Effect of mixing temperature on microstructure of an Al-Si alloy prepared by controlled diffusion solidification

The effects of mixing temperature,i.e.,the temperatures of two precursor melts(pure Al and Al-12Si),on the temperature and solute fields of resultant mixture,the nucleation and growth,and the size and morphology of primary grains during controlled diffusion solidification(CDS) of Al-8Si alloy were investigated by using simulation and calculation.The results indicate that a lower mixing temperature is helpful for achieving more supercooled microscale Al-rich pockets in the mixture,and increasing the width and supercooling degree of supercooling zone in the Al-rich pockets,and thus,the nucleation rate.The nuclei grow up in nondendritic mode,resulting in spheroidal,at least,nondendritic grains.In a successful CDS,the superheat degrees of the two precursor melts should be limited within several degrees,and it is not necessary to extra stipulate the superheat degree of target alloy melt(Al-8Si) when the requirement about Gibbs energies of the three melts is matched.Subsequent observation on casting microstructures shows that the employed simulation and calculation processes are reasonable and the achieved results are reliable.

Effects of Y content on microstructures and mechanical properties of as-cast Mg-Zn-Nd alloys

The effects of Y addition amount on the microstructures and mechanical properties of as-cast MgZn-Nd alloy have been investigated by using an optical microscope, a scanning electron microscope, backscattered electronic imaging technique, an X-ray diffractometer, a differential thermal analyzer and a universal testing machine. There are three kinds of ternary phases in the Mg-Zn-Y system alloys, such as I phase(Mg3Zn6Y), W phase(Mg3Zn3Y2) and Z or X phase(Mg12Zn Y). The experimental results in the present study indicate that the Mg-Zn-RE(RE includes Y and Nd) ternary phases change from the I + W phases in turn to unique W, W + Z and unique Z as the Y content increases from 0% to 3%. Simultaneously, their distribution gradually changes from small particle-like form to continuous network form. The grain size first decreases as the Y content increases from 0% to 1% Y, then increases when the Y content exceeds 1% and finally decreases again when the content exceeds 3% due to the variation of growth restriction factor caused by the increased Y element and the change of the ternary phases. The hardness continuously increases because of the increased ternary phase amount. The ultimate tensile strength and elongation first increase within the range of 0-1% Y, also due to the increased ternary phase amount and grain refinement, and then decreases because of the grain coarsening, porosity formation and continuous network distribution of the ternary phases. The grain bonding strength of the W phase-containing alloys is quite strong and the W phase is an ideal strengthening phase if a given amount of it distributes in discontinuous and small-sized form. The alloy with 1% Y only has one ternary phase of W, but has the best combination of mechanical properties. The fracture regimes of these alloys always present a transgranular mode.

Solidification behavior of 6061 wrought aluminum alloy during rheo-diecasting process with self-inoculation method

The semisolid slurry of the 6061 wrought aluminum alloy was prepared by the self-inoculation method（SIM）. The effects of the isothermal holding parameters on microstructures of rheo-diecastings were investigated, and the solidification behavior of 6061 wrought aluminum alloy during the rheo-diecasting process was analyzed using OM, SEM, EDS and EBSD. The results indicate that the isothermal holding process during slurry preparation has great effect on primary α（Al） particles（α1）, but has little effect on the microstructure of secondary solidification in the process of thin-walled rheo-diecasting. Nucleation is expected to take place in the entire remaining liquid when the remaining liquid fills the die cavity, and the secondary solidification particles（α2） are formed after the process of stable growth, unstable growth and merging. The solute concentration of remaining liquid is higher than that of the original alloy due to the existence of α1 particles, hence the contents of Mg and Si in α2 particles are higher than those in α1 particles.

