Structure and properties of Al–Nb alloys produced by laser surface alloying

Al–Nb alloys present considerable strength and hardness, being promising materials for structural applications and for coatings. This paper reports results of a study of the structure and properties of Al–Nb surface alloys produced by laser alloying. Surface alloys were produced by a two-step process: firstly, commercial purity Al substrates were alloyed with Nb by injecting an Al–25 wt.% Nb powder mixture into the melt pool generated using a CO2 laser, and, secondly, the microstructure was refined by laser melting. In the first step, Nb concentrates near the surface of the melt pool during solidification, resulting in the production of surface layers about 700 μm thick of Al–37.6 (±3) wt.% Nb alloy. These alloys are heterogeneous and present porosity and undissolved Nb particles for all the processing parameters used. Their microstructure is formed of large dendrites of Al3Nb and interdendritic α-Al solid solution. Laser melting of the alloyed layers results in complete elimination of defects and homogenisation of the material. Previously undissolved Nb particles dissolve, raising the Nb concentration in the alloy and increasing the volume fraction of Al3Nb. The microstructure still consists of Al3Nb and α-Al solid solution but the dendrites are finer and the volume fraction of interdendritic α is less than 5%. The Vickers microhardness of the surface alloys is high owing to the large amount of this intermetallic compound. It varies from 480 to 650 HV, depending on the volume fraction and dendrite spacing of Al3Nb. Despite the brittleness of Al3Nb no cracks were observed, probably because of the presence of the thin interdendritic film of ductile Al solid solution, which enables better accommodation of the generated stresses.