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Overview Of Fusing Methods Of Nickel Self-fluxing Alloys
Jul 17, 2018

In the thermal spray field, nickel self-fluxing alloys have gained wide applications. One of the main reason is it can be melted while or after the spraying process, which improved the coating quality a lot, both on the bonding strength and also the coating density etc. Here the different fusing process will be discussed, hopefully can help you optimize your coating process.
Starting with the simple torch spray-fusing, now we have already a cluster of process to do the fusing:
    One-step fusing by flame torch
    Two-step fusing by torches
    Two-step fusing by ring-torch
    Two-step fusing by induction heating
    Fusion in funace
    Centirfugal Casting
    Plasma Trasfered Arc
    High-speed Laser cladding

One-step fusing by flame torch:
    This process has been existed more than 60 years, is the oldest while still the most versatile process. It need to be applied by skilled operator, and typically used for small parts, repair etc. The quality is highly relied on the operator. As you can see, it is very flexible, and with very limited invest.

Two-step fusing by torches:
    For all the 2-step process, they need first to apply the coating to the substrate, either by flame spray, HVOF, or even by arc. Also for all the 2-step process, due to spraying and fusing are 2 separate process, typically for big parts, so you can normally expect the coating thickness is evenly distributed. While the coating before fusion normally have some porosity and the bonding strength is not good enough.
    Since the coating quality is not good enough, so subsequent fusing is necessary. Fusing by torches is also very popular for quite some time, here different torches are used for achieving the eutectic temperature. And depending on the size of the parts, typically 2-5 torches can be adopted. The parts are slowly heated to the eutectic point when the coating surface show a glassy surface. The advantage with the torches include due to the blowing of the torch, then the slag is easy to move to the surface of the coating.

Two-step fusing by ring-torch
    This fusing method is typically for fusing of rollers. In this process, a ring torch with many small torches evenly distributed on the ring is applied. While for this process, the idea is good, while during actual operation, problems with some torches will cause big trouble, and also one ring can only be used for certain size rollers. So this process is not recommended although it is still used some cases.

Two-step fusing by induction heating
    This process is developed about 20 years ago and now it is widely accepted by customers, especially in the steel industry. The good thing is the heating is focused on the surface, where we want to use it to melt the coating, and it is also very easy to control. The frequency, No. of rings are a few critical factors to consider if you are thinking to adopt this process. This process also requires powder with consistent quality.

Fusing in furnace
For the coating to be fused in furnace, you can either use the former mentioned process to apply a typical coating, or you can also choose preforms (cloth) or use a paste to apply a coating, just like in many brazing processes. Actually the nickel-self-fluxing alloy is also one brazing alloy. After we have get the coating on the surface, then you can put the parts in the furnace, and heating at certain temperature graph. Here again the powder property need to be consistent and you can fusing/heat-treat a big number of parts at ease, of course this will depending on your furnace size. This process is widely used for guiding rollers etc.

Centifugal casting:
This is the popular process for plastic machinery. And it is only applicable for internal circular coatings. For this process, Given amount of powders are feeded into the pipe to be coated, then both side are covered while only leaving exit-holes. Then it is rotated and heated to the eutectic point of the powder. With the centrifugal effect the powders become fluid and stick to the pipe and become the coating. This is unique while very effective process.

Plasma Transfered Arc (PTA)
This is actually another version of one-step fusing, but with plasma arc as heating source. Due to the usage of plasma, it is very easy to control, while the disadvantage is due to the pass connection area, this may cause small coating quality variation and not accepted in some cases. Typical coating thickness for PTA is 1-2mm/pass.

High-speed laser cladding
This is a brand new process developed in recent years. In this process, the laser is using as heat source. Differ from standard laser cladding, the laser is focused just before the substrate and melt the powder here, which makes it idea for making thin coatings like 0.2-0.3mm. And the coating efficiency is also highly improved compared with conventional laser cladding while keep a metallurgical bond with substrate. This process will be idea for replace hard chrome plating and also many HVOF coatings.

For more info, you can contact: Antonio Fu

A valve fused by flame torch

A valve fused by flame torch.jpeg

Induction fusing of steel rollers

Induction fusing of steel rollers.jpg

 High speed laser cladding

 High speed laser cladding.jpg

Typical PTA appearance (As coated)

Typical PTA appearance (As coated).jpg

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