Study Insights Characteristics of New Alloys for HPDC Structural Parts
RHEINFELDEN ALLOYS developed new alloys for the future generation of structural cast components in vehicle construction. This article shows all characteristics of the new presented AlFe-alloy family Castaduct®-42 and Castaduct®-18 by discussing mechanical properties, fatigue strength, dynamic test and joining techniques such as riveting and welding.
Today’s casts are getting thinner and larger with more and more functions integrated. There is the need to offer highest strength and ductility for these crash relevant parts and the riveting process. Structural casting parts should ideally be made without or only with T5 heat treatment to avoid distortion and other problems associated with solution heat treatment.
Classic applications are served by the Al-Si family in the range of 7 to 11% Silicon. Varying amounts of Mg (for Mg2Si hardening), low contents of Fe and Mn for die soldering resistance are added as well. If elongations of 10% or higher are required, then a T7 heat treatment is mandatory. In this paper two new alloys are presented offering unique benefits for the production of Structural Casts in HPDC. Castaduct®-42, AlMg4Fe2 offers a dispersoid hardening mechanism which does not need any heat treatment. Castaduct®-18, AlMg4Zn3Fe2 is a cold hardening alloy which may be stabilized by T5, but does not need solutionizing, like T6 or T7 heat treatments.
The alloy Castaduct®-42, AlMg4Fe2 was developed for any kind of Structural Cast such as the cross beam shown in picture 1. A good castability and simple casting process parameters are important characteristics of this alloy. The high Fe content leads to low soldering and a long lifetime of the die. The absence of Si in Castaduct®-42 prohibits any change of mechanical properties due to natural aging. This results in high applicable dynamic loads and an excellent ductility and corrosion resistance. To fulfill the industrial requirement for Structural Casts (YS > 120 MPa and E > 10% from German OEMs) without heat-treatment Mg was added to the Al-Fe-eutectic. The Castaduct®-42 achieves these good mechanical properties already in the as-cast condition. The distortion of the cast during heat treatments is a huge issue which is not applicable here. Castaduct®-42 is a perfect Non-Heat-Treatment (NHT) alloy.
An even further improvement of the strength in order to reduce the weight of constructions is done with the Castaduct®-18. The high strength is achieved through the addition of zinc and the consequential cold hardening process. This cast alloy is suited for all structural components where stiffness is key, like the door frames shown in picture 2. The main advantages of Castaduct®-18 are a high strength up to 180 MPa in the as-cast state, highest dynamic load, good ductility and corrosion resistance. An AlMg4Zn3Fe2-type alloy demands a higher technical level in casting technique than AlMg4Fe2-type of alloy.
Castaduct-42: Enabling New Applications for HPDC Aluminum Parts
Thanks to the simple chemical composition this alloy has the potential to open new applications for HPDC aluminum parts in car bodies. It provides sufficient mechanical properties without the necessity of a post cast heat treatment. Furthermore, the high Fe content of Castaduct®-42 leads to a low die sticking tendency which results in a longer die life. In addition, reduced die wear will positively influence the quality of cast part and reduce cast surface grinding and die costs. Its simplicity has positive effects on alloy manufacturing costs, too.
The main strengthening mechanism in Castaduct®-42, AlMg4Fe2 is solid solution strengthening by Mg solute atoms. Similar to our elder Structural Cast NHT-alloy Castasil®-37, AlSi9MnMoZr this new alloy Castaduct®-42 requires no heat treatment on the casts. Results of tensile tests demonstrate that Castaduct®-42 has a strength and ductility which fulfills requirements for Structural Casts. The yield strength is greater than 120 MPa and the elongation is around 14%.
The absence of silicon has other effects: The alloy shows virtually no aging. This is shown by a heat treatment of 350 °C for three hours without any impact on mechanical properties. So any production step related to heat (i.e. surface treatment) is less critical than in case of AlSiMg alloys. Furthermore, due to the absence of silicon the alloy can be anodized showing an excellent surface quality.
