General rules of metal powder flame spraying (welding) process

Based on the author's more than 20 years' experience in thermal spraying, this paper plays an important role in the selection of materials, coatings, coating quality control and key points of technology 1、 Process selection 1 Understand the performance of the workpiece itself: such as material, composition, surface condition, worn part, worn area, depth, etc 2 Understand the working conditions of the workpiece: working environment (working temperature, main medium concentration and other components), stress conditions (impact load or sliding fit, static fit), failure reasons (processing over tolerance, wear, corrosion, etc.) 3 The spraying process should be adopted in the following cases: 1) for the workpiece whose surface is required to be wear-resistant under static load or low-speed sliding load, but the tensile bond strength between the coating and the substrate is not more than 50N / mm2, the hardness of the spraying layer hv130-350, the thickness of the coating from 0.3 to several millimeters, the surface roughness of the spraying state ra10-25 μ m, and the surface roughness of the coating after grinding can reach ra0.8-0.4 μ M , such as bearing parts of various shafts 2) It is required to be resistant to cavitation and water erosion, and the surface roughness after machining shall not exceed Ra0.4 μ m, such as imported automobile crankshaft, train crankshaft, marine crankshaft, etc 3) Workpieces made of copper alloy aluminum or aluminum alloy and other non-ferrous metals that need to restore size or improve surface wear resistance 4) Copper base powder or aluminum powder is the best choice for spraying on non-ferrous metal workpieces such as copper alloy, aluminum or aluminum alloy 5) If the coating is required to be dense and void free in the corrosive medium, the appropriate sealing agent shall be used to seal the coating after spraying the plastic, ceramic or corrosion-resistant metal that can meet the use conditions according to the working temperature and medium concentration and composition of the workpiece 6) Bonding strength of spraying layer and substrate: the tensile bonding strength of traditional flame spraying layer is generally 20-30n / mm 2 The latest zk8032 powder multi-function high-speed flame spraying gun developed by Shaanxi Zhongke Surface Engineering Co., Ltd is used to spray aluminum clad nickel composite coating on the sandblasted surface of nodular cast iron The shear bonding strength of the coating can reach 63.7n/mm2; Although the bonding strength of spray coating is not as high as that of spray welding coating, it is enough to meet the needs of repairing the parts with friction and failure resistance between lubricated metals According to the calculation of the scientific research institute, when the Jiefang CA141 automobile is in full load operation, the shear stress of seven main journals of crankshaft is only 15N / mm 2 on one main journal; For Dongfanghong 75 tractor, it is only 5.5n/mm2, which is much lower than the shear strength of spray coating 7) Wear resistance of spray coating: for example, the main journal sprayed by automobile crankshaft has higher wear resistance than the new crankshaft Once our company sprayed the main journal repaired by Japanese Hino automobile crankshaft, the wear amount was 0.022-0.03mm after inspection, while the wear amount of the main journal not repaired by spraying was 0.2 - The wear amount of 0.25 mm is almost ten times the same From this example, it can be seen that the spray coating has good wear resistance, and it can run for several hours even when the engine is short of oil without burning the bearing and axle 8) Process difference between spraying and spray welding: ① The combination of spray coating and spray welding layer with base metal forms different Ni-Al coating When heated by spray flame beam, exothermic chemical reaction takes place on the surface of carbon steel with rust removal of SA3 grade RZ > 50 μ m by sand blasting, forming a mechanical bonding coating with micro metallurgical bonding bottom layer and working layer and "anchor hook" effect The bonding between spray welding layer and base metal is purely metallurgical bonding coating ② different spray welding requirements of spraying materials require the use of self fluxing alloy powder, while spraying does not require high self fluxing of the powder, which is not necessarily self fluxing alloy powder All kinds of self fluxing alloy powder can be used for spray welding and spraying, but spray powder without self fluxing can only be used for spray coating and not for spray welding process ③ different preheating temperature before spraying and different heating conditions during spraying and spray welding affect the microstructure and properties of different sprayed workpieces ④ The density of the coating is different from that of the spray welding layer ⑤ The ability of bearing load different spray coating can generally bear large area contact, but the spray