| Al-Si-Cu alloys are the most widely used in Al alloys.Due to its low density,high specific strength,low thermal expansion coefficient and good corrosion resistance,it is often used as a class of functional materials in engine blocks,cylinder heads and chassis.In recent years,the increasing demand for strengthening and toughening of Al-Si-Cu alloys has attracted continuous attention;at the same time,with the increasing application of functional Al alloys with bionic structures,the demand for solid-phase connection of such alloys has also increased.This makes strengthening the properties of Al-Si-Cu alloys by means of optimization of alloy composition and modification,and the solid-phase connection of cast Al alloys have great practical significance.However,the effect of various elements on the solidification behavior of the hypoeutectic Al-Si-Cu alloy is still unclear,and the high-strength and toughness cast Al-Si-Cu alloy developed by the microalloying method is lacking.At the same time,research on the agents of microstructure manipulation of Al-Si-Cu alloys is also needed to meet the needs of efficient micronization of the alloy to achieve the comprehensively optimized microstructure configuration.In addition,strength and plasticity of the cast Al alloys are low,they cannot meet the requirements of friction stir connection.Therefore,it is urgent to develop a high-strength hypoeutectic Al-Si-Cu by means of composition optimization design and simultaneous control of multiple microstructures in order to achieve solid-phase connection.This paper studied the effect of alloying elements on the liquid-solid transformation and microstructure of hypoeutectic Al-Si-Cu alloys;explored the in-situ nanocrystals produced by Fe BSi amorphous crystallization to manipulate the multiple microstructures and strengthened the properties of multiple hypoeutectic Al-Si alloys;Through the optimization of the composition and the synergistic effect of nanocrystalline manipulation,the high-strength eutectic Al-Si-Cu alloy was obtained.The solid-phase connection of the high-strength and toughness cast Al alloy was realized by friction stir connection,and its strengthening and toughening mechanism was revealed;the following main conclusions were drawn: 1.The mechanism of Mg,Ti,and Sb on the refinement of the hypoeutectic Al-Si-Cu alloy and its influence on the mechanical properties of the alloy were revealed.1)It was found that the addition of Mg in the alloy provided supercooling for the ?-Al nucleation to achieve dendrite refinement and improved the eutectic Si morphology;When 0.3 wt.% Mg was added,the tensile properties were best improved at room and high temperature;Among them,the yield,tensile strength,elongation and strain at room temperature reached 301 MPa,451 MPa,6.7% and 8.6%;the yield and tensile strength reached 157 MPa and 175 MPa,but the elongation and strain were decreased to 13.1% and 16.3% at high temperature.2)It was found that after adding Ti,?-Al achieves a larger nucleation subcooling degree,which was beneficial for Al3 Ti to provide more heterogeneous nucleation sites for the alloy and achieve grain refinement.At the same time,after adding Ti,the yield strength of the alloy increased slightly and the strain increased to 9.1%.3)It was found that Sb can provide greater undercooling for the eutectic structure nucleation,and at the same time,it will adhere to the surrounding of the Si phase and hinder its growth.After adding 0.1 wt.% Sb,the yield strength and ultimate tensile strength of the alloy were increased to 334 MPa and 480 MPa,and the elongation and strain reached 6.8% and 8.9%.4)It was revealed that the optimum design of alloy composition for the hypoeutectic Al-Si-Cu alloy was: 0.3 wt.% Mg and 0.1 wt.% Sb,the strengthening mechanisms are: ?-Al dendrite was significantly refined,grain strengthening was significant,and ?? and Mg2 Si precipitated phase formed the second phase strengthening;at the same time,the refined spherical eutectic Si effectively pinned on the grain boundaries and reduced crack initiation,thereby increasing the ductility of the alloy.2.The refinement mechanisms of in-situ nanocrystals on the multi-level microstructure in the hypoeutectic multiple Al-Si-Cu alloy were revealed;and the influence of nanocrystal on the mechanical properties of the alloy and its strengthening and toughening mechanism were revealed.1)It was found that the hypoeutectic Al-Si-Cu alloy achieved the morphology improvement and size refinement of ?-Al,eutectic Si and two precipitated phases(?? and ?-Mg2Si)after the manipulation by in-situ nanocrystalline for 20 min.The refinement mechanism was: Fe2 B and ?-Al had a very low lattice mismatch degree,which can promote the nucleation as the heterogeneous nucleation sites of ?-Al;Fe2B and Fe3 Si were adhered on the solid-liquid interface to prevent the growth of the eutectic phase,and achieved dendritic and eutectic Si morphology improvement and microstructure refinement.2)The in-situ nanocrystalline manipulation can increase the strength of the alloy,greatly improve the plasticity of the alloy,and significantly increase the static toughness of the alloy.The strengthening and toughening mechanism was: the size of ?-Al was significantly reduced after manipulation,and the fine grain strengthening effect was improved;the precipitated ?? phases and ?-Mg2 Si phases were refined and uniformly distributed in the matrix,and the dislocation hindering effect was enhanced,thus the second phase strengthening was significant;at the same time,the improved morphology of eutectic Si effectively reduced crack initiation and stress concentration.3.Based on the hypoeutectic Al-Si-Cu alloy treated by composition optimization and nanocrystalline regulation,the solid phase connection of the casting hypoeutectic Al-Si-Cu alloy was realized,the optimal process parameters were optimized,and the mechanism of the effect of solid phase connection on the microstructure and mechanical properties of the joint was revealed.1)It was revealed that the microstructure changed in different connection areas of the matrix alloy after friction connection.The grains in the HAZ were coarsened and the eutectic Si grew;the grains in the TMAZ wre stretched,some of the grains were dynamically recovered,and the eutectic Si distribution tended to a certain direction;dynamic recrystallization of grains in the NZ showed a regular and fine equiaxed structure,and fine eutectic Si was evenly distributed in the matrix.In addition,the ?? phases and ?-Mg2 Si precipitates at the joints grew to a certain extent,increasing by 72.9% and 65.4%,respectively.2)The change law of tensile properties and its mechanism after friction stir connection were revealed.After the friction stir connection,the plasticity of the joint decreased and fracture occurred in the HAZ.The reason was that the grains in the HAZ are coarsened and the grain strengthening was reduced;at the same time,the size of the precipitated phases of ?? phases and ?-Mg2 Si was increased,and the strengthening effect of the second phase was weakened.When the parameters were 1200 rpm and 75 mm/min,the joint reached the best yield and ultimate tensile strength,280 MPa and 400 MPa,reaching 84.8 % and 83.3 % of the base material;meanwhile,the elongation and the strain were 5.5 % and 7.6 %.3)The hardness of joints and its mechanism after friction stir connection were revealed.The hardness value on the cross section of the connection showed a “W”-shaped distribution,in which the hardness in the HAZ was the lowest,the hardness in the BM was the highest,and the hardness value in the NZ was lower than in TMAZ.The reason was that the grain size in the HAZ was the largest thus the hardness was the lowest;the precipitation phase in the BM was small and the dispersion distribution had the highest resistance to dislocations and the highest hardness;the precipitation phase in the NZ grew up,and the strengthening effect of the second phase was reduced to make its hardness value lower than the TMAZ.When the parameters were 1200 rpm and 75 mm/min,the joint achieved the highest hardness after friction stir connection. |
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