General rules for metal powder flame spraying (welding) process

this article is mainly based on the author engaged in thermal spraying more than 20 years of practical experience, to different workpiece materials, coating selection, coating quality control and process points play a "brick-throwing jade" role, process selection1, understand the performance of the workpiece itself: such as materials, compositions, surface conditions, wear site and wear area, depth, etc2, understand the conditions of use of the workpiece: the use of the environment (working temperature, the concentration of the main media and other components), the force conditions (impact load or sliding fit, static fit), failure reasons (processing differences, wear, corrosion, etc.)3, the following conditions should be used spraying process:1) surface by static load or low-speed sliding load requirements wear but the coating and substrate tensile binding strength requirements not more than 50N/mm2 workpiece, spraying layer hardness HV130-350 The coating thickness from 0.3 to several millimeters, the coating surface roughness Ra10-25 m, the coating after grinding and processing surface roughness up to Ra0.8-0.4 m, such as a variety of shaft-type bearing sites2) Requires gas-resistant, water-resistant and processed roughness not exceeding The surface of Ra0.4 m, such as imported automotive crankshaft train crankshaft, marine crankshaft, etc3) Non-ferrous metals such as copper alloy aluminum or aluminum alloy need to be restored to size or improve surface wear resistance4) It is best to use copper-based or aluminum powder for spraying non-ferrous metal workpieces such as copper alloy, aluminum or aluminum alloy5) The use of the corrosion medium requires the coating to be dense and pore-free shall be closed after the coating is sealed with a suitable sealing agent in accordance with the concentration of the working material used in the ambient working temperature medium and the composition of the plastic, ceramic or corrosion-resistant metal sprayed to meet the conditions of use6) the binding strength of the spray layer and the substrate: the traditional flame spray coating layer anti-pull binding strength is generally 20-30N/mm2 using Shaanxi Zhongke Surface Engineering Co., Ltdthe latest development and production of ZK8032 type powder multi-functional high-speed flame spray gun in the ball ink cast iron sprayed on the blast surface aluminum package nickel composite coating, its coating shear strength can reach 63.7N/mm2; The welding layer has high binding strength, but the parts that are resistant to failure are sufficient for repair work in the friction resistance between lubricated metalsAfter calculation by scientific research units, the shear stress on the seven spindle necks of the crankshaft is concentrated in a single spindle neck when the liberating CA141 car is fully loaded;7) Spraying layer wear resistance: to the car crankshaft spray as an example spray eddy spindle neck than the new crankshaft has a higher wear resistance once my company sprayed the Japanese-made Hino car crankshaft repair spindle neck overhaul when checked wear volume is 0.022-0.0 3mm and unsprayrepaired spindle neck wear is 0.2 -0.25mm its wear is almost ten times different from this example can show that the spray layer has good wear resistance, even in the absence of engine oil can still continue to run for several hours without burning the shaft phenomenon8) The process difference between spraying and soldering:(1) the combination of the spray layer and the spray solder layer and the substrate metal to form a different nickel package aluminum by spraying flame beam heating when the thermal reaction occurs in the sandblasting rust to Sa3 class RZ 50 m carbon steel surface formation micrometallurgical binding layer and the working layer and the work layer produces "anchor hook" effect of mechanical binding coating and the combination of the spray layer and the substrate is purely metallurgical(2) The different spray materials are required to use self-melting alloy powder and the spray is not high self-melting requirements for powder and is not necessarily self-melting alloy powder can be used for both soldering and can be used for spray but the spray powder does not have self-melting can only be used for spray but can not be used for spraying process(3) The composition spraying and pre-spray preheating temperature during the spray welding process are different in the composition affected by heat, the organization and performance of the post-spray workpiece are also different(4) coating has different density of the spray welding layer and a small number of pores in the spray layer(5) the ability to withstand the load different spray coating spraying layer can generally withstand large area contact more in the lubrication conditions of the working surface mating surface and other less stressed conditions using the spray solder layer but can withstand the greater impact force extrusion stress or contact stress4, the following conditions should be selected for the use of soldering process:(1) a variety of carbon steel, low alloy steel workpiece surface load, especially under impact load requires the coating and substrate binding strength in 350-450N/mm2 workpiece solder ingesting hardness HRC150-65 coating thickness from 0.3 to several mm spray welding layer after grinding rough surface roughness of Up to Ra0.4-0.1m(2) used in corrosion media, requires the coating to be dense and pore-free, and(3) the original design of the workpiece surface requires a high hardness of the surface by quenching, carburizing, nitriding, hard-chrome and other processes(4) The working environment of the workpiece is bad, such as wear by strong grinding particles, erosion wear, air erosion and so on(5) Oxygen-acetylene flame alloy powder spray welding process to adapt to a variety of carbon steel, low alloy steel parts surface strengthening or repair but should note that some characteristics of the part material when the base material of the line expansion coefficient and alloy spray welding layer when the difference between the wire expansion coefficient is less than 12 x 10-6 / C greater than 12 x 10-6 / C should be used with caution to avoid cracks if the base metal and the large reverberation of the elements The content of the hemolyn is greater than 3% aluminum, magnesium, cobalt, titanium, molybdenum and other elements of the total content is greater than 0.5% or the sulfur content of steel is more difficult to spray welding because these materials and oxygen action easily generate dense and stable oxide film blocking the melting of the melting gold on the substrate remelting liquid alloy will be beads like "sweat beads" should be carefully applied for this process(6) metal materials that can be soldered without special treatment:(1) carbon structure steel with carbon content of 0.25% (2) Mh, Mo, V, Cr, Ni total content of 3% alloy structure steel(3) 18-8 stainless steel, nickel stainless steel, gray cast steel, forged cast iron, ball-ink cast iron, low-carbon pure iron, copper.
