Molten Metal FlameSpray
molten metal flame spraying, a thermal spraying process variation
in which the metallic material to be sprayed is in the molten
It has long been recognized that fluids may be broken up into very fine particles by a stream of high velocity gas emanating from a nozzle. Early experiments using this atomizing approach appear to have been directed at producing metallic powders rather than coatings. It was left to Schoop to appreciate the possibility that a stream of metallic particles, formed from a molten source, could produce a coating. Myth has it that Schoop developed the concept when playing "soldiers" with his son and observing the deformation of lead pellets being fired from a toy cannon against a brick wall. Whatever the rationale, it can be stated that the pioneer work of Schoop resulted in the discovery and development of metal spraying and subsequently the "Thermal Spray Process".
The first spray technique developed by Schoop was the outcome of experiments in which molten metal was poured into a stream of high velocity gases. Schoop's apparatus consisted of a compressor supplying air to a heated helical tube. The heated air was used to pressurize a crucible filled with molten metal and eject it out as a fine spray that would adhere to a suitable surface. This system was bulky, primitive and inefficient; however, the concept did lead to the development of portable and user friendly equipment.
There are no further accounts of molten metal spraying by Schoop, it appears that his efforts were directed at developing and improving powder and wire flame spraying. However, work by others continued as a 1924 Dutch patent, describing equipment for spraying low melting point metals, was granted to Jung and Versteeg (2). Mellowes Ltd commercialized the process in the UK. Their system consisted of a gun, a furnace, an air compressor and a fuel supply. The gun had many air and gas valves, a heating chamber (burner), nozzle, handle and a melting pot. The pot was bulky having the ability to store 1.8 kg (4 lb) of molten lead. The pot sat atop the heating chamber, which was similar in construction to a Bunsen burner. Compressed air, fed to the burner, intensified the flame. The handle jutted out and downward from the pot; it was insulated using wood and asbestos. Metal exited the pot through a front orifice where it was directed into a nozzle. Compressed air surrounded the nozzle, atomizing the molten metal and propelling it to the surface to be coated.
The molten metal process has advantages and disadvantages. Advantages include: cheap raw materials; use of inexpensive gases; and, gun design is very basic. Noteworthy disadvantages are: gun is cumbersome to use in the manual mode, can only be held in a horizontal plane; high maintenance due to high temperature oxidation and molten metal corrosion; and, useful only with low melting temperature metals.
Uses for the molten metal thermal spray process include the fabrication of molds, masks and forms for the plastics industry, using low melting point bismuth based alloys (the Cerro family of alloys); the deposition of solder alloys to joints that would be coalesced using torches or ovens; and, the production of metal powders.
My gratitude is extended to Dr. Richard Knight, Drexel University,
Philadelphia, PA for providing information relative to the "Cold
Ceramic Rod Flame Spraying
ceramic rod flame spraying, a spraying process in which material
to be sprayed is in ceramic rod form.(1)
cold spray, a kinetic spray process utilizing supersonic jets
of compressed gas to accelerate near-room temperature powder particles
at ultra high velocities. The unmelted particles, traveling at
speeds between 500 to 1,500 m/sec plastically deform and consolidate
on impact with their substrate to create a coating. (1)
A mention is made of "Cold Spray" although the name seems to contradict the concept of "Thermal (heat) Spraying"; however, the method has garnered significant research interest over the last five years. Developed in the former Soviet Union in the mid 1980s by Papyrin (4), the process is, however, now being commercialized in both Europe and the United States. A typical system is shown above.
The basis of the cold spray process is the gas-dynamic acceleration of particulates to supersonic velocities (300-1200 m/sec-1), and hence high kinetic energies, so that solid-state plastic deformation and fusion occur on impact to produce dense coatings without the feedstock material being significantly heated. This is achieved using convergent-divergent, de Laval nozzles, high pressures (up to 500 psi [3.5 MPa]) and flow rates (up to 90 m3/hr) of gases such as helium or nitrogen. The gases are pre-heated to about 800°C (1472°F), or below the melting point of many metals, to increase the velocity. Pre-heating also aids in particle deformation. The spray pattern is roughly 20 to 60 mm2 (0.031 to 0.093 sq in.); spray rates - 3-5 kg/hr (6.5 to 11 lb/hr), with build ups of about 250 µm (10 mils) per pass and DEs of 70 wt %. Feedstock particle sizes are typically of the order of 1-50 µm.
The advantage of cold spray versus the "hot" spray processes, which melt or soften the feedstock, is a significantly reduced level of coating oxidation. Electrical conductivity of cold sprayed copper has been reported at about 90% of wrought material - a significant increase over the <50% typical for other sprayed copper deposits.Cold spray coatings also exhibit improved adhesion, reduced material loss by vaporization, low gas entrapment, insignificant grain growth and recrystallization, low residual stress, phase and compositional stability, reduced masking requirements and improved surface finishes (5) (6).
