Microstructure characteristics and solidification behavior of wrought aluminum alloy 2024 rheo-diecast with self-inoculation method

One important problem in casting wrought aluminum alloys is the high tendency to the formation of hot tears in the solidification process.By using semi-solid metal(SSM) processing,the hot tearing tendency of alloys can be minimized during casting.In the present research,the semi-solid slurry of wrought aluminum alloy 2024 was firstly prepared with a novel self-inoculation method(SIM),and then the microstructure characteristics of the semi-solid slurry and the rheo-diecastings cast with the semi-solid slurry were investigated.The results indicate that finer and more uniform globular primary α-Al particles can be obtained when the semi-solid slurry are isothermally held for a short period within the semi-solid temperature range,and the primary α-Al particles without entrapped liquid are uniformly fine,globular grains in the rheo-diecastings.The holding temperature and time affect the solid fraction,particle size,and shape factor.After the semi-solid slurry is held at 625 ℃ for 3 min and 5 min,the optimal values for the average equivalent diameter are 70.80 μm and 74.15 μm,and for the shape factor are 1.32 and 1.42,respectively.The solidification process of the rheo-diecastings is composed of the following two distinct stages:primary solidification process and secondary solidification process.The secondary solidification process consists further of the following three stages:(1) direct growth of secondary primary(α 2) phase from the surface of the primary α-Al phase particles without re-nucleation,(2) independent nucleation and growth of α 3 phase from the residual liquid,and(3) eutectic reaction at the end.

Microstructure evolution of ZA72 magnesium alloy during partial remelting

The application of some semi-solid forming magnesium alloys is restricted due to their weak mechanical properties. To improve the mechanical properties, it is necessary to research the regularity and theory of semi-solid microstructure evolution of the alloy. In this study, microstructure evolution of ZA72 alloy during the partial remelting, and the effect of holding temperature and holding time on the semi-solid microstructure evolution of ZA72 magnesium alloy were investigated by means of OM, SEM and EDS analysis. The results indicate that the microstructure with small and spheroidal semi-solid particles which are available for thixo-forming can be obtained using proper heating parameters. After being isothermally treated at between 580 and 610 ℃ for 30 min, the equivalent size and shape factor of primary solid phase of ZA72 alloy decrease gradually, while the liquid volume fraction increases. When isothermally treated at 600 ℃ and held for different times from 15 to 60 min, with the increase of holding time, the equivalent size of primary particles decreases at first and then increases gradually; while the shape factor decreases gradually. The best heat treatment parameters in this experiment are to hold at 610 ℃ for 30 min. Compared with as-cast ZA72 alloy, the sizes of the eutectic phase and second α-Mg phase obtained in semi-solid state are smaller due to the higher solidification rate and the higher under-cooling degree than as-cast state. These decrease the fracture probability during tensile stress and improve the properties of the ZA72 alloy by semi-solid forming.

Microstructural evolution during partial remelting of AM60B magnesium alloy refined by MgCO_3

AM60B magnesium alloy was refined by MgCO3 and its microstructural evolution was investigated during partial remelting.The results indicate that MgCO3 is an effective grain refiner for AM60B alloy and can decrease the grain size from 329 μm of the unrefined alloy to 69 μm.A semisolid microstructure with small and spheroidal primary particles can be obtained after being partially remelted.The microstructure evolution can be divided into four steps:the initial rapid coarsening, structure separation, spheroidization and final coarsening.Correspondingly, these four steps result from the phase transformations of β→α, α+β→L and α→L, α→L and two reverse reactions of α→L and L→α, respectively.One spheroidal primary particle in the semisolid microstructure usually originates one dendrite in the as-cast microstructure.The variation of primary particle size with holding time does not obey the LSW law, Dt3?D03=Kt, after the semisolid system is in its solid-liquid equilibrium state.Longer heating duration makes the primary particles more globular, but it makes their size larger at the same time.

