Effect of Semi-solid Isothermal Heat Treatment on Microstructure of VW63Z Alloy

GONG Zhengxuan，PANG Song，JI Song，DONG Xiwang，HOU Xiangwu，CHEN Ge，HUA Xiru，WU Lili，JIANG Shanyao，XIAO Lü

（Shanghai Spaceflight Precision Machinery Institute，Shanghai 201600，China）

Abstract:The effects of isothermal heat treatment on the semi-solid microstructure evolution of VW63Z（Mg-6Gd-3Y-0.4Zr，wt.%）alloy are studied.It shows that the microstructure of VW63Z alloy could transform from equiaxed crystal to semi-solid spherical crystal after isothermal heat treatment above 620 °C.With the heating temperature elevating from 620 °C to 635 °C and the holding time prolonging from 10 min to 35 min，the liquid fraction increases gradually.The semi-solid microstructure evolution of VW63Z alloy can be divided into three stages，i.e.，particle coarsening and spheroidization；particle necking，coalescence，and Ostwald ripening；and dynamic equilibrium.The semi-solid process window of VW63Z alloy ranges from 620 °C to 635 °C，where the best process parameters are holding at 635 °C for 20 min-30 min.The solid fraction，the average particle size，and the shape factor are 41.1%-53.8%，81.5 μm-83.2 μm，and 0.70-0.75，respectively.The maximum relative deviations of the solid fraction，the particle size，and the shape factor at different heights of the same billet are 44.6%，17.4%，and 16.6%，respectively，which means that it should pay attention to the uniformity of edge and core of VW63Z alloy during isothermal heat treatment.The driving force of microstructure is supposed to be the reduction of solid-liquid interface free energy.

Key words:Mg-Gd-Y alloy；semi-solid；isothermal heat treatment；microstructure

0 Introduction

As the lightest metal structural material，mag?nesium alloys have the advantages of low density，high specific strength，high specific stiffness，and ex?cellent weight reduction potential.Semi-solid metal（SSM）processing is a widely accepted form?ing technology in light alloy forming in recent years.SSM processing applies external treatment to the al?loy in the semi-solid state to obtain a solid-liquid mixture where non-dendritic solid particles are uni?formly suspended in the liquid matrix，and uses this state of material to form components.The semi-sol?id forming technology has the advantages of high forming accuracy，few defects，and near-net-form?ing，and can be used to improve the mechanical properties of magnesium alloys.

At present，the application of magnesium alloys in SSM processing is limited to commercial magne?sium alloys such as AZ91D and AM60，the me?chanical properties of which are usually less than 250 MPa.Rare earth（RE）magnesium alloys，which are widely used in aerospace field，are still in the stage of experimental research.The research on RE magne?sium alloys mainly focuses on Mg-RE alloys，Mg-Zn-RE alloys and Mg-Gd-Y alloys，and it has been shown that Mg-Gd-Y is a promising material system with good comprehensive mechanical proper?ties.VW63Z alloy is a kind of Mg-Gd-Y alloys devel?oped by Shanghai Spaceflight Precision Machinery Institute based on the control of the total amount of solute atoms，and has been applied to large compo?nents with high weight reduction demand such as cab?ins.However，no study on the semi-solid application potential of VW63Z alloy has been found.

The SSM processing technology for RE magne?sium alloys mainly includes electromagnetic stir?ring，strain induced melt activation（SIMA），and isothermal heat treatment.Among them，the isothermal heat treatment method requires the alloy to be reheated to the semi-solid temperature and kept for certain time so as to make the dendritic crystal turn into spherical particles.This method has the merits of simple process，less oxidation，and less in?terference factors in the preparation of semi-solid al?loys，making isothermal heat treatment a widely used method in preparing semi-solid magnesium alloys.

