تاثير فرآيند پيرسازي بر خواص مكانيكي و ريزساختار آلياژ مس-بريليم مورد استفاده در پيستون تزريق دستگاه ريخته‌گري تحت فشار بالا

Injection plungers used in cold-chamber high-pressure die casting machines are subjected to repeated thermal an‎d mechanical cycles, direct contact with molten metal, an‎d stresses induced by the injection process. These conditions gradually lead to microstructural changes an‎d deterioration of mechanical properties. Given the high cost of CuCoNiBe alloy used in these components, this study aimed to investigate the feasibility of recovering the mechanical properties of used CuCoNiBe alloy through heat treatment an‎d to examine the relationship between microstructural evolution an‎d mechanical behavior. For this purpose, samples were obtained from new an‎d used injection plungers. The chemical composition of the alloy was determined using optical emission spectroscopy (quantometry) an‎d inductively coupled plasma optical emission spectroscopy (ICP-OES), an‎d the results were compared with the chemical compositions of stan‎dard CuCoNiBe alloys. The used samples were then subjected to solution heat treatment at 950 an‎d 1030 °C for various durations. After determining the optimum solution heat-treatment conditions, aging treatments were performed at 450 °C for different durations. Microstructural characterization was carried out using optical microscopy, scanning electron microscopy (SEM), field-emission scanning electron microscopy (FESEM), an‎d X-ray diffraction (XRD). The mechanical properties were eva‎luated using Vickers hardness an‎d tensile tests. The results showed that the unused sample exhibited relatively fine grains, numerous annealing twins, an‎d fine, dispersed CoNiBe precipitates, with a hardness of approximately 250 HV. In contrast, prolonged exposure to service conditions resulted in grain an‎d precipitate coarsening, a reduction in the number of twins, an‎d a decrease in precipitate density, reducing the hardness of the used sample to 220 HV. Increasing the solution heat-treatment temperature an‎d duration promoted precipitate dissolution into the matrix while reducing precipitate size an‎d number. Grain growth at higher temperatures an‎d longer treatment durations also led to a gradual decrease in hardness, with a minimum measured value of approximately 80 HV. Aging at 450 °C promoted reprecipitation, increased the density of fine precipitates, an‎d resulted in a more uniform precipitate distribution throughout the matrix, thereby substantially improving hardness an‎d tensile strength. The maximum hardness an‎d ultimate tensile strength obtained after aging were 317 HV an‎d 770 MPa, respectively. Analysis of the tensile test results an‎d microstructures indicated that the mechanical properties depended on the extent of precipitate dissolution, grain growth, an‎d subsequent reprecipitation. These findings highlight the critical role of selec‎ting appropriate heat-treatment conditions in achieving a suitable balance between strength an‎d ductility. Based on the results, the optimum heat-treatment cycle for recovering the properties of the used alloy consisted of solution heat treatment at 950 °C for 105 min, followed by aging at 450 °C for 60 min. This treatment provided the best combination of strength, hardness, an‎d fracture behavior. Overall, the findings demonstrated that appropriate selec‎tion of heat-treatment parameters can recover a substantial proportion of the mechanical properties of used CuCoNiBe alloy, potentially extending the service life of injection plungers an‎d reducing component replacement costs in high-pressure die casting industries.

بهزاد نيرومند , محمود مرآتيان اصفهاني

محمدرضا طرقي نژاد , محمدحسين مصلي‌نژاد