Patent Publication Number: US-6902768-B2

Title: Method of producing thermally sprayed metallic coating with additives

Description:
TECHNICAL FIELD 
     This invention relates generally to methods for spray coating the cylinder walls of a light metal engine block using a high velocity oxygen fuel (HVOF) system and more particularly the application of ferrous-based coatings including aluminum. 
     BACKGROUND OF THE INVENTION 
     It is known in the art to thermally spray coat the cylinder walls of aluminum engine blocks with a ferrous-based material using high velocity oxygen-fuel (HVOF) systems. Examples of prior HVOF systems include those disclosed in U.S. Pat. Nos. 5,014,916; 5,148,986; 5,275,336; 4,578,114 and 5,334,235, wherein a jet of oxygen and gaseous fuel is ignited within an HVOF gun to melt a feed wire of ferrous-based material which is expelled from the gun by the jet of burning oxygen-fuel onto the surface of the cylinder wall. The rate of application is limited by the rate of melting of the wire feed material. 
     It is an object of the present invention to increase the efficiency of such HVOF systems. 
     SUMMARY OF THE INVENTION 
     A method of thermally spray coating a cylinder wall of a light metal engine block includes providing high velocity oxygen-fuel (HVOF) device and advancing a feed wire of ferrous-based material into the HVOF device to locate a tip end of the wire in a high temperature zone of the HVOF device. High velocity jet flows of oxygen and gaseous fuel are supplied to the high temperature zone and combusted to generate sufficient heat to melt the tip end of the feed wire and spraying the molten feed wire material onto the cylinder wall of the engine block. According to a characterizing feature of the invention, the supply of the oxygen to the HVOF device is controlled in order to provide an oversupply of oxygen to the high temperature zone of the UVOF device in excess of the oxygen required for stoichiometric combustion of the gaseous fuel. The excess oxygen reacts with an associated fraction of the ferrous-based feed material in the high temperature zone to combust the associated fraction of the feed material as a source of solid fuel to generate a supplemental source of heat to the high temperature zone of the HVOF device. 
     The invention has the advantage of oversupplying oxygen to the HVOF device so as to consume a fraction of the ferrous-based feed material as a source of solid fuel so as to increase the temperature and intensity of heating in the high temperature zone, thereby substantially increasing the rate at which the ferrous-based feed material can be converted by the HVOF device as a sprayed coating on the cylinder walls. Consequently, the method of the present invention provides a more efficient process for thermally spraying ferrous-based coatings onto cylinder wall substrates in an HVOF system, increasing the application rate of the coating material and greatly increasing the number of cylinder wall surfaces that can be coated in a given time, and makes it possible to process a cylinder block using the HVOF system without use of cooling water flow in the water jacket of the block. 
     The invention has the further advantage of providing a simple solution for increasing the efficiency and application rate of HVOF systems with the use of standard materials, namely use of standard oxygen and gaseous fuel types and ferrous-based feed material through control of the oxygen flow relative to the gaseous fuel flow. 
     Still a further advantage of the invention is that the high heat capacity generated from burning the fraction of feed material decreases the dependence on the gaseous fuel as the sole source of heat for melting the feed material in the high temperature zone. The supplemental heat generated through burning of the feed material enables the user of the present invention to select from a variety of gaseous fuels, including some low cost fuels which, on their own, may not provide sufficient heat in an HVOF system for acceptable performance of the system. However, supplemented by the burning of the feed material as a solid fuel source, these otherwise inadequate gaseous fuel sources become viable as low cost alternatives in an HVOF system as the gaseous fuel source. 
     A further advantage of the invention is that the burning of a fraction of the ferrous-based feed material produces iron oxides which are incorporated as part of the thermally sprayed coating. The presence of iron oxide particles increases the wear resistance of the thermally sprayed coating. 
     According to a further aspect of the invention, aluminum may be added to the ferrous-based feed material to lower the oxygen content in the sprayed coating and to alter the form of oxide from FeO to FeAl 2 O 4 . FeO is a metastable oxide phase that can transform over time at engine operating temperatures to Fe 3 O 4  in a volume expanding reacting. FeAl 2 O 4  is a stable oxide phase that is not subject to any transformations at engine operating temperatures. The presence of the aluminum in the oxide further enhances the wear resistance properties of the thermally sprayed coating and is less brittle than a coating having FeO oxides. 
     According to a still further aspect of the invention, additives are included in the iron-based feed material to control embrittlement from impurities such as sulfur. According to the invention, introducing yttrium, calcium, magnesium, titanium, zirconium, hafnium, cerium, or lanthanum has the beneficial effect of tying up impurities so as to eliminate their ability to segregate to interfaces such as grain boundaries to reduce or eliminate embrittlement caused by such impurities. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A preferred embodiment of the invention is disclosed in the following description and in the accompanying drawings, wherein: 
         FIG. 1  is a schematic isometric view of a cast aluminum engine block shown partly broken away and in section and illustrating the process of coating the walls of the cylinders according to the invention; and 
         FIG. 2  is an enlarged fragmentary sectional view of a cylinder of the block being coated according to the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  shows a schematic representation, partly in section and broken away, of an engine block  10  for a four-cylinder engine having four cylinder chambers defined therein by cylinder walls  12 . The block  10  is cast of a lightweight metal, such as aluminum, magnesium or alloys thereof. 
