Thermal Spraying & PTA

Powder flame spraying

Flame spraying is one of a number of thermal coating processes. In powder flame spraying, the spray material, in powder form, is melted with an oxy-fuel gas flame, accelerated towards a component by the combustion gases and sprayed onto the surface of the component.

Metallic, oxide ceramic, carbide and plastic powders can be processed using spray guns specifically designed for those materials. Spray guns that frequently take the form of manual torches, preferably using acetylene as a fuel gas because of its high flame temperature, are chosen for metallic alloys based on nickel, iron or cobalt. The powder particles, which are partially melted by the flame, deform on impact with the surface of the component and are deposited there to form a spray coating with a lamellar structure. The main areas of application for thermal coatings are corrosion protection and wear protection.

Powder flame spraying may be subdivided into cold and hot processes. In cold processes, the powders are only applied by the spray gun, and the spray coating is not subjected to any subsequent thermal treatment. UTP has, accordingly, designated these powders as COLDmelt™. The coatings typically have a porosity between  5 and 15%, depending on the process used.

In hot processes, metal powders of materials known as self-fluxing alloys, based on Ni-B-Si, are employed.

The layer that has been sprayed on is melted by a subsequent thermal treatment. The resulting thermal compaction makes it possible to obtain coatings that are virtually free of pores. Depending on the  application, two processes have emerged for this: simultaneous and subsequent melting.

UTP Maintenance has designated these powders according to the process used.

Rotationally symmetrical parts are frequently coated using a two-stage process (subsequent melting), while a single-stage process (simultaneous melting) is often used for surfaces and edges.

Plasma powder surfacing (PPS/PTA)

Plasma powder surfacing (PPS), also known as the plasma transferred arc (PTA) process, is a thermal coating process. In contrast with the spraying processes, this method is a welding process and so involves metallurgical bonding of the applied material to the base material.

However, if the parameters are set optimally, the degree to which it blends with the base material can be reduced to a minimum. The PTA process is employed primarily for the surfacing of wear resistant and corrosion resistant coatings onto a base material.

The process is characterised by the use of two separately controllable electric arcs. One of these is the (non-transferred) pilot arc. This arc is formed between the non-melting (tungsten) electrode and the plasma nozzle. It accelerates the plasma gas and enables ignition of the (transferred) main arc. This arc burns with a high energy density between the electrode and the workpiece. With the aid of the electric arc, both the base material and the metal powder that serves as the welding consumable are fused together, which then gives rise to the deposited protective coating. Ar, H2, He, or mixtures of gas are employed as a processing gas. This serves, firstly, as a plasma gas and, secondly, as a shielding gas and as a carrier gas for the powder.

Because of its high degree of automation, the PTA process is clearly most suitable for series parts and offers advantages here with regard to:

• High reproducibility
• Low degree of dilution with base material
• Small concentrated heat-affected zone
• High surfacing rates possible
• Alloy multiplicity in powder form
• Material combinations with hard substances

UTP offers these PTA powders as nickel-based, cobalt-based and iron-based alloys. The powders are designated PLASweld™ in keeping with the process for which they are intended. The grain sizes should be chosen according to the type of system; powder grain sizes between 150 and 210 µm are selected for the standard range of PTA.

Another method of using metal powder as a welding consumable is provided by laser powder surfacing. Here, a laser serves as the source of heat for partially melting the surface of the workpiece and fusing the welding consumable in powder form. The high-energy focus of the laser allows precisely targeted surfacing, which makes it possible to provide wear protection at specific places without negatively affecting (e.g. through a high heat input) the properties of the rest of the component. 

Because the coating thicknesses are usually small and the processing times short for laser powder surfacing, PLASweld™ powder of a finer grain size, typically 45-106 µm, can be used here.

Arc spraying with flux-cored wire process

UTP Maintenance offers a high quality range of powders for plasma transfer applications. Our long term plasma transfer experience enables us to provide the best tailor-made powders for several industries as well as for distributors.

Arc Spraying is the highest productivity thermal spraying process. A DC electric arc is struck between two continuous consumable wire electrodes that form the spray material. Compressed gas (usually air) atomizes the molten spray material into fine droplets and propels them towards the substrate. The process is simple to operate and can be used either manually or in an automated manner.