Laser alloying of aluminium and 7175 aluminium alloy for enhanced corrosion resistance

The microstructure and corrosion resistance of aluminium and ANSI 7175 alloy laser alloyed with chromium were investigated. After laser treatment the microstructure contains relatively large amounts of intermetallic compounds dispersed in a matrix of α-aluminium. Most of the intermetallic compounds present needle-like or cubic morphologies, arranged in a dense network or distributed radially. The phases identified by X-ray diffraction were Al₇Cr, Al₁₁Cr₂ and α-aluminium solid solution. When the chromium content exceeds a few percent the laser alloyed layers present cracks, pores, inclusions and undissolved chromium particles, depending on the exact chromium concentration and its particles size. However, homogeneous layers were successfully produced by a two-step process, consisting of laser alloying followed by remelting. The second treatment eliminates porosity and refines the structure. The hardness attains 160 HV in chromium alloyed aluminium and exceeds 300 HV in chromium alloyed 7175 alloy. The corrosion behavior of the above alloys was assessed using anodic polarization techniques. Laser alloying of 7175 and aluminium with chromium improves the pitting corrosion resistance of the alloys. The effect depends on the chromium content of the alloyed layers and is more significant in 7175 alloy.