U.S. patent application number 10/578985 was filed with the patent office on 2007-06-21 for automobile body part.
Invention is credited to Corrado Bassi, Juergen Timm.
Application Number | 20070137738 10/578985 |
Document ID | / |
Family ID | 34429624 |
Filed Date | 2007-06-21 |
United States Patent
Application |
20070137738 |
Kind Code |
A1 |
Bassi; Corrado ; et
al. |
June 21, 2007 |
Automobile body part
Abstract
In a car body or component of a car body with at least one first
component of sheet metal of a first aluminium alloy (A) and at
least one second component of sheet metal of a second aluminium
alloy (B), the first and second aluminium alloy are of type AlMgSi
and in the sheet metal of the second aluminium alloy (B) a
substantial part of the elements Mg and Si, which are required to
achieve artificial ageing in solid solution, is present in the form
of separate Mg.sub.2Si and/or Si particles in order to avoid
artificial ageing. As well as common recycling of process scrap in
the production of the various components and simple scrap recycling
of the body parts from the end of life car, by reduction of the
hardening capacity of the second component during artificial ageing
of the body as part of the paint baking cycle, the car body has an
improved impact protection for pedestrians in comparison with
solutions according to the prior art.
Inventors: |
Bassi; Corrado; (Schweiz,
CH) ; Timm; Juergen; (Steisslingen, DE) |
Correspondence
Address: |
COOPER & DUNHAM, LLP
1185 AVENUE OF THE AMERICAS
NEW YORK
NY
10036
US
|
Family ID: |
34429624 |
Appl. No.: |
10/578985 |
Filed: |
November 22, 2004 |
PCT Filed: |
November 22, 2004 |
PCT NO: |
PCT/EP04/13233 |
371 Date: |
May 11, 2006 |
Current U.S.
Class: |
148/440 ;
428/654 |
Current CPC
Class: |
C22F 1/05 20130101; B62D
25/082 20130101; B62D 25/10 20130101; C22C 21/08 20130101; B62D
25/105 20130101; C22C 21/02 20130101; B62D 25/08 20130101; C22C
21/04 20130101; Y10T 428/12764 20150115; B62D 25/12 20130101; B62D
25/085 20130101 |
Class at
Publication: |
148/440 ;
428/654 |
International
Class: |
C22C 21/06 20060101
C22C021/06 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 20, 2003 |
EP |
03405826.3 |
Claims
1. A sheet metal AlMqSi type aluminium alloy automobile body part,
wherein a substantial part of the Mg and Si in the sheet metal
alloy, which are required to achieve artificial ageing in solid
solution, are present as separate Mg.sub.2Si and/or Si particles in
to avoid artificial ageing.
2. A sheet metal AlMqSi type aluminium alloy automobile body part
according to claim 1, wherein the aluminium alloy consists
essentially of: 0.6 to 1.2 weight percent silicon; 0.3 to 0.8
weight percent magnesium; max. 0.8 weight percent copper; max. 0.4
weight percent iron; max. 0.3 weight percent manganese; max. 0.2
weight percent vanadium; with production-related contaminants and
aluminium as the remainder.
3. A sheet metal AlMgSi type aluminium alloy automobile body part
according to claim 1, wherein the aluminium alloy consists
essentially of: 0.25 to 0.60 weight percent silicon; 0.25 to 0.60
weight percent magnesium; 0.25 to 0.30 weight percent copper; max.
0.40 weight percent iron, max. 0.30 weight percent manganese; max.
0.20 weight percent vanadium; with production-related contaminants,
individually a maximum of 0.05 weight percent, total maximum of
0.15 weight percent, and aluminium as the remainder.
4. A sheet metal AlMgSi type aluminium alloy automobile body part
according to claim 3, wherein the aluminium alloy contains 0.30 to
0.50 weight percent silicon.
5. A sheet metal AlMgSi type aluminium alloy automobile body part
according to claim [3 or] 4, wherein the aluminium alloy contains
0.30 to 0.50 weight percent magnesium.
