INDUSTRIAL A N D ENGINEERING CHEMISTRY
956
Vol. 17, No. 9
Properties and Structure of Some Alloys of Aluminium-Chromiu m' By F. T. Sisco and M . R. Whitmore AIR SERVICE, WARDEPARTMENT, MCCOOK FIRLD, DAYTON, OHIO
T
HE work outlined in this paper is the third part of the
to alloys of aluminium containing less than 10 per cent chroresearch on the light alloys of aluminium with some of mium. By making only these alloys, direct comparison with the rarer metals now being conducted by the Metal- the aluminium-copper alloys would be comparatively simlurgical Branch, Engineering Division, Air Service. As noted ple. Thus it would be possible to determine if chromium in the papers of the previous re~earches,~J the plan followed could be substituted for copper in producing light alloys with high strength. One excepwas to make a few exploration to the schedule .was tory alloys to determine made. A single melt of (1)if the metal would alloy Chromium can be alloyed with aluminium with satisfactorily with aluminaluminium-chromium-silicomparative ease. The solidification point of the alloy con was cast and tested. ium, and the foundry pracincreases rapidly as the proportion of chromium intice and precautions necesMaterial creases from 1 to 5 per cent. Chromium increases the sary in casting the alloy, (2) shrinkage of aluminium to a marked extent. It would The analysis of material the physical properties of a be difficult to pour sound castings of thin and intricate used in the investigation is few of the alloys as cast and sections with these alloys. Chromium a pparently as tabulated below. as heat-treated, and (3) the combines with aluminium to form AlCrs which is inThe aluminium ingot was effect of the chromium on soluble in the metal when cold. This constituent does a very pure (Grade A) aluthe microstructure as cast not go into solution when the alloy is heat-treated at minium purchased in the and as heat-treated. From 600' C. (1100O F.) The AICrs assumes characteristic form of 2-pound notched the results of these exploraforms when viewed under the microscope. bars. The aluminium-chrotory alloys it could be deThe alloys of aluminium-chromium have higher mium hardener was purtermined whether the series tensile strength and less ductility than pure aluminium. chased from a manufacturer was worth a more exhausThe properties are not so good as the aluminium-copwho made a specialty of tive investigation. per alloy. Apparently, chromium cannot be used as a supplying unusual alloys. A search of the literature substitute for copper in the casting of light alloys of The a l u m i n i u m - s il i c o n disclosed the fact that very aluminium with high strength. hardener was made in the little work has been done on McCook Field Foundry. the alloys of aluminium and chromium. Practicallv all Analysis of Material Used the available references are appended a t the end of this Aluminium Iron Silicon Copper Chromium paper. The most important work was that of Hendineot 99.25 0.46 0.17 0.12 ricks, who constructed the equilibrium diagram shown in Aluminium Aluminium-chyomium 17.00 hardener Figure 1. Most of the other investigators have prepared Aluminium-silicon hard12,90 ener and described compounds of chromium with silicon and aluminium. Apparently, only one has investigated the physiFoundry Practice cal properties. Minet and Waldo state that if the alloys aluminium and chromium are to be hammered or rolled the Seven melts of about 12 pounds each were made.2 The comproportion of chromium should not exceed 3 per cent. It was considered advisable to restrict the exploratory work position of the various melts, together with the method of melting, is summarized in Table I. Melt 3, containing 5 per 1 Received April 27, 192.5. Published by permission of Chief of Air cent chromium, was the first one made, which accounts for the Service, War Department. low pouring temperature. The pouring temperature for Sisco and Whitmore, THISJOURNAL, 16, 838 (1924). standard light alloys of aluminium is 704" C. (1300" F.). I Ibid., 17, 15 (1925).
