Metals (May 2020)
Opportunities and Issues in the Application of Titanium Alloys for Aerospace Components
Abstract
The metal titanium (Ti) and its alloys have many attributes which are attractive as structural materials, but they also have one major disadvantage, high initial cost. Nevertheless, Ti and Ti alloys are used extensively in airframes, gas turbine engines (GTE), and rocket engines (RE). The high cost is a deterrent, particularly in airframe applications, in that the other alloys it competes with are, for the most part, significantly lower cost. This is less of a concern for GTE and RE where the cost of titanium is closer to and sometimes even lower than some of the materials it competes with for these applications. In spacecraft the weight savings are so important that cost is a lesser concern. Ti and its alloys consist of five families of alloys; α-Ti, near α-alloys, α + β alloys, β-alloys, and Ti-based intermetallic compounds. The intermetallic compounds of primary interest today are those based on the compound TiAl which, at this time, are only used for engine applications because of their higher temperature capability. These TiAl-based compounds are used in a relatively low, but growing, amounts. The first production application was for low pressure turbine blades in the GE engine (GEnx) used on the Boeing 787, followed by the GE LEAP engine used on A-320neo and B-737MAX. These air foils are investment cast and machined. The next application is for the GE90X which will power the Boeing B-777X. These air foils will be made by additive manufacturing (AM). Unalloyed titanium and titanium alloys are typically melted by vacuum arc melting and re-melted either once (2X VAR) or twice (3X VAR); however a new and very different melting method (cold hearth melting) has recently become favored, mainly for high performance applications such as rotors in aircraft engines. This process resulted in higher quality ingots with a significant reduction in melt-related defects. Once melted and cast into ingots, the alloys can be processed using all the standard thermomechanical working and casting processes used for making components of other types of structural alloys. Because of their limited ductility, the TiAl-based intermetallic compounds are quite difficult to process using ordinary wrought methods. Consequently, the low-pressure turbine blades currently in service are investment cast and machined to net shape. The AM air foils will require minimal machining, which is an advantage. This paper describes some relatively recent developments as well as some issues and opportunities associated with the production and use of Ti and its alloys in aerospace components. Included are new Ti alloys, new applications of Ti alloys, and the current status of several manufacturing processes including a discussion of the promise and current reality of additive manufacturing as a potentially revolutionary method of producing Ti alloy components.
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