Heat Treatment Strengthening Behavior Of Hypereutectic Al-Si-Cu-Mg Alloy Prepared By Semi-solid Extrusion

Hypereutectic Al–Si-Cu-Mg alloys are widely used in transportation fields to produce engine blocks,engine cylinder,piston and other components,due to their high specific strength,high hardness,low linear expansion coefficient,good wear resistance,and good thermal stability.However,coarse and massive primary Si phases seriously affect the mechanical properties of the alloy.As a near-net forming process,the semi-solid squeeze casting process can effectively improve the microstructure and mechanical properties of alloys.However,the current research results of heat treatment process and strengthening mechanism of semi-solid extrusion are still weak.In this paper,hypereutectic Al-17Si-4Cu-0.5Mg alloy is used as the basic experimental material and formed by semi-solid squeeze casting process.The heat treatment strengthening behavior of semi-solid extrusion forming Al-Si alloy is systematically studied,and the advantages of semi-solid forming alloy structure are fully utilized to obtain the short-time heat treatment process of semi-solid extrusion forming Al-Si alloy.The ageing strengthening mechanism of semi-solid extruded Al-Si alloy is deeply analyzed,so as to provide theoretical support for the development of high-performance cast aluminum alloy and its short-time heat treatment strengthening process.The semi-solid slurry is prepared by mechanical stirring,electromagnetic stirring and ultrasonic vibration.It is found that the alloy has more uniform structure after ultrasonic vibration.Pouring at different slurry temperature found that when the slurry temperature is600℃,the alloy has less defects,uniform and fine microstructure and better mechanical properties.The differences in microstructure and properties of Al-17Si-4Cu-0.5Mg alloy under forming processes of gravity casting and semi-solid squeeze casting are further studied.It is found that the number of coarse block primary Si phase is significantly reduced and becomes small,and the lath eutectic Si phase is transformed into fine fiber,α-Al is nearly equiaxed or rosette-like,and the alloy has higher mechanical properties and wear resistance after semi-solid extrusion.The effect of solution treatment on the microstructure and properties of semi-solid extruded Al-17Si-4Cu-0.5Mg alloy is studied.In the early stage of solid solution,the edge and corner of primary Si phase are passivated preferentially,the fibrous eutectic Si phase is necked and fused.The second phases such as Al₂Cu begin to dissolve.In particular,the Cu-rich second phase enriched near the primary Si dissolves rapidly and dissolves into the matrix,the dislocation density in the matrix begins to decrease.In the middle stage of solid solution,the primary Si is further passivated and has a polygonal structure.The surface energy of eutectic Si phase is reduced and tends to be rounded,and the degree of dislocation accumulation in the matrix is greatly reduced.In the later stage of solid solution,both the primary Si phase and the eutectic Si phase are coarsened,which conforms to the LSW model,and the eutectic Si phase exhibits angular facet characteristics.The dislocation density in the matrix is maintained at a reduced level.For semi-solid extruded Al-17Si-4Cu-0.5Mg alloy,solution treated at 525℃for different time and then aged at 180℃for 6h,the experimental results show that the solid solution curve of the alloy exhibits the phenomenon of“double peak”with the prolongation of the solution time.The first peak appears at 525°C for 1h,the tensile strength of the alloy is 278 MPa,The second peak appears at 525℃for 8h,the tensile strength of the alloy is 323MPa.The first strength peak is mainly due to the high dislocation density in the matrix at the initial stage of solid solution,Cu element rapidly dissolves into the matrix with dislocation line as the medium.After ageing treatment,a large number of fineθ’phase and Q’phase are precipitated,which has a strong pinning effect on the dislocation.The second intensity peak is mainly due to the rounding of primary Si phase and the granulation of eutectic Si phase.The content of solute element in the matrix is high,when the solid solution treatment is 8h.After ageing at 180℃for 6 h,the number of nano precipitates increase further,the distribution is more uniform,and the dispersion strengthening effect is further enhanced.The effect of ageing treatment on the mechanical properties and microstructure of semi-solid extruded Al-17Si-4Cu-0.5Mg alloy is systematically investigated.The alloy is aged at 180℃for 0-24h.As the ageing time increases,the strength increases at beginning and then decreases.The peak strength is reached when ageing at 180℃for 6 h.The ageing precipitation process of Al-17Si-4Cu-0.5Mg alloy formed by semi-solid extrusion is mainly divided into three stages.The initial stage of ageing is mainly the precipitation of GP region and Q"phase,the size and number of precipitated phase increase rapidly,which is consistent with the variation of SANS scattering curve and GPPD powder diffraction peak.In the peak ageing stage,θ"phase and Q’phase are mainly precipitated.In the over ageing stage,the precipitation phases are mainlyθ’phase and Q’phase.Combining first-principle calculations,it is concluded that the binding energy and heat of formation of the Q’phase are lower than those of theθ’phase.Therefore,Q’phase is more stable and preferentially precipitates.Compared with gravity casting,the type and sequence of precipitates in semi-solid extruded Al-17Si-4Cu-0.5Mg alloy are the same after treated with 525℃solid solution for8h and then aged at 180℃for different time,the distribution of precipitates is more uniform,dispersed and smaller.The main reason is that the cooling rate of the alloy is faster after semi-solid extrusion,the distribution of solute elements in the matrix is more uniform.Meanwhile,a large number of dislocations are introduced into the matrix after quenching of eutectic Si,which promotes the ageing process ofθ’phase.In the peak-aged state,the tensile strength and yield strength of the semi-solid extruded alloy are increased by 43.6%and 51.1%,respectively,compared with gravity casting.Theθ"phase in the semi-solid extruded alloy has a completely coherent relationship with the matrix,the degree of lattice distortion is strong,the distortion energy is high,and the dislocation is mainly based on the shear mechanism,the alloy has higher mechanical properties.The effect of the content of important alloying element Mg on the ageing precipitation behavior and mechanical properties of semi-solid extrusion Al-17Si-4Cu-x Mg alloy is studied.When the content of Mg in Al-17Si-4Cu alloy increases from 0.5wt.%to 2.5wt.%,In addition to Al₂Cu and Al₅Cu₂Mg₈Si₆phases,the Mg₂Si phase also appears in the as-cast microstructure.It is found that the main precipitates of Al-17Si-4Cu-2.5Mg alloy areθ″,Q’andβ"phases under the peak ageing state,and theβ"phase exists in the form of needles and dots along the<001>_Aldirection.Al-17Si-4Cu-2.5Mg alloy has higher strength value and hardness value,mainly because the formation of Mg₂Si phase and the ageing precipitation ofβ’phase are promoted with the increase of Mg content and strengthens the alloy.