Ductile aluminumdie casting alloy for die castings with demanding requirements in terms of ductility and elongation at failure or with demanding requirements in terms of formability. An overview (without guarantee of completeness) of the alloys of this type and their chemical composition is given in Table 1.
Alloy type Al Mg5Si2Mn
The production of large structural parts with low wall thicknesses sets demanding requirements for the casting alloys which are to be used. In addition to good flowing properties and a low tendency to stick, the material must, above all, fulfill the requirements for optimum ductility. High ductility in a casting requires the use of die casting alloys with a low iron content.
The alloy type Al Mg5Si2Mn covers naturally hard, highly ductile alloys; the term “naturally hard” is taken over from the wrought materials. Strictly speaking, even Al Mg5Si2Mn alloys have a tendency to age, albeit in a very limited way, but this effect ceases after just a few days at room temperature. Depending on the wall thickness, outstanding mechanical properties are achieved without heat treatment. Here, the high elongation at failure in thin-walled parts should be mentioned in particular. The relatively low 0.2% yield strength is typical for AlMg alloys. The wide solidification interval ensures that the mold is filled well, whereby the tendency for warm cracks to form is still low.
Despite the low iron content, the alloys do not tend to stick as, due to the presence of manganese, even thin-walled parts can be demolded very well. When designing the mold inclinations, the solidification shrinkage of alloys of this type, which is high in comparison to that of AlSi alloys, must be considered. These alloys are resistant to corrosion and fantastically weldable. They are used everywhere where castings with very high ductility and good strength without heat treatment are required. By saving the heat treatment, the production costs of the castings are reduced due to:
An example of where they can be used is as safety components in vehicle construction (chassis components, pressure vessels, car seats, structural parts).
The setting of the structure of Al Mg5Si2Mn alloys takes place in the refinery with a special molten metal treatment (Figure 1). Here, the content of alkaline and alkaline earth elements must be reduced as much as possible. Contents of as little as 10ppm of sodiumlead to a brittle structure. When processed further in a die casting foundry, the use of any cleaning salts which contain sodium must therefore be avoided. When circulation material is used, it is recommended that the calcium and sodium contents are monitored. A tried and tested method for the control of the metal quality is thermal analysis. The right structure of these alloys can be proved with a recalescence of 0.6 to 1.2°C.
Alloy type Al Mg5Si2MnCr
As part of a development project, the influences of a chromium additive in a basic alloy of the type Al Mg5Si2Mn was investigated with regard to mechanical properties. The combination of manganese and chromium reduces the tendency to stick in the mold and reduces castingdistortion due to increased high-temperature tensile strength during demolding. The alloy Al Mg5Si2MnCr contains between 0.1 and 0.3% by weight of chromium which reduces the tendency to stick. The 0.2% yield strength, compared with a chromium-free alloy, is around 10 to 15MPa higher. The preferred area of application of Al Mg5Si2MnCr (brand name MAXXALLOY®-Structure© from SAG Aluminium Lend GmbH) is for thin-walled die castings with wall thicknesses of 2 to 4mm.
The very good weldability allows complex components to be manufactured from castings and, for example, extrusion profiles to be manufactured from 5000 and 6000 alloys. Structural parts are an example. The guide values for the mechanical properties given in Table 2 indicate the potential of this alloy under the condition of error-free casting (largely free from pores, cavities and oxides). For this reason, for safety components, suitable measures used in casting technology (e.g. the use of a vacuum) must be put in place to avoid these defects. The weldability is also influenced by the parting agents and lubricants. Impurities when melting the pigs can also have a negative effect on the mechanical properties. Particular care is therefore required for this step of the process.
Alloy type Al Mg5Si2MnCrRE
By adding metals such as zirconium, chromium and vanadium, the 0.2% yield strength can be increased even further. However, the combination of chromium and manganese also leads to the alloy having a higher migration factor here; chromium can therefore not be added without limit. The alloy elements vanadium and zirconium accumulate in the circulation material; zirconium in particular can then lead to hard inclusions which have a negative effect on the mechanical properties and lead to increased wear of the tools in later processing.
The addition of rare earths is possible in principle but leads to undesired side effects. Until now, easily soluble alloy elements such as cerium and lanthanum have only been used to improve the grain refinement in wrought materials and in attempts to use it as a modifier for AlSi casting alloys. The addition of 100 to 500ppm of cerium or lanthanum, dependent on the wall thickness and the solidification speed, increases the 0.2% yield strength from 160MPa to between 190 and 230MPa. This effect can be explained by the formation of intermetallic Al-Ce and Al-La phases which prevent dislocation movement. When lanthanides are used, a special melting technology is required. Due to this metal’s unusually high affinity with oxygen, rapid oxidation occurs immediately. If added incorrectly, very rough diffusions which have a negative effect on the elongation at failure and endurance strength form. Through the further development of melting technology and process technology, it is now possible to add these elements into the melt without strong oxidation and to ensure that they dissolve completely.
The result of these developments is an alloy of the type Al Mg5Si2MnCrRE (brand name MAXXALLOY®-ULTRA© from SAG Aluminium Lend GmbH). This alloy offers the advantages of Mg5Si2MnCr but with a considerably higher 0.2% yield strength and tensile strength. During the further development of the alloy, the contents of magnesium and silicon were also increased a little; in casting tests this resulted in better casting properties. The alloy is characterized by a particularly fine α-solid-solution-Mg2Si-eutectic and is less sensitive in comparison to accompanying elements such as phosphorus. The circulating amount can therefore be slightly higher. The molten metal’s high tendency to oxidize mentioned above is counteracted with beryllium contents of 30 to 40ppm. The melting loss behavior, particularly for longer melt standing times, is therefore comparable with that of Al Mg5Si2MnCr.
The mechanical properties which can be achieved with this alloy type are given in Table 2 (cast condition) and were proved using die cast steps. For even more demanding requirements regarding the 0.2% yield strength, artificial aging can be carried out. To do so, the quenching of the casting must be done immediately after the demolding; the 0.2% yield strength can be increased once more by around 10% by doing so.
Benefits of using Al Mg5Si2Mn alloys
In summary, the use of these alloys makes it possible to create high-strength yet ductile die castings. The special alloy compositions with additives of chromium and rare earths offer high strength and elongation at failure properties, even in the cast state, so that heat treatment is not necessary. The maximum mechanical properties which can be achieved are comparable with forgings from EN AW-6082 in the condition T6. However, the mechanical properties are dependent on the wall thicknesses of the casting. Strength and elongation at failure sink with increasing wall thickness, meaning that these alloys are particularly suitable for the production of thin-walled castings which are subjected to strong loads.