Pulse-Impact on Microstructure of Liquid-Phase-Pulse-Impact Diffusion Welded Joints of Particle Reinforcement Aluminum Matrix Composites at Various Temperatures

doi:10.4236/eng.2013.57069

Engineering, 2013, 5, 567-576

http://dx.doi.org/10.4236/eng.2013.57069 Published Online July 2013 (http://www.scirp.org/journal/eng)

Pulse-Impact on Microstructure of

Liquid-Phase-Pulse-Impact Diffusion Welded Joints of

Particle Reinforcement Aluminum Matrix Composites at

Various Temperatur es

Kelvii Wei Guo

Department of Mechanical and Biomedical Engineering, City University of Hong Kong, Hong Kong, China

Email: [email protected]

Received February 28, 2011; revised January 1, 2013; accepted January 8, 2013

which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ABSTRACT

Investigation was to study the influence of pulse-impact on microstructure of Liquid-Phase-Pulse-Impact Diffusion

Welding (LPPIDW) welded joints of aluminum matrix composite SiCp/A356, SiCp/6061Al, Al2O3p/6061Al. Results

showed that under the effect of pulse-impact: 1) the interface state between reinforcement particle (SiC, Al2O3) and ma-

trix was prominently; 2) the initial pernicious contact-state of reinforcement particles was changed from reinforcement

(SiC, Al2O3)/reinforcement (SiC, Al2O3) to reinforcement (SiC, Al2O3)/matrix/ reinforcement (SiC, Al2O3); 3) the den-

sity of dislocation in the matrix neighboring to and away from the interface in the matrix was higher than its parent

composite; and 4) the intensively mutual entwisting of dislocation was occurred. Studies illustrated that: 1) deformation

was mainly occurred in the matrix grain; and 2) under the effect of pulse-impact, the matrices around reinforcement

(SiC, Al2O3) particles engendered intensive aberration offered a high density nucleus area for matrix crystal, which was

in favor of forming nano-grains and improved the properties of the successfully welded composite joints.

Keywords: Aluminum Matrix Composite; Particle Reinforcement; Pulse-Impact; Microstructure; Diffusion Welding

1. Introduction

The high specific strength, good wear-ability and corro-

sion resistance of Aluminum Matrix Composites (AMCs)

attract substantial industrial applications. Typically, AMCs

are currently used widely in automobile and aerospace

industries, structural components, and heat resistant-wea-

rable parts in engines, etc. [1-5]. The particles of rein-

forcement elements in AMCs may be either in form of

particulates or as short fibers, whiskers and so forth [5,6].

These discontinuous natures create several problems to

their joining techniques for acquiring their high strength

and good quality weld-joints. Typical quality problems

of those welding techniques currently available for join-

ing AMCs [7-15] are as elaborated below.

1) The distribution of particulate reinforcements in the

weld

As properties of welded joints are usually influenced

directly by the distribution of particulate reinforcements

in the weld, their uniform distribution in the weld is likely

to give tensile strength higher than 70% - 80% of the

parent AMCs. Conglomeration distribution or absence

(viz. no-reinforcements-zone) of the particulate reinfor-

cements in the weld generally degrades markedly the

joint properties and subsequently resulted in the failure

of welding.

2) The interface between the particulate reinforce-

ments and aluminum matrix

High welding temperature in the fusion welding me-

thods (typically: TIG, laser welding, electron beam etc.)

is likely to yield pernicious Al4C3 phase in the interface.

Long welding time (e.g. several days in certain occasions)

in the solid-state welding methods (such as diffusion

welding) normally leads to a) low efficiency and b) for-

mation of harmful and brittle intermetallic compounds in

the interface.

To alleviate these problems incurred by the available

welding processes for welding AMCs, a liquid-phase-

pulse-impact diffusion welding (LPPIDW) technique has

been developed [16-18]. This paper aims at providing

C

K. W. GUO

568

some specifically studies the influence of pulse-impact

on the microstructures of welded joints. Analysis by

means of scanning electron microscope (SEM), trans-

mission electron microscope (TEM) and X-Ray Diffrac-

tion (XRD) allows the micro-viewpoint of the effect of

pulse-impact on LPPIDW to be explored in more detail.

2. Experimental Material and Process

2.1. Specimens

Stir-cast SiCp/A356, P/M SiCp/6061Al and Al2O3p/

6061Al aluminum matrix composite, reinforced with

20%, 15% volume fraction SiC, Al2O3 particulate of 12

m, 5 m mean size, were illustrated in Figures 1-3.

Figure 1. Microstructure of aluminum matrix composite

SiCp/A356.

Figure 2. Microstructure of aluminum matrix composite

SiCp/6061Al.

Figure 3. Microstructure of aluminum matrix composite

Al2O3p/6061Al.

