US3915758A

US3915758A - Surface hardening of aluminum alloys

Info

Publication number: US3915758A
Application number: US354619A
Authority: US
Inventors: Richard Martin Otto Weinbaum
Original assignee: Metal Leve SA Industria e Commercio
Current assignee: Metal Leve SA Industria e Commercio
Priority date: 1972-05-04
Filing date: 1973-04-25
Publication date: 1975-10-28
Anticipated expiration: 1992-10-28

Abstract

This invention provides aluminum alloys which have a Vickers surface hardness of about 250 kg/mm2 to 1,400 kg/mm2. This invention also provides a process for preparing such aluminum alloys by exposing said aluminum to a source of cyanide anion at a temperature of about 450*C. to 550*C.

Description

United States Patent Weinbaum [45] Oct. 28, 1975 SURFACE HARDENING OF ALUMINUM [56] References Cited ALLOYS UNITED STATES PATENTS [75] Inventor: Richard Martin Otto Weinbaum, 2,413,929 1 1947 Simpson 148/20 S210 Paulo, Brazil 3,268,372 8/1966 Brotherton et a]. 148/20 [73] Assignee: Metal Leve S.A., Sao Paulo, Brazil Primary Examiner Ralph S Kendall [22] Filed: Apr. 25, 1973 Assistant ExaminerCharles R. Wolfe, Jr.

Attorney, Agent, or FirmLadas, Parry, Von Gehr, [21] Appl 354619 Goldsmith & Deschamps Related US. Application Data [63] Continuation-in-part of Ser. No. 250,126, May 4, [57] ABSTRACT 1972 abandmed' This invention provides aluminum alloys which have a Vickers surface hardness of about 250 kg/mm to [52] US. Cl. l48/6.11; 148/627; 148/20; 400 2 i invention also provides a process 51 I Cl 2 1418/28 148/315 for preparing such aluminum alloys by exposing said 1 lrt. l i to a Source of Cyanide anion at a p Fleld Of Search 1, A, 20, ture of about 450C to 5500C 7 Claims, 19 Drawing Figures US. Patent FIG. l

FIG. 2

FIG. 3

Oct. 28, 1975 Sheet 1 of 8 U.S. Patent Oct. 28, 1975 Sheet 2 of 8 3,915,758

FIG.

\ US. Patent Oct. 28, 1975 Sheet30f8 3,915,758

FIG

US. Patent Oct. 28, 1975 Sheet 4 of8 3,915,758

FIG. ll

U.S. Patent OCL28,1975 Sheet 5 of8 3,915,758

FIG. l3

FIG. l4

FIG.I5

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US. Patent Oct. 28, 1975 Sheet7of8 3,915,758

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if) R SURFACE HARDENING OF ALUMINUMALLOYS This application is a continuation-in-part of my prior copending application Ser. No. 250126 filed May 4, 1972, now abandoned, and the benefit of the filing date of my prior application is claimed.

BACKGROUND OF THE INVENTION a. Field of the Invention It is obvious that the hardening of the surface of aluminum alloys is an extremely desirable phenomenon. Thus, the very best possible use of a lightweight metal such as aluminum could result if the hardness of aluminum could be increased so that it could be utilized in areas where heavier but harder metals (e.g.,-steel) are now used. To overcome thedrawback resulting from the relative softness of aluminum it has been the practice to replace aluminum portions with other metal portions at that point where hardness or surfaceresistance is critical. Thus, the points of severe wear and abrasion in machine parts are made-from metals other than aluminum, whereas, the remaining portions of many machines are made from aluminum.

By having to substitute heavier metals in certain portions of machinery which would otherwise be made from aluminum at least two drawbacks results-The first is that inexpensive single piecesof machinery components cannot be made and expensive piecing together of machine parts have to be carried out. Thesecond drawback is that the more aluminum which has to be replaced by heavier metals the heavier the machinery is. This latter drawback is particularly severe in the aircraft industry.

b. Description of the Prior Art There have been studies of the treatment of aluminum, but none have really been directed to the problem of increasing the surface hardness of aluminum alloys.