In comparison to widely used AlSi cast alloys, Castaduct®-42 has a higher shrinkage which must be taken into account during part and die design. However, first trials in several foundries with cast weights up to 12 kg showed that Castaduct®-42 could be still used for HPDC in dies designed for a standard AlSi10MnMg alloy. An example of the collaboration between RHEINFELDEN ALLOYS and Fonderie 2A (Santena, Italy) is shown in picture 1. This Structural Cast with the length of approximately 60 cm could be cast in a quality that fulfilled requirements of series production. Surface and X-ray examinations showed similar results as in series production. On the other hand, spray and solidification time could be reduced in comparison to series conditions.
Fatigue Strength of Castaduct®-42
Fatigue strength was investigated by the Steinbeis-Transferzentrum BWF (Esslingen, Germany). 3 mm HPDC plates were subjected to bending stress (R = -1) and resulted with a default probability of 10% to 106 MPa. This value is a step higher than values to be found in literature1-3 for AlSi-alloys..
The Institute for Vehicle Concepts DLR (Stuttgart, Germany) carried out static and in addition dynamic bending test up to 5 m/s. As the positive result DLR found an increase of ductility with higher impact speed. This means a static designed component has higher safety in dynamic applications. Overall the dynamic properties of Castaduct®-42 can be classified as good-natured.
MIG Welding Trials of Castaduct®-42
In cooperation with Oerlikon-Wirth (Reutlingen, Germany) MIG welding trials were carried out. A 3 mm HPDC plate in Castaduct®-42 and a AlMg0,5Si0,5 sheet material were joined together using a 1,2 mm wire of AlMg4,5Mn. With the help of their welding power source Fronius TPS 400i weld seams in high quality could be produced. There was no tendency for hot tearing or porosity and generally no change in microstructure. In spite of the mismatch of materials in a welding area, with Castaduct®-42 no significant change in hardness could be measured.
In case of classic AlSi10MnMg type of alloys, strong change in hardness can be found, especially in the heat affected zone near by the weld seam. With Castaduct®-42 there is no significant effect of heat treatment neither on cast nor on sheet material, thus no major change of material properties can be measured.
Riveting Trials of Castaduct®-42
Several investigations for riveting were carried out. One was realized with 3 mm plates produced in the Tech Center of RHEINFELDEN ALLOYS. Trials with the rivet type Henrob Betamate 1640 showed that Castaduct®-42 was the most suitable out of all alloys examined in this investigation.
Shock towers with a shot weight about 5 kg were casted in Castaduct®-42. They passed the production accompanying examination for riveting. The series production for these shock towers ran in AlSi10MnMg, but has to use T6 heat treatment.
Metallurgy of Castaduct®-42
Alloys of the AlFe type are generally known1,2,4-6, some references mention them as 8000 alloy. Structure of Al- Al3Fe eutectic7 and solidification morphology8 are described in literature. Shrinkage of this Si-free alloy is reduced by Fe. With up to 0,4% Si there is a silicon free Al3Fe eutectic. These descriptions and statements could be confirmed by the present investigation.
Micrographs show the very fine structure of AlFe eutectic. However, aluminum dendrite structure remains dominant in this rather simple alloy composition. Due to constrain by the fine dendritic structure in HPDC, the eutectic fraction is much smaller than it would be in gravity casting. Magnesium is solved in the alpha phase, there are no ternary phases AlMgFe. Furthermore, few Fe-rich phases Al13Fe4 could be found.
For Al-Mg-Fe alloy, although the Fe concentration is slightly higher than the eutectic composition, the primary phase forming in the alloys is still the α-Al dendrite structure. This should be due to the skewed eutectic region of the kinetic phase diagram. With some undercooling, the first phase to form will be the α-Al dendrite instead of primary Al13Fe4 or eutectic.
