| Aluminum alloys,with high specific strength,high thermal conductivity,good formability and corrosion resistance,have been widely applied in transportation,aerospace and automotive manufacturing industries,etc.However,the inferiors such as low hardness and poor wear resistance limit their further applications especially in a friction environment.Surface modification can improve the surface properties of aluminum alloys while maintaining their bulk advantages,which provides an effective method in manufacturing industry for high-efficiency and energy-saving development.Various surface modification technologies have been proposed to improve the surface properties of aluminum alloys,including thermal spraying,electroplating,anodizing and micro-arc oxidation etc.With the development of high-power laser in recent decades,laser surface modification has attracted more attentions due to rapid solidification,small impact on the substrate and good metallic bonding.In this paper,several alloying material systems were designed for laser surface alloying on 6061 Al alloy,including "pure Fe+pure Al","Fe,based alloy","Fe-based alloy+B4C","Fe-based alloy+B4C+Ti","Fe-based alloy+h-BN" and "Fe-based alloy+h-BN+Ti".By preplacing the alloying materials on the substrate and then laser treating,Fe-Al intermetallic coatings or in-situ synthesized multi-ceramic(carbide,boride,nitride)reinforced Fe-based coatings were fabricated,significantly improving the surface properties of the substrate,Surface morphologies,phase constituents,microstructures and wear properties of various alloyed coatings were discussed comparatively.Moreover,the interfacial structures,nucleation and growth mechanism of in-situ synthesized ceramic phases were further studied.The alloyed coatings prepared with "pure Fe-pure Al" mixed powders were mainly comprised of Fe4Al13,FeAl,Fe3Al and ?-Al.During laser process,Fe-rich melt segregated at the bottom of the molten pool attributed to higher specific gravity,forming some special microstructures characterized by "white bright stripe" at the interfacial zone.The alloyed coating showed obvious delamination phenomena.Also,the un-uniform exothermic reactions along the width direction of the interfacial zone resulted in the formation of a rough saw-toothed interface.The micro-hardness of the alloyed coating was markedly improved with increasing Fe component in the preplaced powders.The alloyed coating with 70 wt.%Fe+30 wt.%Al showed the best wear resistance,with a 90.8%reduction in volume loss compared with that of the Al alloy substrate.When Fe-based alloy was used as the alloying material,alloyed coatings with uniform and dense structure were fabricated under optimized laser parameters.During laser process,Fe-Al compounds precipitated preferentially as dendrites,then un-solidified liquid filled the intervals and transferred into network eutectic at the interdendritic space.The micro-hardness distribution of the alloyed coating was more unmiform with an average value of 459 HV0.2(6.8 times of the substrate).The wear volume loss was reduced to 1 1.33%of the substrate,indicating a significant improvement in wear resistance.The alloyed coatings fabricated with "Fe-based alloy+B4C" mixed powders were mainly comprised of Fe4Al13,FeAl,Fe3Al,Cr2B,CrB,Cr2B3,AlB2,Cr23C6,Cr7C3,Al4C3,and a-Al.With the addition of B4C from 5 wt.%to 20 wt.%.the micro-hardness of the coatings was greatly enhanced,reaching up to 531 HV0.2(8 times of the substrate).However,much B4C particles led to high brittleness and large defects,resulting in severe spallings during wear test.The "Fe-based alloy+10 wt.%B4C" coating exhibited best wear resistance,with a 18.2%wear mass loss compared with the substrate."Fe-based alloy+B4C+Ti" alloying material system possessed better wettability ascribed to suitable Ti addition.Moreover,in-situ synthesized TiB2 and TiC contributed to further improvement in micro-hardness and wear resistance.With Ti content increased from 15 wt.%to 30 wt.%,the micro-hardness and wear resistance significantly enhanced.The average micro-hardness of "Fe-based alloy+10 wt.%B4C+30 wt.%Ti" coating reached a high value of 520 HV0.2.Moreover,a 92.8%reduction in the volume loss enunciated a tremendous enhancement in wear resistance.Nevertheless,with the increase of Ti component to 45 wt.%,microstructure coarsening and adverse Al3Ti phase restricted the further enhancement of wear resistance.In "Fe-based alloy+10 wt.%B4C+30 wt.%Ti" alloyed coating,small TiC particles nucleated heterogeneously on the surface of short rod-like TiB2,forming TiB2/TiC composites.The relationship of[121O]TiB2//[011]TiC and(0001)TiB2//(111)TiC was observed.The TiB2/TiC interface was well bonded with a low disregistry of 1.049%,indicating a highly effective heterogeneous nucleation.The alloyed coatings prepared with "Fe-based alloy+h-BN" mixed powders were mainly composed of Fe4Al13,(Fe,Cr),AlN,Cr2B,FeNx and ?-Al.During laser process,h-BN floated to the surface and caused severe burn.With the addition of 5 wt.%h-BN,the alloyed coating exhibited smooth surface,uniform microstructures and reasonable wear resistance.When h-BN content raised up to 10 wt.%,the micro-hardness and wear resistance of the alloyed coating were remarkably reduced ascribed to high dilution rate.Adding Ti powder into "Fe-based alloy+h-BN" alloying system can significantly improve the wet behaviors and restrain the buring loss of the preplaced powders.Through the reaction between Ti and h-BN,fine TiN,TiB2 and TiB particles are in-situ generated,contributing to excellent wear preperties.During solidification,small TiN particles precipitated prefertially,then AlN nucleated heterogeneously on the surface of TiN in a metastable cubic structure,resulting in the formation of core-rim TiN/AIN composites.A small disregistry of 9%revealed a moderately effective heterogeneous nucleation."Fe-based alloy+5 wt.%BN+15 wt.%Ti" coating showed superior wear resistance at room temperature,200? and 400?,with a substantial 91.8%,92.6%and 89.9%reduction in wear volume loss compared with the substrate. |
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