The SSM processing potential can be evaluated in three dimensions，i.e.，the solid fraction，the aver?age particle size，and the shape factor.The opti?mum solid fraction for isothermal heat treatment is 40%-60%.When the solid fraction is below 40%，the billet would soften greatly and fail to maintain its original shape.When the solid fraction is high，the solid skeleton would form in the billet，which will af?fect the feeding of the remaining liquid phase and in?crease the difficulty of deformation coordination of solid particles.The average particle size of the solid phase should be small so as to have good fine grain strengthening ability.The shape factor should be as close to 1 as possible，which means that the solid par?ticle is a perfect sphere，so as to obtain good compre?hensive properties.

In this paper，semi-solid VW63Z alloy is pre?pared by isothermal heat treatment.The microstruc?ture evolution is observed，and the semi-solid micro?structure parameters of VW63Z alloy are quantita?tively analyzed.The relationships between the SSM processing parameters and the microstructure are in?vestigated，the semi-solid process window and optimal process parameters are determined，and the mechanism of organizational evolution is also discussed.

1 Experiment

1.1 Starting material

The VW63Z alloy ingot with a nominal compo?sition of Mg-6Gd-3Y-0.4Zr（wt.%）is prepared with pure Mg（purity 99.95%），Mg-20Y（%），Mg-20Gd（%），and Mg-30Zr（%）The raw materials are melted in an electric-resistant furnace，degassed，and then poured into a sand mold at 710 °C.The ac?tual composition of the ingot is analyzed by a plasma emission spectrometer（ICP，PerkinElmer Plas?ma400）and given as Mg-6.05Gd-2.97Y-0.41Zr.The obtained ingot with a diameter of 90 mm is cut into small billets with the dimensions of16 mm ×10 mm and30 mm × 20 mm，and the surfaces of the small billets are polished.

1.2 Isothermal heat treatment experiments and observation of microstructure

The semi-solid isothermal heat treatment is con?ducted in a muffle furnace under the protection of the SFatmosphere.The sample is packed with a sam?pling tool and placed on a steel plate as shown in Fig.1（a）.The steel plate is used to accelerate the heat conduction of samples.The samples are quickly quenched into water at room temperature after they are heated to the determined temperature and held for required time.The quenched samples with the dimen?sions of16 mm × 10 mm are cut along the cross section from the middle position to study the effects of temperature and holding time on the microstructure of semi-solid VW63Z alloy.The quenched samples with the dimensions of30 mm × 20 mm are cut along the cross section every 2.5 mm from the sample bottom to investigate the uniformity of edge and core.The specific sampling location is shown in Fig.1（b）.These samples are ground，polished，and etched in a 5% nitric acid solution before microstructure observa?tion，and are performed with an optical microscope（OM，Axio Scope.A1），a scanning electron micro?scope（SEM，FEI QUANTA 450），and an energy dispersive spectrometer（EDS）.Quantitative metallography is carried out by an image analysis soft?ware called Image Pro Plus to calculate the solid frac?tion，the average particle size，and the shape factor of the semi-solid samples.The shape factor（）is calculated by

Fig.1 Sample layout in the resistance furnace and sampling location for tissue observation

whereandare the area and the perimeter of a sin?gle particle，respectively，andis the number of par?ticles to be calculated.The closer shape factor is to 1，and the closer particle is to a perfect circle.At least 80 particles need to be calculated to obtain the shape factor in the experiment.

In order to determine the temperature of the semi-solid isothermal heat treatment，a differential scanning calorimeter（DSC，MDSC2910）is used to measure the DTA curve of as-cast VW63Z shown in Fig.2.The liquidus of VW63Z is 644 °C，and the solidus is approximately 603 °C.In order to obtain a relatively high liquid fraction，the temperature of the isothermal heat treatment is selected as 620 °C，625 °C，630 °C，and 635 °C，respectively，and the holding time is 35 min.To investigate the effect of holding time，the samples are held at 635 °C for 10 min，15 min，20 min，25 min，30 min，and 35 min，respectively.

