Introduction to laser cladding
① Principle
The important characteristics of laser cladding are heat concentration, fast heating, fast cooling, and small heat-affected zone. The melting between different materials has characteristics that other heat sources cannot match. It is this special heating and cooling process that produces in the casting area. Its structure is also different from other cladding methods (spray welding, surfacing, ordinary welding, etc.), and it can even produce amorphous structure, especially pulsed laser. This is the reason why the so-called laser cladding does not deform or anneal. From a micro level, you see another form.
② Equipment
Laser cladding currently adopts two models in China; CO2 laser and YAG laser. The former is continuous output, and the cladding machine is generally above 3KW; YAG laser is pulse output, generally around 600W. For the equipment, it is difficult for general users to understand and rely heavily on the services of the manufacturer. The purchase price is expensive, maintenance costs, and parts prices are high. In addition, equipment stability and tolerance are generally far behind those of foreign countries. Therefore, laser cladding machines are generally used in special fields, and it is difficult to be effective in general industrial manufacturing and maintenance fields.
③ Process
First-stage treatment: Laser cladding generally only needs to polish the workpiece clean, remove oil, rust, and fatigue layer, etc., which is relatively simple.
The second powder feeding: CO2 laser has high power, generally use argon gas to feed powder; YAG laser has low power, generally use natural powder drop method. Both of these methods basically form a molten pool in a horizontal position during cladding, and the powder cannot be delivered normally if the tilt is slightly larger, and the use range of the laser is limited, especially the YAG laser.
Thirdly, we can see from the state of molten pool formation: due to the high control precision of the laser, constant output power, and no arc contact, the size and depth of the molten pool are consistent.
Fourth, heating and cooling: it affects the uniformity of metal phase formation, and is also detrimental to exhaust scum. This is also an important reason for the formation of pores and uneven hardness in laser cladding, especially the YAG laser tends to be more serious.
Fifth material selection: Due to the different absorption capabilities of different materials for different wavelength lasers, the choice of laser cladding materials is more limited. The laser is more suitable for some materials such as nickel-based self-fluxing alloys, and the cladding of carbides and oxides It's more difficult.
Introduction of micro-plasma cladding
①Technical characteristics:
The plasma beam used by the micro-beam plasma cladding machine is an ionizing arc, which is more concentrated than the arc welding machine, so the heating speed is faster. In order to obtain a more concentrated ion beam, generally high compression ratio aperture and small current are used. In order to control the substrate temperature not to be too high, to avoid annealing deformation. Of course, this is incomparable with the heating speed of YAG laser. Because the plasma arc works continuously, the cooling of the body is relatively slow, and the transition area formed is deeper than that of laser cladding. This will release the stress better for hard surface material cladding.
②Equipment
Micro-plasma cladding equipment is developed on the basis of DC welding machine. Its power supply, spray gun, powder feeder, swinger, etc., have low technical threshold, easy to manufacture, good reliability, simple maintenance and use, and low power consumption. Low cost of use, good versatility, low production cost, good adaptability, convenient for large-scale production, significant benefits, low environmental requirements, and wide adaptation to materials. With the advancement of electrical technology, my country's welding machine technology level has sufficient support capabilities. In addition, the equipment is small in size and weight, and the welding torch can be held by hand, which makes it more flexible and convenient to use, and the cost of auxiliary tooling is cheap.
③Process
First, the pre-treatment is simple: only need to remove the rust, decontamination and fatigue layer.
The second powder feeding: the powder is fed with argon gas, the feeding accuracy is low, and it can have a certain inclination. This allows manual operation, which is more suitable for metal repair.
Third, the micro-beam plasma has good stability: the micro-beam plasma has good stability, the formation of the molten pool is also easy to control, the dressing is fully fused with the body, and the area is excessively good.
The fourth heating and cooling rate is lower than that of the laser: the molten state is maintained for a long time, which is conducive to the uniform formation of the metallographic structure, and the exhaust scum is better. It has been heated during the powder ejection process, and is protected by argon and ion gas , So the uniformity of the cladding layer is better, and there are fewer defects such as slag inclusions in the pores.
