Deep pit repairing of nodular cast iron by laser cladding NiCu/Fe-36Ni low-expansion composite alloy

Abstract

It is very easy to form hard and brittle phases (chilled structure and martensite) in the interface zone during the welding and cladding process of nodular cast iron which always decrease the tensile strength of the interface. Shrinkage during the freezing and cooling process of the weld and the clad also bring high residual stress which results in cracks in the interface. However, it is very difficult to eliminate hard and brittle phases in the interface. In this study, a new method was adopted to reduce the residual stress during the laser cladding process of nodular cast iron. That is, cladding alloy with low expansion characteristic was prepared to repair the nodular cast iron, and the shrinkage of the claddings could be reduced, which could reduce the residual stress and cracks. Fe-36Ni (Invar 36) alloy powders which show very low coefficient of expansion and NiCu alloy powders which could restrain the hard phases in the interface were used as the cladding powders to repair the pit of the nodular cast iron by designing composite claddings. The microstructure characteristics were identified by Optical microscope (OM) and Scanning electron microscopy (SEM). Chemical composition was conducted using Energy dispersive microanalysis (EDS). The results showed that no obvious cracks forming in the composite cladding layers and the interface. The average hardness of the cladding layer was about 190 HV and the tensile strength is beyond 460 MPa. The thermal expansion coefficient of the cladding layer is obviously lower than that of the substrate at temperatures below 230 °C and above 870 °C. The cracks were effectively controlled by using NiCu/Fe-36Ni low-expansion composite alloy during the deep pit repair of the nodular cast iron.