Surface alloying of metals is usually accomplished by welding processes or various flame and plasma spraying techniques. Welding processes utilize an electrical or gaseous energy source with a highly variable power level which is difficult to control. As a result, excess heat is normally directed to the article being coated and the microstructure thereof is often adversely affected so that additional heat treatment is subsequently necessary. While the strength of the bond between the surface coating and article may be sufficient with such welding processes, an excessively thick coating is normally provided because of irregularities in the deposition rate and the lack of the full ability to control its placement. Such excess coating must, in many instances, be machined off to achieve the desired final surface contour at additional expense. Further, welding processes are basically accomplished at a rather slow material deposition rate and the accompanying control problems often lead to impurities in the microstructure, cracks or porosity, or other forms of harmful metallurgical defects which tend to decrease the strength of the coating and its bond to the article.
By way of example, engine valves of austenitic chrome nickel silicon steel with Stellite No. 6 coating material fusibly bonded thereto around the annular valve seat portion thereof have been widely adopted by the diesel engine industry. Such coating material is available from the Stellite Division of Cabot Corp., Kokomo, Ind. These valves are produced by a relatively complex manufacturing process utilizing gaseous torches to extensively heat the valve head prior to, during, and subsequent to depositing the Stellite coating material thereon. This process is known as "puddle welding." The disadvantages of this expensive and relatively slow process include directing an excessive amount of heat into the base material and slow cooling of the coating. This results in a relatively thick overall diffusion zone or region of interstitial bond of approximately 0.020 inch between the base material and the coating as well as dendritic segregation and enlarged crystallization of the coating material with an accompanying decreased hardness thereof. Further, the thick diffusion zone represents a region having neither the desirable properties of the base material or of the coating. For example, it is known that the iron in the steel degrades the Stellite when present in only relatively minute amounts. Accordingly, the service life of these valves is not as high as desired.
On the other hand, flame and plasma spraying techniques apply a coating material to an article without substantial heating of the article, with the result that a relatively poor metallurgical bond occurs therebetween. Such a technique is usually limited to a coating thickness of a few thousandths of an inch having generally low strength. U.S. Pat. No. 3,310,423 issued Mar. 21, 1967 to H. S. Ingham, Jr. discloses a flame spraying process for applying a coating utilizing instantaneous energy bursts from a laser beam to raise the temperature of the previously heated flame sprayed particles as they are propelled toward the surface of an article. The substantially molten particles are purported to adhere to the surface of the article upon contact therewith with a better bond than conventional flame spraying processes, which are generally known to have a relatively weak bond between the coating and the article, and coating porosity. This method is more expensive because it requires dual heating and, further, the thickness of the applied layers is restricted to only a few thousandths of an inch. Because of these deficiencies, flame spraying processes have been considered entirely unsatisfactory for hard facing an engine valve, for example.
Other heating sources such as the possible use of solar energy or electron beam energy appear to raise more difficult problems. With solar energy, a large and expensive heat gathering system is needed in order to obtain sufficient power density energy levels and the broad spectrum energy source is continually changing. Likewise, with electron beam energy a relatively expensive evacuation system is normally necessary, and this greatly restricts the manufacturing process of the parts to be coated. For example, the vapor pressure of either the coating material or the article could be adversely affected and some materials might volatilize, imposing further limits on material choice which is already limited by the electron beam welding process itself.
Illustrative of the wide range of efforts to solve the particular problem of applying a hard surface coating to the head of an engine valve, for example, are U.S. Pat. No. 3,147,747 to LeRoy O. Kittelson; U.S. Pat. No. 3,362,057 to Gerhard Kubera et al.; and U.S. Pat. No. 3,649,380 to Max. J. Tauschek. Also, U.S. Pat. No. 3,478,441 to Alexander Goloff et al. and assigned to the assignee of the present invention discloses a method of friction welding a facing material workpiece to an engine valve to provide an impact resistant seat thereto. However, none of these methods completely solves the various problems, including the frequent need to machine off the excess coating, or solves them in a sufficiently economical manner. One other method, U.S. Pat. No. 3,663,793 to James Petro et al., describes a way of applying a coating to a glass light bulb or the like for decorative purposes, using heat from a laser beam or an electron beam. However, such method employs an additional heating step and makes no mention of improving the wear and impact resistant service life of the coating and its bond with the parent article to which the present invention is particularly directed.