Investigation Of Refining Effect Of Al-Ti-B-Sr Rod By Continuous Rheo-Extrusion

A356 alloy has good casting and welding properties.However,its mechanical properties are reduced by the coarseα-Al phase and eutectic Si phase.The addition of Al-Ti-B refiner and Al-Sr modifier can effectively improve the mechanical properties of A356 alloy.The purpose of this paper is to mix ternary Al-Ti-B and binary Al-Sr into quaternary Al-Ti-B-Sr master alloy,which is processed into rod by continuous rheo-extrusion technology,to achieve the effective combination of refinement and metamorphism,shorten refinement metamorphism time,improve refinement metamorphism.Firstly,ternary Al-2.5Ti-0.5B and quaternary Al-2.5Ti-0.5B-5Sr master alloy ingots were prepared by melting method.Then the molten master alloy was processed into a rod with a diameter of 9.5mm by rheo-extrusion and Accumulative continuous extrusion forming techniques.Finally,Al-2.5Ti-0.5B Al-2.5Ti-0.5B-5Sr in the as-cast rheo-extrusion state and the accumulated continuous extrusion state were used to carry out the refining test of A356alloy,and the refining effect and mechanical properties were compared and studied.By changing the processing state and adding amount of the master alloy,the change rule ofα-Al eutectic Si phase and mechanical properties of A356 alloy was investigated,and the best process scheme was determined.At the same time,it was compared with the method of adding master alloy separately commonly used by enterprises,and the following conclusions were reached.（1）For the alloys prepared by rheo-extrusion and Accumulative continuous extrusion forming,the morphology of Ti Al₃,Ti B₂ and Al₄Sr phases changes significantly after extrusion shear deformation,and the Ti Al₃ blocks are broken into small blocks with smaller size and more uniform distribution.Ti B₂ phase changed from agglomerate to more diffuse and more uniform distribution.At the same time,the Al₄Sr phase was also crushed and changed from a lamellar shape to a fine short rod shape.（2）Al-2.5Ti-0.5B can significantly change the grain size ofα-Al phase,and the refining effect is obvious.Quaternary Al-2.5Ti-0.5B-5Sr composite refinement metamorphism is more significant than the single refinement metamorphism of ternary Al-2.5Ti-0.5B,the coarse acicular lamellar eutectic Si phase in A356 alloy is transformed into fine and round granular short rod,so that the properties of A356 alloy are significantly improved.In addition,the refining effect of the two master alloys in rheo-extrusion and Accumulative continuous extrusion forming state is better than that in as-cast state.（3）The results show that the grain size of Al-2.5Ti-0.5B-5Sr can reach 55.02μm when1wt%of the master alloy is added.The tensile strength and elongation of A356 alloy are208.56MPa and 12.93%,respectively,which are 96.96%,23.41%and 81.44%higher than that of the unrefined A356 alloy.A large number of dimples are present in the fracture.In addition,each index is obviously higher than the traditional three-way Al-2.5Ti-0.5B and the current refining scheme of the enterprise.