Effects of melt treatment temperature and isothermal holding parameter on water-quenched microstructures of A356 aluminum alloy semisolid slurry

The semisolid slurry of the A356 aluminum alloy was prepared by self-inoculation method(SIM),the effects of melt treatment temperatures and isothermal holding parameters on water-quenched microstructures of A356 aluminum alloy semisolid slurry were investigated,and the solidification behavior of the remaining liquid phase(secondary solidification)was analyzed.The results indicate that the melt treatment temperature has significant effects on the final semisolid microstructures.The semisolid slurry which is suitable for the rheological forming can be produced when the melt treatment temperature is between 680 and 690°C.During the isothermal holding process,the growth rate of the primary particles conforms to the dynamic equation of Dt 3-Do3=Kt,and the coarsening rate of the primary particles is the fastest when the isothermal holding temperature is 600°C.Additionally,the isothermal holding time also has obvious effect on the secondary solidification microstructures.The secondary particles are the smallest and roundest when the isothermal holding time is 3 min.The amount of the secondary particles gradually increases with the increase of isothermal holding temperature,and the eutectic reaction therefore is confined into small intergranular areas,contributing to the compactness of the final solidified eutectic structures.

Effects of Cu addition on microstructureand mechanical properties of as-cast Mg-6Znmagnesium alloy

The application of Mg-Zn binary alloys is restricted due to their developed dendritic microstructure and poor mechanical properties. In this study, an alloying method was used to improve the mechanical properties of Mg-Zn alloy. The Mg-6Zn magnesium alloys microalloyed with varying Cu content(0, 0.8, 1.5, 2.0 and 2.5wt.%) were fabricated by permanent mould casting, and the effects of Cu content on the microstructure and mechanical properties of as-cast Mg-6Zn alloys were studied using OM, SEM, XRD and tensile tests at room temperature. The obtained results show that the addition of Cu not only can refine the grains effectively, but also can modify the eutectic morphology and improve the mechanical properties of the alloys. The main phases of the studied alloys include α-Mg, MgZn_2, Mg_2Cu and CuMgZn. When the content of Cu exceeds 0.8wt.%, Mg_2Cu phase appears. Meanwhile, the eutectic morphology is modified into dendritic shape or lamellar structure, which has an adverse effect on the tensile properties. Furthermore, among the investigated alloys, the alloy containing 0.8% Cu shows an optimalultimate tensile strength of 196 MPa, while the alloy with 1.5wt.% Cu obtains an excellent elongation of 7.22%. The experimental alloys under different Cu contents show distinguishing fracture behaviors: the fracture of the alloy with 0.8wt.% Cu reveals a mixed mode of inter-granular and quasi-cleavage, while in other investigated alloys, the fracture behaviors are dominated by cleavage fracture.

Effects of punch velocity on microstructure and tensile properties of thixoforged Mg2Sip/AM60B composite

The effects of punch velocity on the microstructures and tensile properties of Mg2 Sip/AM60 B composite were investigated.In comparison,the tensile properties of the permanent mold casting of this composite were also analyzed.The results indicate that the punch velocity obviously influences the microstructure through changing the secondary solidification behaviors and semisolid deformation mechanisms.The variations of the microstructures and deformation mechanisms are responsible for the changes in tensile properties and fracture modes of the composites.The best comprehensive tensile properties of this composite are obtained under the punch velocity of 60 mm/s.The resulting ultimate tensile strength and elongation of the composite are found to be 198 MPa and 10.2%,respectively.The excellent tensile properties of the thixoforged composite are ascribed to the elimination of porosities and the work hardening.

Effects of Sm addition on microstructural evolution of Mg-6Zn-0.4Zr alloy during semi-solid isothermal heat treatment

The application of segmental semi-solid thixoforming of magnesium alloys is confined due to the dimensional distinction existing in solid particles of the alloy billet from edge to center zones. In the present study, the effects of Sm addition on the microstructural evolution of Mg-6Zn-0.4Zr and Mg-6Zn-4Sm-0.4Zr alloys by semi-solid isothermal heat treatment were investigated, to obtain optimum semi-solid microstructures for the subsequently thixoforming. The results indicate that the grains of the Sm-bearing alloy are evidently refined and gradually evolve from dendritic to globular and elliptic particles. In addition, the distinctly dimensional effect of the Mg-6Zn-0.4Zr alloy is eliminated with 4% Sm addition; the particle sizes in all zones from center to the edge of the billet are almost identical. With the increment of isothermal heat treatment temperature, the dendritic microstructures completely disappear, and meanwhile, the irregular and globular particles gradually form. The size, morphology and the distribution of solid particles mainly depend on the formation and permeation of the liquid phase in the process of isothermal heat treatment. As the isothermal temperature increases from 570 °C to 610 °C, the average size and shape factor of solid particles of both the alloys with and without Sm addition gradually decrease while the liquid fraction gradually increases.

