As jet engine technology advances, materials and materials processing techniques are keeping pace.
Design News: How has jet engine technology changed over the years?
Stanley: Technology developments for the jet engine have been aimed at two things: Increasing the efficiency and power. One trend has been to elevate the temperature inside the turbine—the higher the temperature, the more efficiently the fuel burns and the more power per pound of fuel. As a consequence, the entire inner parts of high- and low-pressure turbines have been undergoing design changes in order to accommodate that rapid escalation of temperature.
A gas stream at 2,600F produces a very hostile environment, and the advancements in materials that can withstand the rigors have been tremendous: An important development has been directionally solidified castings, in which grain boundaries can be oriented in a specific way to impart strength. One of the most recent developments in materials has been the single crystal metal, which can be used to make an entire part. The obvious benefit is there are no weaknesses or grain boundaries.
Q: How do engineers manage these higher engine temperatures?
A: The gas stream temperature in modern, advanced jet engines is actually higher than the melting point of the metals in the air foil. So obviously, you need to worry about cooling. Today's air foil designs include very sophisticated inner cooling chambers that direct the hot air away from the metal. Another trend has been the development of coatings, made of ceramic, zirconium, and other materials.
Q: Sounds like special processing techniques are necessary to make these very complex shapes.
A: One of the most important new processes that we developed here at Howmet is vacuum die casting, which allows us to produce complex parts to a net shape. Typical investment casting requires a ceramic mold and depending on the part shape, a certain amount of machining of the part. In this process, we've eliminated the ceramic mold entirely. Basically we use a vacuum system to inject the metal into a mold which forms to a net shape. It's a process that is analogous to plastic injection molding, although the temperatures are much lower.
Q: What is the impact on part design?
A: For one thing, it will lower the cost. The main reason is that vacuum die casting is a very efficient process. The very best investment casting is only about 60% efficient; vacuum die casting is about 85% efficient. That opens up new options in materials, including titanium.
And just as significantly, this new process allows engineers to start designing parts that are much more intricate in design in terms of air flow and optimization of the actual shape of the air flow.
Q: What do engineers need to know about processing techniques to have successful designs?
A: Engineers learn a lot about materials at school, but there is quite a lot to know about the specifics of various processes. Here at Howmet, for example, we have an engineering training program. It's a very tailored, two-year program for new engineers. We also offer a training program for our customers so they get a better understanding of the issues associated with processing.
Q: Can you provide a sneak preview of future developments?
A: I think we are going to see an evolution in terms of techniques to keep these air foils cool as they go ever higher in temperature. One of the technologies being looked at is steam cooling—air is not a terribly good conductor of heat, and steam would have the capability to extract more heat. We'll also see more process and materials developments.
|James R. Stanley
President and CEO, Howmet Castings, Greenwich, CTJames R. Stanley was appointed president and CEO of Howmet Castings on July 10, 2000. Prior to assuming his new position, he was senior vice president and chief operating officer. Stanley, who has an engineering degree in metallurgy from the University of Cincinnati, has been involved in the aerospace industry all of his professional life including executive positions at Textron and GE's Aircraft Engine Business Group. Howmet, the world's largest manufacturer of precision castings, primarily for jet aircraft and industrial gas turbine engines, was the first company to produce castings of engine air foils in 1941.