CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is related to US Provisional Application No. 60/614,489 filed on October 1, 2004, US Provisional Application No. 60/614,490 filed October 1, 2004, US Provisional Application No. 60/614,483 filed on October 1, 2004, US Provisional Application No. 60/617,161 filed October 12, 2004, US Provisional Application No. 60/646,985 filed January 27, 2005 and International Application No. PCT/US05/OO9841 filed March 23, 2005, each of which are incorporated herein by reference in their entireties. BACKGROUND OF THE INVENTION

Technical Field of the Invention

The present invention relates generally to aluminum alloy bilayer brazing sheet materials including brazing sheet tube stock, and more particularly to increased strength and erosion/corrosion resistant aluminum alloy bilayer brazing sheet materials. The present invention also relates to methods for the manufacture and use of the brazing sheet materials of the invention.

Description of Related Art

Aluminum brazing sheet is widely used to manufacture various heat exchangers such as radiators, charge air coolers, evaporators and condensers in the automotive industry. One of the much-needed improvements in the automotive industry is the overall weight reduction in order to enhance fuel economy. The goal of weight reduction extends to all components of a vehicle including heat exchangers. Accordingly, research and development efforts are continuing to down gage the tube stock in automotive radiators, while increasing strength and erosion/corrosion resistance.

Long life of heat exchangers from the viewpoint of corrosion and/or erosion is of importance not only to improve their performance, but also to down gauging of radiator tube stock. The material properties of interest in this regard are 'post-braze' strength, braze flow, and internal (water-side) and external (air-side) corrosion resistance of the brazing sheet.

Typically, radiator tube material is a composite, with a non-heat treatable core alloy of 3xxx series, which is sandwiched between an inner liner and a 'braze' clad of an Al-Si alloy. The strength is provided by the core alloy, whereas the inner liner improves the coolant-side corrosion resistance. The air-side corrosion resistance is affected by the core alloy and interactions between core and 'braze' clad alloys during the brazing process. Development of a highly corrosion-resistant core alloy would permit manufacture of radiator tube material from bilayer material without liner, thereby simplifying manufacturing.

Formation of an anodic near-surface layer, known as 'brown band' through diffusion of Si into the core alloy during the brazing process is one of the methods of improving the external corrosion resistance. See, e.g., Marshall, et al., "Development of a Long Life Aluminum Brazing Sheet Alloy with Enhanced Mechanical

Performance " SAE paper 940505, (1994). Another way of increasing the air-side corrosion resistance is by adding Ti to the core alloy. See, e.g., Yasuaki, et al., "Development of Corrosion Resistant Brazing Sheet for Drawn Cup Type

Evaporators, Part 2: Application to Evaporator," SAE Technical paper No. 930149,

(1993).

In US Patent No. 4,673,551, Sugiyama et al. provide aluminum alloys suitable for construction of fin stock for super-high pressure service. hi US Patent No. 6,756,133, Palmer et al. provide aluminum alloy brazing sheet materials that have an increased yield strength when the materials have been "peak aged." The term "peak aged" refers to the treatment where a brazing alloy is subjected to a brazing cycle and then aged at various temperatures and times to determine its "peak age," i.e., the time and temperature combination where the maximum strength is observed. The peak-aged alloy of the Palmer et al. invention demonstrated yield strength at 175° C. The core alloy of the Palmer et al. invention comprises in weight percent based on the weight of the core alloy: less than 0.2% Si, less than 0.2% Fe, 1.3-1.7% Mn, 0.4-0.8% Mg, 0.3-0.7% Cu and less than 0.2% Ti, at least one element selected from the group consisting of Cr, Sc, V, Zr, Hf, and Ni, and the balance aluminum and unavoidable impurities.

Previously, it was virtually unheard of to be able to use brazing sheet tubes without waterside clad liners for applications such as radiator tubes for applications

that involve high temperature fluids flowing therein. While use of AA 3005 has been attempted, AA 3005 generally does not have sufficient strength characteristics and the necessary corrosion/erosion resistance.

SUMMARY OF THE INVENTION It was therefore an object of the present invention io obtain an alloy that is capable of being used for heat exchanger applications without requiring the use of a waterside liner. This invention provides bilayer aluminum braze sheets that save on manufacturing costs and also on the weight of the parts made thereof.

