Magsimal-25 - A new High-Ductility Die Casting Alloy for Structural

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Magsimal-25 - A new High-Ductility Die Casting Alloy for Structural
Parts in Automotive Industry
P. Krug, H. Koch, R. Klos
Aluminium Rheinfelden GmbH, Rheinfelden, Germany


Especially for structural parts for the car body a great variety of specifications must be met.
Some of these parts integrating a number of single components to a complex one, which
requires a good castability and fluidity of the alloy. No die sticking should occur because
straigthen reduces productivity. For the same reason a material is of interest which
reaches mechanical properties, i.e. 120 MPa yield strength and elongation greater than
16% in the as cast state (temper F) so heat treatment and distortion during quenching can
be avoided. Since structural parts have to be connected to the car body itself, good
welding behaviour and the possibility to join the castings via self-piercing riveting must be
taken into account. With the development of Magsimal-25, an AlMn1MgCo-type alloy all
these requirements can be fulfilled. It shows superior mechanical properties in the temper
F state, where elongations up to 26% can be obtained. Due to its outstanding ductility not
only a excellent crash-energy absorption can be achieved, also mechanical joining
(riveting, clinching) is no problem. The reduced Magnesium-content leads to a good
welding behaviour, especially by using CO2-Lasers. By adjusting a high content of
Manganese in addition to a Cobalt and Iron content of about 0.3 wt.-% each, die sticking
will not occur, thus leading to a good ejection behaviour, which means that consumption of
release agents during the casting process can be reduced to a minimum. This leads to a
better welding behaviour, too. Silicon-content must be kept below 0.15 wt.-% because it
affects ductility in a negative sense.


Introduction
During the last years there is a growing tendency to use aluminium die casting parts in
body design. The driving forces for that tendency are weight reduction, integrating several
parts (profiles and cast nodes) into one part, applying a high productivity process like high
pressure die casting. Especially in body design such parts must fulfil several requirements
due to their crash relevance. Deformation behaviour should be suited for crash loads.
Fatigue resistance and corrosion resistance should also be sufficient. Special
requirements to the alloy arise from production and assembly processes. An excellent
castability and no tendency for hot cracking are a must as well as the possibility to join the
part either thermally or mechanically to the other car body materials. Reaching the
mechanical properties in the as cast state (Temper F) is desired, because a full heat
treatment (T6) means always higher cost, lower productivity and the danger of blister and
distortion of the cast part during quenching. From the point of view mentioned above one
have to turn to natural hard alloys. As shown in Figure 1 there are still two primary
Magsimal-alloys denoted Magsimal-59 (AlMg5Si2Mn) and Magsimal-22 (AlMg3MnCo).
The average properties are shown for wall thickness of 2 to 4 mm which is of interest for
structural parts. Both alloys provide good elongation in the temper F state. For Magsimal-
59 the resulting yield strength is too high (1,2).
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                                                               Magsimal-22 would fulfill the
                                                               mechanical       requirements
                                                               but     exhibits    a    slight
                                                               tendency for hot tearing.
                                                               This may lead under certain
                                                               circumstances to problems
                                                               during casting real parts
                                                               which had sharp changes in
                                                               wall thickness, especially at
                                                               stiffening rips and led to
                                                               cracks during welding, too.
                                                               Aim of our development was
                                                               to design an alloy which
                                                               provides the same good
                                                               mechanical properties with
                                                               improved       hot     tearing
                                                               resistance and improved
                                                               weldability.

Fig.1: Magsimal-alloys portfolio for a wall thickness between 2 to 4 mm. The coloured
boxes are indicating average values for yield strength and elongation.




Composition
This improvements were achieved by reducing the Silicon and Magnesium content. From
literature it is known that these both elements cause hot tearing problems, especially
during welding (3). For the alloy composition see Table 1.

          Si       Fe       Cu       Mn        Mg       Zn      Ti      Co
   min.               0,10                0,9       0,9                    0,20
   max.      0,15     0,40      0,10      1,4       1,3    0,10    0,20    0,40
Table 1: Chemical composition of Magsimal-25 in weight-%.


