1 Introduction >> Into the colorful world of laser welding Â
With the continuous development of science and technology, many industrial technologies have special requirements for materials, and the materials manufactured by the smelting method cannot meet the needs. Therefore, it is especially necessary to use powder metallurgy to obtain some materials for modern industrial applications. Due to the special properties and manufacturing advantages of powder metallurgy materials, traditional smelting materials are being replaced in some fields such as automobile, aircraft and tool cutting tools. With the development of powder metallurgy materials, its connection with other parts. It is becoming more and more prominent. Brazing and projection welding have always been the most common methods for the connection of powder metallurgy materials. However, due to the low bonding strength and wide heat affected zone, it is not suitable for high temperature and high strength requirements [1], making powder metallurgy materials The application is limited.
In the early 1980s, laser welding entered the field of powder metallurgy material processing with its unique advantages. Compared with the traditional welding method, laser welding has the following characteristics: large aspect ratio, narrow weld seam, high weld joint strength; small heat affected zone, no influence on surrounding structure, small welding deformation; automation of welding process can be realized. high productivity. Therefore, it has developed rapidly in the field of powder metallurgy material processing, and the typical application is in the manufacture of diamond tools. In the early 1980s, Mosca discovered that CO2 lasers can successfully weld certain P/M materials. When the conditions are selected, the weld joint strength is high, the heat affected zone is narrow, and the results of laser welding are found to be sintered. The conditions are very sensitive: the endothermic gas is not suitable for the sintering atmosphere of the laser welding material; the materials sintered in hydrogen, decomposed ammonia and vacuum can be successfully applied in laser welding [2]. The UK's Nimbus Diamond Tools introduced laser welding technology at the end of 1985. So far, the company has invested £250,000 to develop this high-tech method of welding diamond segments. In Germany, Dr. Fritsch Sondermaschinen GmbH has developed a new fully automated laser welding method for welding diamond drill bits and saw blades, which greatly increases the welding strength. In addition, Italy, Japan, and Belgium also reported on this aspect [3][4][5].
In recent years, the number of units engaged in research work in this area has gradually increased. For example, the National Engineering Research Center for Laser Processing at Huazhong University of Science and Technology has successfully applied laser welding technology to the production of diamond saw blades and drill bits, changing the traditional sintering and brazing. The welding process greatly increases the strength and high temperature strength of the joints [7].
2 Laser welding process characteristics
2.1 Main factors affecting welding quality
2 1 1 Material composition The content and type of alloying elements have a great influence on the mechanical properties such as weld strength, toughness and hardness. Sintered low carbon steel, sintered Ni and Cu alloys, and Co alloy can be successfully laser welded under certain conditions. The pre-weld preheating and post-weld slow cooling measures of the sintered medium carbon steel can also ensure the welding quality and reduce the crack sensitivity. Figure 1 shows the microhardness distribution of the weld zone under the conditions of preheating and non-preheating of medium carbon steel. The hardness is lowered during preheating, and the joint toughness is increased because the structure replaces needle-shaped martensite by bainite and a small amount of pearlite.
2 1 2 Sintering conditions The materials sintered in hydrogen, decomposed ammonia and vacuum can be successfully laser welded. The sintered materials in a clean reducing atmosphere show less pores, voids, inclusions and oxides after welding. Suitable sintering temperature, holding time, pressure and temperature-pressure curve are also important guarantees for successful welding.
2 1 3 The number, shape and distribution of pore pores affect the physical properties of the material such as thermal conductivity, thermal expansion and hardenability. These physical properties directly affect the weldability of the material [1], so that the welding is the same as that of the same material. More difficult than it is. For laser welded parts, a large number of voids can reduce the weld strength or even the welding process.
