Intragranular Liquid Droplets Research Articles

Comparison of Semisolid Microstructure Evolution of Wrought Nickel Based Superalloy GH4037 with Different Solid Fraction

The research on semisolid metal forming of Ni-based superalloys is limited because of the difficulties in preparing semisolid billets. Traditional methods for fabricating semisolid billets are SIMA and RAP. In order to decrease the processing procedures, a new method called semi-solid isothermal treatment of wrought superalloy (SSITWS) was used to fabricate semisolid billets in this paper. In order to get the optimal parameters to fabricate Ni-based GH4037 semisolid billets, the effects of isothermal temperature and soaking time on microstructure evolution were investigated. The isothermal temperature ranges from solidus to liquidus, and the soaking time ranges from 10min to 60min. The results show that near-globular grains can be obtained only in the semi-solid state with appropriate solid fractions. When the solid fraction was high, the shape of grains was irregular and the average size of grains was large and inhomogeneous. When the isothermal temperature reached 1350°C, spherical grains appeared, and many small intragranular liquid droplets existed. During the appropriate isothermal temperature range, from 1350°C to 1380°C, the shape of solid grains was globular and the size of solid grains was homogeneous. At the same time, the size of the grains and the roundness of the grains increased with the increasing of isothermal temperature.

Correlation between the Liquid Fraction, Microstructure and Tensile Behaviors of 7075 Aluminum Alloy Processed by Recrystallization and Partial Remelting (RAP)

The recrystallization and partial remelting (RAP) method was applied to obtain the semisolid 7075 aluminum alloy with different liquid fractions. The effects of liquid fraction on the microstructure and tensile properties were determined in detail. The results show that during the semisolid isothermal treatment, the number of the intra-granular liquid droplets increased initially with the melting of the eutectic phases. Extension of isothermal soaking led to the coarsening and spheroidization of the intra-granular droplets. Finally, these liquid droplets merged and moved towards the grain exterior. The room temperature tensile strength of the RAP-processed AA7075 alloy, which were isothermally soaked at 600 and 610 °C, increased with the holding time from 5 to 15 min and then decreased dramatically from 15 to 25 min, whilst that soaked at 620 °C decreased monotonously. The fracture morphology exhibited intra-granular fracture mode at low liquid fractions. However, it transformed to a completely brittle and inter-granular type at high liquid fractions and the cohesive force of the liquid-solid interfaces at the grain boundaries determined the strength of the alloys. The transfer of the intra-granular liquid droplets into the inter-granular liquid phase played a significant role for the different fracture behaviors of the RAP-processed AA7075 alloy. The paper provides some reference for better controlling the microstructure and mechanical properties in semisolid processing.

Microstructure evolution of wrought nickel based superalloy GH4037 in the semi-solid state

Study on semi-solid processing of superalloy is little reported due to the difficulties in fabricating semi-solid billets. In this manuscript, a novel method called semi-solid isothermal treatment of wrought superalloy (SSITWS) was firstly proposed in order to fabricate semi-solid billets of superalloy. The effects of isothermal temperature and soaking time on microstructure of Ni-based superalloy GH4037 in the semi-solid state were investigated. The results show that the near-globular microstructure of wrought GH4037 superalloy can be obtained by the semi-solid isothermal treatment. The average grain size increased and the roundness decreased with the prolonged soaking time at 1350 °C and 1360 °C. When the isothermal temperature was elevated to 1370 °C or 1380 °C, the microstructure evolution was complex due to the combination of breaking up mechanism and coarsening mechanism. At 1390 °C, the dendritic grains were obviously observed in the microstructure. With the increasing of isothermal temperature and soaking time, the liquid boundary film coarsening behavior occurred in the microstructure. With the increasing of soaking time, the size of intragranular liquid droplets increased, but the amount of them decreased. When the isothermal temperature was high, the intragranular liquid droplets disappeared. The coarsening rate constant at 1360 °C was lower compared to that at 1350 °C. However, when the isothermal temperature was higher than 1370 °C, the coarsening kinetics was not suitable for describing the microstructure evolution. The discrete carbide precipitates appeared in the grain boundaries. The distribution of Ti and Mo were higher in the grain boundaries than that in solid grains, and the distribution of other elements was homogenous in the grain boundaries and inside the grains.