In accordance with this and other objects, there is provided a bilayer heat exchanger material comprising a braze layer and a core layer comprising an aluminum AA 3xxx series alloy including Mg in an amount from about 0.19 to about 0.72 weight % sufficient to invoke an improved yield strength and increased core erosion/corrosion resistance. In one aspect the yield strength is at least about 76 MPa. In one embodiment, the bilayer material has a yield strength of at least about 80 MPa at a temperature up to 225 ⁰ C except where specifically noted. In yet another embodiment, the material has a tensile strength of at least about 177 MPa. In yet another embodiment, the post-braze tensile strength is at least 145 MPa. In a particular embodiment, the yield strength and/or the tensile strength is measured at room temperature. The core layer of the material can further comprise at least one dispersoid- forming element. In one embodiment the dispersoid forming element is selected from the group consisting of Mn, Cr, Zr, and combinations thereof. In another embodiment, the dispersoid forming element comprises between' about 0.11 and 0.14 weight % Cr. In one aspect of the invention the core layer comprises between about 1.0 and about 1.7 weight % Mn. The core layer can further comprise about 0.12 weight % Cr.

In another aspect of the invention, the core layer further comprises about 0.16 weight % V. In a particular embodiment, the core layer further comprises about 0.1 weight % Si, about 0.26 weight % Fe, about 0.48 weight % Cu, about 1.5 weight % Mn, and about 0.11 weight % Cr. Moreover, in another particular embodiment, the

core layer further comprises a metal selected from the group selected from Ti, Zn, Ni, and combinations thereof.

In yet another aspect of a bilayer material of the invention, the core layer further comprises between about 0.40 and about 0.70 weight % Cu. A bilayer material of the invention can have a core layer which further comprises between about 0.03 and about 0.11 weight % Si.

In yet another aspect of the bilayer material of ^' the invention the core layer further comprises a metal selected from the group consisting of about 0.15 weight % Ti, about 0.01 weight % Zn, about 0.01 weight % Ni, and combinations thereof. _^ The said braze layer can be any suitable metal or alloy. Among many alloys suitable for the braze layer are AA 4343 and AA 4045 aluminum alloys.

The invention is also directed to devices prepared from a material of the invention. These devices can include a heat exchanger tube, a tube stock, a radiator tube, a condenser tube, an evaporator tube, and other like devices. In yet another aspect, the invention is directed to a method of reducing erosion/corrosion associated with fluid velocity in the interior of a heat exchange tube comprising forming said tube from a bilayer material of the invention.

In still another aspect, the invention comprises a bilayer heat exchanger material having a braze layer and a core layer wherein said core layer comprises an aluminum 3xxx series alloy having about 0.19 to about 0.72 weight % Mg and further comprises in weight percent based on the weight of the alloy: Si, 0.1 max; Fe, 0.2 max; Cu, 0.4 - 0.7; Mn, 1.4 - 1.7; Cr, 0.05 - 0.2; Ti ₅ 0.2 max; and. the balance aluminum and inevitable elements or impurities.

In yet still another aspect, the invention comprises an aluminum alloy suitable for use as a wrought product comprising: up to about 0.11 weight % Si, up to about

0.27 weight % Fe, between about 0.4 weight % and about 0.7 weight % Cu, between about 1.45 weight % and about 1.66 weight % Mn, between about 0.25 weight % and about 0.72 weight % Mg, up to about 0.14 weight % Cr, up to about 0.15 weight %

Ti, up to about 0.15 weight % Zr, up to about 0.16 weight % V, less than about 0.01 weight % Zn and Ni, and the balance aluminum and normal and/or inevitable elements and impurities. The invention also comprises an aluminum alloy further

comprising Cr between about 0.11 weight % and 0.14 weight %. Moreover, the invention also comprises an aluminum alloy which further comprises substantially 0 weight % Zr.

In another aspect the invention comprises a heat exchanger material comprising a braze layer and a core layer comprising an aluminum 3xxx series alloy having about 0.57 weight % Mg, sufficient to increase core erosion/corrosion resistance.

In one aspect, the invention also comprises a wrought aluminum alloyproduct consisting essentially of two layers wherein a first layer comprises the alloy of the invention and a second layer comprises a brazing layer. In one aspect, the invention comprises a wrought alloy aluminum product wherein said product has a yield strength of at least about 76 Mpa. Moreover, in another aspect the invention comprises a wrought aluminum alloy product wherein said product has a post-brazed tensile strength of at least about 177 MPa. There are further provided methods for preparing brazing sheets as described herein as well as methods for use of brazing sheet materials including tube stock and heat exchangers as well as further applications.

