Aluminium metal matrix composites - successes using diffusion bonding

TWI Bulletin, May - June 1998

Wendy Hanson joined TWI in 1992 and is the Technology Manager - Ceramics in the Advanced Materials and Processes Department, where she has responsibility for a number of projects concerned with ceramic joining and metallic diffusion bonding. Before this, Wendy studied Metallurgy and Engineering Materials at the University of Strathclyde, where she also completed a PhD on the fabrication and properties of silicon carbide ceramic matrix composites.

High quality joints using diffusion bonding have been achieved on aluminium metal matrix composites. Wendy Hanson reports on the principles of diffusion bonding, the materials involved, and examines the results of mechanical testing.

Aluminium metal matrix composites (Al-MMCs) have superior mechanical properties compared to un-reinforced aluminium alloys, Table 1 and therefore show potential in a variety of industries including: aerospace, automotive and sporting goods.^[1] However, one problem which has delayed the application of these composites is the joining method.^[2,3] The problem is caused by the different physical and chemical properties of the ceramic reinforcement (which may be particulates, or fibres) and the metal matrix, such as: mismatch in coefficients of thermal expansion, melting points, thermal conductivity etc. A number of joining techniques have been tried, including: arc and laser welding,^[4] or resistance spot welding;^[5] however these fusion processes cause considerable damage at the reinforcement-matrix interface, due primarily to the increase in temperature during joining. There is a requirement to develop a joining technique which does not cause microstructural degradation. Diffusion bonding is a solid state process carried out at moderate temperature and pressure which meets these needs.

Table 1: Selected materials properties of aluminium alloys and Al-MMCs

Aluminium based materials have one serious drawback when bonded in the solid state; an inherent oxide (Al ₂O ₃, alumina) surface layer. It is essential that this is eliminated in order that a strong bond may be produced. One solution to this is the use of metallic interlayers which may remove the oxide film through chemical and/or mechanical means. In this article, work is described where an Al-Li alloy has been used as the interlayer. One advantage of the Al-Li alloy is its superplasticity, which facilitates the deformation step in the diffusion bonding process, thus providing improved interfacial contact. Furthermore, the Li and Mg contents of this interlayer are not only responsible for age hardening in the interlayer and matrix respectively, but are also able to break up the oxide film by a number of mechanisms, described below:

Li + 2/3Al₂O₃ <-> LiO₂O + 4/3Al
Li + 2/3Al₂O₃ <-> LiAlO₂ + 1/3Al
3Mg + 4Al₂O₃ <-> 3MgO + 2Al
3Mg + 4Al₂O₃ <-> 3MgAl₂O₄ + 2Al

Materials

Two Al-MMCs were supplied (courtesy of Duralcan). Their matrix composition is given in Table 2. The reinforcement phase was alumina particulates at weight percentages of 10 and 20% respectively. For the 20% Al₂O₃ MMC, the percentage difference is caused by a larger particle size, rather than an increased number of particles.

Table 2: Matrix composition (AA6061)

The material was received in sheet form in the standard T6 treatment, (ie solution heat-treated and artificially aged). Microstructural studies of both composite materials showed that they were not completely homogeneous. In both, it was possible to find porosity and an heterogeneous reinforcement distribution. In particular, Al ₂O ₃ particles were preferentially aligned along the rolling direction. Some clustering, banding and particle free zones were also observed, Fig.1a and 1b. The defects were bigger in the AA6061/20%Al ₂O ₃ and a large amount of variation in size, shape and distribution was observed. This will have an important effect on the bond since the joint strength depends upon the number and shape of the reinforcement at the joint interface. The Al-Li alloy (AA 8090) used as an interlayer was applied in the form of a 50µm foil. The composition of this Al-Li alloy is given in Table 3.

Table 3: Interlayer composition (AA8090)

Diffusion Bonding

The principles of diffusion bonding are as follows. Two clean and smooth surfaces (average roughness <0.4microns) are brought together under a load, Fig.2. The temperature is raised to ˜80% of that of the lowest melting point material and the parts held for a number of minutes. In this work, the loads used were between three and six MPa, at temperatures of 500 to 530°C for a period of 10 to 60 minutes. After bonding, a solution heat treatment (525°C for 2 hours), cold water quench and age (190°C for 8 hours) was applied.

To obtain the required surface finish, all materials were ground with a 600 grit, followed by cleaning and degreasing using acetone, in an ultrasonic bath. Finally, a chemical etch with NaOH, followed by nitric acid wash was applied. This gave the following surface finishes: Aluminium MMCs - 0.41µm, Al-Li interlayer - 0.59µm.

These values are slightly in excess of the accepted levels; however in this case, a higher roughness will facilitate both the break up of the oxide layer during plastic deformation and will also cause the protruding reinforcement phase (residual from the chemical etch) to become embedded at the bond line, thereby increasing joint strength.