Fifth material selection: Plasma heating has fewer restrictions on materials and a wider selection of materials. It is easier to cladding carbides and oxides.
Several problems in cladding
1. Regarding welding stress:
A concept must be established. No matter what nouns are used (such as welding, surfacing, spray welding, cladding, etc.), it is melted and cast on the metal substrate under heating, then from heating to melting and then to cooling In this process, stress will inevitably occur. Except for very special materials, the shrinkage stress generally affects the most. Different welding methods are nothing more than the heating method speed, the filling material and some other conditions. So reducing the influence of this kind of stress on the substrate and the molten cast layer is an important aspect that engineers must consider when pursuing welding quality. The editor believes that shrinkage stress cannot be avoided, so stress release is the key to solving the welding stress problem. That is to say, where this shrinkage stress is released, and how the stress is distributed from the body to the casting area, is the problem that needs and can be solved.
2. Why is the deformation of laser welding (cladding) small:
Mainly because the casting area is small, the transition area is small, and the shrinkage is small. Then the shrinking force generated by the material in the shrinking process is not enough to deform the entire body. This is the reason for the so-called invariance of laser cladding (so when the body size is too small, deformation will also occur). This is also the advantage of laser welding (cladding). So where does this welding stress go? It is mainly released into the casting area and the transition area. So this creates two problems. One is that cracks are prone to occur in the melting and casting zone, so laser cladding requires relatively high ductility of materials, such as nickel-based powder; the second is that the transition zone has high stress, and the transition zone is too small due to the rapid heating and cooling during the laser welding process , Causing stress concentration in this area, which affects the bonding effect of laser welding (cladding). Especially when the mechanical properties of the substrate and the welding material are quite different, the tendency is more serious, and even the phenomenon of shedding occurs, which requires special attention to the material and thickness design of the transition layer during laser cladding.
3. Why plasma cladding (surfacing welding) is not easy to produce defects such as cracks and pores:
There are three main reasons.
(1) Plasma is used as the heat source for cladding (surfacing) and submerged arc welding and gas shielded welding, etc. The heat is more concentrated, the ion arc stability is better, there is no electrode melting, the output heat is uniform, and it is easy to control, so that the heat in the casting area The distribution is uniform, the material is fully and evenly fused, the exhaust scum is sufficient, and the shrinkage stress is uniformly distributed.
(2) Due to the high control precision of the plasma equipment, the control of the melting and casting zone and the transition zone is convenient, and the uniformity is good, and the stress distribution is easier to control and reasonable.
(3) The use of argon gas protection does not require various additives, and there are no problems such as hydrogen exhaustion and oxidation. Therefore, plasma cladding (surfacing) is more suitable for large-area, large-thickness, and high-quality hard surface casting (such as high manganese ·High chromium ceramic materials, etc.) suitable for manufacturing wear-resistant plates, valves, rolls, etc.
Summarize
Processability of cladding:
Regarding laser cladding and plasma cladding, many colleagues have published many articles, most of which emphasize the advantages of laser, which is also the goal that everyone is pursuing. However, most of the lasers are evaluated by metallographic analysis from a microscopic point of view. But everything has its two sides, and laser cladding also has its disadvantages. There are many limitations in the process, and high operation skills are required in actual production, which causes difficulties for many customers. The editor believes that the main reason is that the melting time of the cladding layer caused by fast heating and fast cooling is too short, resulting in large differences between the outer and inner edges of the light spot, uneven tissue formation, uneven stress distribution, insufficient exhaust scum, and uneven hardness. It is easy to form pores and slag inclusion, and it is difficult to obtain a large-area perfect cladding layer, especially the YAG laser. Therefore, laser cladding should be extremely meticulous from material selection to operation.
Compared with laser, plasma cladding has a larger input heat, and the deformation of the substrate is larger than that of laser. However, the melting is sufficient, the hardness distribution is uniform, and the exhaust scum is thorough. Wide selection of materials, easy operation, easy to obtain relatively intact overall cladding layer, low cost and good benefit. Therefore, it has obvious advantages in large area, large thickness and cladding.
