Key Engineering Materials Vols. 373-374

Abstract: The high-energy laser melts the cladding materials which coagulate onto the surface of the base materials. The laser cladding coatings are produced mainly in two methods: powder feeding and prefabricating. The grain-strengthening composite coatings could be formed more easily by prefabricating method than by powder feeding method. In this paper, Ni/SiC ceramic composite coating on carbon steel (45 steel) was made by prefabricating method. The microstructure and wear-resistance of Ni/SiC cladding coating on the 45 steel were studied using scanning electronic microscope (SEM) and wear test. The results showed that the microstructure of cladding coatings included bonding layer, thermo-affected layer and heat-affected layer after laser cladding. The microstructure of cladding coating was mainly of dendrite and cell-like crystals. The resultant multilayered coating had excellent adherence with the base steel. The addition of SiC particles into cladding coatings significantly reinforced the microhardness of laser cladding coating. Compared with Ni60A cladding coating, Ni60A/SiC cladding coating had high microhardness, which was attributed to not only the dispersion intensification effect of the SiC particle, but also by the new complicated phases. During laser cladding process, SiC particles may decompose and dissolve into the coating and result in solid solution strengthening effect which increases the microhardness of the composite coatings. The base material and Ni-based laser cladding coatings with and without SiC were tested to assess the wear-resistance property. The test results demonstrated that the laser cladding coatings had better wear resistance than the base material. Furthermore, the laser cladding coating with SiC particles had higher wear-resistance than the coating without SiC.

Abstract: This paper presents some results of plasma oxidizing of Al alloys. The oxidation of aluminium alloys was carried out in the mid-frequency pulsed excited plasma. The processing temperature was 450~500°Cand the processing time was 4 hours. The modified layer was characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and scanning electron microscopy (SEM). AFM and SEM observations indicated that there were no evident defects appeared in the modified layer. The immersion experiments in 3.5% NaCl solutions were carried out to test the effect of plasma oxidation on the corrosion resistance of Al alloys. After immersion for 200 hours, the severe pit-corrosion was occurred on the untreated sample surface while for the treated sample no evident corrosion was observed.

Abstract: A numerical simulation is proposed to study the dynamics expansion characteristics during the material irradiated by a high-intensity laser beam.The ionization effect and the local mass and momentum conservations for plasma expansion are considered in this model. As an example of carbon target, the plasma flow dynamics into a vacuum, like ionization degree, plasma number density and space pressure are studied in detail. The results show the plasma temperature strongly affects the ionization fraction and the ionization effect evidently influences the plasma dynamic expansion behavior. The space pressure of plasma decreases along with the plasma expansion.

Abstract: Ultrasonic phase spectrum of reflection coefficient has been used to nondestructively characterize the density, porosity, and microcracks of plasma sprayed Cr2O3 coatings irradiated by high-intensity pulsed ion beam (HIPIB). The ultrasonic measurement was investigated using immersion focusing pulse echo method with a 25 MHz transducer and the phase spectrum of reflection coefficient has been experimentally obtained. The relationship among velocity, density, attenuation coefficient and the phase spectrum was analyzed based on an acoustic transmission model in a multi-layered structure. The velocity relates to the harmonic frequency of the phase spectrum, and the density and attenuation coefficient play roles on the amplitude of the maximum and the shape of the spectrum. The effects of the three parameters (velocity, density and attenuation coefficient) on the amplitude of the maximum of the phase are similar. Some critical value exists which change the relationship between the individual parameter and the maximum of the phase spectrum. When one of the three parameters exceeds the corresponded critical value, the amplitude of the maximum decreases in the form of exp( ) n y = A Bx . For the as-sprayed and irradiated coatings with the thickness of 50 0m, the velocity calculated from the phase spectrum is 2522 and 2772 m/s, harmonic frequency corresponding to 12.61 and 13.86 MHz, respectively. These ultrasonic determinations demonstrated that the coating becomes more densified after irradiation by HIPIB, in agreement with SEM observations. Ultrasonic phase spectrum itself is a good indicator to reveal differences between density and microstructures of coating prepared with various conditions as well as to follow their evolution after HIPIB irradiation.

Abstract: In order to improve the wear resistance of the surface of thick copperplate, a coating of alloy powder is produced on the surface of the thick copperplate with the method of laser remelting plasma sprayed coating. The value simulation of temperature and the experimental results show that, it is difficult to produce ferronickel coating with large area and crackfree on thick copperplate for laser cladding technique. Using transsonic plasma heat spraying method to fix the alloy powder on the surface of thick copperplate, a sprayed coating can be produced, which has certain adhesion strength to the copperplate. In order to prevent crack produced in large area laser remelting coating, a method with band-like and point-like remilting area at intervals of same distance has been taken. The surface appearance and roughness of the coating produced by plasma spraying are compared with that of the coating produced by plasma spraying and remelted by laser, micro organization analysis and wear resistance comparison experiments are taken to the specimen. The microstructure photos show that the coating of the plasma heat sprayed is mainly made up by grains, with a great deal of pores existing. After laser remelting, microstructure of the coating is more compact, and the most of pores disappeared, so the strength of the boundary between the remelted coating and the copperplate is build up and the coating with large area and crackfree on thick copperplate is performed by the method of laser remelting plasma spraying coating in large amount of point-like remilting areas The experiment result shows that the wear resistance of laser remelting plasma heat sprayed coating enhances 3 times more than That of the unremelt coating, and about 14 times compared with the copperplate, and the wear resistance of the surface with point-like laser remelting area is between that of plasma heat sprayed coating and that of the laser remelting coating, it depends on the ratio of the remelted part to the whole in area, however, the wear resistance increases with the value of the ratio

