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Laser cladding with coaxial powder feeding is one of the new ponent to improve performance of its surface. In the process, processes applied to produce well bonding coating on the com- the clad material is transported by the carrying gas through the coaxial nozzle, generating gas-powder flow. The powder feeding process in the coaxial laser cladding has important influence on the clad qualities. A 3D numerical model was developed to study the powder stream structure of a coaxial feeding nozzle. The predicted powder stream structure was well agreed with the experimental one. The validated model was used to explore the collision behavior of particles in the coaxial nozzle, as well as powder concentration distribution. It was found that the par- ticle diameter and restitution coefficient greatly affect the velocity vector at outlet of nozzle due to the collisions, as well as the powder stream convergence characteristics below the nozzle. The results indicated a practical approach to optimize the powder stream for the coaxial laser cladding.

High-entropy alloy layer up to 150 lm in thickness was formed on H13 substrate with a metallurgical bonding at the coating/substrate interface. Simple solid solution phases were formed in the coating layer with a typical microstructure composed of both dendrite and interdendrite. The microstructure at the top of the cladding zone consists of equiaxed grains while that at the bottom consists of columnar grains. The coating layer exhibits great enhancement in microhardness and wear resistance compared with the H13 substrate.

The approximately equimolar ratio A1CrNiSiTi multi-principal element alloy （MPEA） coatings were fab- ricated by laser cladding on Ti-6Al-4V （Ti64） alloy. Scanning electron microscopy （SEM）, equipped with an energy-dispersive spectroscopy （EDS）, and X-ray diffrac- tion （XRD） were used to characterize the microstructure and composition. Investigations show that the coatings consist of （Ti, Cr）5Si3 and NiA1 phases, formed by in situ reaction. The phase composition is initially explicated according to obtainable binary and ternary phase diagrams, and the formation Gibbs energy of TisSi3, VsSi3 and CrsSi3. Dry sliding reciprocating friction and wear tests of the A1CrNiSiTi coating and Ti64 alloy substrate without coating were evaluated. A surface mapping profiler was used to evaluate the wear volume. The worn surface was characterized by SEM-EDS. The hardness and wear resistance of the A1CrNiSiTi coating are well compared with that of the basal material （Ti64）. The main wear mechanism of the AICrNiSiTi coating is slightly adhesive transfer from GCrl5 counterpart, and a mixed layer com- posed of transferred materials and oxide is formed.

A Ni-based composite coating reinforced by in situ synthesized TiB2 and TiC particles was fabricated on Ti6A14V by laser cladding. An attempt was made to correlate the thermodynamic predictions and experimental observation. The micro- structure and the microhardness profile across the coating were investigated by means of X-ray diffraction （XRD）, scanning electron microscopy （SEM）, energy dispersive spectroscopy （EDS）, and a hardness tester. It is found that the coating mainly consists of a large number of reinforcements （black blocky TiB2, flower-like or equiaxial TiC, and fine acicular CrB） and the 7 matrix. The hardness of TiB2, TiC, and CrB reinforcements is much higher than that of the 7 matrix. The dispersive distribu- tion of such high hardness reinforcements causes the increase in hardness of the whole coating. The average value of the hard- ness is approximately Hv0.2 700 in the coating. The hardness of the coating is obviously higher than that of the substrate due to the dispersion strengthening of reinforcements.

FeCoCrNiA1Bx（x=0, 0.25, 0.50, 0.75） coatings were prepared by laser cladding to study the effects of boron on the structure and properties of high-entropy alloys coatings. The microstructure, microhardness, and wear resistance properties of the samples were investigated by scanning electron microscopy, X-ray diffraction, metallographic micro-hardness test, and friction wear test, respectively, and the mechanism of the wear behavior was also analyzed. The results showed that the high-entropy alloys consisted of BCC phase and eutectic structure, which contained FCC phase and M2B. With boron addition, the content of BCC phase increased while that of eutectic structure decreased. The wear resistance of the high-entropy coatings was considerably improved with increasing addition of horon, and accordingly, the FeCoCrNiAlB0.75 coating showed the best wear resistance.

