High-speed engines, particularly those utilized in aircraft, may include rotating elements such as fans, turbines and/or rotors (collectively called “rotors” hereinafter) for compressing air.
FIG. 1A is a simplified, schematic cross section of a jet engine 100 that utilizes rotors 110. Airflow through engine 100 is illustrated by arrows 120. Rotors 110 suck air into an inlet end 102 of engine 100 and compress it into combustion chambers 130. Jet fuel burns with the air to form jet exhaust, which expands and is expelled at outlet end 104 of engine 100. The expanding jet exhaust spins turbines 140 that connect through an axle 150 to turn rotors 110. The assembly formed by rotors 110, turbines 140 and axle 150 may turn at thousands of revolutions per minute. Therefore, rotors 110 must be precisely balanced and of high mechanical integrity, because an unbalanced or broken rotor can fly apart, destroying the engine and possibly generating high speed shrapnel that can damage adjacent objects, such as wings or fuselage of an aircraft. These requirements present challenges for the manufacture and repair of rotors 110.
It is often costly to manufacture a rotor by mounting discrete blades on a common structure. The blades and common structure must be precisely formed and attached so that the final rotor is balanced. However, such structures are compatible with repair of individual nicked or broken blades (e.g., due to an engine aspirating foreign matter, such as a bird). In such cases, the blade that needs to be repaired can be removed and a new blade attached in the same manner as for the initial manufacture of the rotor.
Integrally bladed rotors have recently begun to appear in military aircraft; it is expected that commercial aircraft will also utilize such rotors. FIG. 1B shows one example of an integrally bladed rotor 110(1). Rotor 110(1) is formed of a high strength alloy, the metal of which is manufactured in a “parent-metal” metallurgical state, typically characterized by an equiaxed, fine grain structure. Although initial manufacture of integrally bladed rotors has become practical in recent years, repair of such rotors is problematic. One method of repair called “blending” simply grinds away and polishes damage sites. This improves reliability to a point, because without blending, the damage sites can form nucleation sites for cracks to propagate through the damaged blade. However, by removing material, the blending affects the balance of the rotor, therefore the original equipment manufacturers (OEMs) of engines place strict limits on the amount of blending that can be done. The same OEMs would allow removal and replacement of metal, but they specify that replaced metal must have the metallurgical properties of the parent metal. To date, welding a new blade in place (or adding a piece of a blade by welding) typically results in a metallurgical difference between the metal that melts and resolidifies due to the act of welding (called a “weld nugget” hereinafter) and the “parent” metal (e.g., adjacent unwelded metal). In such a case, the weld may be weaker than the original material, particularly in fatigue strength. Therefore, no welding technology to date has become qualified for repair of integrally bladed rotors.
Many attempts by industry to achieve parent metal fatigue properties in repaired IN-100 and other high strength superalloys have failed. Such attempts have included layer-by-layer laser sintering buildup, and plasma powder deposition approaches. Both of these approaches have failed to produce parent metal fatigue properties. This is at present believed to be due to (a) porosity of the metal thus formed, due to inert gas trapped during powder processing, (b) presence of oxides, carbides or ceramic inclusions, and (c) inability to match the weld zone microstructure to that of the parent metal.
Therefore, at present there is no satisfactory repair method for integrally bladed rotors damaged beyond OEM blending limits. When even a few blades of such rotors become damaged, the entire rotor is typically replaced—at a current cost of about $125K for a Ni superalloy rotor, or about $250K for a Ti rotor. Rotors that are unrepairable according to these standards are presently accumulating at engine overhaul shops awaiting development of an acceptable repair technology. The inventory of presently unrepairable rotors continues to increase as the number of engines that utilizes them increases, and the number of flight hours on the engines increases.
Not only can weld repair sites be problematically weak, but it is known that nickel alloys become difficult to weld at all when hardening agents such as aluminum and titanium exceed an aggregate amount of about 4.5%. FIG. 2 shows a graph 200 that illustrates composition of certain nickel alloys known by the trade names Inconel 702, IN-718, Mar-M-200, Waspaloy, Astroloy, IN-100 and Modified Waspaloy. Percent titanium is shown on the horizontal axis and percent aluminum is shown on the vertical axis; a line 205 indicates the 4.5% boundary between alloys typically considered weldable (below/left of line 205) and alloys typically considered unweldable (above/right of line 205). The direction of arrow 210 indicates decreasing weldability. Some of the alloys considered unweldable (e.g., IN-100) are known as “superalloys” that have excellent properties such as high fatigue strength supporting their use in rotors 110. These same alloys are susceptible to hot cracking when welding is attempted using present methods.
It is also known to provide stress to metal surfaces by utilizing techniques such as shot peening, low plasticity burnishing and laser shock peening. These techniques impart residual compressive stress to a metal surface and thereby enhance resistance to fatigue damage. However, the stresses imparted by these techniques may be limited in depth, such that uniform stress may not exist throughout the metal volume of a treated article.