6. A sheet metal AlMgSi type aluminium alloy automobile body part
according to claim 3, wherein the aluminium alloy contains a
maximum of 0.20 weight percent copper.
7. A sheet metal AlMgSi type aluminium alloy automobile was body
part according to claim 6, wherein the aluminium alloy contains
0.05 to 0.20 weight percent iron.
8. A sheet metal AlMgSi type aluminium alloy automobile body part
according to claim 7, wherein the aluminium alloy contains a
maximum of 0.15 weight percent vanadium.
9. A sheet metal AlMgSi type aluminium alloy automobile body part
according to claim 8, wherein the aluminium alloy contains 0.10
weight percent manganese.
10. A sheet metal AlMgSi type aluminium alloy automobile body part
according to claim 9, wherein the automobile body part is as an
inner panel of a body part, a hood, a trim part, a structural
component or a reinforcing element in the front part of an
automobile body.
11. A sheet metal AlMgSi type aluminium alloy automobile body part
according to claim 9, wherein the automobile panel is as a
deep-drawn body part with good bending behavior.
12. An automobile body or component thereof comprising first and
second AlMgSi type aluminum alloy sheet metal components, wherein
after articifical ageing of the body of component thereof the
second component has lower mechanical strength than the first
component, wherein at least in the sheet metal of the second
aluminium alloy, before artificial ageing of the body or body part,
a substantial part of the elements Mg and Si, which are required to
achieve artificial ageing in solid solution, is present in the form
of separate Mg.sub.2Si and/or Si particles in order to avoid
artificial ageing.
13. An automobile body or component thereof according to claim 12,
wherein at least the first aluminium alloy component consists
essentially of: 0.6 to 1.2 weight percent silicon; 0.3 to 0.8
weight percent magnesium; max. 0.8 weight percent copper; max. 0.4
weight percent iron; max. 0.3 weight percent manganese, max. 0.2
weight percent vanadium: and production-related contaminants with
aluminium as the remainder.
14. An automobile body or component thereof according to claim 13,
wherein the second aluminium alloy component consists essentially
of: 0.25 to 0.60 weight percent silicon; 0.25 to 0.60 weight
percent magnesium; 0.05 to 0.30 weight percent copper; max. 0.40
weight percent iron; max. 0.30 weight percent manganese, max. 0.20
weight percent vanadium, and production-related contaminants,
individually a maximum of 0.05 weight percent total maximum of 0.15
weight percent, with aluminium as the remainder.
15. An automobile body or component thereof according to claim 14,
wherein the second aluminium alloy component contains 0.30 to 0.50
weight percent silicon.
16. An automobile body or component thereof according to claim 15,
wherein the second aluminium alloy component contains 0.30 to 0.50
weight percent magnesium.
17. An automobile body or component thereof according to any of
claims 14 to 16, wherein the second aluminium alloy component
contains a maximum of 0.20 weight percent copper.
18. An automobile body or component thereof according to claim 17,
wherein the second aluminium alloy component contains 0.05 to 0.20
weight percent iron.
19. An automobile body or component thereof according to claim 18,
wherein the second aluminium alloy component contains a maximum of
0.15 weight percent vanadium.
20. An automobile body or component thereof according to claim 19,
wherein the second aluminium alloy component contains a maximum of
0.10 weight percent manganese.
21. An automobile body or component thereof according to claim 20,
wherein the second are inner panels of an automobile body part, a
hood, a trim part, a structural component or a reinforcing element
for the front part of a car body.
22. An automobile body or component thereof according to 20,
wherein the second components are deep-drawn with good bending
behavior.
Description
[0001] The invention concerns a car body part of sheet metal of an
aluminium alloy type AlMgSi, and a car body or component of a car
body with at least one first component of sheet metal of the first
aluminium alloy and at least one second component of sheet metal of
a second aluminium alloy, where the first and second aluminium
alloys are of type AlMgSi, and after artificial ageing of the body
or body part, the second component in comparison with the first
component has lower mechanical strength values.
