Study On Plasma Spray Welding In Repaiirng The Surface Defects Of Die

With the rapid development of modern science and technology, die is becoming a corepart of the manufacturing industry, and has been widely applied to daily life and industrialproductions. Products manufactured by the die are characterized by high accuracy, highconsistency, high productivity and low cost. Die is working in an extremely harshenvironment, which is usually under high temperature and high pressure, and the die willsubject to friction, extrusion, and the alternating stress during the working process, therefore,there will be several defects appearing in the surface of the die, which thus, may cause thefailure of the die and bring about a great deal of waste in the die production process. How toprolong the service life of the die, substantially to reduce the cost and to increase theefficiency of enterprises, has become the primary task of the die manufacturing industry. Inrecent years, developing rapidly, surface engineering technology has become the basis of theadvanced new materials, photonics, microelectronics and aerospace industries, and has beenwidely used in repairing surface defects of die, such as brush plating, vapor deposition,thermal spray, and thermal spray welding. H13steel （parallel to domestic4Cr5MoSiV1）, ahot work die steel, is widely and mainly used in making die-casting die, hot extrusiondie-casting die and hot forging dies and other dies. Because of its special work environment,the main failures are the thermal fatigue failure and wear failure. If we can choose theappropriate surface engineering technologies to restore the die’s function and extend itsservice life, such as the plasma spray welding in repairing the surface defects, economicsignificance and social benefits will be greater.The title of the paper is study on plasma spray welding in repairing the surface defectsof die, which is focusing on the plasma spray welding in repairing the surface defects of thehot work die in order to extend the life of the die. After the die being repaired by the plasmaspray welding, there will be an alloy layer on the surface of the matrix. The alloy layers arecomposed of NiCrBSi, WC-Co12and NiCr-Cr3C2, and the combination mode ismetallurgical bonding. Optical microscopy, scanning electron microscopy and XRD tests are used to analyze the microstructures and chemical compositions of the alloy layer, and thetests of microhardness, wear resistance and tensile property are used to measure the qualityof the layer.（1） The combination mode between the alloy layer and the die surface is metallurgicalbonding, and there is a fusion zone at the interface, which is about20³0μm wide.Appearing in the fusion zone, some cellular dendrites are growing along the direction ofperpendicular to the interface. Element diffusions have token place between the alloy layerand the matrix;（2） The microstructures of the alloy layer are martensite and retained austenite. WithoutWC-Co12, because of the presence of Ni, Cr, B, Si and other elements, Cr23C6, FeNi3, FeCrand other compounds will be produced in the alloy layer and disperse at the grain boundaries.With WC-Co12, there will be a lot of hard phases, such as WC, W3C, in the alloy layer. Thebigger the proportions of WC-Co12and NiCr-Cr₃C₂are, the greater the contents of WC andW3C are;（3） Without WC-Co12, the bigger the proportions of NiCr-Cr₃C₂is, the greater thecontents of CrB and other hard phases are, and the microhardness and abrasive wearresistance of the layer will be enhanced. With WC-Co12, the bigger the proportions ofWC-Co12and NiCr-Cr₃C₂are, the greater the contents of WC, W3C and other hard phasesare, and the microhardness and abrasive wear resistance of the layer will be enhanced; Theaverage tensile strength of the specimen after being repaired by plasma spray welding withNiCrBSi powder is450MPa;（4）Selecting the welding current, plasma gas flow, transfer gas flow as three-factorsorthogonal optimization of plasma spray parameters is made to obtain optimal parameters ofthe plasma spray welding. The optimal parameters are given as below: the welding current is50A, the plasma gas flow is1.5L/min, transfer gas flow is2.5L/min, welding voltage is40V,the protective gas flow is15L/min, and Welding distance is12mm;（5）The alloy layer with NiCrBSi70%+WC15%+NiCr-Cr₃C₂15%has the highestmicrohardness and the abrasive wear resistance.