This article focuses on an ancient art form exhumed by modern engineering tools that makes hyperefficient gas turbines. IGT makers, such as GE, Siemens Westinghouse, and Alstom Power, use Howmet’s blades and vanes in the “hot sections” of their gas engines. Extreme inlet temperatures and the promise of greater efficiencies keep Howmet engineers seeking ways to implement advanced cooling designs, higher- temperature alloys, and stronger airfoils. Developing directionally solidified crystals was an important step in the improvement of turbine blade strength—one taken over 30 years ago. For directionally solidified casting, both the mold and the metal are kept at similar temperatures, around 2700 or 2800°F. Aside from the technological leaps that have been made in lost wax casting to produce ever larger, stronger, hotter, and more complex IGT components, it is still very much an elemental process.
Multigrain, directional grain, single crystal: Described by the names of their crystallographic structures, the turbine airfoil castings manufactured at the Howmet plant in Hampton Roads, Va., have the sound of strange new breakfast foods. Seen in person, the same blades and vanes reflect an old-world beauty of fine sculpture.
Maybe this is just how things ought to be, tor lost wax casting, the process Howmet uses to create these precision forms, is both old and somehow strangely new. It is old in that it dates back at least to 1,000 B.C., when artisans applied its techniques to the manufacture of religious icons. It is new in that technology has pushed it from an art to a science, making it the method of choice for manufacturing many of the components used in advanced power trains.
With GE Power Systems’ announcement last February of the H System and its promise for combined-cycle plant efficiencies of 60 percent, a light shone down upon the folks at Howmet. Long a major manufacturer of airfoils for aircraft engines, Howmet s airfoil business for industrial gas turbines has been growing in the past few years. If they haven’t already, IGT products may soon become the majority portion of Howmet’s business, said Jim Boutot, the company’s IGT business manager.
IGT makers, such as GE, Siemens Westinghouse, and Alstom Power, use Howmet’s blades and vanes in the “hot sections” of their gas engines, Boutot said. Extreme inlet temperatures and the promise of greater efficiencies keep Howmet engineers seeking ways to implement advanced cooling designs, higher-temperature alloys, and stronger airfoils. Many of these gains have come straight from earlier advances in aircraft engine airfoils, Boutot said. Still, stationary turbine airfoils are sufficiently removed from their smaller aircraft cousins that, at times, their design and manufacture have required completely different approaches.
As one of several advanced systems being conceived by IGT makers, the coming H System provides a good excuse for taking another look at this ancient art—and modern science.
Bill Smith, a Howmet engineer who leads a team making nonrotating components, recently conducted a tour of the Howmet IGT plant. The first stop was the pattern-making shop. Here, a forest of pink wax patterns reached toward the ceiling, each one an exact duplicate of the turbine blade or vane for which it would one day melt. Every pattern receives careful attention from a bevy of crafters who inspect, touch up, and wax-weld the patterns. But the story doesn’t begin there.
Smith pointed to a collection of molds. These, he said, are where the wax patterns originate. The molds, installed into wax injection machines manufactured by subsidiary Howmet-Tempcraft of Cleveland, generate what will become the final forms of gas turbine blades and buckets. Tempcraft makes the tooling as well, Smith explained.
Patterns emerge from the injection molds shiny and pink. Some comprise as many as four airfoil segments. Others must be individually assembled to form a multiairfoil vane segment. A great measure of handcraft comes into play here and at other steps along the casting process.
Many turbine blades and vanes rely on internal cooling passages to circulate air through them so they can better withstand the metal-melting heat of the turbine’s hot section. Ceramic cores, installed during the wax injection, form the interior surfaces of these parts during the casting, Smith said.
Patterns for smaller parts, such as the blades for aircraft engines, are commonly wax-welded to a central sprue in order to cast as many parts as possible in a single mold, Smith explained. For larger parts, such as blades for stationary turbines, the number of patterns welded to a single sprue may drop from several dozen, say, to as few as one or two, depending on the final weight of the assemblage, or tree.
The trunk of the tree is wax-welded to a ceramic pouring cup that, in turn, is attached to a metal plate, or pallet. The assembly rests upside down on this pallet as it goes through a series of measurements, additional wax welding, and finishing work in the pattern making shop.
Next Stop, Slurry
Once the wax patterns are made, sprued, and inspected, they move along to the shell application, Smith said. Howmet just began operation this year of a $10 million line capable of processing many patterns at random, eliminating the batching of like patterns that the company had to do before. Central to the shell application line are five robot work cells complete with dip tanks and stucco showers. An overhead conveyor stages individual shell molds between dips and dusts. The new line has increased production significantly, Smith said.
Up to 15 dips are needed to form a ceramic shell. Alternating layers of slurry and stucco surround the wax pattern until a coating almost a half-inch thick builds up. Critical to the process is the robot’s manipulation of the pattern and shell immediately after dipping, while the slurry drains back into the tank. “The robot controls the shell build,” Smith said. A shell uniformly thick provides two benefits. For one thing, it avoids thin spots and their tendency to leak. For another, it ensures even thermal conductivity throughout, he said, leading to balanced solidification of the alloy.