On Mar. 18, 1935 Professor M. H. LeChatelier, published a paper in the Academie des Sciences authored by Paul Laffitte and Pierre Grandadam entitled The Nituration of Several Metals (pages 1,039 to 1,041). A second paper was published Aug. 1, 1936 by M. P. Laffitte, M. M. E. Elchardus and P. Grandadam in the Revue de lindustn'e Minerale No. 375, page 861, entitled Research on the Nituration of Magnesium and Aluminum. A third paper was published in July and August 1935 in the Ann. de Chimie l lth series, t.4pages 1 18 to 123 by Pierre Grandadam, entitled Experiments on the Direct Oxidation of Platinum and the Nituration of Several Metals (Cu, Al, Mg, Zn, Fe, Ni, Ti). In all three of these publications experiments were described as the effect of nitrogen and ammonia at high temperatures on pure aluminum wire. No mention at all is made in these publications with respect to cast aluminum alloys or to cast and mechanically worked aluminum alloys or to the use of sources of cyanide ions as I employ in my invention.

A fourth publication is one published by the German Company Degussa in December 1970 and authored by Bruno Finnein entitled Wear Resistant Surfaces by Treating Titanium and Titanium Alloys in Salt Baths. In this publication titanium or its alloys are treated with salt baths containing cyanide ions at a temperature of 800C. Of course, experiments with titanium bear no relationship with resect to aluminum and, in addition, the use of temperatures in the range of 800C. could not possibly be used on aluminum alloys. Two other 2 reasons explain why the Degussa article bears no teaching whatsoever with regard to alluminum alloys. This Degussa article does not inform one of the compositions of the bath butmerely states that carbon and nitrogen form mixed crystals with titanium, which seems entirely doubtful to me. My opinion is that vanadium carbides and aluminum nitrides may be formed and these compounds may be responsible for the formation of a deep layer. Degussa, however, stages that the composition of the layer is unknown. Further in our case, I know that silicon, magnesium and nickel are necessary. The difference between treatment of titanium and treatment of aluminum appears to mainly consist in that the hard layer, in the case of aluminum, is mostly produced by alloy constituents of the interior of the Al alloy, such as silicon, whereas the hard layer of the titanium alloy is formed by direct reaction between the bath and the surface of the Ti alloy and there appears to be no migration of constituents from the interior to the surface.

SUMMARY OF THE INVENTION In accordance with this invention there is provided a cast alloy of aluminum or a cast and mechanically worked alloy of aluminum containing substantial quantities of silicon, copper, nickel and magnesium and, optionally, small quantities of iron, titanium manganese, zinc and chromium, characterized by Vickers surface hardness of about 250 kg/mm to about 1,400 kglmm depending on the composition of the alloy and its treatment. This is to be contrasted with a Vickers surface hardness for the untreated aluminum alloy in the range of kg/mm This increased surface hardness of said aluminum alloys is effected by treating said aluminum alloy with a source of cyanide ion at a temperature of 450 to 550C.

The alloying elements content in the aluminum alloys which can be treated by the process of this invention are those having the composition:

Si 0.2 to 30.0 Cu 0.2 to 5.6

Ni 0.2 to 6.0

Mg 0.2 to 5.0

Fe 0.0 to 1.0

Ti 0.0 to 0.2

Mn 0.0 to 1.5

Zn 0.0 to 2.0

Cr 0.0 to 0.7

The usual source of cyanide ion is sodium cyanide. Other cyanide sources can be used, for example, potassium cyanide.

The temperature at which the aluminum alloy is kept in contact with the source of cyanide is preferably at about 450 to 550C; At this temperature a time of exposure of the aluminum alloy to the cyanide source is preferably for a period of time of preferably from about 8 to about 12 hours.

In the heating time there is about one half hour incubation. Times longer than 20 hours do. not seem to be practical due to the parabolic relationship of time X depth of the layer. Shorter times in the order of about 4 hours treatment showed good results and a smaller surface layer depth resulted but it had the same hardness as with longer treatments. Furthermore, the zone behind this layer is not completely depleted of silicon crystals. This fact is very important from the point of view of the hardness gradient from the surface to the 3 core and of the anchorage of the surface layer in the matrix.

Studies of the treated alloys were conducted. A review of the micrographs of the treated alloys indicated the growth of an oxide layer simultaneous with the development of the hard particle layer.

X-Ray diffraction studies were conducted on the sur face of treated

alloys

124 and 138. The results showed that the surface of the treated alloys were coated with a spinel type oxide having a lattice parameter of 8.07A iO.2A. Two compounds, NiAl O (8.05A) and MgAl- (8.08A) are potential matches for the data, but electron microprobe data showed that the oxide contains Mg. Thus, MgAl O is the major constituent in the oxide layer. This layer contains numerous metallic particles which have been identified as Al Ni by X-ray diffraction which was consistent with microprobe data showing that these particles contain Ni. Also, the diffraction pattern and microprobe data show elemental Si to be present in the oxide layer. There was no indication of the oxidation of either Si or Ni.

