Superplastic elongation characteristic of fine grained magnesium alloy ZK60
Department of Mechanical Engineering,National Central University
Chun Shan Institute of Science and Technology
作者简介：CHEN Yunghung E-mail: yunghungchen@gmail.com;
收稿日期：7 August 2009
Superplastic elongation characteristic of fine grained magnesium alloy ZK60
Abstract：
This paper employs simple rolling process plus annealing to refine the grain size of magnesium alloy ZK60.This goal is effectively achieved,obtaining grains as fine as ～3.7 μm.Such a specimen shows an elongation of 642%,and its ultimate fracture surface exhibits intergranular separation and significant grain growth.Additionally,the effects of the specimen's geometry and tensile test axis with respect to the rolling direction on superplastic elongation is studied,which has not been done before.
Keyword：
magnesium alloy; superplastic elongation; rolling process; grain refining;
Received： 7 August 2009
1. Introduction
Magnesium alloys are the lightest metal that can be used for structural applications.In recent years,magnesium alloys die casting is prevalent in manufacturing components for automotives,hand tools,computer covers,and mobile phones.On the other hand,sheet forming,especially superplastic sheet forming or quick plastic forming[1-3],offers a potential alternative for making stronger and more economical products.In order to be considered for industry applications,various dimension specs of sheets or plates via extrusion and/or rolling must be made available along with their associated property information.The most significant item among the specs is superplastic elongation.However,highly different elongation data has been frequently observed among the same superplastic materials,e.g.the Mg-6%Zn-0.5Zr alloy(designated as ZK60).Galiyev et al.first prepared the raw material via direct chill casting and,second,adopted an idea of intense plastic straining(IPS)through extrusion.This was,in turn,combined with compression plus rolling to achieve a fine grained structure(~3.7μm).This method achieved a corresponding superplastic elongation that reached~1100%[4].This value is greatly superior to some other published papers regarding ZK60[5-6].In this paper,the same grain refining state has been achieved without the extra compression procedure;however,the elongation is still significantly below Galiyev’s level.While this inconsistency can be due to various reasons,one particular factor has been generally overlooked the specimen’s geometry,which has never been evaluated.It is a common knowledge that when testing specimens at room temperature,the experiment should comply with a specification,such as the ASTM one,in which the specimen’s aspect ratio(length over cross section area)is an important factor to be aware of[7].In this paper,17 ZK60 specimens bearing fine grain structure have been prepared with four different geometrical shapes for tensile test in order to determine whether this factor will influence the superplastic elongation Besides,the effect of the rolling direction with respect to the tensile test axis is also demonstrated.
2. Experimental
The used ZK60 magnesium alloy has a chemical composition of Mg-5.29%Zn-0.59%Zr.The alloy was prepared by chill casting it into a?200 mm ingot.Using this casting method avoids potential macrosegregation.It was then followed by an extrusion at 380?C to obtain a 1000 mm?90mm?6 mm sheet,which corresponded to a 60(25)1 reduction ratio.This extrusion was an intermediate process to facilitate the rolling work that followed.This as-extruded state was processed next by hot rolling,and fine-grained structure was obtained for sheets subjected to 60%and 80%thickness reduction.For the latter one bearing~3.7μm grain size,four types of tensile specimens,different gage width,length,and fillet curvature(Fig.1),were prepared for superplasticity comparison.
Fig.1.Shapes and dimensions of the four types of specimens for superplastic tensile test(unit:mm).
3. Results and discussion
3.1. Microstructure evolution toward refined grains siz-ing below 10μm
(1)As-extruded.
Fig.2(a)is a micrograph of ZK60 after being extruded at380?C.It exhibits a parallel layer-stacking of coarse and elongated grains.This structure shows 262 MPa in tensile strength and an elongation of 12.7%.Subsequent annealing at 365?C for up to 24 h(Fig.2(b))exposed the hidden boundaries not seen in the preceding micrograph.As the annealing temperature was raised to 400?C and maintained for 3 h,the parallelism of stacking was eliminated,and grains were further coarsened(Fig.2(c)).
(2)Rolling sheets with 300 and 350?C initial temperatures.
For a rolling reduction of 20%at these temperatures,the grain size uniformity is low(Fig.3).In some large grains,deformation twins are present.It was also noticed that the300?C specimen was much harder to roll.This indicates that both the slip and twinning mechanisms are temperature dependent.For the 350?C rolling,40%reduction produced a much higher density of twins(Fig.4(a))as compared to the one with 20%reduction.When the rolling adopted 60%reduction,many fine and uniform grains were generated together with net-like stripes(Fig.4(b)),which seems to be composed of even finer grains or recrystallizing nuclei.
Considering the rolling at 350?C for 60%and 80%thickness reductions,followed by annealing,a fine grain structure was obtained.The former yielded grain size of 7.5μm(Fig.5),and the latter gave 3.7μm(Fig.6).The role of annealing is important to provide driving force for recrystallizing the heavily strained one;and a higher temperature(365?C versus 265?C)is more effective than a longer period of time(1 h versus 17 h).