     According to the invention, a spray  14  of atomized ferrous-based material is applied to the cylinder walls  12  to form thereon a thermally sprayed coating  16  of the material. The cylinder walls  12  are initially cleaned such as by water etching according to known practice. The coating  16  is applied by using a high velocity oxygen-fuel (HVOF) thermal spray device  18  and practices which are generally known to the art, but modified according to the invention as will be described below. The HVOF metal spray gun device  18  has one or more tubular coating heads  20  which are extended into the open cylinders of the block  10  in spaced relation to cylinder walls  12  as illustrated schematically in the drawings. According to HVOF practice, a jet flow of oxygen, originating from oxygen source  22 , and a jet flow of gaseous fuel, originating from gas source  24 , are directed through the coating head  20  and ignited to combust the gaseous fuel  24  in a high temperature zone  26  of the coating head  20  adjacent a nozzle  28  of the coating head  20 . Once ignited, the flame of the burning gases is self-sufficient. 
     Material for the coating  16  is supplied to the HVOF spray gun  18  where it is melted in the high temperature zone  26  and blown by the jet of high velocity gases out of the nozzle  28  through a nozzle opening  30  and deposited onto the inner surface of the cylinder walls  12 . The ferrous-based feed material  32  is preferably supplied in wire form and fed, preferably as a single wire, down through the coating head  20 , where its lower tip end enters the high temperature zone  26  and is melted by the burning gases. The coating head  20  is automatically rotated about the feed wire  32  and is reciprocated in the longitudinal direction of the cylinder as generally described in U.S. Pat. No. 5,080,056, which is owned by the assignee of the present invention and its disclosure incorporated herein by reference. 
     The standard HVOF practice in applying such coatings is modified according to the invention as follows. According to one aspect of the invention, the flow of oxygen to the high temperature zone  26  is controlled such that the volume of oxygen supplied to the high temperature zone  26  exceeds the amount of oxygen required for stoichiometric combustion of the gaseous fuel  24  supplied to the high temperature zone  26 . In other words, there is an excess supply of oxygen delivered to the high temperature zone  26  beyond that required to burn the gaseous fuel  24 . According to the invention, this excess oxygen supply reacts exothermically with the ferrous-based feed material  32  in the high temperature zone and actually burns or combusts (not just melts but consumes) a fraction of the feed material  32  in the temperature zone  26  to generate substantial heat, such that the ferrous-based feed material  32  serves as a source of solid fuel as well as a coating material. The burning of the fraction of feed material within the coating head  20  provides a supplemental heat source beyond that provided by the combustion of the fuel gas  24 , greatly increasing the temperature environment in the high temperature zone  26 . The usage of the feed material in part as a solid fuel has several practical advantages which will be discussed below. For the feed material  32  to serve most effectively as a heat-generating solid fuel source, the over supply of oxygen should be about twice the amount needed for stoichiometric combustion of the gaseous fuel  24 . 
     There are several advantages which are recognized by preparing and applying a thermal spray coating of ferrous-based material according to the invention, wherein excess oxygen is supplied to burn a fraction of the feed material as a solid fuel source. The temperature generated from the combustion of the gaseous fuel  24  is hot enough to melt the iron-based feed material (having a melting point less than about 2,800° F.) but does not reach the boiling or combustion temperature of iron, which is about 5,400° F. The over supply of oxygen reacts at high temperature with a fraction of the molten ferrous-based feed material  32  within the coating head  20  and the exothermic reaction reaches temperatures sufficient to burn the fraction of feed material which, consequently, releases heat and increases the overall temperature in the high temperature zone  26 . The increased temperature environment accelerates the wire deposition rate. On studies conducted with and without burning of the iron-based feed material, it was found that about 17 pounds of feed material are deposited per hour according to the invention, whereas about seven pounds per hour are deposited when oxygen levels are kept at about stoichiometric levels. Accordingly, the deposition rate is increased by more than twofold. 
     Another advantage of using the iron-based feed material as a solid fuel is that it is a rather inexpensive source of fuel. Low carbon steel, for example, in wire form is relatively inexpensive and readily obtainable on the market. In addition, using the iron-based feed material  32  as a fuel source presents opportunities to select from gaseous fuel sources  24  which might not otherwise be suitable or sufficient in an HVOF system. Prior U.S. Pat. No. 5,080,056 discloses uses of propylene as the fuel source in an HVOF system which burns at a temperature of about 5,000° F. According to the invention, other more readily available gaseous fuel sources may be used, such as natural gas, which is already supplied to most major manufacturing facilities and would be an inexpensive alternative to the usual propylene. Another readily available inexpensive gaseous fuel alternative is propane. Propylene has a higher heat content than either methane or propane and, on its own, would be more suitable for general HVOF applications. However, it is generally more costly and the relatively high heat content may not be required in the HVOF process according to the invention where oxygen is oversupplied at a rate sufficient to burn a fraction of the ferrous-based feed material as a solid fuel source. Consequently, less costly, more readily available gaseous fuels, such as methane and propane mentioned above, can be used, among others, even though they might on their own lack the heat content of more costlier fuels like propylene. 
