Effect Of Ca On Microstructure And Mechanical Properties Of Rheo-squeeze Casting AZ91-Ce Magnesium Alloy

Magnesium alloys,as the lightest metal engineering materials,have a lot of advantages,which are widely used in many fields.AZ91 alloys are the most successfully developed and widely used,which have fine casting performance and corrosion resistance.However,AZ91 alloys are easy to oxidize and burn during melting,resulting in inclusion,which limits its further application.Therefore,we should find ways to improve the flame retardancy of AZ91 alloys.Ca element is appropriate flame-retardant element.It is also found that the flame-retardant effect of Ce and Ca combined is better than that of single addition.But considering the high price of Ce element,we should not add too many Ce elements.Therefore,1%Ce is added to AZ91 alloys in this paper.The addition of Ca elements can improve the flame retardancy of the alloy,but when the Ca content exceeds1%,the mechanical properties of the alloy rapidly deteriorate.Therefore,it is necessary to find a method to improve the mechanical properties of AZ91-Ce-Ca alloy and ensure excellent flame-retardant properties simultaneously.The semi-solid slurry prepared by gas bubbling stiring can obtain the fine and uniform non-dendrite microstructure.The squeeze casting is a low speed and high pressure casting method filling mold,which can make castings have good mechanical properties.Based on this,AZ91-1Ce alloy was selected as the base alloy in this paper.Firstly,the influence of Ca elements on microstructure,mechanical properties and flame retardancy of AZ91-1Ce-x Ca alloy was studied.In order to ensure its flame retardancy and mechanical properties,the Ca content was optimized to be 2%.Then,how to improve the mechanical properties of AZ91-1Ce-2Ca alloy was studied.The microstructure of semi-solid slurry prepared by gas stirring was studied,and the microstructure and mechanical properties of squeeze casting and rheo-squeeze casting alloys were studied respectively and compared.Firstly,the effect of Ca content on microstructure,mechanical properties and flame retardancy of AZ91-1Ce-x Ca magnesium alloy was identfied.Compared with AZ91-1Ce alloy,the second phases of AZ91-1Ce-1Ca alloys refined and the numbers increased.However,with the increase of Ca content,Al₂Ca phases were coarsened,and the slice spacing between layers also increased.The second phases in the as-cast structure were mainly composed of thick network Al₂Ca phases with nonuniform distribution.When the Ca content increased from 0 to 3%,the mechanical properties of AZ91-1Ce-x Ca alloy first increased and then decreased.When the Ca content was 1%,the yield strength,tensile strength and elongation of the alloy reached the maximum values,124MPa?195MPa and 3.2%respectively.With the increase of Ca content,the ignition point of AZ91-1Ce-x Ca alloy improved gradually.When the Ca content was 2%,its ignition point was 801?C,which was 34.2%higher than that of AZ91-1Ce,which was 597?C.Combined with these two aspects,considering that AZ91 was normally smelted at 700?800?C,so the Ca content was optimized to be 2%.Then study on the microstructure of semi-solid slurry of AZ91-1Ce-2Ca alloy prepared by gas flowing stirring,the influence rules of processing parameters of semi-solid slurry were indentified.The improvement of gas flowing rate is beneficial to the uniform dietribution of concentrate field and temperature field and the increase of heterogeneous core,which can make a large number of nucleation.Therefore,the primary phase changed from dendrites to spherical granule and the size decreased continuously.But when the gas blowing rate was too high,it resulted in the uneven distribution of primary phase.In addition,with the pouring temperature decreasing and stiring speed increasing,it continuously improved the roundness and volume fraction of semi-solid slurry,and decreased the mean size of primary phase.So,the optimized process parameters are the pressue 130Mpa and pouring temperature 600?C and rotation speed 60r/min.So,in this paper,when the gas flowing rate,pouring temperatures and stirring speed are 6 L/min,587?C and 130r/min,we can get the best sem-solid slurry.Next,the effect of squeeze casting parameters on microstructure and mechanical properties of AZ91-1Ce-2Ca alloy was studied,and the influence rules of it were identified.With the pressure increasing,the subcoold temperature and thermal conductivity raised,favorable for massive nucleation.It contribured to the decreasement of the mean size of primary phase.In addition,the defects in the process of casting can be reduced by lowering the pouring temperature properly,so that the microstructure of alloys can be refined and the mechanical properties can be improved.So,in this paper,the optimized parameters of squeezing casting process were the pressue 130Mpa and pouring temperature 600?C.And the yield strength,mechanical strength and elongation of AZ91-1Ce-2Ca alloy reached 113.2MPa,142.9MPa and 1.7%,respectively.Finally,the influence of rheo-squeeze casting parameters on microstructure and mechanical properties of AZ91-1Ce-2Ca alloy was studied and compared with squeeze casting.With the increase of pressure,the microstructure of AZ91-1Ce-2Ca alloy cast by rheological extrusion was continuously refined and mechanical properties improved.It was noticed that in the process of rheo-squeezing casting,the pressure on phase diagram had only effect on nucleation of residual liquid phase,but no effect on nucleation of primary phase.The decline of the pouring temperatue and the increase of the stirring rate also refined the microstructure and improved the mechanical properties.However,the stirring speed had only effect on the process of semi-solid prepared.So,in this paper,the optimized parameters of rheo-squeezing casting process were the pressue 130Mpa,pouring temperature 587?C and stirring speed 120r/min.And the yield strength,mechanical strength and elongation of AZ91-1Ce-2Ca alloy reached 130.1MPa,189.1MPa and 2.4%,respectively.Compared with the squeeze casting,under the same pressure,the grain size of rheo-squeezing casting was obviously smaller than that of squeeze casting,and the mechanical properties were better than that of squeeze casting.The results showed that when AZ91-1Ce alloy was added to 2%Ca and cast by rheo-squeezing,it not only ensured its flame retardancy but also improved its mechanical properties.The optimized parameters of rheo-squeezing casting process were the pressue130Mpa,pouring temperature 587?C,gas bubbling rate 6L/min,and stirring speed120r/min.