Materials Science and Engineering A 516(2009)23–30
Contents lists available at ScienceDirect
纸花的折法Materials Science and Engineering隔着门缝吹喇叭
A
j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /m s e
a
Microstructure and mechanical properties of Mg–10Gd–2Y–0.5Zr alloy recycled by cyclic extrusion compression
T.Peng a ,b ,Q.D.Wang a ,b ,∗,J.B.Lin a
a National Engineering Rearch Center for Light Alloy Net Forming,Shanghai Jiao Tong University,Shanghai 200240,China b
State Key Lab of Metal Matrix Composites,Shanghai Jiao Tong University,Shanghai 200240,China
a r t i c l e i n f o Article history:
Received 26October 2008
Received in revid form 21February 2009Accepted 13April 2009
Keywords:
Cyclic extrusion compression (CEC)Machined chips
Rare earth magnesium alloy Solid-state recycling Mg–10Gd–2Y–0.5Zr
a b s t r a c t
Cyclic extrusion compression (CEC)as a new solid-recycling processing was applied to recycle the Mg–10Gd–2Y–0.5Zr alloy.The microstructure and mechanical properties of the recycled alloy were stud-ied.Results showed that after 6-pass CEC processing at 673K,equiaxed grains of <5m were obtained.Meanwhile,deep cracks caud by interfaces decohesion between chips were almost vanished.More cond pha particles were obrved in the recycled specimens than ingot-procesd ones.High tem-perature CEC of the recycled specimens can accelerate the precipitation of the cond pha particles.Ductility of the recycled alloy mainly depended on the degree of elimination of the interfaces between the chips.After 4-pass CEC processing at 723K,the recycled alloy exhibited a combination of high strength and excellent ductility,and this strength can be ascribed to the grain refinement,uniform distribution of oxide and the precipitation of Mg 24(Gd,Y)5,as the cond pha particles.
©2009Elvier B.V.All rights rerved.
1.Introduction
豹纹陆龟Magnesium alloys currently as the lightest structure materials have been applied to such us as automobile parts and electric appliance cas [1,2].In order to increa the applicability of Mg alloys,it is necessary not only to attain excellent properties,but also to develop their low-cost methods for efficiently reclaiming them from scrap or machined chips.Several recycling process,such as re-melting,electro refining in molten salt and vacuum distillation,have been propod and some have been carried out [3].
Recently,solid-state recycling by hot extrusion has been pro-pod as a new recycling method for machined chips becau its cost is relatively low,also favorable for environment protection [3].Some works on the properties of the AZ31[3–5],AZ91[6,7],AZ80[8]and ZK60[9]magnesium alloys produced by solid-state recy-cling were investigated,and the following common results were obtained,which were that magnesium alloys recycled by this means showed relatively higher strength due to the grain refinement and uniform dispersion of oxide contaminant,but elongation decread compared with tho extruded specimens from the original ingots.Nevertheless,it is difficult to improve both the strength and ductil-ity of the recycled alloys by conventional extrusion means.
∗Corresponding author at:National Engineering Rearch Center for Light Alloy Net Forming,Shanghai Jiao Tong University,Shanghai 200240,China.Tel.:+862154742715;fax:+862134202794.
E-mail address:wangqudong@ (Q.D.Wang).Back pressure during hot extrusion as above has not been exerted during the solid-state recycling in precious articles.How-ever,back pressure was ud successfully in the processing of bulk materials which was hard and difficult to deform at room tem-perature [11–14].Furthermore,it was well-established that the imposition of a back pressure was beneficial in the consolidation of metal particles [13,15,16].In the prent study,a CEC process,where a back pressure is introduced,is applied to in order to improve the ductility of the recycled alloy.Machined chips filled into the chamber are deformed and consolidated as a bulk billet of high density in the die by multi-pass CEC.The recycled alloy exhibits a combination of high strength and excellent ductility by this means.For comparison,CEC is procesd from the original Mg–10Gd–2Y–0.5Zr alloy ingot block under the same conditions as for machined chips operated by CEC.The microstructures evo-lution and mechanical properties with different pass number and deformation temperature during CEC processing are investigated.Improvement mechanisms are also discusd in this study.2.Experimental procedure 2.1.Material preparation
The material ud in this study was a GW102K (Mg–9.95wt%Gd–2.3wt%Y–0.46wt%Zr)magnesium alloy.Two staring forms of the GW102K alloy were ud for CEC.One was chips with average dimensions of 8mm ×4mm ×80m from machining an original ingot (which was solution treated at 490◦C for 8h)in a
0921-5093/$–e front matter ©2009Elvier B.V.All rights rerved.doi:10.1016/j.ma.2009.04.024
24T.Peng et al./Materials Science and Engineering A516(2009)23–30
lathe.For comparison,the other was the original ingot which was also solution treated in the same conditions with the chips ud as above.
