Researchers at the Fraunhofer Institute for Manufacturing Technology and Advanced Materials – IFAM in Germany recently developed a new material combination that can be processed into complex-shaped precision components with magnetic/non-magnetic properties by using Two-component Metal Injection Molding. Metal Injection Molding (MIM) is a manufacturing process that allows producing net-shape metal components by injecting a powder-binder-mixture into a mold on an injection molding machine. After binder extraction the shaped powder particles are densified into net-shape metal part in a sintering process. Due to the reduction of the free space between the powder partciles, the densification causes shrinkage of about 15 – 20 %. By processing two metal powders at the same time on a two-component injection molding machine bi-metal parts can be achieved. However, since the shrinkage during sintering is an intrinsic material property, identifying suitable metals with a compatible shrinkage is a scientific challenge.
Recently Marco Mulser, Georg Veltl and Frank Petzoldt published a new approach to compensate the shrinkage of two powder materials in the International Journal of Precision Engineering and Manufacturing. Exemplarily, they utilized gas-atomized spherical powders of ferritic stainless steel (AISI 430) and austenitic steel (AISI 314). By means of dilatometry measurements the sintering shrinkage was quantified. It shows that the shrinkage differs substantially with the chemical composition of the alloy and the particle size. The published approach allows balancing the shrinkage by combining different particle sizes for the ferritic and the austenitic steel.
Astonishing results were found during tensile tests of the new material combinations after sintering. None of the tensile specimens failed in the joint so that the interface was still intact after testing. In other words, all tested specimens failed in the ferritic base material. This result proves that the joint is stronger than the ferritic stainless steel. Micro-hardness tests indicated a significant increase in local hardness at the interface compared to both base materials.
According to the authors, the strengthening effect during sintering can be traced back to the interdiffusion of alloying elements at the interface. The ferrite and the austenite differ significantly in alloying elements. The austenite is much higher alloyed mainly with chromium and nickel that the ferrite. At the interface a dual-phase ferritic-austenitic microstructure was formed that was visualized by the researchers by means of x-ray measurements and the Schaeffler diagram. It induced strain hardening caused the hardness increase and strengthened the interface in the utilized material combination. According to the article, the mechanical test results also confirm the assumption that no martensite and no brittle sigma-phase were formed.
The authors conclude that the new material combination of ferritic and austenitic stainless steel and the described strengthening effect on its interface is now ready to be processed into net-shape magnetic/non-magnetic precision components. They further state that the Two-component Metal Injection Molding process (2C-MIM) offers multifarious advantages in terms of new design opportunities for engineers and designers by combining unique properties in a single net-shape component such as e.g. high toughness plus wear resistance, local hollow structures in dense components or even flexible with non-detachable connections.
Mulser, M., Veltl, G., Petzoldt, F. Development of Magnetic/Non-Magnetic Stainless Steel Parts Produced by Two-Component Metal Injection Molding, International Journal of Precision Engineering and Manufacturing 17 (2016) 347-353.
Fraunhofer Institute for Manufacturing Technology and Advanced Materials – IFAM Bremen, Germany.Go To International Journal of Precision Engineering and Manufacturing