaces. These are important development projects. Tailored materials with high sintering activity, the process stability and automation have to further advance MBJ to make the costs of AM series production even more attractive. 

cfi: Thank you for talking to us.

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