Research On The Effect Of The Pre-placed Layer Thickness On Microstructure And Properties Of Laser-clad Coating

Titanium alloys are widely used as structural components in aerospace, chemical,petrochemical and marine industries due to their useful characteristics such as low density,high specific strength, exceptional corrosion resistance and high temperature mechanical properties. However, the application range of titanium alloys is limited because of their low hardness and poor tribological properties when they come in contact with other surfaces of same or different materials in the applications of sliding wear. Among the employed methods at present, laser surface techniques have drawn great attention in recent years because of their unique features and capabilities in various applications. As all known that the pre-placed layer thickness has a significant effect on the quality of the laser-clad coating. It determines directly the final thickness of the coating, which affects its service life when applied as an engineering tribological component. Moreover, the pre-placed layer thickness can affect the chemical compositions of the molten pool because of the change in dilution rate. As a result, microstructure and mechanical properties of the coating exhibit the corresponding changes. However, few detailed investigations into the relationship among the pre-placed layer thickness, microstructural evolution and mechanical properties of the coating have been performed because of the uncontrollability in pre-placed layer thickness.In this investigation, first, a novel method was adopted to precisely control the thickness of a pre-placed layer. Ni-based composite coatings with different pre-placed layer thicknesses（0.5, 0.8, 1.0, 1.2, 1.5 and 1.8 mm） were produced with the NiCrBSi alloy powder by laser cladding. The effects of the pre-placed layer thickness on the quality of the coatings（surface morphologies and inner defects） were investigated. The critical value of maximum allowable pre-placed layer thickness was obtained. The result also shows that the addition of NiCrBSi improved the wear-resistance of the coatings. However it cannot play a role of anti-friction in the coatings. The addition content of the sulfides is avery essential factor affecting the anti-friction role due to their easy decomposition characteristic. Then we carried out the study of the effect of the content of MoS₂ on the microstructural evolution and mechanical properties of the laser-clad coatings.Microstructure of the coatings was examined using X-ray diffractometry（XRD）, scanning electron microscopy（SEM） and energy dispersive spectrometry（EDS）. In addition,microhardness and wear behaviors of the coatings were also investigated by a microhardness tester and an ultra-functional wear testing machine.The experiment about laser cladding 6 groups of different pre-placed layer thickness NiCrBSi composite coatings indicated that the critical value of maximum allowable pre-placed layer thickness was 1.0 mm by the analysis of the surface characteristics and inner defects of the coating. For the coatings with 0.5, 0.8 and 1.0 mm in pre-placed layer thickness, the dilution rate, microstructural evolution, microhardness, fracture toughness and wear behavior were measured and analyzed. The dilution rates of the coatings were reduced with increasing pre-placed layer thickness（73.5 %, 54.8 % and 46.0 % for thickness values of 0.5, 0.8 and 1.0 mm, respectively）. The corresponding phase constituents of the matrix in the three coatings evolved as follows: a（Ti）+Ti2Ni,TiNi+Ti2Ni and Ni3Ti+g（Ni）. The main reinforcements in the coatings were transformed from TiC+TiB to TiC+TiB₂, finally to TiC+TiB₂+Cr7C3+CrB. The average microhardness values of the coatings exhibited an increasing trend（817.7, 837.1 and 1078.3 HV0.2） with increasing pre-placed layer thickness. Similarly, the average fracture toughness values were also gradually increased（3.019, 3.526 and 5.055 MPa·m1/2）. The average friction coefficient of the coating with 1.0 mm pre-placed layer thickness was comparatively lower（0.568） and more stable with the change in sliding time compared with two other coatings.This coating also possessed the lowest wear volume（0.2732 mm3）. Wear mechanism of this coating was micro-cutting in a particular area but coupled with the formation and destruction of the transfer layer from the counterpart ZrO2. However, wear mechanism of two other coatings was micro-cutting. The coating with 1.0 mm pre-placed layer thickness possessed more excellent wear resistance because of its higher microhardness（resistance to micro-cutting）, fracture toughness（resistance to brittle debonding） and the protection of the transfer layer.The experiment about laser cladding 5 groups of MoS₂+NiCrBSi composite coatings show that no MoS₂ was found in the coatings. MoS₂ was decomposed and gasificated during the laser cladding. The coatings were all composed of TiNi-Ti2 Ni as the matrix andTiB₂-TiC as the reinforcements. However, a new phase（TiS₂） was in situ synthesized in the coatings when the addition content of MoS₂ exceeded 6 wt.%. With the increase in content of MoS₂ from 8 wt.% to 10 wt.%, the volume fraction of TiS₂ also presented the increasing trend. When more than 6 wt.% MoS₂ was added, the change in friction coefficient of the coatings with the sliding time was more stable and its average values was also lower in the stable wear stage. The wear volume of the coatings with less than 6 wt.% MoS₂ was similar（about 0.6104, 0.6408 and 0.5889 mm3）. The wear volume of the coatings with 8wt.% and 10 wt.% were significantly reduced to 0.3624 mm3 and 0.2686 mm3,respectively. The formation of TiS₂ can significantly improve wear resistance of the coatings due to its significant anti-friction and protection roles.