Second, the typical surface of the spray characteristics
the geometry of the part has a certain impact on the spray (welding) effect, such as the outer round surface spray when the coating is cooled with the tendency of shrinking hugging trend;1, shaft surface spraying process points:(1) before the spray surface to be strictly cleaned treatment, coarse treatment to do according to the process requirements to do the surface to avoid contact with moisture or hand touch (2) strictly grasp the spraying specification control workpiece temperature rise can not exceed 250 degrees C when the coating thicker workpiece smaller continuous spray temperature rise may be more than 250 degrees C should be used intermittent spray method to cool down but stay time can not be too long or the coating will be contaminated caused by poor interlayer binding (3) Repair the shaft neck of excessive angle and edge prism hole or key slot should be blocked and the edge of the keys groove polished into rounded note squirting edge (4) the preheating temperature of the outer round surface substrate should be controlled during the lower limit of the coating process, thereby reducing the shrinkage force of the substrate 2, flat spraying process points: (1) pay attention to repair the edge, angle; (3) spray from the surrounding edge spray and then lead to the plane multiple spray powder after one should be perpendicular to each other with the previous spray direction (4) can be slotted at the edge of the workpiece and blasted coarsebeforeto reduce stress before preventing the coating edge from being skewed 3, stop surface spraying process points: (1) spray should first spray the cylinder part of the stop port after the spray plane part (2) spray gun spray direction into the direction of the workpiece circumference into a 45-degree angle of injection so that may make the powder deposition rate low but can spray clear angle spray operation note not to spray the spray gun perpendicular to the corner of the stop so that the coating tissue at the corner will cause loose or gray (3) when spraying, such as the edge of burrs, the edge should be removed and resprayed in time 4, local serious damage parts of the spraying process points: (1) will be local damage in the ground grinding clean first spray or with other methods to repair and then spray all the plane (2) for the deeper, wider local defects can be seen, can also be first by electric welding leveling and then full ypraying (3) the local defects with cracks first to find the crack start, stop end and at the place to lay a crack opening opening to clean up the cracks and then spray the defect site to be sprayed at the end of the spray surface 5, large-scale equipment guide process points: (1) repair machine guide encountered a large area of oil seepage problem straining machine guide overall uniform heating and oil removal is difficult to achieve and it is difficult to remove all deep oil pollution such as heating temperature and easy to make machine tool rail deformation for the following operations (2) with flame for low-temperature local baking workpiece temperature of about 80-100 degrees C with the roasting with the oil stains so repeated several times and then use high-power spray gun to quickly spray (3) are cleaned or brushed with solvent and then re-sprayed with flame baking (4) depending on the specific circumstances of the rail pull injury can also be used rail repair glue or casting defect repair machine welding process to complete 6, the outer circle surface spraying process points: and shaft parts spray the same coating shrinkage force so that the coating hoop tight base is conducive to the binding strength of the lifting pressure pituitary dam heat temperature in the process of the work part temperature to exceed 250 degrees C must also be explained for the shape, volume and spray parts of different parts of the work piece temperature control should also have different requirements when spraying small parts of the outer circle when the pressure is too high to cool the work part when the stress shrinkage 7, the inner hole spraying process points: (1) the inner hole and shaft type different spray layer in the cooling of the tendency to shrink the coating out of the substrate is not conducive to the increase of strength
