Investigation Of Micro-structure And Mechanical Properties For A356 Alloys With Ti Alloying By Electrolysis

In this paper, two kinds of A356 alloys with different titanium content are produced by directly using electrolytic low-titanium aluminium alloys (LTAA) that produced by electrolysis, named electrolytic A356 alloys (EA356), and by melting Al-Ti master alloys as grain refining agent, named melting A356 alloys (MA356) respectively. The effect of titanium content, the titanium alloying methods and heat treatment process on the microstructure, tensile properties and low-cyclic fatigue properties of EA356 alloys is investigated and compared with each other. The grain refinement mechanism, the fading mechanism and strengthening mechanism are discussed. The testing results must be have an important theoretic and practical signification and may be helpful to the application and popularization of A356 alloys in industries.The experimental results show that the effect of grain refinement and appearance of Si particles of EA356 alloys is superior to that of MA356 alloys. The primary dendrite length, Si particles size and aspect ratio of Si particle of A356 alloys decrease with raising titanium content, while its roundness increases, which means the grain is refined and the morphology of Si particle is improved. But this beneficial effect decreases when Ti content exceeds 0.10%. The analysis results of microstructure and DSC curves during solidification of alloys show that excellent ability of grain refinement of EA356 alloys may be attributed to the large number of Al₃Ti particles existed in LTAA alloys and the lower undercooling and activation energy of nucleant needed in solidifying of alloys. Prolonging the holding time of melt in furnace, the fading behavior of grain refinement of alloys is discovered. The size of α-Al grain and Si particles are obviously coarsened. It may be related to the dissolution and settle down of Al₃Ti and the drastic burning of Ti and Sr. Increasing the cooling rate or stirring melt before casting may be helpful to postpone the occurrence of the fading behavior of grain refinement.The effects of Ti content and Ti alloying methods on the strength of alloys aretrifling but on the plasticity are distinct. The plasticity of EA356 alloys is superior to that of MA356 alloys. The plasticity of all alloys increases with the increase of titanium content. The alloys with 0.1 %Ti have the optimum combination of strength and plasticity. The heat treatment processes have obvious influence on the microstructure and mechanical properties. Raising solution temperature and prolonging the solution time improve the appearance of Si particle. But overly raising solution temperature or prolonging solution time may result in the coarsening of Si particles. The solution temperature almost doesn't have any effect on the strength of alloys. But the plasticity always has the tread that firstly quiekly increases then decreases either raising solution temperature or prolonging solution time. The strength of alloys is insensitive to the change of aging temperature but is very sensitive to the change of aging time. The precipitates can be precipitated from matrix according the GP region—py/—p;—P with prolonging aging time. Both the sub-stable phase $" and p; can be used to improve the strength of alloys. Their volume portion and size of precipitates increase with prolonging aging time. The optimum combination of strength and plasticity of alloys can be obtained when the alloys are treated by the process that solution at 535 °C for 3h then aging at 165°C for 2h.The cycle hardening behavior of A3 56 alloys is sensitive to both the titanium content and titanium method. The alloys with 0.14%Ti always have higher cyclic hardening rate than that of alloys with 0.1%Ti. Both EA356 alloys and MA356 alloys have the similar cyclic hardening behavior when tested at low strain amplitude. While tested at high strain amplitude, the EA356 alloys show the quasi-stable state that cyclic hardening ability approach to saturation. But for the MA356 alloys, the cyclic hardening continuously proceeds. It must be related to that the facts the alloys with high titanium content or titanium alloyed by electrolysis have the finer grain. The low-cycle fatigue life of alloys is only sensitive to the titanium content. The alloys with 0.1 %Ti always have higher low-cycle fatigue life than that of the alloys with 0.14%Ti. The reason is that the lower yield strength and larger plasticity of alloys with 0.1 %Ti is in favor of the relaxing of stress concentration in the discontinuous region at the surface of samples, which can effectively delay the initiation of fatigue crack. And the low yield strength can results in higher plastic-inducing close effect of fatigue crack, which can effectively increase the propagation resistance of fatigue cracks. Whether alloyed by electrolytic low-titanium alloys (EA356) or AHi master alloys (MA356), the EA356 alloys have the similar low-cycle fatigue life as that of MA356 alloys if two kinds of alloys have similiar titanium content It may beattributed that the effect of SDAS on the fatigue life is more evident than the effect of prime dendrite length and grain size. While the titanium alloying method mainly influence the prime dendrite length and almost doesn't have any effect on SDAS. If considering the strain energy consumed during cyclic deformation, the difference of strain energy density may be the another reason that influence the fatigue life of two kinds of A356 alloys. The low-cycle fatigue life of alloys is determined by plasticity strain energy density. The A3 56 alloys with low titanium content can consume higher plasticity strain energy during cyclic deformation than the alloys with high titanium content. If the alloys with the similar titanium content, all the alloys almost consume the same plasticity strain whether the alloys are EA356 alloys or MA356 alloys.