2.2. Experiment

The quench-hardened layer and oxides, as induced by

wire-cut process, on the surfaces of aluminum matrix

composite specimens were removed by careful polishing

using 400 # grinding paper. The polished specimens were

then properly cleaned by acetone and pure ethyl alcohol

so as to remove any contaminants off its surfaces. A DSI

Gleeble®-1500D thermal/mechanical simulator with a 4

× 10−1 Pa vacuum chamber was subsequently used to

perform the welding.

The microstructures and the interface between the re-

inforcement particle and the matrix of the welded joints

were analyzed by SEM and TEM.

2.3. Operation of LPPIDW

Figure 4 illustrated a typical temperature and welding

time cycle of a LPPIDW. It basically involved with: 1)

an initially rapid increase of weld specimens, within a

time of ta, to an optimal temperature Ta at which heat was

preserved constantly at Ta for a period of (tb − ta), 2) at

time tc, a quick application of pulse impact to compress

the welding specimens so as to accomplish an anticipated

deformation δ within a glimpse of 10−4 - 10−2 s, whilst

the heat preservation was still maintained at the opera-

tional temperature Ta; and 3) a period of natural cooling

to room temperature after time tb.

3. Results and Discussion

3.1. Microstructure of Welded Joint

Figure 5 showed the microstructures of welded joints of

SiCp/A356 at various temperatures with VI = 560 mm/s, tI

= 10−4 - 10−2 s, t = 30 s, P0 = 5 MPa, δ = 1 mm, where VI

was velocity of pulse-impact, tI was the impacting time, t

was holding time for heat preservation, P0 was holding

K. W. GUO 569

Figure 4. Schematic diagram of liquid-phase-pulse-impact

diffusion welding.

400 μm

(a)

400

μ

m

(b)

400 μm

(c)

Figure 5. Microstructures of welded joints of SiCp/A356 at

various temperatures by LPPIDW; (a) 563˚C; (b) 565˚C; (c)

570˚C.

pressure during the welding, δ was the horizontal defor-

mation. It elucidated that when the welding temperature

was 563˚C, under the effect of pulse-impact, the liquid

phase matrix alloy wasn’t formed enough to wet the par-

ticle reinforcements. In addition, at this temperature, the

diffusion capability of the atoms within the matrix was

relatively low. As a result, the welding interface between

two specimens could be observed obviously as shown in

Figure 5(a) and followed by the unsuitable strength

(about 118 MPa). Moreover, because of lower welding

temperature, the area of the formed solid-liquid phase

was smaller, which led to some streamlines scattered in

the matrix (Figure 5(a)) after the pulse-impact acting on

the substrates. When the temperature reached 565˚C, the

rate of the atom diffusion in the joint region within the

matrix was accelerated (Figure 5(b)). At the same time,

more liquid phase matrix alloy was formed to wet rein-

forcements (SiC). Therefore, the interface state of rein-

forcement and reinforcement was improved and the re-

inforcements were distributed uniformly to some extent.

Also, the streamlines scattered in the matrix were disap-

peared, and the tensile strength of welded joints was

about 134 MPa higher than that of 563˚C. When the

temperature is up to 570˚C, the formed liquid phase ma-

trix alloy was enough and suitable for wetting reinforce-

ments effectively, and the rate of the atom diffusion was

more active. As a result, for reinforcements the welding

mode in the joint region changed from reinforcement-

reinforcement to reinforcement-matrix-reinforcement.

Con- sequently, the joint was welded successfully (Fig-

ure 5(c)). The average strength of 179 MPa for the

welded joints produced at welding temperature of 570˚C

was about 74.6% of the 240 MPa for the strength of par-

ent aluminum matrix composite.

The relevant fractographs were shown in Figure 6. It

illustrated that when the welding temperature was 563˚C,

the initial morphology of substrate could be detected

obviously, and some sporadic welded locations appeared

together with some rather densely scattering bare rein-

forcement particles as shown in Figure 6(a). With the

welding temperature was increased to 565˚C, more liquid

phase was formed. Under the effect of pulse-impact,

some wet locations in the joint had been excellently

welded and the aggregated solid reinforcement particles

were improved. However, the bare reinforcement parti-

cles were still distributed on the fractographic surface. It

indicated that substrates did not weld ideally the pieces

together and it consequently resulted in a low strength

joint (Figure 6(b)). Figure 6(c) showed the fractograph

of welded joint at 570˚C. It illustrated that the fracture

was dimple fracture. Moreover, SEM of the fracture sur-

face showed some reinforcement particles (SiC) in the

dimple. In order to confirm the state of these reinforce-

ment particles, particles itself and matrix neighboring to

K. W. GUO

570

(a) (b)

(c) (d)

Figure 6. Fractographs of SiCp/A356 at various temperatures; (a) 563˚C; (b) 565˚C; (c) 570˚C; (d) 575˚C.

these particles were analyzed by energy dispersive X-ray

analysis (EDX) respectively. Result was shown in Fig-

ure 7. It indicated that reinforcement particles (SiC) were

wet by matrix alloy successfully suggesting that the re-

inforcement particles had been perfectly wet and the

composite structure of reinforcement/reinforcement had

been changed to the state of reinforcement/matrix/rein-

forcement.

