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1、Science Press Available online at Trans.Nonferrous Met.SOC.China 16(2006)818-823 Transactions of Nonferrous Metals Society of China bi Electron beam welding of SiCp/LD2 composite CHEN Mao-ai(I%?%),WU Chuan-song(i$j$j%),ZOU Zeng-da(%P%)Key Laboratory of Liquid Structure and Heredity of Materials,Mini
2、stry of Education,Shandong University,Jinan 250061,China Received 18 October 2005;accepted 23 November 2005 Abstract:The 2 mm-thick SiC&D2 composite plates were electron beam welded at different heat inputs.The microstructures of welds were investigated by OM,TEM,SEM,and XRD,and the properties of we
3、lds were measured with MTS-810 testing system.The results show that the quantity and size of acicular A14C3 precipitates(interfacial reaction product)decrease with the heat input decreasing.When the heat input lowers to 30 J/mm,the formation of needle-like A14C3 can be prevented.The distributions of
4、 Sic in the fusion zones are more uniform than that in as-received composite.TEM analysis reveals that there are A14C3 crystals on the surface of every survived particle,the needle-like A14C3 observed under the optical microscope consists of many tabular A&C3 crystals which have different orientatio
5、ns.With the increase of heat input,the fracture mechanism changes from ductile one to brittle one,the quantity of fractured particles on the fracture face decreases and the strength and ductility of the weld decrease.Key words:SiCdLD2 composite;electron beam welding;interfacial reaction 1 Introducti
6、on Sic particle reinforced aluminum composites(SiCdAl)possess excellent properties,e.g.high strength-to-mass ratio,high stiffness-to-mass ratio,good wear resistance and higher service temperature,therefore they are considered a kind of promising structural material.With the gradual reduction in prod
7、uction cost,an increase in uses and application may be expected in the next years.For this reason,welding technologies of this lund of material have attracted a lot of attention recently.Previous studies reveal that almost all fusion welding processes produce deleterious microstructural alterations
8、when they are applied to SiCdAl composites l-91.In TIG and MIG welding pool,the strong rejection of Sic particles by solidification front results in microsegregation and inhomogeneous particle distribu-tion,thus reducing the reinforcement efficiency 1-31.The high hydrogen content in the A1 matrix co
9、mposites(especially the one produced by powder metallurgical routes)and low flowability of molten pool tend to result in serious porosity in both weld and HAZ 2-51.The vacuum degassing prior to welding can eliminate the porosities in the TIG and MIG welds2,3,5.During welding,serious interface reacti
10、on between Sic particles and A1 matrix occurs and plate-like A14C3 precipitates form,which are very brittle and deleterious to the pro-perties of the weld2,3,7,8.However,through applying Al-Si filler metal and/or lowering heat input,the interfacial reaction can be reduced or suppressed2,3.By using s
11、olid-state joining,the above-mentioned problems can be eliminated.Therefore much more research work has been done in solid-state joining(such as diffision bonding and friction welding)of Sic particle reinforced aluminum composites and results are encouraging 10-121.However,the low production efficie
12、ncy and suitability limit the application fields of those procedures.Thus,it is still necessary to reconsider the possibility of application of the more productive welding processes,fusion welding.In this paper,the influence of heat input on microstructure and properties of the EB(electron beam)weld
13、s in SiCdLD2 were studied.The emphasis was placed on the analysis of effect of welding cycle on the particle distribution,A1-Sic interface morphologies and the interfacial reaction products in the welds.The influence of interfacial reaction on the properties and fracture mechanism of the EB welds we
14、re also discussed.The mechanism of interfacial reaction was also reviewed.Foundation item:Project(2004BS05010)supported by Shandong Youth Doctoral Foundation,China Corresponding author:CHEN Mao-ai;Tel:+86-53 1-8839271 1;E-mail:chenmaoaisdu, CHEN Mao-ai.et aLTrans.Nonferrous Met.SOC.China 16(2006)819
15、 2 Experimental 2.1 Material The material used in this investigation was 2 mm thick plate of Sic particle reinforced LD2 aluminum(SiCdLD2).The volume fraction of Sic particle was 10%.2.2 Welding machine and procedure The 2 nun thick as-received plate was cut into to-be-welded pieces with 500 mm in l
16、ength and 50 mm in width.Before welding,the pieces were cleaned with acetone thoroughly to eliminate dirt and grease.Welding experiments were performed with EBW2100115-15OCNC.The welding parameters are listed in Table 1.Heat inputs in Table 1 were calculated as the product of accelerating voltage an
17、d beam current divided by traveling speed.2.3 Microstructure analysis and mechanical testing Metallurgical specimens were sectioned from the weldment in transverse to the welding direction.The specimens were first polished with diamond paste and water,and finally polished with diamond paste and alco
