Thermal Spray in the Fabrication Process
Plasma Powder and Systems, Inc.
One definition of fabrication is “to construct by combining or assembling diverse, typically standardized parts.” A coating, especially one applied using thermal spray, is generally not considered a “standardized part”. Therefore, some may question having thermal spray included in the normal fabrication process.
Yet, thermal spray has advanced from a highly specialized, unregulated process to one that is standardized and able to be integrated into a fabrication shop. Herein are summarized standard thermal spray processes now available to the fabricator and a basic procedure for starting a thermal spray operation is outlined.
First, what can thermal spray do for a fabricator? Think in terms of
Initially, thermal spray operations concentrated on
RESTORING: applying like material on like material to restore the worn or corroded surface of a part. Restoration was primarily achieved with a
flame spray gun using powder or wire feed.
With the ability to mix materials, thermal spray served as a
SHIELD, protecting a substrate from corrosion, erosion, temperature, electrical noise, etc. This includes hard facing of drill-bits, thermal barrier coatings on turbine blades, copper coatings in electronic enclosures, tungsten-carbide coatings on hydro-electric turbines and Teflon coatings on frying pans.
This was augmented with BONDING, applying a coating of material that provides a bond between two different materials. This includes coatings to bond a second coating such as tungsten-carbide to a base metal and special coatings such as hydroxylapatite (HA) for bonding a prosthesis to bone.
More recently, thermal spray has been used for
BUILDING. In this case, multiple passes of a thermal spray gun over a form has been used to construct a form used in injection molding of plastic parts. The ability to install cooling passages into a mold is one benefit of building molds using thermal spray.
Thermal spray is also being used for
DECORATING, providing a wide range of colors and textures for the artist.
One last function thermal spray has been used for is
CONTAINMENT. A thermal spray verification process was developed in which molten glass is sprayed on a lead-based coating. The glass encapsulates the paint and, because of the thermal stresses, falls off upon cooling. The collected waste is vitrified so that it can be disposed of as nonhazardous waste or recycled into value-added products. No containment structure is required, as there are no hazardous effluents produced by this process.
Why is thermal spray now reasonable for a fabrication operation?
Initially, thermal spray was like a teenager, exciting, but sometimes not fully responsible. Early guns needed to be test-fired and recalibrated when any change was made. Many operators had their own spray parameters and mistrusted published parameters from material suppliers. Work-piece manipulators were modified decommissioned lathes and gun controllers were adapted from Sears garage-door openers.
Now, with maturity, many improvements have been made. Standardized precision parts make it possible to rebuild and change guns without having to recalibrate each time. Many coating materials are provided with proven parameters that work and accomplish the desired function. Turntables and lathes designed for the thermal spray industry are available as are many reliable, reasonably priced and easily used
Also, ASM International, formerly known as the American Society for Metals, has developed a certification training program for the thermal spray trade to insure the quality of skills in the industry. Through this program, it is possible for the new user to have some comfort when hiring thermal spray operators.
What procedures should be followed?
FAMILIARIZATION. First, become familiar with the industry. A day or two on the internet should provide ample information on any aspect of this business. The information is certainly at your fingertips.
CONNECTIONS. Become acquainted with the movers and shakers. Avenues to achieve this include:
Attending and participating in technical meetings such as those organized by the International Thermal Spray Association (ITSC). Many of the papers may be beyond the interest of one starting off with thermal spray. However, these meetings can provide a sense as to the direction the industry is moving and new products on the horizon.
Attending trade shows such as Fabtech meetings. Here, activities tend to be more towards the application as opposed to theory. This is also an opportunity to become familiar with ancillary processes that may be needed such as grit-blasting and grinding.
RESOURCES. Potential suppliers should be auditioned to find the best fit for your developing operation. Often, we might think of the large organization with international representation. Here, a problem is often encountered. Large suppliers to the thermal spray industry often adopt a business model that concentrates on the multi-million dollar customers, large aerospace and power generation firms, a smart business move by the large suppliers, but a disadvantage to the fabrication house. The new and small shops often find that they cannot get the attention needed.
