Analysis of Magnesium Applications In German Aircraft and Equipment

By F A Rappleyea,
The Dow Chemical Co

Study of new uses of forgings, sand and die castings in BMW-801D and DB-601E engines. and Fungerrat radio reveals Nazis used alloys similar to ours; their techniques showed no superiority.

Analysis of captured Nazi aircraft shows that while the Germans had developed many new applications of magnesium alloys, the compositions of these alloys and their design principles were definitely not superior to those of this country.

This has been proved in earlier studies 1 and again in examination of parts of BMW-801D and DB-601E engines, a Fungerrat radio and a camera support casting, all submitted for study by the Air Technical Service Command.

Magnesium alloys accounted for approximately 10% of the dry weight of both the engines, a ratio similar to that found in the earlier models which were of slightly lighter weight. Magnesium alloy parts of the radio were mostly die castings. This piece of apparatus was standard field equipment and although large and bulky, was apparently portable to some extent. Such service, of course, subjected it to effects of the elements.

Table I shows the magnesium alloys used in these units and also shows the nominal composition and the American equivalent. Most popular casting alloys used in German was AZG and A9, the former being used in the as cast state, the latter usually in the solution heat treated state. The German AZ91 alloy is the principal alloy used for die casting, although alloy A8 is sometimes used for simple die casting liable to impact stresses. Elektron alloys AZM and AM6 were used for forgings. The AM6 alloy is particularly applicable where high temperatures are encountered.

Distribution of Parts

In order to determine the popularity of the alloys used, Table II gives a summary of the number of articles for each alloy, which shows that the more popular sand cast alloys are AZG and A9V, one in the as cast condition, and the other in the solution heat treated condition. Although previously1 some solution heat treated and aged alloys were found, none were seen in this investigation. Only one part — the DB engine oil pan — was found in the as cast and stabilized condition. In the die castings, where resistance to impact was desired, the A8 alloy was used. In general though, the principal alloy for all kinds of die castings was AZ91. Only a relatively small number of the parts studied were forgings; these encompassed a relatively large percentage of the poundage. Alloy AZM is the principal alloy used for highly stressed forgings, except where elevated temperatures are encountered, in which case AM6 alloy, an alloy characterized by good mechanical properties at these temperatures, is used. In the other investigation1 Elektron alloy AZ855 was commonly used for large forgings.

Wherever it was possible to obtain test specimens, mechanical properties were determined. The average results obtained from tests of the castings together with German specifications are given in Table III. All of bars cut from the castings exceeded German specifications for bars cut from castings. In some cases, the average properties were far greater than the specifications.

Since the forgings were used as stressed parts in most cases, a detailed account of the mechanical property data is given in Table IV with a summarization of the averages in Table V. The uniformity of forgings is evident from a comparison of the properties taken at different angles in the forgings. Metallographic examination showed the forgings to be very homogeneous throughout. Quite evidently the transverse properties were improved by restricting segregation and by proper orientation, the latter being the more important factor. These properties, as given, come up to those properties obtained on similar alloys in this country. This statement is based on experience of tests on many American castings and forgings by different investigators.1

Soundness

All castings were inspected radiographically. These radiographs were then evaluated in the following manner.2 A Variac voltage recorder was used so that voltage readings could be made when the porosity was just discernible as seen through the viewing screen. The percentage areas of porosity were also noted and the radiograph evaluated qualitatively as follows:

Variac Reading  
Volts
%Area of  
Porosity
Rating
45-500-100Very good
40-450-40Good
40-4541-100Fair
35-400-40Poor
35-4041-100Very poor
Using the above as a guide, the castings were found to be generally fair to good; a few pieces were excellent and a few were poor. At points of stress concentration and at surfaces which required machining care must have been taken to obtain good sound sections. Elektron AZG showed the greatest amount of microporosity; A9, a lesser amount and AZ31 the least. The surface of the castings did not come up to the standard of surfaces of American castings.

