Study On The Effects Of Al-Ti-B-O Grain Refiner On The Microstructure And Mechanical Properties Of A356 Alloy

In this paper, Al-Ti-B-O composite refiner is used to refine A356 alloy. The effect of the addition amount of Al-Ti-B-O composite refiner, the holding temperature and time on the microstructrue of A356 alloys were studied by metalloscope, scanning electron microscope, X-Ray diffractomer, electronic universal material testing machine and other means. The effect of refinement of A356 alloy with different refiners were Compared. The influence of yttrium on Al-Ti-B-O composite refiner refining effect and the mechanical properties of A356 alloy at both as-cast and heat treatment conditions were also studied.The grain of A356 alloy with Al-Ti-B-O composite refiner can be significantly refined, and the secondary dendrite arm spacing is reduced. The optimum amount, holding temperature, holding time were 0.5wt%, 730℃, 30 min in A356 alloy. When the addition amount is 0.5wt%, the holding temperature is 730℃,the holding time is 30 min, the grain refinement reaches the best, the average grain size is 210μm, the secondary dendrite arm spacing is 13.07μm. When the addition is more than 0.7wt%, the holding tempreature reaches 760℃ or the holding time reaches60 min, it will lead to decreasing the refining effect of Al-Ti-B-O composite refiner on A356 alloy. Besides, the holding time has the most influence on refining effect.Al-Ti-B-O composite refiner shows the best refinement among the Al-5Ti, Al-3B,Al-5Ti-1B, Al-Ti-B-O four kinds of refiner. Al-Ti-B-O composite refiner strengthens the effect of modification of Al-10 Sr on eutectic Si, which makes eutetic Si transform from fine needle-like and small block-like into tiny coral-like.Rare earth yttrium can promote the refining effect of the secondary dendrites when the Al-Ti-B-O composite refiner and Al-10 Y master alloy were added into A356 alloy simultaneously. After adding 0.5wt% Al-Ti-B-O+0.5wt% Al-10 Y into A356 aluminum alloy, the grain size of A356 is well refined and rare earth yttrium is capable of delaying the refinement fading time from 60 min to 120 min. Al-Ti-B-O refiner does not influence the modification of eutectic Si from needle-plate like to small coral-like and its modification mechanism is rare earth yttrium adsorbed on theSi phase, hindering the growth of Si phase. T6 heat treatment of the refined and modificated A356 can promote the spheroidization.of Si phase and make it uniformly dispersed in the Al matrix Alloy, besides, the length-width ratio of eutectic Si of A356 alloy decreased from 3.47 to 1.09.Room temperature tensile testing shows that when adding 0.5wt%Al-Ti-B-O+0.5wt%Al-10 Y in the A356 alloy, the tensile strength and elongation achieve the best at both as-cast and heat treatment condition.Under heat treatment condition, the tensile strength and elongation of refined and modified A356 alloy increased by 45 MPa and 2.0% respectively compared with the unrefined and unmodified. The tesile fracture shows that the A356 alloy with Al-Ti-B-O+Al-10 Y is ductile fracture and changes to ductile and brittle mixed fracture, ductile fracture dominated after heat treatment.