Influence of laser scanning speed on the formation property of laser brazing diamond coating

The pre-placed BNi-2 alloy solder was utilized as braze material to fabricate brazing diamond coating on 65Mn steel substrate by using fiber laser with 5 mm × 5 mm square laser spot. The laser spot was scanned at a steady power of 1.1 kW. The effect of laser scanning speed on the formation mechanism was investigated. Different scanning speed leads to different heating conditions of solder layer, and finally leads to different melting behavior of the coating. At the small laser scanning speed (1 mm/s and 2 mm/s), the solder can be fully melted to form a small liquid ball owing to the long interaction time, and the small ball can quickly flow into the left liquid molten pool because of the high heating temperature and good fluidity. Because of its small mass, the diamond particles float on the surface of the molten pool and gather on the top of the molten pool. With the cooling and solidification of the molten pool, the diamond particles are finally fixed on the top of the coating. With the increase of laser scanning speed (3 mm/s), the solder which has not been completely melted will flip the coating toward the liquid molten ball due to the greater melting imbalance in the solder area. The initial liquid molten balls constantly gather and grow in size and finally contact with the molten pool and merge into it. However, when the scanning speed reaches 4 mm/s, the solder cannot be fully melted due to the shortening of the irradiation time of the laser spot. The graphitization degree of diamond, the exposed height of diamond and the width of coating were also studied and they are related to the previous formation process.