Fraunhofer Institute for Mechanics of Materials IWM
Precipitation hardening is a key mechanism for improving the mechanical properties of materials, particularly copper alloys, which require both high strength and high electrical conductivity. Particularly in contact materials for connectors, small, finely distributed precipitates within a homogeneous mixed-crystal matrix are required to meet the requirements for strength, relaxation properties, and conductivity. The challenge lies in precisely controlling the size and distribution of the precipitates to achieve optimal materials properties. This requires precise adjustment of the annealing and rolling parameters, while taking into account the overall temperature control throughout the manufacturing process. The precipitates are highly sensitive to temperature and time parameters during aging. High temperatures can lead to coarse-grained, less homogeneous particles, while temperatures that are too low result in excessively slow precipitation development.
Control of the precipitation process is based on a thermodynamic-kinetic model that describes the temporal evolution of precipitates in Cu-Ni-Si alloys and enables predictions regarding their size and number at varying temperatures. This model accounts for the nucleation, growth, and dissolution of the particles and is based on known Gibbs energy functions and diffusion coefficients available in databases. The theory allows for the simulation of various annealing parameters and thus the targeted control of the microstructure. This enables the precipitation process to be optimized in such a way that high strength is achieved through finely distributed Ni2Si precipitates while maintaining good electrical conductivity.
To master the process, experimental investigations on Cu samples with varying annealing parameters were conducted as part of a collaborative project, and the precipitation distribution was measured using TEM (transmission electron microscopy) at partner institutes and compared with the model predictions. The results are being incorporated at Fraunhofer IWM into the refinement of a thermodynamic-kinetic model used to calculate the optimal aging temperatures and durations. This method enables the targeted adjustment of process parameters to specifically influence the microstructure and thus achieve an ideal combination of strength and conductivity.