Forged Titanium Domes Will Help Explore the Deep
Funded by the National Science Foundation, owned by the U.S. Navy and operated by the Woods Hole Oceanographic Institution (WHOI) in Falmouth, Mass., the original Alvin gained worldwide renown as the vessel that made possible the 1986 filming of the Titanic 13,000 feet below the surface of the Atlantic. Alvin is also well known in scientific circles for bringing hydrothermal vents to the world’s attention, the discovery of which has led some scientists to hypothesize these vents may have created the conditions for the emergence of organic life on the planet. The deep-sea submersible has also helped to discover previously unknown marine life and document evidence of plate-tectonic activity, among innumerable other achievements.
During four decades of service to science, Alvin has contributed much despite a frustrating constraint. Due to the pressure limitations of the current vessel’s hull, more than one-third of the ocean floor remains out of reach. Scientists are eager to see what exists in the unexplored 38% of the ocean’s floor. To realize that capability, the National Science Foundation (NSF) authorized the development of an RHOV (Replacement Human Occupied Vehicle). This vehicle will be designed to reach 99% of the ocean’s floor. In addition to a capability for reaching greater depths, a larger hull will give the crew of three – traditionally made up of a pilot and two scientists – 23% more elbow room, creating space for additional scientific equipment and extending “dwell time” on the bottom to accomplish research missions.
The Forging ChallengeA number of forgers, including Ladish, were offered the opportunity to bid on the project. However, the technical challenges to fulfilling the design vision for the new vessel, daunting and significant as they were, advised caution. The proposed new hull design pushed the envelope of what traditional forging processes had historically shown themselves capable of producing.
Having the design for what was needed was one thing; transforming it into an actual part was quite another. This was not a job for those who were averse to risk or who lacked confidence in their forging process-development capabilities. According to Christopher A. Roedel, Ladish’s manager of domestic sales, “For us, the uncertainties involved in forging hemispherical shapes as large as those ordered by WHOI and SwRI were enormous. Initially, the potential for success seemed to us to be evenly balanced with the potential for failure.”
Over the course of 18 months, the supplier evaluation and selection process narrowed down to Ladish, one of the small handful of organizations worldwide that might end up on the design team’s short list of companies deemed capable of manufacturing the hemispheres. In the end, Ladish accepted the challenge. In early 2008, SwRI arranged for the shipment of the three ingots of Ti-6-4-ELI they were holding in Texas to Ladish’s Wisconsin forge.
“The RHOV is one of the most exciting projects we’ve worked on in years,” said Jerry A. Henkener, SwRI staff engineer, Marine Structures and Engineering. “Of the three primary hurdles to building the RHOV, the most critical was successfully forging titanium hemispheres bigger than any hemispheres attempted before.”
Technical ConcernsThe concern about achieving high levels of metallurgical and mechanical integrity in the finished forgings arose from knowing that the larger the hull, the more surface area it presents in the high-pressure environments encountered at great depth. The new hull would have to be built from forgings that met demanding standards. To withstand the pressure at 6,500 meters (21,320 feet), which approaches 5 tons per square inch, the design team had selected titanium as the material and a sphere as the shape for the hull.
“The spherical shape provides even distribution of pressure on the hull’s surface,” Henkener said. “We picked a titanium alloy for its strength-to-weight ratio. Titanium is 45% lighter than steel yet every bit as strong. We were also attracted by titanium’s superior anti-corrosion properties – particularly its ability to deliver long service in a saltwater environment.
“Stated another way, the compressive yield strength of the titanium we used for these components is more than 120,000 psi, so the hull will be able to withstand massive underwater pressure and provide decades of service while accommodating a heavy dive schedule. Titanium, especially the selected alloy, has proven its durability over the decades in parts specified for aerospace and desalinization-plant applications as well as in U.S. Navy subsea applications.”
Ladish Forging is well-equipped to supply SwRI with its hemispheres. It operates some of the most powerful pieces of forging equipment in the world. The company has a long track record of forging dome-like shapes and titanium products.
