Ti‐6Al‐2Sn‐2Zr‐2Mo‐2Cr Alloy for High Strength Aerospace Fasteners

Next generation demanding aerospace systems requirements are pushing the titanium alloy performance needs beyond the upper limits of the workhorse alloy Ti 6Al-4V (Ti 6-4), necessitating the use of advanced solutions. This paper provides an overview of Arconic’s lightweight solution to address the needs of future aerospace fastening systems. The key attributes for aerospace fasteners are strength (tensile, double shear, and fatigue) and manufacturability (ability to forge heads and roll threads while meeting metallurgical and dimensional requirements) at an affordable cost. In particular, increasing double shear strength (DSS) while meeting other requirements is very challenging. Typically, DSS is about 60% of the tensile strength for Ti 6-4, restricting Ti applications to moderate strength levels. Limited deep hardenability of Ti 6-4 (≤0.5”) also restricts the usage to smaller diameter fasteners. Beta Ti alloys (e.g. Beta C) capable of achieving high tensile strengths suffer from shortfalls in DSS and producibility. There is a need for an affordable high strength Ti alloy that can extend titanium fastener usage to higher strength levels and larger size (up to 1”), which will enable reduction in number of joints and weight reductions by replacing higher density nickel/steel fasteners. Ti 6Al-2Sn-2Zr-2Cr-2Mo (Ti 6-22-22), a judiciously balanced α + β Ti alloy, designed and developed by RMI Titanium Company in the early 1970s for thick-section aerospace structural applications with a need for higher strengths than Ti 6-4, is capable of meeting demanding fastener requirements of next generation aerospace systems. Superior producibility and ability to tailor processing-microstructure-property relationships in Ti 6-22-22 for achieving performance improvements will be discussed in this paper.

Development of Hardening and Tempering Cycle for High Strength Low Alloy Fastener Grade Steel

Medium carbon low alloy Ni-Cr-Mo steel is used in the fabrication of aerospace fasteners. It finds application in different heat treated conditions to meet the desired strength level. The alloy was realized through double melting route. Heat Treatment studies have been carried out by following different tempering temperatures to obtain varying strength levels ranging from 1200MPa to 1400MPa. Microstructural analysis has been carried out to find out reasons for variation in mechanical properties. Tempering cycle has been suggested to obtain fully tempered martensitic structure. This paper presents the different hardening and tempering cycles studied to obtain the desired strength level for the intended application.

Advances in the Manufacture of Aerospace Fasteners

T. J. Brooks Limited achieved a 42 per cent increase in output in 1981 through the introduction of unique manufacturing processes, developed in-house, to become the European leader in aerospace fastener technology. Creation of the right industrial climate where change is accepted enabled this increase to he achieved with no overall increase in labour.