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ISSN 0031-918X, The Physics of Metals and Metallography, 2009, Vol. 107, No. 2, pp. 206–210. © Pleiades Publishing, Ltd., 2009.Published in Russian in Fizika Metallov i Metallovedenie, 2009, Vol. 107, No. 2, pp. 219–224.

1. INTRODUCTION

There is a great interest in metal matrix composites(MMCs) as potential structural materials in aerospace,defense, automotive and sports goods. MMCs showexcellent properties such as stiffness, high strength,wear resistance and elevated temperature properties.Matrix materials for MMCS are mainly aluminum, tita-nium and magnesium alloys [1, 2]. Reinforcementmaterials are ceramics usually in the form of particu-late, continuous fiber, chopped fiber or whiskers.Majority of investigations on MMCs have focused onparticulate reinforced composites with matrix of alumi-num and its alloys. The preferred reinforcement mate-rial is SiC with the mean particulate size in the range of2–45

µ

m. Although significant progresses have beenmade in the efforts to overcome difficulties about pro-duction costs, fabrication and joining of metal matrixcomposites, there are still some problems related tojoining need to be solved. Fabrication of complex struc-ture requires the joining of the parts. Reinforcementmaterials within the parts greatly affect the ease of thejoining process. A number of joining processes such asfusion welding, electron and laser beam welding, diffu-sion and friction welding, brazing and adhesive bond-ing are used for joining of MMCs [1–5]. Diffusion

welding is a process by which two nominally flat inter-faces can be joined at an elevated temperature using anapplied pressure for a time ranging from a few minutesto longer. The temperature is usually in the range of0.5–0.8 T

m

, where T

m

is the absolute melting point ofthe material being joined. The interfacial pressure gen-erally is sufficiently low to prevent large-scale defor-mation, although localized deformation at the interfaceitself may be substantial [2, 6]. Diffusion bonding pro-cess such as temperature, pressure, holding time andsurface roughness can be controlled accurately with rel-ative ease and at low cost [6]. Diffusion welding ofsome metallic alloys such as copper, titanium and manysteels is easy because their oxide films either dissolvein the bulk of the metal or decompose at the bondingtemperature. However, if the oxide film is chemicallystable as aluminum-based alloys, then achieving ametallic bond is difficult. Hence, to achieve a soundbond it is necessary to remove the surface oxide at leastpartially or to disrupt its continuity [1, 2].

Previous investigations of diffusion welding of alu-minum alloy MMCs have been concentrated on purealuminum. Al-2024, Al-6061 or Al-8090 aluminummetal matrix based composites reinforced with differ-ent amount of SiC [3–5, 7, 8]. Only very limited studiesexist for diffusion bonding of 7075 unreinforced alumi-num alloys [9]. Therefore the present investigation wasundertaken for diffusion joining of 3% SiC particle

The Diffusion Welding of 7075Al-3%SiC Particles Reinforced Composites *

M. Aydin

a

, R. Gürler

b

, and M. Türker

c

a

Dumlupinar University, Engineering Faculty, Mechanical Engineering Department, Kutahya, TURKEY

b

Osmangazi University, Institute of Metallurgy, Eskisehir, TURKEY

c

Gazi University, Technical Educational Faculty, Metallurgy Department, Ankara, TURKEYe-mail: m_avdin@dumlupinar.edu.tr

Received December 26, 2007; in final form, March 18, 2008

Abstract

—A group of 3% SiC particle reinforced Al-7075 alloys was diffusion joined at 560

°

C between 1 hand 2 h durations under 2 MPa applied pressure in a vacuum of

2

×

10

–3

Pa. Optical microscopy and SEM-EDSstudies were used to characterise the weldment and the fracture surfaces of all samples investigated. A non-pla-nar interface formation was observed at the bond interface. The maximum shear strength of 137 MPa wasobtained with the composite 7075-3% SiC joined for two hours, which is 92% of the shear strength of the parentmaterial. The fracture surface of the 7075-3% SiC composites displayed a non-planar fracture surfaces withsome plastic deformation.

Keywords

: Metal-matrix composites, Diffusion bonding, Shear strength. Aluminum alloys.

PACS numbers:

81.20.Vj.81.40.Np

DOI:

10.1134/S0031918X09020136

STRENGTH AND PLASTICITY

*The article is published in the original.

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THE DIFFUSION WELDING 207

reinforced Al-7075 alloy with a particular attentionpaid to the effect of SiC reinforce at the bond inter-face.