As welding temperature increasing to 575˚C, it led to

more and more liquid phase matrix alloy distributing in

the welded interface, meanwhile, more liquid phase ma-

trix alloy reduced the effect of impact on the interface of

the welded joints, subsequently the application of tran-

sient pulse-impacting would cause the relative sliding of

the weldpieces that jeopardized ultimately the formation

of proper joint as shown in Figure 6(d). It demonstrated

that results of fractographs were agreed with the corre-

sponding microstructures well.

Figure 7. Energy dispersive X-ray analysis of the fracture

surface of SiCp/A356.

6061Al at various welding temperatures were shown in

Figures 8-11.

It showed that the microstructure evolutions and its

corresponding fracture surfaces under the effect of pulse-

impact are similar to that of SiCp/A356.

Figures 8(a) and 10(a) showed when the welding tem-

perature was too low to form enough liquid phase matrix

The relevant results of SiCp/6061Al and Al2O3p/

K. W. GUO 571

(a) (b) (c)

Figure 8. SEM micrographs of SiCp/6061Al we ld ed joints at various welding temperatures; (a) 620˚C; (b) 623˚C; (c) 625˚C.

(a) (b) (c)

Figure 9. Fractographs of SiCp/6061Al at various temperatures; (a) 620˚C; (b) 623˚C; (c) 625˚C.

(a) (b) (c)

Figure 10. SEM micrographs of Al2O3p/6061Al welded joints at various welding temperatures; (a) 641˚C; (b) 644˚C; (c)

647˚C.

K. W. GUO

572

(a) (b) (c)

Figure 11. Fractographs of Al2O3p/6061Al at various temperatures; (a) 641˚C; (b) 644˚C; (c) 647˚C.

alloy to wet the reinforcement particles, and the diffusion

capability of the atoms within the matrix was relatively

low. Therefore, the indistinct welding interface between

two specimens could be observed resulted in low tensile

strength (about 240 MPa for SiCp/6061Al and 270 MPa

for Al2O3p/6061Al). When the temperature is higher

(623˚C for SiCp/6061Al and 644˚C for Al2O3p/6061Al),

the liquid phase matrix alloy was formed enough to wet

the reinforcement particles (SiC), together with higher

rate of the atom diffusion in the joint region (Figures 8(b)

and 10(b)). Consequently, the joints could be welded

successfully with the average strength of 260 MPa for

SiCp/6061Al (about 72.2% of the 360 MPa for the

strength of parent aluminum matrix composite) and 282

MPa for Al2O3p/6061Al (about 70.5% of the 400 MPa for

the strength of parent aluminum matrix composite). As

welding temperature increasing further (such as 625˚C

for SiCp/6061Al and 647˚C for Al2O3p/6061Al), more

and more liquid phase matrix alloy would be distributed

in the welded interface, at the same time, more liquid

phase matrix alloy reduced the effect of impact on the

interface of the welded joints, subsequently prompted for

the descending of the joint strength (Figures 8(c) and

10(c)).

Moreover, according to the fractures of welded joints

at various temperatures shown in Figures 9 and 11, it

showed that it agreed with Figures 8 and 10 very well,

and the fractures were all dimple fractures with some

reinforcement particles (SiC, Al2O3) in the dimple. Also,

the results of SiCp/6061Al were better than that of

Al2O3p/6061Al due to a mild interfacial reaction between

the reinforcement and matrix, which released the thermal

mismatch stress to an acceptable extent between the re-

inforcement and matrix to allow load transfer from the

matrix to reinforcement successfully. As a result, it had

advantageous effect of improving the strength of welded

joints further [18].

Based on microstructures of the welded joints with the

optimal parameters (i.e., p

SiC/ A356 = 570˚C, p

SiC/6061Al

= 623˚C, 23p

Al O = 644˚C, VI = 560 mm/s, tI = 10−2 −

10−4 s, δ = 1 mm, t = 30 s, P0 = 5 MPa) and its corre-

sponding fracture surfaces as shown in Figures 5, 6, 8-11,

the welded joint displayed with uniformly distributing

reinforcement particles and microstructure almost similar

to that of its parent composite (Figures 1-3). SEM of the

fracture surface showed that the reinforcement particles

had been perfectly wet and the composite structure of

reinforcement/reinforcement had been changed to the

state of reinforcement/matrix/reinforcement. XRD pat-

tern of the fracture surfaces (Figure 12) did not illustrate

the existence of any harmful phase or brittle phase of

Al4C3. This suggested the effective interface transfers

between reinforcement particles and matrix in the welded

joint that subsequently provided favorable welding

strength [16-18].

