I. Characteristics of Laser Cladding

1. Technical characteristics

The most important characteristics of laser cladding are heat concentration, fast heating, fast cooling and small heat-affected zone. Especially, the melting of different materials has incomparable characteristics with other heat sources. It is precisely this special heating and cooling process that produces different structure in the casting area from other cladding methods (such as spray welding, surfacing welding, ordinary welding, etc.), and even can produce amorphous. The state structure, especially the pulse laser, is more obvious. This is the reason why laser cladding has no annealing and deformation, but I think it is only from the overall macro view of the workpiece, and when you micro-analysis the cladding layer and heat-affected zone, you will see another picture.

2. Equipment characteristics

At present, there are two types of laser cladding in China: CO2 laser and YAG laser. The former is continuous output, cladding power is generally over 3KW; YAG laser is pulse output, generally around 600W.

For equipment, the general users are difficult to understand, rely heavily on the services of the producers, the purchase price is expensive, the maintenance cost and the price of spare parts are very high, and the stability and tolerance of the equipment are generally different from those abroad. Therefore, laser cladding machines are generally used in special fields, and it is difficult to be effective in the field of general industrial manufacturing and maintenance.

3. Process characteristics

(1) Pretreatment of laser cladding, generally only need to grind the workpiece clean, oil removal, rust removal, fatigue layer, etc., is relatively simple.

(2) The power of powder-feeding CO2 laser is high, and the powder is usually fed by argon; YAG laser has low power and is usually fed by natural powder. Both of them form a molten pool in the horizontal position during cladding, and the powders with a slightly larger inclination can not be delivered normally, which limits the use of lasers, especially YAG lasers.

(3) From the state of molten pool formation, the size and depth of molten pool are consistent because of the high precision of laser control, constant output power and no arc contact.

(4) Rapid heating and cooling affect the homogeneity of metal phase formation, and are not conducive to the exhaust of scum. This is also an important reason for the uneven porosity and hardness of laser cladding, especially the tendency of YAG laser is more serious.

(5) Material selection. Due to the different absorption ability of different materials for different wavelength lasers, the choice of laser cladding materials is limited. Laser is more suitable for some materials such as Nickel-based Self-fluxing alloys, and the cladding of carbides and oxides is more difficult.

CHARACTERISTICS OF MICROBEAM PLASMA CLADDING

1. Technical characteristics

The plasma beam used in micro-beam plasma cladding machine is an ionization arc, which is more concentrated than the heat of arc welding machine, so the heating speed is faster. In order to obtain more concentrated ion beams, high compression ratio aperture and low current are generally used to control the temperature of the matrix and avoid annealing deformation. Of course, this is incomparable with the heating speed of YAG laser. Because the plasma arc works continuously, the cooling rate of the body is relatively slow, and the transition zone is deeper than that of laser cladding. For hard surface cladding, the stress release is better.

2. Equipment characteristics

Microbeam plasma cladding equipment is developed on the basis of DC welding machine. Its power supply, spray gun, powder feeder and swing device have low technical threshold, easy manufacture, good reliability, simple maintenance and use, low power consumption, low cost, good versatility, low production cost, good adaptability, large-scale production, remarkable benefits, low environmental requirements and wide adaptability to materials. 。 With the progress of electric technology, the technology level of welding machine in our country has enough support ability. In addition, the equipment is small in size and weight, and the welding torch can be handheld, which makes it more flexible and convenient to use, and the cost of auxiliary tooling is low.

3. Process characteristics

(1) Pre-treatment is simple. Only rust removal, decontamination and fatigue layer removal are needed.

(2) Powder feeding. With argon powder feeding, the requirement of feeding and distributing accuracy is low, and a certain inclination can be obtained. This allows manual operation, which is more suitable for metal repair.

(3) The stability of microbeam plasma is good. The stability of microbeam plasma is good, the formation of molten pool is easy to control, the fusion of dressing material and body is sufficient, and the region is excessive.

