Study On Influences Of WC On Microstructures,Mechanical Properties And Tribolongical Characteristics Of Laser Cladding Layers From Co-Based Alloy

Since it was carried out primarily in 1970s,laser cladding technique has been developed quickly because of its high speeds of heating and cooling,lower hot effect zone and distortion,lower dilution,fine and dense microstructure of the layer,adaptability for materials and controllability for operation.Recently,this technique has been widely applied to the surface strength of special parts and repairing and remanufacturing of key position.In this dissertation,the main researches are focused upon the cracking sensibility in the coatings layers and key technologic problems for property improvement of the laser cladding layers from Co-based alloy.The main results and developments are listed bellow:1.The laser cladding process,solidification structure,compositions and microhardness of the laser cladding layer from the regular Co-based alloy and the special Co-based alloy were studied and developed systematically;2.The residual stresses in the cladding layer were measured,and its effect upon the cracking sensibility of the layer was evaluated;3.The effects of WC particles on the properties of laser cladding layers from Co-based alloy were studied;4.The tribological characteristics under normal and high temperatures were investigated emphatically.In the area of laser cladding technique,up to now,there are only few work was carried out on the research for special alloy for laser cladding and the tribological characteristics of the laser cladding coating with Co/WC complex.Therefore,the present work focuses on this area for the first time,especially the tribological properties of under high temperature.In the present work,the laser cladding samples were prepared according to the demand for strength and tribological characteristics of wear-resistance.And then,the solidification microstructures and chemical compositions of the cladding layers were examined by using high resolution scanning electron microscope(HRSEM),electron probe X-ray microanalysis(EDAX)and energy dispersive spectrometer(EDS)etc.Comparing to the plasma transferred arc(PTA)layer,the present laser cladding layer exhibits a fine,dense and more uniform microstructures,and higher microhardness.These coatings quality improvement is considered to be the results of high energy density of laser beam and lower dilution during cladding.The results showed that laser cladding crack-free complex coatings from Co/WC having were obtained with the optimum laser cladding parameters.YWhen the WC particles were added,the basic solidification microstructure of the laser cladding layers was not altered,but the WC particles affected the solidified pattern of eutectic structure in the dendrite,that is,the amount of eutectic was increased gradually with the increase of WC particles,at the same time,the other complicated faceted dendrites were precipitated and dissociated carbon exists in hypoeutectic.When it was added into the Co-based alloy,the WC particles acted on different matter nucleus so the size of the solidification microstructure was decreased and the grain was became smaller with the amount of WC particle increasing.The better part of the tungsten was solutioned in hypoeutectic and eutectic as carbide which acted as a strengthened element.When the amount of tungsten was large enough,then,it was precipitated in the formation of carbide and distributed in the substrate of solidification microstructure in various dendrites with flowery and star shapes.Some bigger un-melted WC particles existed independently and were distributed on the positions with higher free energy as grain boundary etc,because it was not melted and become not the nucleus for solidified.The hardness of laser cladding complex coatings was improved with the increase of the content of WC particles,therefore,the wear-resistance of the coatings could be improved.The values and directions of the residual stresses in the cladding layers not only affected the properties but also the tendency of cracking sensibility.The results showed that there is a critical thickness of laser cladding layer for producing cracking,when various processes were carried out.The distribution of residual stresses was studied by delaminated X-ray diffractometer and the thermal expansion coefficient was measured by thermal expansion meter for the laser cladding layers.In a word,the cracking tendency was larger in the laser cladding layer of added WC particles than without WC particles.However,the curve of thermal expansion coefficient could be changed by adjusting the amount of WC particle,so that producing and expanding of the cracking in high and low temperature could be controlled.In this paper,tribological characteristics were studied systematically for the laser cladding layers from Co-based alloy and laser cladding complex coatings of Co/WC using a special pin-disc friction and wear machine under the laser cladding layer-silicon nitride pair in the air with different temperature.The morphologies of the worn surfaces were observed with high resolution scanning electron microscope(HRSEM)and three-dimension outline meter.The results showed that the friction factors and wear values for the laser cladding layers were lower than that of the plasma transferred arc welding coating.The friction factors and wear values were decreased with the increase of WC particles content,furthermore the reducing of wear values was very remarkable.The friction factors and wear values were decreased differently with the experiment temperatures raised.The results showed that the wear mechanism was affected remarkably by the experimental temperature:the wear mechanism performed as abrasive wear at normal temperature and the wear mechanism performed as adhesive wear and oxidation wear at high temperature.