Electron beam welding of SiCp/2024 and 2219 aluminum alloy

Chen Guoqing,Zhang Binggang,Yang Yong,Feng Jicai

State Key Laboratory of Advanced Welding and Joining,Harbin Institute of Technology,Harbin 150001,China

Abstract SiCp/2024 matrix composites reinforced with SiC particles and 2219 aluminum alloy were joined via centered electron beam welding and deflection beam welding,respectively,and the microstructures and mechanical properties of these joints were investigated.The results revealed that SiC particle segregation was more likely during centered electron beam welding(than during deflection beam welding),and strong interface reactions led to the formation of many Al4C3 brittle intermetallic compounds.Moreover,the tensile strength of the joints was 104 MPa.The interface reaction was restrained via deflection electron beam welding,and only a few Al4C3 intermetallic compounds formed at the top of the joint and heat affected zone of SiCp/Al.Quasi-cleavage fracture occurred at the interface reaction layer of the base metal.Both methods yielded a hardness transition zone near the SiCp/2024 fusion zone,and the brittle intermetallic Al4C3compounds formed in this zone resulted in high hardness.

Key words SiCp/2024 matrix composite,2219 aluminum alloy,electron beam welding,microstructure,mechanical properties

0 Introduction

Aluminum alloys are the most common type of non-ferrous metallic structural material used in industry.Owing to their high specific strength and specific modulus,these alloys are widely used in various fields including the aerospace and automotive industries.Aluminum-based composite materials are characterized by several positive attributes and have therefore been rapidly developed in recent years.These attributes include a high specific strength and specific modulus,high temperature resistance,corrosion resistance,good electrical and thermal conductivity as well as good dimensional stability[1,2].

The reinforced phase in aluminum-based composites is characterized by a high strength,high melting point,high modulus,low expansion coefficient,and poor weldability.Therefore,the realization of a good connection between the reinforcement phase and the aluminum alloy poses a considerable challenge during the welding of aluminum-based composite materials and aluminum alloys.The presence of the reinforcing phase will affect the weld formation and the interface reaction with the liquid aluminum at 827°C or will generate a brittle Al4C3phase,thereby affecting the joint connection performance.This brittle phase will reduce the toughness of the joint and increase the cracking tendency of the welded joint.Furthermore,Al4C3can react chemically with water,leading to the formation of acetylene and,consequently,poor corrosion resistance of the joint during service[3-6].With tungsten inert gas(TIG)welding,a high-strength titanium alloy can be used as a filler material.This enhances the fluidity of the molten pool and yields significant reduction in the number of non-melted welds and holes in the weld.At high temperatures,titanium alloys react preferentially in situ with reinforced SiC particles,thereby suppressing the brittle phase.The resulting TiC particles were produced and strengthened in the weld[7].

1 Test method

In this study,SiCp/2024(volume fraction of SiC:45%,particle size:～5 μm,material state:as-cast)and 2219 aluminum alloys were used as the base materials.A 50 mm×20 mm×3 mm piece of each material,which was sanded with 400 grit sandpaper before welding,was used as the test piece for welding.After the oxide film was removed from the surface,the surface was washed with acetone and airdried prior to welding.

Vacuum electron beam welding equipment(HIT-951 type)was employed.A fixture developed in-house was used to clamp the base metal in preparation for welding.Butt joint welds were formed.Metallographic and tensile samples were selected from an appropriate part of the weld and prepared for characterization and testing.The structure of each joint was observed via optical microscopy(OLYMPUS(GX71)).In addition,the microstructural morphology,and fracture characteristics were evaluated by means of scanning electron microscopy(SEM;Quanta 200F).Using a spectrometer attached to the scanning electron microscope,the components of each phase were identified through energy dispersive spectroscopy(EDS).The phase composition of each joint was determined via X-ray diffraction(XRD).Furthermore,micro-hardness measurements of the joint and tensile tests were performed using an HXD 1000TM digital micro-hardness tester and electronic universal testing machine,respectively.

2 Test results and analysis

2.1 Surface forming of welded joints

In the electron beam welding process of SiCp/2024 and 2219 aluminum alloys,the porosity and macro-crack tendencies were relatively low.However,the welded joints were poorly formed,especially those formed via center-welding,where the front face of the weld was uneven.The reason was that due to the large heat of the electron beam,the aluminum alloy and the aluminum alloy matrix in SiCp/2024 were easily melted.This leads to evaporation of the aluminum.This left a SiC particle skeleton that melted at high temperatures,thereby affecting formation of the welds.In addition,the mixing of SiC particles with the liquid aluminum alloy increased the viscosity of the molten pool,resulting in poor fluidity of the molten pool.The poor fluidity had a significant effect on the formation of welded joints comprising SiCp/2024 and the aluminum alloy.

2.2 Microstructure of welded joints

Fig.1 shows the typical morphology of cross-sections comprising welded joints of SiCp/2024 and 2219 aluminum alloy obtained via centered electron beam welding and deflection beam welding.The dashed line in Fig.1 represents the fusion line,the area between the two fusion lines is the weld zone,and the region outside of the fusion line is the heat affected zone(HAZ).As shown in the figure,the weld metal and the base metal were well combined,and no defects such as cracks were observed.However,the joint formation of the deflection beam welding process was significantly better than that of the center welding process.