[0002] For car body parts, car bodies or components of car bodies,
artificial ageing takes place for example under the annealing
conditions during paint baking or in a separately performed heat
treatment.
[0003] The growing importance of the production of lighter cars
with the purpose of energy saving has led to the development of a
large number of aluminium alloys for car applications.
[0004] Different components in a car usually require different
properties. For example, an aluminium alloy for outer panel
applications must be easily deformable in order to allow stretch
drawing, deep drawing and bending, and at the same time achieve a
high strength after paint baking.
[0005] In Europe, for outer panel applications in particular for
engine bonnets, already AlMgSi alloys are used, e.g. the alloy AA
6016, to a fairly great extent.
[0006] In particular, with regard to scrap metal reuse and
recyclability, it would be particularly advantageous and suitable
if for all aluminium panel applications in the body, aluminium
alloys could be used which belong to the same family of alloys.
U.S. Pat. No. 4,082,578 and EP-A-0 811 700 disclose aluminium
alloys of type AlMgSi for inner and outer panel applications in car
bodies.
[0007] Aluminium alloys in the structural area of a vehicle improve
the driving behaviour (vehicle rigidity, axle load distribution,
centre of gravity etc.). Such constructions can also have a high
energy absorption capacity in the event of a crash. EP-A-1 165 848
discloses structural components made of sheet metal from an AlMgSi
alloy.
[0008] In particular in Europe, the reduction of injury risk in car
accidents has high priority. Due to improvements in car safety,
this objective has been achieved very effectively. So far, however,
very little has been done to reduce the severity of injuries to
pedestrians and motorcyclists who hit the front of a car in an
accident. Substantial improvements can be achieved by constructing
the front parts of cars with corresponding energy absorption
behaviour.
[0009] Pedestrian protection measures on car bodies can be very
effective in preventing serious and fatal injuries from collisions
in the medium speed range. In most traffic accidents with
pedestrians, a car collides frontally with the victim. The injury
to the pedestrian is only partly caused by the initial impact. In
many cases the pedestrian's torso bends and his head hits the
bonnet.
[0010] Most head injuries are caused in adults by the upper end and
in children by the front part, of the engine bonnet. The front edge
of the bonnet is particularly critical in relation to injuries in
the thigh or hip area. Detailed changes in the panel construction
of the bonnet edge are necessary to reduce the rigidity and create
sufficient crumple depth. This can be achieved by weakening or
taking back the inner panel reinforcements on the bonnet, the
bonnet closure and the closure cross braces.
[0011] By various active and passive measures, front panels and
other large area body elements of cars have been made "softer".
Here, the components are designed or actively modified so that in
an impact they can absorb a large part of the kinetic energy by
plastic deformation. These measures lead to fewer serious
injuries.
[0012] Passive measures include the design, construction and
material. In relation to the material, various material compounds
are known e.g. sandwich constructions with foam materials. So far,
however, no tests have been undertaken on the use of relatively
soft aluminium alloys.
[0013] The invention is based on the object of creating a car body
part and car body or component of a car body of the type cited
initially which, as well as the common recycling of process scrap
in the production of the various components, and simple scrap
recycling of the body part in the end of life vehicle, leads to
improved impact protection for pedestrians in comparison with
solutions according to the prior art.
[0014] In relation to the single skin car body part, the object is
achieved according to the invention by the presence in the sheet
metal of a substantial part of the elements Mg and Si, which are
required to achieve artificial ageing in solid solution, in the
form of separate Mg.sub.2Si and/or Si particles in order to avoid
artificial ageing.
[0015] In relation to the multi-skin car body or components of a
car body produced from an outer and an inner part, the object is
achieved according to the invention by the presence, at least in
the sheet metal of the second aluminium alloy before artificial
ageing of the body or body part, of a substantial part of the
elements Mg and Si, which are required to achieve artificial ageing
in solid solution, in the form of separate Mg.sub.2Si and/or Si
particles in order to avoid artificial ageing.
