Laser Surface Alloying Of Aluminum Alloy With Titanium And Nickel Mixed Powders

A strengthening layer was fabricated on 2024 aluminum alloy by laser surface modification with pure Ti, Ni and Ti+Ni mixed powders in order to obtain in-situ formed aluminum intermetallics for improving the properties of the Al 2024, such as surface hardness, wear resistance, high temperature and oxidation resistance, etc.With proper processing parameters the qualified strengthening layer of about 1～2 mm thick was formed. The analytical results of microstructure and wear properties of the strengthening layer showed that the strengthening layer was composed of the intermetallics such as NiAl, Ni3Al, NiAl3, Al3Ti, AlTi etc. in the form of fine dendrites distributed in a-Al solid solution. The strengthening layer can be divided into several micro-layers:the surface layer is primarily formed of the small cellular-like crystal sticks and a large number of nano-particles; the microstructure of the inner layer is varied and has a strong directional; the microstructure near the substrate is columnar and grows perpendicular to the interface, which improves the bind strength between the strengthening layer and the substrate.The properties of strengthening layer are greatly influenced by laser parameters and the convection strength of melt pool is affected by laser power density. Given laser power density, with a higher scanning speed, the coatings with a finer microstructure, a higher hardness and a thinner thickness can be obtained. However, excessive high scanning speed will result in the crack of the coatings. According to the experimental results, a set of proper parameters of laser power density:700-800W/mm2, scanning speed:12-14mm/s, overlap:30% and argon flow rate:20L/min, was obtained:The results showed that the average hardness of the layer was more than 350 HV, which was much higher than that of the A12024 substrate(less than 120 HV). The wear resistance of the strengthening layer was 4 times higher compared with the Al 2024 substrate. The strengthening layer also has a strong oxidation resistance at elevated temperatures compared with the coatings laser fabricated with TiC50%+SiC50%. Exposed to 400℃for 24 hours, the latter was crack but the former wasn't.