The Influence Of Salt Fog Corrosion To Mechanical Properties Of A356 Aluminum Alloy

The main content of this paper is the study on the influence of salt fog corrosion to mechanical properties of A356 aluminum alloy. Due to its excellent comprehensive performance, easy to near-net shape and low manufacturing cost, casting A356 aluminum alloy is applied in high-speed train, high-speed rail,subway, aviation and aerospace, for load-bearing structure. These load-bearing structure on the one hand all year round service, prone to fatigue damage under cyclic loading, On the other hand,after years of exposure in the atmosphere will be subject to severe atmospheric corrosion, these will eventually affect the load-bearing structure of the life and safety, which salt fog corrosion is a very common type of atmospheric corrosion,cause serious destructive and loss. Therefore, the study on the influence of salt fog corrosion to mechanical properties of A356 aluminum alloy becomes very important.In order to simulate the destruction of salt fog corrosion on A356 aluminum alloy, In this paper, to the A356 aluminum alloy for neutral salt spray test (NSS test), and to A356 aluminum alloy for the tensile properties and fatigue properties of test before and after salt spray corrosion, research about the influence of the salt fog corrosion to mechanical properties of A356 aluminum alloy,analysis corrosion mechanism and fracture mechanism, the results are as follows:The salt fog corrosion no significant affect tensile properties of A356 aluminum alloy. After salt fog corrosion,material yield strength decreased by 2.14%, tensile strength decreased by 2.32%, extension decreased by 4.16% and section shrinkage rate decreased by 2.78%. before and after corrosion of the tensile test data is t-tested, the results show that tensile performance indicators do not exist significant differences before and after salt fog corrosion,show that A356 aluminum alloy of the salt fog corrosion no significant effect on material yield strength, tensile strength, extension decreased and section shrinkage rate.Cast A356 aluminum alloy tensile fracture cracks originated in the core part of the specimens. There are some foundry defects, such as porosities, non-metallic inclusions and dendrites are more prone to cracking under tensile loading, they all seriously reduce the mechanical properties of materials. The tensile fracture surface mainly belongs to quasi-cleavage fracture with secondary cracks and a little cleavage plane, crack propagation is transgranular type, the tensile fracture mechanism is transgranular brittle fracture type.The salt fog corrosion has significant affect the cast A356 aluminum alloy fatigue life. With the reduction of stress, fatigue life increases, under the same stress, salt fog corrosion specimens shorter life than without the salt fog corrosion specimens, the salt fog corrosion significantly reduces the fatigue life. Fitting s-1gN curve equation,without the salt fog corrosion and after specimens of salt fog corrosion fatigue limit were 59.18MPa and 50.08MPa, the corrosion specimens of fatigue limit reduced by 15.38% than the uncorroded specimens.Cast A356 aluminum alloy fatigue crack from the surface of the specimens. Salt fog corrosion generate corrosion pits in A356 aluminum alloy surface, produce stress concentration, there is fatigue crack source easily sprouted, leading to greatly reduce the fatigue life. Crack growth area mainly has the fatigue striation, with secondary cracks, torn edges, quasi-cleavage fracture. There are a few shallow dimples in the fast fracture area, showed that the cast A356 aluminum alloy has lower toughness, the brittle features is obvious, the fatigue fracture mechanism is a typical quasi-cleavage transgranular fracture.Using an optical microscope observe the corrosion layer, A356 aluminum alloy after the salt fog corrosion mainly is pitting and intergranular corrosion, in the part of the corrosion layer occur spalling phenomenon. A large number of chloride ion which in the salt spray solution gathered aluminum alloy oxidation film place, these areas occur corrosion, soon produce pitting. The pitting continue to occur in the depths of the matrix, intergranular corrosion accelerated the development of corrosion pits, lead to the corrosion layer shedding. Spraying a thin layer of polyurethane in the surface of the aluminum alloy specimen, Observed found that, there is no obvious corrosion pits in the A356 alloy specimen whose surface sprayed polyurethane, surface corrosion is light and relatively flat.Therefore. By the surface spray polyurethane coating to reduce the degree of salt spray corrosion, then play an effective role in protection of materials.