18、hol to prevent A14C3 that may be present in the weld from decomposing.After polishing,the specimens were cleaned with acetone.Prepared specimens were observed with Neophto-20 optical microscope.Quantitative metallography was used to measure the mean size of A&in individual weld.X-ray diffraction was
19、 performed by Dlmax-rc diffractometry on the specimens sectioned from the top of the weld along the welding direction.The A1-Sic interface morphology in weld metal was examined with H-800 transmission electron microscope.And the fracture surface was examined with Philips-5 15 scanning electron micro
20、scope.The tensile testing was performed on MTS-8 10 testing system.3 Results and discussion Fig.1 shows the typical microstructures of the EB welds in SiCdLD2 made at different heat inputs.In the weld made at a heat input of 30 Jlmm,the fusion zone microstructure mainly consists of Sic and A1 matrix
21、(Fig.l(a).The A1 matrix possesses a very fine cellular solidification structure(3 to 4 pm apparent cell spacing)because of high cooling rate.Compared with the particles in the as-received composite,the surface of survived Sic particle in the fusion zone is much coarser.This suggests that dissolution
22、 of Sic particle occurs during welding even at a heat input of 30 Jlmm.In the weld made at a heat input of 36 J/mm,some small-sized needle-like precipitates are observed only in the top center(Fig.l(b)of the fusion zone.X-ray diffraction analysis indicates that the needle-like precipitates are A14C3
23、.Similar precipitates are also observed in TIG and MIG weld of SiCd6061Al and this phase is considered to be deleterious to the properties of material2-4.Those precipitates are products of the following interfacial reaction between Sic and molten Al1,21:4A1+3 SiC-A4C3+3Si The average length of the n
24、eedle-like A&in the 36 Jlmm fusion zone is 6-7 pm.Most of the A14C3 precipitates lie around the survived Sic and some of them are continuous with the survived Sic particles.In the weld made at a heat input of 42 J/mm,A4C3 precipitates are observed in the whole fusion zone.The number and average size
25、 of needle-like A14C3 decrease with the distance from the top center of the fusion zone.The A&C3 needles are randomly oriented(Fig.l(c).Their average length are about 20 pm.When the heat input increases to 48 Jlmm,the average length of needle-like A14C3 increases to about 30 pm,and forms interconnec
26、ted networks and the number and the size of survived particles decrease(Fig.l(d).It is apparent from Fig.1 that the amount and size of needle-like A4C3 vary with the heat input.A high heat input results in heavy dissolution of Sic and more and larger A4C3 precipitates.By controlling the heat input,i
27、t is possible to avoid the formation of needle-like A14C3.TEM observation indicates that tabular phases exist at the interface between every survived Sic and Al.Fig.2 shows a typical TEM micrograph of Sic-A1 interface in the 30 Jlmm fusion zone.The electron diffraction pattern indicates that these p
28、hases are A14C3 crystals.This suggests that even welding at a heat input of 30 J/mm,slight interfacial reaction does occur,although no needle-like phases are observed under optical microscope.It can Table 1 Welding parameters Weld No.Accelerating voltageN Beam current/A Focusing current/A Traveling
29、speed/(mms-)Heat input/(Jmin-)1 150 5 350 25 30 2 150 6 350 25 36 3 150 7 350 25 42 4 150 8 350 25 48 820 CHEN Mao-ai,et aVTrans.Nonferrous Met.SOC.China 16(2006)be seen that the tabular A14C13 precipitates disperse along the surface of the Sic particles rather than form a continuous layer around Si
30、c.Generally,most of A14C3 tablets form on the surface of the SIC particle and grow into the A1 region at the same geometric direction(Fig.2(a),while some A14C3 rods grow into the Sic particles(Fig.2(b).Considering that the diffusion rate of A1 in solid Sic is very slow.It can be concluded that the A
31、14C3 forms through the following steps:1)dissolving of Sic in liquid A1 and thus generating of Si and C;2)formation of A14C3 near the interface.The dissolution of the interior part of the Sic particles results from the crystal structure defects.TEM analysis also reveals that needle-like A14C3 precip
32、itates observed under optical microscope consist of a lot of discrete A14C3 tablets(Fig.3(a).These tabular A14C3 precipitates have different crystal orientations(Fig.3(b),although they nearly have the same geometric direction.The interfacial reaction products between Sic and A1 include A4C3 and free
33、 Si,but no Si crystals are found at the Sic-A1 interface.While equiaxed Si crystals,in single or a group,are observed in the A1 region(Fig.4(a).Other researchers5,6 also did not found Si on the Sic surface while studying the interfacial products of SiCp/Al composites with TEM or HREM.Those observati
