TWI Knowledge Summary

Weldability of metal matrix composites (MMCs)

Materials consisting of metallic matrices, reinforced with ceramic particles or fibres, are known as metal matrix composites or MMCs. The volume fraction of the reinforcement is typically in the range 10-70%. Although all metallic alloy systems constitute potential matrices, development activity and applications have been concentrated on aluminium alloys. Aluminium alloy MMCs can offer a range of property enhancement over monolithic alloys, e.g. higher strength and stiffness, improved high temperature properties, better wear resistance and lower thermal expansion.

The most significant property gains are associated with long-fibre materials which are the most expensive to produce and few applications outside space and defence can bear the associated material costs. High volume applications are for particulate reinforced alloys for which relatively modest property enhancement is effected.

Most materials are unsuitable for fusion welding techniques because their carefully engineered structures are destroyed if melting occurs. Indeed, even elevated temperature exposure will degrade some materials. Thus, the most satisfactory joining techniques are often those which are carried out at lower temperatures, i.e. adhesive bonding, brazing or solid phase joining procedures, such as friction welding and diffusion bonding. Friction stir welding (FSW) is considered to be a candidate method for welding of MMCs. However, due to the requirement of sufficiently high hardness on the FSW tool, this method is still on the stage of development. Academic studies showed that FSW of Al-MMCs produced a homogeneous microstructure with a uniform hardness profile when compared to fusion welded Al-MMCs. Reduced porosity and increased depth-of-penetration is reported by using high power Nd-YAG laser welding with a pre-welding treatment of nickel coating. Again, Nd-YAG laser welding has not been used commercially even though metals show a higher absorbability to YAG lasers than CO₂ lasers. Friction welding has proved particularly successful for joining particle reinforced aluminium alloys.

One of the exceptions to the general rule is aluminium alloys reinforced with alumina particulate. Alumina is stable in molten aluminium and MIG welding has been successfully applied to structures such as bicycle frames. The latter is a typical application for this class of materials since, in sporting goods, premium performance can command premium price and advanced technology can be a selling point rather than a cost burden.

Packaging for electronic devices is another area where the costs associated with the production of MMCs can be justified. A high volume fraction of silicon carbide particulate creates a packaging material which is matched in thermal expansion coefficient with the silicon devices. When these packages must be hermetically sealed, appropriate bonding or brazing technology must be applied.

The most relevant industry sectors are:

Aerospace
Defence
Sporting and Leisure goods
Automotive

Further information

Joining of metal matrix composites - a review (Industrial Members Report No. 489 - August 1994)

Aluminium metal matrix composites - successes using diffusion bonding

Composites - a guide to best practice

You can use the Weldasearch literature database to supplement what you find in JoinIT.

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