Direct welding of different metals used ultrasonic vibration

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y209(2009)954–960j 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/j m a t p r o t ecDirect welding of different metals used ultrasonic vibrationShin-ichi Matsuoka a,Hisashi Imai b,∗a Department of Mechanical Systems Engineering,Faculty of Engineering,Toyama Prefectural University,5180Kurokawa,Imizu939-0398,Japanb Materials Joining Mechanism,Composite Materials Processing,Joining and Welding Research Institute,Osaka University,11-1 Mihogaoka,Ibaraki,Osaka567-0047,Japana r t i c l e i n f oArticle history:Received9August2007 Received in revised form 23February2008 Accepted2March2008Keywords:Ultrasonic welding BondingWelding pressure Required duration Welding energy a b s t r a c tThis paper describes an experimental study on ultrasonic welding of aluminum and copper alloy.There are several welding techniques which are expected to replace other welding and brazing processes.In this study the relation between energy density and welding pressure in welding certain types of aluminum alloys was clarified.The welding energy is effectively used in the ultrasonic welding of aflexible,narrow material with a narrow pressurization area.The ultrasonic welding of Al/Cu can be accomplished when the conditions of ampli-tude:15␮m,welding pressure:20MPa,at a required duration of1.0s under the water bath. Furthermore,the oxidefilm and organic coating are periodically removed from bonded inter-faces by ultrasonic wave vibration,and it can be expected to form transition layer of1–2␮m at the bonded interface.©2008Elsevier B.V.All rights reserved.1.IntroductionJoining and adhesion technology is essential in everyfield. Welding in the automotive,shipbuilding and architectural industries,and brazing in electronic components manufac-ture are good representative examples.Miniaturization and weight saving,further,are increasingly the recent trend in the manufacture of electric appliances and automobiles and,here, technology for joining dissimilar materials is indispensable.Traditionally,when joining dissimilar materials is needed, solid phase joining,adhesion and mechanical joining are used.Solid phase joining includes various methods,such as friction welding(Cabello Mu˜noz et al.,2008),explosion bond-ing(Kahraman et al.,2005),ultrasonic welding(Imai and Matsuoka,2005),and diffusion bonding(Fillabi et al.,2008).For example,it is possible to join aluminum and copper together. Bonding material of aluminum and copper is useful material∗Corresponding author.E-mail address:imai@jwri.osaka-u.ac.jp(H.Imai).concerning lightening,and thermal and electrical conductiv-ity.However,hardly and weakly intermetallic compounds are easy to form at welding interface in conventional welding pro-cess in wide composition range.Therefore,the application of the solid phase bonding method has been examined for the welding of aluminum and copper.At present,a novel brazing technique(Timsit and Janeway,1993)and the method using soldering of Zn–Al alloy are influential joining methods.Of these,the ultrasonic welding method allows easy joining in a short time and with a simple facility in any environment such as in water and in the vacuum.Additionally,it has the merit of direct joining of dissimilar materials such as Cu/Al without requiringflux(Matsuoka,1989).However,some of the various materials do pose problems that limit welding possibilities,yet few reports concerning these have been published(Matsuoka,1987;Ibrahim et al., 2005;Kong et al.,2003).0924-0136/$–see front matter©2008Elsevier B.V.All rights reserved. doi:10.1016/j.jmatprotec.2008.03.006j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 209(2009)954–960955Fig.1–Basic principle of ultrasonic welding.The present research took up welding of dissimilar metals using ultrasonic vibration in any environment (in ordinary-temperature atmosphere and underwater),investigated the possibilities of joining,joined strength,temperature and the properties occurring in the joining interface,and studied the mechanism of ultrasonic welding.2.Welding method and conditionsFig.1shows an outline of ultrasonic welding.Welding pro-cesses are as follows;Materials are held by anvil and horn in horn’s pressurization,and ultrasonic vibration is applied to the horn in the condition to held materials.The out-put of the welding equipment is 600W and 1200W ,with a frequency of 19kHz and 15kHz,respectively.The required duration t :0.1–8.0s,and welding pressure P c :5–60MPa.An anvil was formed sink-like in the underwater welding.The system which calculated the energy (the electric power value)utilized in the welding was introduced in order to observe the resistance energy in the welding to the horn.The temperature on the welding interface was estimated from an electromotive force between a thermocouple K and materials.The welded strength of the welded material was evaluated from a shearing tensile test,and welding char-acteristics and welding mechanism were studied by SEMTable1–Properties of aluminum alloys used for ultrasonic welding Al alloysThickness (mm)Hardness (Hv25)SymbolA1050H-240.3,1.0,1.538.28AH A1050-O 1.027.99A1-O A3003-O1.035.77A3-OFig.2–Relation between welding strength and required duration of ultrasonic welded products of AH(0.3)/AH(1.0)(amplitude:30␮m (p-p)).observation of the welding interface,EDX ray analysis,or Auger analysis.3.Kind and welding characteristics of Al alloysTable 1shows the characteristics of Al material used for weld-ing.AH alloys are described with AH (thickness).Fig.2shows the relationship between the welding pres-sure and required duration affecting thewelded strength of an AH(0.3)/AH(1.0)welded material.Fig.2further shows that the welded strength increases with the increase of the required