H. N. Chou, Boeing Phantom Works, St. Louis, MO
Metal Injection Molded (MIM) casting can be used to produce low cost affordable components for aerospace applications. Phantom Works co-worked a MIM development program with Advanced Manufacture Research Center (AMRC) in U.K.
This presentation describes the behavior of MIM specimens in heat treated and hot isostatic pressing (HIP) conditions when they were tested in stress life fatigue test. HIPed and heat treated castings usually exhibited longer fatigue life than those that was not HIPed. However, AMRC reported that the HIPed MIM fatigue specimens had shorter fatigue life than heat treated ones. A failure analysis was initiated to investigate the cause and to solicit ate any recommendations.
The optical and image mapping examinations of the fractured specimens determined the HIPed specimens had less amount of porosity than the heat treated ones. Scanning Electron Microscope examination observed typical fatigue striations in both HIPed and heat treated specimens. Fatigue origins were observed to be close to the internal porosity in the only heat treated specimens. Photomicrographs taken from the prepared etched mounts revealed carbide particles in the grain boundaries, which can serve as stress riser for fatigue initiation.
Further investigation revealed that the HIP process for the specimens was performed at a higher temperature than the solution heat treatment temperature. This HIP process facilitated the formation of carbides in the grain boundaries. So, the damage caused from carbides resulted from HIP contributed more to the fatigue initiations. This damage overrides the benefit from pore closing and shortened the fatigue life for the HIPed specimens. It was decided that new MIM specimens are needed and shall be HIPed at a temperature lower than that used for heat treatment to minimize the carbide formation to correct the problem. The new specimens shall be submitted for tests to validate the process.
Metal Injection Molded (MIM) casting is a manufacturing process that can be used to produce low cost secondary structural components for aerospace applications. Phantom Works in Boeing Enabling Technology co-worked a MIM development program with Advanced Manufacture Research Center (AMRC) and Sheffield University in U.K.
This presentation describes the behavior of MIM specimens in heat treated and hot isostatic pressing (HIP) conditions when they were tested in stress life fatigue test. HIPed and heat treated castings usually exhibited longer fatigue life than those that was not HIPed. However, AMRC reported that the HIPed MIM fatigue specimens had shorter fatigue life than heat treated ones and the specimens made from four (4) different MIM suppliers all behaviored in the same fashion. A failure analysis was initiated to investigate the root cause and to solicit ate any recommendations.
The optical examination and image mapping of the fractured specimens determined the HIPed specimens had less amount of porosity than the heat treated ones, which proved the conventional observation that HIP can close internal flaws. Scanning Electron Microscope examination observed typical topographic features of fatigue striations in both HIPed and heat treated specimens. Fatigue origins were observed to be close to the internal porosity in the only heat treated specimens. Photomicrographs taken from the prepared etched mounts revealed carbide particles in the grain boundaries. X-ray diffraction determined the presence of carbide particles in the grain boundaries, which can serve as stress riser for fatigue initiation.
Further investigation revealed that the HIP process for the MIM fatigue test specimens was performed at a higher temperature than the solution heat treatment temperature and the specimens were slowly cooled. This un-conventional HIP process facilitated the formation of carbides in the grain boundaries. So, even HIP closed the internal flaws, which did enhance the fatigue life, but the damage caused from carbides more significantly contributed to the fatigue initiations. This damage overrides the benefit from pore closing and shortened the fatigue life for the HIPed specimens. The same phenomena were observed in the fatigue specimens made from four (4) different suppliers since they all were HIPed at a higher temperature than the solution heat treatment temperature.
It was decided that new MIM specimens are needed and shall be HIPed at a temperature lower than that used for heat treatment and the specimens shall be fast cooled to minimize the carbide formation to correct the problem. The new specimens shall be submitted for tests to validate the process.