Manganese and Cobalt are necessary to reduce Iron activity in the melt, because a Iron
content of maximum 0.40 % is not sufficient enough to prevent die soldering. Iron itself is
necessary to a certain extent to provide strength to the alloy by precipitating between the
dendrites. This will also reduces hot tearing. By limiting the Silicon content to a very low
value hot tearing is reduced, too. Last but not least Titanium is added for grain refinement
and due to smaller grain size assures a good fluidity. Because of corrosion resistance Zinc
and Copper content should kept low. Elemental distribution can be recognised from Figure
2a and 2b. In Figure 2a the microstructure of a die cast part is shown. One will see alpha
dendrites and precipitations between their branches. SEM analysis (Fig.2b) revealed that
this precipitations are of the kind Al6Mn, where Manganese is partially replaced by Cobalt
and Iron. In the dendrites itself there is a segregation of Titanium which cannot be resolved
in the SEM, but is visible in light microscopy as a certain shade.
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Fig.2: a) left, microstructure of die cast plate with 3 mm wall thickness; b) right, SEM
picture; BSE; showing (Al,Mg)6(Mn,Fe,Co) precipitations (light, between dendrite-
branches.


No other phases could be found in the microstructure. This would be supported by looking
to the cooling curve of Magsimal-25 as shown in Fig 3. The derivative evinces no further
reaction during cooling than the both mentioned above.




Fig.3: Cooling curve of Magsimal-25. Note the high solidus temperature of 625°C and the
small solidification range of 27K.


Due to the low alloying content Ma-25 exhibits a small liquidus-solidus-interval of only
27 Kelvins. This small solidification range is one of the main reasons of the hot tearing
behaviour. i.e. no cracks occurred at test parts with stiffening rips.
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Mechanical Properties
Table 2 shows the mechanical properties of Magsimal-25. Specimens were taken from flat
plates (220x60mm) as well as from boxes (120x120x60mm). Yield strength has a slight
dependence on wall thickness while elongation and ultimate tensile strength are quite
unaffected.

      Design        Wall thickness       Rp0.2             Rm              A5
                         (mm)           (MPa)            (MPa)            (%)
   Plate                   2                    124             206            18,4
   Box                     2                    124             213            17,8
   Plate                   3                    123             219            18,1
   Box                     3                    120             211            17,5
         1)
   Plate                   3                    114             213            20,5
   Plate2)                 3                    119             211            20,0
   Plate                   4                    105             204            17,4
  1)
     Tensile tests performed by Fraunhofer Institut für Werkstoffmechanik, Freiburg
  2)
     Tensile tests performed by TUHH Arbeitsbereich Produktionstechnik, Hamburg

Table 2: Mechanical properties of die cast test samples.


Since structural parts are crash relevant parts, dynamic tests were performed, too. The
results are shown in Figure 4 in comparison to results of static tests. Increasing the strain
rate from 0.0004 s-1 to 200 s-1, i.e. related to the specimen size a cross beam speed of
about 10 m/s, the yield strength increases slightly while elongation and reduction in area
show a dramatic increase. That means, that by holding the stress level nearly the same,
energy consumption is increased by a factor of 1,5.




Fig. 4: Results of dynamic tests performed at Fraunhofer Institut für Werkstoffmechanik,
Freiburg. Values shown are an average of five specimens.
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Low cycle fatigue was tested by strain controlled tests. The resultant cyclic stress strain
curve and the parameters of the curve are shown in Fig.5.




Fig.5: Cyclic stress strain curve of Magsimal-25 showing strengthening effect of cyclic
deformation. The parameters describing the curve are given in the box in the lower right.

Magsimal-25 strengthens during cyclic strains, which leads to a cyclic yield strength of
200 MPa. This strengthening behaviour may also be responsible for the superior
endurance limit of above 100 MPa at 106 cycles (Fig.6).




Fig. 6: Wöhler diagram for Magsimal-25. Endurance limit is 103 MPa at 106 cycles. For
higher cycle numbers there is an usual decrease of about 5% per decade in stress
amplitude.
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Corrosion resistance
Corrosion resistance plays an important role in car manufacturing especially for structural
parts. Resistance against intergranular corrosion (IGC) and stress corrosion cracking
(SCC) has been tested. SCC test followed ASTM G47-90 specification, i.e. periodical
immersing the specimens into 3.5% NaCl-solution. Load was calculated on basis of 75%
of yield strength in temper F state and were applied for 30 days. IGC-tests were performed
according to ISO 11846 which means periodical rinsing with a mixture of 3% NaCl-solution
and 1% hypochloric acid. Two states were tested, temper F and a sensitised state (150°C
for 120h), respectively, as well as two surface conditions: machined and as cast. While the
machined surface exhibits only a pitting depth of about 50 µm the as cast sample shows a
much higher pitting depth of about 120 µm. This was also recognised along with other
alloys investigated (4,5). We suggest that this effect is based on residues of the die casting
release lubricant and we recommend washing and pickling of the parts. All SSC-
specimens (machined as well as cast surface) withstand the test without failure.


