High temperature devices and applications employing pure aluminum braze for joining components of said devices

Abstract

The present applicant presents a structure intended for high temperature use above 30 C. comprising multiple components having metal-to-metal or metal-to-ceramic contacting surfaces wherein the surfaces are joined by a braze composed of pure aluminum. Anticipated devices include but are not limited to igniters as well as electronic applications in the automotive and aerospace industries.

Claims

1. A method for the joining of components of a structure, wherein a first components is comprised of iron or iron alloys and a second component is comprised of a non-metal, for use at high temperatures comprising; positioning pure aluminum braze in the joint between mating surfaces of the components and subjecting the components of the structure and the braze to a temperature, in a non-vacuum environment, sufficient to melt the pure aluminum braze.

2. The method of claim 1 wherein the pure aluminum braze is in foil form.

3. The method of claim 1 further comprising pre-heating the structure.

4. The method of claim 1 wherein the braze material is melted through immersion in a thermal plasma plume.

5. The method of claim 1 wherein the braze material is melted in an ionic atmosphere.

6. The method of claim 1 wherein the non-metal of the second components of the structure is comprised of a ceramic.

7. The method of claim 1 wherein the non-vacuum environment is comprised of air.

8. The method of claim 1 wherein the pure aluminum braze is in direct contact with the surfaces of the components.

9. The method of claim 1 wherein the first component is further comprised of nickel.

Description

DRAWINGSFIGURES

(1) FIG. 1 is an overall view embodiment of a structure having multiple components joined together with pure aluminum braze.

(2) FIG. 2 is a cut-away view embodiment of a structure having multiple components joined together with pure aluminum braze showing the braze in the joint between components.

DRAWINGSREFERENCE NUMERALS

(3) TABLE-US-00001 10. high temperature 20. core structure 30. lead 40. braze joint

DETAILED DESCRIPTION

(4) The use of, and need for, an effective braze for turbomachinery, ignitors, electronics, structural parts, etc., especially when high-temperature use is required, has grown in recent years. The increasing use of natural gas in automobiles and home heating requires ignitors that work at temperatures above 1000 C. The applicants have found that igniters (ignitors) assembled with pure aluminum braze exhibit very good performance in terms of temperature, cyclability and life.

(5) In the past, eutectic or similar alloys were preferred for the reasons of low temperature and high fluidity for brazing. This, however, had taught away from pure materials because brazing then required special protective atmospheres. However, pure (even 99%) aluminum presents a distinctly different case. Aluminum oxide (a thin layer) protects both solid and liquid aluminum from oxidation especially in constrained conditions like attaching terminals to heaters. Pure aluminum is protected well by its oxide unlike many other metals.

(6) An embodiment of such an igniter would comprise leads or contacts made from iron or some other metal. Between these leads would be positioned an element or filament comprised of a ceramic such as silicon carbide or nitride. Typically, the ceramic would be of a much smaller diameter than that of the leads, however the ceramic could be much larger than the leads or comprised of other thin filaments embedded in ceramic or ceramic like materials. The leads may be hollowed out at the ends nearest the ceramic. Pure aluminum or aluminum alloy would be placed in the hollows as well as the ends of the ceramic. The aluminum may be melted to affix the ceramic to the leads. Friction welding may be used to melt the aluminum in some applications to apply the braze uniformly.

(7) There is considerable energy used when a protective atmosphere must be provided in brazing furnaces. Some of the atmospheres are created by fluxes which could be very toxic as they contain chlorides and fluorides. There is a loss of productivity when using furnaces with atmospheres or when cleaning residual fluxes. Such problems can be avoided if just pure aluminum is used without the use of fluxes. Aluminum is low melting and can maintain high flowability when protected against oxidation. When reacted, the products are conductive and high melting. Therefore, in constrained situations and with the use of foils, pure aluminum is effective in brazing applications with high productivity and lower energy usage. Table 1 shows how reactive pure aluminum behaves with many materials that either contain iron, silicon or other commonly used ceramics in heaters.

(8) One embodiment envisions igniters with leads attached with foil inserted prior to braze between metal and non-metal. Such igniters may be used for gas ignition or black body applications. Normally leads are brazed with AgCu commercial grade (e.g. a braze paste, power or foil of CuSil (a tradename) of materials made by Morgan Materials, Cleveland Ohio). The applicants have found that with a thin foil (20 microns to 200 microns) of pure aluminum or aluminum alloy one can braze, joining and wet silicon nitride, as well as many other ceramics mentioned above, with metal having better integrity than when made with the commercial braze alloys.

(9) One method for the application of the aluminum based braze is with the use of the cascade e-ion rapid process (thermal plasma) that enables rapid braze joints since aluminum brazes can additionally have multifunctional functionally graded layers. Joint conductivity was found to be maintained over 1300 C. when aluminum was used as the braze. As an example, the items to be brazed and the braze are immersed in a thermal plasma plume containing ions (open plasma). The thermal plasma methods and devices contemplated are those disclosed in U.S. Pat. No. 9,643,877, issued on May 9, 2017 entitled Thermal Plasma Treatment Method and US patent application Ser. No. 15/600,824, filed May 22, 2017 entitled Method and Apparatus Employing Fermion and Boson Mutual Cascade Multiplier for Beneficial Material Processing Kinetics, each by the present applicants, the disclosures of which are both incorporated by reference herein in their entireties.