2.2.Pre-compaction of machined chips
In order to avoid over-shrinkage distortion phenomenon of the chips in the pressure during this CEC processing,the chips were firstly converted low intensity blocks by pre-compaction method before CEC.Pre-compaction technology was as follows:the chips were placed in a cylindrical container with a diameter of29.5mm, and then compresd at573K with a pressure of200MPa in air. 2.3.Cyclic extrusion compression(CEC)
Fig.1shows the CEC die ud in this investigation and its oper-ation procedure.The CEC processing was carried out by pushing a specimen from one cylindrical chamber with a diameter D,into the cond chamber with the same dimensions,through a die with the smaller diameter d.For thefinal extrusion,the opposite Ram was removed.In the prent study,D and d are30mm and 20mm,respectively.The pre-compaction blocks and the original ingots had the geometry of29.5mm(diameter)×42mm(length). The number of extrusion pass was defined as the number of the specimen pass through the die.The number of extrusion pass for the two forms was1,2,4,and6,respectively.At the final pass,one Ram was removed so that the other Ram could extrude the specimen in a rod shape with20mm in diameter. The CEC processing was carried out from673K to773K with a constant processing rate of25mm/S.The samples from the original ingot and compacted chips by CEC were named the ingot-procesd specimens and the recycled specimens,respectively,in this study.
2.4.Microstructural analysis and mechanical properties test
Theflat tensile specimens with a gauge ction of 10mm×3mm×1.5mm were cut from the samples procesd by CEC with an electric-sparking wire-cutting machine.Tensile tests at room temperature were carried out at strain rates of 5×10−4/s using the Zwick T1-Fr020TN.A50.The recycled and the i
ngot-procesd specimens were etched in a solution of1ml nitric acid+20ml acetic acid+19ml distilled water+60ml ethylene alcohol for the microstructure obrvation.The grains size was determined using a linear intercept method from a large number of nonoverlapping measurements.Fractographs were done by scanning electron microscopes(SEM,Jeol2100).X-ray diffraction, operated at40kV and200mA,was also ud to analyze the precipitation of the cond pha
particles.Fig.1.Diagram of the CEC facility and procedure(D=30mm,d=20mm):(a)initial state,(b)extruding with a pushing pressure on the Ram A,(c)end of Ram A,(d) rever extruding,(e)end of Ram B,(f)final extruding to obtain a rod.
3.Results and discussion
3.1.Microstructures
Fig.2shows the optical photographs of the original GW102K Mg ingot and the chips before CEC.The average grain size of the ingots and chips was determined to be120m and70m,respectively. Eutectic phas in grains boundaries were dissolved in the matrix during solution treatment.There were many twins in the chips caud by induction of vere plastic strain during machining[17] or processing of samples preparation.CEC processing was carried out from673K to773K for the ingot and the compacted chips in this study.Fig.3shows the longitudinal microstructures of the recycled specimens procesd by CEC with1-,2-,4-and6-pass at673
K, Fig.2.Optical photographs of the GW102Mg alloy just before extrusion:(a)the original ingot(b)the machined chips.