duration.For example,the welded strength becomes roughly constant when the required dura-Fig.3–Relations between energy density and welding pressure of ultrasonic welded products (amplitude:35␮m(p-p)).956j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y209(2009)954–9602Welding materials K nAH(0.3)/AH(1.0) 3.73×103−2.66AH(0.3)/A1-O 4.72×103−3.19AH(0.3)/A3-O 3.99×103−2.70tion exceeds0.5s with the welding pressure P c of8MPa,and2.0s with the welding pressure P c of6MPa.The area wherematerial transferred to welded interface after the tensile testis defined as a true welding area.A true welding area is oneof the guideposts of stabilization of the welded strength,andthe true welding area was confirmed to be about70%or morefor horn area in any case.Fig.3shows the relationship between the welding pres-sure and the energy density required,therefore,for a weldedmaterial providing the stable welded strength.Fig.3is for(a)AH(0.3)/AH(1.0),and for(b)AH(0.3)/Al-O.Here,the energy density means a value obtained by sub-tracting energy at the time of no-load from the charged energy,and by dividing the value by a pressurizing area(16mm2).The line in Fig.3shows boundary condition of the weldability.Welding is possible under the condition of higher energy andpressure than the line condition.Fig.3shows that,in any welded material,the weldingenergy required for welding decreases as welding pressureincreases.In ultrasonic welding,weldability has a close rela-tionship with the hardness of a material.The smaller thehardness,it is said,the more efficient is the transfer of thewelding energy.Fig.3shows that use of a soft material pro-vides a satisfactory welded material.The following relation isintroduced from these results:E=KP n c(1)Here,E is the energy density;P c is the welding pressure.Table2shows values of K and n.And,following equations are obtained when relationbetween welding strength and P c in the previous report(Imaiand Matsuoka,2005)and this result are combined:B=EKK(2)This shows that the welded strength is largely affected by the magnitude of the welding energy(E).Since E seems to be resistance value on the horn,the relational expression between strength and energy E is similar to the relationship between welding pressure and strength in friction welding (Sahin et al.,2008).Further,Fig.4shows the investigative results of the rela-tionship among the magnitude of amplitude,welding pressure and required duration in the AH/Al-O welded material.Fig.4 further shows a range allowing satisfactory welding.The required duration can be shortened with the increase of weld-ing pressure in any welding.Under the same amplitude,the required duration canbe shortened for larger amplitude.Fig.5shows the investigative results for influence of a plate thickness on the welding property.Fig.5further shows that the welding pressure increases with the increase of plate Fig.4–Relation between required duration and welding pressure in ultrasonic welding.(AH(0.3)/A1-O).thickness on the horn side,with the required duration of 0.5s.Further,the ultrasonic vibration scatters oxidefilms and organicfilms on the material surface to complete welding (Imai and Matsuoka,2005).Fig.6shows the investigative results of presence or absence of oxides on the welding interface of the AH/Al-0welded mate-rial.Besides the SEM observation of(a)in Fig.6,(b)and(c) in Fig.6are the results of surface analysis.SEM observation shows that the welding interface has no defect such as exfo-liation or cracks,and that welding is,therefore,satisfactory. While the presence of oxides or agglomerate can be confirmed in an incomplete welding region in(b)of Fig.6,the presence of these are not found in weld(c),showing a good interface. Thus,oxides and organicfilms are inferred to be removed in the welding interface,causing adhesion between the newly formed faces to weld(Matsuoka,1994).Fig.7shows one example of microscopic property obser-vation in the vicinity of the weld.Fig.7shows plasticflow of the material surface by pressurizing and frictional heat of the horn,causing pleat-like traces at the edge.The pleats are found to have built up about every0.1␮m,and synchronized with the ultrasonic frequency.The surface layer part softened by the frictional heat due to vibration is considered to haveFig.5–Relations between welding pressure and thickness of ultrasonically welded products of AH/A1-O.j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 209(2009)954–960957Fig.6–SEM showing internal structure (a),and surface analysis by electron probe X-ray micro-analyzer for ultrasonically welded products (b)and (c).Fig.7–Cleft surface of ultrasonic welded products of AH(0.3)/A1-O observed by SEM.sink by a pressurizing force and built up in pleats by strong lateral vibration.4.Quality of underwater welding propertyAlthough ultrasonic welding is possible in any environment,an example of underwater ultrasonic welding cannot be found in published literature,leaving various points unclear in terms of the possibilities or welding characteristics of underwater welding.Direct underwater welding of thin metal plates was investigated.Fig.8shows the outline of the weld,and Table 3shows the characteristics of the welded material.In this study,underwater welding of Cu/Al which could be welded in the atmosphere was tried.Fig.9shows the relationship between the required duration and weld strength of the Al/Cu welded material.Fig.9shows that the weld strength increases with the increase of required duration up to 0.5s in the atmospheric welding,and up to 1.0s in the underwater welding.As shown in Fig.7,ultrasonic weld-ing is possible by removing covered film of material surface toFig.8–Basic principle of ultrasonic welding under the water bath.958j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 209(2009)954–9603DescriptionThickness (mm)Tensile strength (MPa)Hardness (HV50)Al(A1050O)0.2112–12039.18Cu(C1020)0.2212–22895.25Fig.9–Relation between welding strength and required duration for ultrasonically welded products of Al/Cu (a:15␮m (p-p),P c :20MPa).synchronize with the ultrasonic frequency.The reason to need much time to obtain stabilized strength in underwater welding is that required duration is also needed to drain water on the interface in underwater welding.On the other hand,no differ-ence is found in the strength of both after stabilization.It is considered that the stable strength of the ultrasonic welding material is not dependent on the junction environment.Fig.10shows the relationship between vibration energy and welding pressure for the welded material obtaining a sta-ble strength.Fig.10shows in (a)that a high welding pressure is required to obtain a satisfactory welded material in underwa-ter welding,and that the vibration energy required for welding demonstrates an increasing tendency.Fig.11shows the result of temperature occurring on the welding interfacefound from the electromotive force occur-ring between Al and Cu.Fig.11further shows that the temperature greatly varies across the required duration of 1s in both welding cases.In welding at room temperature,Fig.11–Measured temperature of welding interface in ultrasonic welding and water bath.the temperature of the welding interface rapidly increases with the increase of the required duration.In underwater welding,the temperature slowly increases up to 1s,and sta-bilizes thereafter,while the temperature of the water rapidly increases beyond 1s.These results show that,in atmospheric welding,the tem-perature of the interface increases even after the welded strength has stabilized,while in underwater welding,only the temperature of the water increases at the same time when the welded strength has saturated.In the former case,the fric-tional heat occurring in the interface is inferred to cause an increase of the temperature,while in the latter case,the tem-perature of water is inferred to cause an increase due to heat radiation to the water.The investigative resultsof the crystal state of Cu near the welding interface show that,in both atmospheric and under-water welding,the crystal grain becomes significantly finer than in the material in the vicinity of the welding interface,Fig.10–Relation between energy density and welding pressure for ultrasonically welded products of Al/Cu (a:15␮m (p-p)).j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y209(2009)954–960959and the grain size tends to get larger with the increase of distance from the interface.In atmospheric welding,the tem-perature of the welding interface increases to nearly400◦C due to frictional heat and plastic deformation heat,exceeding the recrystallization temperature of Cu.In underwater welding,conversely,since frictional heat at the interface is radiated into the water,the temperature does not reach the region of recrystallization temperature,and crys-tal grains have been found to solidify in afine state.Fig.12shows the results of investigation of the hardness of the welding interface.The plus side distance from the inter-face is for the hardness distribution of Cu,and the minus side distance is for that of Al,respectively.Fig.12shows the increasing tendency of the hardness in the vicinity of the weld in both atmospheric and underwater welding.This shows that the crystals becomefine,causing strain hardening in the vicin-ity of the weld and an increase of hardness.Inside the welded material,atmospheric welding increases temperature,the material is recrystallized by the annealing effect,and the hardness is lowered.These results show that underwater welding has the effect of suppressing heat at the heat-affected part,compared with atmospheric welding.Fig.13shows the results of SEM observation and EDX ray analysis of the welded material.Fig.13furthershows Fig.12–Relation between distance from welding interface and Hardness.that both welded materials have satisfactory welding inter-faces causing neither hollow pores nor exfoliation.The EDX ray analysis confirms that both have transit layers of about 1␮m,and formation of diffusion and reaction layers can be expected.Although underwater welding requires a somewhat larger welding pressure compared with atmosphericwelding,Fig.13–Scanning electron micrographs showing the internal structure,and line analysis by electron probe X-raymicro-analyzer for ultrasonically welded products of Al/Cu.960j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y209(2009)954–960no difference from the welding pressure in atmospheric weld-ing is found in the range of obtaining a satisfactory welded material.5.ConclusionUltrasonic welding mainly using an Al alloy was performed, the possibilities of welding,characteristics of the welded materials and welding mechanism were studied to obtain the following results:(1)Tests of welding of Al alloys with different properties showthat satisfactory welding is possible in any test and,in par-ticular,that the magnitudes of the welding pressure and plate thickness largely affect the welding properties.The welding pressure P c and vibration energy E largely con-cern the welded strength,and the following relation was introduced:E=KP n c(2)Underwater welding requires a somewhat larger weld-ing pressure and longer welding time compared with atmospheric welding,however,the obtained strengths of the welded materials are equal and can suppress heat at the heat-affected part in the vicinity of the welding interface.(3)SEM observation of the welding interface shows that oxidefilms and organicfilms on the material surface can be removed along the frequency of vibration,allowing weld-ing of newly formed 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