Joint development of VST2-based alloys in Russia.
Titanium’s properties in strength versus density make it one of the most desired metals for aerospace applications. But the cost considerations have prevented its widespread use. Improvements to Ti processes and alloys that lower cost and make use of the metal more feasible could allow for game-changing innovation in aircraft design.
Around 40 percent of the cost of an aerospace titanium part is the cost of sponge used in ingot fabrication. Within the cost of sponge, only about 5 percent is the cost of the ore (titanium dioxide); the rest of the cost owes to the process used to reduce the oxide to metallic form.
At the same time, the buy-to-fly ratio (the difference between the amount of material that is purchased and amount that is thrown away as scrap) for titanium parts is typically so high that around 70 percent of the metal in a semi-product goes to chips during machining. Recycling titanium scrap, therefore, warrants serious attention from titanium suppliers.
However, with the conventional method of ingot fabrication—vacuum arc remelting (VAR)—scrap reuse accounts for only around 30 percent of total ingot production because the cost to clean and sort the scrap would not be compensated by the savings on sponge beyond that level.
The Boeing–VSMPO Innovation Center— a Boeing partnership with VSMPO-AVISMA, the world’s largest titanium producer—was established in 2000. VSMPO supplies its products to the markets of 50 different countries and is deeply integrated into the world’s aerospace industry.
About 10 years ago, Vladislav Tetyukhin, then general director of VSMPO-AVISMA, and his research team, created a concept of a new lower cost alloy, VST2, aimed at replacement of Ti 6Al-4V, which could be produced with the use of scrap of various titanium alloys serially produced by VSMPO at the level of about 75 percent.
The key enablers for this concept were the technology of scull melting of ingots (available at VSMPO and characterized by much higher than VAR scrap refining ability) and development of the VST2 chemistry range, which is wide enough to accommodate the increased oxygen and the alloying elements getting into charge with scrap of various alloys and yet capable of providing for mechanical properties comparable with these of Ti 6Al-4V.
Also, an important factor to the advantage of using this concept at VSMPO was the opening of the joint venture Ural Boeing Manufacturing in 2009, a machining facility at the VSMPO site, supplying tons of chips of Ti 6Al-4V and Ti 5553. In 2010, Tetyukhin presented the concept of VST2 to Boeing. Soon, the research teams from Boeing and VSMPO started development of compositions that could be produced with high scrap utilization and hence reduced ingot cost.
By Natalia Mitropolskaya and Robert Briggs, materials, processes and physics engineers, Boeing Research & Technology