*

Key Lab of Electromagnetic Processing of Materials,Ministry of Education,Northeastern University,314Mailbox,Shenyang 110819,People's Republic of China

Effects of casting process on microstructures and f l ow stress behavior of Mg-9Gd-3Y-1.5Zn-0.8Zr semi-continuous casting billets

Xuan liu,Qichi Le*,Zhiqiang Zhang,Lei Bao,Zhengxing Fan,Jianzhong Cui

Key Lab of Electromagnetic Processing of Materials,Ministry of Education,Northeastern University,314Mailbox,Shenyang 110819,People's Republic of China

Mg-9Gd-3Y-1.5Zn-0.8Zr alloys own high strength,good heat and corrosion resistance.However,it is diff i cult for the fabrication of large-scale billets,due to the poor deformation ability and strong hot-crack tendency.This work investigated the casting process on the microstructures and f l ow stress behaviors of the semi-continuous casting billets for the fabrication of large-scale Mg-9Gd-3Y-1.5Zn-0.8Zr billets.The casting process(electromagnetic intensity and casting speed)shows outstanding effects on the microstructures and f l ow stress behavior of the billets.The billets with the specif i c casting process(I=68 A,V=65 mm/min)exhibit uniform microstructures and good deformation uniformity.

Mg-Gd-Y-Zn-Zr;Semi-continuous casting;Microstructures;Flow stress behavior

1.Introduction

Recently,the lightening of materials has become a spot for research,due to the energy and resource limitation.Magnesium alloys become one of the most attractive metallic materials because of their low density,high specif i c stiffness and good damping capacity[1].However,the poor mechanical properties frustrate magnesium alloys'applications.

As we know,magnesium alloys with RE additions could enhance the mechanical properties drastically,especially at elevated temperatures.It is reported that Mg-Gd-Y-Zr alloys exhibit the higher specif i c strength at both room temperature and elevated temperatures.Meanwhile,they own more excellent creep resistance than WE54[2,3].Thus,it provides an effective approach to improve the strength of Mg alloys.Recently,a Mg97Zn1Y2with excellent tensile yield strength above 600 MPa were manufactured through rapidly solidif i ed powder metallurgy[4].It should be attributed to the f i ne grains and distribution of long period stacking order phases(LPSO).It was reported that the LPSO phases showed outstanding effects on the mechanical properties of Mg-Gd-Y-Zn-Zr alloys[5-10].Meanwhile,a research has been developed about the effect of Zn addition on the aging hardening of Mg-2.0Gd-1.2Y-0.2Zr(at.%)[11].As a result,Mg-Gd-Y-Zn-Zr alloys could be the potential candidate to broaden the application of magnesium alloys.

However,the fabrication of Mg-Gd-Y-Zn-Zr alloys was notwidely reported.Although the fabrication of Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr has been achieved by semicontinuous casting[12],it is still rarely covered that external f i eld like electromagnet(EM)was introduced during the semicontinuous casting of Mg-Gd-Y-Zn-Zr alloys.Lowfrequency electromagnetic casting(LFEC)has been applied in the semi-continuous casting of Al-Zn-Mg-Cu-Zr alloys[13],which could restrain the macrosegregation and uniform the structure[14].

Fig.1.Schematic for the casting process.

In this work,Mg-9Gd-3Y-1.5Zn-0.6Zr alloys are fabricated under electromagnetic f i eld by semi-continuous casting.Investigations were carried out around the effects of casting speed and electromagnetic intensity on the microstructures of the alloys.Meanwhile,f l ow stress behavior will be also discussed.

2.Experimental

An alloy of chemical composition Mg-9.3Gd-3.2Y-1.5Zn-0.8Zr(wt.%)waspreparedfromhigh-purity(99.9%)Mg andZn,Mg-25Gd(wt.%),Mg-50Y(wt.%)andMg-30Zr(wt. %)masteralloysbymeltinginresistancefurnaceandcastingwith ahot-topcrystallizerunderashieldinggasofmixedCO2andSF6at690°C.Fig.1 shows the schematicfor the casting process and casting apparatus are set as shown in Table 1.The billets have approximately a diameter of 190 mm and length of about 1000 mm,as shown in Fig.2.