In practice the cast aluminum alloy or cast and mechanically worked aluminum alloy was immersed partially or completely in the salt bath for the period of time and, after the exposure was completed, it was convenient to rinse the test piece with water. Hardness tests were then run on the treated aluminum surfaces utilizing the Vickers hardness test E: 92-55 as described in the 1955 Book of ASTM Standards, Part 1, Ferrous Metals, published by the American Society for Testing Materials, l9 16 Race Street, Philadelphia, Pa., at pages 1694 to 1699, hereafter referred to as Vickers Hardness. The magnitude of the test load used was 35 grams.

In preparing the salt bath, it is convenient to mix with the cyanide salt fluxes or other auxiliary agents which enable the salt bath to be kept conveniently at the 450 to 550C. temperature. Examples of such agents are the use of boron oxide or the use of a mixture of sodium hydroxide and sodium carbonate. These auxiliary agents do not enter into the reaction and merely permit the cyanide ion to be brought into a convenient form at the 450 to 550C. range in contact with the aluminum aly.

THEORY OF THE PROCESS While I do not want the interpretation of the claims for my invention to be restricted in any manner by any theory it seems that the phenomenon of surface hardness is connected with the migration of silicon from the interior to the surface, leaving behind the surface a zone of lower silicon content. The mobility of silicon atoms increases with the silicon content to a maximum of approximately 18 (e.g., alloy 138). With a further increase of silicon content this mobility decreases as for example with alloy 244 which has a silicon content of about 24. In addition to the silicon migration, the surface hard layer appears to comprise some Al Ni. Although the increase of hardness appears to be due mainly to the increase in silicon content in the hard surface layer, the hardness of alloy Y must be considered from a different point of view. The silicon content of the Y alloy is only 0.5 percent and con sequently'there is no essential migration of silicon. The hardness of the surface layer of the Y alloy can be attributed mostly to the formation of MgAl O which presumably occurs as a result of oxidation during the salt bath treatment.

Obviously a combination of concentrating silicon and/or Al Ni on the surface, as well as, the formation of MgAl O can occur to produce the surface hardness of the aluminum alloy as a result of the treatment of this invention. In addition, other undiscovered effects may occur.

That there are several different particles in the hard layer is really a very favorable aspect as far as wear resistance is concerned. Different hardness is parallel to different elasticity, which reduces the danger of removing brittle particles.

The oxide layer on the extreme outside has a different composition, mostly MgAl O as compared with the hard particle layer, which consists chiefly of Si crystallites. An exposure time of merely 4 hours produces layers with less thickness but practically the same hardness. It should be pointed out that hardness is a property of the crystallites 'formed while thickness of the hard surface layer depends on exposure time. Wear resistance, on the other hand, depends on both these factors: increased hardness usually increases wear resistance, and greater thickness implies in longer abrasion time of the layer.

The oxide layer consists mostly of MgAl O The oxide layer may contribute to the overall hardness; but its hardness is much below that of the Si layer. The hard layer consists of Si and some Al Ni. Also here Al Ni has a lower hardness than Si particles.

Even a short exposure time of 4 hours produces a hard layer of reduced thickness in

alloys

124, 138 and 244.

Time of exposure and temperature have a certain influence on the hard layer thickness. I found a parabolic relation for dependence of thickness on exposure time:

p=K V? DESCRIPTION OF THE FIGURES FIGS. 1 through 12 and 14 and 16 are photomicrographs of the alloys treated in accordance with this invention.

FIGS. 13 and 15 are photomicrographs of untreated alloys.

The data with regard to these Figures are as follows:

Unetched x Alloy l24 Treatment: NaCN 4% B 0 Temperature 530C; Time 4 hours Thickness of the hard layer 13 microns.

'fFIG. 2

Unetched 150x Alloy 124 Treatment: NaCN 4% B 0 Temperature 530C; Time 8 hours Thickness of the hard layer 18 microns.

FIG. 3

Unetched 150x Alloy l24 -continued Treatment: NaCN 4% B Temperature 530C; Time 12 hours Thickness of the hard layer 20 microns. FIG. 4

Unetched 150x Alloy 138 Treatment: NaCN 4% B 0 Temperature 530C; .Time 4 hours Thickness of the hard layer 8 microns.

