Estudo do efeito do ambiente no comportamento em fadiga de novas ligas 'AL' de grau aeronáutico

Abstract

Lithium-containing aluminum alloys are known to be attractive materials because of their low densities, high elastic modulus, high strengths and good corrosion resistance. Material degradation due to fatigue and corrosion are two major factors that contribute to the aging of an aircraft, and the prediction of corrosion and corrosion-fatigue are very important for the structural integrity of aircrafts. The presence of corrosion pits can significantly shorten the fatigue crack initiation life and decrease the threshold stress intensity of an alloy by as much as 50%. The purpose of this work was to compare mechanical, fatigue and electrochemical behavior of the new Al alloys, 2198 T851 and 7081 T73511 that are on the market, with those used by EMBRAER, named 2525 T3 and 7050 T7451. The comparisons also considered fatigue resistance in pre-corrosion confining in salt environment. Therefore, air and 15-day-confined 5% salt environment specimens were submitted to crack nucleation tests to obtain S-N curves. Potentiodynamic polarization curves were also plotted to study the electrochemical behavior of this alloys in a corrosion environment. Tensile and hardness tests were performed. The obtained results for the 2XXX series showed that these alloys are less susceptible to pitting corrosion due to more positives E pite compared to 7XXX series. However, their j corr values are also more positive, increasing the corrosion speed once it has begun. This happens due to the presence of some precipitates and a few elements that react in a way to increase the potential to higher values. Optical and electronic microscopy analyses were performed to determine the nature of precipitates of each alloy. The fatigue behavior of the salty-pre-confined environment specimens was much inferior compared to air test specimens according to each corrosion environment. Pits work as tension concentration, increasing fatigue crack nucleation. Air fatigue resistance of the new alloys was slightly inferior than the base alloys, but when the tests under preconfined environment were conducted, the four curves overlapped. This implies that the pit action governs crack nucleation mechanism, while in air, surface finishing is an additional factor.

Effect of salt-water fog on fatigue crack nucleation of Al and Al-Li alloys

“…Further investigation is on the way using transmission electron microscope to make a detailed characterization of the microstructure. Difference in roughness would be another possibility, however as verified by Gamboni 21 the specimens from both alloys presented similar roughness.…”

“…It is observed that in air, without the preconditioning, the AA2524-T3 alloy presented slightly better fatigue resistance than the AA2198-T851. The fracture surface analysis carried out by Gamboni and presented elsewhere 21 has shown that all the crack nucleation sites were located at the specimens' surface. The surface is weaker than the interior metal in the same state, since at the surface the atoms do not have neighbors, and it is likely that the stress to cause plastic flow for these atoms is slightly lower than at interior.…”

“…The fracture surface analysis using SEM confirmed that, in the presence of corrosion pits, the fatigue cracks always initiated from any of these pits, see the examples in Figure 4. Pitting is a highly localized type of corrosion that occurs in the presence of aggressive chloride ions 21 . According to Lin and Yang 13 , corrosion pits on the surfaces of axially fatigued specimens may become stress concentration sites, thus enhancing the onset of corrosion and resulting in a short fatigue life.…”

Estudo da tenacidade e fadiga em meio assistido da liga de Al-Li de grau aeronáutico AA2050-T84

“…Segundo Gamboni (2011) para aplicações práticas, o alumínio possui elementos de liga como Cu, Mg, Mn, Zn, Li e Si com a finalidade de aumentar sua resistência mecânica ou ainda sua resistência específica, entretanto estes elementos formam compostos intermetálicos, os quais tem um efeito prejudicial nas propriedades de corrosão do material, devido a formação de um filme óxido mais fino e mais condutor que permite a ocorrência de reações catódicas.…”

Fadiga e comportamento tensão-deformação cíclico da liga de alumínio 7050-T7451

“…Posteriormente, as placas foram submetidas a carregamento axial com amplitude constante e razão de carregamento igual a 0,1. A pesquisa revelou que o Al 7050-T7451 é susceptível ao surgimento de pites quando imerso em ambiente corrosivo e que há uma redução significativa da vida em fadiga devido a presença dos pites Gamboni (2011). conduziu em ensaios de fadiga no Al 7050-T7451 após sua exposição, por 15 dias, à névoa salina com concentração de 5% de NaCl.…”