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SAG: Optimisation of the Mechanical Properties

of AlMgSi high pressure diecasting alloy

 

Optimisation of the Mechanical Properties of AlMgSi high pressure diecasting alloy 

AlMgSi die cast alloys offer the major advantage that excellent mechanical properties can be achieved just with casting.
Tensile strength and ductile yield in particular are at a level that could previously only be achieved through the use of heat treatable alloys and application of heat treatment. Increasing the relatively low yield strength of this alloy type was the goal of a research project at the SAG subsidiary Aluminium Lend GmbH + Co KG.  

1.      AlMg(Si) – high pressure diecasting alloy 

1.1 PERALUMAN® - 90 
The use of AlMg(Si) alloys in die casting is not new: The alloy PERALUMAN® - 90 (EN AB-51200, AlMg9) has been in use for decades. The alloy PERALUMAN® - 90 contains up to 2.5 % silicon by weight, which serves primarily to improve the casting properties.


GD-AlMg9 [100x]

The high excess magnesium content of the alloy leads to high saturation of the proeutectic, meaning that the Mg2Si formed exists almost completely in the eutectic. Heat treatment or artificial ageing are therefore not productive. The disadvantage of this magnesium-rich alloy is the strong tendency toward hydrogen uptake as well as the low 0.2% yield strength. The cast parts produced from PERALUMAN® - 90 are characterised by excellent corrosion resistance. 

Si

Fe

Cu

Mn

Mg

Zn

Ti

Other

0.1 - 2.5

0.50

0.02

0.2 – 0.5

8.5 – 10.5

0.10

0.05-0.15

each 0.02

       
Chemical composition of PERALUMAN®-90


Material

 

0.2% Yield Strength

RP0.2[MPa]

Tensile Strength

RM [MPa]

Ductile Yield

A5[%]

PERALUMAN®-90

AlMg9, EN AB-51200

130 - 150

200 - 250

1 – 4

Mechanical properties of PERALUMAN®-90 in the cast state 

1.2 MAXXALLOY® - 54

The alloy MAXXALLOY®-54 (AlMg5Si2MnCr) was developed specially for use in die casting. Cast parts produced from MAXXALLOY®-54 have excellent mechanical properties such as strength and ductility already in the cast state. The combination of manganese and chromium reduces the adhesive tendency in the mould and reduces the deformation of the cast part thanks to higher thermal resistance during demoulding. The relatively low 0.2% yield strength is typical for AlMg alloys. 

Wall Thickness

[mm]

0.2% Yield Strength

RP0.2 [MPa]

Tensile Strength

RM [MPa]

Ductile Yield

A5 [%]

2 – 4

150 - 195

270 - 310

10 - 18

4 – 6

130 – 175

250 - 280

9 - 16

6 - 12

100 – 145

210 - 250

7 - 10

  Mechanical properties of MAXXALLOY®-54 in the cast state 

2. Improving the Mechanical Properties 
The two die cast alloys presented achieve high strength values with average to good 0.2% yield strength. However, the properties profile limits the range of applications. In comparison, heat treatable AlSiMg alloys (such as AlSi9MgMn, SILAFONT®-36) have a high strength and 0.2% yield strength once fully heat-treated. The combination of the advantages of the alloy MAXXALLOY®-54, such as the high tensile strength, high ductile yield, very good welding properties and excellent corrosion resistance with the yielding point of heat treated AlSiMg alloys appears difficult to achieve at first glance.

A development project at SAG Aluminium Lend GmbH + Co KG set out to achieve this goal. 


2.1 Solution Approaches
2.1.1 Alloy Elements

Through the addition of metals such as zirconium, chromium and vanadium, the 0.2%  yield strength can be increased. MAXXALLOY®-54 contains between 0.1 and 0.3% chromium by weight, which reduces the adhesive tendency; compared with a chromium-free alloy, the 0.2% yield strength is higher by approximately 10 to 15 MPa. However, the combination of chromium and manganese leads to a higher sludge factor[1] for the alloy, meaning that chromium cannot be added without limits.

The alloy elements vanadium and zirconium become enriched in the circulation material; zirconium in particular can eventually lead to hard inclusions that negatively affect the mechanical properties or result in increased wear on tools during subsequent machining.

