Thermal degreasing in reactive atmospheres and subsequent brazing of aluminum-based sheets or parts

Aluminum-based sheets or parts, such as formed aluminum assemblies contaminated with forming die lubricants, are cleaned by a solventless, thermal degreasing process at controlled temperature (about 300.degree.-400.degree. F.), and in specified atmospheres (reactive gas which is at least at atmospheric pressure) prior to brazing. The organic contaminates on the aluminum-based sheet or part are reacted with the reactive gas (air or oxygen) and removed from the surface of the sheet or part without disruption of the underlying protective oxide layer. Subsequent brazing is thereby facilitated.

BACKGROUND OF THE INVENTION 
The present invention generally relates to a solventless, thermal 
degreasing process and, more particularly, it relates to solventless, 
thermal degreasing of aluminum-based sheets or parts in a reactive gaseous 
atmosphere and subsequent brazing of the cleaned aluminum-based sheets or 
parts. 
Degreasing of metal is well established in the art and has been used 
extensively in processing metallic scrap material. Representative of the 
prior art patents relating to the degreasing of metallic scrap are U.S. 
Pat. Nos. to Kennedy (4,508,564), Fitzpatrick (4,654,088), Ruthven 
(2,104,102), Mathis (3,650,830), Erman (3,627,289), Ripoche (2,595,411), 
and Stephens (4,010,935). 
However, none of these references disclose a method whereby degreasing 
takes place in a solventless atmosphere. For example, Ruthven (2,104,102) 
discloses a thermal degreasing method which involves dipping a heated 
article in a solvent such as trichloroethylene at such a rate and in such 
quantity that the article will obtain a temperature of at least the vapor 
temperature of the solvent after the article leaves the bath of boiling 
solvent so that the article is devoid of a film of solvent as well as of 
greasy material when it leaves the vapor atmosphere. The use of solvents, 
such as trichloroethylene, for the reactive atmosphere during degreasing 
is an expense and may pose environmental concerns. 
In addition, a problem which has occurred with degreasing methods at 
elevated temperatures especially when using aluminum-based parts has been 
the further oxidation of the underlying aluminum alloy surface or the 
complete destruction of the protective oxide layer which normally exists. 
U.S. Pat. No. (2,856,333) Topelian and U.S. Pat. No. (4,684,411) Johnson 
et al both specifically disclose oxidizing the underlying aluminum-based 
alloy as part of their thermal degreasing method. By increasing the 
thickness of the initial thin, protective oxide layer of aluminum-based 
alloys in this manner, subsequent brazeability of the metallic alloy is 
severely compromised. That is, the additional oxidation, that can occur in 
high temperature thermal degreasing (i.e., around 500.degree. C. and 
above) forms a layer which is not readily brazeable. With magnesium 
bearing alloys, oxidation produces a duplex oxide on the outermost surface 
which degrades brazability. Careful maintenance of the temperature during 
thermal degreasing is necessary in order to prevent disruption of the 
existant protective aluminum oxide layer and thereby preserve the 
brazability of the aluminum-based sheet or part. 
In this regard, U.S. Pat. No. (4,016,003) Stauffer discloses the careful 
maintenance of temperatures to avoid ignition of volatile materials which 
could also conceivably preserve the underlying protective oxide layer of 
the metallic alloy in cleaning aluminum scrap. However, Stauffer does not 
control the temperature for this purpose but rather for the purpose of 
avoiding the ignition of volatiles. Moreover, Stauffer does not 
subsequently braze the metallic alloy. Rather, Stauffer subsequently melts 
the alloy as part of the scrap recycling process which clearly destroys 
the underlying protective oxide layer. 
Another degreasing method is shown by Hetherington in "ULVAC Aluminum 
Brazing Furnaces" Supp. to ULVAC Sales brochure E 3009 "Aluminum Vacuum 
Brazing Furnace" FB Series, which discloses a degreasing method that has 
been implemented in the Japanese automotive industry. Hetherington's 
solventless degreasing method for aluminum based alloys prior to brazing 
requires drawing a vacuum which precludes oil and grease from completely 
reacting. This method depends upon the complete evaporation of surface 
contaminants for surface cleaning. Dissociation of the surface contaminant 
during evaporation can produce residual surface contaminant species. 
Moreover, because vacuum conditions are required, Hetherington's 
degreasing method is not as economical as is desirable. 
Accordingly, there remains a need in the art for a degreasing method for 
aluminum-based alloys which preserves the underlying protective oxide 
layer so as not to compromise any subsequent brazing and which removes 
surface contaminants which can not be completely removed by evaporation 
alone. Further, the degreasing method should be solventless so to reduce 
cost and do as not to pose any adverse toxicological and/or environmental 
problems. Also, there is a need for a degreasing method which does not 
require a drawing a vacuum. 
