An rf-excited gas-dynamically cooled carbon monoxide laser with unstable resonator (M equals 2) has been developed. The extracted laser beam shows an astigmatic phase distortion which is corrected by use of an extracavity cylindrical mirror. A 4.5 kW laser beam with a total divergence of 1.3 mrad is obtained with an efficiency of 9% and a Strehl ration of 0.5. The beam quality is 1.3 times the diffraction limit. First welding experiments on the aluminum alloy AlMgSi1 (6082) show higher penetration depths compared to carbon-dioxide and Nd- YAG laser welding at the same power level. This is a result of the smaller focal spot size with the CO laser leading to higher intensities in the welding zone. The aluminum weld seams obtained with the CO laser are very homogenous and regular at the surface in contrast to the weld seams obtained with the CO laser. The process parameters in CO laser aluminum welding can be changed in a wide range. This is a consequence of the shorter 5 to 5.6 micrometer wavelength compared to the carbon-dioxide laser resulting in a reduced beam- plasma interaction. Spectroscopic investigation of the CO laser induced aluminum welding plasma show a strong decrease of the intensities of Al (II) lines and no appearance of Al (III) lines as in case of carbon-dioxide laser welding of aluminum.