Corrosion

Although no corrosion was observed on the engine parts, some was noticed on eleven diecast parts, but in no case was the corrosion severe or extended over a large area. The basic corrosion resistance of the parts was determined by testing ground specimens in a 3% NaCl solution using alternate immersion at 95° F. Table VI gives the analyses of the parts as determined spectrographically, together with the basic corrosion rate in mg per sq centimeter per day. Here it is seen that even though serviceability of the parts was good, the basic corrosion rates are high and are above the American standard which is:

For castings., 0.5-5.0 mg/cm2/day
and for forgings 0.2-0.3 mg/cm2/day
(The rates on forgings apply to those compositions in which the impurities have been controlled.) Very little galvanic corrosion due to the use of dissimilar metals, especially at inserts, was observed. In general, it can be said that no serious corrosion was observed on any of the magnesium alloy parts examined.

Surface Treatment

In general, it was found the Germans used but one surface treatment, one corresponding to the American Chrome-Pickle. There were a few isolated cases, ie, the supercharger impeller, aid scoop grid and some other small parts, on which no treatment had been applied. Previously, the Germans had the practice of Chrome-Pickling before machining only, and after machining, rather infrequently. However, the trend as indicated by the study of these engines changed so that now usually all parts get a Chrome-Pickle treatment after machining.

In most cases the parts had no primer. Only a single coat of black paint was used for the engine parts. Seven of the die castings were given a primer; five, a rust colored primer; one, a white and one a light blue. Total thickness of coating varied from 7 to 30 mils as compared to the American standard four coat system of 3 to 3.5 mils. Adhesion of the coatings varied considerably, ranging from poor to fair. On many of the parts the coating could be chipped or flaked off quite easily. A spectrographic analysis, giving from 1-10% Cr in the black top coat would quite possibly indicate the use of an inhibitive chromate pigment.

Six different colored top coats were observed on the die castings, making a total of ten different systems. This is quite unusual and may be attributed to various manufacturers of the items. The general conclusion is that the painting practice of the enemy is below acceptable American standards. However, even with the poor basic corrosion resistance, a chemical treatment inferior to American standards, and the poor paint used, almost no evidence of corrosion was observed.

Metallography

The microstructures of all parts were normal with grain size about the same as observed in American practice, that on the forgings running about 0.0005 in. Heat treatment of the castings was usually satisfactory as evidenced by the solution of the magnesium-aluminum compound. All the forgings were very homogeneous, although in the case of the supercharger impeller on the BMW there was a little evidence of the as cast dendritic structure still remaining. No fusion voids of the castings were noted.

A general summary of the design features studied shows the following principles:

The BMW supercharger impeller was keyed to the drive shaft by means of splines in the magnesium alloy forging. When compared to a similar American design, the splines on this impeller were apparently overstrength.

A detailed study of the forgings showed the following general German practices: Draft angles 5-10°, fillet radii ¾", corner radii 1/8".

Small rivets were found and by spectrographic analysis were determined to be similar to 565 aluminum alloy in composition. Ball bearings were mounted in steel inserts in several items. When pushed out, the force required was determined and was found to correspond to an interference of 0.0015"/in of diameter.

A study of thicknesses showed a minimum sand casting thickness of 5/32" and die casting thicknesses varying from 0.0947" to 0.1470", corresponding to American practice. Some lips, however, were found to be only 0.076" thick.

Conclusions

  1. Rather extensive use of forgings in magnesium alloys was made by the Germans and care was given to choice of alloy for high temperature applications.
  2. Compositions used for magnesium did not differ materially from those in the United States, although usages of the alloys in various conditions do differ. For example, the Germans use AZG in the as cast condition, where the corresponding American alloy, Dowmetal H, is used in the solution heat treated condition, and Elektron A9, similar to Dowmetal A or R approximately is used for sand castings and is usually used in the solution heat treated condition. These are the two major German sand casting alloys.
  3. No serious corrosion resulted in service even though the basic corrosion rates of the metals were greater than those of the alloys of the United States.
  4. Design principles in the use of magnesium alloys by the enemy are similar to those used in the United States.

