Titanium Advanced Manufacturing at TITANIUM 2012
Titanium is the ninth most abundant metal in the Earth’s crust, but despite its many advantages, its high cost has limited its applications. These presentations from Tricor Metals, Cambridge University, the Boeing Company, and RTI International Metals show that costs can be reduced by finding more cost-effective ways to extract the ore as well as improved manufacturing methods. However, Charles S. Young of Tricor Metals made the case in “Titanium is Not Too Expensive,” that titanium is really less expensive than many other corrosion-resistant alloys. He showed that rather than price per pound, the cost should be based on a more realistic measure that considers the cost of protecting a given surface area, a cost he designates the “normalized cost.”
In other words, it is the cost of the amount of material required for a part that is important, not the cost per pound. Because each metal or alloy has a different density, a square foot of a metal with lower density would weigh less at a given thickness than one with higher density. In addition to density, yield strength needs to be considered when attempting to compare the true cost of different alloys. Therefore, the normalized cost takes into account both density and yield strength. Normalized cost can be calculated by this equation: Normalized Cost = Price/lb x Density x (YS of alloy/YS of titanium) This equation shows that the normalized cost of Titanium Grade 2 is significantly lower than that of nickel alloys 625 and C276. This is a very significant difference, and the titanium industry should educate industrial end-users about this difference.
This approach can provide a quick comparison of metals and alloys, but ASME Design Allowables (Section VIII Div 1) should also be taken into account. This can easily be done by substituting Design Allowable strengths at operating temperature for the Yield Strength in the Normalized Cost equation. The equation works to compare any metals at any operating temperature. In fact, this same type of analysis is suitable for heat transfer equipment, and it shows that currently titanium is more cost-effective than copper-nickel alloys. A process for titanium extraction that would reduce costs even further was explained by Dr. Daniel Jewell of Cambridge University. He discussed “The Chinuka Process: Titanium Metal Production via Oxycarbide Electrorefining.”
This technology replaces the expensive multi-step Kroll process with one in which refining and electro-deposition take place simultaneously, with ores and concentrates as feedstock. It allows impure metal oxides to be reduced and refined to high-purity metals. Initial work has focused on refining natural rutile concentrates through three simple steps: Reacting the concentrate with carbon at 1700°C to form oxycarbides;
Making these oxycarbides the anode in an electrolytic bath at around 800°C; and Ionizing and dissolving metals in the oxycarbide in the electrolytic bath according to their electrode potentials. Once in the electrolytic bath, the metal ions again deposit in accordance with their electrode potentials, with the result that the impurities either remain at the anode, or are retained in the electrolyte, from which some evaporate.
In this way, it is possible to reduce and electroextract-refine impure titanium concentrates to pure titanium. Deposits consist of pure titanium metal crystallites, sponge, or powder with a particle size range of 1 to 30 microns. In addition to standard grade concentrate, the process has been successfully applied to fine and ultra-fine concentrates, as well as to combinations of the concentrates plus ilmenite, a less expensive form of titanium dioxide feedstock. Unlike carbo-chlorination and treatment in sulfuric acid, the particle size and calcium oxide content are immaterial. Dr. Jewell noted that the electro-extractive FFC-Cambridge Process, the flagship Kroll replacement technology, is unable to accept any significant cationic impurities in the feedstock, as these would be retained in the final product. However, “although this is a bane for pure metal production, it is a boon for producing alloys, where the FFC-Cambridge process excels.” In 2013, research will continue at Cambridge University, where an experienced team of academics and researchers has been assembled.
Whatever the method of production, titanium has become an important component for both military and civilian jet aircraft at the Boeing Company, according to a presentation by Daniel Sanders. In addition to low density and high strength, titanium alloys are very compatible with composite materials, such as graphite-containing plastics, because of their similar galvanic properties and coefficient of thermal expansion.
However, the manufacture of titanium aerospace components can be very expensive by traditional methods. Therefore, Boeing engineers are exploring several technologies that offer advantages over existing options. For example, airframe castings have traditionally been net forging geometries, descended from sheet metal geometries. Consequently, one goal is to redefine structure geometries to optimize the 3-D capabilities of titanium castings. Additive manufacturing technologies enable direct fabrication of parts without part-specific dies. In fact, such parts are qualified for flight and are in production. However, the ability to reduce cost is very dependent on geometry.
Cost savings depend on faster deposition rates and dimensional control. Nearer-net shape die forgings are also being developed. Forging titanium above the beta transus allows for lower flow stresses and the ability to form a nearer-net-shape part with equivalent press capacity. Larger forging presses and additional blocker dies result in improved definition. In addition, advanced forging modeling allows for optimizing the forging sequence. Another effort is development of superplastic forming of complex contoured structures, with friction stir welding as the joining method. As a solid state weld, friction stir welding involves no melting, and produces a fine grained microstructure.
It results in components with a low occurrence of defects such as cracks and porosity, as well as exceptional static and fatigue properties. Superplastic forming was also the focus of the presentation by Ernie Crist of RTI International Metals. He reported on developments in superplastic forming/diffusion bonding of Ti-64 and Ti-6242 alloy sheets. Deep alpha case forms, and this requires removal by expensive and non-environmentally friendly chemical milling. Therefore, reducing SPF temperatures and cycle times would enable more cost-effective fabrication of parts.
Toward this end, RTI engineers have devoted significant effort to developing Ti-64 sheet that is superplastic at temperatures in the range 1450-1500°F (788–816°C). In addition, aero-engine components currently manufactured from Ti-64 sheet are being converted to Ti-6242 to meet the demands of elevated temperature requirements. Consequently, OEMs and Tier 1 suppliers have expressed an interest in Ti-6242 sheet that exhibits “low-temperature” SPF behavior at or below 1550°F (843°C). As with Ti-64, lowering the SPF temperature would improve die life and eliminate the need for chemical milling.
As steps toward this goal, engineers have utilized pilot-scale facilities to establish critical process parameters; designed computer simulations to validate load and torque requirements for the production rolling mill; and completed characterization of SPF behavior by established ASTM practices, such as ASTM E-2448. Properties of production-size sheets have been validated in SPF presses at RTI Advanced Forming. Today, Ti-6Al-4V production-scale finegrain sheets (FGS) have been fabricated, and successful SPF trials have been carried out at 1425-1475°F (774-800°C). Commercialization of Ti-64 sheet is on-going at multiple customer sites. Pilot-scale work on Ti-6242 FGS shows that it is capable of being superplastically formed at temperatures as low as 1550°F (843°C). Production sheets of Ti-6242 have been manufactured, and customer validation plans have been established.