An Investigation On Microstructure And Properties Of Ni-based Alloy By Laser Cladding And Laser Cladding Forming

Laser cladding (LC) is a kind of surface modification technology by laser processing. It can produce high-performance coating on low-cost substrate, which results in the saving of large amount of precious alloys. This technology is suitable for the repair of tools, dies, ma-chine parts and the processing of coating with anti-friction and corrosion-resistant properties and so on. Laser cladding forming (LCF) is a non-mold manufacturing technology and can directly produce dense parts with functional properties or mold. It is further development of LC. LCF can save costs of die, shorten the manufacturing cycle and produce the complex structure parts that traditional technology is unable or difficult to process. LC and LCF has become a research focus of laser processing areas in Western developed countries and China. However, LC and LCF is also faced with many problems to be solved, such as interface is-sues, accuracy problems of metal parts, implementation of computer control of parameters during entire process and the application research, etc. The existence of these problems greatly limits the wide application of this technology.Nickel-based alloy powder with excellent performance of wear resistance, corrosion re-sistance and heat resistance has been widely used as a surface hardening material. This paper selects the Nickel-based alloy powder as cladding powder and aims at some existent prob-lems such as unstable forming quality, process controllability to be improved and interfacial bonding, etc. and firstly discusses the effects of process parameters on macro-size, micro-structure, properties, etc. of laser cladding single-layer. In order to obtain large area and a certain thickness of cladding layer, overlapping-layer cladding and multi-layer cladding may realize. Overlapping-layer cladding and multi-layer cladding are also the basis of the forma-tion of block parts. Therefore, different process parameters for overlapping-layer cladding and multi-layer cladding are used in experiment and this paper systemically studies the clad-ding forming capability, microstructure of bonding interface and the effects of process pa-rameters on microstructure of bonding interface. In order to improve the wear resistance of cladding layer, the hard phase of WC particles are added. The microstructure of Ni/WC thin wall, bonding interface morphology between the hard phase WC particle and the nickel-base alloy and its influence on the properties are discussed.Macro-dimension will affect the forming accuracy and quality. The results show that the varieties of parameters directly affect the forming capability and the shape characteristics of laser cladding single layer. The clad width (W) depends on the size of molten pool. The clad height (H) depends on the numbers of powders entered into molten pool on unit of time and directly affects processing accuracy and efficiency of part. The clad width and height de-crease with the increase of scanning speed and increase with an increase of laser power. The clad width changes little with the variety of powder feeding rate and the clad height in-creases with an increase of powder feeding rate. The depth of fusion (h) decreases with an increase of scanning speed and powder feeding rate, and increases with an increase of laser power. Contact angle (θ) reflects the integrative effects the cladding materials, laser tech-nology and other factors on the shape of cladding layer. It decreases with the increase of the ratio of H/W.With an increase of scanning speed and laser power, the ratio of the height and the width decreases, and increases with an increase of powder feeding rate. The ratio (h/H) helps qualitatively estimating the rate of dilution. With an increase of scanning speed and powder feeding rate, the ratio of the depth of fusion and the clad height (h/H) decreases and increases with an increase of laser power. Effective utilization coefficient of powders (λ) increases with scanning speed and powder feeding rate increase, and increases with laser power increases in definite range. The results provide an experimental basis for the control of process and the improvement of quality of cladding layer.The microstructure will affect the integrative performance of cladding layer. Hence, this paper studies the forming mechanism of microstructure and the rule that the impacts of process parameters on the microstructure and the performance. The investigated results show that the microstructures of transverse cross-section of cladding layer are planar growth, cel-lular crystal, coarse columnar dendrite and shifted fine dendrites from the interface bonding with the substrate to the top, respectively. With the scanning speed increases, the temperature gradient (G) and solidification rate (R) ratio decreases and microstructure of cladding layer is refined. With the powder feeding rate increases, the temperature of substrate and specific energy per unit mass decrease and microstructure is refined. With an increasing laser power, specific energy inputting into the cladding powders increases and microstructure is coars-ened. With the scanning speed and powder feeding rate increase, the hardness of cladding layer increases and with the increase in laser power, the hardness reduces. This is because that the refined cladding layer resulted in the increase of grain boundary area, which resulted in the increase in the ability of deformation resistance. An intuitive and facilitative method which combining the change and the line that joining the turning points of the hardness pro-file to show the effect rules of the laser scanning and the powder feeding rate on the range of dilution and the region of heat affect zone (HAZ) is adopted. The results provide an experi-mental and theoretical basis for the control of process and the improvement of performance of cladding layer.In engineering practice, it is often to obtain large area of cladding layer. Large area of cladding layer can obtain by overlapping laser cladding. The investigated results show that the quality of sigle cladding layer, overlapping rate and the match between cladding powder and laser beam all directly affect the quality of overlapping layer. It should be done to situ monitor and adjust the process for the cladding layer with good quality. The bonding inter-face of overlapping layers will affect the overall performance. Hence, the effects of process parameters on the morphology of bonding interface between the overlapping cladding layers and forming mechanism are discussed. In overlapping cladding, substrate has changed. One part is the prior layer with tilted surface and another is original substrate with plane surface. Overlapping interface does not appear white layer and shows different morphology. One case is continuous epitaxial growth. The other case is discontinuous epitaxial growth. This is re-lated to the depth of remelting region of former shifted dendrite. The results provide an ex-perimental and theoretical basis for the control of bonding interface and the improvement of quality of overlapping cladding layer.The thickness of single layer often does not meet the practical requirements. In order to obtain a certain thickness of the cladding layer, multi-layer overlapping is necessary. From a consideration of the integrity of structure and performance of multi-layer, the bonding inter-face morphology between individual multi-layers and forming causation are discussed. The results show that the bad match between cladding powders and laser enengy affects forming speed and quality of thin wall. In multi-layer overlapping cladding, substrate is the surface of prior layer with bending surface. It is found that the bonding interface morphology between the first layer and second layer is different from the rest. It is a flat bonding interface that interaction is unobvious. This is related to the effects of substrate on the composition and cooling rate of the first layer, which affect the microstructure of the first layer. Most of inter-face of layers are continuous epitaxial growth of the dendrite. The interlayer combination is the epitaxial growth with mutual occlusion and help to the improvement of bonding intensity. Laser remelting can improve the surface quality of thin wall. The results provide an experi-mental and theoretical basis for the improvement of quality and overall performance of part by laser cladding forming.The investigated results of Ni/WC thin wall by multi-layer overlapping cladding show that the height of thin wall appears difference resulted from fluctuation of powder feeding rate. It is necessary to in situ monitor the process of cladding forming and to take compensa-tory approaches in time. The hard phase of WC particles distribut equably in the Nickel-based alloy substrate and can greatly improve the wear resistance of nickel-based al- loys. An ideal gradient interface appearred, which not only improved the wettability and consistency, but also was beneficial to the metallurgical bonding. At the same time, the ideal gradient interface decreased microstructure stress and forming probability of crack, and made the effect of framework well exerted. In laser cladding process, in order to control and design the ideal cladding layer, the size of cladding powder particle should be adjusted based on the relationship between the size of particle and the heating temperature. The results pro-vide an experimental and theoretical basis for the improvement of wear resistance of clad-ding layer.In laser cladding and laser cladding forming, the match between cladding powders and laser enengy has an important effect on the forming quality. Hence, it is necessary to in situ monitor and to adjust the processing for the good match.