Fig.2.The photo of the billet fabricated by semi-continuous casting.

Table 1Casting parameters in this study.

The samples for the microstructure observation were cut from the center,1/2 radius and near the surface of the alloy billets along axial and radial directions.The microstructures were etched in 4%Nital and observed using optical microscope(OM).Linear intercept method was applied to determine the grain size of the alloys.Flow stress behaviors were studied on a Gleeble-1500D thermal mechanical simulator in the temperature 400 and 450°C,at constant strain rates of 0.1 s-1and 1 s-1.In addition,the Gleeble samples were 15 mm in diameter and 8 mm in height.

3.Results and discussion

3.1.Surface quality of the billets

Fig.3 shows the surface qualities of billets.Structures of the hot-top crystallizer make severe inf l uence on the surface of billets.Obvious lengthways streaks(Fig.3a)are observed on the surface of the billet,and the depth of them could reach as much as about 3 mm.It would drastically decrease the f i nal yield of billets.As the hot-top mould is mainly made up of refractories with a rough skin(Rags and scratches spread along the inner wall of the mould),large friction force generate on the inner wall of the hot-top mould during semicontinuous casting.Consequently,it gives rise to the poor surface quality of the billets.Moreover,the melt adhered and solidif i ed on the inner wall of the mould at the primary stage. Thus,the billet(Fig.3b)has a cup-like tail.

3.2.Microstructures of the as-cast billets

Fig.4 shows the microstructures of the as-cast samples from edge to center of the billets.The microstructures of the billets are mainly composed of α-Mg and large amount of discontinuous eutectic around the grain boundaries.EM treatment reduces the grain sizes distinction ranging from edge to center of the billets.Fig.5 illustrates the grain sizes of the different positions with and without EM treatment.The billet without EM inf l iction owns a wide grain size distribution from edge to center.Grain size for the edge position is as small as 27 μm,while that for the center reaches as large as 58 μm. This should be ascribed to the large temperature gradient and variant cooling rate between the edge and center,during the semi-continuous casting process.

Fig.3.Surface quality of the as-cast billets.(a)surface of the billets;(b)cup tail.

The microstructures of the billet become uniform after casting under an EM f i eld(f=30 Hz,I=68 A),as shown in Fig.4b,e and h.Grain size from edge to center are also uniform,as shown in Fig.5.A forced convection in the liquid sump gives rise to the initial solidif i ed grains detached following the induced liquid f l ow[15,16].On the other hand, temperature gradient is also decreased by the induced convection,and it also make sense that the driving force bringing the overheated melt from the center to the periphery[17]. Moreover,the role of the hot top is also important.Its low heat convection coeff i cient ensures slow cooling of the melt from the edge in the initial stage of the cast.Consequently,the microstructures become uniform.While the EM treatment parameters change tof=30 Hz,I=90A,microstructures of the billet are ref i ned apparently,as shown in Fig.4c,f and i. Grain size from edge to center are also uniform.The average grain size are less than 30 μm,also shown in Fig.5.It reveals that EM plays an important role on the solidif i ed structure of the billets.The increasing of the current intensity enhances the force convection.Thus,electromagnetic stirring effect inhibits the grain growth eff i ciently.But 68 A is better than 90 A in the view of structure uniformity.What worth mentioning is that the grain size of the edge with EM(I=68 A)is a bit larger than that without EM treatment.Interpretation should be given as the change of heat convection coeff i cient led by soft contact with the mold.

