Study On The Effect Of Cu Addition On Microstructure And Properties Of NiTi Based Alloy Fabricated By Laser Cladding

Because of its superior performance,Ni Ti alloy has been widely used in various fields,including automobile industry,civil engineering industry,aerospace,energy and electric power and so on.However,the properties of Ni Ti alloy are greatly affected by the atomic ratio of two main elements,nickel and titanium.For example,the change of the composition of nickel 1% will lead to the martensitic transformation temperature change for more than 100 K.Among most common means,the addition of copper in the alloy can reduce the sensitivity of the Ni Ti alloy to its composition to a certain extent.At the same time,the addition of copper can further enhance the properties of the Ni Ti alloy.Laser cladding is a material preparation method developed rapidly in recent years.It has the characteristics of high efficiency and fast molding.And combining it with Ni Ti alloy has strong practical application prospect.In this paper,laser cladding of Ni Ti alloy and the effect of copper addition on the microstructure and properties of the cladded layer are studied.The TA2 titanium alloy is chosen as the substrate and the mixed powder is prepared by ball milling method.The cladded layer has very nice forming with no obvious defects and excellent properties has been obtained.The microstructure of the cladded layer has been characterized by XRD,SEM,EDS and other techniques.From the fusion line to the surface,the microstructure gradually changes from the coarse grains,to island structure in the middle and the dendrite structure on the top.The EDS results show that the coarse grain structure is Ti2 Ni phase,and the island structure is Ni Ti phase in the Ti2 Ni matrix,the top part of the dendritic structure is Ti2 Ni precipitates in the Ni Ti matrix.The addition of copper elements is enriched at the grain boundary,forming a Ti Ni0.8Cu0.2 ternary phase.Through the DSC,AFM and hardness,friction and wear properties tests of the laser cladded coating,it is found that adding copper significantly improves the hardness of the cladding layer.It also raises the change of the surface roughness before and after the phase transformation.And the friction wear resistance is also improved.The reason of the increased hardness is that the ternary Cu enriched phase at the grain boundaries of the cladded layer brings the grain boundary strengthening effect.On the other hand,copper also changed the phase transformation behavior,so that the phases of the coatings are not exactly the same when the hardness is tested at room temperature,which is also another of the reasons for the hardness rising.The friction wear properties of the coating is also increased due to the addition of copper,which results from the improvement of its hardness.At the same time,the ternary phase formed in the grain boundary will transfer to hard particles during the friction process and changed the wear form of the coating.