Effects of isothermal treatment parameters on the microstructure of semisolid alloys

ABSTRACTCold-rolled 6061 aluminium alloys are used to prepare semisolid billets by recrystallisation and partial melting. The effects of isothermal treatment parameters on the microstructures were investigated. The results indicated that the high isothermal holding temperature increases the experimental liquid fraction, average grain size and shape factor. A long isothermal holding time also increases the experimental liquid fraction and improves the spheroidisation degree of solid grains, but the average grain size also increases. Moreover, when the isothermal holding time at high temperature is increased, the size of intragranular liquid droplets increases, but their quantity decreases. The optimal isothermal holding temperature and time during semisolid isothermal treatment were 635°C and 5 – 10 min, respectively.

Thixoforming of AM50 magnesium alloy

To obtain AM50 magnesium alloy slurry with fine microstructure, a new process including equal channel angular extrusion (ECAE) and partial remelting treatment has been developed. The evolution of grains from the ECAE-formed AM50 alloy in the semi-solid stage has been studied. Furthermore, thixoforming of ECAE-formed AM50 alloy was conducted, and the mechanical properties of thixoformed components were investigated. Instead of involving dendritic in the as-cast alloy, the microstructure of alloy produced by ECAE contained plenty of recrystallized grains along the deformation direction. During the partial remelting, the liquid phase penetrates not only to the grain boundaries but also to the inside of solid grains. An increase in reheating temperature leads to a corresponding enhanced liquid fracture, and the result improved the degree of spheroidization. Prolonged the holding time, both the solid grains and intragranular liquid droplets had undergone coarsening. The tensile properties of thixoformed from ECAE state are much higher than those from as-cast state. The research encourages the belief that magnesium alloy semi-solid billet with fine and spheroidal grains, which is suitable for semi-solid processing, could be obtained by the combination of ECAE and partial remelting.

Microstructure evolution of SiCp/ZM6 (Mg–Nd–Zn) magnesium matrix composite in the semi-solid state

The first reported microstructural study of Mg-RE alloy matrix composites in the semi-solid state was presented. Mg–Nd–Zn (ZM6) alloy matrix composites reinforced with SiC particles were fabricated by mechanical stir casting and extrusion. Semi-solid slugs of SiCp/ZM6 composites were prepared by semi-solid thermal transformation (SSTT) route and recrystallization and partial melting (RAP) route, respectively. The microstructure evolution of as-cast and as-extruded SiCp/ZM6 composites during semi-solid remelting was examined under the same conditions. The coarsening mechanisms of SSTT and RAP composites were discussed. The results show that the microstructures of RAP SiCp/ZM6 composites consist of fine and spheroidal solid grains surrounded by intergranular liquid, while the SSTT composites consist of inferior microstructures with coarse solid grains and high proportion of intragranular liquid droplets. The solid grain coarsening during partial remelting is dominated by coalescence and Ostwald ripening for SSTT and RAP SiCp/ZM60 composites, respectively. The formation of intragranular liquid droplets and entrapped SiC particles were also discussed.

Effect of accumulative plastic deformation on generation of spheroidal structure, thixoformability and mechanical properties of large-size AM60 magnesium alloy

