Laser welding offers high processing speeds, ease of automation and low distortion. However, much remains unknown about the process and the response of aluminium alloys to it, e.g. the hot cracking of welds in particular is a potential problem.

Thus five different aluminium alloys were welded using four distinct laser sources: CO2, CO and Nd:YAG (pulsed and high powered continuous). It was evident that high quality welds could be produced using all four laser sources and good mechanical properties with high joint efficiencies (up to 95%) could be obtained.

Weld metal strengths were found to be dependent upon welding speeds. Higher welding speeds were shown to reduce alloying element losses and to produce finer microstructures, both resulting in increased strengths. Hot cracking in the welds was studied and attempts were made to correlate cracking behaviour with material and process parameters. The cracking was not observed to be dependent upon the equilibrium Brittle Temperature Range (as had been reported in earlier studies) but showed some correlation with cooling rate for some alloys. Transverse cracking was observed and increased at high welding speeds, whereas longitudinal cracking did not.

Coupling, the efficiency of energy transfer from laser beam to workpiece, governs the heat input to the workpiece and is a major variable required in the modelling of welding behaviour. Values for coupling (20-45%) were determined for the five alloys and four lasers used; it was found to depend strongly upon the laser wavelength, increasing as wavelength decreased.

All lasers could be used to produce acceptable welds on most materials. The continuous Nd:YAG laser offers the highest welding speeds; the CO laser offers few unique advantages.

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