The Evolution Regulation And Control Of Iron-rich Intermetallics In A356 Alloy

As a typical material of the Al-Si casting aluminum alloys,A356 alloy has been extensively applied on the crucial components production,such as automobile and motorcycle hub,engine block and large aerospace thin-walled structural parts.During production process,the iron?Fe?is an unavoidable impurity element for aluminum alloys,and it will form the iron-rich intermetallics in the alloy to deteriorate the mechanical property.In industrial production,some techniques are used to reduce the Fe content in the aluminum alloy.However,the Fe content standard for A356 alloy in abroad is higher than domestic standard.Therefore,the investigation of the iron-rich intermetallics in aluminum alloy has important theoretical and practical significance to improvement the mechanical properties,the service ability and the safety performance of components,and reduction the production cost of A356 alloy.In this work,the different compositions of iron-containing A356 casting alloys were prepared.Combined with the mechanical tensile test and microstructure characterization techniques of scanning electron microscopy?SEM?,quantitative metallographic statistical analysis,energy dispersive spectroscopy analysis?EDS?,X-ray diffraction analysis?XRD?and?High-resolution?transmission electron microscopy?TEM?,the evolution of the iron-rich intermetallics in the A356 alloy were researched.Besides,the effects of the neodymium?Nd?addition on the iron-rich intermetallics in alloys and the mechanical properties were investigated.Meanwhile,based on the thermodynamic analysis,the formation and evolution mechanisms of the iron-rich intermetallics in alloys were discussed as well.The main conclusions of this research are as follows:?1?When the iron content increased from 0.1 wt.%to 0.2 wt.%,the dominant iron-rich intermetallics in alloys transformed from the isolate coarsened script-like?-AlSiMgFe phase to the isolate needle-like?-AlFeSi phase,and the script-like?-AlSiMgFe phase would form on the coarsened needle-like?-AlFeSi phase surface when the iron content further increased to 0.3 wt.%.Besides,the 3D morphologies of the?-AlSiMgFe and the?-AlFeSi phase were multilayered seaweed-like and irregular polygonal platelets-like respectively.The nucleation of the?-AlFeSi phase is mainly promoted in 0.2Fe alloy,while the sufficient growth of the?-AlFeSi phase in 0.3Fe alloy may be beneficial to the?-AlSiMgFe phase formation.So the amount of the?-AlFeSi phase in alloy increased with the increasing iron content,but the?-AlSiMgFe phase decreased firstly and then increased.With the increasing Fe content,the mechanical properties values decreased steadily,and the decline of the mechanical properties was affect by the mount change of the?-AlSiMgFe phase in alloy.Under stress state,the?-AlSiMgFe phase which form on the?-AlFeSi phase surface can generate and induce the microcracks extending to the?-AlFeSi phase,resulting in the reduction of the harmful effects of the?-AlFeSi phases on mechanical properties of alloys.And the change of the mechanical properties was similar with the amount of the?-AlSiMgFe in alloys.?2?The Nd addition had no significant influence on the?-AlFeSi phase amount,but the trend of the?-AlSiMgFe phase amount increased initially and then decreased.Besides,the maximum amount of the?-AlSiMgFe phase was obtained in 0.03Nd alloy.With the increasing Nd addition,the morphology of the Nd-rich particles became coarsen and formed on the?-AlFeSi phase surface.In addition,the Nd addition could promote the formation of the?-AlSiMgFe phase by influence the solidification behavior of the alloy.After T6 heat treatment,the coarsened irregular Nd-rich particles in alloys turned to fragmented and spherical,and the?-AlSiMgFe phases transformed to the?-AlFeSi phases leading to the amount of the?-AlFeSi phase increased.Moreover,combined with the mechanical properties test,tensile strength and yield strength of the as-cast alloy had small fluctuations with increasing Nd addition,while the elongation increased firstly and then decreased.After T6 heat-treatment,the tensile strength and yield strength increased clearly while the elongation was lower than that of the as-cast alloy,and the trends of the mechanical properties values increased firstly and then decreased with increasing Nd addition.Finally,it confirmed that the optimum Nd content for A356-0.3Fe alloy was 0.03 wt.%,and the mechanical properties of the alloy were closed to A356-0.2Fe alloy.?3?The?-AlFeSi phases in A356-0.3Fe alloy became cross-linking,and the dominant?-AlSiMgFe phases formed in both the platelet interconnected area and the platelet center area.It should be pointed that the microhardness of the?-AlSiMgFe phases?71.1 HV?was between the?-AlFeSi phases?97.3 HV?and the alloy matrix?50.4 HV?by microhardness analysis.Besides,the crystallographic orientation relationship between the?-AlMgFeSi phase and the?-AlFeSi phase was[1?10]_?-AlFeSi?[12?10]?_-AlSiMgFe,and the interface was an amorphous transition area which the interfacial width was about 0.4 nm.The quarternary peritectic reaction interface was flat and smooth at the microscopic scale.In addition,the growth and interconnection of the?-AlFeSi phase in alloy implies the solute diffusion in the residual melt during solidification process,resulting in the surface defects and solute enrichment areas forming on the?-AlFeSi phase surface.It increased the constitutional undercooling which is caused by the solute enrichment,and improved the Gibbs free energy of the the quarternary peritectic reaction.Finally,it promoted the formation of the?-AlMgFeSi phase on the?-AlFeSi phase.