[0016] The essential core of the invention lies in the use of
"soft" components with a prespecified structure, so that--in
contrast to `hard` components--under the normal paint baking
conditions no or a decreased artificial ageing, respectively,
occurs and consequently there is no further or a decreased
increase, respectively, in the chemical strength values, but the
soft components retain the values previously set by the
prespecified structure or do not reach the maximum possible
strength level during artificial ageing.
[0017] As a hard first aluminium alloy, an alloy is preferred which
contains
[0018] 0.6 to 1.2 w. % silicon
[0019] 0.3 to 0.8 w. % magnesium
[0020] max. 0.8 w. % copper
[0021] max. 0.4 w. % iron
[0022] max. 0.3 w. % manganese
[0023] max. 0.2 w. % vanadium
[0024] and production-related contaminants and aluminium as the
remainder.
[0025] The hard first aluminium alloy comprises in particular the
usual body outer skin materials e.g. AA 6016 and AA 6111.
[0026] In principle as a soft second aluminium alloy, an alloy
identical to the first hard aluminium alloy is used, but in general
a composition is preferred with a substantially lower strength
level.
[0027] As a soft second aluminium alloy an alloy is preferred which
contains
[0028] 0.25 to 0.60 w. % silicon
[0029] 0.25 to 0.60 w. % magnesium
[0030] 0.05 to 0.30 w. % copper
[0031] max. 0.40 w. % iron
[0032] max. 0.30 w. % manganese
[0033] max. 0.20 w. % vanadium
[0034] and production-related contaminants, individually max. 0.05
w. %, in total max. 0.15 w. %, and aluminium as the remainder.
[0035] For the individual alloy elements of the second aluminium
alloy, the following preferred content ranges apply:
[0036] 0.30 to 0.50 w. % silicon
[0037] 0.30 to 0.50 w. % magnesium
[0038] max. 0.20 w. % copper
[0039] 0.50 to 0.20 w. % iron
[0040] max. 0.10 w. % manganese
[0041] max. 0.15 w. % vanadium.
[0042] The desired strength or softness of the soft second
component is set mainly by way of the Mg and Si content of the
second aluminium alloy in combination with heat treatment of the
sheets produced from the alloy before their shaping into the second
components. Heat treatment ensures that the desired low mechanical
strength values of the soft second component are substantially
unchanged or may only reach a strength level being higher but lying
below the maximum possible values, respectively, even after
performance of a paint baking cycle on the car body. Depending on
performance, the heat treatment causes: [0043] precipitation of a
substantial part of the alloy elements Mg and Si from the solid
solution in the form of Si and Mg.sub.2Si particles and their
coarsening so that the said alloy elements are no longer available
in is entirety for the subsequent artificial ageing, and/or [0044]
prevention of redissolution of the separated Mg.sub.2Si and Si
particles so that the alloy elements Mg and Si are also no longer
available in is entirety for further ageing during subsequent
artificial ageing during a subsequent paint baking cycle.
[0045] It is also conceivable to use, instead of a "hard" first
component, a "soft" one i.e. a component which cannot be
artificially aged, and to adjust the different strength values of
the first and second components by way of the concentration of the
alloy elements Mg and Si.
[0046] "Soft" panels, or sheets of the second aluminium alloy, can
be produced in a conventional manner by way of continuous or strip
casting with subsequent hot and/or cold rolling, with or without
intermediate annealing.
[0047] With the conventional manufacturing process of car body
sheet from AlMgSi materials attention is paid that alloy elements
which are relevant for the precipitation are practically completely
in solid solution after solution heat treatment or before
artificial ageing, respectively, and only a part which is
unavoidable with the selected manufacturing process and which may
be designated as unessential at best is present in precipitated
form.
[0048] The car body sheet according to the invention differs from
this. The part of alloy elements which are relevant for
precipitation which are present in precipitated form after solution
heat treatment or before artificial ageing, respectively, causes a
change of the mechanical strength values which lies outside the
deviations from a given nominal value lying within the scope of
manufacturing tolerances with a conventional production process.