Additional objects, features and advantages of the invention will be set forth in the description which follows, and in part, will be obvious from the description, or may be learned by practice of the invention. The objects, features and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate particular embodiments of the invention, and, together with the general description given above and the detailed description of certain embodiments given below, serve to explain the principles of the invention.

Chart 1 depicts the effect of temperature on the yield strength of certain aluminum alloys. Chart 2 depicts the effect of temperature on the tensile strength of certain aluminum alloys.

Chart 3 depicts the through-thickness corrosion profiles of certain aluminum alloys.

Chart 4 depicts the through-thickness corrosion profiles of certain aluminum alloys. Chart 5 depicts the effect of temperature on the yield strength of certain aluminum alloys.

Chart 6 depicts the effect of temperature on the tensile strength of certain aluminum alloys.

Chart 7 depicts the effect 'of temperature on the yield strength of certain aluminum alloys.

Chart 8 depicts the effect of temperature on the tensile strength of certain aluminum alloys.

Chart 9 depicts the effect of temperature on the tensile properties of certain aluminum alloys. Chart 10 depicts the effect of temperature on tensile properties of certain aluminum alloys.

Chart 11 depicts the through-thickness corrosion potential profile of certain aluminum alloys as a function of the depth from the surface.

Chart 12 depicts metal fatigue of two aluminum alloys. Chart 13 depicts through-thickness corrosion potential of as-brazed and peak- aged alloy Al 1.

Chart 14 depicts metal fatigue of two aluminum alloys. Chart 15 depicts creep data of two aluminum alloys at 150°C. ■ Chart 16 depicts creep data of two aluminum alloys at 200°C. Chart 17 depicts creep data of two aluminum alloys at 25O ⁰ C.

Figures 1-8 illustrate microstructure and grain structure of aluminum alloys 1- 5.

Figures 9-12 illustrate SWAAT corrosion damage of aluminum alloys 1-4.

Figure 13 illustrates microstructure and grain structure of aluminum alloys A1-A8.

Figure 14 illustrates SWAAT corrosion damage of aluminum alloys A1-A8. Figure 15 illustrates microstructure of aluminum alloy Al 1. Figure 16 illustrates grain structure of aluminum alloy All.

Figure 17 illustrates corrosion damage in aluminum alloy All. Figure 18 illustrates corrosion damage in peak-aged aluminum alloy Al 1. DETAILED DESCRIPTION OF A PARTICULAR EMBODIMENT Definitions Aluminum Association alloy designations are used as appropriate. Thus the notation 3xxx is used to indicate an aluminum alloy in the 3000 series. Certain non- standard alloys are designated with proprietary numbers.

Ultimate Tensile Strength (UTS), Yield Strength (YS) and Elongation (E) of alloys were determined according to ASTM B557. As disclosed herein, "heat exchanger tube materials," can include, but are not limited to radiators, charge air cooler, condensers, evaporator tubes, and the like. In particular, the invention provides improvements in heat exchanger tube core alloys that have increased 'post-braze' strength at both room temperature and normal heat exchanger tube operating temperatures, and core materials which have improved water-side erosion/corrosion resistance.

"Core layer" is used to describe the aluminum alloy layer adjacent to the braze layer.

"Erosion/corrosion," as used herein means substantially simultaneous mechanical and chemical action. For example, in charge air coolers and other heat exchanger applications, the tubes used in such applications are subject to both mechanical erosion through the high velocity of water or other liquid running therethrough, as well as, outside forces of rocks hitting the tubes when in use. At the same time, the tubes are subjected to chemical erosion due to environmental forces (e.g, salt or sand) as well as other contact with chemicals on their inner sides. The dual interaction, (that is mechanical and chemical), often leads to more rapid damage

of the heat exchange material (such as radiator tubes). The extent of erosion/corrosion in applications such as radiator tubes is affected by variables such as fluid velocity, test temperature and mechanical properties of the material.

"CAB" is controlled-atmosphere brazing. Metric and English units are used interchangeably.

As used herein and in the following claims, articles such as "the", "a" and "an" can connote the singular or plural.

The components of the alloys, when present in elemental or combined forms, are generally referred to by standard symbols, including Cu for copper, ^' Cr for chromium, Fe for iron, Mn for manganese, Mg for magnesium, Si for silicon, Ti for titanium, V for vanadium, Zr for zirconium, and so forth. Compositions are provided in units of weight percent (weight % or weight %) based on the total weight of the composition.