Fig.6 shows the microstructures of the billets with different casting speeds.It can be seen that grains are more ref i ned with a lower casting speed.That is because the liquid sump increases with the enlargement of the casting speed.Thus,the stirring effect made by EM is short and weak,and the temperature gradient is also increasing.As a result,the grainsbecome coarser,and the structures of the billets could be ref i ned with a low casting speed.Even so,microstructures become uniform with a casting speed of 65 mm/min,as shown in Fig.5.

Fig.4.Optical microstructures of the as-cast billets(from left to right:edge to center).(a),(b)and(c)without EM;(d),(e)and(f)with EM(f=30 Hz,I=68 A); (g),(h)and(i)with EM(f=30 Hz,I=90 A).

Fig.5.As-cast alloy grain size of different electromagnetic conditions.

3.3.Flow stress of the as-cast alloy

In the previous chapter,the low frequent electromagnet could ref i ne the microstructures of the billets.The different deformation ability between the edge and the center of the billets should be a key to the subsequent processes.Thus,the f l ow stress characters of the billets are now investigated to f i nd the optimist casting parameters for the uniform deformation. Fig.7 shows the true f l ow stress-true strain curves of the billets with different casting parameters.In general,low temperature and high strain rate will increase the f l ow stress,while higher temperature and lower strain rate will decrease the f l ow stress of the alloy[18].This is due to the thermal activation processes that become intense by raising the deformation temperature.

Flow stress curves at 400°C exhibit a sharp increase of the initial stage of strain and then slowly increase up to a transient equilibrium.When stress reaches the maximum value at,the stress decreases gradually and then drives to a steady value with the increasing strain,as shown in Fig.7a,c,e and g.As the deformation temperature increases to 450°C,f l ow stress behaviors become quite different.After the sharp increase on the stress,slow stress growth to the maximum value is missing as shown in Fig.7b,d,f and h.Meanwhile,the f l ow stress behaviors are similar as the strain rate are 0.1 s-1and 1 s-1.

Fig.8 shows the optical microstructures of samples with different strains at 400°C.Large amount of lamellar phase are observed inside the grains.This lamellar phase should be identif i ed as LPSO phase[11],which are formed during the hot compression process.From the microstructures of the sample with a strain of 0.1,some lamellar LPSO phase traverse the grains partially,while most of the LPSO run throughout the grains in the larger strained samples(ε=0.3 and 0.6).Thus,it could be seen that the LPSO phases gradually run through the grains as the increasing strain.In other word,these lamellar phases play an important role on the f l ow stress behavior of the as-cast billets.It can be found that the f l ow stress curve is mainly composed of three stages: stage I(work hardening stage),stage II(softening stage)and stage III(steady stage)[19].In the work hardening stage,the increasing number density of dislocations gives rise to the strong work hardening.On the other hand,the dynamic recovery is too weak to absorb the accumulating dislocations. Consequently,the f l ow stress quickly increases with the increase of strain.In the softening stage,the accumulated dislocation density exceeds the critical strain,giving rise to the formation of the lamellar LPSO phase.The nucleation and growth of LPSO phases need to absorb dislocations, which decreases the f l ow stress.In the short steady stage,the f l ow stress keeps a steady state due to the dynamic balance between the work hardening and dynamic softening.The nucleation and growth of LPSO phases is obviously signif icant to the soften stage.

Fig.6.Optical microstructures of the ingots from edge to center with different casting speeds(from left to right:edge to center).(a),(b)and(c)v=55 mm/min; (d),(e)and(f)v=65 mm/min.

Fig.7.True stress-strain curves of billets with different casting parameters.

Fig.8.The optical microstructures of compressed samples with different strains at 400°C.(a) ε=0.1;(b) ε=0.3;(c) ε=0.6.