The part of the alloy elements which are relevant for precipitation
which are present in precipitated form is therefore to be
designated as substantial.
[0049] The desired precipitation state of the alloy elements Mg and
Si in the sheets of the second aluminium alloy can be achieved in
various ways which are already known. Preferred process stages
which deviate from the conventional production procedure of AlMgSi
body materials and lead to the desired pre-separation of the alloy
elements Mg and Si which are relevant for artificial ageing,
include the following steps which can be performed individually or
in combination: [0050] No homogenisation annealing of the casting
bar, merely heating to hot rolling temperature and immediate hot
rolling. [0051] Performance of a "partial solution annealing" on
the sheet, rolled to the final thickness, for a short period at
relatively low temperature with continuous annealing in a strip
passage oven at a temperature range from around 450.degree. to
520.degree. C. for max. 30 seconds, where applicable using mild
cooling conditions. [0052] Performance of a "partial
heterogenisation" annealing of the sheet, rolled to the final
thickness, with annealing of coils in a chamber oven with a
retention time from 1 to 4 h in a temperature range from around
330.degree. C. to 400.degree. C.
[0053] In principle the second aluminium alloy is selected
primarily on the basis of a prespecified strength. The temperature
and duration of performance of the above-mentioned annealing which
is necessary to achieve a structure state which does not lead to a
further or only to a defined limited rise, respectively, in the
mechanical strength values on subsequent artificial ageing, are
determined for each alloy or application individually from a simple
test series.
[0054] The lowest strength level results if the part of alloy
elements present in solid solution and contributing to artificial
ageing is so small that it is to be neglected. For example, in case
a specification for a car body part made from sheet requires a
defined strength level lying above the -minimum strength level for
a given alloy composition, the strength level can be adapted with
the same alloy composition by selecting a higher part of alloy
elements present in solid solution and contributing to artificial
ageing or controlling the artificial ageing treatment that only a
small part is precipitated as Mg.sub.2Si and/or Si particles,
respectively. The car body part is then somewhat less "soft" in
favour of a higher strength.
[0055] Preferably, the soft second components are inner panels of a
body element, in particular a bonnet, and trim parts or structural
components or reinforcing elements arranged in the front area of
the body. The soft second components can however also be body
elements which in conventional car bodies are formed from hard
first components. A substantial area of use of the soft second
component is hence deep-drawn body parts with good bending
behaviour.
[0056] A soft component can for example be used as an inner panel
of a steel or plastic bonnet, a trim part in the front area of a
car (e.g. radiator grille, bumper cover, spoiler etc.) or a
structural component or reinforcement panel in the frontal area
(e.g. reinforcement panel in the bonnet closure area, support
panels for radiator, headlights and other assemblies in the front
area etc.).
[0057] A further application which is not known in this manner in
body construction can also be "curtain-type" protective panels. In
this case the improved bending behaviour which is achieved is
particularly important as, on an impact, it prevents cracking or
splintering in the folds, further minimising the risk of
injury.
[0058] Further advantages, features and details of the invention
arise from the description below of preferred embodiment examples
and with reference to the drawing which shows:
[0059] FIG. 1 a diagram with the yield strength of a first and a
second aluminium alloy in different ageing states;
[0060] FIG. 2 a diagram with the differences between the yield
strength of the first and second aluminium alloys of FIG. 1 in
different ageing states and the yield strength of the alloys in
delivery state T4;
[0061] FIG. 3 and 4 pictures taken from metal cuts of sheet samples
with different part of precipitated Mg.sub.2Si particles under a
scanning electron microscope (SEM) in compo modus;
[0062] FIG. 5 the dependence of the yield strength on the volume
part of precipitated Mg.sub.2Si particles of an AlMgSi alloy by
means of a model calculation.