34、ons suggest that Si produced by interfacial reaction have diffused into the A1 matrix at a high rate.In addition,Al-Fe-Si phases are found on the grain boundary ofAl in the EB weld(Fig.4(b).Fig.5 shows the particle distributions in EB welds and as-received SiC,/LD2 composite.The particles in the Fig
35、.1 Typical microstructures of EB welds in SiC,/LD2 made at different heat inputs:(a)30 J/mm;(b)36 J/mm;(c)42 J/m;(d)48 J/llUn Fig3 TEM images of Sic-A1 interface in 30 J/mm fusion zone:(a)AI4C3 tablets gowing into A1 region;(b)A14C3 tablets growing into Sic particle CHEN Mao-ai,et al/Trans.Nonferrou
36、s Met.SOC.China 16(2006)82 1 Fig3 TEM morphologies of needle-like A14C3 in 42 J/mm fusion zone and its diffraction pattern:(a)TEM morphology of needle-like A&;(b)Diffraction pattern Fig.4 Si crystals(a)and Al-Fe-Si phase(b)in A1 region as-received composite exhibit a slight laminar distribu-tion alo
37、ng the rolling direction(Fig.S(a).And the Sic particles in the EB weld are distributed much more uniformly(Figs.S(b)and 5(c).The uniform distribution of particle in weld may be attributed to the key-hole effect and the rapid cooling rate of the EB pool.During welding,the vaporizing of A1 matrix and
38、colliding of the atom to the composite result in a formation of keyhole.The melted A1 around the keyhole is subjected to a violent stirring movement which might make the Sic particles distribute uniformly in the weld pool.The particle distribution in weld depends on the particle distribution in molt
39、en pool and the rejection of the solidification.It is generally accepted that solidification rate influences the rejection effect of solidification front to the particle.As the solidification rate increases,the rejection effect decreases 151.According to the estimation of Loloyd,the cooling rate of
40、the EB welds is in the range of approximate 3 000-6 000“C/s at a heat input of 33.5 J/mml6.With such a high cooling rate,almost no particle rejections exist,as a result,the particle distributions in the EB fusion zones are fairly uniform.Hydrogen porosities often observed in TTG welds and MIG welds
41、are not found in the fusion zone of the EB welds.The possible reason may be as follows:the hydrogen in the molten pool can escape easily because of the vacuum environment of EB welding and the violent stirring of the molten pool;furthermore,no cracks are observed in the EB welds due to the rapid coo
42、ling of molten pool.When welded at a heat input of 30 J/m,the work-pieces were not completely penetrated,therefore no mechanical property tests were carried out for the 30 Jlmm weld.All other weldments were tested under as-welded condition,and failed in the weld metal during testing.The maximum tens
43、ile strength measured is about 60%of the as-received composite strength,the maximum elongation is about 40%of the as-received composite elongation.This indicates that the interfacial reaction exerts more effect on the ductility than on the tensile strength.The tensile testing data for individual EB
44、weld are summarized in Table 2.It can be seen from Table 2 that the tensile strength and ductility(elongation)decrease with the heat input increasing During tensile fracture of SiCdAl composite,the crack propagation path involves Sic particles,A1 matrix and particle-matrix interface 171.SEM examinat
45、ion of different fracture surfaces shows that the extent of ductile dimpling decreases with the heat input increasing and the failure mechanism varies with the heat input.Fig.6 shows 822 CHEN Mao-ai,et aVTrans.Nonferrous Met.SOC.China 16(2006)Fig.5 Particle distributions in EB welds and as-received
46、composite:(a)As-received composite;(b)30 J/mm EB weld;(c)42 J/mm EB weld Table 2 Tensile ProDerties of EB welds on SiCJLD2 Elongation/Remark Heat input1 strength/%Serial No.(J.mm-)MPa 303 5.6 T6 As-received O composite-Incomplete 1 30 penetration 2 36 204 2.1 As-welded 198 2.3 196 2.6 3 42 169 1.3 A
47、s-welded 153 1.4 160 1.6 4 48 120-As-welded 112 107-Fig.6 SEM fractographs of EB welds metal and as-received composite:(a)As-received composite;(b)36 Jimm weld;(c)48 J/mm weld the typical fracture morphologies of as-received composite and individual EB welds.The fracture surface of the as-received c
48、omposite exhibits typical ductile morphology with a lot of fine dimples in the A1 matrix region between particles,there are some fractured particles on the fracture surface(Fig.6(a).This indicates that the fracture process of as-received material is controlled mainly by matrix ductile failure and pa
49、rticle fracture mechanism.The fracture surface of the 36 J/mm weld also exhibits evident ductile morphology,the quantity of dimples is much less than that of as-received composite.Although most particles survive the welding cycle,few fractured particles are observed on the fracture surface(Fig.6(b).
50、This is because that the A14C3 tabular and the dissolution of the Sic surface during welding reduce the strength of Sic-A1 interface.The cracks progress along the Sic-A1 interface when they meet the particles.In this CHEN Mao-ai,et al/Trans.Nonferrous Met.SOC.China 16(2006)823 case,the weld fracture