Laser Cladding Steel Substrate Situ Composites

Combining laser cladding technique and in-situ metal-matrix composite producing technique, In-situ Ni-TiB2 ceramic-metal composite coatings have been cladded on the surface of 45steel by CO2 laser melting a precursor mixture of nickel based alloy powder, boron and titanium powders. The composition, microstructure, and hardness of cladding layers were system studied by EPMA, TEM and XRD. The dependence of micro-hardness on depth are studied.According to the heat transfer feature during the melting process of laser cladding, the model of the heat transfer was established, and the calculated results were agreed with the experimental ones quite well.In laser cladding process, the chemical reaction of TiB2 is analyzed by thermodynamic theory. The tendencies of thermodynamics for possible formed phase were analyzed. The reason why TiB2 can be formed optimally were explained. At the same time, the reactive mechanisms between titanium and boron were testified. The results show that in proper processing method, titanium is molten firstly because of its relative lower melting point. TiB2 is generated on the surface of boron. Further produced TiB2 depends on titanium's diffusion.The microstructure analyses show that three regions with different microstructure exist in the surface of layer of clad samples: the cladding layer, the heat influence and substrate. There is a metallurgical-bonding interface between the cladding and substrate. Under optimized cladding parameters, the cladding layer with smooth surface, fine microstructure and without crack is made. Combining system studied by EPMA, TEM and XRD, the result shows that the coating mainly have Y -Fe, Y -Ni dendrite solid solution and TiB2 ,TiB particles and a small a mount of Ni4B3.The effect of laser beam parameters such as laser power and laser scanning speed on the microstructure and the performance of cladding layers was also discussed, which indicates that laser translation speed influenced the microstructure and the performance is as same as laser power.The multi-pass laser claddings were investigated. The overlapping coefficient is defined in theory. The good experimental result is got when coefficient equals 0.3. Like the microstructure of single cladding layer, the microstructures of overlapping cladding layer also have three regions. The composite layers are dense and free of cracks with a good metallurgical bonding between the layer and substrate. Microstructure of coating consists of dendritic Austenite and spinal Martensite. There is a metallurgical-bonding interface between the tow tracks of overlapping. Combining system studied by EPMA and XRD, the result shows that the microstructure of layers is mainly composed of Y -Fe, Y -Ni dendrite solid solution and compound of B, in which TiB2 is dominant phases. There is also TiB, Ni4B3 and Fe3B particles in layers.