| Magnesium alloys possess many advantages, such as low density, excellent damping capacity, good electromagnetic shielding and high specific strength. Being regarded as the most promising green material of the 21st century, it has bright promise for magnesium alloys to be widely used in aviation and spaceflight, vehicle, electrommunication. But because of its poor plastic deformation ability and corrosion resistance, the use of magnesium and magnesium alloys is limited for a long time. Large-scale(d350mm) AZ70 magnesium alloy ingot which is producted by semi-continuous cast has been used to forge into automobile wheel. While the grain of ingots is inclined to be coarsen and there are many foundry defects in ingots. As a result, the forging property of ingots is bad.In this dissertation, the influences of thermal-rate treatment, A13B master alloy and Al-Ti-B master alloy(A13Ti4B and A15TiB) on the microstructure, mechanical property and corrosion resistance of AZ70 magnesium alloy are systematically studied by using measurements of optical microstructure, X-ray diffraction, scan electric microscopy, microstructure at elevated temperature, microhardness, tensile test, impact toughness, corrosion potential, polarization curves and salt-spray corrosion rate.The results of thermal-rate treatment(TRT) showed that the optimized TRT technics was as follows: Ts=850 ℃, Vc=2.0 ℃/s. The average grain size of alloy after such TRT technics(alloy A7) decreased from 140 μm to 47 μm. The β-Mg17Al12 phase was small and distributed dispersedly. The microhardness, tensile strength and yield strength of as-cast alloy A7 was 10.4%, 18.1% and 17.9% higher than that of un-thermal-rate treated alloy(alloy Al). The energy to fracture of alloy A7 was 2.16 times that of alloy Al. The corrosion potential of alloy A7 was 15 mV higher than that of alloy Al. And the salt-spray corrosion rate of alloy A7 was 21.9% lower than that of alloy Al. The atomic diffusion rate of alloy A7 was higher than that of alloy Al at the temperature of solid solution so that the β phase was easy to solid solution and the efficiency of heat treatment got higher.The results of ABB modification showed that: (1) the microstructure of AZ70 alloy was refined remarkably by small addition of boron and the average grain size decreased with the increase of boron addition. The average grain size was 40 urn by adding 0.15%B. It was concluded that high melting point compounds(AlB2) which have HCP crystal structure can serve as nucleating sites of cc-Mg during solidification. This was the main cause of the refinement of a-Mg grains. (2) The microhardness, tensile strength and yield strength of alloy with 0.15%B(alloy C4) was 13.1%, 19.5% and 22.0% higher than that of alloy without boron(alloy Cl). And the energy to fracture of alloy C4 was 2.3 times that of alloy Cl. The corrosion potential of alloy C4 was 25 mV higher than that of alloy Cl. And the salt-spray corrosion rate of alloy C4 was 31.7% lower than that of alloy Cl. (3) The deposition and segregation of boron was disappeared by adding boron in the form of ABB ribbon. The average grain size of alloy with 0.15% boron which added in the form of ABB ribbon(alloy C5) was 34 urn. The salt-spray corrosion rate of alloy C5 was 0.25 mg-cm'^day"1, which was the best in all alloys.The results of Al-Ti-B modification showed that:(l) the microstructure was refined remarkably by addition of 0.3%A13Ti4B. The average grain size decreased from 140 um to 30 um. High melting point compounds(TiB2 and AIB2) which have HCP crystal structure can serve as the nucleating sites of a-Mg during solidification and the heterogeneous nucleation ability of TiB2 was better than that of AIB2. Heterogeneous nucleation was the main cause of the grain refinement of a-Mg grains. (2) The tensile strength, yield strength and elongation percentage of alloy with 0.3%ABTi4B(alloy F3) was 36.6%, 76.3% and 3.9% higher than that of alloy without A13Ti4B(alloy Fl). And the energy to fracture of alloy F3 was 2.7 times that of alloy Fl. The corrosion potential of alloy F3 was 30 mV higher than that of alloy Fl. And the salt-spray corrosion rate of alloy F3 was 0.26 mg-cm"2-day"', which was 36.6% lower than that of alloy Fl. The microhardness of alloy increased with the increase of A13Ti4B addition and the microhardness of alloy with 1.0%A13Ti4B was 19.6% higher than that of alloy Fl. (3) The Al-Ti-B master alloy with different m(Ti)/m(B) showed different refinement ability. A15TiB had poor refinement effect to the AZ70 alloy and the mechanical properties of alloy with 0.3A15TiB was lower than that of alloy Fl. The different refinement ability ofA15TiB and A13Ti4B was due to the different second-phase in them.The results of industry experimentation showed that the grain of semi-continuous ingot with 0.3%A13Ti4B(alloy F9) was small and uniformity and the average grain size was 45 urn. The as-cast mechanical properties of alloy F9 was as follows: ab=206 MPa, 8=7.0 %, which was 30.4% and 33.3% higher than that of semi-continuous ingot without A13Ti4B(alloy F8). And the corrosion stability of alloy F9 was good. When the extrusion ratio(X) was 50, the mechanical properties of alloy F9 reached a high level: ab=3O5 MPa, |
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