By Francisco Aristizabal
There is a rumor going around that there is “high risk” in the spray metal processes for rebuilt/recovered engine hard parts such as blocks, cranks, and others. A misunderstood concept is that a recovered part by these methods has the same mechanical properties as the original component — this is not true. These are processes to only to recondition the surface and not the recovery of mechanical properties such as hardness or fatigue. This process must be avoided in parts subject to punctual and or impact loads.
Different kinds of equipment and materials exist to be used in these operations, and the right selection will depend basically on the understanding of the type of wear, the material of the parts to be recovered, and the filler material used as well. This article looks for an approach to some metal spray processes, features, and some recommendations.
Let’s talk about the different wear types – abrasion, impact, friction, corrosion, erosion, cavitation, and heat. In an engine, most of the components are subject to a minimum of two of these at a time. Their dynamics and effects over the parts are different, no matter if the end the result will be same: wear/lack of material. It is important to be aware that not only friction and heat as a common belief are involved in the engine part running, so as erosion and abrasion are equal.
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The process of applying powder or any coating material at low temperature is known as thermal spraying, metallization, plasma spray, etc., regardless of the type of projection system used. We can define the process as the projection of particles, metallic or not (could be organic or ceramic, too), plasticized or casted/melted, over a cleaned and roughed micro surface.
This process is considered a “low temperature” operation because the part to be recovered will take between 100°C and 250°C as a max temperature, considered as a “stable temperature” avoiding deformations or any other change in the material base. The projection systems are based in the different sources of energy (gas or electrical type) to heat the filler material and the projection against the base material or “substrate” (previously cleaned, roughed and preheated) is through a gas stream (often compressed air).
When molten particles impact the surface, they are crushed, forming little lentils that get stuck to each other with surface irregularities (for these reasons the component must be cleaned and blasted). Then, the lentils cool fast and solidify, creating a structure that grows until the desired measurement is achieved.
The bond between the substrate (material base) and the coating (filler material) can be mechanical, metallurgical, chemical or a combination of these. Sometimes a subsequent heat treatment of the coating increases the resistance of binding to the substrate by diffusion or chemical reaction.
There are a great variety of flame spraying equipment that can use wire, powder, rod or cord as a filler material. Some are low speed projection (subsonic) others are supersonic speed. The speed is a crucial topic and is determinantal in the final product in a spray metal process related to the density and porosity. Higher speed results in a higher density and less porosity (material particles impact stronger and build a better coating).
And finally, the filler material – the right selection, in combination with all the above-mentioned topics is the key. Most are based in alloys Ni Cr Mo, however, the cermet materials (ceramic/metallic) that are being widely used in the automotive industry, are harder and with higher resistance to abrasion, fretting and erosion.
As a summary, the bullet points are:
- Surface preparation deeply cleaned and gross surface roughness (blasting or grinding), cylindrical parts threaded.
- Preheating is necessary up 40°C room temperature.
- Apply the right coating with the selected projection equipment at the right speed.
- Lightly post heat treatment to a stable 200°C through the workpiece to release residual stresses.
What was outlined in this article is intended to serve as a general guide; specific procedures may vary depending on the equipment manufacturer and specific application. Always check with their literature and any other OEM service recommendations. The AERA tech line is available to answer any questions you may have.
Read this article with all images in the digital issue of Engine Professional magazine https://engineprofessional.com/2024EPQ2/#p=94