The 'post-braze' strength of 3xxx aluminum alloys can be improved to some extent through alloying modifications. The applicable strengthening mechanism is primarily solid solution strengthening. Grain size strengthening is another mechanism, wherein smaller grain size can contribute to an increase in strength at lower temperatures. Mg is an element of interest for solute strengthening. "Solute strengthening" is a metallurgical term, whereby solute atoms are of a size and lattice parameters that they allow for strengthening of the alloy to occur. Precipitation hardening to a small extent is also possible if Mg is present in the core alloy. The mechanism of age hardening involves precipitation of Mg ₂ Si during the 'post-braze' aging treatment. The gain in strength through age hardening, however, is not stable at elevated temperatures because of coarsening of precipitate particles. Alloying additions, such as Mn, that result in fine dispersion of intermetallic particles (e.g. MnAl ₆ ) may result in some dispersion strengthening. Intermetallic dispersoids, being thermally stable, may provide some elevated temperature strengthening, which is of interest to enhance the operating temperature of charge air coolers. Starting with a typical 3xxx aluminum alloy, the compositions were modified by magnesium addition to invoke a solute strengthening effect. Advantageously, Mg is present in an amount from 0.01 to about 0.72% based on the

weight of the alloy, particularly from about 0.19-0.72 weight %. Modifications involving dispersoid-forming elements such as Mn, Cr and Zr are also disclosed. The dispersoid forming elements are particularly present in total in an amount from about 0.75 to about 2.15 weight %. Mn, if present, is preferably present in an amount from about 0.1-2.0 weight %, particularly preferably from 1.0 to 1.7 weight %, Cr if present is from about 0.01 to about 0.5 weight %, particularly from 0.05 to 0.2 weight %, and Zr if included is preferably included in an amount from about 0.01 to about 0.16 weight %, particularly from O.I to 0.16 weight %.

Alloys of the present invention can function as liners themselves. Different alloy compositions of 3xxx aluminum alloys are disclosed which improve strength at ambient and elevated temperatures. The present invention is not limited to an aluminum alloy that does not have an inner liner however.

In accordance with one particular embodiment of the present invention, there is provided a heat exchanger material having a core layer having alloys of the following composition: Mg up to 0.72 weight % maximum, Si from 0 to 0.11 weight %, Fe from 0 to 0.2 weight %, Cu from 0.4 to 0.7 weight %, Mn from 1.4 to 1.7 weight %, Cr from 0.05 to 0.2 weight %, Ti from 0 to 0.2 weight %, Zr from 0 to 0.25 weight %, and Zn up to 0.1 weight % maximum, with the remainder, aluminum and inevitable elements or impurities. In accordance with another particular embodiment of the present invention, there are provided alloys suitable for use as a wrought product and having the following composition: Mg up to about 0.72 weight % maximum, Si from 0 to about 0.11 weight %, Fe from 0 to about 0.27 weight %, Cu from about 0.4 to about 0.7 weight %, Mn from about 1.4 to about 1.7 weight %, Cr from about 0.05 to about 0.2 weight %, Ti from 0 to about 0.2 weight %, substantially 0 weight % Zr, V up to about 0.16 weight % maximum, and less than about 0.01 weight % Zn and Ni, with the balance, aluminum and inevitable elements or impurities.

A. wide range of different alloys can be used in connection with the present invention. Exemplary alloys are listed below: Alloy a, comprising 0.07 weight % Si, 0.17 weight % Fe, 0.70 weight % Cu,

1.45 weight % Mn, 0.257 weight % Mg, 0.139 weight % Cr, 0.147 weight % Ti,

0.144 weight % Zr, and about 96.92 weight % Al and inevitable elements or impurities.

Alloy b, comprising 0.03 weight % Si, 0.04 weight % Fe, 0.461 weight % Cu, 1.655 weight % Mn, 0.717 weight % Mg, 0.1 18 weight % Cr, and about 96.98 weight % Al and inevitable elements or impurities.

Alloy c, comprising 0.106 weight % Si, 0.263 weight % Fe, 0.475 weight % Cu, 1.524 weight % Mn, 0.571 weight % Mg, 0.006 weight % Zn, 0.003 weight % Ni, 0.110 weight % Cr, 0.05 weight % Ti, 0.158 weight % V, and about 96.73 weight % Al and inevitable elements or impurities. EXAMPLES Example 1

The K328 core alloy (see Table IA- IB for the compositions), was modified with Mg, Cr, Cu, and/or Zr, to prepare alloys with modified properties. The elevated temperature performance of the new alloys was measured. K328 ingot slices were re-melted and. their compositions modified. The K328 and modified alloy ingots were machined and then brazed with AA 4045 alloy (cladding). The composites with the desired layer thickness of braze clad were processed to a final H24 temper. Pre- and post-braze tensile properties were measured at room temperature. In addition., elevated temperature tensile tests, braze flow and core erosion, corrosion potential measurements and SWAAT corrosion tests were made.

