Microstructure and Mechanical Property of Al?Cu?Li Alloy Joint by Laser Welding with Filler Wire

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College of Materials Science and Technology，Nanjing University of Aeronautics and Astronautics，Nanjing 211106，P.R.China

Abstract:The Al-Cu-Li alloy is welded by using laser beam welding，and the welding wire ER4043 is used as filler metal.The microstructure and mechanical property of welded joints are systematically investigated.Microstructure analyses show that the fusion zone is mainly composed of α-Al matrix phase and some strengthening phases including T，δ′，θ′，β′ and T1，etc.During welding，the weld formation and joint quality are obviously improved by the addition of Al-Si filler wire.The measurements of mechanical property indicate that，compared with that of the base metal（BM），the microhardness in the weld zone is decreased to a certain extent.Under the appropriate welding parameters，the tensile strength of welded joint reaches 369.4 MPa，which is 67.8% of that of the BM.There are many dimples on the joint fracture surface，and it mainly presents the fracture characteristic of dimple aggregation.

Key words：Al-Cu-Li alloy；laser welding with filler wire；microstructure；mechanical property

0 Introduction

Al-Li alloy has excellent comprehensive proper?ty such as low density，high specific strength and specific stiffness，and good formability，and they are widely used in aerospace field［1］.Because they are of?ten used as welded structure，some welding proce?dures such as friction stir welding（FSW）were used to weld these alloys［2］.Compared with that of the fu?sion welding，the process flexibility of FSW is insuf?ficient，and it is difficult to weld the component with complex shape.During the aircraft manufacturing process，if the riveted structure was replaced by the laser-welded structure，the structural weight and production cost could be greatly reduced［3］.At pres?ent，the laser beam welding（LBW）of Al-Li alloy has been paid more attention.As an advanced weld?ing technology，the LBW was also used to weld the high-silicon aluminum alloy and the aluminum ma?trix composite，respectively［4-5］.Moreover，the LBW of newly developed materials and dissimilar materials were increasingly applied，for example，the LBW of high temperature shape memory alloys such as precipitation strengthened Ni-rich NiTiHf and H-phase strengthened Ni-rich NiTi-20Zr，and the dissimilar LBW of a CoCrFeMnNi high entropy alloy to 316 stainless steel［6-8］.

Examilioti et al.［9］studied the effect of laser welding parameters on the weld formation of 2198 Al-Li alloy.With the increase of laser power，the morphology of weld cross section changed from nar?row V shape to Ⅰshape.The 2060 Al-Li alloy was welded by using LBW，and the effect of welding pa?rameters on weld formation，porosity and micro?hardness was investigated［10］.Cui et al.［11］analyzed the microstructure of LBW joint of Al-Li alloy.Liu et al.［12］conducted the LBW of 2060 alloy，and the relationship between microstructure and mechanical property of the joint was investigated.

Because the active element Li was added to the Al-Li alloy，it was prone to generating welding de?fects such as weld porosity and hot cracking during welding［13］.The 2198 alloy was welded by using LBW［14］，and the susceptibility to hot cracking could be decreased to a certain extent by preheating，pre?loading and optimizing welding parameters.Here，preloading means that a compressive load was added to the weldment during welding，the effect of strain localization due to solidification shrinkage would be reduced or even compensated.Compared with that of the autogenous LBW，some alloying elements can be added to the weldment during laser welding with filler wire.The chemical composition of weld metal can be adjusted，and the morphology and distribution of low melting point eutectic are improved，thus the hot cracking in Al-Li alloy weldment is greatly de?creased.In the 2195 Al-Li alloy laser-welded joint，the susceptibility to hot cracking was obviously de?creased by using the Al-Si filler metal，which could increase the amount of eutectic structure and im?prove the fluidity of molten pool［15］.Lukin et al.［16］stated that the hot cracking in the 1420 Al-Li alloy joint could be effectively reduced by the addition of Al-Mg filler wire.The AlSi12 filler wire was used to weld the 2060 alloy，and the irregular distribution of LiAlSi phase in weldment could contribute to break?ing the Al2Cu eutectic phase at grain boundary，thus the joint mechanical property was improved［17］.The Al-Li alloy joint by the addition of new CW3 filler wire had higher tensile strength and lower suscepti?bility to hot cracking［18］.

At present，investigation on the laser welding with filler wire for the Al-Cu-Li alloy is still not sys?tematic and in-depth.It is necessary to investigate the welding technological characteristics and the ef?fect of filler wire on the microstructure and property of the welded joint.In present work，the Al-Si filler wire is used to weld the Al-Cu-Li alloy.The effect of welding parameters on the microstructure and me?chanical property of the welded joint is investigated，and some results can be provided for the application of the welded structure.

