Research On The Microstructure And Reactive Behavior Of Ti-BN System By Laser Cladding On Ti-3Al-2V

Poor wear performance of titanium and titanium alloy is the one of the main factorswhich restrict its widely used in some special field. Lase cladding on titanium alloybecome one of the important surface strenthening methods due to its economy, efficientand speedy. In situ synthesized ceramic particle reforced metal matrix composites iswidely used as the claddding layer because of its outstanding wear performance. However,researches on the relationship among microstructure-processing–properties are notsystematic, especially on the in-situ synethesized mechanism. Besides, rare repotres havebeen published on the Ti-BN system by laser cladding. Therefore, there is meaningful tohave a research on the relationship among microstructure-processing–properties, it canbe used to control microstures and properties and have further study on in-situ synthesizedmechanism.The effect of laser power, scanning speed, powder feeding rate, the ratio of BN to Tiand the grain size of BN on phases’ morphology and distribution have studied systemly.The results show below: at the proper cladding paramters, the cladding layers of TiN-TiBcomposite have been in-situ synthesized, the cladding layers have good metallurgicalconnection with substrate without obvious pores and cracks. In situ synthesized TiN wascompared with dendrite/equiaxed grains while and TiB exhibit needle platelet type. Withthe laser power increased, the cladding width, the thickness of clad layer, the penetrationdepth and the depth of HAZ increased, but the dilution rate decreased; the grain of TiNand TiB grew coarse. with the laser power changed as600W�'1000W�'1400W�'1800W,the size of TiB phase alter as follow: undetected TiB�'small whisker microstucture withsize of15μm100nm�'unever needle microstructure with grain size range from15μm100nm to20μm1μm�'uniform needle microstruct with size of20μm1μm. The grain size of synthesized become small with the scanning speed increased. With thescanning speed increased from smm/s to12mm/s, the microstrue of TiB at the top fieldvaried much: uniform needle microstructure with mean grain size of15μm2μm�'agglomerated whisker with grain size range from10μm1μm to10μm500nm�'mixed neele and whisker micrsture. The high scanning speed leads to unevermicrostructure becasued of high cooling speed. Powder feeding rate plays little role on themacrostructure and microstructure of phases. However, the ration of BN to Ti has greateffect on the microstructure and distrubituion of phases. The results shows that TiNnucleat and grow in the grain but TiB at the grain boundary at low BN content,; with BNcontent add the nucleated amount of TiB increased, TiB and TiN distribut uniformly in theclad layer with the ratio of BN to Ti being1:4. The content of B play an important role onthe crystal of TiB. Nucleation and growth of TiB depend on solubility of B satisfy to formTiB. Besides, BN with small size used as rwa material is benfit to synthesize uniformmicrostructure.The solidification of Ti-BN system is different with the equilibrium solidifactionprocess at the BN content of20%by laser cladding. TiB formed in the clad layer withoutTiB2. The solidifation process should be the follow:（Ⅰ）Precipitation of primary phaseTiN will nucleate from the liquid phase by the reaction of L�'TiN and grow into thecoarse cellular or dendritic shape due to the high cooling rate and space to grow in theliquid alloy;（Ⅱ）Formation of the α-Ti by the peritectic reaction L+TiN�'α-Ti at thesurface of TiN. Because of the limited react time and the difficulty of solid diffusion,core-shell structure of TiN-β-Ti formed;（Ⅲ）Primary phase TiB (phase (c) in Fig.3(b))will nucleate on the surface of TiN and grain boundary.（Ⅳ）Formation of the binaryeutectic of β-Ti and TiB(phase (d) in Fig.3(b)) as the temperature gradually reduces by thereaction of L�'TiB+β-Ti.（Ⅴ）Formation of α-Ti. β-Ti will be transformed into α-Ti bythe allotropic transformation reaction β-Ti�'α-Ti.The character and forming mechanism of in-situ synthesized TiB and TiN have beenanalysed systemly. In situ synthesized TiN is fcc structure with lattice constant beinga=0.4245nm. The high isotropic structure and high cooling speed of laser cladding lead to TiN with dendrite/equiaxed grains. Little coarse TiB shows needle platelet with hollowshape, and the hollow is not regular with different wall thickness. TiB is B27structurewith lattice constants beingα=0.628nm, b=0.312nm and c=0.461nm。TiB grow fast in thedirection of [010]and its stacking faults being (100). The eutectic TiB exhibit needleplatelet with small size. TiB can nucleated and grow with the twin csystal. In-situsynthesized TiB have semi-coherent relationship with Ti.Effect of the cladding parameters on the in-situ synthesized mechanism have beeninvestigated systemly.the higher laser power, lower scanning speed, fewer BN content andsmaller BN grain size, the acquired reforced ceramic partiles more uniform and dispersive,the main mechanism control by Dissolving-precipitation mechanism. In contrast, thelower laser power, higher scanning speed, more BN content and coarser BN grain size, theacquired reforced ceramic partiles distribute more unever and “flower” structure observedat the top area of cladding layers. The main mechanism is diffusion mechanism. When usesmall BN as the raw material, Dissolving-precipitation mechanism become the mainreaction mechanism at the laser power density bigger than64.8W·h/m. Otherwise, thediffusion mechanism become the main reaction mechanism at the laser poer densitysmaller than64.8W·h/m.The forming mechanism of special “flower” structure has been discussed.Microstructures of the cladding layer on the Ti-3Al-2V are vared much at different field.“flower” structure tends to formed at the following situations: low laser power, fastscanning speed, high BN content and coarse BN as raw material. The forming processtends to follow the way (1) forming equilibrium phases TiNx and TiBx betweenliquid/solid Ti and solid BN,(2) the heat relased during the reaction of Ti-BN and laserpower energe input make the solid to melt and form liquid,(3) atom B and N from BNdiffuse into the liquid Ti and form TiN at the surface of TiNx ndTiBx, atom B diffused intothe liquid. TiNx ndTiBx works as the nucleation of TiN,(4) TiB from as sigle phase oreutectic phase with Ti when the solubility of B satisfy to form TiB.In-situ synthesized TiB-TiN reinforced Ti based coating exhibited excellect hardnessand wear performance. With the laser power increase, scanning speed desease, BN content add, the distribution of reinforced particles in the cladding layer more uniform, the amoutincrease and the micohardness increased with the highest of1250HV0.5，which is fivetimes more than the substrate; at the same rotating wear time, the wear depth is less halfthan the substrate. The mian mechanism of the sbustrate is severe adhesive wearmechanism. However, the wear mechanism of cladding layer is a combination of adhesiveand abrasive wear characteristics, and the abrasive wear characteristic is the main wearmechanism.