Tom Furman is one of the many engineers who worked on the team that designed the forging process for the two Alvin hemispheres. According to Furman, “Ladish has produced many dome shapes, mostly for rockets, missiles and NASA launch vehicles. Our expertise also includes the forging of titanium parts for jet engines, aerospace structures and a range of defense applications. However, this strong background notwithstanding, we have never attempted to forge a dome of titanium as large as that called for by this design.”
The Engineers Go to Work“We faced many tests en route to designing a process for this large, relatively thin, difficult-to-forge shape,” Furman said. “Just to cite one example: To achieve the specified mechanical properties, we had to plan for a sequence of forging operations in order to thoroughly work the material before deformation into its final hemispherical shape. Maintaining tight control of temperatures during these material preparation stages was a key concern. Because titanium cools quickly, we had to pay extraordinary attention to cooling rates in our modeling and process-development phases to ensure that the workpiece was never in danger of cracking.”
The engineering team spent weeks designing and orchestrating the entire operation, from the setup of the furnace to the travel speed of the giant manipulators that would remove the hot workpiece and deliver it to the press, to the forging process and tooling parameters. Furman said, “We drew on our extensive history of lessons learned during prior experience.”
Experience CountsLadish relied heavily on its experience forging scores of dome shapes and thousands of titanium components. Engineers devised a plan for integrating multiple forging steps with an extensive materials’ database to produce a computer simulation of the proposed forging process. Computer simulation was a valuable tool that enabled the engineers to move forward with relative speed and confidence regarding how the titanium alloy for WHOI’s submersible would behave during the forging process. After reviewing the process design with the customer, company engineers were gratified to learn that they had addressed all of the problems that had challenged another supplier years earlier when the previous generation of smaller titanium hemispheres were forged.
According to Furman, “Past experience of our own forging process and learning of the experiences of others helped, but ultimately we had to manufacture the hemispheres using specific pieces of equipment that are unique to us. For this reason, in addition to knowing how to transform a cast titanium ingot into a workpiece of the correct size and how to work with a specific alloy, we also had to have complete confidence that we understood the performance capabilities of our furnaces and our large hydraulic press.”
The engineering team performed multiple tests on the computer model they developed. They wanted to troubleshoot every material-behavior prediction. It was during this phase that Ladish designed and built the tooling for the forging operation based on the WHOI/SwRI design.
Two 7-ton cast ingots of titanium provided the raw material for the forging process. Both ingots were heated to forging temperatures in one of the company’s large furnaces before undergoing hot-work across a series of intermediate forging operations. These forging steps refined the as-cast titanium into an optimal microstructure for the forging process. In this phase, ingots were converted into large pancakes – circular slabs about 6 inches thick with enough diameter to enable the forging of the desired hemispherical shape.
Bearing WitnessWhen the time for the final hot-forming operation was determined, schedules were coordinated among a number of stakeholder organizations so interested parties could witness the final forming of the two hemispheres. WHOI sent veteran pilots of the Alvin, other representatives and a film crew that would film the event for a possible documentary. Southwest sent members of its design team and technical associates. More than 50 Ladish employees, most of whom had been involved in different aspects of the dome design, planning, quality or manufacturing, were also on hand to witness the culmination of the large team effort.
Ladish forged the first hemisphere on June 26, 2008, and the second on June 27. After being brought up to forging temperature, a company-designed manipulator, with forks more than 17 feet in length, scooped up the heated slabs of titanium and transported them to the forging press. The press selected to produce the titanium domes is remarkable for the more than 17 feet of “daylight” between its ram and base. The manipulator deposited the workpiece, glowing orange from its prep-time in the furnace, on the forge tooling. After adjustments for positioning, the pressman activated the press and the forming punch descended under intense hydraulic pressure to form the slab into the specified shape.
According to Furman, “Our predictive models turned out to be very accurate. At every step of the process the material behaved almost exactly as our model said it would. From an engineering standpoint, the two forging operations went perfectly. The entire team behind the design, forging and quality-control operations deserves kudos for carrying out a complex and challenging plan.”
Although the forging operations appeared to have been performed without a glitch, the engineers waited for testing to be completed before declaring the forging process a success. Extensive tests over the following four months were needed to confirm that the two hemispheres met all the targeted properties enumerated in the customer’s specifications.