SUMMARY OF THE INVENTION 
The present invention provides a thermal degreasing method which satisifies 
all of the aforementioned needs. In particular, the present invention 
provides a solventless, thermal degreasing process for cleaning an 
aluminum-based sheet or part to produce a more readily brazable sheet or 
part. A method for subsequent brazing of the cleaned aluminum-based sheet 
or part is also provided. 
In the degreasing portion of the instant process, the aluminum-based sheet 
or part is heated in the presence of a reactive gas such as air or oxygen, 
ammonia, hydrogen, etc., at least at atmospheric pressure and at a 
temperature of between 300.degree. and 400.degree. C. for a sufficient 
period of time, preferably between about 10 minutes to about 30 minutes, 
to volatize and remove any organic contaminants, such as oils or grease. 
The process is preferably useful in removing forming die lubricants from 
formed aluminum assemblies prior to brazing to form heat exchangers for 
automobile radiators. The reaction of the organic contaminants on the 
aluminum-based sheet or part with the reactive gas occurs without 
significant distruption of the underlying protective oxide layer and does 
not, therefor, interfer with the brazability of the sheet or part. The 
aluminum-based sheet or part is thereafter allowed to cool prior to 
brazing or it may be brazed immediately. 
Thus, in the preferred process, the aluminum-based sheet or part is brazed 
relatively shortly after having been cleaned. For that reason it is 
necessary, as stated above, to preserve the underlying aluminum oxide 
layer. In order to do so it is essential that the temperature of 
solventless, thermal degreasing be at or below about 400.degree. C. Any 
signficantly higher temperatures will alter the aluminum oxide layer and 
may oxidize any magnesium which is present in the aluminum-based alloy. 
Preferrably the aluminum-based alloy contains some magnesium in order to 
render it capable of fluxless brazing, which is the preferred brazing 
process. 
The preferred brazing process utilizes a filler metal alloy, such as one 
containing 89% aluminum, 1.5% magnesium, and 9.5% silicon, in a vacuum at 
around 600.degree. C. to join the formed, cleaned aluminum-based sheets or 
parts and form, for example, a heat exchanger for automobile radiators. 
Ideally the solventless, thermal degreasing process of the present 
invention is used in the automobile parts plant and is followed shortly 
thereafter by the preferred fluxless brazing process. 
Accordingly, it is an object of the present invention to provide a 
solventless, thermal degreasing method for aluminum-based alloys which 
preserves the underlying protective oxide layer so that subsequent brazing 
is not compromised. Also, it is an object of the present invention to 
provide a less costly reactive atmosphere for the degreasing process. 
Finally, it is an object of the present invention to provide a method of 
brazing aluminum-based sheets or parts which have been cleaned by 
solventless, thermal degreasing. 
Other objects and advantages of the invention will be apparent from the 
following description and the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the preferred embodiment of the present invention an aluminum-based 
sheet or part such as a formed aluminum assembly which is covered with 
various forming die lubricants, i.e., oils and/or grease, is heated in the 
presence of a reactive gaseous atmosphere such that the underlying 
protective oxide layer of the aluminum-based sheet or part is not 
disturbed but the organic contaminants are reacted, volatilized, and 
removed. The cleaned part is thereafter brazed. 
The preferred temperature range is from about 300.degree. to 400.degree. C. 
and the preferred heating time is for about 10 minutes to about 30 
minutes. Further, the preferred reactive atmosphere is air at atmospheric 
pressure although other reactive gases such as oxygen, ammonia, hydrogen, 
etc., and other pressures greater than atmospheric may also be used. 
The process of the invention will now be more particularly described by way 
of examples which particularly illustrate the removal of lubricating oils 
and grease from the aluminum-based sheet or part as the sheet or part 
undergoes the present solventless, thermal degreasing process. The 
following examples also compare the various process parameters, such as 
time, temperature, type of oil and reactive atmosphere components. It 
should be noted that it is preferred to tailor aforementioned parameters 
to the specific organic contaminant sought to be removed from the sheet or 
part. Using air as the reactive atmosphere is preferred since it removes 
most all the forming die lubricants used in the automotive industry. 
