The Properties And Microstructures Of Carbide Reinforced Metal Matrix Composites Fabricated By Lasers

Laser cladding of Metal Matrix Composites (MMC) or Metal Ceramics (MC) layer is an effective method to remanufacture wear resistant structural parts. The aim of the present work is to in situ laser clad nickel-based MMC coatings and to invent new powders and technology for surface damage repairing of wear parts. Then, MMC components are to be fabricated by laser forming with a new type double hoper feeder. Cr3C2-NiCr composites and metal ceramics were firstly in situ laser clad with Ni-Cr-graphite element powders mixture. Then, the technology of laser cladding, powders composition (adding Cr3C2 and rare earth) as well as mechanical alloying (MA) of raw powders were researched. Also, the microstructure, composition, microhardness, wear and corrosion resistance of laser clad coatings were investigated with optical microscope, scanning electron microscopy, X-ray diffraction and transmission electron microscopy, microhardness tester, wear tester and electrochemical test system. Finally, the mechanisms of in situ laser cladding of element powders were discussed and the following the results were obtained:At first, crack-free Cr3C2-NiCr composites scattered with graphite was in situ laser clad with Ni70%-(Cr-graphite)30% element powders mixture, and the results revealed that microstructure and carbide reinforced composition of the composites were determined by atomic ratio of Cr/C in the raw powders mixture. There was only one type of carbide, that was Cr3C2, formed in the coating when Cr/C ratio<3/2, and content of carbide in the coating increased with the Cr/C decreased. Graphite appeared and scattered in the coating when C content in raw powders was more than 2.7mass%. Furthermore, microhardness and wear resistance of the coatings were enhanced with Cr/C ratio declined, in detail, average microhardness of the best was 55OHV0.3 Galvanic corrosion were formed between graphite/carbide and matix, so the corrosion resistance of the coatings in 0.2M/L H2SO4 solution were reduced with Cr/C ratio increased, and the corrosion potential and current density of the best were -67mV,1.23×10-7A.cm-2 respectively. The resolve process of graphite in laser melt was studied and it was found that carbide type and content determined by the graphite dissolve level in laser melts and then influenced performances of the composite.On the basis of the previous research, Cr3C2-NiCr coatings were in situ laser clad with the element powders mixture of Ni25%-Cr65%-graphite10%(Cr/C=3/2), afterwards, the processing parameters of in situ laser cladding was studied to improve the technique. The results showed that the laser clad shape characteristics were determined by the input energy volume density of laser which was relevant to the cooling rate of laser melts (9.3×104?1.2×l06?/s). The phase composition of metal ceramics consisted of NiCr matrix and carbide (Cr3C2 and Cr17Ca) which was depend on ?. With the decrease of ?, the content of carbides increased and Cr3C2 content grew, but Cr7C3 content reduced, furthermore, the microhardness, wear and corrosion performances were affected accordingly. Wear resistances of the coatings were enhanced when the microhardness improved with ? decline, in detail, the microhardness of the best processed coating was 1230HVo.3, and wear type of the coatings was abrasive wear. The corrosion resistances of Cr3C2-NiCr coatings were better than the laser clad coating of nickel based alloy in 3.5% NaCl aqueous solution, and the best corrosion potential and corrosion current density were -271mV and 1.8x 10-8 A.cm-2 respectively. The processing parameters were normalized to be volume density of laser input energy with the help of solidification investigation. It is concluded that the shape, microstructure and the performance of laser clad coating is determined by the composition of laser melt and the input energy volume density of laser.With the guide of laser cladding processing study, O3C2 particles and rare earth were added into the powders mixture of metal elements, as well as mechanical alloying were utilized to optimize microstructures and performances of the coatings made by in situ laser cladding. The study of laser cladding of the powders of Ni25%-Cr65%-graphite10%with different addition of Cr3C2 particles showed that Cr3C2 particles can promote the in situ nucleation of Cr3C2 in the coatings, refine Cr3C2 grain size and enhance the thickness of the laser clad coating. In detail, the microhardness of coating clad with 25% content of Cr3C2 powders were enhanced to 1051HV0.1 from 710HV0.1. The effects of Y2O3 on the coating made by laser cladding of Ni25%-Cr65%-graphite10% powders are:refining microstructures, minimization of cracks and enhancement of microhardness. The content of Y2O3 was suggested to be 1%. At the same time, Ni50%-Cr43.33%-graphite6.67% powders mixture was mechanical alloyed for different time to become NiCr saturated solid solution, from which the coatings were laser clad. The effects of MA on the coating made by laser cladding are:refining microstructures, increasing microhardness and improving corrosion and wear resistance, although the enhancements are all small. So it is obvious that the effect of MA on laser cad coating is positive.Finally,316 stainless steel (SS) single panels, H13 steel square tubes and H13 hollow blades were laser formed with blown powders. The research results revealed that thickness of single panel grew and grain size became coarser with laser power increase, meanwhile, the surface roughness was not affected by laser power, but controlled by the layer height. Own to the difference of heating process, the shapes and microstructures of the components made by pulsed and continuous CO2 laser are not the same. And the minimum surface roughness of laser fabricated component can be controlled to be 21.6?m.After the study of blown powders laser form fabrication, double hopers feeder was developed to laser fabricate MMC parts. WC-316SS composite components were successfully laser fabricated, in which WC was uniformly distributed and the total volume content was over 56vol%. At the same time, cenosphere and 316SS powders were fed by the double feeders to make a cenosphere-316SS composite, in which the broken cenosphere was also well distributed. So it can be concluded that the double feeder system can uniformly mix two powderss with big gap of density differences, and deliver them to laser head for laser forming.