Fabrication Of NiTi Alloy Coating On Stainless Steel

Steam turbine last stage blades can be easily damaged under water droplet erosion, which influence the safety and efficiency of steam turbine. The nearly equiatomic NiTi intermetallic compounds posses excellent cavitation erosion resistance, and is a kind of ideal surface modification material. As a new local strengthening technology, laser cladding embodies the obvious superiority. Fabrication of cavitation erosion resistance alloy coating on blade inlet edge could be controlled automatically, and replace some commonly used method such as brazing satellite alloy, thermal spraying, and bead weld, etc.A systematic study of the cladding NiTi alloy on 17-4PH stainless steel by 500W plused Nd:YAG laser and 6kW CO2 laser machining system has been conducted. SEM, XRD, EDS and micro sclerometer were carried out to analysis the microstructure, phase, element composition, and microhardness, friction and wear characteristics of NiTi cladding layer is measured by MMW-1 friction and wear tester.The results show that three typical microstructures (B2 phase + tetragonal structure of precipitates, dendrites and grains) are precipitated in the NiTi alloy during rapid solidification in different pulsed energy using the pulsed Nd:YAG laser. The average microhardness of the clad layer in C-2, C-4 and C-5 samples are about 670HV, 700HV and 615HV, respectively, which are much higher than substrate's 300HV.In CO2 laser irradiation conditions, cladding area mainly contains B2 NiTi and tetragonal precipitates, alloying area are dendrites and the bonding zone between substrate and coating is composed of columnar and planar. HAZ is quenched martensitic structure. The cladding layer mainly constituted by B2 and only a few TiN. There were Ni4Ti3 phase precipitated at energy density of 100J·mm-2. The average microhardness of the coating in cladding area, alloying area and HAZ are about 600～750HV, 500～550HV and 350HV, respectively.Wear resistance of NiTi cladding layer improved 7 times than that of 17-4PH. Wearing mechanism of 17-4PH steel is the compound form of grinding abrasion and adhesive wearing, while wearing mechanism of laser cladding layer is mainly grinding abrasion. Wearing resistance of the coating was greatly improved maybe attributed to the existence of hardening phase Ti compound.