Abstract: In this paper, boron films were deposited on W6Mo5Cr4V2 high speed steel (HSS) via ion beam sputtering of boron target and implanted with polyenergetic(50 keV, 30 keV, 10 keV) nitrogen ions. The mechanical properties of the implanted layer were tested by Vicker`s hardness and SKODA wear tester. The microstructure of the implanted layer was analysed by X-ray Photoelectron Spectroscopy (XPS) and Fourier Infrared (IR) spectroscopy. Experimental results showed that the HSS substrate was intensified obviously after the implantation of boron and nitrogen ion. When the nitrogen dose equals to 12.8×1017 ions/cm2, the hardness reaches its highest value, HV1982 and the relative wearability was increased for about ten times under the load of 30N. XPS depth concentration profiles show an interfacial mixing between film and substrate, and compared with monoenergetic ion implantation, polyenergetic ion implanted layer have an even N/B distribution. Combined with XPS and IR analysis results, It can conclude that boron exists in the form of boron nitride and first as a-BN or h-BN, with the depth decreasing, it has a tendency to transform to c-BN.

Abstract: A remanufacturing system based on robot MAG surfacing has been developed recently. In this paper, the work principle, functions and composition of this system are introduced. A worn metal part to be remanufactured should be preprocessed firstly, and the defective model of the part gained by reversing engineering technology is compared with normal model of the metal part, then the prototyping path layout is carried out combined with MAG welding process, finally the remanufacturing prototyping is implemented. The remanufacturing system is composed of robotic system (as executing machine), 3D laser scanner (as reversing scanning device), digital pulse MAG welding power source (as prototyping equipment), desk computer (as central control unit) and software modules that support various functions. The functions of the remanufacturing system comprise calibration of system, part reversing measurement, data processing, defective model reconstruction, welding remanufacturing prototyping path layout and etc. It is indicated that the exploitation of the remanufacturing system will provide an effective way for the remanufacturing of metal defective parts.

Abstract: Compressor rotor blade is one of the critical parts of aero-engine. Because of the complex structure and bad work environment, it often went wrong. More over, the trouble of blade can result in serious accident of engine even the aircraft. But the conventional shot peening strengthen method couldn’t take ideal effect, so a new method, laser shock peening on aero-engine compressor rotor blade was studied in this paper. The principle of laser shock peening strengthening was presented first, and the appropriate shock parameters of a certain type blade are fixed on by modeling. The parameters were as follows: laser pulse width is 20ns, output energy is 25J, the spot diameter is 8mm, and peak value is at GW level, and the laser peak value power is bigger than material dynamic yield intensity. Based on these parameters, an experiment was performed by shocking the blade through a repeated Nd: glass LASER supplied by Jiangsu University. Through contrastive performance test, an analysis on the high frequency fatigue life and surface rigidity of the blade fore and after laser shock was performed. The result was concluded that laser shock peening can significantly increase the high frequency fatigue life of aero-engine compressor rotor blade, and enhance the surface rigidity to increase the performance of contuse resistance. At last the reasons were analyzed from two aspects: laser shock can impart compressive residual stress on blade and reduce the size of crystal grain. Surface compressive residual stress can change the blade distribution of load when working; and reduce the size of crystal grain can bring in more crystal interface, for the more interface the crystal has the higher dislocation moving resistance and yield stress is produced.

Abstract: According to analyzing the principle of atmospheric pressure plasma arc (APPA) cleaning metal surface, a model of heat and mass transfer is put forward with using transient-state heat transfer equation about interior heat source and Arrhenius equation of chemical reaction kinetics theory. With finite volume method, the one dimensional control differential equation is transformed into discrete control equation, which is calculated numerically and analyzed with the using of implicit scheme. Taken cleaning lubricant film as an example and analyzed temperature distribution of cleaning film on metal surface, the result indicates that the temperature of film has a strong effect on its removal rate which improves with increasing temperature. In order to both avoid damaging the workpiece surface owing to higher temperature and ensure removal rate of the film, there exists an appropriate temperature under given calculation conditions.

Abstract: The preparation and characterization of a nickel-based composite coating reinforced by TiB2 particles produced in situ and WC particles on stainless steel by laser cladding have been investigated. The experiment results show that the coating obtained at suitable parameters epitaxially grows from the substrate. The coating is mainly composed of γ-Ni dendrite and dispersed TiB2 and WC particles. In the upper part of the coating, the white phase dispersed in the coatings is mainly fine batt-shaped TiB2 particles. In the middle and bottom parts of the coating, the white phase is mainly composed of comparatively coarse equiaxial WC particles, only an small amount of fine TiB2 particles are present. The different distribution of TiB2 and WC should be attributed to the density difference of TiB2, WC and Ni-based alloys and the comparatively slow scanning rate. No cracks were observed throughout the whole coating, especially in the interface, after the coating was deal with by the thermal shock experiment, which shows that the coating processes a good metallurgical bond with the substrate and the high cracking resistance.