Accumulative plastic deforming (APD) at multistep temperatures was induced into the recrystallization and partial melting (RAP) route as a novel predeformation method to replace conventional upsetting or extrusion. AM60 magnesium alloy ingots with diameter of 520mm were produced by semi-continuous casting, and semi-solid slugs with diameter of 320mm were prepared by partial remelting of as-cast and APD formed alloys. The microstructure evolution during semi-solid remelting was investigated. The tensile mechanical properties of AM60 alloy at room temperatures were determined. The results show that large semi-solid AM60 alloy slugs with fine and homogeneous microstructures can be produced by employing APD in RAP route, and the microstructure consists of spheroidal solid grains surrounded by intergranular liquid matrix. The as-cast AM60 alloy during partial remelting showed inferior microstructures with coarsened solid grains and high proportion of intragranular liquid droplets. During partial remelting, coalescence and Ostwald ripening are dominant in as-cast alloy and APD formed alloy, respectively. The formation and migration mechanism of intragranular liquid droplets were discussed. Although both of APD formed and as-cast AM60 slugs could fill the die during thixoforming process, the tensile mechanical properties of the components thixoformed from APD formed alloys far exceed those from the as-cast alloys.

Microstructural coarsening of 7005 aluminum alloy semisolid billets with high solid fraction

In the present study, 7005 aluminum alloy billets were supplied in the warm extruded and T6 heat treated state. They were then reheated to the semisolid state and the microstructural evolution and coarsening investigated. This is necessary groundwork if the billets are to be formed to shape by semisolid processing, an innovative manufacturing route which has recently gained wide use for a range of aluminum alloys but which has not yet been applied to 7005. The results showed that the average spheroid size, liquid film thickness and liquid fraction of 7005 aluminum alloy increased with increase in soaking time. Intragranular liquid droplets were present in solid grains, and coarsened with increase in soaking time and isothermal temperature. The roundness of globules of solid in the semisolid state at 600°C–615°C was in the range of 1.5 to 2.0 after soaking for periods up to 12min at 620°C, but for times greater than 15min was more than 2, which is unlikely to be suitable for the semi-solid forming process, where smooth flow is required and therefore relatively round globules. When the isothermal temperatures were 600°C, 610°C, 615°C and 620°C, the coarsening rates were 542μm3s−1, 606μm3s−1, 683μm3s−1 and 688μm3s−1, respectively. The coarsening rate K increased with the increase of isothermal temperature. These values place this alloy on the dividing line between typical coarsening rates for normally cast alloys and typical coarsening rates for normally wrought alloys. This is attributed to the fact that the alloy has a relatively simple precipitation hardening system in comparison with other 7xxx alloys and the hardening precipitate (MgZn2) melts below the temperature the experiments have been carried out at, hence preventing the precipitates inhibiting liquid film migration and diffusion along narrow liquid films at spheroid boundaries. Coarsening tends to occur via Ostwald ripening and coalescence, but Ostwald ripening plays an increasing role with the increase of isothermal temperature.

Study on Microstructural Evolution of Deformed Magnesium Alloy during Partial Remelting

Microstructural evolution of ZK60 Mg alloy during partial remelting was investigated in this paper. The results show that ZK60 Mg alloy semi-solid billets were successfully produced by recrystallisation and partial remelting (RAP) process. Following partial remelting in each case, grain coarsening had occurred in the semi-solid state with increasing holding time. Liquid was present, during partial remelting all four temperatures, in the form of isolated intragranular droplets. These liquid droplets were produced by intragranular eutectic phase and entrapped liquid. Intragranular liquid droplets migrated and merged into some large rounded liquid droplets. After the spheroidization was completed, coarsening and coalescence occurred. When temperature is relatively low (the thickness of liquid is relatively thin), grains contacted with each other, which produced irregular-shaped grains.

Observation of the formation of liquid droplets in steep temperature gradients

Rods of a homogenized coarse-grained Al-4wt.%Cu alloy were subjected for a short time (in the order of seconds) to a steep temperature gradient and (partial) melting. θ-phase or eutectic particles are visible in the resolidified micros true ture. These microstructural features are interpreted as traces of former liquid regions, particularly intragranular liquid droplets. The former droplets were characterized with respect to their morphology and size. The bimodal size distribution indicates two classes of nucleation sites in the solid.

