Abnormal distribution of microhardness in tungsten inert gas arc butt-welded AZ61magnesium alloy plates
Nan Xu,Jun Shen ?,Weidong Xie,Linzhi Wang,Dan Wang,Dong Min
College of Material Science &Engineering,Chongqing University,Chongqing 400044,PR China
A R T I C L E D A T A
A B S T R A C T
Article history:
Received 20January 2010Received in revised form 21March 2010Accepted 6April 2010In this study,the effects of heat input on the distribution of microhardness of tungsten inert gas (TIG)arc welded hot-extruded AZ61magnesium alloy joints were investigated.The results show that with an increase of heat input,the distributions of microhardness at the top and bottom of the welded joints are different because they are determined by both the effect of grain coarsening and the effect of dispersion strengthening.With an increase of the heat input,the microhardness of the heat-affected zone (HAZ)at the top and bottom of welded joints and the fusion zone (FZ)at the bottom of welded joints decreased gradually,while the microhardness of the FZ at the top of welded joints decreased initially and then increased sharply.The reason for the abnormal distribution of microhardness of the FZ at the top of the welded joints is that this area is close to the heat source during welding and then large numbers of hard β-Mg 17(Al,Zn)12particles are precipitated.Hence,in this case,the effect of dispersion strengthening dominated the microhardness.
?2010Elsevier Inc.All rights reserved.
Keywords:
AZ61magnesium alloy TIG welding Microstructure Microhardness
1.Introduction
Magnesium alloys are used widely in the transportation,aerospace and nuclear industries because of their high specific strength,high damping capacity,excellent shielding property against electromagnetic interference,and good recycling ability [1–5].However,because of their poor deformation ability at room temperature [6],the welding technology of magnesium alloys needs to be researched deeply in order to expand the applications of them.Recently,the welding technologies of magnesium alloys,including friction stir welding,laser welding,TIG welding and laser hybrid welding process,have been reported widely in the literature [7,8].Since microhardness is an important mechanical property for a welded joint,the distributions of microhardness of welded magnesium alloy joints also have been studied.Zhang et al.[9]pointed out that,during TIG welding,whether a coating of CdCl 2is on the surface of AZ31B magnesium alloy plates or
not,welded joints have a similar distribution of microhard-ness (that is,the microhardness of the base metal (BM)is higher than in the fusion zone (FZ)and heat-affected zone (HAZ),and the microhardness of the FZ is higher than HAZ).Ding et al.[10]found that,after coating SiC on the surface of AZ31magnesium alloy plates during TIG welding,the micro-hardness of the FZ was enhanced to 100–150Hv,much higher than that of BM (55Hv).With an increase of the heat input,the microhardness of the FZ decreased because of the coarsening of grains.Hence,generally speaking,the microhardness of the FZ is higher than that of the BM and the HAZ in a welded joint due to the effect of dispersion strengthening of second phase particles.With an increase of the heat input,the microhard-ness of the FZ decreases due to the coarsening of grains [11].
In a TIG welding process of a magnesium alloy,the top and bottom of a welded joint undergo different solidification processes due to their different distances from the heat source.Hence,this will lead to the formation of different microstructures
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?Corresponding author.Tel.:+86138********;fax:+862367084927.E-mail address:shenjun2626@https://www.doczj.com/doc/ab7796935.html, (J.
Shen).
a v a i l a
b l e a t w w w.s
c i e n c e
d i r
e c t.c o m
w w w.e l s e v i e r.c o m /l o c a t e /m a t c h a r
between the top and bottom of the welded joint.In this study,TIG welding tests were given to AZ61magnesium alloy plates and the distribution of microhardness of different areas of welded joints was obtained with different heat inputs.The reason for an abnormal distribution of microhardness in the TIG welded AZ61magnesium alloy plates is
discussed.
Fig.1–(a)SEM image shows the typical microstructure of a welded joint.(b)High-resolution SEM image shows the microstructure of the
FZ.
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2.Experimental Condition
In this study,hot-extruded AZ61magnesium alloy plates with dimensions of120×30×2mm and an AZ91D welding wire with a diameter ofΦ2mm were used for the welding tests. Before welding,the top surface of each specimen was cleaned with acetone to remove grease,and then they were polished with a stainless steel wire to remove oxides.The samples were welded on the top of a20mm wide copper plate with a semi-circular groove.The welding currents(I)were60A,75A,90A and105A respectively.Other welding parameters were constant(welding speed(v)was15mm/s,flow rate of argon gas was5l/min and welding voltage(U)was10V).The heat input(L)of the welding was given by L=ηUIν,here,η=0.65[12].