Processing Details

TABLE IA. Elemental analysis of modified K328 core alloys

IO

These composites were roll-bonded by hot rolling to 0.110" gage. The hot band of 0.110" gage was processed to H24 temper making use of the following steps.

Step No. 1 involved annealing the hot band at 710 ⁰ F for 2 hrs with a heat-up rate of 5O ⁰ F/ hour and then air cooling to form O-temper.

Step No. 2 involved cold rolling the alloy to 0.012" gage.

Step No. 3 was annealing at §40°F for 4 hrs with a heat-up rate of 50 ⁰ F/ hour and then air cooling to form H24 temper.

The braze clad thickness of different materials was measured by Image Analysis of optical metallography. The resulting data are listed in Table IB.

TABLE IB. Composite (AA 4045 / modified K328 core) layer thickness details

The following evaluations were performed for the modified aluminum alloys: pre- and post-braze tensile properties at room temperature, pre- and post-braze microstrucrure and grain structure, post-braze tensile properties at elevated temperatures (up to 350 ⁰ C), braze flow and core erosion, corrosion potential measurements, and SWAAT life and corrosion damage.

EXPERIMENTAL DETAILS FOR EXAMPLE 1

Coupons of (i) 2-3/16" width and 4-7/8" length and (ii) 2-1/2" width and 8- 10" length were brazed by CAB using the following braze cycle: (1) Ramp to 572 ⁰ F in the course of 15 minutes, (2) hold at 572 ⁰ F for 3 minutes, (3) ramp from 572 ⁰ F to 950 ⁰ F in the course of 8 minutes, (4) hold at 950 ⁰ F for 1 minute, (5) ramp from 95O ⁰ F to 1067 ⁰ F in the course of 6 minutes, (6) hold at 1067 ⁰ F for 1 minute, (7) ramp from 1067 ⁰ F to 1112 ⁰ F over 2 minutes, (8) hold at 1112 ⁰ F for 3 minutes, and then (9) pull and air cool.

Tensile tests. Post-braze tensile tests were performed according to the procedures of ASTM:B557-94. Tensile specimens were machined from brazed

coupons. Elevated temperature tensile tests were performed at temperatures up to 350°C by heating in a resistance heating furnace.

Metallography. Metallographic examination of pre-braze and post-braze samples was carried out using standard methods of specimen preparation. The samples were anodized using Barker's etch for observing the grain structure.

Braze flow and core erosion. Standard drip strips were brazed by CAB and their braze flow was evaluated from the weight of the drip. It is expressed as percent of the total amount of clad available for melting and flow.

The core thickness before and after brazing was measured metallographic ally by Image Analysis. The core erosion is calculated from:

Core erosion (%) = [1 - (T _c / T _co )]*100 where T _c is core thickness in the post-braze sheet and T _co is the original core thickness in the pre-braze material.

Corrosion potential measurements. Through - thickness corrosion potential measurements were carried out in the post-braze condition from the surface (bxaze clad side and water-side) to the center of the sheet according to the procedures of ASTM - G69.

SWAAT (salt water acetic acid) Corrosion Test. Brazed drip strips were corrosion tested according to the SWAAT, ASTM G85-A3 test procedure. The corrosion damage in SWAAT tests was evaluated by preparing failed SWAAT coupons and examining them metallographically.

RESULTS OF EXAMPLE 1

Pre- and post-braze tensile properties of various materials at room temperature are listed in Table 2. Among the different materials, highest post-braze tensile strength of ~ 178 MPa is exhibited by material # 2 at room temperature, whereas the corresponding strength of un-modified material # 3 is 161 MPa. See also Charts 1 and 2.

TABLE 2. Pre-braze and Post-braze tensile properties of modified K328 materials

The elevated temperature tensile data are presented in Tables 3-7 and Charts

1-2.

