Study On The Improvement Of ZL114A Alloy Microstructure By Microalloying And The Strengthening Mechanisms

ZL114A alloy has the advantages of high specific strength,low coefficient of thermal expansion and excellent casting performance.It is widely used in aerospace to manufacture large thin-walled complex structural castings,like missile pylon,etc.However,such castings did not have good comprehensive mechanical properties and short service life.At present,the aviation industry has put forward higher requirements for the service life of castings,and there is an urgent need to improve the comprehensive mechanical properties of ZL114A.On the premise of ensuring the excellent casting properties of ZL114A alloy,in this paper,the effects of Y and Sb on the eutectic solidification process of ZL114A alloy under different cooling rates were studied;the effects of Zn and Cu on the precipitation behavior of ZL114A alloy were studied;the effects of Sc and Zr on the grains of ZL114A alloy were studied.It provides theoretical support for improving the comprehensive mechanical properties of ZL114A alloy by microalloying.The main results are as follows:(1)When the cooling rate is low,the mechanism of Y modifying eutectic Si is that during eutectic solidification,Y is enriched near eutectic Si to produce bigger component supercooling,resulting in the increase of the number of nucleation cores of eutectic Si;When the cooling rate is high,the mechanism of Y modifying eutectic Si is that during eutectic solidification,Y is distributed on eutectic Si to increase the number of twins,resulting in the change of eutectic Si growth mode and branching.When V=0.16℃/s,Y changes the eutectic Si from plate to fiber.The tensile strength of as-cast ZL114A+0.18 wt.%Y alloy is 153 MPa and the elongation is 5.3%;When V=3.20℃/s,Y changes the eutectic Si from fibrous to coral.The tensile strength of as cast ZL114A+0.18 wt.%Y alloy is 169 MPa and the elongation is 8.9%.(2)The mechanism of Sb modifying eutectic Si is that Sb reduces the nucleation temperature of eutectic Si,reduces the solid solubility of Si in α-Al during eutectic solidification,increases the chemical potential of Si atoms discharged from the front of solidification interface,increases the discharge rate of Si atoms,accelerates the growth rate of Si phase,and similarly accelerates the growth rate of α-Al,that is,the eutectic solidification rate increases,reduces the eutectic spacing,and then achieves the effect of modifying eutectic Si.When V=0.16℃/s,Sb changes the eutectic Si from plate to fiber.The tensile strength of as-cast ZL114A+0.09 wt.%Sb alloy is 158 MPa and the elongation is 7.0%;When V=3.20℃/s,Sb changes the eutectic Si from fibrous to flake.The tensile strength of as-cast ZL114A+0.09 wt.%Sb alloy is 184 MPa and the elongation is 9.6%.(3)When ZL114A alloy is aged,Zn atoms enter GP zones and β" Some Si atoms and Mg atoms are replaced in the phase,and Zn makes the enthalpy of phase formation and activation energy of GP zones and β" reduced,so Zn makes the number and size of GP zones and β" increased and the aging process is accelerated.By optimizing the heat treatment process,Zn can slightly improve the tensile strength and yield strength of ZL114A alloy while ensuring high elongation.When the two-stage aging process is 165℃/1h+120℃/12h,the tensile strength,yield strength and elongation of ZL114A+0.35 wt.%Zn alloy without modification are 324 MPa,236 MPa and 7.8%.(4)The effect of Cu on the precipitated phase of ZL114A alloy mainly has two aspects:By optimizing the heat treatment process,in ZL114A alloy,θ" precipitation phase will appear,to achieve β" and θ" double precipitation phase strengthening,and the sequence of precipitates is from GP zones→β"→β′→β change to GP zones → β"+θ"→β′+θ′→β+θ.During aging treatment of ZL114A alloy,Cu atoms combine with some vacancies,resulting in the reduction of the number of vacancies combined with Mg and Si atoms.The number of β" phase and β′ phase decreases to a certain extent.(5)The strengthening effect of θ" phase>β" phase>β′phase,the strengthening of β" and θ" double precipitation phases significantly increases the strength of ZL114A alloy.When the aging process is 180℃/8 h,there are β" phase and θ" phase in ZL114A+0.54 wt.%Cu alloy,the length of β" phase is less than 20 nm,and the length of θ" phase is greater than 50 nm.The strengthening mechanism of the precipitation phase is the cut-through mechanism.The tensile strength of the unmodified alloy reaches 384 MPa,the yield strength reaches 338 MPa,and the elongation is 2.2%.(6)For the ZL114A alloy whose grains are refined by Al-Ti-B,the combined addition of Sc and Zr can further refine the grains,and the grain size is reduced from 319.2 μm to 287.8 μm;Adding Sc alone or adding Zr alone,due to the formation of intermetallic compounds of Al-Si-Sc-Ti and Al-Si-Zr-Ti respectively,the Ti content in the ZL114A alloy was consumed and the number of primary α-Al grain nucleation cores Al3Ti and TiB2 decreased,resulting in grain coarsening.