Ⅰ. Characteristics of laser cladding
1. Technical features
The important features of laser cladding are concentrated heat, rapid heating, and rapid cooling, and a small heat-affected zone. Especially for melting different materials, it has characteristics that other heat sources cannot match. It is this special heating and cooling process that generates heat in the melting and casting area. The organizational structure is also different from other cladding methods (spray welding, overlay welding, ordinary welding, etc.), and can even produce amorphous structures, especially with pulsed lasers. This is the reason why laser cladding does not deform or anneal. But I thought this was only from a macro perspective of the workpiece as a whole, and when you conduct a microscopic analysis of the cladding layer and heat-affected zone, you will see another picture, which I will talk about later.
2. Equipment features
Laser cladding currently uses two types of machines in China: CO2 laser and YAG laser. The former is continuous output, and the cladding machine is generally above 3KW; the YAG laser is pulse output, generally around 600W. For equipment, it is difficult for ordinary users to understand it thoroughly. They rely heavily on the services of the manufacturer. The purchase price is expensive, and the maintenance cost and the price of parts are high. In addition, the stability and tolerance of the equipment are generally different from those in foreign countries. Therefore, laser cladding machines are generally used in special fields, and are difficult to be effective in general industrial manufacturing and maintenance fields.
3. Process characteristics
Preliminary treatment: Laser cladding generally only requires polishing the workpiece, removing oil, rust, fatigue layer, etc., which is relatively simple.
Second powder feeding: CO2 laser has high power and generally uses argon gas to feed powder; YAG laser has low power and generally uses natural powder falling. Both methods basically form a molten pool in a horizontal position during cladding. If the tilt is slightly larger, the powder cannot be delivered normally, and the use range of the laser is limited especially the YAG laser.
Third, from the perspective of the state of molten pool formation: due to the high control accuracy of the laser, constant output power, and no arc contact, the size and depth of the molten pool are consistent.
Fourth, fast heating and fast cooling: affect the uniformity of metal phase formation, and are also detrimental to exhaust scum. This is also an important reason for the formation of pores and uneven hardness in laser cladding, especially the tendency of the YAG laser to be more serious.
Fifth material selection: Due to the different absorption capabilities of different materials for lasers of different wavelengths, the selection of laser cladding materials is greatly restricted. Laser is more suitable for cladding of carbides and oxides on some materials such as nickel-based self-fluxing alloys. A little more difficult.
Ⅱ. Characteristics of micro-beam plasma cladding
1. Technical features:
The plasma beam used by the micro-beam plasma cladding machine is an ionization 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, a high compression ratio aperture, and a small current are generally used. In order to control the substrate temperature from being too high to avoid annealing deformation. Of course, this is incomparable with the heating speed of the YAG laser. Since the plasma arc operates continuously, the cooling of the body is relatively slow, and the transition zone formed is deeper than that of laser cladding. This means that the stress will be released better for the cladding of hard surface materials.
2. Equipment features:
Micro-beam plasma cladding equipment is developed on the basis of DC welding machines. Its power supply, spray gun, powder feeder, oscillator, etc. have a low technical threshold, easy to manufacture, have good reliability, simple maintenance and use, and have low power consumption. Low cost of use, good versatility, low production cost, good adaptability, convenience for large-scale production, significant benefits, low environmental requirements, and wide adaptability 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 gun can be held by hand, which makes it more flexible and convenient to use, and the cost of auxiliary tooling is cheap.
3. Process characteristics:
The preliminary treatment is simple: just remove rust, dirt, and fatigue layer.
Second powder feeding: Argon gas is used to feed powder. The powder feeding accuracy is low and can have a certain inclination. This allows manual operation, which is more suitable for metal repair.
Third, micro-beam plasma has good stability: Micro-beam plasma has good stability, the formation of the molten pool is easy to control, the dressing material and the body are fully integrated, and the area transition is good.
