Look at almost any coated surface and you are most likely looking at the result of a thermal spraying technique. Flame Spraying is one of the Thermal Spraying processes. The concept of Thermal spraying has been around for many of years and the process continues to be evolved and improved. New applications for the old process develop constantly and it’s a bet that Flame Spraying will be around for many years to come. A testament to the Flame Spraying process’s staying power is its fundamental principle, its long history, great benefits and continual technological advances.
Flame spraying is a process of coating a substrate by using combustion of a fuel gas (usually propane or acetylene) with compressed air or oxygen to melt a wire, rod, or powder that’s fed into a spray gun. It is heated and propelled on a prepared surface at a rate of about 10 to 20 meters per second. The particles that hit the surface flatten and conform to the surface forming a coating. Amount of layers required, temperature and speed of particles, the distribution of particles, the type of feedstock used and substrate material are factors that contribute to the quality of the end product. If several layers are required as such in metals and alloys and the under layer is too thick it could result overheating and cracking. It is better to lay out less per pass than more. The Adhesion of particles to the substrate depends on the substrate and coating material, temperature and kinetic energy of droplets.
Flame Spraying is the oldest of the thermal spraying processes and the concept was developed by Max Ulrich of Switzerland in 1910. It wasn’t until two years later that he developed a device based on the principle that if a metal rod were fed into an intense steady flame that it would melt and if encompassed by streaming compressed gas then the molten metal would become atomized and easily propelled onto a surface to create a coating. This process was originally called the “metallizing” or the “schoop process” and Max later sold his rights to the process to a German company named Metallizer. In the United States two companies were first to adopt the technology they were Metal Coatings Company and Metalworld. Early applications included coating railroad tank cars and navy ship boats. It wasn’t until the end of the Second World War that “metalizing” was a major industrial process. It is now used is numerous industries including but not limited to: Aerospace, Automotive, Marine, Hydraulics, textile, Oil & petroleum, Biomedical, Pumping, Cement & coal, Power and Electrical.
Flame Spraying has become so popular because of its many advantages over other coating processes. The main advantages are that is safer, cheaper and more cost efficient compared to other coating methods. Repairing parts using flame spraying coatings cost less than replacing with new parts. An example of this is worn bearing areas that can be reformed and substantiated with steel or bronze alloys. The coatings have lubricants absorbed into them so this enhances the performance of the bearing.
Flame Spraying offers the most tolerable work environments and ease of use since there is minimal dust and fume levels compared to arc spraying. These coatings are also good for corrosion and heat resistance as well as insulation.
The advantages of Flame spraying cannot be discussed without touching on the risks associated with it as well. All the thermal spraying methods produce dust and fumes therefore adequate ventilation and breathing equipment is crucial. If there is no means of extraction of the dust it can build up and can lead to fire and/or explosion. It is important to keep the dust from getting damp since that could result in an evolution of hydrogen gas. Rotating equipment produces static charges that should also be avoided since friction is a source of ignition.
There has been much advancement in the process since it invention in 1910. Some recent developments include high speed flame spraying. These high velocity processes use internal combustion of the oxygen gases to create higher gas velocities resulting in higher particle velocity. The benefits of the higher particle velocities are its improved adhesive properties in the final coating produced.
Another advancement includes the concentration on increasing the quality of feedstock powders creating new ergonomically-produced metal alloys and ceramic formulations. These new process innovations allow us to apply more wear resistant materials onto less wear resistant parts.
Flame Spraying is truly a lasting invention. It has evolved and contributed to how we improve materials over the decades from the WWII to the present day. It not only is cost effective but durable and versatile. Many different factors go into the flame spraying process any of which can be altered to develop a different product. It will be interesting to see what the next innovation to flame spraying will be.
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