1 Materials and Procedure

The base metal（BM）is the Al-Cu-Li alloy plate and its heat treatment condition is T8（solution treatment，cold working and then artificial aging）.The BM is machined to welding sample with the di?mensions of 150 mm × 50 mm × 4 mm.The joint type is butt joint.The filler metal is ER4043 welding wire with the diameter of 1.2 mm.The chemical composition of BM and filler wire is listed in Table 1.

Table 1 Chemical composition of base metal and filler wire (in weight) %

Before welding，the welding samples are pre?treated to remove thoroughly the oxide film and grease contamination on the sample surface.The la?ser beam welding machine is TruDisk-12003 type disc laser with the maximum output power of 12 kW.The KUKA KR30HA type welding robot is used to control the angle and movement of the incident laser beam during welding.The equipment of laser weld?ing with filler wire is shown in Fig.1.Welding is car?ried out along the longitudinal direction of the sam?ple，and the angle between the laser beam and verti?cal direction is 10°.After some attempt welding，combined with the observation of weld formation and the measurement of joint performance，the welding procedure is determined.The amount of defocus is 0 mm and the wire feeding speed is 5 m/min.In or?der to avoid oxidation and reduce the weld porosity，argon gas is used to protect the top and the root of the joint during welding.The argon purity is 99.99%，and the gas flow rate is 15 L/min and 10 L/min.The main parameters of laser welding with filler wire for the Al-Cu-Li alloy are given in Table 2.

Table 2 Parameters of laser welding with filler wire for the Al?Cu?Li alloy

Fig.1 Equipment of laser welding with filler wire

After welding，the microstructure of the weld?ed joint is observed by using Leica DMILM inverted optical microscope.The chemical composition of mi?cro-region in weldment is analyzed by using energy dispersive spectrometer（EDS）.The phase constitu?ent of the weld metal is identified by using D8 Ad?vance type X-ray diffractometer（XRD）.The precip?itated phases in weldment are observed by using JEM-2100F type transmission electron microscope（TEM）.The TEM specimen is prepared by the fol?lowing steps：Mechanically grinding to the thickness of about 50 μm，and then punching thin foils with the diameter of 3 mm，and double-jet electrolytic thinning with the electrolyte of 30% nitric acid meth?anol solution（volume ratio）.The microhardness in the weld zone is measured by using HXS-1000AC type hardness tester with the load of 200 g and the duration time of 15 s.The tensile property of the welded joint is tested by using CMT-5105 type elec?tronic universal material testing machine with the loading rate of 1 mm/min.The fracture morphology of the joint is observed by using JSM-6360LV type scanning electron microscope（SEM），and the ten?sile fracture characteristic of the joint is analyzed.

2 Results and Discussion

2.1 Weld morphology and microstructure

The macrographs of welded joints under differ?ent welding parameters are shown in Fig.2.Due to the relatively low laser power，the incomplete pene?tration is generated to joint 4#.Compared with those of the other joints，the weld formation of joint 2# is good，and no welding defects such as undercut and lack of fusion are generated in the weldment.Conse?quently，the appropriate welding parameters are de?termined.Fig.3 shows the cross section morphology and microstructure of joint 2#.During the welding of the Al-Cu-Li alloy，hot cracking was probably oc?curred in weldment［13］.Moreover，in the autogenous LBW process，due to the feature of the Al-Li alloy and the evaporation and burning loss of alloying ele?ments，the welding defects such as underfill and un?dercut were easily generated［9］.In present work，the Al-Si filler wire is added during LBW，and the weld formation is obviously improved.As shown in Fig.3（a），the joint with good morphology is ob?tained，no welding defects such as gas pores and mi?cro-cracks are produced，which is advantageous to the mechanical property of the welded joint.

Fig.2 Macrographs of welded joints

Fig.3 Morphology and microstructure of joint 2#

The microstructure of BM is shown in Fig.3（b）.It is the strip rolled structure with preferential orientation，and many strengthening phases distrib?ute within grain and at grain boundary.The main strengthening phases in Al-Cu-Li alloys were T1（Al2CuLi），δ′（Al3Li）and θ′（Al2Cu）phases［19］.There is a fine equiaxed grain zone（EQZ）between the fusion zone（FZ）and heat-affected zone（HAZ），and the grain size in EQZ is 4—8 μm，as shown in Fig.3（c）.The formation mechanism of EQZ had studied by many researchers.Reddy et al.［20］stated that the grain morphology in BM basically had no ef?fect on the formation of EQZ.The Al3Zr（β′）parti?cles only existed in EQZ，and the β′ particles played an important role on the formation of EQZ.Gutierrez and Lippold［21］concluded that the formation of EQZ was mainly due to the heterogeneous nucleation of Al3Zr and Al3（Li，Zr）particles.From fusion line（FL）to weld center，the grain morphology in FZ changs from columnar crystal to equiaxed dendrite.During welding，the columnar crystals nucleate at the surface of EQZ grains and grow towards the FZ by the pattern of epitaxial growth.With the growth of columnar crystals，the temperature gradient in FZ is gradually decreased.The large constitutional su?percooling is generated，thus the equiaxed dendrites are formed in the weld center，as shown in Fig.3（d）.

