L. W. Weihmuller, Bell Helicopter Textron, Hurst, TX; S. J. Mashl, Bodycote HIP, Andover, MA

Summary:

Aluminum castings offer a low cost manufacturing approach for many rotor-craft components because complex shapes can be cast net. However, the usage of castings for fatigue or flight critical applications is limited because of the concern over the internal integrity of the casting. Internal defects, such as porosity, can severely reduce the mechanical strength and fatigue life of the component. Hot Isostatic Pressing (HIP) has been used for decades to improve the internal integrity of titanium and nickel base cast alloys, however the use of HIP to densify aluminum cast structure is not as common because of the relatively high cost of HIPing versus the perceived benefits. This paper examines the metallurgical benefits provided by HIPing of aluminum cast structure, and compares the effectiveness of Densal II ®, a lower-cost proprietary HIP process, to traditional HIPing.

In this study, A357 sand cast and A356 investment cast materials were put through either a conventional aluminum HIP cycle or the proprietary Densal II ® HIP process. Additionally, non-densified material was processed to provide a baseline for comparison. All material was heat treated to the T6 condition. Non-destructive inspection was performed prior to densification and after heat treatment. After all processing, tensile, fracture toughness, and fatigue testing was performed on the six groups of material. Additionally, a group of the fatigue specimens were shot-peened to introduce compressive surface stresses and assess their effect on fatigue life. Results indicate a significant improvement in fatigue strength with the introduction of either HIP or Densal II ® processing. Post-processing metallography and fractography were employed to evaluate as-processed microstructures and fatigue fracture initiation sites. This paper reviews the work described and draws conclusions regarding the factors affecting fatigue life.