welding layer can bear large impact extrusion stress or contact stress when it is used on the working surface with lubrication conditions and other working conditions with small stress 4 Spray welding process should be selected in the following cases: ⑴ the surface load of various carbon steel and low alloy steel workpiece is large, especially the impact load requires that the hardness of spray welding of workpiece with the bonding strength of coating and substrate of 350-450n / mm2 hrc150 ≤ 65, and the surface roughness of spray welding layer from 0.3 to several millimeters after grinding and processing can reach above ra0.4-0.1 μ M (2) when used in corrosive medium, the coating is required to be dense without pores; (3) the original design of the surface of the workpiece adopts quenching, carburizing, nitriding, hard chromium plating and other processes, which require that the surface has high hardness; (4) the working environment of the workpiece is poor, such as strong abrasive wear, erosion wear, cavitation, etc (5) the oxyacetylene flame alloy powder spray welding process is suitable for surface strengthening or repair of various carbon steel and low alloy steel parts, but some characteristics of the parts should be noted When the linear expansion coefficient of the base material is quite different from that of the alloy spray welding layer, it should be careful when it is less than 12 × 10-6 / ℃ and more than 12 × 10-6 / ℃ If the content of elements with high affinity to oxygen in the base metal is more than 3%, such as tungsten and molybdenum, the total content of elements such as aluminum, magnesium, cobalt, titanium and molybdenum is more than 0.5%, or the sulfur content in steel is more than 0.5%, it will also bring difficulties to spray welding, because these materials are easy to form compact and stable oxide film to prevent the wetting effect of molten alloy on the base metal remelting The liquid alloy will roll off like "sweat bead" when spraying, so the adaptability of this process to the sprayed matrix material should be paid attention to when using spray welding process (6) metal materials that can be spray welded without special treatment: ① carbon structural steel with carbon content ≤ 0.25%; ② alloy structural steel with total content of MH, Mo, V, Cr and Ni < 3% (3) 18-8 stainless steel, nickel stainless steel, gray cast steel, malleable cast iron, nodular cast iron, low-carbon pure iron, copper Second, the spray characteristics of typical surfaces The geometry of parts has a certain impact on the spray (welding) effect For example, when the coating is cooled, the spray on the outer circle surface tends to shrink and tighten; The coating cooling on the inner circle surface tends to be separated from the workpiece The simple surface spray powder is easy to deposit, while the complex surface spray powder is difficult to deposit in some zigzag parts Therefore, it is of practical significance to study and excavate the spray (welding) characteristics of various types of parts and correctly use the spray (welding) process to ensure the spray quality 1 Key points of spraying process for shaft surface: ⑴ before spraying, the surface preparation shall be strictly controlled for purification treatment, and roughening treatment shall be conducted according to the process requirements to avoid contact with moisture or touch by hand (2) strictly control the spraying specification and control the temperature rise of the workpiece to be no more than 250 ℃ When the temperature rise of the relatively thick workpiece is smaller and the continuous spraying may be more than 250 ℃, the intermittent spraying method shall be used to cool down but the residence time shall not be too long, otherwise the coating will be polluted and cause poor interlayer bonding (3) after finishing the excessive angle and the journal with holes or keyways, the holes shall be blocked and the edges of keyways shall be polished into rounded corners Pay attention to the sprayed edges (4) the preheating temperature of the substrate on the outer circle surface shall be taken as the lower limit, and the substrate temperature shall be controlled during the spraying process so as to reduce the shrinkage force of the substrate 2 Key points of plane spraying process: ⑴ pay attention to the repair of edges and corners; ⑵ preheating of large parts with uniform preheating requirements can be used at both ends (3) during spraying, powder shall be sprayed from the surrounding edge and then led to the plane for several times, and the later spraying direction shall be perpendicular to the previous spraying direction (4) in order to prevent the edge of the coating from warping, the edge of the workpiece can be slotted before spraying and the stress can be reduced by sandblasting 3 Key points of nozzle surface spraying process: ⑴ when spraying, the cylinder part of nozzle shall be sprayed first and then the plane part (2) it is better to spray the spray gun at an angle of 45 degrees along the circumference