Microstructure And Properties Of Nickel-based Superalloy K438 Laser Cladding Repairing

Based on the problem that the defects in K438 nickel-based superalloy are liable to occur during foundry or using processing which need to repair, the repairing process, of K438 nickel-based superalloy was studied by using the method of laser cladding and using the Ni60 powder as the filler material. The repairing process of laser cladding for single layer, overlaping layer and multilayer was discussed respectively. And the macro morphology, microstructure and mechanical properties of the cladding layer were analyzed. The solidification characteristics of the laser cladding repairing microstructure were observed by using the optical microscope(OM), scanning electron microscope(SEM), X-ray diffraction(XRD) and energy dispersive spectrometer(EDS) with different process parameters. The mechanical properties such as micro area residual stress and mechanical properties and wear resistance were tested, and the wear mechanismwas discussed.The results of process experiment show that the depth, width and contact angle of the cladding layer are increased and the height of the cladding layer is decreased with the increase of the laser power or the decrease of scanning speed and powder feeding speed.The powder feeding speed has little impact on the depth and width. The surface of the cladding layer will be lumpy if the laser scanning interval is too large or too small.The main cause of the crack in the cladding layer is attributed to the high heat input,which may caused by high laser power, scanning speed, low powder feeding speed or small scanning interval.Scanning path has little impact on the quality of cladding layer.The best cladding repairing parameters are:laser power P = 1200 w, scanning speed v =5 mm/s, powder feeding speed VF = 0.7 g/s, scanning interval DS = 1.8 mm, with which can obtain good forming repair structure without defects.The analysis of mocrostructure shows that the temperature gradient G and the solidification rate of the molten pool R have a great influence to the crastalline morphology. From the bottom to the surface of the cladding layer, the morphplogy of microstructure turns by the columnar crystal into isometric crystal with the decrease of the G/R.The heat input is a main factor of the crack of K438 superalloy. The crack in thealloy shows to be the liquation type, and the main cause is attribute to the liquation of phases such as MC carbides, γ-γ’ eutectic, and γ’precipitates in the grain boundary.The study of mechanical properties shows that the hardness of the cladding layer improved a lot campared with the base material. The microhardness of the cladding layer can improve with the decrease of the heat input. The residual stress is higher at the overlapping part which can up to about 500 Mpa, while the residual stress is lower at the unoverlapping part, only about 50~200MPa.The wear of K438 nickel-based superalloy mainly caused by abrasive wear, adhesive wear and oxidation wear these three kinds of wear mode, while the wear of the cladding layer mainly caused by adhesive wear.The wear resistance of the cladding layer has high correlation with the average microhardness. And wear resistance of the cladding layer improved a lot compared with K438 base material. The wear rate can be increased caused by the increase ofd the average friction coefficient, with the increase of the scanning interval.