3.2. Superplastic elongation analysis
(1)Geometry factors.
The reference specimen was tested at 350?C and 10-4/s initial strain rate,and its elongation reaches 642%(Fig.7).For this specimen,its geometrical features are short,wide,and small fillet curvature.Based on this specimen,another one of a longer length with the same width(Fig.1(b))was tested at identical conditions,350?C and strain rate of 10-4/s.The elongated length is obviously shorter,as shown in Fig.8Thus,it is asserted that length does not play a significant role.Then,about the width,a specimen with a narrower width(Fig.1(c))was tested at the same conditions,exhibiting higher elongation,566%(Fig.9),relative to the immediately preceding one.Thus,width factor is just like that of length,i.e.insignificant.Two identical specimens except fillet curvature,seen Figs.1(c)and 1(d),were tested under the same conditions.The one with gradual curvature exceeds its opponent(Fig.10).Tentatively,it is assumed that fillet curvature is a factor.However,comparing again the one containing gradual curvature with the one with the highest elongation that bears the sharpest fillet(Fig.7),gradual fillet curvature is not favored.Overall judgment is that in superplastic deformation test,the specimen’s geometrical effect is not significant.When some variation of superplastic elongation among tested specimens appears,it should be mainly due to the testing technique or material itself.Another confirmation is that two geometrically identical specimens tested at the same condition can obviously give different elongations(Fig.11).
Fig.2.Optical micrographs of ZK60 after being extruded at380?C:(a)single extrusion;(b)plus annealing at 365?C for 24 h;(c)plus annealing at 400?C for 3 h.
(2)Rolling direction with respect to the tensile test axis.
Two sets of specimens with the testing axis parallel and perpendicular to the sheet rolling direction were compared,as shown in Fig.12.It is seen that those with the tensile axis perpendicular to the original rolling direction show inferior elongation.
Fig.3.Optical micrographs of rolled sheets by 20%:(a)300?C initial temperature;(b)350?C initial temperature.
Fig.4.Optical micrographs of rolled sheets at 350?C:(a)40%rolling reduction;(b)60%rolling reduction.
Fig.5.Fine grains obtained by rolling at 350?C with 60%re-duction and followed by(a)365?C/1 h and(b)265?C/17 h annealing.
Fig.6.Fine grained structure of 3.7?m obtained by rolling at350?C with 80%reduction and subsequent annealing of265?C/17 h.
Fig.7.Specimen(type A shape)bearing a grain size of 3.7?m attaining a 642%elongation when tested at 350?C and 10-4/s.
(3)Superplastic versus quick plastic flow.
It is a common recognition that fine grain alloys may exhibit remarkable superplasticity under suitable temperatureand strain rate.However,the latter is generally low enough to be an obstacle for realistic applications.A relative new concept is to explore another higher strain rate regime where the dislocation creep mechanism dominates and still produces sufficient amount of elongation.A specimen of 3.7μm in grain size,tested at 300?C and 10-3/s,attained 420%elongation(Fig.13).The other one as tested at 300?C and10-2/s can yield 240%elongation in 4 min,which is fas enough for realistic applications.Their stress-strain curves(Fig.14)indicate the dominant plastic deformation mechanism is dislocation creep rather than grain boundary sliding.
Fig.8.Longer specimen(type B shape)tested at 350?C and10-4/s in comparison with the one described in Fig.7.
Fig.9.Specimen with a narrower width(type C shape)tested at the same conditions,exhibiting a higher elongation of 566%relative to the one in Fig.10.
Fig.10.Two specimens tested under the same conditions showing that the one with gradual curvature(type C)exceeds its opponent(type D).
Fig.11.Two geometrically identical specimens tested at the same conditions(350?C,strain rate 10-4/s)exhibiting obviously different elongations.
Fig.12.Two sets of specimens,I(type B)and II(type D),whose testing axis parallel(upper one of each set)and perpendicular(bottom)to the sheet rolling direction,for comparing elongation capability.
Fig.13.Two specimens tested at 300?C,and strain rates of(a)10-3/s and(b)10-2/s showing 420%and 240%elongations re-spectively.
Fig.14.Stress-strain behaviors of the specimens described in the preceding figures:(a)10-3/s;(b)10-2/s.
4. Conclusions
An extrusion ratio of 60:1 on ZK60 Mg alloy at 380?C cannot produce fine grain structure.The subsequent rolling at 350?C induces deformation twins.The following annealing for the 60%or 80%rolled generates fine grain structure below 10μm.For the 80%specimen,its~3.7μm fine grain structure also gives impressive superplastic elongation reaching 642%.Based on this specimen,others of differen shapes are tested just to pronounce that geometrical factor is not significantly relevant to the specimen’s final elongation values.On the other hand,the rolling direction with respec to the tensile test direction is effective.
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