ENI is known for taking on difficult projects that are critical to the security of the United States. But few required more innovative engineering than the upgrades to the EDO Corp.'s MK 105 Helicopter Towed Minesweeping Sled.
The helicopter towed minesweeper is a significant part of the US Navy’s effort to clear mines in harbors and contested sea areas. Its first use was to clear Haiphong Harbor of mines in 1973, following the end of the Vietnam conflict. It has been used in every Navy mine-clearing operation since then.
The minesweeping system is based on a hydrofoil sled platform which carries sophisticated electronic equipment to create an electromagnetic field resembling the signature of a ship, and is powered by a gas turbine/generator installed on the sled. Towed at high speeds by a MH-53E helicopter, the system can safely clear a path through a minefield much faster than conventional surface ships.
The hydrofoils that provided the lift to support the sled, along with their hubs and struts, were often damaged in service after hitting objects on or just below the surface of the water. This damage degraded lift capability and increased the strain on the tow cable and the helicopter itself.
We participated in a joint effort with EDO Corporation to minimize weight, increase tow speeds and increase the reliability and sustainability of the hydrofoil system.
The Challenge
Our role was to fabricate both the redesigned hydrofoils and the redesigned supporting struts and hubs, as well as to identify opportunities to improve manufacturability and reduce costs.
EDO chose to base the redesign on the use of high-strength corrosion resistant alloys, which were still considered exotic materials at the time. The hydrofoils were made from light-gauge titanium alloy sheets. The redesigned struts and hubs that maintain the alignments of the foils were made from 15-5 PH martensitic precipitation-hardening stainless steel.
15-5 PH material is prized for its resistance to salt water corrosion, strength and toughness, which make it perfect for seawater applications. It is, however, more difficult to machine and weld than most stainless steels. It can be gummy, so it requires special tooling and special cutters, and it must be welded under carefully controlled conditions to achieve dimensional control and prevent weld cracking.
The Journey
Based on our previous experiences with fabricating titanium by welding, we knew that distortion control was going to be the key factor in meeting drawing requirements.
Since distortions in fabrications are caused by the buildup of residual stresses as parts are formed and welded, we needed to address and mitigate these stresses at every stage of manufacturing.
The Discovery
We determined early on that the hydrofoils were going to be far too stiff for post-weld straightening. That meant that we needed to make every piece of each structure as close to its finished shape as possible before welding to minimize the problems later.
The tapered skins were considered to be the most problematic parts so we needed a repeatable way to get them ready for assembly.
The welding processes used for the project were well established by this point, but the extensive welding of long, thin sections was a major concern. It is easy for weld shrinkage to cause dramatic bending and curving in such sections. Figuring out welding sequences to keep the hydrofoils straight and square was going to be an interesting learning process.
The Solution
The EDO design required that we put a 3D bend in almost every piece of the hydrofoil structure. The skin parts were curved in two directions — over their length and also over their width - so forming the skins was a complex process.
The resolution was to precision form the tapered skins by pressing them to their final shape between contoured top and bottom dies. Each curved surface required its own upper and lower die. The die halves were welded together to maintain the shape, then the closed die assembly was stress relieved at a high temperature in an inert atmosphere furnace.
Welding the internal structures to the formed sheets was performed on the dies, and carefully sequenced and controlled to maintain the correct shape, using the die as a contour reference. Strategically placed weld beads were used to tune the shapes to the dies as well.
Welding the closing skins in place required even more care in weld sequencing to minimize the accumulation of weld shrinkage and distortion over the required long welds. Lessons learned during welding the internals were successfully applied to the closing skin welds.
All of the welding was performed in the shop environment using special weld shielding to prevent contamination and oxidation. After all the welding was completed, the fixture/inert atmosphere stress-relief process was repeated to produce a stable structure meeting all print requirements.
The 15-5 stainless steel struts and hubs were also redesigned to match the expected capabilities of the foils. The geometry was revised to break down both the struts and hubs into smaller components that could be welded together to minimize the extent of machining required to achieve the finished condition. The individual components were welded together in the solution annealed condition. Each weldment was machined to near finish, leaving material to account for possible distortions from heat treatment. The weldments were then subjected to a complete solution heat treatment plus an aging treatment to reach the maximum material properties. Finally, the weldments were machined to finish dimensions.
The Results
Tests showed that the revised foils were a significant improvement over the existing design. However, the projected costs of the redesigned system package were higher and it was not incorporated into production due to government cutbacks at the time.
Our team gained valuable insight from the project. In the ensuing years, we have incorporated the high-temperature/inert atmosphere (or vacuum) stress relief process, using fixtures to produce specific shapes, into so many subsequent titanium projects that we now consider it a standard practice.
The expanded understanding of weld shrinkages and distortion in titanium weldments has resulted in better control of distortions. It has also allowed for more reliable use of weld shrinkage from strategically placed weld beads to induce changes in the contour of parts for dimensional or fit-up purposes.