Laser cladding of cast aluminum-silicon alloys for improved dry sliding wear resistance

AA333 specimens were hardfaced with mechanically mixed Mn-Al bronze powder by laser cladding to investigate its possible use in a boundary lubricated sliding application for better wear resistance. The coatings needed to be more seizure resistant than the AA333 currently in use. Like other cast aluminum alloys, AA333 possesses a hard, silicon rich secondary phase which helps impart good wear properties. However, in starved lubrication conditions, the relative softness and low melting point of the Al-rich matrix can result in its scuffing and even seizure. The laser used was a 6 kW cw CO2 laser, in an oscillating beam process. The experimental group of coatings consisted of a 3 X 3 matrix of three process parameters-laser power, traverse speed, and powder feed. The coatings were mechanically sound, relatively free of small cracks and porosities, with a strong, tough metallurgical bond. The coatings were 3 to 4 times harder than the AA333 substrate matrix which was about 90 Vhn. Many wear specimens were prepared from the coatings for comparison with AA333 properties, and for property versus process parameter correlations within the experimental matrix of coatings. The coating microstructure was primarily single phase, consisting of fcc columnar-type grains of varying size produced by rapid and directional solidification, with a homogeneous distribution of the alloying elements in solid solution (by wt %: 75 Cu, 12 Mn, 8 Al, 3 Fe, 2 Ni). Analysis of coating microstructures was carried out using optical and scanning electron microscopy, x-ray energy dispersive spectroscopy, and x-ray diffractometry. The interface microstructure was found to contain alpha-Al (fcc) in the remelt layer and theta-CuAl2 (tetragonal). Tribological tests showed that the seizure resistance of the coatings was superior to AA333 in dry sliding tribological tests, though the wear rates of the coatings show some dependence on the process parameters. The coatings exhibited up to 1/3 the wear rate of the AA333 substrate under the same dry wear test conditions of 4.5 kg normal load at 600 rpm, 30.3 mm radius of sliding, simulating actual component operation. However, the major benefit of the coating is increased resistance to an abrupt, work-stopping failure by seizure. Necessary repair and replacement of parts as a result of seizure can be very costly. The wear mechanisms of coatings and substrate are discussed, as are the coating properties versus process parameters. (C) 1998 Laser Institute of America. [S1042-336X(98)00202-2].