Cold spray, owing to its principle of impact-fusion coating build-up, is limited to the deposition of ductile metals and alloys (Zn, Sn, Ag, Cu, Al, Ti, Nb, Mo, NiCr, Cu-Al, nickel alloys and MCrAlYs) and polymers, or blends of >50 vol % ductile materials with brittle metals or ceramics. The absence of a heated jet also yields a low heat input to the substrate.
Obvious disadvantages to the cold spray process include the use of high gas flows, increased gas costs, especially in the case of helium, recycling would be needed. Consequently, lower cost gases as nitrogen are being investigated as alternatives. Also, high gas pressures have required the development and modification of powder feeders. Solid materials traveling at high velocities are abrasive, so the lifetime and dimensional stability of key components are emphasized. Nozzle lifetimes in excess of 100 hours have been reported (7).
Applications for cold spray coatings include corrosion protection, where the absence of process-induced oxidation may offer improved performance; deposition of electrical conductors and solders; and, the application of metallic coatings to ceramic and glass substrates.
Detonation Flame Spraying
ceramic rod flame spraying, a spraying process in which material
to be sprayed is in ceramic rod form.(1)
Electric Arc Spraying
electric arc spraying, a thermal spray process in which an arc is
struck between two consumable electrodes of a coating material.
Compressed gas is used to atomize and propel the material to the
As early as 1914, Schoop in collaboration with Bauerlin (2), an electrical engineer, experimented with electrical heating for spraying. Initial attempts were unsuccessful as they attempted to tailor their spray apparatus on the lines of molten metal equipment rather than wire. One pole was a graphite crucible, loaded with the consumable, the other a carbon rod. An arc was struck between the crucible and the rod causing the metallic consumable to melt and flow through an orifice. On exiting, the molten metal was atomized by jets of compressed gas. Eventually, a device was built utilizing two wires, insulated from each other, made to advance and intersect at some point. Generally, the wires were given a difference of electrical potential of about 89 V that caused the wires to melt and; in the presence of a gas stream, spraying was produced. Later guns, developed by Schoop, do not radically differ from those used today.
Electric arc spraying has the advantage of not requiring the use of oxygen and/or a combustible gas; it has demonstrated the ability to process metals at high spray rates; and is, in many cases, less expensive to operate than either plasma and/or wire flame spraying. "Pseudo" alloy coatings, or those constructed by simultaneously feeding two different materials, are readily fabricated. An example would be copper-tin coatings constructed by feeding pure copper and tin wires into the arc to produce a heterogeneous mixture of each in the coating. Also, the introduction of cored wires has enabled the deposition of complex alloys (such as MCrAlYs) as well as carbide-containing metal alloys that were only attainable using powdered materials as feedstock. Some materials produce "self-bonding" coatings that are sprayed in a "superheated" condition. The overheated, hot particles tend to weld to many surfaces thereby increasing the coatings' adhesive strength.
High Velocity Oxy/Fuel Spraying (HVOF)
Nontransferred Plasma Arc Spraying
plasma spraying, a thermal spray process in which a nontransterred arc is a source of heat that ionizes a gas which melts the coating material and propels it to the workpiece.(1)
If a gas is heated above 5,000°C (9,032°F)
chemical bonds are broken down and its atoms undergo violent random
movements. This results in atomic collisions that cause some electrons
to become detached from their nuclei. Electrons are the negatively
charged constituents of atoms; so having lost an electron the heavier
nuclei, with any remaining electrons, become positively charged.
When a gas undergoes this disruption it is said to be ionized and
the cloud it has become is identified as plasma. Its behavior involves
complex interactions between electromagnetic and mechanical forces.
Plasma is present in any e
Plasmas have been known for a considerable time. In commercial technology they are considered as hot streams of particles attaining temperatures greater than 10,000°C (18,032°F). Today's plasma spray guns are sufficiently robust to produce temperatures from 5,000°C (9,032°F) to 16,000°C (28,832°F) for long periods. These guns are referred to as "nontransferred arc plasma generators". The generator is essentially an electric arc working in a constricted space. Two electrodes, front (anode) and rear (cathode), are contained in a chamber, as is the arc through which the effluent (the operating gas) passes, a concept developed by H. Gerdien (2) of Germany in the1920s. However, at that time, it was afforded little interest, as there was no apparent need for such high temperatures. The advent of the space age changed this and workable systems were commercially introduced in the 1950s.