2 1 4 Samples with dense density and good mechanical properties have better weldability under the same conditions than loose ones with poor mechanical properties. Materials below a certain density (<6.5g/cm3) can hardly be welded by fusion welding because low strength and torsional strength do not allow the material to absorb energy; medium density (<6.9g/cm3) materials can be used. Melt welding, but welding methods such as electric resistance projection welding and friction welding are preferred, and the welding success rate is high; the high density (>7.0 g/cm3) sintered material has almost the same weldability as the smelting material. The density not only has a great influence on the welding strength but also on the welding defects, especially the pores, and the sintered material below a certain density has low strength after welding and many pores. A material with a low density will have a large gap after welding. The lower the density, the deeper the gap will affect the fatigue strength. In addition, the density has an effect on the weld penetration. When the laser power and welding speed are constant, the density is higher. The lighter. Figure 2 shows the effect of density on weld shrinkage and penetration on the laser power and welding speed. (a) shows the effect of density on penetration, and (b) shows the effect of density on weld shrinkage.
2 1 5 Preparation before welding Since the laser spot is small, the clearance tolerance is required to be high. The gap is generally required to be less than 0.1 mm during the docking. In addition, in order to reduce welding defects such as porosity, the weld must be descaled and oily. dry. Figure 2(b) Effect of density on weld seam shrinkage
2.2 Main welding process parameters
The main process parameters of welding quality are: laser power, welding speed, lens focal length, focus position, shielding gas and so on. Laser power and welding speed are the most important parameters affecting the welding quality. The welding thickness depends on the laser power, which is about 0.7 power of the power (kW). Generally, the power increases and the welding depth increases. The speed increases, the penetration depth becomes shallow, and the welding speed increases. The seam and heat affected zone are narrowed and the productivity is increased. Excessive welding speeds and welding power will increase the tendency of the pores and holes. Table 1 is the reference value of the welding power and speed of several materials (material thickness 3mm).
Table 1 Reference welding power and speed materials for several materials
Material grade raw material powder density / g · cm-3 sintering atmosphere laser power / kW welding speed m · min-1 ASC100 atomized iron powder 6.94
Vacuum 6.55 0AISI316L stainless steel powder 6.73
Vacuum 5.55.5PASC45 phosphating (0.45%) iron powder 7.23
Vacuum 3.52 0C10 medium carbon steel (0.10% C) 7.85 reducing atmosphere 7 05.5NC100 sponge iron powder 6.51 reducing atmosphere 5.54 0ASC100 atomized iron powder 7.02 reducing atmosphere 6 04 0ASCCu Ni0.4%Cu5.6%Ni alloy iron powder 7.08 reducing atmosphere 5.54 0
The focal length of the lens is determined by the spot diameter of the output laser, and there is a best match between the two. Generally speaking, the deeper the depth to be welded, the longer the focal length of the lens, the higher the focus of the short focal length lens, and the greater the spatter of the powder metallurgy material during welding, the lens is heavily polluted; the lens with too long focal length makes the focus due to diffraction When it becomes larger, the energy density at the focus cannot reach the maximum value. The lens focusing optical system is generally used in China. The system can only be used in applications where the laser power is small. The higher laser power will cause the lens focus to drift, resulting in poor weld formation and quality. Most of the higher power occasions in foreign countries use the mirror focusing optical system. Because of the good cooling conditions, the thermal stability is good, the weld bead is evenly and beautifully formed, and the welding quality is reliable. The laser focus can obtain the maximum welding depth at a special position below the workpiece, that is, about 1/3 of the thickness of the plate; the horizontal focus position depends on the situation, and for the diamond circular saw blade, the base side should be selected. At a position where the amount of shift is about 0.1 to 0.2 mm, Fig. 3 shows the fusion depth at different offsets. Figure 4 shows the position of the laser and the workpiece of the laser circular saw blade laser welding. The protective gas acts to protect the focusing lens from oxidation of the weld and is protected by an inert gas. In China, because He is expensive, argon is generally used, and the gas flow rate should be controlled. If it is too small, it will not work. If it is too large, it will waste gas and roll the weld pool. There will be a wave of waves on the surface of the weld. Raised.
3 Welding quality inspection and analysis
3.1 Welding quality inspection
3 1 1 Appearance inspection Observe the weld surface for obvious defects such as holes, cracks, undercuts, and incomplete penetration.
3 1 2 Non-destructive testing Non-destructive testing methods include: penetrant testing; magnetic particle testing; radiographic testing; ultrasonic testing, etc., should be selected according to needs.
3 1 3 Mechanical properties test According to the working state of the parts, tensile, bending, hardness, impact and other tests are carried out. If the weld is broken, the welding strength is lower than that of the base metal.