Microstructure of partially remelted billet of AM60 alloy prepared with self-inoculation method

The billets of AM60 alloy, prepared with self-inoculation method, were partially remelted into semisolid state.Effects of process parameters on remelting microstructure of semisolid billet were investigated.Experimental results show that the solid particles obtained with self-inoculation method are in smaller grain size and globular shape after partial remelting, compared with those prepared with other casting methods.In the optimized process conditions, the average size of solid particles of partially remelted billet is 65 μm, and the shape factor is 1.12.The process parameters, i.e.pouring temperature, addition amount of self-inoculants, and the slope angle of multi-stream mixing cooling channel have influence on the microstructure of partially remelted billet.The optimized temperature is from 680 °C to 700 °C, addition amount of self-inoculants is between 5% and 7% (mass fraction), slope angle of multi-stream mixing cooling channel is between 30° and 45°, with which the dendritic microstructure of as-cast billet can be avoided, and the size of solid particles of remelted billet is reduced.

Phase-field model of isothermal solidification with multiple grain growth

This paper develops a new phase-field model for equiaxed dendrite growth of multiple grains in multicomponent alloys based on the Ginzberg-Landau theory and phase-field model of a single grain. Taking Al-Cu and Al-Cu-Mg alloys for example, it couples the concentration field and simulates the dendrite growth process of multiple grains during isothermal solidification. The result of the simulation shows dendrite competitive growth of multiple grains, and is reapplied to the process of dendrite growth in practical solidification.

Commercial AM60 alloy for semisolid processing:Effects of continuous rheoconversion process on microstructure

In order to verify the effect of CRP(continuous rheoconversion process) in the preparation of magnesium alloy semisolid billets,AM60 alloy billets were fabricated with CRP,and effects of pouring temperature,slope angle,and length of the reactor on the microstructure of AM60 alloy were investigated.The results show that the grain size is reduced with the decrease of pouring temperature.A small block/rosette grain is obtained when the pouring temperature is less than 680 °C.Therefore,the available temperature process interval/window has to be required.To change slope angle of reactor(from 30° to 45°) is helpful for formation of the small and block/rosette grains.Meanwhile,the reactor with a length of 500 mm is enough for copious nucleation of primary phase.CRP changes the solidification microstructure of billets by controlling the nucleation and growth of the primary phase in melt.

Effects of increase extent of voltage on wear and corrosion resistance of micro-arc oxidation coatings on AZ91D alloy

The effects of increase extent of voltage on the wear resistance and corrosion resistance of micro-arc oxidation(MAO) coatings on AZ91D magnesium alloy were investigated in silicate electrolyte.The results show that with increasing extent of voltage, both of the thickness and bonding force of MAO coatings first increase,and then decrease.These parameters are all up to their maximum values when the increase extent of voltage is 20 V.The roughness of the coatings always increases.The coating has the best corrosion resistance when the increase extent of voltage is not below 25 V,and the coating has the best wear resistance when the increase extent of voltage is 10 V.The wear mechanisms for the micro-arc oxidation are abrasive wear and micromachining wear. These are related to their microstructures.

Grain refining technique of AM60B alloy by Al-Ti-B master alloy

The effects of grain refining parameters on grain size of AM60B magnesium alloy have been investigated using an Al-5Ti-1B master alloy as refiner; and an appropriate refining technique has been developed. The results indicate that the Al-Ti-B master alloy is an effective grain refiner for AM60B alloy and the grain size can be decreased from 348 μm to 76 μm. Raising the addition temperature or the pouring temperature is beneficial for grain refinement; while for the addition amount and holding time, there is an optimal value. The appropriate grain refining technique is that 0.3% Al-Ti-B master alloy is added at 780oC and then the melt is held for 30 min before pouring. The above phenomena can be explained by the refining mechanisms that have been proposed from the related studies on Al and Mg alloys and theoretical analysis.

