I. Features of laser cladding
1. Technical resources
The most important feature of laser cladding is concentrated heat, rapid heating, rapid cooling and small heat-affected zone. It has incomparable characteristics for fusing between different materials.
This special heating and cooling process results in a structure in the molten area that is different from other coating methods (such as spray welding, plating, common welding, etc.) and can even produce amorphous structures, especially in the case of pulsed lasers. .
This is the so-called reason for laser cladding without annealing and deformation. But I believe this is only from the macro perspective of the entire workpiece. When you perform a microscopic analysis of the coating layer and the heat-affected zone, you will see another scene.
2. Equipment features
Currently, there are two types of machines used in laser cladding in China: CO2 lasers and YAG lasers. The first has continuous output with coating power generally above 3KW, while the YAG laser has pulsed output, generally around 600W.
For equipment, it is difficult for general users to fully understand, and it largely depends on the producer's service. The purchase price is expensive, the maintenance cost and parts price are high.
Furthermore, the stability and durability of the equipment often lag behind foreign counterparts.
Therefore, laser cladding machines are generally used in special fields, and it is difficult to be economical in general industrial manufacturing and maintenance fields.
3. Process features
(1) Preliminary treatment
For laser cladding, it is generally only necessary to polish the part, remove oil and rust, remove the fatigue layer, etc., which is relatively simple.
(2) Powder feed
CO2 lasers have higher power and typically use argon for powder feeding; YAG lasers have lower power and typically use a natural powder falling method.
These two methods basically form a weld pool in a horizontal position during coating. If the slope is a little higher, the powder cannot be distributed normally, limiting the use of lasers, especially YAG lasers.
(3) From the point of view of the molten pool formation state
Due to the laser's high control accuracy, constant output power and no arc contact, the size and depth of the molten pool are consistent.
(4) Rapid heating and cooling
This affects the uniformity of metal phase formation and is also detrimental to exhaust slag removal. This is also an important reason for the formation of pores and uneven hardness in the laser coating, especially in the case of YAG lasers, which are more severe.
(5) Material selection
Due to the different absorption capabilities of different materials for lasers of different wavelengths, the choice of materials for laser cladding is quite restricted. Lasers are better suited to some materials, such as nickel-based self-fusing alloys, and coating with carbides and oxides is more difficult.
II. Features of Microbeam Plasma Coating
1. Technical resources
The plasma beam used in the microbeam plasma cladding machine is an ionization arc, which is more concentrated than the arc welding machine, so its heating speed is faster.
To obtain a more concentrated plasma beam, high compression ratio aperture and low current are generally used to control the base temperature to become too high and avoid backfire deformation.
Of course, this cannot be compared to the heating speed of a YAG laser. Because the plasma arc runs continuously, the machine cools relatively slowly, forming a transition zone deeper than the laser cladding. This results in better stress release for hardfacing materials.
2. Equipment features
Microbeam plasma cladding equipment is developed on the basis of DC welding machines.
Its power source, spray gun, powder feeder and oscillator have low technical limit, are easy to manufacture, reliable, simple to maintain and use, consume less electricity, have low usage costs, good versatility, low production costs, good adaptability, and are easy to scale up for production, offering significant benefits.
It has low environmental requirements and wide adaptability of materials.
With the advancement of electrical technology, the level of welding technology in our country has sufficient support. Furthermore, the equipment is small in size and weight, and the welding gun can be held manually, making it more flexible and convenient to use. The cost of auxiliary tools is also low.
3. Process features
(1) Preliminary treatment is simple
Only rust removal, degreasing and fatigue layer removal are required.
(2) Powdered Feed
Argon gas is used for powder feeding. Feeding accuracy requirements are low and a certain degree of skew is permitted. This allows manual operation, suitable for metal repairs.
(3) Microbeam plasma has good stability
The stability of microbeam plasma is good and the molten pool formation is easy to control. The filling material and machine body are fully fused, and the transition area is good.
(4) Heating and cooling speeds are lower than laser
The molten state is maintained for a long time, which favors the uniform formation of metallurgical structures. Exhaust slag is better. The powder is heated during the spraying process and is protected by argon gas and ionized air.