FIG. 5

Unetched 150x Alloy 138 Treatment: NaCN 4% B 0 Temperature 530C; Time 8 hours Thickness of the hard layer 15 microns.

FIG. 6

Unetched 150x Alloy 138 Treatment: NaCN 4% B 0 Temperature 530C.; Time 12 hours Thickness of the hard layer 17 microns.

FIG. 7

Alloy 244 Unetched 150x Treatment: NaCN 4% E 0 Temperature 530C; Time 4 hours Thickness of the hard layer 12 microns.

FIG. 8

Unetched 150 x Alloy 244 Treatment: NaCN 4% H 0 Temperature 530C; Time 8 hours Thickness of the hard layer 14 microns.

FIG. 9

Unetched 150x Alloy 244 Treatment: NaCN 4% B 0 Temperature 530C; Time 12 hours Thickness of the hard layer 25 microns.

FIG. 10

Unetched 150x. Alloy 244 Treatment: NaCN 4% B 0 Temperature 520C; Time 12 hours.

Thickness of the hard layer 10 microns.

FIG. 11

Unetched 150x Alloy 124 Treatment: NaCN 4% B 0 Temperature 520C; Time 12 hours The start of migration of Silicon to the surface of the test piece can be observed.

FIG. 12

Unetched 590x Alloy 124 Treatment: NaCN 4% B 0 Temperature 530C.; Time 12 hours Indentation of micro-hardness (35 grams) Hardness of matrix 104 kg/mm Hardness of the hard layer 1200 kg/mm FIG. 13

Alloy 124 X120 not etched Condition: as cast FIG. 14

Alloy 124 X120 not etched Conditions of treatment: Bath =-90% NaCN 3% NaOH 1% Na CO 6% H O; Temperature 540C; Time 13 hours Thickness of the hard layer 15 microns FIG. .15

Alloy 138 X120 not etched Condition: as cast FIG. 16

Alloy 138 X120 not etched Bath 90% NaCN 3% NaOH 1% Na CO 6% H O; Temperature 540C; Time 9 hours Thickness of the hard layer 6 microns.

Conditions of treatment:

FIGS. 17 and 18 show the relation of the layer thickness to the exposure time.

FIG. 19 shows the results of wear tests made with Alloy 124.

Alloy 124 was treated for four hours at 540C with the bath of example 8 and the friction test used ASTM test D27 14-68.

DESCRIPTION OF THE PREFERRED EMBODIMENTS All the alloys which were employed were sand cast or 5 cast in permanent molds or cast and mechanically worked.

Examples of the alloys used and the analysis of their contents are as follows:

DESIG- ALLOY ALLOY ALLOY ALLOY NATION: Y 124 138 244 COMPO- s1 0.5 11-13 17-19 23-26 SITION: Cu 3.5-4.5 0.8-1.5 0.8-1.3 1.0-1.7 Ni 1.75-2.25 0.8-1.3 0.8-1.3 0.8-1.3 Mg 1.25-1.75 0 8-l 3 0.8-1.3 0.5-1.0

Fe 0.6 0.7 0.7 0.7 Ti 0.2 0.2 0.2 0.2 Mn 0.2 0.2 0.2 0.2

Zn 0.2 0.2 0.2 0.2 Cr 0.3-0.5 Al -93 82-85 76-80 69-73 These alloys are well known and are designated as indicated above or are designated by other means as shown in the following table.

The above four types of alloys were subjected to the exposure of cyanide ions by immersing the alloy partially or completely into the appropriate salt bath, for a period of time of 8 to 12 hours at a temperature of 500 to 530C. After the exposure the test piece was rinsed with water and the hardness of the treated surface at several areas was determined. 1

EXAMPLES l 7 Examples of salt baths employed are as follows:

SALT BATH A 98 percent By weight of sodium cyanide and 2 percent by weight of boron oxide (B 0 SALT BATH B 90 percent By weight of sodium cyanide, 3 percent by weight of sodium hydroxide, 1 percent by weight of sodium carbonate and 6 percent by weight of water vapor.