Melt 1
'
COMPOSITION AS MIXED Si AI Cr 1 99
Table I-Summary -COMPOSITION, ACTUALA1 Cr Si 0.80
1A
1
99
0.98
2
3
97
2.53
2A
3
97
2.54
3
5
95
Kot analyzed
3A
5
95
5.60
4
3
96
3.17
of Foundry Practice -TEMPERATUREMelting Pouring REMARKS 788O C. 760° C. A1 and AI-Cr hardener charged and melted together. 1450' F . 1400° F. Small amount of slag on metal. Metal not too hot 1010" C. See ReAI-Cr added after AI was melted. Poured one moid of 1850" F. marks test bars a t 927O C. (1700' F.). Cooled to 721 '2. (1339' F . ) and poured second mold 843" C. 816' C. AI and AI-Cr hardener charged and melted together. 1550' F. 1500' F. Considerable slag on metal. Metal not too hot S43O C. 788' C. AI-Cr hardener added after metal was melted. Metal cooled too much so was reheated to 843' C. (1550' F.) Molds ooured satisfactorily. Metal sluggish 788' C. 718' C. AI and A1-Cr hardener melted together. Metal'much 1450'F. 1325OF. too cold. Undissolved material in bottom of pot and heavy slag 8 1 6 O C. AI and AI-Cr hardener melted together. Metal too 954' C. cold. Nearly a t freezing point. All liquid but 1500" F. 1750'F. sluggish. Heavy slag 871' C. 871" C. All materials melted together. Metal hot enongh. 1600' F. 1600" F . Poured well. Very little slag ~~
1
Not analyzed
INDCSTRIAL AND ENGINEERISG CHEAMISTRY
September, 1925
The equilibrium diagram shown in Figure 1 was not located until after Melt 3 was made. It was evident from the fact that Melt 3 was not completely melted and alloyed that chromium raised the solidus line of the alloys considerably. This n-as verified when the diagram (Figure 1) became available. Accordingly, the melting and pouring temperatures of the wxeeding melts were increased as noted in Table I. In some of the melts, notably 2A and 3A, the metal was kept as cold as possible and yet have the mold completely poured. The metal was poured in standard TB-1 test bars. These bars have been described and illustrated in two previouq
0
I
20
40
60
80
100
Per cent chromium by weight
Physical Tests
The physical properties of the alloys as cast and as heattreated are given in Table 111. Each result is the average of three or six tests. For comparison, the physical properties of pure aluminium and aluminium containing 5 per cent copper are included. Metallography
S p e c i m e n s for metallographic examination were taken from the end of t h e 1 . 2 8 - e m . (0.505-inch) sandcast test bars. Sp e c i m e n s were taken from Melt 1 containing 0.80 per cen t c h r o m i u m , Melt 2 containing 2.53 per cent chromium, and Melt 5 containing 5.60 per cent chromium. A specimen of the pure aluminium was also Figure 2 - A l u m i n i u m - C h r o m i u m Hardener taken for coinpara17.0 Per c e n t C h r o m i u m . X 500 tire purposeq. -4s it was desired to study the characteristics of the aluminium-chromium constituent, a sample of the hardener containing 17 per cent chromium was also examined. The specimens were photographed unetched at magnifications of 100 and 500 diameters.
Figure 1-Equilibrium Diagram of A l u m i n i u m C h r o m i u m Alloys
Results
papers.?,3 Three molds of three bars each were cast on each melt. The bars were numbered lA, IB, lC, 2A, 2B, 2C, etc.; the bars numbered 1were poured first, those numbered 2 second; and those numbered 3 last. Some of the bars were tested as cast; the rest were heat-treated as noted below. Heat Treatment
An electric muffle furnace was used. The temperature was controlled to *9.4" C. (15" F.) by a recording potentiometer and automatic current regulators. Furnace temperature and time, quenching mediums, and aging temperatures and time are given in Table 11. Fixing the temperature to which bars were heated was largely a matter of conjecture. The temperature for quenching the aluminium-copper and other standard alloys ranges from 496" C. (925" F.) to 524" C. (975" F.). Probably no low-melting constituents are present in the aluminium-chromium alloys, as is the case of the aluminiumcopper alloys; consequently the bars could, theoretically, be heated to slightly below the melting point of aluminium. It was realized that unless the chromium-bearing constituents could be made to go into solution, heat treatment would have no effect on the properties and structure. These assumptions led to fixing 580" C. (1075" F.) to 593" C. (1100" F.) as the temperature for quenching. Table 11-Summary Melt 1
Xold
Bar .-1, B , C
2
A , B, C
3.1
A , B, C
4
A , B, C
Furnace temD 593' C . 1100O F. 5800
c. c.
