Mandrel-Free Spin Forming of Aluminum Alloy Domes

Mandrel-free spin forming techniques were investigated for manufacturing hemispherical aerospace aluminum alloy domes. This supports a NASA goal to reduce manufacturing costs for cislunar space hardware. The study focused on process parameter optimization and geometric accuracy for incrementally deforming flat circular blanks. Mandrel-free spin forming offers significant advantages for producing dome-shaped components via the use of rotating tooling, rather than custom mandrels. Large-scale domes (>15 ft.) will require the use of friction stir welded blanks because of commercial plate size limitations. Extensive experimental trials on approximately 12-inch diameter blanks were conducted on Aluminum (Al) alloy 6061-O, Al 2024-O, and aluminum-lithium (Al-Li) alloy 2050 using multiple roller geometries and a variety of processing conditions. In Al-Li alloy 2050, blanks were taken from both monolithic plate and friction stir welded material. Initial mandrel-free spin forming trials of constant thickness blanks consistently resulted in cone-shaped geometries instead of the target hemispherical profiles. This was a result of plastic hinge formation and "sink-in" instabilities below the ideal arc-shaped profile.

The most significant breakthrough involved tailoring the starting blank design, which facilitated achieving target geometries. Contoured thickness blanks reduced the sink-in instabilities that compromised hemisphere formation. Higher strength alloys like Al 2024 showed improved geometric conformance compared to Al 6061, but at the expense of reduced formability and increased cracking susceptibility. During parametric studies to optimize final part quality, the effects of feed ratio, number of forming passes, elevated temperature processing, and roller geometry were explored. Force analysis of in-situ monitoring data provided insights into material flow behavior and failure mechanisms. Friction stir welded blanks were successfully formed, expanding manufacturing possibilities for large-scale components.

This research establishes a fundamental understanding of mandrel-free spin forming limitations and solutions. The results provide useful guidance to aerospace manufacturers seeking cost-effective alternatives to customary fabrication of domed structures.