References
  1. Schmidt, H. W. Magnesium in German Aircraft, Aeronautical Engineering Review, June 1943.
  2. Busk, R S A Correlation of the Mechanical Properties and Radiographic Appearance of Magnesium Alloy Castings. ASTM Transactions, 1943.
    Additional sources are as follows:
    Erlenbach, A and Beck, A, "Some German Aircraft Light Alloy Material Specifications", British Air Ministry Translations, 1654.
    Beck, A, "The Technology of Magnesium Alloys", F A Hughes and Co, Ltd, 1940.
    Cave, M V, "Design Details of the BMW 801A Engine", Aviation, Vol 41, Oct 1942 and Nov, 1942.
    Anonymous, "The German BMW 801A Engine", Aero Digest, Aug, 1942.
    Gordon, R A, "Captured Axis Equipment", Electronics, Mar, 1944.
    Bourdon, M W, "Some Features of the DB 601 Liquid Cooled Aircraft Engine", Automotive Industries, June 15, 1941 and July 15, 1941.

TABLE I — Designation And Composition Of German Alloys

Nominal Composition
German
 Designation  
 Al %  Zn %  Mn %  Ce % ConditionUS Equiv
Dowmetal Alloy
AZ31310.3Sand CastFS *
AZF 430.2Sand Cast
AZG630.2Sand CastH
A9 8.50.50.2Sand CastR **
A9V8.50.50.2Sand Cast and
solution heat treated
R HT **
AZ919.50.50.2Die CastR **
A880.50.2Die CastA ***
AZM610.2ForgingJ
AM62.06Forging
AZ31310.3ExtrusionFS
* FS is commonly used as an extrusion alloy in American practice.
** Nominal composition of R alloy is 9% Al, 0.6% Zn and 0.2% Mn.
R alloy is commonly used for die castings in American practice.
*** Dowmetal A alloy is commonly used for sand castings in American practice.

TABLE II — Number Of Articles For Each Alloy And Condition

AlloyNominal
Composition
StateNumber of
Articles
AZ313 Al, 1 ZnSand Cast4
AZF4 Al, 3 ZnSand Cast1
AZG6 Al, 3 ZnSand Cast14
AZG6 Al, 3 ZnSand Cast, and
solution heat treated
1
AZG6 Al, 3 ZnSand Cast and
stabilized
1
A98½ Al, ½ ZnSand Cast6
A9V8½ Al, ½ ZnSolution heat treated23
AZ919½ Al, ½ ZnDie Cast13
A88 Al, ½ ZnDie Cast10
AM66 Ce, 2 MnForging2
AZM6 Al, 1 ZnForging7
AZM6 Al, 1 ZnPlate4
AZ313 Al. 1 ZnExtrusion2
__
88

TABLE III — Average Mechanical Properties And German Specifications Of Magnesium Alloy Castings From Enemy Equipment

* Ultimate  
Tensile
Strength
* Tensile
Yield
  Strength  
Elongation
% in 2"
* Compressive
Yield
Strength
Brinnell
Hardness
Number
Alloy   TestSpec
Min
TestSpec
Min
TestSpec
Min
TestTestSpec
Min
AZ3124.511.48.65.76.01.59.641.9
AZG19.012.813.411.43.01.017.751.250-60
AZGs21.014.51.552.4
A923.915.112.151.8
A9V30.017.115.512.86.22.014.650.150-60
AZ9125.219.620.353.6
A820.714.219.811.41.61.018.957.845-55
* 1,000 psi
German specifications are for bars cut from castings.

