Study Insights

Characteristics of New Alloys for HPDC Structural Parts

| Author/ Editor: S. Wiesner, Rheinfelden Alloys Germany; Y. Saka, Nikkei MC Aluminium Kunshan / Isabell Page

Rheinfelden Alloys developed new alloys for the future generation of structural components in vehicle construction. This article shows all characteristics of AlMg4Fe2 and AlMg4Zn3Fe2 by discussing mechanical properties, metallurgy investigations, fatigue strength, static and dynamic 3-point bending test and joining techniques such as riveting and welding.

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The study shows that the development goals, such as strength for lightweight design, good castability or the reduction of production costs, have been improved.
The study shows that the development goals, such as strength for lightweight design, good castability or the reduction of production costs, have been improved.
(Source: gemeinfrei / Unsplash)

Today’s castings are getting thinner and larger with more and more parts 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 structure parts in HPDC. AlMg4Fe2 (Castaduct-42) offers a dispersoid hardening mechanism which does not need any heat treatment. AlMg4Zn3Fe2 (Castaduct-18) is a cold hardening alloy which can be stabilized by T1 or T5 but does not need T6 or T7 heat treatments.

The Alloy AlMg4Fe2 (table 1) was developed for any kind of structure part such as the cross beam shown on figure 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 structure parts (YS > 120 MPa and E > 10 %) 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 casting during heat treatments is a huge issue which is not applicable here.

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 structural components where stiffness is key, like the door frames shown in figure 10. The main advantages of Castaduct-18 are a high strength in the as-cast state (up to 180 MPa / 26 ksi), highest dynamic load, good ductility and corrosion resistance. An AlMg4Zn3Fe2-type alloy demands a higher technical level in casting 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 production issues and die costs. Its simplicity has positive effects on alloy manufacturing costs, too.

The main strengthening mechanism in Castaduct-42 is solid solution strengthening by Mg solute atoms. Similar to Castasil-37, AlMg4Fe2 requires no heat treatment. Results of tensile tests demonstrate that Castaduct-42 has a strength and ductility which fulfills requirements for structural components (table 2). 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 AlSi alloys. Furthermore, due to the absence of silicon the alloy can be anodized showing an excellent surface quality.

In comparison to widely used Al-Si 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 casting weights up to 12 kg (25lbs) 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 is shown in figure 1. This structure part 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
Fatigue strength was investigated by the Steinbeiss-Transferzentrum (BWF, Esslingen, Germany). 25 samples out of 3 mm HPDC plates were subjected to bending stress (R = -1). Default probability of 10 % was 106 MPa (15.37 ksi). This value is slightly higher than values to be found in literature1-3.

3-Point Bending Test
The Institute for Vehicle Concepts (DLR Stuttgart, Germany) carried out static and dynamic 3-point bending test. Figure 3 shows the experimental setup. In figure 4 a curve for 3 different impact velocities can be seen: Quasistatic, 1 m/s and 5 m/s. There was a high reproducibility of these curves. The maximum force in each curve is quite similar, a dynamic decrease of the breaking stress does not occur. There is no dynamic embrittlement, but 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
In cooperation with the German company Oerlikon-Wirth GmbH & Co. KG MIG welding trials were carried out. A 3 mm HPDC plate in AlMg4Fe2 and AlMg0,5Si0,5 sheet material were joined together using a 1,2 mm wire of AlMg4,5Mn. With the help of a welding power source Fronius TPS 400i weld seams in good quality could be produced. There was no tendency for hot tearing or porosity. Microstructure of the weld seam was very fine (see figure 5).

Figure 6 shows a hardening profile from casting across the weld seam into the sheet material (measured by Welding and Joining Institute, TU Brunswig, Germany). In spite of the miss match of materials (which is typical for welding) no significant change in hardness could be measured. In case of classic AlSi10Mg type of alloys, strong change in hardness can be found, especially in the heat affected zone near by the weld seam. There is no significant effect of heat treatment neither on casting nor on sheet material, thus no major change of material properties can be measured.

Riveting Trials
Several investigations for riveting were carried out. One was realized with 3 mm plates produced in the Tech Center of Rheinfelden Alloys. 14 trials with the rivet type Henrob Betamate 1640 showed that the alloy AlMg4Fe2 was the most suitable out of all alloys examined in this investigation.

Shock towers with a shot weight about 5 kg were casted in AlMg4Fe2. They passed the production accompanying examination for riveting. The series production for these shock towers ran in AlSi10MnMg with T6 heat treatment.

Metallurgy
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 alloy is reduced by Fe. With up to 0.4 % Si there is a silicon free Al3Fe eutectic, Si can be found in the alpha phase, and there is no AlFeSi phase6. These descriptions and statements could be confirmed by the present investigation.

Figure 7 shows the Al-Fe phase diagram (Thermocalc calculation by Prof. Y.Li, NTNU, Trondheim, Norway). The eutectic point is at 1.74 wt.% Fe with liquidus temperature of 655 °C. Solidification path of an Al - 1.6wt.% Fe binary alloy starts with liquid, goes across liquid + alpha Al into alpha Al + eutectic (Al13Fe4). In other words, an Al- 1.6wt.% Fe binary alloy is hypoeutectic.

Eutectic behavior together with relatively low eutectic temperature and low die sticking tendency are positive characteristics that lead to good castability in HPDC. Negative is its low strength of the AlFe eutectic (about 75 MPa / 11 ksi), so Magnesium was added to improve solid solution strengthening.

Due to the Mg addition (4.5wt.%), the Al-Fe phase diagram is much changed, see figure 8. The eutectic temperature is further reduced from 655 °C to 629 °C; the eutectic point is changed from 1.74 to 1.31 wt%. Solidification path of Al-1.6wt.%Fe-4.5wt.% Mg ternary alloy starts with liquid, crosses the zones Al13Fe4 + liquid and Al13Fe4 + Al + liquid and ends in the solidification structure primary Al13Fe4 + eutectic. In contrast to an Al-Fe1.6wt.% binary alloy an Al-1.6wt.%Fe-4.5wt.% Mg ternary alloy is hypereutectic.

Micrographs of AlMg4Fe2 (figure 9) 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 could be found.

Scanning electron microscope photograph (SEM) together with electron probe micro analysis (EPMA) of eutectic zone showed an average concentration of alloying elements in the eutectic region about Fe 6.86 wt% and Mg 2.67 wt%. Average constitution of Fe-rich particles was Al 64.5 wt.%, Fe 35.5 wt% and nearly no Mg. Arise of an Al13Fe4 phase is reasonable.

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 alpha 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 alpha Al dendrite instead of primary Al13Fe4 or eutectic.

Castaduct-18: Particularly High-Strength HPDC Alloy for Structural Parts

Recently introduced alloy is Castaduct-18 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. Temperature-time profile starting from the oven trough all 3 HPDC phases to the solidification has an influence on mechanical properties of the component. Unlike Castaduct-42 heat treatments have an influence on Castaduct-18.

Table 3 shows mechanical properties achieved 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 strength ductility moderately.

First trials of this alloy were done by Fonderie Cervati spa, Brescia, Italy. A door frame (see figure 10) 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
The alloy Castaduct-18 showed very high fatigue strength compared to other HPDC alloys for structure parts1-3. The Steinbeiss-Transferzentrum (BWF, Esslingen, Germany) tested 25 samples from 3 mm HPDC plates for bending stress (R = -1). Default probability of 10 % was 123 MPa (17.84 ksi, see figure 11).

MIG Welding Trials
Similar to the trials with Castaduct-42, the company Oerlikon-Wirth GmbH & Co. KG (Reutlingen, Germany) ran MIG welding trials with 3 mm HPDC plates in AlMg4Zn3Fe2 and AlMg0,5Si0,5 sheet material, see figure 12. Microstructure of the weld seam was very fine.

The hardening profile is shown in figure 13. 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 AlMg4Zn3Fe2, 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 casting, sheet and filler material can be found. As a result hardness lies between casting 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
First riveting trials were realized with 3 mm plates produced in the Tech Center of Rheinfelden Alloys. It was possible to rivet the alloy AlMg4Zn3Fe2. 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
The addition of zinc turns the alloy AlMg4Fe2 in a quaternary alloy system. Quaternary alloy systems can be very complicated. In this case the phase diagram of AlMg4Zn3Fe2 is quite similar to ternary alloy AlMg4Fe2. Figure 14 shows the phase diagram (Thermocalc calculation by Prof. Y.Li, NTNU, Trondheim, Norway). The addition of Zn has also the influence of moving the Al-Fe eutectic point to lower Fe content. For the Al-4.2Mg-3.5Zn-XFe alloy, the eutectic point is further lowered down to 1.1%. If the Fe content is higher than 1.1 wt%, the Al13Fe4 phase will be the first phase to form during solidification. The eutectic temperature is 626 °C; still lower than in case of AlMg4Fe2. Solidification path of an Al-4.2Mg-3.5Zn-1.6Fe quaternary alloy starts with liquid, goes across liquid + alpha Al into alpha Al + eutectic (Al13Fe4). In other words, the AlMg4Zn3Fe2 quaternary alloy remains hypereutectic.

One reason why this quaternary phase diagram is not too complex is the absence of a ternary AlMgZn phase. According to calculation of Prof. Li, formation of a β-AlMgZn phase is possible, but this phase is not stable and will most likely dissolve at elevated temperatures. So the calculated phase diagram is still accurate enough without this β-phase.

Microstructure of the eutectic is very fine (see figure 15). It is even difficult to depict it by optical microscopy. The fraction of eutectic particles is slightly higher than AlMg4Fe2. Also, a fraction of skeletal Zn-rich intermetallic particles formed in the alloy. The fraction of coarse rod shaped Al13Fe4 particles is less than in the AlMg4Fe2 alloys without Zn. The Al13Fe4 particles tend to be smaller in the AlMg4Zn3Fe2. The reason could be that the growing Al13Fe4 particles have a lower growth rate in the present of zinc.

Electron probe micro analysis (EPMA by NTNU, Trondheim) on 5 particles showed that all the rod shaped particles are Al13Fe4 phases. The average constitution of the particle is about Al 64.5 wt.%, Fe 35.5 wt% and nearly no Mg. EPMA 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.

References

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2) Aluminium Zentrale Düsseldorf: “Aluminium Taschenbuch”, 15. Auflage, Aluminium Verlag Düsseldorf (1996)
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8) Liang, D., W Jie, H Jones: “The effect of growth velocity on primary spacing of Al3Fe dendrites in hypereutectic Al-Fe alloys“, Journal of crystal growth (1994)

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