The numerical analysis of thermal effect and weld shaping of DE-GMAW under composite heat source

The action of the composite heat source model in DE-GMAW has been carried out according to the characteristics of the DE-GMA W process, and the distribution of surface and body heat source was analyzed. The weld temperature field distribution has been derived from the appropriate boundary conditions and the thermal physical property parameters by COMSOL software. The effects of the positions of the surface and body heat source on the cross- sectional shape were investigated by studying the experimental welding thermal cycle curve. The simulated results are fully compliant with the experimental data under the same conditions. This illustrates that the composite heat source model is correct. It reflects the thermal mechanism of DE-GMA W process, and reveals the internal influence of weld shaping.

Microstructure and crystal growth direction of Al-Mg alloy

The microstructures and crystal growth directions of permanent mould casting and directionally solidified Al-Mg alloys with different Mg contents have been investigated. The results indicate that the effect of Mg content on microstructure is basically same for the alloys prepared by these two methods. The primary grains change from cellular crystals to developed columnar dendrites, and then to equiaxed dendrites as the Mg content is increased. Simultaneously, both the cellular or columnar grain region and the primary trunk spacing decrease. All 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 parallel than those of the permanent mould casting alloys. 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.

Thermodynamic analysis of Ti powder synthesized by SHS thermitreaction

According to the thermodynamics theories,the reactive Gibbs free energies,the reactive adiabatic temperature,the melting rate of Ti and the gasification mass of Mg in the Mg-TiO2 and Al-TiO2 systems were theoretically calculated and analyzed respectively. The results show that the reactions of Mg-TiO2 and Al-TiO2 are very easy to take place and the reaction of producing various suboxides of Ti may occur in Mg-TiO2 and Al-TiO2 reaction system;the adiabatic temperature of Mg-TiO2 becomes lower with increasing mass fraction of Mg. The adiabatic temperature is below 1 800 K when the mass fraction of excessive Mg exceeds 25%;The adiabatic temperature of Al-TiO2 also becomes lower with increasing mass fraction of Al,but it becomes higher with the preheat temperature increment. The adiabatic temperature plateau is the result of Ti melting endotherm;owing to the gasification of a great deal of Mg in Mg-TiO2 reaction process,Mg should be properly excessive in order to get Ti.

Commercial AM60 alloy for semisolid processing:Alloy optimization and thermodynamic analysis

Semisolid processing is now a commercially successful manufacturing route to produce net-shape parts in automotive industry.The conspicuous results of alloy optimization with thermodynamic simulations for semisolid processing of commercial AM60 alloy were present.The results indicate that the available processing temperature range of AM60 alloy is 170 °C.The temperature sensitivity of solid fraction decreases with increasing solid fraction or with decreasing temperature above eutectic reaction temperature of AM60 alloy.When the solid fraction φs is 0.4, corresponding processing temperature is 603.8 °C and the sensitivity -dφs/dT is 0.0184.The effects of various alloying elements on the solidification behavior and SSM processability of AM60 alloy were calculated with Pandat software.

Electro-catalytic degradation properties of Ti/SnO_2–Sb electrodes doped with different rare earths

Ti/SnO2–Sb electrode has a good effect on the removal of organic pollutants. But its short service life limits its large-scale application in industry. Electro-catalytic degradation performances and service life of the electrode can be significantly improved by doping rare earth（RE） ions into the oxide coating of Ti/SnO2–Sb electrode. Ti/SnO2–Sb electrodes doped with different RE elements（Ce, Dy, La, and Eu） were prepared by the thermal decomposition method at 550 ℃. Electro-catalytic degradation performances of electrodes doped with different RE elements were evaluated by linear sweep voltammetry（LSV） and Tafel curves. During the electrolysis,the conversion of p-nitrophenol was performed with these electrodes as anodes under galvanostatic control. The structures and morphologies of the surface coating of the electrodes were characterized by scanning electron microscope（SEM）. The results demonstrate that the electro-catalytic degradation performances of Ti/SnO2–Sb electrodes are improved to different levels by doping different RE ions. Improved Ti/SnO2–Sb electrodes by the introduction of different RE have higher oxygen evolution potential, better electro-catalysis ability, better coverage,and longer electrode life.