Electron beam and laser surface alloying of Al-Si base alloys

Surface alloying on aluminium-base alloys is achieved either by using an electron beam or a laser beam, in order to improve the mechanical properties of the near-surface region. A predeposit of nickel is first realized by plasma spraying. Melting of both the coating and part of the substrate produces a surface alloy with a fine, dendritic microstructure with a high hardness. Enhancement of this property requires an increase in the nickel content. Various problems occur during the formation of nickel-rich surface layers: incomplete homogenization owing to a progressive increase of the liquidus temperature, cracks owing to the brittleness of this hard suface alloy, formation of a plasma when experiments are carried out in a gaseous environment (laser surface alloying). Nevertheless, various kinds of surface layers may be achieved; for example very hard surface alloys (HV0.2=900), with a thickness of about 500-600 μm, or very thick surface alloys (e>2 mm), with a fairly good hardness (greater than 350 HV0.2). Thus, it is possible to obtain a large variety of new materials by using high energy beams on aluminium substrates. (orig.)