Castaduct-18: Particularly High-Strength HPDC Alloy for Structural Parts
A recently introduced alloy is Castaduct®-18, AlMg4Zn3Fe2 which was developed on the base of Castaduct®-42. Substantial increase of strength is caused by precipitation hardening with the help of the elements Zn in combination with Mg. This natural aging alloy demands a higher technical level in casting than AlMg4Fe2-type of alloy. Unlike Castaduct®-42, heat treatments have an influence on Castaduct®-18.
Mechanical properties were examined with 3 mm test plates in the Tech Center of RHEINFELDEN ALLOYS. Yield strength about 180 MPa and an elongation 7-8 % was measured in the hardened state. Additional elements such as zirconium or chrome increase strength and lower ductility moderately.
First trials of this alloy were done by Fonderie Cervati spa (Brescia, Italy). A door frame (see picture 2) could be realized in good casting quality: Surface condition and X-ray quality showed no negative conspicuity. The casting trials confirmed the results of lab-scale mechanical tests; yield strength of 180 MPa and an elongation of 7% could be achieved.
Fatigue Strength of Castaduct®-18
The alloy Castaduct®-18 showed very high fatigue strength compared to other HPDC alloys for Structural Casts1-3. The Steinbeis-Transferzentrum BWF (Esslingen, Germany) tested 25 samples from 3 mm HPDC plates for bending stress (R = -1). With a default probability of 10% the measurement was 123 MPa.
MIG Welding Trials of Castaduct®-18
Similar to the trials with Castaduct®-42, the company Oerlikon-Wirth ran MIG welding trials with 3 mm HPDC plates in AlMg4Zn3Fe2 and AlMg0,5Si0,5 sheet material. Microstructure of the weld seam was very fine.
Unlike the welding trials with AlMg4Fe2, there was an influence of the welding process on hardness noticeable. This effect is typical whenever heat treatable alloys are joined by arc welding. The reason behind is the hardening of the alloy Castaduct®-18, especially in the heat affected zone close to the weld seam (in this case about 1-3 mm away from the weld seam). 0,5 mm from the weld seam a loss of hardness can be seen. An annealing effect takes place here; the material has a T4-heat-treatment-like characteristic. In the weld seam a quite homogenous material mix out of cast, sheet and filler material can be found. As a result hardness lies between cast and sheet material. This effect of a heterogeneous hardening distribution can be reduced either by welding technique measures or by a T5 heat treatment.
Riveting Trials of Castaduct®-18
First riveting trials were realized with 3 mm plates produced in the Tech Center of RHEINFELDEN ALLOYS. It was possible to rivet the alloy Castaduct®-18 with standard methods. Generally, rivetability decreases with increasing strength of the material. It is possible to compensate this effect by using rivets and a die with a suited geometry.
Metallurgy of Castaduct®-18
The addition of zinc turns the alloy AlMg4Fe2 in a quaternary alloy system. Quaternary alloy systems can be very complicated. One reason why this quaternary phase diagram is not too complex is the absence of a ternary AlMgZn phase.
In this case the phase diagram of AlMg4Zn3Fe2 is quite similar to ternary alloy AlMg4Fe2. But the addition of Zn moves the Al-Fe eutectic point to lower Fe content and keeps the Castaduct®-18 quaternary alloy still hypereutectic.
Microstructure of the eutectic is very fine. It is even difficult to depict it by optical microscopy. Electron probe micro analysis on particles in the eutectic region shows that both Al13Fe4 and β-Al13Mg5Zn2 have formed. The β-AlMgZn phase has a skeletal structure and is mostly located in the interdendritic region. The fraction of eutectic particles is generally in Castaduct®-18 slightly higher than in Castaduct®-42. Positively is here again the low presence of Al13Fe4 particles. The fraction of coarse rod shaped Al13Fe4 particles is less than in the Castaduct®-42, the alloys without Zn. Even the Al13Fe4 particles tend to be smaller in the Castaduct-18. The reason could be that the growing of Al13Fe4 particles has a lower growth rate in the present of zinc.
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