Therefore, the uniformity of the overlay layer is better and there are fewer defects such as pores and inclusions.
(5) Material Selection
The plasma heating method has fewer restrictions on material choice, offering a wider selection and is easier to overlay carbides and oxides.
III. Various overlay issues
1. About welding stress
We must establish a concept that regardless of the terminology used (welding, coating, thermal spraying, overlapping, etc.), it is casting on a metal substrate under heating.
Therefore, stress inevitably occurs in the process from heating to casting and then to cooling.
Apart from very specific materials, the most significant influence is usually shrinkage stress. Different welding methods vary in heating methods, speed, filler materials and some other conditions.
Therefore, minimizing the impact of this stress on the substrate and molten layer is an important aspect that we consider when looking for welding quality.
I believe contraction stress is inevitable. Therefore, the key to solving the welding stress problem is stress relief. In other words, where this shrinkage stress is relieved and how the stress is distributed from the substrate to the casting area is a problem we need and can solve.
2. Why is there less deformation in laser welding (overlap)?
The main reasons are small casting area, small transition area and minimal shrinkage.
Therefore, the contraction force generated during the material contraction process is not enough to deform the entire machine body.
This is the reason why laser overlay does not deform (therefore, when the size of the machine body is too small, deformation may still occur), and it is also an advantage of laser welding (overlay).
So where does this welding voltage go? It is mainly released to the casting area and transition area. Therefore, two problems arise:
First, the casting area is prone to cracking. Therefore, laser overlay requires high ductility of material such as nickel-based powder.
Secondly, the transition zone presents great stress. Due to the rapid heating and cooling in the laser welding process, the size of the transition zone is very small, causing stress concentration in this area, which affects the bonding effect of laser welding (overlap).
Especially when there is a significant difference in mechanical properties between the base material and the welding material, the tendency is more severe, and delamination may even occur. This requires special attention to material design and transition layer thickness during laser overlay.
3. Why doesn't plasma cladding (welding overlay) easily produce defects such as cracks and pores?
There are three main reasons:
One is that plasma as a heat source for cladding (welding overlap) is more concentrated than submerged arc welding, gas shielded welding, etc.
The stability of the plasma arc is better, there is no electrode consumption, the output heat is uniform and it is easy to control.
This results in uniform heat distribution in the casting area, complete and uniform melting of the material, sufficient exhaust slag and uniform distribution of shrinkage stress.
Secondly, due to the high control precision of plasma equipment, it is convenient to control the casting area and transition area, the uniformity is good, and the voltage distribution is easier to control reasonably.
Third, the use of argon gas protection does not require various additives, and there are no problems such as hydrogen scavenging and oxidation.
Therefore, plasma cladding (weld overlay) is more suitable for large-area, thick, high-quality hard surface casting (such as high-manganese, high-chromium ceramic materials, etc.), suitable for manufacturing wear-resistant plates, valves, rollers, etc.
4. Coating Processability
Regarding laser cladding and plasma cladding, many peers have published various articles, most of which emphasize the advantages of lasers, which is also the goal pursued by everyone.
However, the majority are evaluated using the metallographic analysis method from a microscopic perspective.
Everything has two sides and laser cladding also has its disadvantages.
There are many limitations in the process and higher operational skills are required in actual production, which causes difficulties for many customers.
I believe that this is mainly due to the short melting time of the coating layer caused by rapid heating and cooling, resulting in a large difference between the outer and inner edges of the site, the formation of uneven organization, uneven stress distribution, insufficient exhaust slag , resulting in uneven hardness, easy pore formation and slag inclusion problems, it is difficult to obtain a large-area perfect coating layer, especially for YAG laser.
Therefore, laser cladding must be particularly careful from material selection to operation.
Compared with laser, plasma coating provides more heat and the substrate deformation is greater than that of laser.
But its melting is sufficient, the hardness distribution is uniform, the exhaust slag is complete, the material selection range is wide, easy to operate, and it is easy to obtain a relatively intact overall coating layer, low cost, good benefits, Therefore, it has obvious advantages in large areas, thick coating.