The addition of these elements is possible in principle, but leads to undesired side effects.


Fe-Mn-Cr – sludge / alloy AlMg9

2.1.2 Rare Earths - Lanthanides 
Easily soluble alloy elements, such as cerium and lanthanum were previously used only to improve the fineness of the grain in wrought materials or in experiments as an improving agent for AlSi cast alloys. 
The addition of 100 – 500 ppm cerium or lanthanum increases the 0.2% yield strength of MAXXALLOY®-54  FROM 160 MPa to between 190 and 230 MPa, depending on the wall thickness or the hardening speed. This effect can be explained by the formation of intermetallic Al-Ce or Al-La phases that prevent the displacement motion [2].

 


'
Phase diagram Al-Ce    


Phase diagram Al-La

A special melting technology is required with the use of lanthanides. Due to the exceptional affinity of these metals for oxygen, rapid oxidation occurs immediately. In event of improper addition, very course deposits form that have a negative effect on the ductile yield and fatigue strength. 

Through comprehensive further development of the melting technology at SAG Aluminium Lend, it has become possible to add these elements to the molten metal without high oxidation and to ensure complete dissolution. 

The result of these developments is the alloy MAXXALLOY®- ULTRA. This alloy offers the advantage of AlMg5Si2MnCr but with significantly increased 0.2% yield strength and tensile strength. Within the framework of the alloy development, the magnesium and silicon contents were increased somewhat, but improved casting properties were verified in die casting tests as a result.

 MAXXALLOY®-ULTRA is characterised by a particularly fine proeutectic α – Mg2Si eutectic.  The alloy becomes less sensitive to disruptive companion elements, such as phosphorous, meaning that the circulation share can be increased somewhat.


The aforementioned higher oxidation tendency of the molten metal is compensated by a beryllium content ranging from 30 to 40 ppm. The melting loss is therefore comparable with that of MAXXALLOY®-54, particularly for longer standing times of the molten material. 

Si

Fe

Cu

Mn

Cr

Mg

Zn

Ti

Ce

La

Other

2.4 – 3.0

0.15

0.02

0.2 – 0.5

0.1 – 0.3

5.5 – 5.8

0.10

0.05-0.15

0.01 – 0.03

0.01 – 0.03

each 0.02

  Chemical composition of MAXXALLOY®-ULTRA (% by weight) 

The mechanical properties that can be achieved with MAXXALLOY® - ULTRA are given in the following table (casting state): 

Wall Thickness

[mm]

0.2% Yield Strength

RP0.2 [MPa]

Tensile Strength

RM [MPa]

Ductile Yield

A5 [%]

2 – 4

220

 350

9 – 12

4 – 6

200

330

14 – 16

The mechanical properties that can be achieved with MAXXALLOY®-ULTRA were verified with die cast step plates. In event of even higher requirements for the 0.2% yield strength, artificial ageing can be performed. In this case, it is necessary to quench the die cast part immediately after demoulding, and the 0.2% yield strength can be increased by approximately 10%. 

3. Summary
The mechanical properties, in particular the 0.2% yield strength of AlMg5Si2MnCr, can be improved through the addition of lanthanides. Cast parts of MAXXALLOY® - ULTRA are ductile thanks to the formation of a particularly fine eutectic. A special melting technique must be used during production of the alloy in order to prevent melting losses in the melting plant and later in the die casting plant. The alloy MAXXALLOY®-ULTRA is characterised by a significantly increased 0.2% yield strength and tensile strength in comparison with the standard alloy. The increase of the silicon and magnesium content leads to improved casting properties.

The new alloy MAXXALLOY®-ULTRA is therefore particularly well suited for the production of die cast components capable of withstanding higher loads. 

SAG Aluminium Lend GmbH + Co KG has applied for patent for the discoveries regarding the chemical composition and melting technique for production of the alloy MAXXALLOY® - ULTRA. 

Literature:
[1] E. Brunhuber: Praxis der Druckgussfertigung, 4th edition, 1991, page 415+
[2] F. Vollertsen, S. Vogler: Werkstoffeigenschaften und Mikrostruktur,
    1989, page 63+

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