(10) With the use of plasma immersion, brazing with aluminum did not require inert atmosphere when ignitors were assembled or in other anticipated brazing applications. This is a distinct departure from normal brazing requirements. However, an ionic, nitrogen or nitrogen-hydrogen environment may also be employed. When inert gas required brazing is performed, the process can take many hours and require an enclosure. When using aluminum and just air, or nitrogen, more than 100 parts can be made easily per hour with just the use of 10 KWhr of electrical energy compared to over 500 KWhr energy amount requirement in batch brazing processes. The aluminum processes allow continuous operation. Traditional furnace-based batch treatment for brazing, as described above, could be expensive in terms of energy consumption. Batch processes are often inefficient and expensive, additionally, because of high furnace and facilities costs. Other costs from batch brazing include frequent maintenance and repair with specialized labor. Extra steps, such as, plasma cleaning or blanking parts may be required, but are not usually required with aluminum-based brazing. Conventional furnace brazing is often done with a 100-300 kW batch furnace. A continuous aluminum brazing machine may be rated for just 15 kW. The one-hour operational cost for a 300 kW machine at 10c/kWh is $30. The one-hour electric operational cost for a 15 KW braze machine is about $1.50. For 200 days, this equates to $144,000 vs. $7,200 in electric cost.

(11) A plausible reason that aluminum works well with oxides, carbides and nitrides is that, effectively, any detrimental interface phase is easily reduced to a conducting phase with high-temperature composite microstructures by the aluminum. Silica and/or iron chromium oxides are reduced easily to form a conducting phase and a reinforcing phase. The presence of nitrogen can provide an additional reinforcing conductive phase. Multifunctional layers are contemplated as well.

(12) It appears that aluminum is quickly able to reduce oxides like silica or iron oxides and produce a conduction silicon or iron phase. If using nitrogen, a silicon nitride phase may be produced as shown below in Table 1.
3SiO.sub.2+4Al+2N.sub.2(g)=2Al.sub.2O3+Si.sub.3N.sub.4

(13) TABLE-US-00002 TABLE 1 The reaction of aluminum and silica in a nitrogen environment. H is enthalpy, S is entropy, G is Gibbs free energy and K is the equilibrium constant all pertaining to the reaction. T is temperature. T delta H delta S delta G C. kcal cal/K Kcal K Log(K) 0.000 345.752 107.744 316.321 1.296E+253 253.113 100.000 346.632 110.524 305.390 7.552E+178 178.878 200.000 347.086 111.620 294.273 8.648E+135 135.937 300.000 347.257 111.953 283.091 9.020E+107 107.955 400.000 347.373 112.139 271.887 1.906E+088 88.280 500.000 347.693 112.577 260.654 4.858E+073 73.686 600.000 348.691 113.780 249.344 2.607E+062 62.416 700.000 358.969 124.796 237.524 2.226E+053 53.347 800.000 358.808 124.640 225.051 6.856E+045 45.836 900.000 358.808 125.782 212.471 3.848E+039 39.585 1000.000 359.726 125.532 199.905 2.083E+034 34.319

(14) This is a notable invention because, in the past, it was assumed that aluminum had too low a melting point to be an effective braze and because brazes were always designed for flowability that a eutectic offers. However, as shown by V Gopalakrishna and J A Sekhar, in Thermal Analysis and Flowability Correlations in IronCarbonSilicon Alloys, I and II, Transactions of the American Foundrymen's Society, 1987, in certain instances, pure materials can also provide high flowability. In applications such as ignition and heating, where resistance is a key factor, the use of pure aluminum as a braze has proven to be effective. When subjected to an electrical current, the pure aluminum braze in a jointed structure, such as an igniter, is less resistant than alloyed aluminum brazes and therefore heats up and oxidizes less. However, the use of alloying elements of up to one atomic percent of the pure aluminum is contemplated as well.

(15) The use of steam, steam ions, hydrogen, inert gasses and nitrogen and their ions and combinations during processing or cooling is also contemplated. Argon or other inert gasses, nitrogen hydrogen mixture gasses or hydrogen or other reducing or oxidizing gasses as required are contemplated as well. Braze joints made in this way can hold up to high-temperatures over 30-100 C. with high cyclability. Uses are also contemplated for joining purposes in the food and antibacterial industry, especially where the preservation of nanostructures may become important in the structure.

(16) An embodiment of an anticipated device is depicted in FIGS. 1 and 2. A high temperature structure 10 is shown comprising a core 20, leads 30 and braze joints 40. The leads 30 are secured into the core 20 by the braze material of the braze joint 40. The leads may be comprised of iron or other suitable materials listed in the background section above. The core 20 may be comprised of metal or non-metals, such as ceramic (anticipated ceramics include molysilicides, silicon carbides and Si.sub.3N.sub.4 (silicon nitride)) or other materials also listed above in the background section. The leads 30 may be positioned in-line in the core 20 or be side-by-side. In this embodiment the leads 30 are separated from each other by core material and brazing material. When assembled, the braze joint 40 may be initially comprised of pure aluminum braze material in the form of a foil. The structure is subsequently subjected to heat allowing the aluminum foil to melt and for the braze joint 40. The melting heat source may be a thermal plasma or through a friction process. Other types of braze materials and forms are anticipated as well.

(17) In operation, a power source (not pictured) is connected to the leads 30. Electrical current is run through the leads 30 and the core 20 thereby heating the structure 10. The electrical resistance of the core material produces a heat useful in ignition, heating as well as other applications.

(18) The above descriptions provide examples of specifics of possible embodiments of the application and should not be used to limit the scope of all possible embodiments. Thus, the scope of the embodiments should not be limited by the examples and descriptions given but should be determined from the claims and their legal equivalents.