Effect of RE elements on the microstructural evolution of as cast and SIMA processed Mg–4Al alloy

In the present article, the effect of Rare Earth elements on the microstructural development of as cast and semisolid Mg–4Al alloy produced by SIMA process is studied. Investigation conducted by metallographic observation, scanning electron microscope and quantitative metallographic methods. Results showed that alloy's dendrites turn into larger fully dendritic shape with sharp and narrow arms from equiaxed rosette type as the amount of RE elements increased from 0 through 4%. The effect of RE elements on the microstructure of the treated-alloys was detectable through elimination of vast number of intragranular liquid droplets as well as by decreasing kinetic of microstructural changes. It was shown that the trend of grain coarsening decreased drastically by addition of RE elements to the Mg–4Al alloy during partial remelting at 610 °C. Moreover, the effect of REs on the other parameters such as fraction of liquid, shape factor and particle size was studied.

Microstructural evolution and tensile mechanical properties of thixoforged ZK60-Y magnesium alloys produced by two different routes

Semi-solid processing of magnesium alloys is generally based on conventional magnesium-based casting alloys such as Mg–Al series. However, these casting alloys do not give such high mechanical properties as the alloys that are conventionally wrought such as Mg–Zn series. In this paper, a ZK60 magnesium alloy with the addition of Y was thixoforged. The semi-solid thermal transformation (SSTT) route and the recrystallisation and partial melting (RAP) route were used to obtain the semi-solid feedstocks for thixoforging. Microstructural evolution during partial remelting was studied at temperatures for times. Tensile mechanical properties of thixoforged components at room temperature were examined. Results show that a fine spheroidal microstructure can be obtained by the RAP route. Compared to the RAP route, the SSTT alloy shows coarsened solid grains with a relatively high proportion of intragranular liquid droplets. With prolonged holding time, the solid grain sizes of the SSTT alloy and the RAP alloy increased. Coalescence was dominant in the SSTT alloy and Ostwald ripening was dominant in the RAP alloy. Thixoforging for the SSTT alloy and the RAP alloy resulted in successful filling of the die. The tensile properties of the thixoforged RAP alloy were satisfactory and exceeded those of the thixoforged SSTT alloy. However, the mechanical properties of both the thixoforged SSTT alloy and the thixoforged RAP alloy decreased with prolonged holding time.

An investigation of microstructure evolution of RAP processed ZK60 magnesium alloy

The effect of compression ratio on the microstructure evolution of semi-solid ZK60 magnesium alloy produced by the recrystallisation and partial melting (RAP) process was investigated. The microstructure evolution within the semi-solid zone of a ZK60 alloy through a direct isothermal holding process from as-received and as-deformed conditions was also investigated. The results show that as-received alloys reheated to the semi-solid state had solid particles predominantly elongated along original extrusion direction. For as-deformed alloys, the compression deformation gradually refined the recrystallised α-Mg grains by increasing their nucleation to growth ratio. During isothermal holding, intragranular liquid droplets were present in a large number of solid particles, reducing in number and increasing in average size with increasing holding time. The coarsening behavior of solid particles in the semi-solid state obeyed an Ostwald ripening mechanism. The coarsening rate constant obtained was 556 μm 3s −1 at 590 °C.

Isothermal grain coarsening of spray formed alloys in the semi-solid state

Grain coarsening of alloys in the semi-solid state is important in controlling grain sizes in spray forming and in other processes in which an alloy is formed or cast in the semi-solid state. Until now, this coarsening has been analysed in terms of classic LSW theory or in terms of the migration of grain boundary liquid films. These analyses suggest that the coarsening rate should increase with increasing solid fraction, f S, but this suggestion conflicts with previous experimental results which show that coarsening rate decreases with increasing f S for f S>0.7. This paper shows, for the first time, that coarsening rate does increase with f S for f S less than approximately 0.75 in agreement with the liquid film migration model, and then decreases again with further increasing f S for f S greater than approximately 0.75. A modified model of liquid film migration is proposed which takes into account the reducing area of the liquid film as f S increases for high f S. The formation of intragranular liquid droplets, and the pinning of grain boundary liquid films by dispersoids during coarsening are also discussed.