Effect of ultrasonic treatment and Sr addition on microstructure of Al-20%Si alloy

Many investigations have revealed that transition of melt structure can effectively infl uence the fi nal solidifi cation microstructures.In this study,ultrasonic treatment was applied to Al-20%Si melt and Sr-modif ied Al-20%Si melt at 720℃(i.e.above liquidus of about 690℃)for 60 s,and then the melt was quickly quenched to room temperature.Experimental results show that ultrasonic treatment can ref ine the primary Si phase andα-Al of Al-20%Si alloy;strontium can make the morphology of Si phase spheroidized and refi ned as Sr addition changes the faceted growth characteristic of Si phase;however,the ref inement effect of ultrasonic treatment on the primary Si phase andα-Al is weakened by Sr addition.

Study on typical hole defects in AZ91D magnesium alloy prepared by low pressure lost foam casting

The hole defects can easily occur in magnesium alloy castings that are prepared by low pressure lost foam casting(LP-LFC)process when the process parameters such as vacuum,pouring temperature and f illing velocity are not properly selected.In this study,the forming mechanism of the hole defects in AZ91D magnesium castings by LP-LFC process was investigated.The shape,location and surface appearance of the hole defects were observed using optical microscopy and scanning electron microscopy,and the chemical composition on the surface of the holes was analyzed using energy spectrometer.The result indicates that there are two types of hole defects,i.e.,the pyrolysis products related hole defects,including concentrative hole and blow hole defects,and slag related hole defects.The concentrative hole and the blow-hole defects were formed either by the liquidEPS degradation products entrapped in the molten metal under the condition that the pouring temperature is equal to or lower than 730℃,or by the hindered transport of EPS pyrolysis products.Some irregular shape hole defects were caused by slag or by coating slough entrapment when the pouring temperature is equal to 750℃and the f illing velocity is equal to or greater than 100 mm·s-1.To reduce or eliminate the hole defects,the vacuum and f illing velocity must be properly chosen to ensure that the metal front prof ile exhibits convex shape and in laminar current state,and the pouring temperature should be just high enough to ensure that the molten melt has adequate heat energy to complete the foam pyrolysis and to fully occupy the mould.For AZ91D magnesium castings in this study,the parameters should be 730℃pouring temperature,0.02-0.03 MPa vacuum and 80mm·s-1f illing velocity.

Evolution of rheocast microstructure of AZ31 alloy in semisolid state

Semisolid rheoforming(SSR)is a promising technology for the production of Mg wrought alloy in foundry settings.In order to realize SSR,it is necessary to characterize the grain structure evolution during slurry preparation.In this paper,slurry of AZ31 alloy was produced by a novel rheocast process known as self-inoculation method(SIM).Interrupted quenching technology was applied to investigate the primaryα-Mg evolution during continuous cooling and isothermal holding.Results indicate that the initial microstructure of slurry produced by SIM is a mixture of irregular grains,which becomes ideally globular when the slurry slowly cools to 620℃and isothermally held for at least 30 s.The local solute diffusion leads to dendritic fragmentation and forms separated particles.During prolonged holding,the particle surface gradually becomes smooth because of protuberance melting and groove advancement.Coarsening ofα-Mg grains in isothermal holding was analyzed using Lifshitz-Slyozov-Wagner theory.Results suggest that coalescence is most likely the dominant coarsening mechanism in the early stage while Ostwald ripening tends to be the principal one later.The EDS results indicate that a longer holding time leads to Al solute element segregation at the grain boundaries,but Zn distribution within liquid matrix has no obvious change.

Microstructure evolution of an Mg-Zn-Nd-Zr magnesium alloy during recrystallization and partial remelting process

To obtain a spheroidal microstructure of a semi-solid Mg-Zn-Nd-Zr alloy,which is favorable for the subsequent thixocasting process,the recrystallization and partial remelting(RAP)method was used in this study,and the microstructure evolution of the alloy and the mechanism of the shape factor change during the RAP process were invesigated.The as-cast Mg-Zn-Nd-Zr alloy was prepared in a metal mold and then extruded into bars with an extrusion ratio of 16:1.Partial remelting was carried out on the extruded samples at 589℃(at a heating rate of 0.5℃·s-1)for different holding times ranging from 0 to 90 min.To examine the effect of heating rate,partial remelting of samples at the heating rate of 2℃·s-1was also performed.Results show that the extruded microstructure rapidly evolutes into recrystallized grains in the semi-solid state;the liquid f ilm initially forms at grain boundaries during the partial remelting,and then gradually changes from continuous into discontinuous state with the increase of holding time;this results in the agglomeration of adjacent grains and the decrease in shape factor.The value of shape factor increases continuously with holding time at f irst and reaches the maximum 0.62 when holding for 60 min,while decreases rapidly after a prolonged holding time.Moreover,local melting,mainly due to the inhomogeneous deformation during extrusion,becomes more signif icant and less uniformly distributed at a relatively higher reheating rate.The Mg-Zn-Nd-Zr magnesium alloy components have been thixo-cast successfully using the RAP method,which strongly proves the feasibility of RAP process in Mg-Zn-Nd-Zr alloy.