Efficient fabrication of semisolid nondendritic Al alloy slurry with high quality

Subjecting a normal mechanical vibration to a cooling slope plate,is a proposed method for preparing semisolid nondendritic slurry,named shear-vibration coupling sub-rapid solidification(SCS).Taking Al-8Si alloy as model material,the temperature field and distribution field of solid or liquid phase during SCS were simulated using COMSOL Multiphysics software to primarily choose the optimal processing parameters.Subsequently,the slurries were prepared with the parameters selected according to the simulation results and the microstructures of the slurries were experimentally investigated.Results indicate that the simulation results could provide a basis for roughly choosing the processing parameters,although the calculated solid fractions are always higher than the experimental ones.The processing parameters affect the primary grain size,shape factor and solid fraction mainly through altering the contact duration of melt on the plate,and thus affecting the cooling effect on the melt,nucleation rate,and grain dissociation and proliferation.Experiments with optimized processing parameters show that the primary grains in the slurry have an average size of about 32μm and shape factor of 1.38,and are quite uniform,even at the highest pouring rate of 2.81 kg·s^(-1),the size and shape factor are about 46μm and 1.7,respectively,which implies that the proposed SCS is a promising technology for efficient fabrication of high-quality Al slurry available for engineering applications.

Microstructure and crystal growth direction of Al-Mg alloy

The microstructures and crystal growth directions of permanent mould casting and directionally solidified Al-Mg al oys with different Mg contents have been investigated. The results indicate that the effect of Mg content on microstructure is basical y same for the al oys prepared by these two methods. The primary grains change from cel ular crystals to developed columnar dendrites, and then to equiaxed dendrites as the Mg content is increased. Simultaneously, both the cel ular or columnar grain region and the primary trunk spacing decrease. Al of these changes are mainly attributed to the constitutional supercooling resulting from Mg element. Comparatively, the cellular or columnar crystals of the directionally solidified alloys are straighter and more paral el than those of the permanent mould casting al oys. These have straight or wavy grain boundaries, one of the most important microstructure characteristics of feathery grains. However, the transverse microstructure and growth direction reveal that they do not belong to feathery grains. The Mg seemingly can affect the crystal growth direction, but does not result in the formation of feathery grains under the conditions employed in the study.

Comparison of contribution of sub-rapid cooling and shear deformation to refinement of Fe-rich phase in hypereutectic Al-Fe alloy during rheo-extrusion

A hypereutectic Al-3Fe(wt.%)alloy was subjected by rheo-extrusion,and the effect of sub-rapid cooling and shear deformation on the refinement of Fe-rich phase was investigated.The results showed that both the primary Fe-rich phase and eutectic Fe-rich phase in the solidified Al-Fe alloy were finer than the platelike Fe-rich phase in the as-cast Al-Fe alloy with the same content of Fe.The solidified Al-Fe alloy was subjected to three stages of shear deformation,and both the primary Fe-rich phase and eutectic Fe-rich phase were fractured and the average length was refined to 400 nm,while Fe-rich phase in the as-cast Al-3Fe(wt.%)alloy was platelike and its average length was 40 pm.The tensile strength and elongation of the hypereutectic Al-3Fe(wt.%)alloy containing nanosized Fe-rich phase were 162 MPa and 25.78%while those of the as-cast AI-3Fe(wt.%)alloy containing coarse platelike Fe-rich phase were 102 MPa and 16.84%,respectively.In the refineme nt of Fe-rich phase in hypereutectic Al-Fe alloy during rheo-extrusion,the three stages of shear deformation contributed more than sub-rapid cooling.