TABLE 3. Effect of temperature on post-braze tensile properties of material # 1

The tensile results are summarized below:

(1) The yield strength decreases with temperature above 200 ⁰ C. (2) The tensile strength materials # 1 and # 2 are nearly the same as each other over the entire temperature range. (3) Relative to the base material (# 3), materials ^• # 1 and # 2 have ~ 20 MPa greater tensile strength than base material #3 in the temperature range below 225 ⁰ C. (4) The tensile strength of material # 2 is about 135 MPa at 225°C ~ 100 MPa at 250 °C. (5) The tensile strength of base material # 3 and that of materials # 4 and # 5 are similar over the entire temperature range. (6) Mg appears to be a more potent strengthener than Cr and Zr.

The microstructures and grain ^; structures of different materials are illustrated in Figures 1- 8. The braze flow and core erosion data of various materials shown in Table 8 indicate good braze flow and absence of core erosion (< 10%) in all materials.

Through-thickness corrosion potential profiles of various materials from braze clad side are shown in Charts 3 and 4. In general, an anodic layer with a potential difference of ~ 20 mV is observed at a depth of ~ 25 microns from the surface in all materials.

The SWAAT life of the tested materials is better than 650 hours. See Table 9.

SWAAT data are not available for alloy #5. SWAAT corrosion damage of different coupons is illustrated in Figures 9 - 12. Formation of an anodic "brown band" layer near the surface and lateral progression of corrosion damage can be noted from these

Figures in all materials. In a few coupons, shorter SWAAT life is associated with intergranular mode of corrosive attack [See Figure 11 (a)] .

TABLE 4. Effect of temperature on post-braze tensile properties of material #

2

TABLE 5. Effect of temperature on post-braze tensile properties of material # 3

TABLE 6. Effect of temperature on post-braze tensile properties of material # 4

TABLE 7. Effect of temperature on post-braze tensile properties of material # 5

TABLE 8. Braze flow and core erosion data of modified K328 materials

TABLE 9. SWAAT data of materials with modified K328 core alloys

+ Test terminated without failυre

Example 2

Another series of 3xxx aluminum alloys were prepared having the compositions shown in Table 10. The alloying additions were made with a view toward enhancing the alloy strength at room and elevated temperatures and thereby identify the suitable operating temperatures of the alloys. The alloying additions included dispersoid forming transition metals and also solute strengtheners. Since low Mg compositions are of interest for brazing by CAB, alloys A9 and AlO were prepared in which the Mg was kept at a low level.

The, braze sheets were processed to the H24 temper as follows: 1-side AA 4045 braze clad (10%); hot band gage, 0.110"; anneal hot band (700°F/2 hours); cold roll to 0.012" final gage; and H24 temper (540°F/4 hours).

Parameters of the new alloys were compared to a clad aluminum alloy designated CA43 in which the core was 0.08 Si, 0.20 Fe, 0.50 Cu, 1.05 Mn, 0.25 Mg, 0.02 Ti by weight % and the balance Al, and the braze was an alloy having 10.5 weigfϊt % Si, 0.3 weight % Fe, 0.1 weight % Cu, 0.1 weight % Mn, 0.05 weight % Mg ₅ 0.1 weight % Zn, 0.0005 weight % sodium and the balance aluminum and inevitable or unavoidable elements (K400).

RESULTS OF EXAMPLE 2

Various braze sheets were vacuum brazed making use of a standard brazing cycle shown below and their post-braze tensile properties determined over the temperature range of 68°F-617°F (20°C-325°C). In order to explore the extent of age hardening that may occur in these alloys, the alloy samples were peak-aged at 347°F (175°C)/48 hours and their tensile properties evaluated at different temperatures. Age hardening in alloys A9 and AlO was not evaluated. The vacuum brazing cycle was as follows: preheat to 45O ⁰ F; ramp to 800°F for 9 minutes; ramp from 800°F to 1070°F for 5 minutes; hold at 1070°F for 5 minutes; ramp from 1070 ⁰ F to 1100 ⁰ F for 2 minutes; hold at UOO ⁰ F for 3 minutes; and then pull and air cool.

Tensile property data of various alloys are shown in Tables 11-21. Plots of the tensile strength of various alloys in as-brazed and peak-aged states vs. temperature are

presented in Charts 5-8. The CA43 data are also shown in each of the plots for comparison.

Among the investigated alloy modifications, alloy # A8 exhibited the most improvement in strength at room and elevated temperatures. See Chart 9. The room temperature strength enhancement of alloy # A8 is - 5-6 Ksi relative to- CA43. Alloys # A2, A3 and A8 exhibited the best peak-aged strength at room temperature. By comparing the strengthening effect of different alloying additions consisting of transition elements, the most and least effective strengtheners are noted to be Zr and Sc, respectively. The tensile properties of alloys A9 and AlO are plotted in Chart 10 as a function of temperature. The alloys A9 and AlO are relatively low Mg alloys.