Fourth, the heating and cooling speed is lower than that of a laser: the molten state is maintained for a long time, which is conducive to the uniform formation of metallographic structure, the exhaust scum is better, it is already heated during the powder ejection process, and it is protected by argon gas and ion gas, so the cladding layer has better uniformity and fewer defects such as pores and slag inclusions.
Fifth material selection: The plasma heating method has fewer restrictions on materials, wider material selection, and easier cladding of carbides and oxides.
Ⅲ. Several issues in cladding
1. Regarding welding stress: We must establish a concept. No matter what terms are used (such as welding, surfacing, spray welding, cladding, etc.), they are all melting and casting on the metal substrate under heating, so from the process from heating to casting and then cooling, stress will inevitably occur. Except for very special materials, shrinkage stress generally has the greatest impact. Different welding methods are nothing more than different heating methods, speeds, filling materials, and some other conditions. So reducing the impact of this stress on the substrate and the cast layer is an important aspect to consider when we pursue welding quality. I think shrinkage stress is unavoidable, so stress relief is the key to solving the welding stress problem. In other words, where this shrinkage stress is released and how the stress is distributed from the body to the casting area are the issues we need and can solve.
2. Why laser welding (cladding) has small deformation: mainly because the casting area is small, the transition area is small, and the shrinkage is small. Then the shrinkage force generated by the material during the shrinkage process is not enough to deform the entire body. This is the reason for the so-called invariance of laser cladding (so deformation will occur when the body size is too small). 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 transition area. So this creates two problems. First, cracks are prone to occur in the casting area, so laser cladding requires relatively high ductility of materials, such as nickel-based powders; second, the stress in the transition zone is large. Due to rapid heating and cooling during the laser welding process, the size of the transition zone is too small. , 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 greatly different, the tendency is more serious and may even occur, which requires special attention to the material and thickness design of the transition layer during laser cladding.
3. Why plasma cladding (surfacing) is not prone to defects such as cracks and pores: There are three main reasons. First, when plasma is used as a heat source for cladding (surfacing) and submerged arc welding, the heat is more concentrated, the ion arc is more stable, there is no electrode consumption, the heat output is uniform, and it is easy to control, which makes the heat distribution in the casting area even. , the material is fully and evenly fused, the exhaust and scum are fully exhausted, and the shrinkage stress is evenly distributed. Second, due to the high control accuracy of the plasma equipment, it is easy to control the casting zone and transition zone, and the uniformity is good, so the stress distribution is easier to control and reasonable. Third, the use of argon gas protection does not require various additives, and there are no problems such as hydrogen emission and oxidation. Therefore, plasma cladding (surfacing) is more suitable for large-area, large-thickness, high-quality hard surface casting (such as high manganese, high Chromium ceramic materials, etc.) are suitable for manufacturing wear-resistant plates, valves, rollers, etc.
4. The 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 use metallographic analysis to evaluate lasers from a microscopic perspective. But everything has its two sides, and laser cladding also has its disadvantages. There are many limitations in terms of technology, and high operational skills are required in actual production, causing difficulties for many customers. I think the main reason is that the melting time of the cladding layer caused by fast heating and cooling is too short, resulting in a large difference between the outer edge and the inner edge of the spot, uneven tissue formation, uneven stress distribution, insufficient exhaust scum, resulting in uneven hardness. It is easy to form problems such as pores and slag inclusions, and it is difficult to obtain a perfect cladding layer over a large area, especially with the YAG laser. Therefore, laser cladding should be extremely meticulous from material selection to operation. Compared with laser, plasma cladding has a larger heat input, and the deformation of the substrate is larger than that of the laser. But its melting is sufficient, the hardness is evenly distributed, and the scum is completely exhausted. The material selection range is wide, easy to operate, easy to obtain a relatively complete overall cladding layer, low cost, and good benefits. Therefore, it has obvious advantages in large areas, large thicknesses,es and cladding.