TABLE IV —- Mechanical Properties Of Forged Elektron Alloys From Enemy Equipment

Alloy Part* Ultimate
Tensile
  Strength  
* Tensile
Yield
  Strength  
Elongation
% in 2"
* Compressive
Yield
Strength
Brinell
Hardness
Number
AZM  DB 601E
Engine Mount
(Flange 30°)
47.231.615.022.255.6
DB 601E
Engine Mount
(Web-long)
46.231.614.118.355.6
DB 601E
Engine Mount
(Web-trans)
44.927.415.056.0
Engine Shock Pad
and Mounting Ring
BMW801D (0°)
46.029.811.816.060.3
Engine Shock Pad
and Mounting Ring
BMW801D (90°)
46.329.615.020.063.4
Oil Cooler Fan (0°)
BMW801D
45.033.120.265.5
Oil Cooler Fan (30°)
BMW801D
45.331.613.020.265.5
Oil Cooler Fan (60°)
BMW801D
45.932.413.021.265.5
Oil Cooler Fan (90°)
BMW801D
45.832.813.022.764.5
Butterfly Valve
from
Oil Blower
BMW801D
44.733.414.522.362.1
AM6Cam Followers
Guide and Housing
BMW801D
40.531.97.021.297.4
Supercharger Impeller
BMW801D
36.430.83.020.860.8
* 1,000 psi
All values are averages of two or more tests

TABLE V — Average Mechanical Properties And German Specifications Of Magnesium Alloy Forgings

* Ultimate  
Tensile
Strength
* Tensile
Yield
  Strength  
Elongation
% in 2"
* Compressive
Yield
Strength
Brinnell
Hardness
Number
Alloy   TestSpec
Min
TestSpec
Min
TestSpec
Min
TestTestSpec
Min
AZM L  45.940.029.527.113.86.020.063.150
AZM T  44.928.527.417.115.03.020.063.150
AM637.731.14.321.063.0
* 1,000 psi

TABLE VI — Composition Of Articles From Enemy Equipment

Alloy   % Al  % Cu  % Fe  % Mn  % Ni  % Si  % Zn Corrosion Rate
mg/cm2/day
AZ31Max3.30.1100.0400.780.0090.081.357.83
Min2.70.0420.0010.250.0010.010.722.82
Average 3.00.0760.0200.510.0050.041.035.32
AZFMax3.90.0180.0100.200.0060.0525.41.49
Min3.00.0110.0010.110.0010.012.21.36
Average 3.50.150.0050.150.0030.032.31.42
AZGMax6.00.1900.0220.270.0060.152.6022.90
Min5.00.500.0070.110.0010.011.702.91
Average 5.50.1200.0150.190.0030.082.1512.90
A9Max9.30.1700.0070.260.0060.150.9839.30
Min6.60.0170.0010.160.0010.010.351.85
Average 8.00.0930.0040.210.0030.080.6720.57
A9VMax9.20.1300.0380.300.0080.130.085515.0
Min7.00.0100.0050.120.0010.010.0302.08
Average 8.30.0700.0200.210.0030.070.058258.54
AZ91Max11.50.3500.0400.350.0110.170.787.04
Min8.90.0700.0010.170.0010.010.411.07
Average 10.20.2100.0200.260.0060.090.604.05
A8Max8.90.1900.0220.410.0080.180.8512.60
Min7.60.0110.0010.120.0010.010.480.55
Average 8.10.1000.0210.260.0040.090.676.58
AZMMax7.10.0790.0120.280.0110.041.1075.30
Min5.60.100.0030.050.0010.010.675.95
Average 6.30.0450.0070.160.0060.020.8940.67
* AM6Max0.010.0100.0051.80.0010.010.010.35
Min0.010.0100.0041.60.0010.010.010.29
Average 0.010.0100.0041.70.0010.010.010.32
* Total rare earths including cerium 6.35% Max, 6.15% Min, 6.25% Average

This article was originally published in the August, 1945, issue of Aviation magazine, vol 44, no 8, pp 144-148.
The original article includes 19 photos of various magnesium castings and forgings described in the article.
Photos are not credited.