     Another significant advantage of using the ferrous-based feed material as a solid fuel source is that it results in a greater application rate of the thermally sprayed coating material, and thus a greater number of cylinder bore walls can be coated in a given time as compared to operating an HVOF system without usage of the feed material as a solid fuel source. Still a further advantage recognized by the present method is that the higher deposition rate on the walls of the cylinder allow the coating to be applied in a shorter time duration, and thus there is less heating of the substrate block material as a result of the coating process than that caused when using only a gaseous fuel source in an HVOF system. Consequently, it is possible to coat the walls of the cylinder liners without providing auxiliary cooling to the block. 
     A further advantage of burning a fraction of the ferrous-based feed material is that the byproducts of the consumption of the solid fuel are metallic oxides, which get incorporated into the spray coating and increase the wear resistance of the coating  16 . When low carbon steel wire is used as the feed material  32 , Wustite (FeO) is the predominant oxide generated from burning of the feed material and which gets incorporated into the coating  16 . However, it is preferred to incorporate aluminum into the steel wire  32  which has the effect of reducing the oxygen content in the spray coating  16  and altering the oxide formed. With an iron-based feed wire  32 , the spray coating  16  has about 8-12 wt. % FeO or about 35-55 vol % FeO but most preferably 10-12 wt. % FeO. By adding about 1.5 to 3.0 wt. % aluminum (and preferably 2.0-2.5 wt. %) to the iron-based feed wire  32 , the oxide is altered from Wustite (FeO) to predominantly Hercynite (FeAl 2 O 4 ). Everything else being equal, the Hercynite is present in the coating in a range of about 3-7 wt. %. The addition of the aluminum thus has two advantages. Firstly, by reducing the oxygen content, the overall metal oxide content is reduced from 8-12 wt. % FeO to 3-7 wt. % FeAl 2 O 4 . While oxides have beneficial wear characteristics, they also make the coating more brittle, and the 3-7 wt. % range retains beneficial wear properties while reducing the brittleness of the spray coating. Secondly, Wustite (FeO) is a metastable oxide phase that can transform over time at engine operating temperatures to magnetite (Fe 3 O 4 ) with a corresponding volume expansion. Hercynite (FeAl 2 O 4 ) is a stable oxide phase (spinel) that is not subject to any transformations at engine operating temperatures. 
     A {fraction (3/16)}-inch diameter low carbon wire fed to the HVOF coating head  20  in which methane is fed at a rate of 100-150 SCFH and oxygen fed at a rate of 600 SCFH produced a consumption rate of the wire feed at about 36 inches per minute, as compared to a stoichiometric flow rate of oxygen of 250 SCFH with the same gas flow producing a consumption rate of the wire feed material at about 14 inches per minute. 
     The preferred coating  16  has a thickness of about less than 0.2 mm and preferably in the range of 0.050-0.175 mm, and the cycle time for thermal spray coating the wall of a cylinder of an aluminum block with about 0.150 mm finished coating thickness is about 60 seconds when using the feed wire  32  as a solid fuel source, as compared to a cycle time of about 160 seconds for HVOF coating where stoichiometric combustion of gas is employed. 
     In addition to the aluminum, other additives may be added to the low carbon iron feed stock wire  32  to inhibit impurity embrittlement of the thermal spray coating. As the molten droplets of coating material are sprayed onto the surface of the cylinder walls  12 , they immediately quench and solidify, with the droplets building upon one another to produce a dense coating. However, the presence of sulfur and other relatively large impurity atoms may be particularly damaging as embrittling agents if present in the coating materials, as they tend to segregate to the internal interfaces of the coating (such as grain boundaries and the surfaces of the individual droplets) which can inhibit the adhesion properties of the coating and can lead to spalling. The embrittling effects of such impurities can be lessened or eliminated by the addition of yttrium, calcium, magnesium, titanium, zirconium, hafnium, cerium and/or lanthanum. For example, it has been found that the addition of less than 1 wt. % of yttrium is sufficient to eliminate the effects of sulfur embrittlement in the steel thermal spray coating of the invention. Similar percentages of the other anti-embrittlement agents are contemplated. The aluminum and anti-embrittlement additives may be supplied to the high temperature zone  26  of the HVOF coating head as an alloyed feed stock wire  32 , as a coating applied to the low carbon feed stock wire, or may be separately added as a composite wire. 
     The disclosed embodiments are representative of presently preferred forms of the invention, but are intended to be illustrative rather than definitive thereof. The invention is defined in the claims.