Fig.2 DTA curve of as-cast VW63Z

2 Results

2.1 Microstructure of as-cast VW63Z alloy

The optical and SEM images of as-cast VW63Z are shown in Fig.3.It can be seen that the experiment alloy is mainly consist of the Mg（Gd，Y）phase.The α-Mg exhibits a typical fine equiaxed microstruc?ture with an average grain size of 60.2 μm.The long trip or whisker-like Mg（Gd，Y）phase distributes along the grain boundary，and tends to grow into the crystal.The contrast at the grain boundary in the SEM image is significantly higher than that inside the grain，as shown in Fig.3（b），indicating that the con?tent of the RE elements at the grain boundary is sig?nificantly higher than that inside the grain.

Fig.3 Microstructure of as-cast VW63Z

Continue Fig.3 Microstructure of as-cast VW63Z

2.2 Effects of the isothermal temperature on the microstructure of VW63Z alloy

The semi-solid microstructure of VW63Z is re?heated to 620 °C，625 °C，630 °C，and 635 °C and held isothermally for 35 min，respectively，and the microstructure is shown in Fig.4.It is shown that the microstructure is mainly consist of spheroidal primary α-Mg particles and liquid matrix.The holding temper?ature has a significant effect on the semi-solid micro?structure of the alloy.At the heating temperature of 620 °C（see Fig.4（a）），the eutectic phase dis?solves，the liquid film appears along the grain bound?ary，while tiny liquid droplets appear inside the α-Mg particles.The α-Mg particles grow larger but still re?tain obvious equiaxed crystal characteristics.At the heating temperature of 625 °C（see Fig.4（b）），the liquid film intersects and connects to form a continu?ous liquid matrix.The number and area of the liquid droplets inside the α-Mg particles increase remark?ably.Small equiaxed α-Mg crystals are transformed into large spherical particles.The size of the sphericalMg particles exhibits a“bimodal size”characteris?tic.Large particles with the diameter over 120 μm and small particles with the diameter less than 10 μm can be found simultaneously.

With the further elevation of temperature（see Fig.4（c）），the liquid fraction continues to in?crease，and the liquid droplets in the α-Mg particles converge to form a liquid pool.Most α-Mg particles have been fully spheroidized and uniformly dispersed in the liquid matrix，the size of which decreases slightly due to the dissolution caused by the high tem?perature，but the“bimodal size”characteristic can still be observed.When the holding temperature reaches 635 °C，a large number of α-Mg particles dis?solve，which leads to the decline in the particle size and the rise in the liquid fraction.

Fig.4 Microstructure of semi-solid VW63Z after isothermal heating for 35 min

The quantitative analysis results of the solid frac?tion，the average solid particle size，and the shape factor with heating temperatures varying from 620 °C to 635 °C are shown in Fig.5.In the same holding time，the solid fraction is negatively correlated with the heating temperature.With the temperature rising from 620 °C to 635 °C，the solid fraction gradually decreases from 83.9% to 37.1%.The average parti?cle size first grows from 60.2 μm to 106.9 μm at 625 °C，and then decreases to 78 μm at 635 °C.The size difference of the α-Mg particles at the heating temperatures of 625 °C and 630 °C.The shape factor ascends gradually with the increase in the tempera?ture and reaches the highest value at 0.88 at the heat?ing temperature of 630 °C.However，at a higher heating temperature，a deterioration of spheroidiza?tion can be found.

Fig.5 Effects of the isothermal temperature on the microstructure of semi-solid VW63Z

2.3 Effects of the isothermal holding time on the microstructure of VW63Z alloy

Based on the criterion that qualified semi-solid structure requires a moderate liquid phase rate，a small particle size，and a high shape factor，635 °C is selected to study the effect of the holding time on the semi-solid structure.The macrograph of semi-solid VW63Z after isothermal heating at 635°C for 10 min-35 min is shown in Fig.6.As shown in Fig.6，the samples soften along with the progress of the isother?mal heat treatment.When the isothermal holding time is 20 min，the sample can be deformed easily under the action of external force.When the isother?mal holding time is extended to more than 30 min，the sample is unable to maintain its original shape and collapses under its own gravity，indicating that VW63Z shows a certain degree of fluidity.