(2) when the inner hole shallow and large can use the outer round spray gun to take a certain spray angle for spray for the inner round area of the spray mostly use the outer round spray gun but the pre-spray treatment using the car thread should be noted to cause "covering effect" to prevent the coating (3) for the deep hole of the spray (aperture greater than 200mm) to use the long tube inner hole spray gun but the hole depth should not exceed 2 times the aperture, otherwise the effect on the spray quality is greater to prevent the long tube inner hole spray gun temperature is too high application 0.4-0.6MPa pressure air cooling (4) workpieces should speed slightly faster than when the spray outer circle to prevent the rotation direction of the coating overheated workpiece should make the spray-oriented to avoid scattered powder into the flame of the surface to be sprayed slightly higher than the workpiece center line as well (5) the control workpiece temperature is not higher than 250 degrees C for thicker coating can be intermittent lying (6) The inner hole surface of the spray part due to the shrinkage stress of the substrate has a tendency to make the base considerate tight coating trend, such as the workpiece warm-up temperature is not enough workpiece temperature is low, due to the coating and the substrate shrink stress is too large to cause the coating to shed warm-up, the temperature should be appropriately increased the preheating temperature of the substrate so as allowed by the deformation conditions of the workpiece (7) using two-step method before remelting and spraying similar to the specific situation combined with the requirements of the spray welding process to determine the main points of all types of parts of the welding 8, casting iron parts of the soldering process points: (1) in the vehicle and machinery equipment there are a considerable number of parts made of cast iron in the manufacturing or use of various problems in the manufacturing or use of various problems using oxygen-acetylene flame spray welding process is not only an effective way to strengthen casting iron parts, but also to repair castings of various defects and injuries (such as casting sand eye, pores or wear and other damage in use of the ideal means; (2) Spray welding for the local defects of cast iron parts repair and defect size sits different depths are suitable for one-step spray welding according to the workpiece and welding parts should be used as far as possible small power spray gun so as to reduce the amount of heat input to the substrate generally selectqH-1/hQH-2/hQH-4/h , etc.; (3) Due to the poor weldability of cast iron parts should be preferred to select nickel-based alloy powder (Ni-B-Si series) nickel-based alloy powder melting point is generally 950 to 1050 degrees C heavy melting base will not be melted at the same time low carbon content of the nickel-based spray powder layer hardness is low, plasticity can relax the pressure to prevent cracks when the operation is not skilled (4) When selecting the spray welding specification, we should consider the casting iron material, the size of the defect site condition requirements and other factors to minimize the heat input oxygen to the substrate under the premise of ensuring the necessary flame energy, acetylene gas pressure removal limit for the appropriate spray powder, remelting appropriate adjustment of spray, melting distance control heat input ; (5) Welding local small defects such as pores, sand-eye spray before welding can not preheat to minimize the heat area around the spray welding layer so that the high temperature area as small as possible (6) for local small and deep defect solder ingesting is suitable for continuous spraying welding method powder deposition rate high thickness growth fast efficiency, but requires operational skills to make powder feed and spray, melting speed coordination to achieve spray, melt ing; (7) The larger area and deep defects to prevent the base from being too hot to cause thermal stress increase can be used intermittent method of spray welding if necessary, can also be used welding, spray welding composite process with welding rod filling the bottom of the spray welding such as casting wall thickness can not only increase the binding strength to prevent the spray welding layer and the mother material peeling but also can eliminate a part of the of soldering heat; (8) When soldering large complex castings with large thermal stress, measures should be taken to reduce stress accumulation, such as heating and subtraction, segmentsymmetry, etc.
; (9) Controlling the remelting temperature is too high not only the alloy element burning loss substrate overheating or even the substrate is melted into the cast iron carbon into the spray weld ingress will increase the carbon content in the spray welding layer to increase the plasticity reduction, and the substrate is prone to white mouth due to the decrease in carbon content and the temperature is too high will cause large rheumatity caused by the crack remelting temperature can not Too low otherwise easy to form a sandwich or melting effect of the combined strength for the remelting temperature control mainly depends on the operator's attention to observe the change of coating surface state when remelting with the help of the "mirror reflector" when remelting to control the presentation of "mirror reflector" indicates that the powder has been melted slag has floated at this time the remelting gun should immediately remove the remelting part should avoid repeated heating