D. C. Van Aken, Missouri University of Science and Technology, Rolla, MO; R. J. Lederich, Advanced Manufacturing R&D, Boeing – Phantom Works, St. Louis, MO
Corrosion prevention of friction stir welded joints is an enabling technology for aerospace structures. In this paper, a lap joint sealant based upon the polyamide nylon -11 will be discussed. Nylon-11 is a thermoplastic that can be applied to the faying surfaces of a lap joint and friction stir welded without masking the weld tool path. The nylon-11 melts during welding and bonds the faying surfaces together forming a seal. T-joint assemblies were produced with 2024-T8 top-skins welded to cast A357-T6 stringers in both bare and anodized conditions, with and with out sealants. Joints with the nylon-11 sealant had higher stiffness resulting from adhesive bonding of the lap joint. Bond strength was greatest for the bare aluminum and decreased as the thickness of the anodized layer increased. Limited fatigue testing was performed on nylon-11 sealed friction stir welded T-joints. These assemblies were made using standard sulfuric acid anodized aluminum components and as such the nylon-11 bond was prone to failure. However, these sealed joints had 1 % failure lives comparable to the bare aluminum welds and markedly better fatigue lives than the anodized aluminum welds without sealants. The improved fatigue performance of the sealed joints is attributed to the increase in joint stiffness created by bonding the faying surfaces with the thermoplastic sealant. This bonded configuration also shields the notches created by the junction of the faying surfaces and the weld nugget. In situ corrosion-fatigue tests were also conducted by immersing the bare and anodized specimens in a neutral 3.5% salt solution during fatigue loading. The in situ testing produced a 75% reduction in cyclic life. Addition of a sealant produced only a marginal improvement in life.
Summary: Corrosion prevention of friction stir welded joints is an enabling technology for aerospace structures. In this paper, a lap joint sealant based upon the polyamide nylon -11 will be discussed. Nylon-11 is a thermoplastic that can be applied to the faying surfaces of a lap joint and friction stir welded without masking the weld tool path. The nylon-11 melts during welding and bonds the faying surfaces together forming a seal. T-joint assemblies were produced with 2024-T8 top-skins welded to cast A357-T6 stringers in both bare and anodized conditions, with and with out sealants. Joints with the nylon-11 sealant had higher stiffness resulting from adhesive bonding of the lap joint. Bond strength was greatest for the bare aluminum and decreased as the thickness of the anodized layer increased. Limited fatigue testing was performed on nylon-11 sealed friction stir welded T-joints. These assemblies were made using standard sulfuric acid anodized aluminum components and as such the nylon-11 bond was prone to failure. However, these sealed joints had 1 % failure lives comparable to the bare aluminum welds and markedly better fatigue lives than the anodized aluminum welds without sealants. The improved fatigue performance of the sealed joints is attributed to the increase in joint stiffness created by bonding the faying surfaces with the thermoplastic sealant. This bonded configuration also shields the notches created by the junction of the faying surfaces and the weld nugget. In situ corrosion-fatigue tests were also conducted by immersing the bare and anodized specimens in a neutral 3.5% salt solution during fatigue loading. The in situ testing produced a 75% reduction in cyclic life. Addition of a sealant produced only a marginal improvement in life.
The following attributes of the nylon-11 sealant were demonstrated during this research program:
(1) Sealant can be applied to joint by plasma spray or electrostatic spray and successfully friction stir welded without the need for masking (2) Environmental assessment of welding practice indicates sealant is non-hazardous (3) Sealant is repairable by heating joint to 200°C for 15 minutes (4) Sealant shows excellent room temperature static properties when applied to bare aluminum or aluminum with a thin sulfuric acid anodized surface (5) Joints sealed with nylon-11 are stiffer as a result of adhesive bonding (6) Welded and bonded joints have higher fatigue resistance, since the bonded joint carries load and shields the weld nugget.
