Optimierung der Diffusionsschweißparameter von Hastelloy® C-22® zur Herstellung von Mikrowärmetauschern

The subject of this work is to optimize the diffusion bonding parameters for Hastelloy® C-22® by means of a diffusion bonding model for the production of a cross flow micro heat exchanger for chemical pplications. Due to the corrosion resistance, the microstructures of the material should not be strongly changed during the diffusion bonding process. The mechanical properties of the bonding unit (e.g. tensile strength, ductility) should be as high as possible. To achieve this objective the preliminary investigations concerning the micro structural and mechanical properties are necessary. ... mehrThe investigations of the micro structural properties show that a diffusion bonding process is impossible in the temperature range of 700 °C < T < 1050 °C without the recipitation of Mo-rich phase and the formation of metal carbide at the welding seam, which can worsen the ductility of the bonding sample. At temperature above T > 1050 °C a substantial grain growth takes place which lowers the strength of the material. Below a temperature of 700 °C a diffusion bonding process can be conducted only with very high compressive force and bonding time. It was found that the optimal temperature for the bonding process is at 1050 °C. Additionally, it can be recognized that the sheet metal surface in the as-received condition is contaminated with 600 nm thick Al2O3 layer, which has to be ground before the bonding process. By the investigation of the mechanical properties the temperature dependence of yield strength and the creep law will obtained, which served as input parameters for the diffusion bonding model. For the prediction of an optimized parameter set a computer program based on the diffusion bonding model is implemented, which contains all transport mechanisms also present in the sinter process of metals. The model calculation yielded an optimized parameter set of 1050 °C, 26 MPa and 3.6 h for the surface roughness produced by a P-2500 abrasive paper. This parameter set is verified in the diffusion bonding experiments on the unstructured sheet metal samples for different values of compression stress, bonding time and surface roughness. A short bonding process is not possible due to the carbide formation at the welding seam as identified by the AES analysis. The bonding with this parameter set results a good welding seam with small remaining pores. The bonding specimens show that the tensile strength is as high as that of base material and the ductility is being more than 50 % of the base material. By using this optimized parameter set on the cross flow micro heat exchanger similar part, the inhomogenous distribution of the compression stress at the joint interface is considered by means of a FEM computation. Hence the compression stress on the total area of the part amount to 11.7 MPa. The diffusion bonding experiments with the structured sheet metals show likewise high strength and ductility, with microstructures and welding seam quality comparable to the diffusion bonding with sheet metals without channel structures