Mechanism Reaserch On Spray Deposition Coating Of High Temperature Melt Based On Thermit Reaction

Surface cladding coating technology is one of the main technologies for improving the corrosion and wear resistance of components.With the increasing requirements for corrosion and wear resistance of large structural components such as large moulds,heavy-load dump truck bodies,ship decks,and pressure vessels,while laser cladding,electron beam coating,thermal spraying and other commonly used surface depositions technique have limitations in preparing large area coating.Therefore,developing a new type of efficient large and thick alloy cladding coating will be promising that a large area coating is achieved in a single pass,thichness is controllable over a wide range,and bonding with matrix is firmed.Future of the technology is bright.In this paper,the process of metallothermic reduction of alloys was introduced into the preparation of spray-deposited coatings,and an aluminum-thermal reaction/spray-deposited alloy and its in-situ oxide-enhanced cladding coating technology were developed.High carbon steel and high carbon alloy steel were prepared using this technique.The phase structure and microstructure of the coating were investigated by optical microscopy(OM),scanning electron microscopy(SEM)with energy dispersive spectroscopy(EDS),and X-ray diffractometer(XRD);the strength of the coating was tested by a tensile machine;the hardness of the coating was tested using a micro-Vickers hardness tester.SFT-2M ball-disk friction wear tester was adopted to test the dry wear resistance of the coating.CHI660C electrochemical workstation was used to test the corrosion resistance of the coating in a 3.5%solution of NaCI in order to investigate the effect of spray deposition process parameters(spray distance,atomization pressure,and substrate preheat temperature)on the microstructure and properties of substrate and the high carbon steel coating.Meanwhile,effects Cr and Ni on microstructure and properties of high carbon alloy steel were investigated.Research shows:It is feasible to prepare a large-area thick coating by using the spray-coating technique combined with aluminum thermal reaction and spray deposition.The coating has a good metallurgical bonding with the substrate.The coating is a typical rapid solidification structure,with the grain size of about 5-20?m,mainly fine interlayer spacing pearlite(sorbite)structure.The coating hardness is 410HV9.8.The bonding strength between the coating and the substrate is about 300-400MPa,and the bonding mechanism includes cleavage fracture,quasi-cleavage fracture and plastic fracture.Through several experiments,the best process parameters were obtained:spraying distance:150mm,atomizing pressure:0.3-0.4MPa,Preheating temperature of the substrate:650-700?,flow diameter:5mm,atomizing angle:5°.The preparation of needed thermionic agent for high-carbon alloy steel coating is guided by calculation.When the total amount of metal added exceeds 10%,it is necessary to add a small amount of additional reaction auxiliaries to ensure that the reaction process has a high thermal effect,so that the aluminum heat agent can be vigorously and spontaneously performed,ensuring good separation between superalloy melt and residue after reaction.After adding Cr and Ni,the microstructure of the coating changes from pearlite to upper bainite.With the increase of Cr and Ni,the microstructure of the coating gradually transforms into austenite.At the same time,a small amount of micron and submicron Al2O3 particles were found in the coating.The hardness of the coating by adding Cr element and Ni element reached 413HV(5%Cr-5%Ni),450HV(10%Cr-10%Ni)and 475HV(10%Cr-20%Ni)respectively.The corrosion current in 3.5%NaCl solution reached 2.926×10-6 A/cm2(5%Cr-5%Ni),1.425×10.6 A/cm2(10%Cr-10%Ni)and 1.377×10-6 A/cm2(10%Cr-20%Ni)respectively.The wear rates were 1.87×10-7 mm/mN(5%Cr+5%Ni),1.74×10-7mm3/mN(10%Cr+10%Ni),and 1.86×10-7mm3/mN(10%Cr+20%Ni).Compared with the substrate and the high carbon steel coating,all these parameters have been greatly improved,indicating that spray cladding high carbon alloy steel coating has a good protection effect on the components.