Welding
There is a still ongoing examination on MIG- as well as on CO2-Laser welding of
Magsimal-25 alloy. Two combinations were used in a lap joint geometry: Magsimal-
25/Magsimal-25 and Magsimal-25/AA6181. AlMn1, AlMg4,5Mn and AlMg4,5Mn were used
as filler metal. Both combinations has been welded successfully. With the MIG process no
hot cracking occurred during the welding trials and porosity is about 5% although no joint
preparation has been applied to the plates. CO2-laser weldments exhibits no cracks, too,
but porosity is much too high. The trials will be continued with different joint preparations
(brushing, washing, pickling), because the influence of the die release lubricant on porosity
must be excluded, especially for CO2-laser weldings.


Mechanical Joining
Due to it’s high deformability, Magsimal-25 can also be joined by mechanical joining
techniques, e.g. by self-piercing riveting or by clinching. Figure 7a and b gives examples of
clinch joints of Magsimal-25.




Fig 7: a) left, clinch joint between Magsimal-25 and AA6181 using a round die b) right,
cross section of a Magsimal-25/Magsimal-25 clinch joint.


The alloy was successfully joined to a big variety of other wrought materials as
AlMg4,5Mn, Anticorodal 120, Al99,5, AlMg3, DC-04 (with and without cataphoretic
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painting) and casting alloys as Magsimal-22, Magsimal-59 and Silafont-36, T7
(AlSi9MgMnSr). Cohesion forces in shearing tensile tests are comparable to joints of usual
body steel sheet. Magsimal-25 can be clinched either with a round die (without cutting) or
with a rectangular shaped die (with cutting). Riveting trials conducted elsewhere showed
that joining by self-piercing rivets is an additional method of connecting Magsimal-25 to
other body materials.


Ageing behaviour
Due to the high solidus temperature of 625°C one can assume that Magsimal-25 shows a
good thermal stability. To proof that stability ageing experiments were performed at
various temperatures and times. This experiments revealed that no change in mechanical
properties arose from heat treatment up to a temperature of 300°C. After 30h at 500°C a
decrease in yield strength occurred and thus elongation increased (see Figure 8a and b).




Fig.8.: Ageing behaviour of Magsimal-25 a) left, yield strength versus time for various
temperatures b) right, elongation versus time for various temperatures.


Casting trials
For real parts conditions are more complicated than in simple test specimens. Thus the B-
pillar of AUDI A2 was cast to examine the mechanical properties. This part is of very
complex shape and embodies a lot of sharp changes in wall thickness at stiffening rips.
Although the die were not adopted for the AlMnMgCo-type alloys the results are very
promising. No hot tearing occurred after optimising the die casting process. Figure 9
shows the part together with the AUDI A2 space frame and the achieved mechanical
properties. Since the gating system is not designed for AlMnMg-alloys there were a lot of
porosity in the castings. Due to the porosity, requirements on mechanical properties are
not met at some locations. Casting trials will be repeated after adopting the die design to
the alloy.
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Fig. 9: Mechanical properties of die cast B-pillar of AUDI A2 space frame. Values are
taken from different parts. Due to high porosity caused by a non-appropriate gating
system, requirements on mechanical properties especially for elongation is not met at
some locations.


Summary
With Magsimal-25 a new high-ductility die casting alloy was designed for structural parts in
automotive industry. The alloy meets all requirements concerning mechanical properties,
castability, hot tearing resistance, corrosion resistance (IGC and SSC) as well as
weldability. It exhibits a superior fatigue strength and shows a high energy absorption in
dynamic tests. It can be joined mechanically by self-piercing riveting or by clinching and no
thermal altering will occur. Casting trials on a real part revealed the high potential of this
alloy and will be continued with an optimised die to overcome porosity resulting from the
non-appropriate gating system.



References
(1) U. Hielscher, H. Sternau, H. Koch, R. Klos; Giesserei 85, 1998, Nr.3
(2) P. Krug, H. Koch, R. Klos, Diecasting World, March 2000, pp. 30-34
(3) L. Dorn; Mat.-wiss. u. Werkstofftech. 29, 1998, pp. 412-423
(4) Ch. Deola, Untersuchungsbericht ALTEC-98/012, 1998
(5) Ch. Deola; Untersuchungsbericht ALTEC-99/0103, 1999