of the workpiece, so that the powder deposition rate may be low, but the spray can spray clear edges and corners Pay attention not to spray the spray gun perpendicular to the corner of the stop, which will cause the coating structure at the corner to be loose or mixed with ash (3) if there is burr or edge warping on the edge during spraying, it shall be removed and re sprayed in time 4 Key points of spraying process for parts with severe local damage: ⑴ grind the parts with local damage clean, spray repair first or repair with other methods, and then spray all surfaces (2) for deep and wide local defects, it can be repaired by electric welding first and then fully sprayed (3) for the local defects with cracks, the start and end points of the cracks shall be found first, and the crack stopping holes and bevels shall be made at this place to clean all the cracks, then spray repair or weld repair the defects, and finally spray the surface to be sprayed 5 Key points of large-scale equipment guide rail technology: ① it is difficult to repair the guide rail of machine tool to meet the problem of large-scale oil leakage, so that the whole guide rail of machine tool can be evenly heated and degreased, and it is also difficult to remove all the deep oil stains If the heating temperature is easy to deform the guide rail of machine tool, the following operations can be carried out ② use the flame for low temperature local baking, the temperature of the workpiece is about 80-100 ℃, and then wipe off the oil stains after baking Repeat for several times, and then use the high-power spray gun for rapid spraying ③ use solvent cleaning or brush plating treatment, then use flame baking and spray repair ④ depending on the specific situation of the rail pull, the rail repair glue or casting defect repair machine welding repair process can also be used to complete 6 Key points of outer circle surface spraying process: the coating shrinkage force is the same as that of shaft parts spraying, which makes the coating hoop the substrate, which is conducive to the improvement of bonding strength The lower limit of the thermal temperature of the hypophyseal dam is taken, and the workpiece temperature is more than 250 during spraying ℃ it must also be stated that different parts with different shapes, volumes and spraying positions have different requirements for the temperature control of the workpiece When spraying the outer circle of small parts, avoid that the shrinkage stress of the workpiece will lead to the coating falling off or large deformation when the workpiece is cooled too high 7 The key points of inner hole spraying process: ① the shrinkage force of different spraying layers of inner hole and shaft is inclined to separate the coating from the substrate when cooling, which is not conducive to the improvement of the strength; ② When the inner hole is shallow and large, the outer round spray gun can be used to take a certain spray angle for spraying The outer round spray gun is mostly used for spraying the inner round part, but the "covering effect" caused by turning the thread should be paid attention to before spraying to prevent the coating from loosening ③ for the spraying of deep holes (the aperture is greater than 200 mm), the inner hole spray gun of the extension pipe shall be used, but the hole depth shall not be more than twice the aperture, otherwise the spraying quality will be greatly affected to prevent the inner hole spray gun of the extension pipe from being cooled by air with a pressure of 0.4-0.6 MPa if the temperature is too high ④ the rotation speed of the workpiece shall be slightly faster than that when spraying the outer circle to prevent the coating from overheating The rotation direction of the workpiece shall be such that the spraying face rotates downward to avoid the scattered powder from bringing into the flame on the surface to be sprayed to be slightly higher than the center line of the workpiece ⑤ intermittent spraying can be used for thick coating when the temperature of workpiece is not higher than 250 ℃ ⑥ the shrinkage stress of the substrate on the surface of the inner hole of the sprayed parts tends to make the substrate close to the coating If the preheating temperature of the workpiece is insufficient and the temperature of the workpiece is low, the coating will fall off due to the large shrinkage stress difference between the coating and the substrate Therefore, the upper preheating temperature can be used to properly increase the preheating temperature of the substrate and increase the shrinkage force of the substrate when the deformation condition of the workpiece permits ⑦ before two-step spray welding and remelting, the key points of spray welding for various parts can be determined according to the specific conditions and the spray welding process requirements 8 Key points of spray welding technology for iron castings: ① there are quite a lot of parts in vehicles and mechanical equipment, which are made of cast iron, being manufactured or