Feature study of body-fitted Cartesian grids used in casting numerical simulation

A type of mesh called a body-fi tted Cartesian mesh,very different from the traditional structured body-fi tted mesh,is established.At f irst,the right parallelepiped mesh is generated,then,a feature analysis is done on the cross sections.These cross sections are the intersections of the casting shape with the right parallelepiped grids(under the Cartesian coordinate system).On the basis of the feature analysis,two sorts of body-f itted boundary grids,shape-keeping grids and shape-distortion grids,are def ined.Shape-distortion grids can be removed or weaken by increasing the number of grids or moving the coordinates of the mesh generation region,so actually the body-fitted Cartesian mesh generation is to get shape-keeping grids.A shape-keeping grid mainly consists of two sorts of surfaces(I type face and II type face),and each of them is joined by two types of points(I type point and II type point).If only these two types of points were given,the shape-keeping mesh would be constructed.In this paper,the cases of the above two boundary grids being generated were discussed.An algorithm was put forward to get the shape-keeping grids.Several body-fi tted Cartesian meshes generated on castings show the validity of the algorithm.The mesh generation examples show that the body-fi tted Cartesian mesh is more excellent than the right parallelepiped mesh in aspects of decreasing grids number and being closer to the shape of the casting solid.

Effect of inter-critical quenching on mechanical properties of casting low-alloy steel

For some casting low-alloy steels,traditional quenching and tempering heat treatments can improve the strength;however,sometimes the ductility is not satisf ied.Therefore,some kind of effective heat treatment method seems necessary;one which could improve the ductility,but not seriously affect the strength.In this paper,the effect of inter-critical quenching(IQ)on the mechanical properties of casting low-alloy steel was studied.IQ was added between quenching and tempering heat treatment;and the microstructure and mechanical properties were compared to the same steel with the traditional quenching and tempering treatments.The experimental results show that the microstructure comprises small-size ferrite and martensite when the IQ is adopted;and that different temperatures can control the ferrite quantity and distribution,and,as a result,infl uence the mechanical properties.In the case of IQ,the tensile strength decreases just a little,but the ductility increases a lot;and the strength-ductility product(its value is the arithmetic product of elongation and tensile strength)increases by between 6%and 10%,which means the IQ heat treatment can improve comprehensive mechanical properties.

Hot cracking susceptibility of AlSi7MgCu alloys and effects of alloying elements magnesium and copper

Hot cracking during solidification can be a serious problem in aluminium casting alloys under certain conditions.This feature is well known,but still insuffi ciently investigated in shape casting.This study gives a brief overview of the factors inf luencing hot cracking during shape casting.Five different AlSi7MgCu alloys with varying Mg and Cu contents were examined.Theoretical models,including the cracking susceptibility coeffi cient(CSC)from Clyne and Davies,were considered.Thermodynamic calculations(terminal freezing range,TFR)of the behavior of the solid fraction during solidif ication were compared to an experimentbased hot cracking indexing(HCI)method.Scanning electron microscopy(SEM)was used to compare the existing microstructure and precipitated thermodynamic phases using the software ThermoCalc Classic(TCC).Furthermore,SEM was used to investigate crack surfaces initiated by a dog-bone shaped mold during casting.A good correlation between theoretical models and the experimental hot cracking index method was observed.

Modeling and analysis of porosity and compressive strength of gradient Al_2O_3-ZrO_2 ceramic lter using BP neural network

BP neural network was used in this study to model the porosity and the compressive strength of a gradient Al2O3-ZrO2ceramic foam f ilter prepared by centrifugal slip casting.The inf luences of the load applied on the epispastic polystyrene template(F),the centrifugal acceleration(v)and sintering temperature(T)on the porosity(P)and compressive strength(σ)of the sintered products were studied by using the registered three-layer BP model.The accuracy of the model was verif ied by comparing the BP model predicted results with the experimental ones.Results show that the model prediction agrees with the experimental data within a reasonable experimental error,indicating that the three-layer BP network based modeling is effective in predicting both the properties and processing parameters in designing the gradient Al2O3-ZrO2ceramic foam f ilter.The prediction results show that the porosity percentage increases and compressive strength decreases with an increase in the applied load on epispastic polystyrene template.As for the inf luence of sintering temperature,the porosity percentage decreases monotonically with an increase in sintering temperature,yet the compressive strength f irst increases and then decreases slightly in a given temperature range.Furthermore,the porosity percentage changes little but the compressive strength f irst increases and then decreases when the centrifugal acceleration increases.