Plasma generators work on the concept that if sufficient voltage is applied to two electrodes, separated by a small gap, the difference in potential at that moment causes electrons to be extracted from the cathode. The electrons accelerate and speed toward the anode. If a gas is inserted in the gap between the two electrodes, its atoms will collide with the ensuing electrons and themselves, causing more electrons to detach and travel towards the anode. Meanwhile, the nuclei stripped of their electrons, and positively charged, move to the cathode. Thereby, the gas in the gap has been ionized, becoming electrically conductive - a plasma arc; it exits through an orifice in the anode as a plasma stream, containing only electrons and ionized gas is formed (7). Meanwhile the issuing plasma stream, reaching temperatures exceeding 9,000°C (16,232°F), begins to cool and the once ionized gas begins to recombine.
Most commercial plasma guns are fundamentally simple in design, consisting of a chamber and front nozzle (anode) in which there is an orifice. The chamber and nozzle are water-cooled. At the rear of the chamber is nother electrode, also water-cooled. This rear electrode is non-consumable and is fashioned from thoriated tungsten (see graphic above). A port, somewhere within the chamber, allows the high-pressure plasma forming gas, or gases, to enter. A high-frequency spark initiates operation and is discontinued upon ignition. It should be noted that the high-pressure gas cools the outer layer of the plasma arc so extreme heat is kept away from the nozzle bore.
Typical plasma forming gases include argon, nitrogen,
hydrogen and helium. They may be used either alone or in combination:
viz, argon-hydrogen, argon-helium, nitrogen-hydrogen, etc. Argon
and nitrogen are generally utilized as primary plasma gases and
hydrogen is favored as a secondary as it aids in producing a "hotter"
plasma. Nitrogen is less expensive than argon so, based on economics,
is more widely used than argon. Helium tends to expand the plasma
and when used in combination with argon produces a "high velocity
plasma" that exits the nozzle at about 488 m/sec (1,600 ft/sec).
Argon/hydrogen and nitrogen/hydrogen exit velocities have been measured
at roughly 366 m/sec (1,200 ft/sec). As most plasma guns are designed
to spray powders, the powder is introduced through an external port
at the nozzle orifice. Hardware is also available for injecting
powder internally upstream into the nozzle bore. The primaryplasma
forming gas is usually used as a carrier to transport the powder
to the plasma stream.
Powder Flame Spray
Powder flame spraying, a thermal spray
process in which the material to be sprayed is in powder form.
This concept was developed by Fritz
Schori(2) in the early 1930's. However, the amount of
powder that can be supported by a gas stream depends on many factors
including powder characteristics. If air is not used then the density
of the supporting gas influences the feed rate and, for any particular
powder there is an optimum amount that can be carried in a gaseous
stream. It depends upon the velocity and volume of the gases used.
The usefulness and criticality of flowmeters and pressure gauges
are governing factors.
(1) Frank J. Hermanek, Thermal Spray Terminology and Company Origins,
First Printing, 2001, ASM International, Materials Park, Ohio
RF Plasma Spraying
RF plasma, a system in which the torch is a water-cooled, high frequency
induction coil surrounding a gas stream. On ignition a conductive
load is produced within the induction coil, which couples to the
gas, ionizing it to produce a plasma. (1)
Induction occurs when a conductor is placed in an alternating magnetic field. When the effect is sufficient, great eddy currents are set up in the conductor, which rapidly gets hot or even melts, the magnetic linkages necessary being increased with the frequency. To be used for thermal spraying, a water cooled helix of several turns is fashioned from OFHC copper. It is wrapped around a quartz tube that is closed at its top end and fitted with two inlet ports to feed a spray material and a plasma forming gas. Releasing gas into the tube and energizing the copper helix by a high frequency current that sets up an intense magnetic field inside the tube causing ionization of the gas. Continuous feeding of the gas causes it to escape through the open bottom of the tube. Powder fed into the plasma filled tube is melted and relying on either gravity or the plasma flow is conveyed to the work surface.
Coatings produced using RF plasma has shown to be generally homogeneous and not porous. This method, using neutral atmospheres, can deposit reactive and toxic metals including calcium, uranium, niobium and titanium.
Wire Flame Spraying
wire flame spraying, spray process in which the feed stock is
in wire or rod form.(1)
Schoop approached this problem by using a turbine to actuated gears and drive rolls that pulled the wire into the nozzle. This apparatus appeared to him to be similar to a pistol or gun, and because of this, he and we, refer to thermal spray devices as "guns" or "pistols" and never "torches". A typical wire spray gun is shown in the graphic above. Schoop' concepts of spraying solid metals has given rise to the thermal spray industry and for this reason it is sometimes referred to as the "Schoop Process". Regardless, the wire flame spray gun has not radically changed since the days of Schoop. While there have been changes in nozzle and air cap design, replacement of the air turbine with an electrical motor and even the use of barrel valves the basic principal, however, remains the same "push or pull a wire into a flame, melt and atomize it and deposit the molten droplets to form an adherent coating".