Fig.4 shows the microstructure of the joint tran?sition zone（TZ）under different welding parame?ters.The grain size in EQZ is basically the same.With the decrease of heat input，the width of EQZ decreases.The average width of EQZ in the middle part of joint 1#，joint 2# and joint 3# is 88 μm，72 μm and 34 μm，respectively，as shown in Figs.4（b，e，h）.It was consistent with the results in Ref.［21］.Under the condition of different heat in?puts，for the joint 1#，joint 2# and joint 3#，the av?erage width of HAZ is 3.3 mm，2.5 mm and 1.7 mm，respectively，and the average width of FZ is 2.0 mm，1.8 mm and 1.5 mm，respectively.

Fig.4 Microstructures of the joint transition zone

Liu et al.［12］found that the width of EQZ in the middle part of the 2060 alloy joint by LBW was the maximum，while those in the upper and lower parts were relatively narrow，which was related to the combination effect of thermal buoyancy and Maran?goni convection.As a result，the width of EQZ in different regions was different from that of the joint by autogenous LBW.The microstructures of joint 1# at the top，middle and root are shown in Figs.4（a—c），respectively.The width of EQZ in the upper part of joint 1# is the maximum，and its average width is 110 μm and the maximum width 169 μm.The average width of EQZ is 88 μm in the middle part.The EQZ is the narrowest in the lower part，and its average width is 72 μm.

2.2 Distribution of alloying elements and phase constituent

Fig.5（a）shows the SEM image of FZ，and it is mainly composed of eutectic structure.Some sec?ondary phases precipitate at the grain boundaries，and there is few strengthening phase within the grain.As shown in Fig.5（a），EDS analysis is car?ried out along the line scanning path（red line）.The distribution curves of alloying elements Al，Cu，Si and Mg are shown in Fig.5（b）.The contents of ele?ments Si and Cu increase greatly at grain boundar?ies，while their contents within grains decrease obvi?ously.The content of element Cu or Si increases correspondingly in the region with the lower content of element Al.The distribution of element Mg uni?forms relatively.During the fusion welding of the aluminum alloy，the low melting point eutectic is prone to aggregation at grain boundary to form liq?uid film.Hot cracking was easily to generate in weldment under the action of the welding stress［13］.In present work，by the addition of Al-Si filler wire，the amount of eutectic structure increases.Moreover，the fluidity of the molten pool is greatly improved.Due to the effect of crack healing，the hot cracking is effectively avoided in weldment.

Fig.5 SEM image of FZ and EDS analysis

XRD analysis of the weldment is carried out，and the result is shown in Fig.6.There are mainly α-Al matrix phase and some strengthening phases such as T1（Al2CuTi），T（AlLiSi），δ′（Al3Li），θ′（Al2Cu）and β′（Al3Zr）in the weldment.In the welding process，the rapid cooling of the molten pool easily leads to segregation of alloying elements in weldment.During the welding of Al-Cu-Li al?loys，alloying elements，especially Cu element，were prone to segregation at the dendrite and grain boundaries［22］，thus the supersaturation degree of al?loying elements in weldment was probably insuffi?cient.In the subsequent natural aging process，the quantity of precipitated phases in weldment is not enough，which leads to the softening effect in the weld zone.

Fig.6 XRD analysis pattern of the weldment

TEM images of the precipitated phases in weld?ment are shown in Fig.7.Combined with the chemi?cal composition of BM and the result of XRD analy?sis，the different precipitated phases can be inferred.As shown in Fig.7（a），the triangular strengthening phase is the T（AlLiSi）phase［17-18］.The formation temperature of T phase is higher，the Si-rich phase is easily to aggregate and grow.The acicular strengthening phase is the T1（Al2CuTi）phase［23-24］，as shown in Fig.7（b）.The T1phase is the main strengthening phase in the Al-Cu-Li alloy.Due to the semi-coherent relationship between the T1phase and α-Al matrix，there is a large mismatch between them.The movement of dislocations will be inhibit?ed to a certain extent，thus good strengthening ef?fect is generated.The bulk strengthening phase is the β′（Al3Zr）phase［25］，as shown in Fig.7（c）.The existence of β′ phase contributes to form the EQZ in the welded joint.