Table I shown the various aluminum-based sample sheets which were initially 
coated with various oils: 
TABLE I 
______________________________________ 
SAMPLE PREATION 
Sample 
______________________________________ 
Al-1 As received (not cleaned) 
Al-2 Ultrasonically cleaned in detergent and vapor 
degreased 
Al-3 Coated with press-working oil (#7B-1 for the 
radiator header from Oak International 
Chemical); degreased with trichloroethane 
Al-4 Surface pretreatment of Al-3; thermally 
degreased 1/2 hour at 400.degree. C. in vacuum 
Al-5 Coated with Anderson oil (Windsor-Durel #92385C 
oil from Anderson oil and Chemical Company); 
thermally degreased 1/2 hour at 400.degree. C. in vacuum 
Al-6 Coated with press-working oil; thermally 
degreased 1/6 hour at 400.degree. C. in vacuum 
Al-7 Duplicate of Al-3 
Al-8 Surface pretreatment of Al-3; thermally 
degreased 1/2 hour at 400.degree. C. in air 
Al-9 Surface pretreatment of Al-5; thermally 
degreased 1/2 hour at 400.degree. C. in air 
Al-10 Surface pretreatment of Al-6; thermally 
degreased 1/2 hour at 400.degree. C. in air 
______________________________________ 
The study of this example determined the thermal degreasing residues under 
both vacuum and air reactive atmospheres. Subsequently, surface analyses 
were performed with X-ray Photoelectron Spectrosopy (hereinafter XPS) 
which is a technique that provides an analysis of the top 40 angstroms of 
the surface. 
Two oils were used on the samples which were all made from aluminum braze 
sheet, MD-177, with both sides clad. One oil was Oak International 
Chemical press-working oil #7B-1 for radiator headers (referred to in 
Table I as press-working oil), and the other oil was Windsor-Durel #92385C 
thermal degreasing oil from Anderson Oil & Chemical Company (referred to 
in Table I as Anderson oil). Because the Anderson oil has a lower 
viscosity in comparsion to the press-working oil, significantly lighter 
loadings were obtained therewith. The oils were applied to one surface of 
the sample by swabbing. Table I describes the surface pretreatment and oil 
loading combinations studied. Table II below illustrates the oil coating 
weights: 
TABLE II 
______________________________________ 
OIL COATING WEIGHTS 
Weight of Oil 
Weight Loss in 
Samples Coating (g/cm.sup.2) 
Thermal Degreasing (g/cm.sup.2) 
______________________________________ 
(In Vacuum) 
Al-5 0.072 0.109 
Al-6 0.240 0.277 
(In Air) 
Al-9 0.047 0.052 
Al-10 0.211 0.239 
______________________________________ 
Table II would indicate that, the present invention's thermal degreasing 
process removed more volatile material from the surfaces of the samples 
than can be accounted for by the oil coating weight. However, a much more 
sensitive measure of the residue remaining after degreasing has been 
obtained with XPS, and the results are shown in Table III: 
TABLE III 
__________________________________________________________________________ 
SURFACE CHEMICAL ANALYSIS OF OIL 
CONTAMINATED Al BRAZE SAMPLES 
%C as 
%Al as 
% Mg % Na 
% Al 
% Si 
% S 
% C 
% N 
% O 
% Cu 
% P 
% Cl 
% F 
carbonyl 
metal 
__________________________________________________________________________ 
Al-1 1.1 1.3 
1.7 
.51 
84.4 
.53 
10.4 7.2 0.0 
Al-2 
2.1 21.6 
.49 38.9 
.72 
35.4 .59 24.0 7.6 
Al-3 
1.6 .55 19.3 
.37 
.24 
44.5 
.87 
31.3 .31 .80 
25.4 10.6 
Al-4 
6.5 .26 17.7 
.54 
.80 
50.2 
.83 
20.1 
2.0 19.0 
Al-5 
6.2 .29 19.4 
.56 41.6 30.4 13.9 8.7 
Al-6 
5.9 .52 17.0 
.57 36.3 34.1 3.6 13.3 7.5 
Al-7 
1.8 .37 19.4 
.56 
.57 
40.4 
1.4 
35.0 .52 32.6 10.7 
Al-8 
9.8 .31 16.6 
.33 33.0 
1.3 
38.6 22.0 3.1 
Al-9 
11.5 16.4 
.52 
.59 
33.3 37.7 21.0 4.7 
Al-10 
10.2 
1.4 15.8 
.36 29.8 41.0 1.5 20.8 5.2 
__________________________________________________________________________ 
With regard to Table III and each aluminum-based sample therein, the 
following composition characteristics of the surfaces were noted. The 
sample Al-1 had a contamination layer composed mainly of 84% carbon (7% 
carbonyl type binding and 93% hydrocarbon). The small aluminum 
concentration detected and the absence of metallic aluminum indicates that 
the hydrocarbon layer is roughly as thick as the escape depth of the 
secondary electrons being used in the analysis which is approximately 40 
angstroms. 
Sample Al-2 had a contamination layer composed of 38% carbon (24% carbonyl 
and 76% hydrocarbon), 35% oxygen and 22% aluminum (see Table III). Sample 
Al-2, which was not coated with an oil layer, was first ultrasonically 
cleaned in detergent and subsequently vapor degreased. The results 
indicate that the contamination has been reduced in thickness (about 40 
angstroms) by the degreasing process. 