1075' F. 5800
10i5' F. 580' C.
1075' F.
of Heat T r e a t m e n t -AGING--Time Quenching TemperHours medium ature Cold 149' C. 300' F. water Boiling 149' C. water 300' F. Roiling 149' C. water 300' F. Boiling 149" C. water 300' F.
957
Time Hours
FovsDRY-The alloys of aluminium-chromium containing up to 5 per cent chromium melt satisfactorily. In general, it is better practice to charge the chromium as a hardener into the pot with the ingot aluminium than to melt the aluminium and add the aluminium-chromium hardener after the aluminium is molten. The alloys are subject to a h ea v y shrinkage, which would practically e l i m i n a t e their use for casting thin and intricate sections. The alloys must be poured considerably hotter than standard alloys of aluminium-between 760" C. (1400" F.) and 870" C. (1600" F.), instead of 705" C. (1300" F.) which is standard for most Figure 3-Average Structure-0.80 Per of the aluminium allow.
cent Chromium.
X 100
A"pparently, chromium alloys with aluminium completely and without difficulty in amounts up to 5 per cent. The fact that Melts I, 2, and 2A contained only about 80 per cent of the added chromium was probably due to the chromium being segregated in the hardener. This is borne out by Melt 3.4, containing more than the calculated percentage added,
Table III- -Physical Properties of A l u m i n i u m - C h r o m i u m Alloys -AS CAST 2 2.4 3 3.4 4 Pure aluminium 1.4 2 ?
1161,~
Percentage chrome
T'ol. 17, KO.9
I S D C S T R I A L A X D E,YGISEERING CHEMISTRY
958
Average diameter, inches Ultimate strength, Ibe./sq. in. Elongation in 5.08 cm. (2 inches), per cent Character of fracturr
--
A s HEATTREATED--3A 4 AI-Cu 5% (1) 5.60 3% C: Heat-treated 510' C . (950' F . ) 1 % SI
1
1..l
.SO
.9S
0.503
0.500
0.503
0.504
0.505
0.502
0.501
0.505
14.700
14,000
11,300
11,500
10,210
9,700
9,300
11,750
17.33
13.50
7.60
4.50
3.66
2.00
1.70
37.50
Fine.
Granu- Coarse. Coarse. Coarse, Coarse, Coarse, Fibrous Fibrous Fibrous Medium Coarse, Coarse, Woody lar grangranl flaxs grangrangrangrangranular ular ular ular ular ular ular 24.i , . . 26 0 28.4 32.4 36.2 20.6 ... . . . 30.9 3 3 . 2 3 4 . 6 64
of 2.53
2.54
i")
5.60
37' Cr 1% s i
GradeA GradeA (1) (2)
ci,
0.98
2.53
0.505
0.504
0.499
0.501
0.502
9,280
15.600
12,800
9,773
13,356
25,700
74,600
13.50
6.50
2.00
2.50
2.50
2.50
35.17
0.503
0.501
Woody
ELK~US
Brinell. 500 kg. 3 0 . 7 a Not analyzed.