Through-thickness corrosion potential profiles were generated for the alloys

A9 and AlO making use of corrosion potential measurements (according to ASTM —

G69 procedures) at different depths from the surface (braze clad side) of the braze sheet and the results are shown in Chart 11. The potential difference between the surface and interior is ~ 20 mV.

The post-braze grain structures of various alloys (Al to A8) are illustrated in Figure 13. Elongated grains can be noted.

SWAAT corrosion tests were performed according to ASTM G85-A3 procedure, and the results are shown in Table 22. It is noteworthy that Sc addition increases SWAAT life significantly. Alloys A2 (Zr), A6 (Mn) and A8 (Mn+Mg+Cr) also exhibit long SWAAT life. Among the various alloys, the SWAAT life of that with Ni is relatively poor. The SWAAT life in general is either improved by the peak-age anneal, or substantially unaffected. SWAAT corrosion damage was assessed metallographically and the nature of damage in various alloys is illustrated in Figure 14. The mode of corrosion damage is mostly of the lateral type. In the alloy with Ni, a layer-like attack of corrosion is seen (Fig. 14 a/b); but it spreads rapidly through-thickness of the braze sheet.

The fatigue response of alloy # A8 was investigated. Constant - stress amplitude axial fatigue tests were performed according to ASTM Designation: E 466-

82. The tests were conducted at a stress ratio (R = σ _m j _n / σ _max ) of 0.1 and test

frequency in the range of 10-40 Hz. The fatigue data shown in Chart 12 indicates that the fatigue response of A.8 is similar to that of CA43.

SUMMARY OF RESULTS OF EXAMPLE 2

In summary, the highest strength at room and elevated temperatures was obtained by ihe modified core alloy # A8 involving addition of Mn (1.66), Mg (0.72) and Cr (0.12) to K320. The strength enhancement of alloy # A8 at room temperature is ~ 5-6 Ksi relative to CA43. Maximum age hardening effect was ^" also observed in the A8 alloy. Moreover, superior strength, both in as-brazed and peak-aged states, can also be noted in the alloy with Zr addition (A2). Among the various alloying additions, Sc is least effective in strengthening the braze sheet. Alloys A2 to AlO exhibited good SWAAT corrosion resistance.

TABLE 10. Compositions (weight %) of modified core alloys

TABLE 11. Effect of temperature on tensile properties of as-brazed CA43 Basel ine data

TABLE 12. Effect of tem erature on tensile ro erties of Al allo

TABLE 13. Effect of temperature on tensile properties of A2 alloy

TABLE 14. Effect of temperature on tensile properties of A3 alloy

TABLE 15. Effect of temperature on tensile properties of A4 alloy

TABLE 17. Effect of temperature on tensile properties of A6 alloy

TABLE 18. Effect of temperature on tensile properties of A7 alloy

TABLE 20. Effect of temperature on tensile properties of as-brazed A9 alloy

TABLF _^ 21. Effect of temperature on tensile properties of as-brazed AlO alloy

TABLE 22. SWAAT life results

* 6 samples tested in each case.

Example 3

A further core alloy is shown in Table 23. A new braze sheet (designated as brazed Al 1) was prepared with the alloy. Evaluations of Al 1 included pre-braze and post-braze tensile properties, microstructure, SWAAT life and corrosion damage, and through-thickness corrosion potential measurements, fatigue tests, and creep tests.

The pre-braze yield and tensile strength of All are 11.61 Ksi and 27.09 Ksi, respectively. The peak-aging resulted in an increase of ~ 4 Ksi in the yield strength, but the tensile strength remained the same. The average SWAAT corrosion life of the braze sheet was 254 hours in the as-brazed state. On peak-aging, the SWAAT life increased to 450 hours and the mode of corrosion damage changed to lateral type. Fatigue and creep properties of Al 1 are better than those of CA27.

TABLE 23. Alloy chemistry (weight %) of CAC alloy # All

The cast ingot was further processed into a braze sheet by: clad rolling (1-side AA 4045 braze clad of 10% thickness); hot band gage: 0.110"; anneal hot band (700°F/2 hrs); cold roll to 0.012" final gage; and H24 temper (540°F/4 hrs).

