Plasma Transferred Arc (PTA)
The plasma transferred arc (PTA) weld hardfacing process was developed to produce high quality weld overlays with relatively low heat input and very low dilution of the substrate into the weld overlay.
The PTA process is essentially a hybrid process containing elements of thermal spray (i.e. powder consumable melted in a continuous plasma heat source) and welding (i.e. melting of and the formation of a metallurgical bond with the substrate). The torch manipulation is generally automated and hence can achieve uniform overlays on complex parts. The PTA process is typically used to apply relatively thick (i.e. 2-3 mm) wear and/or corrosion resistant cobalt (Stellite and Triballoy ) and nickel (Inconel 625 ) alloys. Stellite 6 is a well know overlay in the pump and valve industry. It is also possible to apply tungsten carbide containing nickel based overlay, although the high melt-pool temperature places limitations on carbide size, volume fraction and binder alloy composition.
The major disadvantage of PTA is that it is essentially a welding process with relatively high heat input into the base material compared to thermal spray and laser cladding, resulting in potential distortion issues and the possible need for post-weld heat treatment. Because PTA is a welding process the weldability of the base material also needs to be considered.
Weld overlay refers to the application of a welding process to deposit one or more layers of metal with specific characteristics onto the surface of a component. This is done to improve desirable properties, such as corrosion or wear resistance, or to restore original dimensions. The procedure involves the deposition of several weld beads side by side which results in the formation of a continuous surface layer.
Weld cladding generally refers to the deposition of corrosion resistant layers. Weld hardfacing refers to the deposition of a weld matrix with the addition of wear resistant particles.
Since a metallurgical bond is formed between the overlay and substrate, the technique is suitable for high stress and/or impact environments. Another advantage is that the overlay can be relatively thick.
Spray and Fuse/Spray-Fuse Hardfacing
During Spray and Fuse/Spray-Fuse Hardfacing, a low melting point nickel or cobalt alloy is heated, deposited and fused onto a substrate. Spray and fuse is distinguished from spray-fuse by the fact that the depostion and fuse occur simultaneously during spray-fuse, whilst during spray and fuse, a powder is deposited to the required thickness before fusing either in a furnace or with a flame in a subsequent step. The distinction between these processes and welding is that spray and fuse acts more like a braze than a weld and generally does not dilute with the base metal.
Spray and Fuse
The original powder flame spray processes developed towards the middle of the previous century were not able to produce coatings with sufficient density, cohesion and adhesion to make them suitable for use in corrosive or high-wear applications. To overcome these limitations a special range of nickel-chrome and cobalt-chrome alloys were developed which allowed for post-spray densification and fusing of the coating. This was made possible by the addition of alloying el:w ements such as boron, iron and silicon, which significantly reduced the melting point of the coating alloy, making it possible to melt the coating during post-coating processing at temperatures below the melting point of the substrate. The post-coating heat treatment, which involves the heating of the part using one of several techniques, such as high energy gas flame or torch, induction of vacuum, inert of hydrogen furnaces, results in the densification of the coating and the formation of a metallurgical bond (i.e. fusing) to the substrate. Using this technique it is possible to produce nickel-chrome alloys with a hardness of up to 60HRC, but even harder coatings have been developed through the addition of tungsten carbide particles into the coating alloy.
In the spray-fuse process, the powder gun and torch are combined. Powder is introduced into a torch flame and sprayed in a semi-molten state onto the preheated part, for fusion. Bonding is achieved by diffusion of the alloys into the base metal. Bonding of the coating alloy and base metal is similar to brazing, where a liquid phase is linked with a solid phase by diffusion.
The spray and fuse/spray-fuse processes produce dense corrosion resistant coatings of between 0.5 and 1.5 mm with good wear resistance and damage tolerance. The main disadvantage of these coatings is the fact that the coating's hardness and thickness is somewhat limited compared to other competing technologies (e.g. PTA and laser), and because of the high temperature processing there is a limitation on suitable substrate materials.