一醉
T.Peng et al./Materials Science and Engineering A 516(2009)23–30
25
Fig.3.Longitudinal microstructures of the recycled specimens procesd by CEC with:(a)1-pass,(b)2-pass,(c)4-pass and (d)6-pass at 673K.
respectively.The average grain size of the recycled specimens was determined to be 20m,10m,8m and 4m,respectively.The grain refinement after CEC proved that the processing pass number was an important factor to control the microstructure of the recy-cled specimens,and the more processing pass,the finer grains.Up to 6-pass,the equiaxed grains of <5m were obtained and homogeneously distributed,which suggested that dynamic recrys-tallization took place during CEC processing.Furthermore,effect of the processing temperature on grains sizes of the recycled alloy was also obvious during CEC,and the lower processing temperature,the finer grains.The longitudinal microstructures of the recycled speci-mens procesd by CEC with 2-and 6-pass at 723K are also shown in Fig.4.The average grain sizes of tho recycled specimens was 14m and 9m,while that of the recycled ones procesd with 2-and 6-pass at 673K was correspondingly 10m and 4m.Nev-ertheless,relatively low processing temperature such as 673K with relatively fewer pass was difficult to thorough cohesion between the chips.Just as shown in Fig.3,the interfaces between the chips in the specimens procesd by CEC at 673K could be obrved,almost vanished up to 6-pass which would be also clarified by fractographs in this essay.
For the specimens procesd at 723K as shown in Fig.4,the interfaces between the chips almost vanished by only 2-pass CEC deformation.
Grains of the ingot-procesd specimens were obviously coarr than tho of the recycled ones as shown in Figs.3and 5.This could be expected for three reasons:(1)grain size of the original ingot was larger than that of the machined chips,(2)huge stress between the chips and a lot of twin boundaries within the chips during the CEC processing induced dynamic recrystallization easier and (3)huge stress between the chips accelerated the precipitation of the c-ond pha particles which could inhibit growth of the recrystallized grains during CEC
processing.
Fig.4.Longitudinal microstructures of the recycled specimens procesd by CEC with:(a)2-pass and (b)6-pass at 723K.
26T.Peng et al./Materials Science and Engineering A 516(2009)
大理沙溪古镇住宿23–30
肉汤面Fig.5.Longitudinal microstructures of the ingot-procesd specimens procesd by CEC with:(a)1-pass,(b)2-pass,(c)4-pass and (d)6pass at 673K.
Besides,more cond pha particles were obrved in the recycled specimens than ingot-procesd ones.The GW102K alloy mainly included three ,␣-Mg,␥and Mg 24(Gd,Y)5,respectively [10].XRD patterns of the original ingot and the recy-cled specimens procesd by CEC at 673K are shown in Fig.6.High temperature CEC of the recycled specimens could acceler-ate the precipitation of the cond pha particles.The intensity of detected peaks incread gradually with the increasing of extru-sion pass,that was,Mg 24(Gd,Y)5,as the cond pha particles [10]were more precipitated during CEC processing,which would be also discusd in the fractographs of the recycled specimens in this essay.
3.2.Mechanical properties
3.2.1.Specimens from machined chips recycled by CEC
加入九三学社Mechanical properties of the recycled specimens with various extrusion pass and temperature at room temperature are listed in Table 1.For the specimens procesd at 673K,the more extrusion pass,the better mechanical properties.The ductility for 1-pass CEC specimen procesd at 673K
was 0.83%,to reflect the fact that the chips had been little welded.For the 6-pass CEC spec-imens procesd at 673K,the elongation incread to 13.18%from 2.53%of the 2-pass CEC ones.This big improvement was mainly attributed to the further elimination of the interfaces between
the
Fig.6.XRD patterns of the original material and recycled specimens procesd by CEC at 673K.The ellip indicates the characteristic peaks of the cond pha.
