| 7050 alloy exhibits high strength,high toughness,low density and good anti-fatigue performance etc.and they are widely applied in the automobile industry,building materials and aerospace field.7050 alloy is a kind of deformable aluminum which could be reinforced by the hot treatment.Grain refinement is not only beneficial to the plastic deformation,but also could reinforce alloys.The element of Zr plays an indispensible role in 7050 alloy,such as restraining the recovery and recrystallization process,reinforcing the aging precipitation and so on.A large amount of literatures about refining to the 7050 alloy are reported.However,the essential problem that refined poisoning isn't solved which has a greatly limitations for the application of these alloys.Based on this,a novelty master alloy of Al-Ti-B-C is fabricated in our work,which is applied to the refining experiments for 7050 alloy.Meanwhile,the principles of poisoning resistance for Al-Ti-B-C master alloy are analyzed.By adjusting the compositions,the master alloy which possesses highly refining efficiency is fabricated.At last,a new refining process which is suitable for 7050 alloy has been exploited.The major research results of the present work are as follows:(1)The refining behaviors of the AI-5Ti-0.8B-0.2C to 7050 alloy7050 alloy could be refined by master alloys of Al-5Ti-1B,Al-4Ti-1C and Al-5Ti-0.8B-0.2C.But when the 7050 alloy contains Zr element,it is revealed that the existence of Zr reduces grain refining efficiency of Al-5Ti-lB and Al-4Ti-1C master alloys,but hardly influences the refinement of AI-5Ti-0.8B-0.2C master alloy.The reason is that Al-5Ti-0.8B-0.2C containing doped TiC,TiB2 particles can resist Zr-poisoning,and after adding 0.2%(mass fraction)Al-5Ti-0.8B-0.2C,the average grain size of 7050 alloy is reduced from about 200?m to(60±5)?m,the ultimate tensile strength increases from 405MPa to 515MPa,increased by 27.2%,and the elongation rate increases from 2.1%to 4.1%.However,after adding 0.2%Al-5Ti-1B and Al-4Ti-1C master alloys,it appears larger grain size and uneven distribution,exhibiting "refinement poisoning" phenomenon.(2)The composition optimization of AI-Ti-B-C master alloy and its refining effects to 7050 alloyBased on the above research,such conclusion could be draw that the master alloy of Al-5Ti-0.8B-0.2C has the abilities to resist Zr-poisoning.However,the refining effects and anti-degeneration capacity required to improve by adjusting the compositions of Al-Ti-B-C master alloy.Therefore,we fabricate Al-3Ti-0.8B-0.2C?Al-5Ti-0.3C-0.2B and Al-5Ti-0.3B-0.2C three kinds of master alloys.Results of refining experiments suggest that these alloys are all presenting excellent refining effect after 5 minutes' dwell time.When the dwell time extends to 60 minutes,only the Al-5Ti-0.3B-0.2C keeps the good refining effect and presents great resistance to the Zr-poisoning.Thus,Al-5Ti-0.3B-0.2C master alloy could be applied to the refining tests of 7050 alloy.(3)Influence of the addictive amount of Al-5Ti-0.3B-0.2C on the microstructures and mechanical properties of 7050 alloyOn account of above analysis,in this section,we primarily study the influence of addictive amount of Al-5Ti-0.3B-0.2C on the microstructures and mechanical properties of 7050 alloy.Different additive amounts of Al-5Ti-0.3B-0.2C are adopted in refining tests for 7050 alloy.Adding 0.5%,the average grain size is minimum reaching up to the refining limitation.Without refined and 1%additive 7050 alloys achieve completely recovery and recrystallization.While adding Al-5Ti-0.3B-0.2C master alloy of 0.2%and 0.5%,the alloys remain fibrous textures which are formed in the process of extrusion.Redundant Ti will restrict the influence of Zr on the recovery and recrystallization process.By adding 0.5%master alloy,the mechanical properties of 7050 alloy attains the best performance.Compared with the no refining alloy,the ultimate tensile strength reaches to 697 MPa,increased by 7.2%;the elongation rate reaches 16.4%,increased by 57.5%. |
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