2. EXPERIMENTAL STUDIES

7075 aluminum alloy and SiC were used to producemetal matrix particulate composites. The compositionof the Al-7075 alloy was Al-1.7Cu-2.45Mg-5.87Zn-0.3Mn-0.22Cr (all in wt %). A constant value of 3 wt %SiC approximately 44

µ

m in size was added to 7075alloy in liquid state. SiC particles were dispersed in theliquid aluminum alloy by vortex method under argonatmosphere at about

675–700°C

. A plain steel die wasused for casting composites. The samples weremachined in cylindrical forms with a 10 mm diame-ter and a 10 mm height for diffusion welding stud-ies. Their sizes were reduced by machining to an8 mm diameter with an 8 mm height for shear test-ing. Diffusion welding was carried out at

560°C

between 1 h and to 2 h durations under 2 MPaapplied pressure in a vacuum of

2

×

10

–3

Pa. Thesame experimental conditions were applied to sam-ples prepared from unreinforced Al-7075 alloy forcomparison of the diffusion joining results withthose of the 7075-3% SiC composite. Following thediffusion welding the samples were allowed to coolin air on a chilled surface.

Optical microscopy and scanning electron micros-copy either in secondary electron image or back-scat-tered electron image modes equipped with energy dis-persive spectrometry (EDS) were used to study thebond interface and matrix characterizations.

Microhardness measurements of the weldment weremade using a pyramidal diamond indenter having a faceangle of

136°

and 50 g indenting load. The durationtime of the indent was 20 s.

As the bonded cylindrical specimens were not largeenough for normal shear lap testing, a speciallydesigned specimen holder was used to measure bondshear strength as shown in Fig 1. The specimen holderwas placed on a tensile test machine, with a full load of40 kN, and then tests were carried out at an ambienttemperature of

20°ë

with a loading speed of0.5 mm/min. The bond fracture strength was deter-mined from the maximum load recorded on the load-deflection graph. Three samples were tested for eachbonding condition.

3. RESULTS AND DISCUSSION

A polished but unetched microstructure of the as-cast composite 7075-3% SiC (parent material) is shownin Fig 2. Finely dispersed SiC particles in 7075 matrixare clearly seen. Optical and scanning electron micros-copy examinations either in secondary electron modeor back-scattered electron mode on metallographicallyprepared samples demonstrated that the unreinforced

Al-7075 alloy welds had a planar interface (Fig 3a),whereas a non planar interfaces was observed at the Al-7075 alloy-3% SiC composite bond interface as seen inFigs. 3b–3d. The presence of the non-planar interfaceobserved on cross-sections of the present diffusionbonded 7075-3%SiC composites are in agreement withreported investigations of diffusion bonded some Alalloy-SiC particle composites [1, 2]. The formation ofthe non-planar interface could be explained by assum-ing that, during diffusion bonding of the compositesome of hard SiC particles on each faying surface caneasily penetrate into the much softer aluminum matrixon the other surface (Fig. 3c). The protrusion of SiCreinforcements causes formation of the non-planarinterface (Fig. 3d). High bond strengths would beexpected because of the local rupture of the oxide layeron each faying surface by the protruding SiC particlesfrom the other faying surface.

Load

Weldedjoint

Weldedsamples

Fig. 1.

Schematic diagram of the assembly for the bondshear strength testing.

200 µm

SiC

Matrix

Fig. 2.

A polished but unetched microstructure of the as-castcomposite 7075-3% SiC with finely dispersed SiC particlesin 7075 matrix.

208

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Microhardness measurements across the weldmentshowed that in Figure 4. There is no significant hard-ness variation. The measured hardness values arealmost the same as for the base composite 7075-3%SiC. Because the reinforcement particle ratio in thematrix was small amount (3% SiC), the matrix hardnessvalue was not raised by SiC particles. As shown inFig. 3a, the unreinforced Al-7075 alloy welds had a pla-nar interface, whereas a non planar interface wasobserved at the Al-7075 alloy-3%SiC composite bond

interface as seen in Figs. 3b–3d. Microsegregation orinhomogeneous distribution of reinforcement was notformed in the diffusion bonding process, but that isformed in fusion welding process [3]. For this reason,as shown in Fig 4, the hardness of the Al-7075 alloy-3%SiC composite was not changed bond interface linethrough.

