Fastening & Joining
 Innovator's Notebook  

Aluminium alloys can get it together

Aluminium alloys, or alloys and metal matrix composites, can be joined without loss of shear strength. Tom Shelley reports

Temperature Gradient TLP bonding Very high integrity bonds can be made in aluminium by sandwiching a thin layer of copper between the parts and imposing a temperature gradient.

The technique, called TLP (transient liquid phase) bonding, can be applied to any aluminium alloy, including aluminium-lithium alloys and aluminium-silicon carbide composites.

Aluminium alloys offer many challenges in welding, because of problems with oxidation, precipitation and age hardening effects. Established non-fusion methods include diffusion bonding – in which two parts are heated and pressed together for hours or even days – and transient liquid phase bonding, in which two parts are pressed together with an interspersed copper foil, to briefly form a low melting point aluminium/copper liquid phase at the interface.

Most bonding methods lead to some kind of weakness at the interface, but Dr Amir Shirzadi, of the University of Cambridge’s Department of Metallurgy, has found a way of minimising this by imposing a temperature gradient.

Transient liquid phase (TLP) bonding
uses a temperature gradient to
ensure a strong bond between alloys

If an upper part remains cool, while the lower part is heated to 550 deg C, the upper and lower boundaries of the liquid layer both move downwards. As they move, they leave behind aluminium with a small amount of copper, until the liquid layer runs out of copper and solidifies. Further, as the interface moves downwards, it tends to wobble, leaving any inclusions or other undesirable phases in a sine wave pattern, instead of across a flat interface. This greatly reduces any tendency for the bond to fail when subsequently tested in shear.

Conditions have to be carefully controlled for best results. If the interface is formed too quickly, and is too unstable, it leads to the formation of finger-like projections at the interface called dendrites.

Ideally, the wobble should be limited to a region 5 to 10 microns across. If this is done, it is possible to bond parts made of ordinary aluminium alloys, such as 6082 aluminium-magnesium-silicon alloy, so that shear failure does not occur until around 250 MPa, similar to the strength of the unbonded metal. It has also been found possible to produce satisfactory weld bonds in UL40 aluminium-4% lithium alloys, one of the strongest light alloy materials per unit weight known, and between Al-6082 and Al 359. Al 359 is aluminium alloy with 20 per cent silicon carbide particles. Bonding both materials present considerable problems by other methods.

The technique is patented and has just won its inventor the 1998 Henry Granjon prize from the International Institute of Welding based in Paris. Research is continuing with the aim of developing the approach and also extending it to other materials. 

Design Pointers
Aluminium alloy parts, including those made of aluminium lithium alloys and aluminium and aluminium metal matrix composites, can be joined without loss of shear strength

Process time with small parts is normally about 10 minutes


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