Study On Ni-based Tenacious Wear-resistance Gradient Coating In-situ Prepared By Laser On The Surface Of Crystallizer Copper Alloy

As China's steel production continues to increase, the copper alloy consumed in continuous casting process is also much more than before. This is not only a cost increase of iron and still enterprise, but also a waste of precious copper resource. So, it is needed to study on a new surface coating preparing technique to improve the high temperature wear resistance of the continuous casting copper alloy. And using laser surface treatment technique to develop a new kind of strong and tough gradient coating which is well bonded with the substrate, wear resistant and high temperature corrosion resistant on the surface of copper alloy, would lay the foundation for the application of the high temperature wear resistant technique on the continuous casting copper alloy.In this work, by using the laser in-situ preparing method, a Ni-based 4 layer gradient in which strengthening elements distribute gradiently is designed, and successfully prepared by pulsed Nd:YAG solid laser, after optimizing the processing parameters. Then the surface morphology, micro-structure, phase compositing and micro-hardness of coating are characterized systematically. The results show:The surface of gradient coating is complete and smooth, with the unique "ripple" morphology of pulsed laser. There is no crack or pore on the surface and inside structure of the coating, and the interface between the coating and the substrate is metallurgically bond. The micro structure of the coating is alternate distributed plate grain, columnar grain and ultra-fine grain. The ultra-fine grain has a fine-grained strengthening effect to the coating. Rich-chromium composite carbide particles were in-situ generated during the preparation of the coating, and dispersedly distributed in the coating, and has a second-phase particles strengthening effect to the coating.XRD analysis shows that there are Ni,Co,Cr-Ni-Fe-C,Ni4(W,Mo),FeNi and Ni3Al phases generated in the gradient coating. The coating has a multi-phases concomitant structure.The micro-hardness of the gradient coating increase gradually with the direction from substrate to the surface. The micro-hardness of outermost layer reaches 834HV. It is 8.3 times than that of Cu substrate.The wear resistance of the gradient coating was analyzed by friction and wear test. It showed that during the same wear time, the wear extent of the coating was only one eighth of the Cu substrate. And the friction coefficient of between the coating and the steel friction disk was much smaller than that of Cu substrate, only one-half to one third. The test proves that the wear resistance of gradient coating in-situ prepared by laser is very good.The bond strength and the high temperature oxidation resistance of the coating were analyzed by 250℃and 750℃heat shock test. After the test, no crack was observed in the coating and no piece of coating flaked off the substrate, and the coating was still metallurgically bonded with the substrate. A thin composite oxide layer which contained CoCr2O4 phase was generated on the surface of coating. It was compact and well bonded with the Cu substrate, and prevented the coating from continuing being oxidated. The heat shock test proves that the gradient coating in-situ prepared by laser has both good heat shock toughness and high temperature oxidation resistance.