3 1 4 Microscopic examination Metallographic analysis of weld formation, microstructure, weld defects, testing of the microhardness distribution of the weld zone, scanning electron microscopy analysis of changes in the composition of the weld zone.
3 1 5 Special Performance Testing Special performance tests such as corrosion resistance and fatigue are required for parts working in a special working environment. Of the above five methods, the first two are mainly used in welding production lines, and the latter three are mainly used in experimental research and sample survey.
3.2 Defect analysis
3 2 1 Porosity and hole are compared with smelting materials in laser welding of powder metallurgy materials. The most obvious defects are pores and holes. The pores and holes not only affect the appearance quality, but also weaken the effective bearing area of ​​the weld, generate stress concentration and reduce joint strength. Common pore shapes are linear, round, honeycomb, stripworm, and the like. The pores inside the sintered material adsorb a large amount of gas, and in the rapid welding, it is too late to escape and remain in the weld. The hole is mainly due to the fact that the sintering atmosphere is not clean, and oxygen cannot be removed to a sufficient extent. Other impurities such as oxides absorb more laser energy than the base material and cause overheating. Mn, Si, Ti, Al and other alloying elements with strong affinity for oxygen can effectively remove oxygen during welding, reduce the tendency of pores and pores; optimize the sintering process, improve the compactness of the workpiece, reduce the pores inside the material, and thus reduce the adsorption gas. An effective way to reduce pores; improving sintering conditions, such as vacuum or sintering in a reducing atmosphere, also helps to reduce pores and pores; in addition, it is also beneficial to reduce the content of low-melting materials in the weld zone and to do pre-weld cleaning.
3 2 2 Cracks mainly have cold cracks and hot cracks, and interlaminar cracks are also likely to occur in diamond tools. Cold cracks are mainly produced in materials with high carbon content and high alloy composition. These materials produce brittle martensite after welding, which generates high internal stress and causes cracks. The solution to this type of crack is preheating before welding, slow cooling after welding, or using small gauge welding parameters. The occurrence of interlaminar cracks in laser welding of diamond tools is mainly due to the large difference in the coefficient of linear expansion between the diamond layer and the transition layer. Under the action of welding thermal cycle, large internal stress is generated to produce shear, or is contained in the material. The low melting point material is large and segregated between the layers, and cracks are generated under the action of welding thermal stress. One way to solve the hot crack is to improve the alloy system of the material reasonably according to the nature of the crack. For example, adding certain Mn, Mo, W, and Cr can effectively prevent the crack, and use the modifier to refine the primary crystal structure of the weld. The weld crystal crack also has a certain effect; the second is to limit the content of harmful impurities S and P. The alloy with higher Ni content should pay attention to the limitation. This is because Ni and S can form sulfides with lower melting point and their total Crystal.
3 2 3 Inferior strength components, sintering conditions and post-heat treatment can affect joint strength. In addition to material factors, excessive pores and pores are important causes of low joint strength. Secondly, the density of the material is too low, and the weld is loose and the strength is low.
4 Development prospects and existing problems
Laser welding technology enters the field of powder metallurgy material processing with its unique advantages, which opens up new prospects for the application of powder metallurgy materials, such as diamond circular saw blades and diamond drill bits, which greatly improves the bonding strength and high temperature strength. It can be used in dry cutting or poor water supply conditions. Automated production increases productivity and reduces cost, which enhances the market competitiveness of the product. No filler is added during welding to ensure continuous and consistent weld metal. It is very important for the magnet assembly. However, the research and application of laser welding technology in the field of powder metallurgy materials is still very limited, mainly because it is difficult to avoid the occurrence of pores and holes during the welding of powder metallurgy materials, so that the appearance of welds and welding quality are affected; welding process and materials The selection is also more difficult than the general smelting and casting materials; in addition, although the weld strength is higher than that of brazing and projection welding, the requirements for fixtures, fitting precision and pre-weld preparation work are high, and the one-time investment is large, so the application is affected. limit. Reducing the price and operating cost of lasers, and more basic research on laser welding processes, materials and welding behaviors of powder metallurgy materials are important prerequisites for promoting the application of laser technology in the processing of powder metallurgy materials.
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