Investigation Of Alloying Mechanism And Properties Of 2519 Aluminium Alloy

The mechanical, welding and corrosion properties of 2519 aluminumalloy were systematically investigated with tensile, micro-hardness,x-raydiffraction, intergranular corrosion,exfoliation corrosion, slow strain ratetensile and double cantilever beam tests etc. The microstructures of theinvestigated alloy at different temper and their elvolution were examinedby means of optical microscopy, scanning electron microscopy andtransmission electron microscopy. By combining the experimental resultswith microstructure analysis, the following conclusions can be drawn:i) By investigating the microstructure evolution of the investigatedalloy during hot deformation, a model of geometric dynamicalrecrystallization for 2519 aluminum alloy has been proposed, Therelationship among the flow stress strain rate and the deformationtemperature can be expressed as follows:ε= 1.44×10^-14[sinh(0.0108σ)]^7.7563exp(-189.83/(RT)) The deformation temperature range for the investigated alloy was determined to be in the range of 350-400℃.ii) A TTT model for decomposition of the supersaturated solid solution of 2519 aluminum alloy was established by measuring the ageing characteristic of the investigated alloy,and the ageing regime of the investigated alloy has been proposed according to this model. The mechanism for improving mechanical properties by pre-deformation was revealed, and the range of pre-deformation has been determined. The ageing hardening response of the investigated alloy was very slow when the alloy is aged at 135℃and 150℃,but it showed the evident three stage aging characteristic when it was aged at 165℃,180℃and 200℃. And it also showed that the hardness, strength and peak-aging time reduced along with the increase of aging temperature, so the favorable ageing temperature for the investigated alloy was 165℃.The deformation before ageing accelerated the precipitation of the investigated alloy and refined the precipitated substantially. And along with the increase of deformation strain favors the dispersion and refinement of theθ' precipitates and reducing precipitates free zone, but the elongation reduced along with the increase of deformation strain. The favorable deformation strain before aging is 10-15%.iii) The influence of the addition of Cu,Mn,Zr,Ag,Mg,Y onmicrostructure and properties of the investigated alloy and corresponding mechanism has proper been studied,and the content of Cu,Mn,Zr,Y in the investigated alloy has been determined.The ageing hardening rate was accelerated with the increase of the content of Cu when it is aged at 165℃, i.e., the peak ageing time was reduced, but the elongation rate decreases when the content of Cu is smaller than 6.0%, the aging hardening increased along with addition of Cu, but further increase will lead to decrease in ageing hardening effect. The same tendency was found for addition of Mn, and the critical value for Mn is 0.29%. The addition of Zr lead refining of as-cast and recrystallized grains, therefore the strength, hardness and ductility increased along with the increase of Zr, but further increase of Zr will lead to the primary phase Al₃Zr which will deteriorate the mechanical properties of the investigated alloy. The proper addition of Cu, Mn and Zr is 6.0%, 0.29%, 0.19% respectively.The addition of trace element Ag improved the peak ageing hardness and the ultimate strength and yield strength at room temperature increased by 14 MPa and 38 MPa respectively when the addition of Ag is 0.30%. And with the addition of trace Ag the ultimate strength and yield strength at 200℃increased by 78 and 96 MPa respectively. The elongation rate decreased along with the addition of trace Ag. All of these may due to thermal stable precipitates ofΩphase.The addition of trace element of Y will leads to the inter-metallic Al₆Cu₆Y, which distributed along the grain boundaries. This inter-metallic compounds improved the high temperature mechanical behavior of the investigated alloy. But further increase of Y deteriorated the mechanical behavior. The proper addition of Y is 0.10 %. iv) The welding characteristics of the investigated alloy has been studied,the influence of the components of welding wire on microstructure and properties of welded joint of the investigated alloy has been revealed.The tensile strength of the welded joint of investigated alloy is higer with ER2319 filler wire than the one with ER4047 filler wire.The welded zone is the weakest point of welded joint, the soften zone in heat-affected zone is the sencond weakest point of welded joint,which was formed due to the coarsening, of the main strengthening phaseθ' particles.v) Influnence of heat treating regime on intergranular corrosion resistance,exfoliation corrosion resistance and stress corrosion cracking in the investigated alloy has been investigated.The model of intergranular corrosion resistance has been estabished.And the mechanism of improving the stress corrosion cracking resistance by pre-deformation in the investigated alloy has been revealed. Obvious intra-granular corrosion is found for the under-aged alloy at 165℃, but it is very trivial for the peak-aged and over-aged alloy. Intergranular corrosion resistance of the investigated alloy is better in T8 temper than in T6 temper.Exfoliation corrosion resistance of the over-aged is better than the under-aged and peak-aged,and exfoliation corrosion resistance of the investigated alloy is better in T6 temper than that in T8 temper.And T6 temper is more susceptible to SCC than T8 temper in 3.5% NaCl water solution at same strain rate,and the K_ISCC of the investigated alloy in T8 temper is 32M Pa·m^1/2.