The results achieved with the treated alloys after rinsing with water are set forth below and stated in Vickers Hardness (HV) in kglmm The hardness of untreated aluminum alloy (HV) is to kglmm SURFACE HARDENING OF TREATED ALUMINUM ALLOYS MICROHARDNESS VlCKERS EXAMPLE 1 4 6 7 ALLOY-Y ALLOY-124 ALLOY-l 38 ALLOY244 BATH A BATH B BATH A BATH B BATH A BATH B BATH A BATH B HV(kg/rnm HV(kg/rr1rn HV(kg/rnm HV(kg/mm HV(kg/mm") HV(kg/mm HV(kg/mm HV(kg/mm 560 860 400 640 1200 510 400 520 760 540 360 1390 760 401 540 900 600 860 l 150 260 940 320 680 950 260 500 320 l 100 480 820 1000 620 535 940 1 100 l lOO 610 1400 2 2. A process according to claim 1, wherein sa1d aluminum alloy has the following alloymg element con- EXAMPLE 8 Alloy 124 Alloy l 38 We claim:

1. A process for increasing the surface hardness of cast or cast and mechanically worked aluminum alloys wherein said aluminum alloy has the following alloying element content:

Si 0.2 to 30.0

Cu 0.2 to 5.6

Ni 0.2 to 6.0

Mg 0.2 to 5.0

Fe 0.0 to 1.0

Ti 0.0 to 0.2

Mn 0.0 to 1.5

Zn 0.0 to 2.0

Cr 0.0 to 0.7 which comprises immersing said aluminum alloy into a molten source having a major amount of cyanide at a temperature of about 430 to 550 C. for a period of time of at least about one-half hour.

tent:

Si 11 to 13 Cu 0.8 to 1.5

Ni 0.8 to 1.3

Mg 0.8 to 1.3

A1 82 to 85.

3. A process according to claim 1, wherein said aluminum alloy has the following alloying element content:

Si 17 to 19 Cu 0.8 to 1.3

Ni 0.8 to 1.3

Mg 0.8 to 1.3

Al 76 to 80.

4. A process according to claim 1, which comprises using as a cyanide source a bath of 98% sodium cyanide and 2 percent boron oxide.

5. A process according to claim 1, which comprises using as a cyanide source a melt of percent sodium cyanide, 3 percent sodium hydroxide, 1% sodium carbonate and 6 percent water.

6. A process according to claim 1, which comprises carrying out the treatment for a period of 8 to 12 hours.

7. A process according to claim 1, wherein the surface hardness is increased from about 250 kg/mm to about 1400 kglmm

Claims

1. A PROCESS FOR INCREASING THE SURFACE HARDNESS OF CAST OR CAST AND MECHANICALLY WORKED ALUMINUM ALLOYS WHEREIN SAID ALUNINUM ALLOY HAS THE FOLLOWING ALLOYING ELEMENT CONTENT: SI % 0.2 TO 30.0 CU % 0.2 TO 5.6 NI % 0.2 TO 6.0 MG % 0.2 TO 5.0 FE % 0.0 TO 1.0 TI % 0.0 TO 0.2 MN % 0.0 TO 1.5 ZN % 0.0 TO 2.0 CR % 0.0 TO 0.7 WHICH COMPRISES IMMERSING ALUMINUM ALLOY INTO A MOLTEN SOURCE HAVING A MAJOR AMOUNT OF CYANIDE AT A TEMPERATURE OF ABOUT 430* TO 550*C. FOR A PERIOD OF TIME OF AT LEAST ABOUT ONE-HALF HOUR.

2. A process according to claim 1, wherein said aluminum alloy has the following alloying element content: Si % 11 to 13 Cu % 0.8 to 1.5 Ni % 0.8 to 1.3 Mg % 0.8 to 1.3 Fe % < 0.7 Ti % < 0.2 Mn % < 0.2 Zn % < 0.2 Cr % A1 % 82 to 85.

3. A process according to claim 1, wherein said aluminum alloy has the following alloying element content: Si % 17 to 19 Cu % 0.8 to 1.3 Ni % 0.8 to 1.3 Mg % 0.8 to 1.3 Fe % < 0.7 Ti % < 0.2 Mn % < 0.2 Zn % < 0.2 Cr % A1 % 76 to 80.

4. A process according to claim 1, which comprises using as a cyanide source a bath of 98 percent sodium cyanide and 2 percent boron oxide.

5. A process according to claim 1, which comprises using as a cyanide source a melt of 90 percent sodium cyanide, 3 percent sodium hydroxide, 1 percent sodium carbonate and 6 percent water.

7. A process according to claim 1, wherein the surface hardness is increased from about 250 kg/mm2 to about 1400 kg/mm2.