| Ni/TiC has become the research hotspots of high-temperature structural materials due to its high specific strength, high-temperature resistance, corrosion resistance and etc. This paper adopts the method of the CO2 laser cladding direct fabrication and mainly aims at the preparation of the interface interlayer between Ni (at one side) and TiC (at the other side) which contains both Ni and TiC advantages. The thermal physical properties difference between the outer ceramics and the inner metal can be obtained full compensation due to the interface interlayer, hoping to get the advances materials with properties of higher strength, better fracture toughness and the thermal stress relaxation, and providing the significant references for the FGM.At present, the researches on FGM are mainly concentrated in the design, preparation and performance evaluation of materials. The three aspects have a close correlation and complement one another. The design of materials is the foundation of researches. It provides the FGM with the optimal compositions and distribution of the structural gradient. The preparation of materials is the core of the investigation. The advantage and disadvantage of the preparation method determines whether the design of the FGM can be finally realized or not. The performance evaluation of materials is the guarantee of the FGM wide use, the standard test method of the accurate evaluation FGM characteristics. The accuracy of the performance evaluation is the basis of ascertain whether it satisfies with the using demand or not. According to this standard, the FGM database is established and fed back the message to the materials design department. In this dissertation, based on the main line of this study, Ni/TiC FGM is studied deeply. The following several research results are achieved:(1) The finite element analysis method is utilized successfully. Stress relaxation design methods of preparation FGM have been proposed which has practical operational significance. The regularity has been found that the gradient composition exponent p, the gradient layer number n and the gradient single layer thick h of Ni/TiC FGM prepared by direct laser fabrication affect on residual stress. FGM can get the optimal stress relaxation effect, when p=1, n=11 and h is as small as possible, respectively.The residual stress analysis results indicate that:â—‹1 the maximum residual stress decreases rapidly when the gradient composition exponent of FGM p<1; when p>1, the maximum residual stress increases slowly with the increase of p. The maximum residual stress is the smallest when p=1, moreover the distribution of the ideal maximum residual stress in X and Y directions shows the ideal performance state of the stress distribution. That is to say, the side rich in Ni of the FGM is tensile stress, while the side rich in TiC of the FGM is compression stress.â—‹2 the maximum residual stress reduces with the increase of the gradient layer number n. When n is in the ranging from 2 to 10, the maximum residual stress reduces rapidly. When n=3, the maximum tensile stress and the maximum compression stress are the biggest in Z direction. When n=2, the maximum tensile stress and the maximum compression stress are the biggest in X and Y directions.â—‹3 the maximum tensile stress and the maximum tensile and compression stress in different directions all decrease with the decrease of the single gradient layer thick h.(2) The concept of the volume powder feeding rate is proposed. The volume powder feeding rate must be controlled for the control of formability when FGM is prepared by using CO2 laser cladding direct fabrication. At the same time, the author states that the same volume powder feed rate of each layer is the necessary conditions to obtain the good forming for the gradient materials.It is discovered from the technology experiment that the laser forming quality has close relation with the laser technological parameters. With the increases of the laser power and carried gas flow, the width and height of the cladding layer increase first and then decrease. Both the width and height of the cladding layer decrease with the increase of the scanning speed. While the width and height of the cladding layer increase with the increase of the powder feeding rate. The surface roughness of single layer multi-tracks cladding surface is guaranteed by good overlap rate. The carried gas quantity does not affect the powder feeding rate, but it can change the position of the powder convergent point and powder particles array density in the power flow. The powder flow volume increase when the power feeding rate increases. The common role of the forced convection caused by surface tension gradient, the natural convection caused by horizontal temperature difference gradient of the molten pool and the wettability is the cause of cladding layer uplift. The Z-axis increment that is smaller than the first layer thickness can assure the forming of 11 single layers thin parts. Forming bulk materials, it can effectively avoid genetic defects and ensure the surface of the formation between two adjacent layers adopting the vertical stacking technologies. It is not appropriate to change the specific energy by change of the spot size and the scanning velocity when preparing the FGM. It should adopt the method of the change the laser power, and the laser power should increase with the Ni volume fraction decrease. DLF processing parameters which can make FGM stable form are obtained by the same powder feeding rate and the changed laser power.