Sample Al-3, which was coated with press working oil, was degreased with 
trichloroethane. The contamination layer thickness was similar to the 
ultrasonically degreased surface of sample Al-2. The sample Al-3 layer was 
composed of 44% carbon (25% carbonyl and 75% hydrocarbon), 31% oxygen and 
19% aluminum (see Table III). Similar results are shown for Sample A-7. 
Again, the resulting thickness of the organic residue and the underlying 
oxide layer was approximately 40 angstroms. 
For comparison, samples Al-4, Al-5, and Al-6 were thermally degreased under 
vacuum at 1.times.10-5 Torr (1.3.times.10-3 Pa) and at a temperature of 
400.degree. C. The contamination layer of Al-4 was composed of 50% carbon 
(19% carbonyl and 81% hydrocarbon), 20% oxygen, 18% aluminum and 6% 
magnesium (see Table III). The combined thickness of the organic residue 
and oxide layer is larger than that of Al-1 and Al-3. The resulting larger 
carbon and smaller oxygen concentrations indicate that a thicker organic 
layer is responsible for the difference. 
Sample Al-5 was coated with Anderson Oil and also thermally degreased under 
vacuum conditions at 400.degree. C. The low-level contamination layer 
consisted of 42% carbon (14% carbonyl and 86% hydrocarbon), 30% oxygen, 
19% aluminum and 6% magnesium (see Table III). XPS results indicated that 
the combined thickness of the organic residue and oxide layer was minimal, 
and similar to samples Al-2 and Al-3 which were solvent degreased. 
Sample Al-6 also had a low-level contamination layer which consisted of 36% 
carbon (13% carbonyl and 87% hydrocarbon), 34% oxygen, 17% aluminum, 6% 
magnesium and 4% phosphorous (see Table III). Similar to samples Al-2 and 
Al-3, the combined thickness of organic residue and oxide layer was 
minimal. A small concentration of phosphorous was detected but this is 
attributed to the residue characteristic of press-working oil. 
Samples Al-8, Al-9 and Al-10 were thermally degreased in air at 400.degree. 
C. for 1/2 hour. Sample Al-8 was coated with press-working oil and then 
thermally degreased at the aforementioned conditions. The low-level 
contamination level consisted of 33% carbon (22% carbonyl and 78% 
hydrocarbon), 39% oxygen, 17% aluminum, and 10% magnesium (see Table III). 
As indicated by the relative concentration of metallic aluminum, the 
combined thickness of the organic residue and oxide layer is only 
marginally larger than for the degreased samples Al-2 and Al-3 and the 
samples thermally degreased in vacuum, Al-5 and Al-6. The larger oxygen 
and combined aluminum and magnesium concentrations indicate that a 
slightly thicker oxide layer is present, presumably composed of a duplex 
of magnesium and aluminum oxide. The results indicate that the oxide layer 
is is only marginally thicker than found with the vacuum thermal 
degreasing process and of further importance, the organic residue is 
minimal. Subsequent brazing test have shown that the small amount of 
oxidation occurring during thermal degreasing in air does not degrade 
fluxless braze performance. 
Sample Al-9 which was coated with Anderson Oil also had a low-level 
contamination layer primarily consisting of 33% carbon (21% carbonyl and 
79% hydrocarbon), 38% oxygen, 16% aluminum and 11% magnesium (see Table 
III). The results indicated that the precent of carbon with carbonyl type 
bonding and the concentration of magnesium were increased by thermally 
degreasing in air relative to vacuum which indicates that oxidation of the 
organic contaminant and the magnesium occurs more readily in air 
processing. Relative thicknesses of the organic and oxide layers were 
similar to those observed with sample Al-8. 
Sample Al-10, which was coated with press-working oil and thermally 
degreased in air at 400.degree. C., had a contamination layer consisting 
of 30% carbon (21% carbonyl and 79% hydrocarbon), 41% oxygen, 16% 
aluminum, and 10% magnesium (see Table III). The results indicated some 
oxidation of the organic contaminant and magnesium. The thickness of the 
organic and oxide layers were similar to samples Al-8 and Al-9. Also, the 
residual phosphorous level was approximately half the level resulting from 
vacuum processing which implies that oxidation had assisted in the removal 
of the residue. 
The aforementioned results indicate that thermal degreasing in air is 
preferrable over a vacuum or solvent vapor. Thus, solventless thermal 
degreasing in air provides a viable alternative to vapor degreasing using 
a solvent such as trichloroethylene and also provides a more 
cost-effective reactive atmosphere. It should be noted that the preferred 
aluminum-based sheet or part will be one which is in need of cleaning so 
as to improve its subsequent brazability. 
Having thus described the thermal degreasing process of the present 
invention in detail and by reference to a preferred embodiment thereof, it 
will be apparent that certain modifications and variations are possible 
without departing from the scope of the invention defined in the appended 
claims: