US2454312A - High-strength corrosion-resistant aluminum alloy sheets - Google Patents

Description

Patented Nov. 23 1948 vrmsnsw Reynolds Metals 00. tion of Delaware No Drawing.

The object of the present invention is to provide a high-strength corrosion-resistant aluminum al- 10y sheet particularly adapted for aircraft.

v 'OFFlCE onaosron-nnsrs'ran'r Annoy snnn'rs 2 Claims. (cl. ee- 191.5)

It is recognized that sheets rolled from Duralumin types of aluminum alloys, generally containing silicon, copper, manganese and zinc as important constituents, possess high tensile and yield strengths together with suitable elongation factors. Of the alloys of suchclass, that in very en eral use, is commercially known as 245 (Cu 4.5%, Mn 0.6%, Mg 1.5%. balance aluminum and normal impurities). By reason of its relatively low resistance to corrosion the said alloy is, for certain aircraft use, clad with pure aluminum. It has, when fully heat treated, a specified tensile strength of 59,000 lbs. per square inch, with a yield strength of 39,000 lbs. per square inch. The

product, however, has a marked difference An electrical potential between the core and cladding. In light gauges it requires critical control in solution heat treatment and quenching, and, in a number of other respects, presents difficulties in treatment.

By means of the present invention, an aluminumalloy composition sheet is provided which not only possesses high corrosion resistance but has greater tensile and yield strengths than the said pure aluminum clad alloy, and less difficulty in treatment. The potential difference between core and cladding are relatively low in the "sheet when solution. and precipitation heat treated. My product is much less critical with respect to overheating in solution heat treatment.

The characteristic of my aluminum alloy composite sheet is that it consists of a Duralumintype core and a cladding equal to 5%-25% of the composite thickness, the cladding consisting of an aluminum alloy itselfof high tensile strength and wholly compatible with the core, in that the core and cladding have substantially equal response to solution heat treatment at given high temperatures, time, quenching conditions and aging conditions.

yIn making my product, a Duralumin core is pre- Thomas L. Frltzlen, Florence, designer to Richmond, Va, a corpora- Application omit. 2a, 194:,- Serial No. mg:

2 5 The cladding alloy is as follows: I Tun: H Percent Si .50- .90 Ms .a 4.: Mn .25- .75 Al and impurities balance NorI.-When certain other metals are brought into the composition, as b scrap imfiuritiesf' it is preierred that the pro be contra ed to the to owing: Fe .6% man, Cu .05% man, I 35% man, and that other "impurities be controlled to .05% max.

As an example, liner-slabs of the cladding alloy;

each amounting to 5% of a core slab, are placed on opposite faces of the latter. and the composite structure rolled into sheet of, .064 This sheet may then be subjected to a solution heat treatment which may be at 930-950" F. for about 15.20 m I0110We 1 y quenching.

For my clad sheet of various gauges. I prefe solution heat treatment for time periods according to the following table:

auze Specifically, the cladding and other operations may consist of the following:

A core ingot may be cast, say 10" x-40".x 72",

rblled to 7", and scalped to 6 /2". The cladding ingot may be cast, say 10" x 40" x 72", analyzed, homogenized, scalped to 9 hot rolled to .650". cold rolled to .4257 and sheared to the size of the scalped core ingot. The core and cladding may then be strappe'dtogether, reheated to about 800 F., hot rolled to .265" at said reheating tem perature, side trimmed and coiled, cold rolled as in a one stand, four high mill, such as the subject of Steckel's-Patent No. 1,779,195, annealed at about 750 F. for two hours, followed by. cooling to 500 C. at a rate of 50". per hour, their air quenched, cold rolled in a three stand tandem pared, the following general composition being preferred: v

. Team I Per cent 81 .75-1.25 Cu 4.0 -5.0 Mg .25- .55 Mn .7 -1.0 A1 and impurities" 1. balance Nor:.When certain other metals are brought into the composition, as b scrap be control ed to the owing: Fe .7 man, Cr 35% max.

"lm urities," it is referred that the pro rtions at of, 2.. .m, man, and that other "impurities" be controlled to .0595

r max mill, and cut in sheets, ready for solution heat treatment.

7 After solution heat treatment, my product may be quenched as in cold water, r'olled level and flattened, stretched, side sheared and subjected I to a precipitation heatfltreatment of about 18 hours at 320 F.

rtions assesses:

the ingot being analyzed, homogenized, hot

Both the core and the cladding obtain high Strength through the said solution heat treatment and precipitation heat treatment at elevated temperatures. They are both affected by quenching conditions in the same manner.

. My aluminum alloy composite sheet derives its excellent resistance to corrosion for two reasons, namely:

(1) A. slower diffusion rate, during solution heat treatment, through the alloy cladding than through a pure aluminum cladding, as determined by heat treating tests and microscopic examination.

(2) Less difference in electric potential between the core and cladding as compared to the same alloy core and a pure metal cladding, as determined through comparative potential measurement, and accelerated corrosion tests in standard NaCl-Hzoz solution.

In addition, the following advantages are present in my aluminum alloy composite structure over the currently usedaluminum clad 24S alloy:

(a) Easier to hot-roll composite slab and plate.