T T

T

3.2. Distribution of Dislocation in the

Welded Joint

The distribution of dislocation in the matrix neighboring

to the interface of the welded joint by LPPIDW in com-

parison with its parent composite was shown in Figure.

13. The clearly distinctive interface between reinforce-

ment particle and matrix indicated that the integration

between the reinforcement particle and matrix was

prominent. The effect of pulse-impact subsequently led

to dislocation in the matrix lattices and showed sign of

mutually entwisting to give higher welded strength.

Comparatively, its dislocation distribution in the matrix

neighboring to the interface was relatively denser than

that in its parent composite (cf. Figures 13(a) and (b)).

K. W. GUO 573

(a)

(b)

(c)

Figure 12. XRD pattern of the fracture surfaces; (a) SiCp/A356; (b) SiCp/6061Al; (c) Al2O3p/6061Al.

Similarly, the density of dislocation and dislocation en-

twisting in the matrix away from the welded interface

was also higher than that of its parent composite (cf.

Figures 14(a) and (b)). Such favorable characteristics

ultimately gave relatively superior strength of the welded

joint to that of conventional diffusion welding [16-18].

3.3. Formation of Nano-Grains in the Weld

SEM micrograph (Figure 15) of a weld by LPPIDW

displayed some newly-formed nano-grains in the lattices

of the joint. These nano-grains would seat in the intersti-

ces of crystal lattices and create new grain boundary in

hindering the movement of neighbouring grains and

subsequently improved obviously the properties of the

welded joints. The formation of new nano-grains was the

advantageous effect of pulse-impact in LPPIDW. In ad-

dition, XRD pattern of the fracture surface (Figure 12)

did not illustrate the existence of any harmful phase or

brittle phase of Al4C3. This suggested the effective inter-

face transfers between reinforcement particles and matrix

in the welded joint that subsequently provided favorable

welding strength [16-18].

4. Conclusions

Results of this study on the microstructures of welded

joints of particle reinforcement aluminum matrix com-

posites (SiCp/A356, SiCp/6061Al, Al2O3p/6061Al) using

liquid-phase-pulse-impact diffusion welding process show

that:

1) Pulse-impact in liquid-phase-pulse-impact diffusion

welding in joining particle reinforcement aluminum ma-

trix composites (SiCp/A356, SiCp/6061Al, Al2O3p/6061Al)

resulted in higher density of dislocation in the matrix

neighboring to and away from the interface than their

parent composite. Simultaneously, the dislocation en-

twisted mutually and intensively in the welded joint pro-

pitious to improve the strength of welded joints.

2) There was distinctly clear interface between reinfor-

cement particle and matrix. It overcame some diffusion

problems normally encountered in conventional diffusion

K. W. GUO

574

SiC

Matrix

0.25μm

SiC

Matrix

0.25μm

SiC

Matrix

0.25μm

SiC

Matrix

SiC

(a) welded joint (b) parent composite (a) welded joint (b) parent composite

(A) (B)

0.25μm

Matrix

Al

2

O

3

0.25μm

Matrix

Al

2

O

3

Al

O

(a) welded joint (b) parent composite

(C)

Figure 13. Distribution of dislocation in the matrix neighbori ng to the interface of the welded joint and parent composite re-

spectively; (A) SiCp/A356; (B) SiCp/6061Al; (C) Al2O3p/6061Al.

Matrix Matrix

0.25μm 0.25μm

0.25μm

Matrix

0.25μm

Matrix

(a) welded joint (b) parent composite (a) welded joint (b) parent composite

(A) (B)

0.25μm 0.25μm

Matrix Matrix

(a) welded joint (b) parent composite

(C)

Figure 14. Distribution of dislocation in the matrix away from the interface of the welded joint and parent composite respec-

tively; (A) SiCp/A356; (B) SiCp/6061Al; (C) Al2O3p/6061Al.

K. W. GUO 575

0.25μ

0.25μm

(a) (b) (c)

Figure 15. Nano-grains formed in the weld of particle reinforcement aluminum matrix composites during the LPPIDW; (a)

SiCp/A356; (b) SiCp/6061Al; (c) Al2O3p/6061Al.

welding, and prevented the formation of harmful micro-

structure or brittle phase in the welded joint.

3) The joint by LPPIDW process would form nano-

grains. The newly-formed nano-grains would improve

the properties of welded joints resulted in higher tensile

strength.

5. Acknowledgements

This work is supported by City University of Hong Kong

Strategic Research Grant (SRG) No. 7002582.

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