Patent Application
20150360326
This invention relates generally to the field of materials technology, and more particularly to high temperature material removal processes such as laser machining.
Many types of material removal processes are known in the art; for example, mechanical processes such as drilling, sawing, grinding, etc. and thermal processes such as oxy-fuel cutting, electrical discharge machining (EDM), laser cutting, etc. One disadvantage of thermal processes is the re-solidification and deposition of vaporized and/or molten material proximate the area of material removal. The deposited material is generally known as recast material. Recast material can cause the edge of a cut to be geometrically inconsistent and to have an undesired rough surface finish. Recast material has been known to partially block cooling holes formed by laser drilling in gas turbine components. Moreover, recast material can contain cracks formed as a result of the configuration and/or rapid solidification of the material, and such cracks may grow into the underlying substrate material during subsequent operation of the component, thereby limiting component life. Accordingly, it is common to remove recast material by a further mechanical, thermal or chemical process, but such processes add time and expense to a manufacturing operation, and they may be incomplete or they may cause the undesired removal of material other than recast material.
The invention is explained in the following description in view of the drawings that show:
The present inventors have developed a technique for handling recast material formed during thermal material removal processes. The technique generally utilizes a flux material to react with the thermally displaced material to resolve it into a friable, easily removable recast slag.
The term “friable” is used herein when referring to a recast slag material to mean that the material can be broken apart and easily detached from a surface to which it is adhering by mechanical impact such as with a hammer, file or other tool, or by blasting such as with glass beads. Fluxes that create detachable slags are known for conventional welding processes; however, to the knowledge of the present inventors, fluxes have not been contemplated for material removal processes for the purpose of eliminating recast material. This application teaches such concept.
Thermal material removal methods that create recast material may utilize a variety of heat sources. The terms “energy beam” and “beam energy” are used broadly herein to describe such heat sources collectively, and they are meant to include laser beams, ion beams, electron beams, plasma beams, and superheated gas streams such as produced by an oxy-fuel cutting torch.
The flux material 12 is chosen in view of the material of the substrate 10 such that a high temperature reaction of the two materials produces a recast composition that exhibits properties making it easy to remove from the substrate 10. For example, the recast slag material 18 may be brittle and may have a different coefficient of thermal expansion than the substrate 10, resulting in the build-up of stresses there between upon solidification and cooling of the assembly. In some embodiments the recast slag material 18 and substrate 10 may have a high differential coefficient of thermal expansion. For example, it is known that from 1,000° C. to room temperature, typical substrate and deposited iron and nickel alloys have linear thermal contraction values of the order of 0.014 mm/mm. Corresponding welding slags over such alloys can have values of the order of 0.004 mm/mm. Such a large differential can be useful in detaching such slag from the substrate, and such slag may even be self-removing due to the developed thermal stresses generated as the recast slag 18 cools and solidifies. Embodiments of the invention select the flux material such that the ratio of the thermal contraction value of the substrate to the thermal contraction value of the recast slag is at least two or at least three from 1,000° C. to room temperature. The recast slag material 18 will also have a different crystal structure and chemical makeup than the substrate 10, thereby minimizing chemical bonding there between.
Many stainless steels and nickel based superalloys contain significant amounts of chromium. Thermal removal of such materials in the presence of oxygen will produce a recast material containing oxides of chrome. Such recast material is known to be tenacious and difficult to remove from the underlying stainless steel or superalloy substrate because of the formation of spinels (e.g. MgAlCrO4) that embed in the metal deposit and anchor and lock the recast material in place. A similar problem occurs when thermally removing material from a titanium bearing substrate due to the formation of perovskites. Embodiments of the invention may provide a material in the flux material 12 that reduces the formation of such problematic compounds. This may be accomplished by selecting the flux material 12 to contain a material which will form an oxide having a lower Gibbs free energy change than a known problematic compound, such as spinels or perovskites. If two metals are present, two equilibriums must be be considered. The oxide with the more negative Gibbs free energy change (delta G) will be formed and the other oxide will be reduced. For example, by including aluminum (or alumina) in the flux material 12, oxides of chrome can be reduced when compared to a prior art process without the flux. Removal or reduction of a problematic compound in the recast slag material 18 will facilitate its removal from the underlying substrate 10.
An exemplary embodiment useful for thermal material removal from an aluminum substrate 10 may include zirconium in the flux material 12. When aluminum is melted and vaporized in a prior art material removal process, the recast material is formed as alumina (Al2O3) which will bond tightly to the aluminum substrate 10. By performing a thermal material removal process of aluminum under a covering flux containing zirconium, the melted zirconium will scavenge oxygen from the region of material removal and will form a recast slag material 18 containing a relatively larger quantity of zirconia and a relatively smaller quantity of alumina. This composition of recast slag material 18 will not adhere well to the aluminum substrate 10 due to its different coefficient of thermal expansion (10.3/° C. for zirconia versus 24/° C. for aluminum) as well as its different crystal structure(monoclinic for zirconia versus face centered cubic for aluminum).
Carbon may be useful in the flux material 12 for some applications. Coke is widely used in steelmaking for oxide reduction. The Ellingham curve for the reaction 2C(s)+O2(g)>2CO(g) slopes downward and falls below the curves for all of the metals. Hence, carbon can normally act as a reducing agent for all metal oxides at very high temperatures. Note that carbon may not be desired to reduce chromium oxides because it will form unwanted chromium carbides.
Table 1 identifies materials that are often thermally removed and the resultant forms of recast material, along with flux compounds that may be useful to resolve those recast materials into a fugitive slag compound.
Fluxes useful in embodiments of the present invention will likely contain little or no calcium fluoride because of its deleterious effect on detachability due to cuspidine formation and the interaction with silica. Alternative fluorides may be utilized for slag fluidity control.
Process parameter controls in conjunction with the invention include power, travel speed, pulse controls and flux thickness, such as may result in high differential thermal contraction and relatively slow cooling rates to optimize slag detachability.
While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.