Fig.7 TEM images of precipitated phases in weldment

2.3 Microhardness distribution

The microhardness distribution curve of joint 2# is shown in Fig.8.The hardness distribution in the weld zone presents the shape of“V”.The aver?age hardness in FZ is 115 HV，and it is lower than those of HAZ（145 HV）and BM（170 HV）.Soft?ening effect occurs to the Al-Cu-Li alloy joint，and FZ is the weak area of the joint.In as-welded（AW）condition，the weld metal is under-aging，and the quantity of precipitated phases is less，thus the hardness in FZ decreases obviously.

Fig.8 Microhardness distribution of joint 2#

Due to the effect of the weld thermal cycle，the strengthening phases in HAZ aggregate and grow，the quantity of strengthening phases decreases，and the spacing among them increases.According to the Orowan mechanism，the strengthening effect of sec?ondary phases is inversely proportional to the spac?ing among strengthening particles.The effect of pre?cipitation strengthening weakens，as a result，the hardness in HAZ decreases to a certain extent.The hardness distribution is related to the microstructure of different regions.

2.4 Tensile strength and fracture analysis

Results of tensile tests for BM and welded joints are shown in Table 3，and the data in Table 3 are the average of three measurements.The corre?sponding stress-strain curves are shown in Fig.9.The incomplete penetration is generated to joint 4#，as shown in Fig.2（h）.Consequently，the me?chanical property of joint 4# decreases greatly，and it has the lowest tensile strength of 262.1 MPa.Un?der the appropriate welding parameters，the tensile strength of joint 2# is the maximum.It reaches 369.4 MPa，which is 67.8% of that of BM.In pres?ent work，the appropriate welding parameters are as follows：4.5 kW of laser power，3.5 m/min of weld?ing speed，and 5 m/min of wire feeding speed.

Fig.9 Stress-strain curves of BM and welded joints

Table 3 Results of tensile tests for BM and welded joints

Compared with that of BM，the tensile strength of the welded joint decreases to a certain ex?tent.In AW condition，the weld metal is under-ag?ing，and the quantity of strengthening phases precip?itated in FZ is insufficient，while HAZ is over-aged.The joint fails in weldment during stretching，and FZ is the weak area of the joint.It is consistent with the microhardness distribution in weld zone.

From Table 3，the elongation of the welded joint is relatively low.The similar results were also obtained in some Refs.［18-19］.It is probably that the heat treatment condition of BM is T8 and the welded joint is in the AW condition.The ductility of joints mainly depends on their chemical composition and heat treatment condition.After appropriate postweld heat treatment（PWHT），the ductility of the welded joint can be improved to a certain extent.

The tensile fracture morphology of joint 2# is shown in Fig.10.Fig.10（a）shows the overall mor?phology of the fracture surface.There are a large number of equiaxed dimples on the fracture surface，and some secondary phase particles distribute inside the dimples，as shown in Fig.10（g）.The tearing edges appear in the local area of fracture，as shown in Fig.10（c）.The tensile fracture of joint is charac?terized by the intergranular fracture of dimple aggre?gation.As described above，there are eutectic struc?tures and secondary phase particles at grain boundar?ies.During the tensile process，dislocations are ac?cumulated around the eutectic structure，which leads to the local stress concentration.When the ex?ternal stress exceeds the ultimate tensile strength of eutectic structure at grain boundary，micro-pores will be generated.Subsequently，they gradually change into microcracks，as shown in Fig.10（d）.With the increase of the tensile stress，microcracks will expand along the eutectic structure at grain boundary，which leads to the joint fracture finally.

Fig.10 SEM images of the joint tensile fracture

3 Conclusions

（1）Under the appropriate welding parame?ters，by the addition of Al-Si welding wire during the LBW process，the joint with good morphology is obtained.In present work，the optimized welding parameters are as follows：4.5 kW of laser power，3.5 m/min of welding speed，and 5 m/min of wire feeding speed.

（2）During the welding of the Al-Cu-Li alloy，alloying elements Cu and Si are prone to aggrega?tion at grain boundaries.By the addition of Al-Si fill?er wire，the weld formation and the distribution of precipitated phases improve obviously.

（3）There are mainly α?Al matrix phase and some strengthing phases such as T1（Al2CuLi），T（AlLiSi），δ′（Al3Li），θ′（Al2Cu）and β′（Al3Zr）in weldment.Under the appropriate welding parame?ters，the tensile strength of the welded joint reaches 369.4 MPa，which is 67.8% of BM.There are many equiaxed dimples on the fracture surface，and the joint mainly presents the fracture characteristic of dimple aggregation.