Fig.6 Macrograph of semi-solid VW63Z after isothermal heating at 635 oC for 10 min―35 min

The microstructure of semi-solid VW63Z reheated to 635 °C and held isothermally for 10 min-35 min is shown in Fig.7.When it is heating at a relatively high temperature for 10 min（see Fig.7（a）），the α?Mg particles are partially spheroidized，and begin to coalesce.Tiny droplets appear inside the α?Mg particles，and the liquid film in the grain boundaries forms a coarse liquid matrix.It can be seen that in the early stage of isothermal heating，the liquid area is separated by the α?Mg particles，and dispersed discontinuously.

With the holding time increasing from 10 min to 15 min（see Fig.7（b）），the liquid phase grows thicker and becomes dispersed continuously along the grain boundaries.More α?Mg particles are coalesced and coarsened，and exhibits the“bimodal size”characteristic.With the prolongation of holding time，the liquid fraction further rises.The liquid pool inside the particle connects to the liquid phase outside the particle，which results in the coalescence and necking of the α?Mg particles and thus the serious reduction in the shape factor of the α?Mg particles.When the holding time is longer than 30 min，the necking and coalescence of the α?Mg particles reach a dynamic equilibrium，and the α?Mg particles gradually spheroidize and coarsen slowly.

Fig.7 Microstructure of semi-solid VW63Z after isothermal heating at 635 oC

The variation curves of the solid fraction，the average solid particle size，and the shape factor with holding time are shown in Fig.8.In the same heating temperature，the solid fraction is negatively correlated with the holding time.With the holding time rising from 10 min to 35 min，the solid fraction gradually decreases from 87.2% to 37.1%.The average particle size first grows to 85.2 μm at 15 min，and then slightly decreases to 81.5 μm at 25 min.As the isothermal heat treatment continues，the average particles keep on growing.The shape factor first descends and then ascends with time，and the minimum value is obtained at 0.68 when the heating temperature is 630 °C.The best semi-solid process of VW63Z should be obtained in heating at 635 °C for 20 min-30 min.At this process，VW63Z has proper deformation ability，moderate solid fraction（41.1%-53.8%），relatively small particle size（81.5 μm-83.2 μm），and high shape factor（0.70-0.75）.

Fig.8 Effects of the isothermal holding time on the microstructure of semi-solid VW63Z

Contine Fig.9 Microstructure of semi-solid VW63Z after isothermal heating at 635 oC for 30 min：the distance to the sample bottom is 2.5 mm-20.0 mm

2.4 The uniformity of the edge and core of VW63Z during isothermal heat treatment

With the enlargement of the billet size，it is more difficult to ensure that the edge part and the core part of the sample are heated evenly during iso?thermal heat treatment.Besides，the solute transpor?tation and solid particle distribution of VW63Z will also be affected by gravity，leading to the inhomoge?neous microstructure of the edge and core of the bil?let.The microstructure evolution of semi-solid VW63Z（large billet，30 mm × 20 mm）along with the distance from the sample bottom after iso?thermal heating at 635 °C for 30 min is shown in Fig.9，and the corresponding numerical statistics are shown in Fig.10.

Fig.9 Microstructure of semi-solid VW63Z after isothermal heating at 635 oC for 30 min：the distance to the sample bottom is 2.5 mm-20.0 mm

Fig.10 Effects of distance to the sample bottom on the microstructure of semi-solid VW63Z

From Fig.9，it can be seen that at the sample bottom，the particle size is relatively large，the solid fraction is low，and the solid fraction is the lowest.In the middle part of the VW63Z billet，the semi-solid characteristic is better，the solid fraction and shape factor increase，while the particle size decreases.Near the sample top，the solid fraction and shape factor decrease slightly.However，the particle diameter increases dramatically.The maximum relative deviations of the solid fraction，the particle size，and the shape factor at different heights of the same billet are 44.6%，17.4%，and 16.6%，respectively，which means that the uniformity of the edge and core of the sample is poor.

