| Unique thermal mechanical coupling effects in friction stir processing(FSP) procedure can be used to improve the thermal sprayed coatings organization and comprehensive performance. Providing a solution to expand the application of thermal spray coatings and extend the service deadline, it will be immeasurable economic benefits from the experimental stage to industrial production. In this paper, Q235 steel substrate nickel chrome aluminum composite layer and high-speed steel composite layer prepared by FSP, using OM, SEM, EDS, HVS and other technics to analyze and test was studied. To verify the feasibility of reinforced composite coating layer by FSP, the mechanism of microstructure changes and mechanical property improvement was the key point. Then analyzed the effects of process parameters on the microstructure of the composite layer and element rapid interdiffusion behavior between coating layer and substrate, discovered the law of microhardness increased significantly and bond strength improved in the composite layer.Main conclusions obtained through data, testing, analysis and summary:(1) The combination of thermal spraying coating and substrate before FSP, the Ni-Cr-Al coating layer was mechanical combination with metallurgical combination, while high-speed steel coating layer was mainly mechanical bond. After FSP, the two interfaces can be eliminated to form a dense composite layer, but the mechanism was different: Ni-Cr-Al composite layer is filled with dynamic recrystallization tiny grains, while high-speed steel composite layer is mixed with a portion of the substrate and coating layer plastic flow welding. The most significant feature of macroscopic morphology in the composite layer after FSP was onion ring morphology, which was a heterogeneity microstructure phenomenon. Different softening degree plastic coating material in chronological order superimposed on each other, leaving the track in adjacent position line. The onion rings can be eliminated to some extent after multi-pass processing.(2) Lateral comparison results: stirring head speed w increase can expand the area of fine grain area and the heat-affected area; the feed speed on horizontal direction v should be within a certain threshold, heat input reduced when v increased; stirring passes n add can increase the density of the composite layer and homogenization area increase, but no influence on the grain size; according to the coating layer thickness, increasing vertical down quantity change Dh within a certain can increase the vertical forging force and the thickness of composite layer. Plastic deformation amount F directly affected the microstructure morphology: although not obtained the four parameters in the mathematical relationship between F, within a certain range, F and,,hnw D was proportional, while was inversely proportional to v.(3) Vertical comparison results: according to the plastic deformation and heat affected zone, FSP process zone can be divided into: FSZ、TMAZ、HAZ. In accordance with the microstructure change after the FSP Ni-Cr-Al composite layer from the coating layer surface to the substrate division: BZ、RLZ、FRZ、ECZ. The most affected area ideal specimen thickness was about 260mm, the optimum parameters for in this test group was =rwmin/600, =mmvmin/16,n=4, =Dh3. High-speed steel composite layer in this parameters was divided into: mixing zone, fine grain zone, grain deformation zone, the impact zone thickness was nearly 300mm. According to the processing area there was a corresponding relationship divide to the microstructure, the ability of plastic deformation difference led to the difference of microstructure. The coating and the substrate element rapid interdiffusion phenomenon occured after FSP, elements enrichment in the original interface. The hardness of both the Ni-Cr-Al composite layer and high-speed steel composite layer had improved significantly. Compared to the respective control sample, after FSP combined strength of Ni-Cr-Al composite layer and high-speed steel composite layer had greatly improved.(4) The mechanism of organization change impacted by FSP: in the action of force and the cumulative plastic deformation, under the joint action of dislocation motion and multiplication of continuously, there was an obvious cellular organizations or dislocation of the product, at the same time the thermal mechanical dynamic recrystallization temperature drop, dynamic nucleation occured on grain boundary, fine crystal and then grow up step by step eaten the original rough grain, formed nano-sized grain. The mechanism of strengthen the composite layer was the four sorts of comprehensive effect: firstly FSP can eliminate the original coating defects, then it was fine-grain strengthening, and hardening are stronger than soften effect of strain, finally the reinforcement of element diffusion. Mechanical behaviour would decompose into horizontal direction and vertical direction of the ideal mechanical model, explained the plastic flow mechanism. |
PDF Full Text Request