Effects Of Partialremelting Treatmentand Yttrium On Microstructureand Properties Of Zl205A Alloy

In this paper, effects of partial remelting treatment and Y on microstructure, mechanical properties and hot tearing resistance of ZL205A alloy were studied in detail. The variation of viscosity, solidification characteristic, mechanical properties and hot tearing resistance of ZL205A alloy after adding returns or Y were investigated. The influence mechanisms of returns, Y, as well as their contents, returns and Y on microstructure and mechanical properties of ZL205A alloy were discussed. The hot tearing susceptibility of ZL205A alloy decreased significantly, meanwhile, maintained the basic mechanical properties by adding returns and Y.Effects of returns on as-cast and heat treatment microstructure and mechanical properties of ZL205A alloy by partial remelting treatment were investigated. The results show that the grain size of ZL205A alloy decreases at different degree. The grain size increases first and then decreases with increasing of returns content. The effect of returns on microstructure of ZL205A alloys under the condition of heat treatment was also studied. Analysis reveals that precipitation of second-phase of ZL205A alloys after adding small and excess amount returns is similar with that of primary ZL205A alloy which precipitates along the grain boundary. The most refinement is achieved when the content of returns increased up to 20 wt.%. The average grain size of the primary ZL205A alloy was measured to be about 60μm, and the good result can be got of the ZL205A alloys with the average particle size ofα(Al) phase being about 33μm after adding 20wt.% returns. The precipitation of second-phase in the matrix of ZL205A alloy with 20 wt.% returns is dispersed and uniform. The mechanical properties of the alloys increase first and then decrease with increasing of returns content. The ZL205A alloy with 20 wt.% returns has a considerably high tensile strength and yield strength of 525MPa and 445 MPa, respectively, which is much higher than 501 MPa and 421 MPa of primary ZL205A alloy, meanwhile the elongation level is up to14%. However, the number of nucleations increased after adding returns which results in the decreases of nucleation undercooling for primaryα(Al) and the increases of liquidus temperature. Therefore, the hot tearing susceptibility of alloy increases after adding returns due to the wider crystallization range.The main phases of ZL205A alloy do not changed after adding returns. The changes of microstructure and properties of alloys are caused by the heredity of returns to ZL205A alloy through analyzing the effects of returns without or with refiner on microstructure and mechanical properties of ZL205A alloy. In addition, Al-Ti-B refiner has heredity in certain during remelting process of the alloy, but is not a major factor.The viscosity of ZL205A alloy melt with different returns content was measured by a torsional oscillation viscometer and the effect of returns on the viscosity of ZL205A alloy melt was studied. It suggests that the viscosity of primary ZL205A alloy increases exponentially as temperature decreases, which follows the Arrhenius formula. At temperature range of 968 K-1088 K, the viscosity of ZL205A alloy melt with different returns content follows the Arrhenius formula, however, the increasing extent is higher than that of primary ZL205A alloy. At 923 K-953 K range, the increasing extent of viscosity is smaller than that of primary ZL205A alloy, and the trend deviates from Arrhenius formula. It shows that addition of returns causes the liquid structural change of ZL205A alloy.Based on the researches of viscosity, as-cast microstructure, thermal analysis results and liquid quenching analysis of ZL205A alloy by partial remelting treatment, the main reason of grain refinement of ZL205A alloy was due to the increase of the number of atom clusters which can act as nucleation after adding different returns contents. Certain amount atom clusters formed in the melt when adding the appropriate amount of returns, a large number of particles are excited for nucleation and growth once the undercooling reaches a certain extent, which results in refinement significantly. However, the atom clusters will segregate to meet or exceed the critical size of nucleation and growth for the grain when adding excessive returns, and resulting in lower refining effect.Y addition into as-cast ZL205A alloy leads to the refined microstructures. The grain size decreases with increasing of Y content. TEM results show that the morphology of AlCuY phase exhibits bulk structure, the crystal structure is close-packed hexagonal (HCP) structure, and located in triangular grain boundary. DSC analysis shows that AlCuY phase formed at 592.1℃which is higher than the liquidus temperature. The formation of AlCuY phase took up Cu atoms which results in the reduction of the quantity of eutectic structure. In addition, theory calculation shows that the electronegativity difference and interaction strengths between Y and Ti, V are higher than that of between Al and Ti, V, which makes them segregated easily. Therefore, the precipitation of AlCuY phase decreases the nucleation frequency of primaryα(Al) and increases the nucleation undercooling, finally, reduces the liquidus temperature.The effect of Y on hot tearing resistance of ZL205A alloy was studied. It is found that the hot tearing resistance of ZL205A alloy with 0.05 wt.%Y and 0.1 wt.%Y achieves up to 380 N and 450 N, respectively, while that of primary ZL205A alloy was 330 N. The reasons of Y reduces the hot tearing susceptibility were investigated. Firstly, Y addition causes a depression of the begin-solidifying temperature ofα(Al) which shortens the crystallization range. Secondly, Y addition refines grain size which increases bonding areas between grains and results in large intergranular bonding. Finally, addition of Y reduces the quantity of eutectic structure and changes the eutectic structure from mesh to strip. The critical stress of fracture propagation is improved due to the decrease of grain boundary weakness which due to the decrease of effective surface energy caused by the existence of liquid in the grain boundary.The effects of returns and Y additions on the mechanical properties and hot tearing susceptibility of ZL205A alloy were investigated. It indicates that the requirement of ZL205A alloy properties can be achieved by controlling returns and Y addition. The addition of returns and Y has very strong effect on microstructure refinement of ZL205A alloy, and the grain size decreases from 60μm to 25μm when contents of returns and Y are 20 wt.% and 0.1 wt.%, respectively. It shows that the hot tearing susceptibility of ZL205A alloy with 20 wt.% returns and 0.1 wt.%Y decreases significantly, and the hot tearing resistance increases from 330 N to 670 N.