Compared with small billet（16 mm × 10 mm），big billet has higher solid fraction，larger parti?cle size，and lower shape factor，which means that the semi-solid microstructure of big billet is inferior to that of small billet.The above results reveal that it is of vital importance to pay attention to the uniformi?ty of the edge and core of VW63Z during semi-solid isothermal heat treatment.

3 Discussion

According to the above research，the semi-solid microstructure evolution of VW63Z during isothermal heat treatment is summarized，as shown in Fig.11.

Fig.11 Schematic illustration of the semi-solid microstructure evolution

It can be divided into three steps：①coarsening and spheroidization of the particles；② necking，coalescence，and Ostwald ripening of the particles；③dynamic equilibrium and dominant by Ostwald ripening.

3.1 Particle coarsening and spheroidization

At the initial stage of isothermal heat treatment，the α-Mg particles coarsen and spheroidize，while ti?ny liquid droplets appear in the α-Mg particles and slim liquid film appears outside the particles disturb?ing along the grain boundary.The content of RE at?oms at the grain boundary is higher than that in the crystal，as shown in Fig.3（b）.Under the solution concentration gradient and energy fluctuation，RE at?oms diffuse from boundaries to the α-Mg particles，and the grain boundaries merge spontaneously to re?duce the free energy of the solid and liquid system，resulting in the growing of α-Mg particles.Melting usually occurs at low melting point phases where the content of RE is high，and then brings about liquid droplets in the α-Mg particles and liquid films along the grain boundaries.According to the EDS line-scan?ning results（see Fig.12（b）），the content of Mg at?oms in the liquid phase is lower than that in the α-Mg particles，but the content of RE atoms is higher，which proves that the liquid phase appears first in the region where RE atoms are enriched.

Fig.12 Microstructure of semi-solid VW63Z holding at 625 oC for 35 min

The dihedral angle between two adjacent α-Mg particles depends on the ratio of the solid/solid and solid/liquid interfacial energy，and thus the surface curvature of solid particles is different.According to

whereis the equilibrium melting point，is the solid/liquid surface tension，is the curvature of the solid-liquid interface，and，，and Δare con?stants related to the solid-liquid interface.The“pro?truding edges”of the α-Mg particles have higherbut lower.Therefore，these“protruding edges”will preferentially melt during isothermal heat treat?ment，which makes the α -Mg particles transform from equiaxed to spheroidized.

3.2 Particle necking,coalescence,and Ostwald ripening

In the middle stage of isothermal heat treatment，the liquid phase continues to expand and the α-Mg particles begin to dissociate in the liquid phase.At this stage，the melting of the low melting point phase and the diffusion of the RE atoms happen con?tinuously.According to the Gibbs-Thomson formu?la，the RE concentration in the matrix corre?sponds to the site where the curvature is large.Thus，the liquid phase inside the α-Mg particles will gradually coarsen and connect to the liquid phase out?side the particle.As a result，particle necking appears（see Fig.13（a））.Another phenomenon can be found is particle coalescence，as shown in Fig.13（b）.The liquid film migrates，and the dissociated α-Mg parti?cles approach each other and gradually merge into one particle.By reducing the total area of the solid/liquid interface，the interfacial energy reduction is re?alized.Normally，particle coalescence is easier to happen when the dihedral angle between two adja?cent α-Mg particles is large.This is because the solid and liquid interfacial energy can be expressed by the following formula：