Results and Discussion

AA6061/10%Al₂O₃

The effect of the diffusion bonding parameters was studied for the AA6061/10% Al₂O₃. The results can be seen in Figs.3a-c. Voids and porosity existed in the bond after 10 minutes at temperature, since plastic deformation was not complete. After 30 minutes, the bond line showed good continuity without any porosity; however, diffusion was not complete, as demonstrated by the presence of a large number of lithium precipitates near the bond.

In the sample held for 60 minutes at temperature, the band where the lithium content had decreased (precipitate free zone) was larger than that observed after 30 minutes. This is evidence of the Li diffusion which has occurred across the bond line.

After an ageing heat treatment, the composition was more homogeneous (Fig.4); however, the existence of a low Li content zone was still clear and it was possible to observe large magnesium silicide precipitates. The loss of lithium from the Al-Li to the MMC was shown by the increase of the grain size in the AA8090 and its decrease in the MMC matrix. The best joints in terms of zero porosity at the interface were obtained at 520-530°C, 6MPa for 60 minutes.

AA6061/20%Al₂O₃

For the 20%Al₂O₃, the best bonds were also produced at 520-530°C and 6MPa (Fig.5). Bonds made in this condition showed an interface with good continuity and no observation of voids or defects. The existence of large size particle clusters in the bond had an important effect over the bond (Fig.6). In this zone the porosity was greater since their elimination by plastic flow was more difficult. This porosity appeared in the triple points between matrix-particle-AA 8090 and could have a detrimental effect on the bond strength.

Post-bond thermal treatment

The effect of the post-bond thermal treatment was different depending on the bond quality achieved. If the bond was poor and contained porosity, the residual oxygen (from the furnace atmosphere) was enough to penetrate into the bond through the voids and deteriorate the interlayer, thus destroying the bond. When the bond was good, as in the conditions mentioned above, the thermal treatment dissolved almost all of the precipitates and improved the bond quality.

When the interlayer thickness decreases (through diffusion effects), the reinforcement may move through the interlayer and produce a very homogeneous microstructure. In this case, the interface of particle-interlayer is very important because, to achieve a strength improvement, both a mechanical and a chemical bond must form.

The good quality of the joints bonded with high temperatures and high pressure, is revealed when they are observed after applying the post-bond heat-treatment (Fig.7). As is shown in the figure, the dissolution of the phases precipitated into the interlayer and the composite matrix during the bonding cycle is almost complete, and the recrystallisation and grain growth across the original bond interfaces revealed.

Mechanical Testing

Hardness profiles were taken across the bond for the samples after the thermal treatment, Figs.8a and 8b. The results were similar for both metal matrix composites. In all the profiles, a deterioration of hardness in the proximity of the bond interface occurs. The diffusion of lithium from the AA8090 to the MMC decreased the lithium content near the bond line causing a decrease in hardness. The same effect occurred at the other side of the bond due to the migration of Mg and Si from the MMC to the AA8090.

At high temperature and pressures (520-530°C and 6MPa), the high plasticity of the AA8090 alloy at the bonding conditions caused a plastic flow of the foil which, together with the diffusion, made the foil almost disappear in some zones (Fig.9). This effect has been observed previously with this interlayer. ^[6,7]

Shear testing was also used to evaluate the mechanical strength of the MMC diffusion bonds. Table 4 shows the shear strength values obtained under different bonding conditions.

Table 4: Shear test results

As expected, the highest shear strength results were obtained with a temperture of 530°C. The smaller than anticipated incremental rise in strength for the 20%Al₂O ₃ MMC was most likely caused by the difficulty in achieving full diffusion with the higher particle loading. An increase in either the bonding temperature, or load should improve the strength results still further.

Conclusions

• Diffusion bonding of Al-MMCs was achieved using Al-Li interlayers which accelerated the break up of the protective oxide film by means of chemical and mechanical mechanisms. The main role played by Li was related to its high reactivity, which not only favoured the break up of the alumina barrier, but also improved the properties of the MMC due to the strengthening of the matrix.
• High quality joints were obtained with minimal deformation, at 520-530°C, 6MPa over a period of 60 minutes.
• Diffusion bonding of Al-MMCs with interlayers is successful since, in addition to avoiding the problems associated with fusion processes:
  - The reinforcement avoids high plastic deformation thus allowing the application of high bonding pressures which favour the break up of the alumina oxide layer.
  - The particles act as a reinforcement in the bond, which cross the interface and act as 'roots'. When the interlayer thickness decreased, the reinforcement diffused through it and provided a more homogeneous microstructure, thus avoiding particle free zones.

Acknowledgements

The author gratefully acknowledges Dr John A Fernie (Morganite Thermal Ceramics) and Ms Maria D Escalera (University of Madrid) for their assistance in this work.

References