Laser surface treatment of electroless Ni–P coatings on Al356 alloy

In the present study, laser surface treatment of electroless Ni–P coating on Al356 alloy was performed using Nd:YAG pulsed laser at different scanning rates. The microstructure, phase, composition, microhardness and corrosion behaviour of the modified surface structure were analyzed using scanning electron microscope, X-ray diffraction, energy dispersion analysis X-ray (EDAX), microhardness testing and electrochemical techniques. It was shown that laser surface treatment produced a relatively smooth, crack free and hard surface layer consisting of Ni–Al intermetallic particles. The composite coating had a very pronounced hardness gradient along the melt depth direction and pure metallurgical bonding to the substrate. Hardness of surface was significantly increased due to the formation and uniform distribution of fine Ni–Al intermetallic phases in remeleted layer. In addition, an amorphous structure can be formed and higher corrosion resistance can be obtained. The thinner the coating thickness and the lower the laser scanning rate, the higher the dilution with Al and degree of crystallization of the alloy and considerably, reduces its corrosion resistance and increases its surface hardness. The specimens with 18 μm Ni–P coating that laser treated in 37 mm/min scanning rate had the best hardness and corrosion resistance simultaneously.