H13 powder is cladded on steel P20 （base） by continuous COe laser, and the influence of technological pa rameters such as the laser power is analyzed. The 3-D model of synchronous powder feeding is built under Gauss heat source. The simulative results in the heat affected zone are compared with the experimental ones, and the average er rors of width and depth are 15% and 4.5%, respectively. It is found that the simulative results provide basic data for investigating of laser cladding further.

Ti-based composite coatings reinforced by in situ synthesized TiB and TiC were deposited on Ti6AlaV substrates by laser cladding. The effects of Y2O3 on the microstructure and cracking susceptibility of the coatings were investigated in details. It is shown that a small amount of Y2O3 addition can significantly refine the microstructure of the coatings by hastening spheroidization of the primary phase structure. The maximum refinement in microstructure was obtained with the optimum （2 wt%） addition of Y2O3. Moreover, it can increase the volume fraction of TiC and reduce the residual stress of the coatings due to the decrease in lattice distortion of the α（Ti） matrix. All of these factors lead to the reduction in cracking susceptibility of the coatings containing Y2O3 on the premise that the hardness of the coatings is improved. The fracture toughness of the coatings without and with Y2O3 （2 wt%） is 8.32 and 17.36 MPa.m1/2, respectively. Scanning electron microscope examination reveals a transition of the fractured surfaces from cleavage fracture to quasi-cleavage fracture resulting from the Y2O3 addition.

Ni-Ag/TiC composite coating was prepared on the 45 steel substrates by means of laser cladding. Microstructure and wear properties of composite coatings were analyzed using optical microscopy, field emission scanning electron microscopy and wear machine. The experimental results show that defects, such as cracks and pores, do not occur in the laser-cladded Ni-Ag/TiC composite coating and 45 steel substrate, and they present good metallurgical bonding between them. Compared with Ni/TiC composite coating, micro- hardness values of the two coatings do not present evident differences. The wear experiment result shows that Ni-Ag/TiC composite coated with Ag possesses low friction coefficient and good wear resistance compared with Ni/TiC composite coating.

Fe-based alloy layer reinforced by Ti（C,N） particles was produced on the surface of cast steel. X ray dif fraction （XRD） was used for phase identification in the composite coating. The microstructure of laser cladding layer was analyzed by means of optical microscope （OM）, electron probe microscope analyzer （EPMA）, scanning electron microscope （SEM） and transmission electron microscope （TEM）. The results show that Ti（C0.3 N0.7 ） particle is in-troduced by an in situ metallurgical reaction of TiN particle and graphite powder in the process of laser cladding. The shape of lots of Ti（C0.3 N0.7） particle is irregular. The sizes of Ti（C0. 3N0. 7） particles range from 0. 1 to 6.0 μm, and they are dispersed evenly in the matrix, which is fine dendritic or cellular crystal. A new kind of phase named Ti（C0.3 N0. 7 ） parti-cles are tightly bonded with α-Fe microstructure, and there is a clean and smooth phase interface between ceramic re-inforcement phase and the matrix.

To improve the surface properties of magnesium alloys, a study was conducted on Cu-Zr-A1 composite coatings on AZ91HP magnesium alloy by laser cladding. The influence of laser scanning speed on the microstructures and properties of the coatings was discussed. The coatings consist of amorphous phase, CusZr3, and Cul0ZrT. With the increase of laser scanning speed, the amorphous phase content of the coatings increases and reaches 60.56wt% with the laser scanning speed of 2.0 m/min. Because of the influence of laser scanning speed on the amorphous and crystal phases, the coatings show the maximum elastic modulus, hardness, and wear resistance at the laser scanning speed of 1.0 m/min. At the laser scanning speed of 2.0 m/min, the coatings have the best corrosion resistance.