(4) The heating and cooling rate is lower than that of laser. The melting state can be maintained for a long time, which is conducive to uniform formation of metallographic structure, better exhaust slag, heating in the process of powder ejection, and protection by argon and ion gas. Therefore, the uniformity of the cladding layer is better, and the defects such as blowhole inclusion slag are less.

(5) Material selection. The plasma heating method has less restriction on materials, wider choice of materials and easier cladding of carbides and oxides.

3. Several Problems in Cladding

(1) About Welding Stress

We must establish a concept that no matter what terms are used (such as welding, surfacing, spray welding, cladding, etc.), they are cast on the metal matrix under heating conditions. From heating to melting and casting to cooling, stress will inevitably occur.

In addition to very special materials, shrinkage stress is generally the most important factor. Different welding methods are different from heating mode, speed, filling material and some other conditions. So, reducing the effect of this stress on the matrix and the casting layer is an important aspect to consider when we pursue the welding quality.

(2) Why the deformation of laser welding (cladding) is small

The main reason is that the casting area is small, the transition area is small and the shrinkage is small. So the shrinkage force produced in the shrinkage process is not enough to make the whole body deform, which is the reason why the so-called laser cladding does not deform (so when the size of the body is too small, it will also produce deformation), which is also the advantage of laser welding (cladding).

So where does this welding stress go? It is mainly released to the casting and transition areas. This raises two questions:

First, the casting zone is prone to cracks, so laser cladding requires high ductility of materials, such as nickel-based powder.

Secondly, the stress in the transition zone is large. Because of the rapid heating and cooling in the laser welding process, the size of the transition zone is too small, which results in the stress concentration in this region, which affects the bonding effect of laser welding (cladding). Especially when the mechanical properties of the matrix and welding material are quite different, the tendency is more serious, and even the phenomenon of falling off occurs. This requires that special attention should be paid to the material and thickness design of the transition layer in laser cladding.

Fourthly, there are three main reasons why plasma cladding (surfacing) is not easy to produce cracks, blowhole and other defects.

First, plasma as a heat source for cladding (surfacing) and submerged arc welding gas welding and other heat more concentrated, ion arc stability is better, there is no electrode melting consumption, heat output is uniform, easy to control, so that the heat distribution in the melting zone is uniform, the material fusion is fully uniform, the exhaust slag is full, the shrinkage stress distribution is uniform.

Secondly, because of the high control precision of plasma equipment, the control of melting and casting zone and transition zone is convenient, and the uniformity is good, the stress distribution is easier to control reasonably.

Third, argon protection does not need various additives, and there are no problems of hydrogen 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 ceramics) and for manufacturing wear-resistant plates, valves, rolls, etc.

V. Technological properties of cladding

About laser cladding and plasma cladding, many peers have published many articles, most of which emphasize the advantages of laser, which is also the goal of everyone. However, most of them evaluate laser from microscopic point of view by metallographic analysis.

But everything has its two sides. Laser cladding also has its disadvantages. There are many limitations in the process. In the actual production, high operational skills are needed, which makes many customers difficult. In my opinion, it is mainly due to the short melting time of the cladding layer caused by the fast heating and cooling that the difference between the outer and inner edges of the spot is large, the formation of the structure is uneven, the distribution of stress is uneven, the exhaust slag is insufficient, the hardness is uneven, and the pore inclusion slag is easy to form. It is difficult to obtain a large area of perfect cladding layer, especially YAG laser. Therefore, laser cladding from material selection to operation should be particularly meticulous.

The input heat of plasma cladding is larger than that of laser cladding, and the deformation of matrix is larger than that of laser cladding. However, it has the advantages of full melting, uniform hardness distribution, exhaust scum thoroughly, wide material selection, easy operation, easy access to a relatively complete overall cladding layer, low cost and good benefit, so it has obvious advantages in large area and large thickness cladding.