ABSTRACT
Due to its high strength, high temperature resistance and good wear resistance, the laser cladding composite coatings has been widely used as mechanical parts and components in the special and harsh field such as machinery manufacturing, petrochemical, nuclear engineering,automotive industry, aerospace. However, some kind damages of composite coatings such particle broken and particle/matrix interface debonding will occur under different loading conditions. On the other hand, the defects of composite coatings such as micro-void introduced during preparation will accelerate the failure of material. Further and deeper researches on the mechanical properties and damage mechanism of laser cladding composite coatings were necessarily required.
In this dissertation, macro-mechanical properties and micro-damage mechanism of laser cladding H13-TiC composite coatings has been studied. The mechanical properties of particle and matrix phases at the microscopic scales were measured by nanoindentation. The particle cracking induced by nanoindentation was observed and the critical fracture strength of particle was obtained by the combination of experiment and finite element analysis. Based on the micromechanics, elastic-plastic mechanics and experimental results, a new damage constitutive model of composite coatings was presented. The model was embedded in the finite element software ABAQUS subroutine UMAT, focusing on investigation of damage and failure behavior of composite coatings. The research work will provide theoretical, numerical and experimental support for the preparation of laser cladding composite coatings. The main work and achievements are as follows:
1. With appropriate laser processing, the H13-TiC composite coatings was cladded on the H13 die steel. The mechanical properties of composite coatings containing different particle volume were obtained by uniaxial tensile experiment. The initial and evolution microstructure of material as well as damage mechanism were observed by scanning electron microscopy (SEM). The results showed that micro-defects such as micro-void were introduced during preparation, particle fractures was the main reason of crack formation in the composite coatings,the particle/matrix interface debonding appeared in the later tensile experiment. Such results provided the experimental basis of damage model.
2. The mechanical properties such as hardness and modulus of TiC particle and H13 matrix were measured by nanoindentation. In addition, the influence of distance between indentation position and particle center, indentation depth and the particle morphology on the indentation results were investigated; The critical fracture stress of TiC particle was obtained by the combination of experiment and finite element analysis characterizing of the strength measurement of ceramic particle.
3. The elastic-plastic properties of composite coatings with different particle volume were studied by Mori-Tanaka mean-field homogenization scheme. Based on the experimental results,the parameters of Weibull damage principal, which characterized the damage mode of particle,were obtained.
4.Coupling Mori-Tanaka(M-T) mean-field homogenization scheme with the Gologanu–Leblond–Devaux (GLD) yield criterion, the damage model accounting for the effect of void shape, void size, void volume and particle damage on tensile behavior of laser-processed composite coatings was developed. At last, the prediction results of this model was compared with experimental results, and the consistence demonstrated that the model well described the damage process of laser cladding composite coatings.
Key Words: laser cladding composite coatings, mechanical properties, nanoind -entation,damage behavior, M-T mean-field homogenization scheme, GLD damage model
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