(3) Ni/TiC (in-situ synthesis) system single layer materials and Ni/TiC FGM have been prepared successfully by DLF technique by adjusting Ni, Ti and C powder mixed ratio. The materials with the same Ni-Ti-C mixed ratio have the same phase composition and microstructure morphology, which is confirmed not only in single layer materials but also in multi-layers materials in the DLF process. The XRD analysis shows that Ni/TiC FGM and Ni/TiC single layer materials prepared by DLF technique are both composed of TiC ceramic phase (f.c.c) and Ni metal phase (f.c.c). The results are consistent with the thermodynamic analysis.The lattice parameters of TiC and Ni are higher than that of TiC and Ni equilibrium phases because of super saturated solid solution produced by laser cladding. The phase compositions of the FGM gradient layer and the optimization design result have the good uniformity. Ni/TiC FGM gradient layer interface is good. Microstructure of FGM transits continuously from Ni to TiC. Ni/TiC FGM shows that the macrostructure is non-uniform and the microstructure is continual. In the Ni/TiC system materials, TiC particles are round or round-like. TiC particles size increase with the decrease of Ni volume fraction. There are a small amount of micro-porous in the inner of the TiC grain, among TiC particles and between the Ni metal and TiC particles caused by the changes in the molar volume of materials, impurities volatile, thermal transfer, high hardness and high melting point of TiC. Ni/TiC FGM and Ni/TiC single layer materials have the similar microstructures. The change of the microstructure with Ni volume fraction change is uniform. The results are obtained which the performance evaluation method of Ni/TiC single layer materials can evaluated performance gradual of Ni/TiC FGM. In laser molten pool, the dissolution-precipitation growth mechanism of in-situ synthesis TiC is the main reason to finally induce the TiC particles round or round-like morphologies.(4) The relevant physical, mechanical and mechanical properties of Ni/TiC single layer materials and Ni/TiC FGM are analyzed deeply, including density, thermal expansion properties, and thermal conductivity properties, bending strength and wear resistance performance. It provides the theoretical and experimental basis for the materials system.Both the density and relative density of Ni/TiC single layer materials prepared by DLF increase with the increase of the Ni volume fraction. The relative density is the smallest but also reaches 95.19 % when Ni volume fraction is 10 vol. %. In the temperature range from 373 K to 1273 K, the thermal expansion coefficient increases with the increase of Ni volume fraction and the rising of temperature. And it is consistent with the thermal stress design goal. When the temperature is in the range from 298 K to 973 K, the thermal conductivity of the Ni/TiC single layer materials increase with the increase of Ni volume fraction and the temperature. The microhardnes changes of Ni/TiC FGM and Ni/TiC single layer materials are consistent with Ni volume fraction change. The microhardness of FGM presents the continuous gradient distribution characteristics. The materials microhardness increase slowly in the region of rich in TiC materials which indicates the influence of material composition and densification on the microhardness. The elasticity modulus and bending strength decrease with the increase of Ni volume fraction. But the fracture toughness increase with the increase of Ni volume fraction. The wear resistance of Ni/TiC single layer materials decreases with the increase of Ni volume fraction. And the wear mechanism is mainly ploughing. The good wear resistance of composites reveals that the TiC particles are the main wearable phase. In the support of high tenacity Ni, the wear resistance of composites increases with the increase of TiC volume fraction because of hard particles (shadow effect). According to the analysis of the physical and mechanical properties of Ni/TiC single layer materials and FGM, it is known that the physical and mechanical properties of Ni/TiC single layer materials with different Ni-Ti-C ratio are a continuous gradient transition with the composition and temperature change. So, Ni/TiC FGM prepared by DLF cladding has the characteristics of stress relaxation. It is proved that the purpose of the design and preparation has been obtained. The fracture morphology analysis of Ni/TiC shows that the fracture morphology has significant changed with composition change in the regions of rich in Ni and rich in TiC. The fracture morphologies are dimples in the region of rich in Ni. The fracture is mainly plastic fracture whether from a macro or micro view. The fracture morphology is brittle fracture in the region of rich in TiC materials. The mode of the brittle fracture is mainly along the grain boundary.(5) The thermal fatigue test of simulation the reality environment simulated and the thermal shock experiments of the great temperature difference indicate that Ni/TiC FGM has better thermal fatigue properties and thermal shock resistance than N-FGM. It fully demonstrates that the composition gradient transition of Ni/TiC FGM is effective in alleviating the thermal stress destructive. Thus it is verified that the Ni/TiC FGM stress relaxation characteristics and the design are correct.In a whole, the paper adopts the high flexibility and toughness processing characteristics to realize the continuous gradient change transition region between the TiC ceramics and Ni metal, which makes the FGM has the good thermal stress relaxation and the resistance to heat shocks and avoid the interface stress due to the mismatch of the thermal physical properties difference between the outer ceramics and the inner metal. The experiment results fully verify the feasibility of the materials system, the design composition distribution and the used fabrication method. It can make full use of the advantages of the ceramics high temperature resistance and the metal strong bearing. It reaches the aim of the complementary among the properties and accords with the research standards of the FGM. It will have a good base for further development and application of the Ni/TiC FGM prepared by DLF.
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