The weld was mainly composed of branched crystals formed via melting of the aluminum matrix.Moreover,many needles phase occurred in the weld area of the centerwelded sample,as shown in Fig.2a.Compared with the center-welding process,the deflection beam-welding process generated only a small number of needles phase(i.e.,Al4C3bodies,as determined via energy spectrum analysis combined with XRD measurements)in the upper part of the weld.These needles were fewer and smaller than those in the center-welded sample,as shown in Fig.2b.

Fig.3 shows the metallographic structure of the heat affected zone on the composite material side in the centerwelded and deflection beam welded joints.During both processes,acicular Al4C3phases were generated in the fusion zone,but(compared with the beam welding process)the center-welding process produces a large number and wide distribution of these bodies.In the beam welding sample,needle-shaped bodies were formed only in a small area of the fusion zone.Significant segregation and growth of the SiC particle-reinforced phase occurred inthe fusion zoneof the butt-welded sample,and gray primary silicon was generated.

Fig.1 Cross-sectional microstructure of the EBW joints(a)Centered electron beam welding(b)Deflection beam welding

Fig.2 Acicular Al4C3 phases of the weld(a)Upper region of deflection beam weld(b)Centered electron beam weld

2.3 Mechanical properties of joints

The tensile strength of joints obtained for different parameters is shown in Fig.4.As shown in the figure,a welding current of 16 mA,the tensile strength of the 0.3 mm polarized beam on the aluminum side welded specimen was only 78 MPa,owing mainly to the poor wettability of liquid aluminum to SiC particles.In deflection beam welding,the joint between the weld metal and the lower part of the SiCp/2024 base material was poor.When the welding current was increased,the temperature of the molten pool increased,thereby improving the wettability of the aluminum to the SiC particles.The maximum strength realized(131 MPa)is 54% of the strength obtained for the SiCp/Al base metal.

Fig.5 shows the typical fracture path and fracture morphology of SiCp/2024 and 2219 aluminum alloy electron beam welding joints.As shown in Fig.5a,the joints were fractured mainly along the reaction layer,which is located in the fusion zone on the SiCp/2024 side.Due to the interfacial reaction,many needle-like brittle Al4C5phases were generated,as shown in Fig.5b,leading to a decrease in the strength of this zone.

Fig.4 Tensile strength of weld sample under different parameters

Fig.5 Fracture location of joints(a)Fracture path(b)High-magnification view

Fig.6 Fracture morphology and fracture characteristics of the joints(a)Fracture morphology(b)Fracture characteristics

The tensile fracture surface is observed via SEM.The fracture morphology and fracture characteristics are shown in Fig.6.As shown in the figure,dimples and significant plastic deformation were absent,but many step-like cleavage surfaces,(consistent with the characteristics of brittle fracture)were observed.A high-magnification view of the fracture is shown in Fig.6b.The blocky features in the fracture maybe SiC particles,as suggested by the energy spectrum analysis results(Table 1).This indicated that the fracture occurred at the joint of the particles and the weld metal.

The micro-hardness of joints by the center-welded and polarized beam samples is shown in Fig.7.As shown in the figure,the two types of welding yielded similar hardness values.The hardness increased gradually from the 2219 alu-minum alloy to the base metal to the weld.Compared with those of the aluminum alloy base material,the grains were finer and denser,and the hardness was higher.From the weld to the SiCp/2024 base material,the hardness changes suddenly in the HAZ on the SiCp/2024 side,and then decreased gradually to the hardness of the SiCp/2024 base material.This sudden change in hardness may have resulted from the segregation and growth of the SiC particle-reinforced phase in the HAZ zone of the SiCp/2024 side.

Table 1 EDS analysis results of points in Fig.6

Fig.7 Hardness distributions of the EBW joints(a)Centered electron beam welding(b)Deflection beam welding

3 Conclusion

(1)The forming of SiCp/2024 and 2219 aluminum alloy joint by electron beam centering welding was poor.Welds and composite fusion lines containedmany Al4C3brittle phases,and segregation of primary silicon and SiC particles occurred.Defection beam welding generated only a few needle-shaped Al4C3bodies.Moreover,compared with those resulting from centered electron beam welding,the Al4C3brittle phase occurs in a narrower area above the middle of the HAZ.

(2)The tensile strength of the joint was poor.Furthermore,the maximum tensile strength(131 MPa),which is 54% of the strength determined for the SiCp/2024 base material,which obtained via the beam welding process.Typical of brittle fracture,occurred in the reaction layer between the base material interfaces,and resulted from the weakening of the connection between the aluminum alloy and the SiC particles.

(3)Similar trends were observed for the cross-sectional hardness distributions of welded joints obtained through center and partial beam welding.The hardness increased gradually from the aluminum alloy base material to SiCp/2024,changed abruptly near the fusion zone of SiCp/2024,and then decreased to the SiCp/2024 base material hardness.