Between each dip the shell must harden, Smith explained. For that, the robots suspend the molds from an overhead conveyor. A programmable logic controller tracks the status of the dozens of molds drying there, every one conceivably different, monitoring how far along each one is in its multidip sequence and how much rack time each has logged.
The first few dips skip a pass through the stucco chamber, Smith said. This guarantees that the mold’s insides will duplicate the smooth texture of the wax pattern. After several slurry baths, however, imparting strength to the mold becomes a chief concern. That’s where the stucco comes in. A finished mold can weigh several hundred pounds empty. Pour weights can reach 1,500 pounds. The dusting with dry stucco strengthens the slurry as aggregate strengthens concrete.
After the final layer of slurry and stucco hardens, the shell-coated pattern is dewaxed in a high-pressure, high-temperature chamber. Smith said the dewax furnace reaches operating temperature and pressure in mere seconds. If it didn’t come up to temperature fast, the expanding wax could easily crack the mold. “It’s the same effect you’d have if you left a shell out in the sun,” Smith said. “The wax would melt slowly and crack the mold.”
After the bulk of the wax is forced from the core, the shell remains in the dewax furnace for some time to allow any residual wax to burn off. “After dewaxing, we inspect the interior of the mold to make sure the ceramic core hasn’t broken,” Smith said. “We don’t want to pour good metal into a bad mold. We also apply insulation wrap. We attack certain areas of the mold in order to get the alloy to solidify at a fairly uniform rate.”
The Die is Cast
The molds are ready for pouring at this time, Smith explained. But whether they are filled in vacuum or ambient furnaces depends on the kind of alloy used. Filling in an environment that’s been pumped free of air keeps impurities from affecting the properties of the alloy, Smith said.
Sitting alone in an air conditioned compartment, the furnace operator oversees each pour through a thick glass porthole. The pour itself goes quickly, but the operator watches the whole process with great care.
According to Boutot, blades are processed to form one of three types of grain patterns: equiax, directionally solidified, or single crystal. An equiax pattern allows for a multitude of grains in the finished casting, but places a tight restriction on their size, depending on where they fall in the part. A directionally solidified casting restricts the path of grain growth in a part, but not necessarily size. A single crystal, technically the most challenging of the three to grow, restricts the number of grains to one, Boutot said.
Howmet installed a new furnace this past year, built by another subsidiary, MTech of Cleveland. It is the first furnace able to produce either equiax or directionally solidified crystals, Boutot said.
The main difference in pouring for equiax, directionally solidified, or single crystals comes after the mold is filled. Both directionally solidified and single crystals withdraw from the furnace through a second heating coil. That ensures that the bottom cools first, Smith said, so the grain or grains begin growing from the bottom and work their way upward.
“After the pour, we apply exothermics in the pour cups, which keeps the gating hotter longer than the other areas of the casting,” Smith said. Exothermics are high flash point elements, he added. “The intention is for any shrinkage to go up into the gating and for the gating to remain hottest longest. The casting is able to draw from the gating to get the metal it needs to solidify without shrinkage or high porosity.”
Once the mold has cooled, any accessible sprue and gating is cut away. Then, through a combination of hammering, blasting, and etching, the remaining shell and ceramic core come off the finished part. Boutot said that any hammering is kept light to avoid part damage. But high-pressure water jet blasting and chemical etching gnaw away at any tenacious bits of shell and ceramic core to reveal a smooth casting.
Finally, it’s on to a series of nondestructive tests, such as ultrasonic wall thickness checks and fluorescent dye penetrant exams. Dimensional inspection follows. Then, a combination of heat treating, hot isostatic pressing, machining, or coating completes the journey.
The Growth of Crystals
In an equiax turbine blade, the grain boundaries control the strength, said John Corrigan, Howmet’s vice president of engineering. “Imagine a turbine blade spinning in a wheel at elevated temperatures and considerable rpm,” he said. Creep is the principal failure mode. “For equiax crystals, failure occurs ultimately at the grain boundaries. An equiax turbine blade that is spinning around with its grain boundaries normal to the stress axis represents the worst-case scenario,” he said.
Developing directionally solidified crystals was an important step in the improvement of turbine blade strength—one taken over 30 years ago. The challenge then “was to grow these crystals so that all the grain boundaries were parallel to the stress axis of the bucket. That gave us a kicker in strength and life,” Corrigan said.
He described the casting process in detail: “For equiax airfoils, we typically heat the mold to 1,900°F and the alloy to 2,600°F. We pour alloy into the mold, it starts to cool down and solidify, and we start to get nuclei through the melt.” Eventually, the alloy solidifies into small grains oriented at random.
For directionally solidified casting, both the mold and the metal are kept at similar temperatures, around 2,700 or 2,800°F. “We have a hot mold above the melting point of the alloy sitting on a water-cooled copper chill,” Corrigan said. “When we pour, we get growth in the vertical direction.” Unlike equiax casting, though, d.s. uses a graphite cylinder surrounded by an induction coil to keep part of the casting hot. “On a d.s. part, we have a starter about 2 to 3 inches long and that’s where the d.s. grains start to grow. We start to withdraw the chill plate and the mold. We go from the hot zone into the cold zone,” Corrigan explained. As the mold is withdrawn from the graphite cylinder, which Corrigan called a “susceptor,” the alloy solidifies from the bottom up.