From the f l ow stress curves in Fig.7,it could be seen that billets with different casting process own distinct deformation uniformity.On condition that the strain rate is 1 s-1and temperature is 400°C,the f l ow stress difference between the edge and the center of the billet without EM is enlarged after the strain increases from 0.12 to 0.7,as shown in Fig.7a,and similar situation occurs when the strain rate is 0.1 s-1.Stress distinction between the edge and the center of the billet (Fig.7c)with EM(I=68 A,V=65 mm/min)is much smaller than that without EM.The difference is not enlarged until the strain increases to 0.36,when the strain rate is 1 s-1.At a lower strain rate of 0.1 s-1,f l ow stress curves are nearly synchronous for the edge and the center as the strain rises up. While the EM intensity increases to 90 A as shown in Fig.7e, the f l ow stress difference is not narrowed further.It proves that large electromagnetic intensity could in turn do harm to the uniform deformation of the billets.

After deformation temperature rises up to 450°C,EM intensity makes weak inf l uence on the f l ow stress difference between the edge and the center of the billet.The higher temperature gives rise to the easier nucleation and growth of the lamellar LPSO phases.Thus,uniform deformation occurs at 450°C.

Fig.7 also shows the f l ow stress curves of billets with a casting speed of 55 mm/min.When deformation temperature is 400°C and EM intensity is a constant of 68 A,the f l ow stress distinction between the edge and the center is very small as the strain increases.Moreover,the f l ow stress-strain curves of the edge and the center of the billet made by a casting speed of 55 mm/min is almost overlapped,as shown in Fig.7g.It suggests a uniform deformation.When the casting speed increases to 65 mm/min and strain rate is 1 s-1,the difference rises up after strain increases to 0.36,as shown in Fig.7c. While the temperature increases to 450°C,billet with a casting speed of 55 mm/min show a great difference in the f l ow stress between the edge and the center(Fig.7h),however, it is better for the billet fabricated by a speed of 65 mm/min,as shown in Fig.7d,and the f l ow stress difference(strain rate of 1 s-1)is small when the strain is less than 0.34.It could be seen that the casting process shows an outstanding effect on the f l ow stress behavior of the billets in the viewpoint of deformation uniformity.Overall consideration,the billet with casting process(I=68 A,V=65 mm/min)shows the best deformation uniformity.Meanwhile,it also owns the largest maximum f l ow stress over others.

4.Conclusion

This work investigated the casting process on the microstructures and f l ow stress behaviors of the semi-continuous casting billets,aimed at the fabrication of large-scale Mg-9Gd-3Y-1.5Zn-0.8Zr billets.The casting process (electromagnetic intensity and casting speed) shows outstanding effects on the microstructures and f l ow stress behavior of the billets.The microstructures could be ref i ned by increasing the electromagnetic intensity as well as decreasing the casting speed.The LPSO phase has outstanding effects on the f l ow stress behaviors of the alloy. Billets with proper EM intensity and casting speed own good deformation uniformity.The billets with the specif i c casting process(I=68 A,V=65 mm/min)exhibit uniform microstructures and good deformation uniformity.

Acknowledgment

This research was f i nancially supported by National Basic Research Program of China(Grant No.2013CB632203),the Liaoning Provincial Natural Science Foundation of China (Grant No.201202072),National Key Technology R&D Program of China(2012BAF09B01),and the Fundamental Research Foundation of Central Universities(Grant Nos. N120509002 and N120309003).

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Received 18 September 2014;revised 18 November 2014;accepted 19 November 2014 Available online 8 December 2014

*Corresponding author.Tel.:+86 24 83683312;fax:+86 24 83681758.

E-mail address:qichil@mail.neu.edu.cn(Q.Le).

Peer review under responsibility of National Engineering Research Center for Magnesium Alloys of China,Chongqing University.

http://dx.doi.org/10.1016/j.jma.2014.11.007.

2213-9567/Copyright 2014,National Engineering Research Center for Magnesium Alloys of China,Chongqing University.Production and hosting by Elsevier B.V.All rights reserved.

Copyright 2014,National Engineering Research Center for Magnesium Alloys of China,Chongqing University.Production and hosting by Elsevier B.V.All rights reserved.