Shear tests were carried out using the speciallydesigned specimen holder. Shear strengths of bondedmaterials the unreinforced Al-7075 alloys and the com-posites Al 7075-3% SiC together with data for the as-cast unreinforced 7075 and the unwelded as-cast com-posite 7075-3% SiC are displayed in Fig. 5. The shearstrengths of 146 MPa and 149 MPa were obtained forthe as-cast unreinforced Al-7075 and the as-cast com-posite Al 7075-3%SiC respectively. A mean shearstrength value of 99 MPa was measured for unrein-forced Al-7075 alloys joined for two hours. This valueis significantly less than the shear strengths of the un-bonded parent material and the bonded composite7075-3% SiC. This discrepancy can be attributed to theformation of a planar interface and continues aluminumoxide film at contacting surfaces as seen in Fig. 3a.However the shear strength of 99 MPa obtained in thisstudy for the bonded Al-7075 alloy is similar toreported shear strength values between 30–173 MPa forsimilar Al alloys [9, 10]. Under the experimental condi-tions the maximum shear strength of 137 MPa was

500 µm 100 µm

25 µm 10 µm

(a) (b)

(c) (d)

Welded joint

Welded joint

Welded joint

Welded joint

Welded joint

Matrix

Matrix

Sci

Sci

Sci

Sci

Fig. 3.

Optical microscopy images of the weldments. (a) the unreinforced Al-7075 alloy, (b) and (c) the composite 7075-3% SiCfollowing two hours joining, (d) SEM back scattered electron image of the 7075-3% SiC composite bond interface at higher mag-nification.

–400 –300 –200 –100 0 100 200 300 400Distance from welded joint, µm

7075-3%SiC + 7075-3%SiC

40035030025020015010050

0

Hardness

,

HV

Fig. 4.

The hardness of 7075-3%SiC + 7075-3% SiCbonded samples at 560

°

C, 2 h.

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THE DIFFUSION WELDING 209

obtained with the composite 7075-3% SiC joined fortwo hours, which is 92% of the shear strength of theparent material (Al-3%SiC unbonded). The signifi-cant improvement in the bond strength of the com-posite 7075-3% SiC is probably the result of a non-planar interface formation and disruption of the alu-minum oxide film at the interface as shown inFig. 3b–3d.

SEM images of the fracture surfaces of the as-castunreinforced Al alloy 7075, the as-cast 7075-3% SiCcomposite and the diffusion joined 7075-3% SiCcomposites are given in Fig. 6. The unreinforcedalloy 7075 shows an interdentritic fracture and dis-plays ductile fracture with a uniform distribution ofdimples and voids (Fig. 6a). The as-cast 7075-3%SiC composite and the diffusion joined 7075-3%SiC composite display a non-planar fracture sur-faces with some plastic deformation (Figs. 6b–6c).The area that underwent static compression and dif-fusion bonding reveals oxidized surfaces with somebonding around the boundaries. This area observedin the peeled surface exhibited breakage of oxidesthat may have contributed to deformation and bond-ing. This peeled surface is typical of the diffusionbonded specimen that underwent significant plasticdeformation [11]. Although the surface of the grainsappear to be oxidized, the grain boundaries in con-tact with other surfaces form a bond, as shown byductile tearing around the boundaries.

4. CONCLUSIONS

Vacuum diffusion joining of the 7075-3% SiC at

560°C

between 1 and 2 h under 2 MPa pressure in avacuum of

2

×

10

–3

Pa resulted in producing sound join-ing with a non-planar interface. The maximum shearstrength of 137 MPa was obtained with the composite7075-3% SiC joined for two hours, which is 92% of the

shear strength of the parent material. The non-planarinterface formation and disruption of the aluminumoxide film at the interface were considered to be rea-sons for the significant improvement in the bondstrength of the composite 7075-3% SiC.

160

140

120

100

80

60

40

20

0

Shear strength

, MPa

A B C D E FType of a sample

Fig. 5.

Shear strengths of the unreinforced Al-7075 alloy(A), the composite Al 7075-3% SiC (B), diffusion bondedAl-7075 alloy for 1 h (C), for 2 h (D) and the composite Al7075-3% SiC for 1 h (E) and for 2 h (F).

(a) 500 µm

10 µm

200 µm

(b)

(c)

Fig. 6.

SEM secondary electron images of fractured sur-faces (a) an interdentritic fracture of the as-cast Al-7075alloy, (b) the as-cast 7075-3% SiC composite and (c) thediffusion joined 7075-3% SiC composite, a non-planar frac-ture with some signs of plastic deformation after 2 h join-ing).

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ACKNOWLEDGMENTSThe research was supported by Osmangazi Univer-

sity (Eskisehir), and Gazi University (Ankara)Research Foundation, Turkey.

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