EXAMPLE 1
[0063] From a first aluminium alloy A (AA 6016) and a second
aluminium alloy B with the chemical compositions given in table 1,
strips of thickness 1.2 mm were produced in a conventional manner
by vertical continuous casting, homogenisation annealing, hot and
cold rolling. TABLE-US-00001 TABLE 1 Al- loy Si Fe Cu Mr Mg Cr Zn
Ti V A 1.14 0.21 0.08 0.07 0.55 0.013 0.003 0.033 <0.005 B 0.42
0.17 0.08 0.07 0.40 0.018 <0.003 0.024 0.006
[0064] The strips were subjected to solution annealing (alloy A)
and partial solution annealing (alloy B) in a strip passage
annealing oven, then quenched by moving air and artificially aged
for several days at room temperature to delivery state T4. For the
two aluminium alloys A and B the following solution annealing
conditions were selected: TABLE-US-00002 Alloy A 550.degree. C./30
seconds Alloy B 500.degree. C./20 seconds
[0065] A paint baking cycle was simulated on sheet samples of
aluminium alloys A and B in delivery state T4, with annealing at a
temperature of 185.degree. C. for a period of 20 min. To test the
influence of cold forming (CF) on the yield strength R.sub.p0.2,
tensile strength R.sub.m and elongation at fracture A80, the sheet
samples in delivery state were 2% further cold formed. A further
series of specimens were 2% cold formed in delivery state and then
subjected to the above-mentioned annealing treatment. The
mechanical strength values given in table 2 for the two aluminium
alloys A and B in the various states tested, and the values also
shown graphically in FIGS. 1 and 2 for the yield strength
R.sub.p0.2, for both aluminium alloys A and B in delivery state
with 2% cold forming, show a slight and proportionally
approximately equal increase in yield strength. If merely a paint
baking annealing is performed at the delivery state, for alloy A
there is a clear increase in the yield strength whereas for alloy B
there is practically no artificial ageing effect. The differing
behaviour of the two aluminium alloys A and B under paint baking
conditions is even clearer under combined application of cold
forming 2% with subsequent annealing at 185.degree. C. for 20
minutes, as often occurs in practice in the production of car body
parts. TABLE-US-00003 TABLE 2 R.sub.p0.2 R.sub.m A.sub.80 .DELTA.
R.sub.p0.2 Alloy State [MPa] [MPa] [%] [MPa] A Delivery state T4
115 225 25.4 185.degree. C. .times. 20 min 195 271 20.8 80 2% CF
140 251 24.3 25 2% CF + 185.degree. C. .times. 20 min 245 295 15.4
130 B Delivery state T4 70 129 27.7 185.degree. C. .times. 20 min
74 130 25.9 4 2% CF 90 133 25.3 20 2% CF + 185.degree. C. .times.
20 min 94 149 18.6 24
EXAMPLE 2
[0066] On 2 tensile test pieces of alloy B in example 1 having a
thickness of 0.85 mm and a width of 12.5 mm in different artificial
ageing conditions tensile strength R.sub.m, yield strength
R.sub.p.02 and elongation at fracture A.sub.50 have been determined
in tensile tests. The examined artificial ageing treatments are
given in Table 3. The solution annealing was carried out in a salt
bath at the given temperature for the given time. Subsequently the
test pieces were quenched in water, aged for 24 h at room
temperature and subsequently aged for 24 h at a temperature of
65.degree. C. This ageing treatment leads to a simulated T4
condition. A part of these test pieces A to L was given an
artificial ageing treatment at 205.degree. C. for 1 h,
corresponding to a T6 condition. TABLE-US-00004 TABLE 3 Test piece
Solution annealing A 520.degree. C./5 s B 520.degree. C./10 s C
530.degree. C./0 s D 530.degree. C./5 s E 530.degree. C./10 s F
530.degree. C./20 s G 540.degree. C./0 s H 540.degree. C./5 s I
540.degree. C./10 s J 540.degree. C./20 s K 540.degree. C./60 s L
540.degree. C./10 min
[0067] The results of tensile tests carried out on 2 test pieces
each are given in table 4 for the test pieces in the T4 condition
and in table 5 for the test pieces in the T6 condition.