After welding,the specimens were cross-sectioned,ground and polished.The mounted samples were etched in a solution comprised of5ml acetic acid+5g picric acid+10ml water and 100ml ethyl alcohol for20to60s until the microstructures were revealed.The microstructures were characterized by an optical microscope(MDJ200)and scanning electron micros-copy(TESCAN,Inc.VegaIILMU SEM).The precipitated phase in the FZ was determined by energy dispersive X-ray spectrom-eter(OXFORD,Inc.ISIS3000)analyses.A Vickers hardness tester(V1000)was used for microhardness tests.3.Results and Discussion
3.1.Microstructures of the Welded Joints
Fig.1shows a typical microstructure of the welded joints.A welding seam consists of the base metal(BM),a heat-affected zone(HAZ)and a fusion zone(FZ).A white phase and a black phase were formed both in the HAZ and the FZ.EDS analysis results show that the white phase contains higher quantities of Al and Zn elements than that of the black phase(the content of Al and Zn at point B is4.84wt.%and1.13wt.%,which is lower than that at point A(21.33wt.%and5.78wt.%))(See Fig.1(b).). According to the Mg–Al phase diagram[13],the white phase is eutecticβ-Mg17(Al,Zn)12,while the black phase isα-Mg.
Figs.2and3show the microstructures of the HAZ at bottom and top of the welded joints.The grain size ofα-Mg and volume fraction ofβ-Mg17(Al,Zn)12were determined by a quantitative metallographic method[14]and the results are collected in Table1.It was found that whether at the bottom or the top of the HAZ of the welded joints,the grain size of the α-Mg increased with an increase of the heat input,while the volume fraction of theβ-Mg17(Al,Zn)12phase,which was distributed both in the grains and at the grain boundaries, also increased with an increase of the heat input(See Table1.). This is because the higher heat input achieved due to
the
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longer period of overheating during the welding coarsened the α-Mg grains and increased the amount of theβ-Mg17(Al,Zn)12 phase precipitated during solidification.
Figs.4and5show the microstructures of the FZ at the bottom and top of the welded joints.Similar to that in the HAZ, at the bottom of the FZ of the welded joints,the grain size of the α-Mg and the volume fraction of theβ-Mg17(Al,Zn)12increased with an increase of the heat https://www.doczj.com/doc/ab7796935.html,pared with that in HAZ, the grain size of theα-Mg in the FZ is smaller.At the top of the FZ of the welded joints,the grain size of theα-Mg only increased slightly with an increase of the heat input.However,the volume fraction of theβ-Mg17(Al,Zn)12increased significantly with an increase of the heat input(Note that theβ-Mg17(Al,Zn)12phase in the bottom of the welded joints mainly precipitated at the grain boundaries,while it precipitated both at the grain bound-aries and in the grains at the top of the welded joints.).In addition,with the same heat input achieved,the amount of the β-Mg17(Al,Zn)12phase precipitated at the top of the welded joints is much larger than that at the bottom of the welded joints.
3.2.Distribution of Microhardness of the Welded Joints
Fig.6shows the distribution of microhardness of the welded joints with different heat inputs.It can be seen that the microhardness of the HAZ at the bottom and top of the welded joints and microhardness of the FZ at the bottom of the welded joints decreased gradually with an increase of the heat input.However,interestingly,the microhardness of the FZ
at Table1–The average grain size of theα-Mg phase and the volume fraction of theβ-Mg17(Al,Zn)12phase in the welded joints.
HAZ FZ
Heat input J/mm The average grain
size ofα-Mgμm
Volume fraction
ofβ-phase%
The average grain size
ofα-Mgμm
Volume fraction
ofβ-phase%
26Bottom16±1.2 2.810±1.411.1 Top18±1.7 5.613±1.219.4
32.5Bottom18±1.5 2.812±1.111.1
Top20±2.0 5.616±2.113.9
39Bottom21±2.0 5.615±1.611.1 Top23±1.48.319±1.713.9
45.5Bottom23±2.78.617±2.113.9
Top25±2.311.121±1.416.7
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the top of the welded joints decreased initially and then increased sharply with an increase of the heat input.
