Research On Semi-solid Slurry Compound Fabrication And Rheo-forming Of Hypereutectic Al-Si Alloy

Hypereutectic Al–Si alloys have been considered as a good candidate for structural applications in automotive and aerospace due to their low density,good wear resistance,low thermal expansion coefficient and excellent casting ability.The presence of coarse primary Si particles in the microstructure of Al–Si hypereutectic alloys has been identified as the main limitation for their industrial use as it may lead to premature crack initiation and low mechanical properties.Because of these problems,fine and uniformly distributed primary Si particles need to be prepared to improve the mechanical properties of hypereutectic Al–Si alloy.In this paper,a new method named MRB with modification compound fabrication was used to refine the primary Si.By using optical microscopy(OM),X-ray diffractometer(XRD),scanning electron microscopy(SEM)with energy dispersive X-ray analysis(EDX)and transmission electron microscopy(TEM),the present work investigated the modification treatment of A390 hypereutectic alloy for the first time.Then the preparations of A390 semi-solid slurry via mechanical rotational barrel(MRB)process and MRB with modification were studied systematically.The optimization of technical parameters and mechanism of compound fabrication were discussed.With the modification of Al-P alloy,the morphology of primary Si drastically changed from polygonal shape to fine granular with smooth edges and uniformly distributed,the average particle size decreased from50?m to 14?m,the eutectic Si had a change trend to the bar shape.When the content of modifier was 0.035 wt.%P,the average particle size of primary Si reached its minimum.With more modifier added,the primary Si particle got coarsening and had a bad modification effect.With the increase of modification temperature,the size of primary Si first decreased and then increased,while with the increase of holding time,the size of primary Si increased.The optimization modification parameters was content of modifier 0.035 Wt.%P,modification temperature 810 ~oC,holding time 10 min.After the MRB process,the morphology of primary Si drastically changed from polygonal shape to fine granular,the morphology of?-Al changed from dendrite to non-dendrite.With the decrease of pouring temperature,the increase of rotation speed,the increase of slope angle,the average particle size of primary Si all first decreased and then increased.When the pouring temperature was 720 ~oC,the rotation speed was 90r/min and the angle was 15~o,the primary Si presented in fine granular with minimum size of 17?m,the eutectic Si was short-bar shape,the?-Al was suborbicular.Compared with the microstructure of alloys made by modification or MRB process,the primary Si by compound fabrication was finer and distributed more uniform.Decreasing the pouring temperature,the average particle size of primary Si first decreased and then increased,and reached its minimum of about 9?m at 720 ~oC.The refinement and uniform distribution of primary Si by compound fabrication were mainly attributed to the presence of AlP particles provided by the phosphorus addition,and chilling and stirring effects induced by the rotational barrel.The refinement was complicated by controlling the outbreak of heterogeneous nucleation and inhibiting the growth of primary Si.More primary Si nuclei exist in the slurry prepared by compound fabrication than that prepared by single MRB process or modification treatment,which lead to the drastically decrease of primary Si size.In the compound fabrication,the influence of modification paid a more important role.Compared with the microstructure and mechanical properties of die-casting A390 alloy and rheo-forming A390 alloy poured at different temperature,it was obviously that the primary Si in rheo-forming and die-casting alloy all presented in fine granular,but in rheo-forming alloy,there was little air hole and the?-Al presented in near suborbicular.When the pouring temperature was 720 ~oC,the ultimate tensile strength(UTS)and elongation of rheo-forming A390 alloy was 296 MPa,0.93%which is increased 10%and 138%respectively compared with the die-casting A390alloy(270 MPa,0.39%).The fracture mechanisms of die-casting and rheo-forming A390 alloy both were the brittle fracture of primary Si particles.