In about two hours for small parts and five hours for large ones, “what you create is a turbine blade with all the grains parallel to the stress axis of the bucket,” he said.
Growing single crystals requires one more step. A mechanism of single crystal solidification was discovered in the 1960s at Pratt & Whitney by Barry Piearcey, Corrigan said. As Piearcey was growing d.s. crystals in a mold having a hard right-angle bend, he noticed that 25 crystals reduced down to five as they negotiated the turn. By adding a second right-angle bend, Piearcey coaxed five crystals down to one, Corrigan said.
On the production floor, Corrigan said, Howmet uses a corkscrew-shaped pigtail to achieve the result that Piearcey discovered. “A single crystal grows from the same kind of starter that d.s. crystals use,” he said. But as the crystals attempt to thread the twists of the pigtail, only certain ones, growing quickly and with preferred orientation, survive. Eventually, one crystal prevails.
The crystal follows one of two paths into the airfoil mold. A triangular ramp leads from the pigtail to the forward and trailing edges of the airfoil, Corrigan said. “You have to be careful if you have an appendage sticking off the airfoil,” he added. “You can nucleate another grain. If that happens, you’ll see it when you etch.”
On Land and in the Air
For aircraft turbines, lightness, high strength, and stability are key attributes of the blades and vanes, Corrigan said.
Weight is less of a concern on stationary gas turbines. “You worry about weight in that it might load up a disc or a shaft, but there’s more forgiveness there than the flight engine guys have,” he added. On the other hand, corrosion resistance is a big factor for stationary turbines.
“About the mid-1960s we started to get involved with IGT for power generation and pumping,” Corrigan said. At the time, mainly aircraft alloys were available. “For some of the older alloys we would add maybe 15 percent chromium for corrosion resistance, but that reduced strength,” he said. The chromium in aircraft engine alloys typically alights below 10 percent, he added.
“As you start to increase the firing temperature, to increase efficiency, strength becomes the overriding factor,” Corrigan said. “Both equiax and d.s. castings have grain boundaries. The superalloy metallurgist has to be concerned with the grain boundaries; that’s where the weakness is. So we add strengtheners like carbon, boron, and hafnium,” he said.
“The superalloys can be operating up near 2,300°F, and be turning over at 3,000 or 3,600 rpm—and for some military jets, at 20,000 or 40,000 rpm. Very big revs. With that temperature and stress, you have to be sure you don’t have phase instability—the microstructural phase changes—going on. If you don’t get the chemistry balance right, you can form needle-like structures that can embrittle the alloy,” Corrigan said.
Russ Vogt, technical director for Howmet corporate engineering, explained that IGT users want TBO— time between overhauls—of 50,000 hours. In the aerospace industry, he said, TBO is more like 5,000 or 10,000 hours. For military jets, TBO is a couple of thousand hours. Long periods of use and fuels of less purity make corrosion resistance important in IGTs.
Added Corrigan: “Increased firing temperatures drive increased strength and increased cooling efficiencies in the design.” Putting together single crystal casting with high-strength alloys and heat removal features has let Howmet develop the blading and vanes for the GE H System and other advanced turbines.
“You reach a limit, as inlet temperatures continually increase, where you have to begin improving the efficiency of the cooling scheme. You can go up to four-wall blades and get fancy with the air cooling like they do on aircraft engines. You can get into special coatings. Or, you can go to a medium other than air for cooling. A more efficient medium is steam, but that brings with it other problems: Now you have to produce a rotating pressure vessel. It’s a closed loop. For stationary parts, you have to deal with corrosion,” Corrigan said.
Cooling systems for IGTs “have basically mirrored aircraft,” he said. Historically, solid blades came first, followed by holed castings. Next came stem drilling, using electric discharge machining through both the tip and the root of the airfoil. Cooling passages today are cast using ceramic cores, which are removed from the finished part, Corrigan explained.
The larger component sizes required for stationary turbines called for casting methods that hadn’t ever been tried for aircraft engines, Vogt said. “A small 6- or 7-inch blade weighs less than a pound. Today, we are making d.s. blades that are 32 inches long and weigh 40 to 45 pounds,” he said.
“We had to re-engineer the shell system,” Vogt said. As blades begin reaching 12 to 15 inches in length, the proven methods used to make those many aircraft parts stopped working. As blade length continued to grow, other troubles surfaced. “Imagine this tall shell at 2,800°F,” Vogt said. “You get a lot of head pressure on this shell and it starts to bulge.” It was “chemistry and reinforcement” that would improve shells eventually, he said.
Aside from the obvious technological leaps that have been made in lost wax casting to produce ever larger, stronger, hotter, and more complex IGT components, it is still very much an elemental process. The world of the Hampton Roads foundry is one that glows with fire and brims with molten metal.
“Howmet’s technology really is not to be able.to make a fancy model or a single part,” Corrigan said. “We manufacture advanced, complex hardware in volume production. The strength of Howmet is that we’re good manufacturing engineers.”