TABLE-US-00005 TABLE 4 Test piece Rp.sub.0.2 [MPa] R.sub.m [MPa]
A.sub.50 [%] A1 43.9 115.6 16.3 A2 44.6 114.5 23.3 B1 43.9 114.9
20.2 B2 44.2 117.3 23.2 C1 44.1 116.4 24.2 C2 40.6 112.9 26.8 D1
45.2 114.8 30.9 D2 43.6 116.0 22.0 E1 44.0 119.5 15.6 E2 45.3 117.2
25.5 F1 48.5 125.2 19.0 F2 48.4 124.9 26.6 G1 41.5 112.1 26.1 G2
42.9 111.1 25.1 H1 43.7 115.3 25.1 H2 43.9 114.0 20.2 I1 44.0 119.0
21.7 I2 45.3 118.7 24.9 J1 48.3 127.6 15.1 J2 47.6 126.1 24.4 K1
56.8 137.8 15.6 K2 56.4 137.9 16.2 L1 63.1 152.4 20.7 L2 61.7 144.1
18.1
[0068] TABLE-US-00006 TABLE 5 Test piece Rp.sub.0.2 [MPa] R.sub.m
[MPa] A.sub.50 [%] A1 47.1 117.2 25.1 A2 46.5 116.1 21.6 B1 52.5
119.9 24.8 B2 54.3 123.4 25.3 C1 40.9 111.0 26.1 C2 41.4 111.2 27.9
D1 49.9 119.6 24.4 D2 53.2 120.4 25.2 E1 50.6 121.4 25.3 E2 57.2
123.5 23.9 F1 61.5 130.9 24.7 F2 61.7 129.1 22.9 G1 44.7 114.1 28.1
G2 44.0 113.3 26.5 H1 45.4 119.9 20.5 H2 47.5 118.4 19.2 I1 55.6
125.7 25.0 I2 52.6 124.5 25.4 J1 65.9 135.1 18.5 J2 64.5 135.1 18.9
K1 98.3 154.6 10.6 K2 98.2 153.5 11.3 L1 138.4 177.3 9.0 L2 137.4
178.0 11.4
[0069] From the test pieces C and L in table 4 metal cuts have been
made. Under a scanning electron microscope in the compo modus the
volume part of the precipitated Mg.sub.2Si particles related to the
total volume has been determined by measuring the corresponding
area parts in 12 area regions. Particles having a diameter <0.1
.mu.m are designated as precipitated Mg.sub.2Si particles.
[0070] The mean values for the test piece C resulted in a volume
part of 0.444.+-.0.077% corresponding to a part of about 50% of the
theoretically possible Volume part. For the test piece L the mean
values resulted in a volume part of 0.071.+-.0.029% corresponding
to a part of about 8% of the theoretically possible volume
part.
[0071] The SEM picture in compo modus of test piece C shown in FIG.
3 and of test piece L shown in FIG. 4 let the heavy iron containing
precipitates appear as bright spots and the light-weight Mg.sub.2Si
particles as dark spots. The higher volume part of precipitated
Mg.sub.2Si particles of test piece C in comparison with test piece
L is clearly perceptible.
[0072] With the values for the yield strength R.sub.p0.2 measured
on the test pieces A to L of table 5 the dependence of the yield
strength R.sub.p0.2 on the volume part of the precipitated
Mg.sub.2Si particles has been determined by means of a model
calculation and is graphically shown in FIG. 5. The values on the
x-axis correspond to the ratio of the volume part of the Mg.sub.2Si
pre-precipitates to the theoretically possible volume part.
[0073] The diagram clearly shows that the yield strength R.sub.p0.2
selected here as a measure for the "softness" of the alloy can be
varied within broad limits by controlling the pre-precipitation of
Mg.sub.2Si.
* * * * *