The microhardness of an alloy is influenced mainly by two factors.According to the Hall –Petch equation,a decrease of the grain size increases the hardness of an alloy.On the other hand,the Orowan hardening mechanism indicates that a distribution of second phase strengthening particles in an alloy matrix may improve the hardness of it [11].In this study,the result of micro-structural analysis showed that both the grain size of the α-Mg and the amount of the hard β-Mg 17(Al,Zn)12phase formed in the HAZ and FZ of the welded joints increased with an increase of the heat input.However,the microhardness of the FZ at the top of the welded joints showed a different distribution compared with that in the HAZ of the welded joints and at the FZ of the bottom of the welded joints.The reason is that at the HAZ of the welded joints and the FZ of the bottom of the welded joints,although the amount of the hard β-Mg 17(Al,Zn)12phase increased with an increase of the heat input,the microhardness of the welded joints was dominated by the effect of the α-Mg grains coarsening.However,for the FZ at the top of the welded joints,when a relatively higher heat input was achieved,the amount of the β-Mg 17(Al,Zn)12phase precipitated during solid-ification increased significantly.This is because the FZ at the top of the welded joints was far from the cooling plate but close to the heat source during the welding.Hence,it experienced a relatively slow solidification process.Because of the diversity of solubility increased due to the relative low temperature during solidifica-tion.Hence,the Al element,which precipitated from the bottom of the welding pool during solidification,transferred to the top of the welding pool and increased the concentration of the Al element in it (the maximum solid solubility of Al is 12.7wt.%at 620°C and 2.0wt.%at room temperature [15]).So,this increased the amount of the β-Mg 17(Al,Zn)12phase at the FZ of the top of the welded joints greatly.
3.3.The Mechanism of the Abnormal Distribution of Microhardness at the Top of the FZ
As discussed above,the microhardness increases with a decrease of size of the α-Mg grains and the measured microhardness values can be fitted to a Hall –Petch type relationship as [16,17]:
σHP =σ0+αHP d ?1=2
g
e1T
where σ0and αHP are material constants,and d g is the average grain size of α-Mg.
In addition,by passing of incoherent dispersoid β-Mg 17(Al,Zn)12particles can be expressed by the Orowan equation as:σOR =
0:13G m b λln r
e2T
Fig.5–The microstructures of the FZ at the top of the welded joints with different heat inputs,(a)26J/mm,(b)32.5J/mm,(c)39J/mm and (d)45.5J/mm.
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are constants,and σOR is determined only by λ(dispersoid spacing)[18].
Fig.7shows the linear regression analysis dependence of the microhardness on the reciprocal square root of d g for the FZ of the top of the welded joint.Fig.8shows the relation-ship between the microhardness and the dispersoid spacing of β-Mg 17(Al,Zn)12particles.The σOR and σHP can be considered as Hv HP and Hv OR .With an increase of heat input,the dispersoid spacing λis from 13,11,10to 2μm.One can see that,with the increase of grain size of α-Mg,the microhardness decreases linearly.However,with a decrease of dispersoid spacing of β-Mg 17(Al,Zn)12particles,the microhardness increased very slowly (when the dispersoid spacing of β-Mg 17(Al,Zn)12is 13,11and 10μm)initially and then increased sharply (when the dispersoid spacing of β-Mg 17(Al,Zn)12is 2μm in Fig.8).
Hence,for the FZ of the top of the welded joint,when the heat inputs increased from 26to 39J/mm,the grain size of α-Mg because a large number of β-Mg 17(Al,Zn)12particles were distributed in/around the α-Mg grains discretely,the effect of dispersion strengthening of the hard β-Mg 17(Al,Zn)12particles dominated the microhardness and thus the microhardness increased obviously.
4.Conclusions
The distribution of microhardness of TIG welded AZ61mag-nesium alloy joints is determined by both the grain size of the α-Mg phase and the volume fraction of hard β-Mg 17(Al,Zn)12particles.With an increase of heat input,both the grain size of the α-Mg phase and the volume fraction of β-Mg 17(Al,Zn)12particles increased.The microhardness of the HAZ at the top and bottom of welded joints and the FZ at the bottom of welded joints decreased gradually due to its being dominated by the effect of coarsening of the α-Mg grains.However,for the FZ at the top of the welded joints,the microhardness decreased initially and then increased sharply with
an
Fig.6–The distribution of microhardness of the welded joints,(a)bottom and (b)
top.
Fig.7–Hall –Petch relationship between grain size
and microhardness at the FZ of the top of the welded
joint.718
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increase of the heat input.This is because this area is close to the heat source during welding and then large numbers of hardβ-Mg17(Al,Zn)12particles become precipitated in it. Hence,in this situation,the microhardness was dominated by the effect of dispersion strengthening.
Acknowledgements
This research was financial supported by a Research Fund for the Doctoral Program of Higher Education of China(Project No. 20070611029)and the Key Scientific and Technological Project of Chongqing(Project No.CSTC,2009AC4046).
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