T.Peng et al./Materials Science and Engineering A 516(2009)23–30
27
13英文Table 1
Mechanical properties of the recycled specimens by CEC with various extrusion pass and temperature at room temperature.CEC temperature/number of pass YS (MPa)UTS (MPa)Elongation (%)673K/1-pass 211.72222.380.83673K/2-pass 253.64274.83 2.53673K/4-pass 268.06313.85 6.54673K/6-pass 258.21322.9613.18723K/2-pass 202.97272.3911.25723K/4-pass 215.99293.8620.73723K/6-pass 222.26303.8519.66773K/2-pass 179.56270.4516.53773K/4-pass
206.78
279.13
15.22
chips from 1-to 6-pass.Since the interfaces,originally consist-ing of oxide films and pores,could retard the welding between the chips,their existence could result in poor ductility.Conventional extrusion could make oxide contaminants disperd in the recycled specimen parallel to extrusion direction [17].As CEC was carried out,specimens were deformed in the pressure from three different directions.As a result,the oxide film could be more easily broken into small particles and disperd in the matrix,and moreover the pores were clod by high pressure.Hence,the ductility of the recy-cled specimens could be restored to a high level after multi-pass CEC processing.This can be obrved by comparing microstructure evolution (interfaces shown in Fig.3)and ductility trend (shown in Table 1).Besides,the tensile strength and yield strength were improved by 21.9%and 45.2%,respectively,from 1-to 6-pass at 673K.As Table 1shown,the increa in tensile strength was directly related to the increas in ductility since the stress–strain curve was raid and extended to give a greater tensile strength.Unlike the tensile strength,the yield strength incread significantly by 23.58%from initial 1-pass to 2-pass and slightly for the latter 4-pass to 6-pass at 673K.According to Hall–Petch formula,this strength due to the grain refinement was mainly improved from initial 1-pass to 2-pass,just as the microstructures evolution shown in Fig.3above.It was to be expected,since the introduc-tion of vere plastic strain during machining [3],deformed grains easily became equiaxed shape by deformation-induced recrystal-lization in the recycled specimens during initial 1-pass to 2-pass CEC processing.
In addition,the processing temperature was an important factor to mechanical properties of the recycled specimens.The mechan-ical properties of the recycled specimens with various CEC pass at 723K and 773K,respectively,are also listed in Table 1and plot-ted in Fig.7.For the specimens procesd with the same 4-pass,the elongation firstly incread to 20.73%at 723K from 6.54%at 673K,then decread to 15.22%at 773K.This was bad on the
Table 2
Mechanical properties of the ingot-procesd specimens by CEC with various extru-sion pass at 673K at room temperature.CEC temperature/number of pass YS (MPa)UTS (MPa)Elongation (%)T4heat-treated 130.58235.17 3.54673K/1-pass 205.59257.26 4.06673K/2-pass 194.15277.3213.71673K/4-pass 238.93317.1022.11673K/6-pass
232.14
296.73
24.89
fact that low extrusion temperature was detrimental to cohesion of the interfaces between the chips.
Meanwhile,high extrusion tem-perature caud relatively coarr grains although the interfaces cohesion between the chips was easier.This meant that the extent of ductility restoration in the recycled specimens mainly depended on the degree of elimination of the interfaces between the chips.For the same 2-pass,with the increasing of extrusion temper-ature,the yield strength decread,the elongation incread,but the ultimate tensile strength maintained the trend of unchanged which was caud by the fact that some loss in strength was due to the poor grain refinement at relatively high temperature during CEC processing.
3.2.2.Specimens from ingot-procesd by CEC
Mechanical properties of the ingot-procesd specimens with various extrusion pass at 673K are shown in Table 2.Unlike the other properties,the yield strength incread significantly by 57.9%from the original ingot to 1-pass CEC specimens and slightly for the other 2-,4-and 6-pass CEC processing.As for the elongation and ultimate tensile strength,there was a continuous improvement up to 4-pass CEC.Both the yield strength and tensile strength decread slightly when CEC pass were incread to 6-pass from 4-pass,which could be caud by deforming texture soften-ing [18].The overall improvements of the yield strength,ultimate tensile strength and elongation from original ingot to 6-pass CEC specimens were up to 77.8%,26.2%,and 6030%,resp
ectively.This also indicated that the properties were more nsitive to the grain refinement.
Comparing the recycled with the ingot-procesd specimens procesd at 673K as shown in Table 1and Table 2,the relation-ships between room temperature mechanical properties of the recycled and that of the ingot-procesd specimens with differ-ent CEC pass
at 673K are plotted in Fig.8.The results indicated the yield strength of the recycled specimens was superior to that of ingot-procesd ones.This disagreement was mainly related to different ability of the grain refinement with the same extrusion pass just as shown Figs.3and 5,also to the precipitation of the
Fig.7.Mechanical properties of the recycled specimens as a function of extrusion temperature with the same (a)2-pass and (b)4-pass at room temperature.
