Determining the heat treatment behaviour of metal injection moulded and wrought alloy 718 using differential scanning calorimetry

The heat treatment behaviour of metal injection moulded (MIM) and wrought alloy 718 was investigated using DSC, microscopy and hardness measurements. Reheating solutionized samples in the DSC resulted in a distinct exothermic peak around 725 degrees C, which was attributed to gamma precipitation. This was followed by a broad endothermic region (-750-900 degrees C), which was attributed to the dissolution of gamma' and gamma precipitates. No DSC peak was observed corresponding to gamma' precipitation upon reheat. The endothermic dissolution region (750-900 degrees C) ended with an abrupt exothermic shoulder around 900 degrees C, which was attributed to the solvus of the gamma'/gamma precipitates and an acceleration of 6 phase precipitation. Ageing the samples at progressively higher temperatures and for longer times resulted in a decrease in the 725 degrees C exothermic gamma precipitation peak and a deepening of the 750-900 degrees C endothermic dissolution region. Also, samples with higher Al content exhibited a deeper dissolution trough and a small bump in the exothermic shoulder at the end of this trough (-880 degrees C), which were attributed to the higher levels of gamma' observed in the FESEM for these samples. A shallow endothermic trough peaking between 1000 and 1045 degrees C was observed in samples treated below the 6 phase solvus and was attributed to dissolution of the 6 phase. Hardness measurements showed an increase in hardness with the amount of gamma' and gamma precipitated as expected. Differences in heat treatment behaviour appeared to be driven largely by differences in chemical composition rather than by processing route (wrought vs. MIM). However, the MIM material had smaller, more evenly distributed carbides and additional oxides not present in the wrought material which acted to restrict grain growth in samples solutionized up to 1232 degrees C.

Automated summary

By investigating the heat treatment behavior of metal injection molded (MIM) and wrought alloy 718 using DSC, microscopy, and hardness measurements, it was observed that increasing temperatures and times led to an increase in hardness due to the precipitation of gamma' and gamma. The differences in heat treatment behavior appeared to be mainly influenced by variations in chemical composition rather than processing route.