Fig.13 Particle necking and particle coalescence of semi-solid VW63Z

whereis the solid/solid grain boundary energy，is the solid/liquid interfacial energy，andis the misorientation angle of two particles.The largeris，the higheris，and the stronger the driving force of particle coalescence is.At this stage，the particle size will show the“bimodal size”characteristic as present?ed in Fig.4（b），which is the result of the Ostwald ripening mechanism.Ostwald ripening can also be ex?plained by the Gibbs-Thompson effect.Smaller parti?cles have greater curvature and surface tension，and thus the solute atoms will migrate from large parti?cles to small particles.Small particles will melt while large particles continue to grow larger，which results in the bimodal-size particles.WU et al.believes that particle coalescence and Ostwald ripening operat?ed simultaneously and independently.At a high solid fraction，the dominant mechanism is particle coales?cence，and Ostwald ripening plays the major role when the liquid fraction is high.The necking and co?alescence of solid particles gradually merge two solid particles into one solid particle，which destroys the spheroidization degree of the solid particles and makes it the reason for the significant decrease in the shape factor（see Fig.8）.However，particle coales?cence does not increase the average particle size，which is the result of solid particle dissolution with the heat preservation and bimodal-size solid particles caused by the Ostwald ripening mechanism.The av?erage particle size is reduced statistically.

3.3 Dynamic equilibrium and coarsening with time

With the extension of holding time，the solid fraction reaches a dynamic equilibrium.The“doublepeak”characteristic of the particle size disappears，and the spheroidized α-Mg particles coarsen with time.The Lifshitz-Slorovitz-Wagner（LSW）theory can be used to calculate the growth rate of the semi-solid particle size at low solid fraction：

whereis the time，dis the average particle size at time，is the average particle size at the initial time，andis the coarsening rate constant.At low solid fraction，the particle size is approximately pro?portional to time，which is in great consistent with the result in Fig.8（b）.

The Scheil equation can be used to calculate the theoretical liquid fractionat Stage ③as follows：

whereis the melting point of the pure metal，is the liquidus of the alloy，is the isothermal heat tem?perature，andis the equilibrium distribution coeffi?cient.Generally，calculated by the Scheil equation is slightly higher than the liquid fraction measured by the OM.The reason is that the quenching can hardly completely“freeze”the semi-solid structure，and part of the liquid phase will be solidified into solid crystal during quenching.In addition，the quenching cooling rate will also greatly affect the semi-solid morphology.

The microstructure of VW63Z holding at 635 °C for 35 min after quenched in hot water is shown in Fig.14.Compared with quenching in cold water as shown in Fig.4（d），the α-Mg particles grow accord?ing to the preferred orientation，leading to the equiaxed crystal.Small α-Mg grains also nucleate from the liquid phase，which makes the solid fraction difficult to count by the metallographic analysis.

Fig.14 Microstructure of semi-solid VW63Z holding at 635 oC for 35 min after quenched in hot water

4 Conclusions

The semi-solid VW63Z alloy is prepared by iso?thermal heat treatment.The main conclusions are as follows.

1）The microstructure of VW63Z alloy trans?forms from equiaxed to semi-solid spherical after iso?thermal heat treatment above 620 °C.The micro?structure evolution can be divided into three stages：①particle coarsening and spheroidization；②parti?cle necking，coalescence，and Ostwald ripening；and③dynamic equilibrium.

2）The semi-solid process window of VW63Z is 620 °C-635 °C.The best semi-solid process parame?ter of the alloy is holding at 635°C for 20 min-30 min.The solid fraction，the average particle size，and the shape factor are 41.1%-53.8%，81.5 μm-83.2 μm，and 0.70 -0.75，respectively.The billet is partly softened，and has great potential of SSM processing.

3）The uniformity of the edge and core of semisolid VW63Z should be improved.When heating at 635 °C for 30 min，the maximum relative deviations of the solid fraction，the particle size，and the shape factor at different heights of the same billet are 44.6%，17.4%，and 16.6%，respectively.