| 2A14 aluminum alloy as a traditional aerospace forging of aluminum alloy, widely used in national defense manufacturing industry. This article through to the two welding of longitudinal seam(ls) and circumferential seam(cs) cutting sampling, compared macro organization, the common tensile properties, the slow strain rate tensile properties under stress corrosion condition; focusing on the circumferential seam, research on the micro organization, impact performance, hardness distribution, exfoliation corrosion performance of the of three different zones of parent metal(pm), heat affected zone(haz), weld metal(wm); also determinate the threshold stress intensity factor for susceptibility to stress corrosion cracking KIscc of the weld metal, at last compared the stress corrosion of C-ring specimens with parent alloy.Using metallographic microscope observe the welding joint macroscopic organization, which find that along the center line of the transverse direction, the base area of grain was rolling slits, haz grain similar to the parent metal, fusion line is in the form of directional columnar crystal, wm is isometric crystal, there are more welding pores in cs than ls. Along the direction of the weld center line, the top is disorderly distribution of developed dendrite,middle is almost isometric, central grain 20 to 30 μm in diameter, grain assumes the circular uniformly distributed, without air hole, the lower is a bulky columnar crystal direction, the cs have some welding hot crack in lower.By scanning and transmission analysis methods to study the welding joint microstructure, which shows in the pm, there are thick eutectic structure, intergranular second phase particles(block Al2 Cu phase), the 200 nm elongated needle θ phase precipitation in the intragranular, and θ phase have the orientation relationship with matrix:(010)α//(110)θ, [001]α//[001]θ. In the haz, for welding heat affected, Al2 Cu phase increases in the intergranular and connected into reticular.. Al2 Cu phase and matrix orientation change:(111)α//(211)θ,[-110]α//[-111]θ, at the same time found within the grain dislocation line and dislocations spots. in the wm, rapid solidification, in the grain boundary there are(α-Al+Al2Cu) and(α-Al+Si) divorced eutectic, Al2 Cu phase in the direction of [001]θ present 10×3μm black pole, in the direction of [-1-12]θ present 100 nm diameter elliptic. Al7 Fe Mn Si(ω phase) in the direction of [010]ω present 1μm diameter round piece. Al4Cu9(γ phase) precipitate in the grain become much smaller, in the direction of [-110]γ present 500 nm pole.There are differences between the welded joint of different regional organizations, which leads to the different mechanics performance. In the tensile experiments, the yield strength, tensile strength, elongation rate of pm was significantly higher than that of the ls, ls is slightly higher than the cs. Pm tensile fracture have obvious necking phenomenon, belongs to the ductile fracture, there is a lot of toughening nest group; the cs tensile fracture belongs to brittle fracture, pores, crack initiation extension; the ls fracture is similar to cs, but crack is shorter, less quantity. In the impact experiments, the parent metal is the highest impact toughness, heat affected zone and wm the worst; Weld zone of fracture exist a large number of pores, many long crack and small secondary porosity distribution around the crack and heat affected zone of fracture have parallel cracks, crack down on bottom, parent metal fracture evenly distributed a large number of toughening organization. In the hardness test, haz grain precipitation strengthening effect is best, the highest hardness, weld, lowest parent metal.Use exco solution to research theexfoliation corrosion behavior of the welded joints, which shows that the corrosion pit extends to the inside grain in the weld are. The pitting corrosion pit diameter and intergranular corrosion pit width less than the weld area. In the parent metal area, less intergranular corrosion, pitting corrosion pit number, diameter, after 48 h grain was completely corrosion. The whole specimen weightlessness rate had a linear relationship and the corrosion time, before the 24 h, corrosion quickly, 24 h after exfoliation corrosion speed slow down obvi ously.For accurate characterization of stress corrosion resistance of welding joint sensitivity, slow strain rate tensile test is used in the experiment, which showed that compared with corrosive medium brine, weld fracture in the silicone oil with high intensity, long time, high elongation, elongation at break, that weld stress corrosion in the corrosive medium saline sensitivity were higher than in inert medium silicone oil. The relative plasticity loss Iscc(δ) of the cs 4.6% higher than the ls, and the relative breaking strength loss Iscc(σ) of the cs 1.1% higher than the ls, which means that the cs stress corrosion in the corrosive medium sensitivity is higher than the l s. After 840 h of stress corrosion in the double cantilever beam experiments, the crack propagation rate is close to 1.0x10-6 mm/s, when the stress intensity factor KI is equal to the sample KIscc=20.56MPa·m1/2, higher than the threshold of stress corrosion of the base metal strength factor 12.43MPa·m1/2, so the stress corrosion sensitivity of the welded joint is lower than the base metal, means the weld joint more resistant to stress corrosion. In the C-ring experiment, pitting corrosion and stress corrosion resistance of weld specimen is higher than the parent metal, but not resistant to intergranular corrosion. At the beginning of the corrosion, the intergranular corrosion cracks extending in all directions along grain boundary, the late connected into network; Intergranular corrosion crack long strip Al2 Cu phase surrounded by corrosion product Al(OH)3 and Al2O3, with 30μm secondary cracks. Parent metal C-ring specimen resistant to intergranular corrosion, but not resistant to stress corrosion. The internal stress corrosion crack initiation crack has obvious characteristics of the stress corrosion cracking, because it′s direction along the direction perpendicular to the axial load stress, the crack tip for typical type I open crack, crack length more than 600μm, less than 10μm in diameter, crack internal full of corrosion products Al2O3. |
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