RS
and to Melt l A , containing exactly the calculated amount. on polishing. Its hardness is responsible for the increaqe in I n addition, the alloys melted clean and apparently free Brinell hardness of the alloy, as noted in Table 111. The heat treatment used for some of the melt. has not from excessive slag on top of the metal and from undissolved affected the solution of this constituent. Hence, there was or solid residue in the bottom. PHYSICAL PROPERTIES-In the as-cast condition chromium no improvement in physical properties. This is in contrast exercises a hardening effect on aluminium. I n the 1 per cent to the aluminium-5 per cent copper alloy, which responds to alloy the ultimate strength has increased approximately heat treatment with increased strength and hardness and 35 per cent and the ductility as measured by the elongation decreased ductility. decreased more than 50 per cent (Table 111). It also produces Conclusions brittleness, as is evidenced by the inferior properties of Melts Chromium in small amounts alloys satisfactorily with 2, 2A, 3, 3A, and 4 in Table 111. Chromium increases the aluminium, but the resulting alloys have such a heaby shrinkBrinell hardness of aluminium about 35 per cent. Heat treatment has not improved the physical properties age that they are of the alloy in any way. The properties of the various melts not suitable for castafter heat treatment check closely with the properties of the ing thin sections. corresponding alloy in the as-cast condition (Table 111). Chromium acts as a With the exception of Melt 4,containing silicon, heat treat- hardening element, ment has evidently had no effect on the alloy as indicated by but p r o d u c e s s o its strength and ductility. A possible exception to this is the much b r i t t l e n e s s Brinell hardness, which is slightly higher after heat treatment. that from a comIt is evident from Table I11 that the physical properties of mercial Ptandpoint the aluminium-chromium alloys containing 1,3, and 5 per cent the alloys may be chrome are much inferior to the physical properties of alumin- considered w o r t h ium containing 5 per less. A heat treatment consisting of cent copper. S T R U C T U RoEr quenching in water THE A L L O Y S - T ~ ~ after 5 hours a t 580" e f f e c t of a d d i n g C. (1075" F.) to chromium and chro- 595" C. (1100" F.) mium-silicon to alu- and aging a t 150" minium was quite C. (300"F.) does not apparent from the affect the uhvsical Structure-Chromium broken test bars. properties or struc- Figure 5-Average a b o u t 5 Per cent. X 108 The fractures of the ture appreciably. 5.60 per cent chroAcknowledgment mium and the 3.17 The authors wish to acknowledge the assistance of D. 31. per cent chromium, 1.0 per cent silicon Warner, Clifford Mchlahon, and J. L. Hester. alloy were very References coarse grained. The c h r o m i u m I-Hendricks, 2. anorg. Chem., 19, 433 (1908). Hendricks investigated occurs in the micro- the thermal transformation occurring in the alloy of aluminium-chromium the equilibrium diagram. structure as a con- and constructed 2-Minet, "Production of Aluminium," translated by Waldo, 1905, Figure 4-Average Structure-2.53 Per c e n t stituent having a p. 171, states "Wohler obtained an alloy of this kind by reducing the violet C h r o m i u m . X 100 characteristic a n d chromium chloride by means of aluminium; there is a metal regulus whose easily recognizable appearance. The appearance of this con- composition is approximately expressed by the formula AlCr. If the is to be hammered and rolled, the proportion of chromium should stituent is shown a t 100 diameters in Figure 3 containing 0.80 alloy not exceed 3 per cent." per cent chromium, Figure 4 containing 2.53 per cent chro3--Moisson, Comfit. rend., 121, 621 (1895). 4--DeChalmot, Am. Clrcm. J . , 19, 69 (1897). mium, and Figure 5 containing 5.60 per cent chromium. 5-Zette1, Comfit. tend., 126, 883 (1898). Figure 2 a t 500 diameters shows the average structure of 6-Liheau and Figueras, Ibid., 186, 1329 (1903). the hardener containing 17.0 per cent chromium. i--Vigouraux, I b i d . , 144, 83 (1907). The constituent noted in the photomicrographs is probahly References 3 to 7 refer to compounds of chromium with silicon and with dlCra. It is evidently very hard, as it stands sharply in relief silicon and aluminium. -
1