Key points that a new thermal spray user should look for in a supplier is:
Does the supplier have knowledge and experience in the specific thermal spray technology of interest (industrial, medical or aerospace)?
Does the supplier have representation reasonably close in order to support your operations?
Does the supplier have or have access to facilities that can be used for trial runs and development?
Does the supplier have a track record of being able to support customer equipment and material needs on a timely basis?
Does your supplier have access to a thermal spray shop that can serve as backup in case problems are encountered with spray operations?
SELECTION. Discussions with your supplier can help in determining which thermal spray process can best provide the desired function.
Thermal Spray operations fall into one of two large categories: combustion and electric.
Under combustion processes, we find flame spraying, either
HVOF. With flame spraying, the flame can be adjusted for oxidizing, reducing or neutral conditions.
For wire flame spraying, materials include steel, aluminum, zinc and copper. These systems are significantly cheaper than arc systems and are also more portable. Powder flame spray has a wider selection of materials. The guns are lighter since a wire-drive is not needed. However, the application rates are lower than for wire flame operations. The HVOF and HVAF processes use more kinetic energy and less thermal energy to produce the coatings. The coatings are dense and therefore ideal for erosion protection.
A fourth combustion process is the D-gun (detonation spraying). Here, a combustible mixture is pulse-detonated by a spark in a chamber providing a pulsing action that heats and propels the feed stock.
Electric thermal spraying processes include arc spraying and plasma spraying. In wire arc spraying, spray rates tend to be higher and energy costs are lower. The higher application temperatures associated with wire arc spraying makes this technique popular for the application of corrosion protection.
For very high temperatures, plasma spraying is often the process chosen. A pressurized inert gas passing through an arc becomes a plasma gas used for both the heating and transfer of the powder being fed.
There are variations on some of the processes, such as the process where the atmospheric pressure of the spray chamber is reduced and also chemically altered. Such processes are specialized and generally not selected for a fabrication operation.
EVALUATION. Having selected the process of choice, the next step is to determine the cost, at least on a budget basis. This is more than the price of equipment. A fabricator needs to be modest with the initial vision. A number of possibilities have never come to fruition because a goal of putting in a fully automated line far exceeded any benefit that would be realized. In one case, a power plant owner wanted to achieve self-sufficiency with respect to maintenance of gas turbines. The assumption was that one could buy a turn-key operation, train workers and be in business coating turbine parts. It really was unrealistic and never succeeded. It would have been best to start modestly, spraying only stationary parts before taking on the more demanding rotation parts, where a set of turbine blades can have a value as high as $5 million.
A shop that is looking to invest in thermal spray may already have a good start in meeting that objective. If a welding operation already exists, there probably is a familiarity with containment of radiation, contamination, noise and fumes along with material accountability and disposal regulations.
A qualified supplier should be able to assist with a cost/benefit analysis. Costs that need to be evaluated include:
1) Price of the basic equipment. A thermal spray operation for a first time user could start with just a hand-held gas gun for use outside, for example, with bridge structures. However, it would not be unreasonable to consider a fully integrated internal facility consisting of:
a) Spray System
b) Acoustical Room
c) Dust Collector
2) Price of the ancillary equipment. This includes any equipment that may be required but is not presently available in the shop. It may include:
a) Grit Blast System
b) Masking Station
3) Cost of
installation. This can be significant, especially if permits are required. One rule-of-thumb is to estimate the installation cost as 30% of the equipment cost.
4) Area for installation. What is the anticipated footprint? This needs to include an area for staging parts into and out of the spray operation.
5) Cost of manpower. Included is the training of operators on the operation of the spray units, manipulators and dust collectors.
6) Finally, a cost/benefit analysis based on a projected production. For this, you would need to have a good anticipation of what is required now and in the immediate future.
Using a well structured analysis, a fabricator should be able to prove the benefits of introducing thermal spray into their operations and be confident in the process selected.