Synthesis And Characterization Of Ni-Based Amorphous Composite Coating Using Diode Laser Processing

Laser cladding technology is a novel technology developed in1970s dueto the development of high power laser equipment. It is a multidisciplinarytechnology with laser heating and melting, mass exchange in the melted pooland rapid solidification. Laser melting or laser remelting is a technique that ahigh power density laser beam scans on surface of metals or alloys, leads tothe melting of surface metals, and solidifies followed. The cooling rate isusually at102～106K/s, even to1012K/s, so fine and homogeneous structurescould be obtained at the surface. It is possible to form composite amorphouslayer in the surface crystalline materials when using laser cladding andremelting process. The laser cladding process can give low dilution claddedcoating to guarantee the composition; laser remelting process can lead to highsolidification rate.In this paper, an amorphous composite coating was fabricated by usinghigh power diode laser cladding and remelting process on mild steel surface.Design and preparation of Ni-Fe-B-Si-Nb amorphous alloys systems wascarried out. The microstructure and phase were observed and analyzed. Themicrohardness and wear resistance properties of the coating were also tested.Some of main experimental results and conclusions are listed as follows.For Ni-Fe-B-Si-Nb alloy systems, based on the three empirical rules anddeep eutectic rule for high glass-foming ability in metallic glasses, the mixingenthalpy and mismatch entropy were calculated, the alloys phase diagrams were analyzed. A preliminary composition of the alloy is obtained.Furthermore, considering the characteristics of laser processing andexperimental studies, the final composition of (Ni_0.6Fe_0.4)₆₈B₁₈Si₁₀Nb₄（at.%）was determined. The biggest amorphous volume fraction could be obtained inthe laser cladding and remelted coating.The influence of laser cladding and remelting processing parameters onamorphous volume fractions of the remelted coating was studiedsystematically. It can be seen that dilution ratio was affected by laser powerinput. With the increasing of laser power, the penetration and dilution willincrease. A high amorphous volume fraction could be obtained at low dilutioncoating where the nominal composition of the powder was kept. While forhigh dilution coating, the amorphous volume fraction was relatively low,even no amorphous phase was formed in the remelted coating. When thenominal composition was guaranteed, the structure and composition weremore homogeneous at higher laser power input. The amorphous volumefraction was also at higher standards in the remelted coating. The amorphousphase was not formed when the laser scanning speed is4m/min and5m/minduring the laser remelting process. For6m/min or more, the amorphous phasewas observed, and the amorphous volume fraction increased with theincreasing of laser scanning speed. The amorphous volume fraction was64%for6m/min.Structure of amorphous phase matrix and NbC particles were observedin the remelted coating when the laser cladding power was800W, scanningspeed was0.36m/min, laser remlting power was3500W and remeltingscanning speed was8m/min. The fine equiaxed grains, nano-grains anddendrites/amorphous structures could also be obtained in the remeltedcoating for low amorphous volume fraction remelted coatings. The thermalcycle of the coating during laser remelting process was calculated using FEM method. Combined with the thermodynamics and kinetic aspects, theformation characteristics and mechanisms of amorphous phase for lasercladding and remlting process were studied. The results showed that thecritical cooling rate was10118.8K/s for (Ni_0.6Fe_0.4)₆₈B₁₈Si₁₀Nb₄alloyfabricated by laser cladding and remelting process which is higher than thatof BMGs. In thermodynamics aspect, the semi-melted zone atremlted/cladded interface and NbC particles in remelted zone are both thesites of heterogeneous nucleation. The barrier energy needed forheterogeneous nucleation is reduced. So much more cooling speed is neededto suppressing crystallization within the supercooled liquid. In kinetics aspect,the liquid flow and solute transport behavior in remelted pool can not beneglected, which accelerated the element diffusion. Therefore, the Surfacetention gradient and holding time at high termperature should be decreasedby increasing the scanning speed （cooling rate） during the laser remeltingprocess.The microhardness and wear resistance properties of the coating weretested systematically. The graded microhardness distribution was exhibited inthe coating, the microhardness increased from the interface of clad/substratesurface to the top of the remelted coating. The microhardness was higher forcoatings with bigger amorphous volume fraction. The mean microhardnessand elastic modulus are1227.9HV and277.4GPa, respectively which arehigher than that of the BMGs with same compositions. The strengtheningmechanism of this phenomenon was also studied. Wear tests results showedthat the wear losses increased with increasing of the wear rotation speed, loadand time. Friction coefficient decreased with the increase of load. The ratio ofH3/E*2can represent the changing rule of wear resistance properties of thecladded coating, remelted coating and BMGs. Wear resistance propertiesincreases with the increasing of H3/E*2ratio. In addition, the increase of amorphous volume fraction also leads to the increase of wear resistanceproperties of the remelted coating.