Study On Formation Mechanism And Grain Refining Performance Of The Al-ti-c-sr Master Alloy

A356 alloys share a major portion of cast components in aeronautic, automotive and marine industries with the advantage of good casting capability, gastightness and corrosion resistance. However, A356 alloys have coarse grain and large needle/plate eutectic silicon, which cause low mechanical properties. The change of the eutectic silicon morphology and fineα-Al grains can improve the microstructure and mechanical properties. Presently, grain refinement and modification have been studied concentratively to improve properties and performance of A356 alloys.Presently, Al-Ti-C alloys as refiner and Al-Sr master alloys as modifier of aluminium alloy are used extensively now. In order to take an advantage of grain refiner Al-Ti-C and modifier Sr, hence, in this paper, a new Al-Ti-C-Sr master alloy which plays a dual role both as grain refiner as well as modifier at a time was prepared by liquid solidification reaction method. And A356 alloys were inoculated with the Al-Ti-C-Sr master alloy.In this paper, the microstructures of the Al-5Ti-0.5C-8Sr,Al-5Ti-0.5C and Al-10Sr alloys, the formation mechanism and grain refining performance of the Al-5Ti-0.5C-8Sr master alloy have been investigated by using modern analysis and test facilities such as optical microscopy (OM), scanning electron microscopy (SEM) with energy dispersive spectrometer (EDS), x-ray diffraction (XRD) and whilst the test of their tensile test. It offers some theoretical reference and experimental basement for developing novel dual-purpose master alloys.The results show that the microstructure of the Al-5Ti-0.5C-8Sr alloy is composed ofα-Al,non-continuous lath-like Al₄Sr,particle-like or cluster-like TiC,enwrapped-like TiAl₃,blocky-like and rim Al-Ti-Sr.The formation mechanisms of the Al-5Ti-0.5C-8Sr master alloy are as follows: TiAl₃ is formed through the reaction between K₂TiF₆ and Al melt at 850℃, and some Ti atoms are released from K₂TiF₆ at high temperature; When the temperature of melt was heated up to 1200-1300℃, TiC is formed through the reactionTi+C(s)â†'TiC(s) and TiAl₃+Câ†'TiC+Al; Al₄Sr is formed through the binary uniform reaction after the pure Sr is added at 850℃; After the following solidification process in the peritectic reactionL(Al, Sr)+a(TiAl₃)â†'β(A1-Ti-Sr), the enwrapped structure is formed with the outer layer for Al-Ti-Sr phase and the internal layer for TiAl₃ phase. After holding for 18h at 620℃, the internal layer TiAl₃ of the enwrapped structure has almost exhausted, and the enwrapped structure just contains Al-Ti-Sr phase, it can be explained that the rim Al-Ti-Sr phase is formed by peritectic reaction mechanism.When refining the A356 alloy, the macrostructure of the A356 alloy inoculated with 0.5%Al-5Ti-0.5C is obviously refined compared with that of the A356 alloy without grain refiner, the average grain size of A356 alloy with 0.5%Al-5Ti-0.5C decreases from 1720μm to 230μm, and part of needle/plate like eutectic silicons turn to short rod-shaped silicons. The tensile strength of A356-T6 alloy with 0.5%Al-5Ti-0.5C increases from 221.7 MPa to 231.2 MPa, which has increased 4.3%, and the elongation increases from 9.5% to 10.4%, which has increased 9.5%. The average grain size of A356 alloy with 0.4% Al-10Sr decreases to 530μm, and the morphology of eutectic silicons are changed from needle/plate like to fibrous/globular like.The tensile strength of A356-T6 alloy with 0.4% Al-10Sr increases to 228.7 MPa, which has increased 3.2%, and the elongation increases to 11.1%, which has increased 16.8%.Both satisfactory grain refinement and modification effects are obtained by the addition of 0.5% Al-5Ti-0.5C and 0.4% Al-10Sr, the average grain size of the A356 alloy is about 230μm, and the morphology of eutectic silicons turn to fibrous/globular like totally. The ultimate tensile strength of A356-T6 alloy is 249.3MPa, which has increased 12.4%, and the elongation is 12.8%, which has increased 34.7%, the effects of grain refinement and modification are more obvious than that of sole Al-5Ti-0.5C or sole Al-10Sr. After adding 0.5wt.% Al-5Ti-0.5C-8Sr master alloy, the average grain size of A356 alloy is about 220μm, and the morphology of eutectic silicon also totally turns to fibrous/globular like. The ultimate tensile strength of A356-T6 alloy is 251.7MPa, which has increased 13.5%, and the elongation is 12.5%, which has increased 31.5%, the effects of grain refinement and modification are considerable with adding the same amount of Al-5Ti-0.5C and Al-10Sr. The fracture mode of the A356-T6 alloy without grain refinement and modification is quasi-cleavage fracture; The fracture mode of the A356-T6 alloy with 0.5% Al-5Ti-0.5C or 0.4% Al-10Sr is quasi-cleavage/dimple mixed fracture; The fracture mode of the A356-T6 alloy with 0.5% Al-5Ti-0.5C-8Sr or 0.5% Al-5Ti-0.5C,0.4% Al-10Sr is dimple fracture.