(b) Easier to cold-roll composite slab' and plate.

Lower mechanical properties in annealed condition with subsequent better formability.

(d) Solution heat treatment less. critical with respect to solution of soluble constituents and overheating.

(e) Solution heat treatment less critical with respect to quenching.

(I) Quenching of-formed aircraft parts after solution heat treatment can be carried out in oil at room temperature or in hot water, avoiding distortion.

(g) Stretching, rolling, and forming can be performed easier in the solution heat treated and quenched condition. I

.02.) Better welding characteristics.

(1) Lower cost due to use of low Mg content.

(7) Higher endurance limit or fatigue strength.

(It) Less spread in tensile mechanical properties in the fully heat treated condition.

Formed aircraft parts may be made from the sheet, prior to heat solution and precipitation treatment, and these parts given solution heat treatment, say at 935 F., followed by controlled, relatively slow, quenching, as a guard against distortion and subsequent aging, say for 18 hours at 320, or for 8 hours or for 6-10 hours at 340- 350 F.

Cladding may range from 5% to of the composite thickness of the sheet, and the solutlon heat treatment, quenching and aging may vary in details from the examples given.

In discussion of my clad sheets hereinafter they will be designated R 301. In the solution heat treatment thereof the tensile properties increase with increasing temperature up to 960 F. and decrease at temperatures over 970 F.

The symbol W hereinafter used defines the heattreated, but not artificially aged, condition of the sheet, and the symbol T defines the heat treated and artificially aged condition of the sheet. The symbol 0 signifies annealed and soft condition.

Stretching R-301-Wsheet 3 /2 and 5 /2 per cent raises the tensile and yield strengths appreciably and lowers the elongation when stretching and testing with grain.

Stretching R-301-W 3 and 5 /2 per cent with grain raises the with grain" yield strength and lowers the elongation when aged at 320F. as compared to RP301-T not prestretched. However, aging at 350 and 375 F. following prestretching R-301-W 3 and 5 per cent with grain does not produce consistently higher yield strength.

Maximum tensile properties can be obtained upon precipitation heat treatment at any time after solution treatment and quenching. Also, no consistent effect upon corrosion resistance has been noticed when precipitation heat treating at various periods after solution heat treatment.

The precipitation heat treating procedures recommended depend upon several factors; namely, whether further working, dimpling, or forming is to be performed after this treatment and the temperature uniformity of the furnace employed. If further forming is to be performed after precipitation heat treatment, a practice of aging 18 hours at 320 F. is desirable; however, if no further forming is to be carried out, aging for five hours at 360 F. is recommended, if the furnace employed is uniform to plus or minus 5 F. If the furnace to be used has a differential of plus or minus 10 F., or greater, a practice of aging for 6 hours at 350 F. is recommended. A somewhat lov'er elongation is obtained by the higher temperature-shorter time aging practices.

Unlike 248-0, my product can be formed, hammered, rolled, or drawn, then solution and precipitation heat treated to develop maximum tensile properties in the T" condition.

Tests have shown that sheets of my product have superior drawing characteristics to annealed 24S. Average Erichsen values obtained on R-301-0 sheet are as follows:

Gauge 'y g g Erlchsen Mm. .040" 12 8.54 .051" 12 3.12 .064 18 9.0a

The formability .of sheets of my product is superior to that of 248T or Pureclad 24S-T, offering an advantage in that a portion of those parts now requiring 248-0 or Pureclad 243-0 can be made using my product in the solution heat treated (W) condition, thereby eliminating subsequent heat treatment and distortion. For example, .064" R-301-W sheet can be bent on a power brake both with and across grain over a radius of 1 5" or IT. Average Erlchsen values obtained on R-30l-W sheet are:

Gauge ggg Erlchsen The Rockwell E hardness for the varioustempers of my clad sheet have been determined as follows: I R-301-0 45 to 50 R-301-W to R-301-T 100 to It is to be pointed out that R-301-T sheet possesses a harder surface than pure aluminum clad (A .I S sheet and subsequently is more resistant .to 7 same gauge (.032") and under the same condi- 'scratching and abrading. tions. as follows:

As stated above, in my product there less diflerence in electrical potential between th more P c and the cladding as compared to Lthe'same alloy 5 1 2 2%" P" Cent Per Cm core and a pure metal cladding. Comparative pog 120m mail? {gag tential measurements or my solution and. precipitation heat treated sheet shows the following: 1 7 5m 6 02 7o l 68 I P02453 1" .18 I20 10 I02 I Volts 1o a 001"" core- 0.72 '1 .084" cladding 0.82 The corrosion tests applied to my sheets and I p 4" composite 0,32 24ST and P. C. 24S-T were calculatedas equivalent to 2-4 years exposure to ocean salt spray mist The precipitation heat treatment 01 the tested (Hampton Roads).

sheet consisted of eighteen hours'total time in Precipitation heat treatment, 1. e. artificial ag- Lindberg furnace at 320 F. preceded by soluing, increases the strength or the core as well as tion heat treatment of twenty-two minutes in I cladding, and the corrosion resistance of the clad- Lindberg furnace at 940 F. soaking nineteen 5 ding is not materially lessened. Thus in .040"

minutes at 940 F. followed by cold water quench. ge maximum tensile Strength was Obtained.