The following evaluations of the braze sheet were performed: pre- and post- braze metallography; pre- and post-braze tensile properties: tensile properties in peak- aged (PA) condition; SWAAT corrosion testing; corrosion potential measurements; fatigue testing; and creep testing. Details of the methods are provided below.

EXPERIMENTAL

Vacuum Brazing. Coupons of (i) 2-3/16" width and 4-7/8" length and (ii) 2- 1/2" width and 8" length were vacuum brazed using the following braze cycle:

Preheat to 450°F

Ramp to 800 °F over the course of 9 minutes Ramp from 800 °F to 1070°F over the course of 5 minutes Hold at 1070°F for 5 minutes Ramp from 1070°F to 1100°F over the course of 2 minutes

Hold at 1100°F for 3 minutes Pull and air cool.

Metallography. Metallographic examination of pre-braze and post-braze samples and SWAAT corrosion damage was carried out using standard methods of specimen preparation. The samples were anodized using Barker's etch for observing the grain structure.

Tensile tests. Pre- and post-braze tensile tests were performed according to the procedures of ASTM:B557-94. Tensile specimens were machined from brazed coupons of 2-1/2" width and 8" length. SWAAT and Corrosion potential measurements. The brazed coupons of 2-

3/16" width and 4-7/8" length were exposed to SWAAT conditions according to the ASTM G85-A3 procedure. Post-braze corrosion potential measurements were also carried out from the surface to the mid-thickness position of the braze sheet according to the procedures of ASTM - G69. _' Fatigue testing. Constant stress amplitude axial fatigue tests were performed on post-braze sheet specimens according to ASTM Designation: E 466 - 82.

Specimens of 0.5" width and 2" uniform gage section, with tangentially blending fillets between the uniform test section and the ends (grip section), were used for these fatigue tests. All tests were conducted at a stress ratio (R = σ _m i _n / σ _max ) of 0.1 and test frequency in the range of 20-40 Hz. For comparison, fatigue data were also generated for CA27 braze sheet which is AA 3003 core with AA. 4343, cladding at 7.5%.

Crεψ testing. Constant load creep tests were performed at temperatures of 302 ⁰ F, 392°F and 482°F to assess their creep behavior. For comparison, tests were performed on CA27 as a reference material. RESULTS OF EXAMPLE 3

Pre- and post-braze microstructures and grain structures of the samples are illustrated in Figures 15 and 16.

The tensile properties listed in Table 24 indicate that the post-braze yield and tensile strength of alloy #A11 are 11.61 Ksi (80 MPa) and 27.09 Ksi (186.8 MPa), respectively. On peak-aging, its yield strength increased to 15.88 Ksi and the tensile strength to 27.18 Ksi (Table 25). An increase in yield strength of - 4 Ksi occurs on peak-aging, but the tensile strength remains the same. By comparison, the yield and tensile strength of CA27 are 6.31 Ksi and 17.22 Ksi, respectively in the as-brazed condition. SWAAT. results are summarized in Table 26. The SWAAT life of as-brazed . coupons was 254 hrs. As a result of peak-aging, the SWAAT life increased to 450 hrs. From the micrographs showing SWAAT corrosion damage, it is seen that the lateral type of corrosion damage is sustained to a greater degree in peak-aged state than in as-brazed condition. Compare Figures 17 and 18. The through-thickness corrosion potential profile of the braze sheet from the surface to the mid-thickness position is shown in Chart 13. In the peak-aged condition, the corrosion potential difference between the surface and the interior is ~ lO raV.

Fatigue data in the form of plots of peak stress (S) vs. number of cycles to failure (N) is presented in Chart 14. An improvement in fatigue strength (at a given

number of cycles to failure) of alloy #A11 can be noted relative to the response of CA27.

Constant load creep test data are presented in Table 27 and Charts 15-17 at different temperatures and stresses. From these data, the creep resistance of Al l is seen to be better than that of CA27

Thus the core alloy Al 1 offers superior features that make it advantageous for brazing and use as a bilayer heat exchanger material.

AdditionaU advantages, features and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, and representative devices, shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of tlie general inventive concept as defined by the appended claims and their equivalents.

All documents referred to herein are specifically incorporated herein by reference in their entireti es .

TABLE 24. Pre- and Post-braze tensile properties of CAC alloy # All

TABLE 25. Tensile properties of CAC alloy # A11-H24 in Peak-aged condition

TABLE 26. SWAAT results of CAC alloy # All -H24

TABLE 27. Creep data of CAC alloy # All vs. CA27