. The potential measurements employed a, satafter solution heat treatment 15 minutes at 940 urated calomel electrode and solution, containdegrees F. with pr cipita ion heat treatment ing 57 grams sodium chloride and 100 cubic cenabout 20 hours at 320 F., or 12-14 hours at 340 timeters of 3% hydrogen peroxide solution per r about 7 h rs t 36 F.

liter. I Having described my invention, what I claim Even after severe corrosion tests my product and desire to secure by Letters Patent is as folretains substantially greater tensile properties O SI V than possessed by the said 24-S pure aluminum '1. A clad aluminum alloy composed oi a core clad alloy. The following 'table shows examples, or a. Duralumin type alloy having a cladding of the treated sheets,being of various gauges, the an alloy composed substantially entirely oi. Si .50-

composition oi the cladding being Si 310%, Mn 00%, Mn 25-35%. Mg .80-l.2% and the balance Mg 1.00%, balance aluminum, and the aluminum, the alloys of the core andcladding claddin ing 5% each side. having similar responsiveness to the same heat I TABLE I Corrosion tests on R-301-W and R-301-T sheet F Tensile Strength 1 Yield Strength I Elongation Percent Lbs/Sq. Inch Percent Lbs/Sq. Inch Perm, in 2 Ins. Perm, Loss in Guage I Temper Change I I Change Change gigs Original Corroded Original Corroded Original Corroded I y. molew 61,800 01,000 -1.3 41,000 40,000 -2 1 10.3 10.5 +1.0 .051" R30l-W 60, 200' 69, 800 0, 7 39, 200 38,300 2.3 21.3 20.3 0 .064" R30l-W 00, 700 59, 700 '1. 0 40, 100 39, 800 0. 7 l9. 2 19.8 I +3. 1 l8 66, 000 66, 600 -l. 0 59, 600 68, 200 2. 2 9. 0 9. 0 0 75 08,000 07,100' -l.3 60,300 00,600 +0.6 9. 0 8.7 -8.4 -.46 08, 900 06, 200 1. 0 60, 800 69, 400 --0. 7 l0. 0 9.8 2.0 24

horas l. Corrosion consisted of a 6-hour immersion in a solution oi-- 57 grams 01' sodium chloride 10 ml. of 30% hydrogen peroxide Balance of literdistilled water Specimens were cleaned rior to corrosion by immersion for 1 min. at 05 degrees 0. in-

50 ml. of concentrate nitric acid (70%) 50 ml. of hydrofluoric acid (48%) I Balance of liter-distilled water Alter cleaning, specimens were rinsed in distilled water, immersed one minute in concentrated nitric acid. rinsed in distilled water and dried: Alter corrosion, sglelcimens were cleaned by immersion in 70% nitric acid, rinsed in distilled water and dried by air and dessication. All of above spec ens exhibited pitting type of attack only upon microscopic examination. 1 I J Corrosion carried out at constant temperature of 75 degrees F. I .040 REM-W sheet was heat treated n plant air iumace receiving a 12-minute soak at 030 to 045 degrees F. .Obl" and .064" R30l-W sheet was heat treated in plant air furnace receivinga 15-minute soak at 030 to 946 degrees F. R30l-W sheet was aged in plant air furnace, receivingan 18-hour soak at 32) degrees F. r

The same corrosion tests applied to my clad treatments, there being not over .6 Fe in any sheets (R301-T) of .032" (5% each side) showed impurity content present in the cladding alloy.- the following maximum losses: 2. A clad aluminum alloy composed of a core of Per Cent Per Cent Percent Percent ire-1.25%, Cu 40-50%, Mg 25-55%, Mn 14.0% gaff; 005 Loss in Loss in and the balance aluminum, the said core alloy Strength Strength Emmi .having a cladding of an alloy composed substan- I tially entirely of Si .50-.90 Mg .8-l.2%, Mn

m- 11-42 .25-.75%-and the balance aluminumthe alloys of the core and cladding having similar respon- The above is comparative with corrosion resisti siveness to the same heat treatments, there being ancnf 24S-T and Pureclad 24S-T sheets of the not over .75% Fe in anyimpurity content present an alloy composed substantially entirely of Si 7 in the core alloy and not over 15% Fe-in any im- Number purity content present in the cladding alloy. 2,106,259 THOMAS L. FRIIZLEN. 2,122,535

REFERENCES CITED 5 The following references are of record in the Number file of this patent: 207,559-

UNITED STATES PATENTS Number Name Date 10 1,805,448 Frary May 12, 1931 1,865,089 Dix June 28, 1932 8 Name Date Stockmur Jan. 25, 1988 Noel: July 5, 1938 FOREIGN PATENTS Country 1 Date Sweden Feb. 16, 1940 OTHER REFERENCES Metals Handbook, 1939 Edition, Published by American Society For Metals, 7016 Euclid Avenue, Cleveland, Ohio, pp. 1270-71.